NASA and general aviation

FOREWORD

In 1915 a far - seeing United States Congress created the Na - tional Advisory Committee for Aeronautics (NACA). Its purpose: improve and develop American aviation, which was then in its in - fancy.

Today NACA’s successor, the National Aeronautics and Space Administration (NASA), has a much broader mission. But a s its name implies, NASA is still very much involved with aeronautics.

Yet until now, the agency’s important role in “general aviation” has never been fully told.

1 find this rather astonishing, for in sheer numbers of planes, pilots, and air operations, general aviation is by far the largest single component of all U.S. aviation - civil and military.

NASA and General Aviation should do much to correct any possible misconception about the fine working relationship be - tween the agency and the aircraft industry. Though perhaps not generally known, the industry continues to rely heavily o n the basic technology that NASA and its predecessor have developed during decades of research. It is this traditional government- industry relationship that propelled America to a position of leadership in world aviation and space.

But a s the author clearly points out, our leadership in some key technical areas is eroding. This is especially true in general avia - tion. While it is the biggest segment of the total aircraft industry, it still gets the smallest share of any federal funding budgeted for aerospace research.

Foreign nations, o n the other hand, have different priorities.

Several of them are striving for preeminence in all fields of flight.

Consequently, a subsidized foreign industry puts U.S. manufac - turers at a distinct disadvantage. If we wish to compete effectively in world markets, then we must dedicate the talent and resources necessary to advance our technology.

The selection of Jeff Ethell to tell this enlightening story is com - mendable. Few full - time authors have the skills to communicate technical subjects in other than highly complex terms. Jeff Ethell, a n experienced pilot and an accomplished writer, is such a profes - sional.

Besides exploring the intricacies of aerodynamics, energy, and safety, the author reveals a number of little - known facts about s o m e truly incredible achievements in the history of aeronautical experimentation and progress. H e not only details what NASA and NACA have done for general aviation over the years, but he also treats us t o a n impressive look ahead.

Anticipated improvements in tomorrow’s aircraft utility, whether the use is for business, agriculture, or some other com - mercial application, appear just a s exciting as the scientific developments in technology.

New concepts in composite materials and metallic structures, along with revolutionary advances in systems integration, will pro - vide the next generation of aircraft with significantly higher per - formance, reliability, and efficiency. And new innovations in com - puter technology and production techniques may result in a total - ly new kind of airplane.

But t o achieve these goals will take more than the imagination and ingenuity of design engineers and builders. It will take broad- based public support, a recognition that general aviation is a ma - jor contributor t o America’s economic security.

This is a splendid work, the definitive book on the many technical wonders of a dynamic and vital national asset. That is why I hope it receives the widest exposure possible. In fact, it should be required reading for anyone with even a modicum of in - fluence in shapicg our nation’s future.

J a m e s R. Greenwood Retired Senior Vice President Gates Learjet Corporation viii

PREFACE

General aviation remains the single most misunderstood sector of aeronautics in the United States. Airline companies make the news on a regular basis and travel on the major carriers is taken for granted, yet they comprise but 10% of all US. nonmilitary flight operations. The remainder belongs to general aviation, one of the major keys to American business success.

This book will give you a detailed look at how general aviation functions and how NASA helps keep it o n the cutting edge of technology in airfoils, airframes, commuter travel, environmental concerns, engines, propellers, air traffic control, agricultural development, electronics, and safety. N o doubt s o m e of you will be surprised to find that there is so much activity in general avia - tion. This is not simply the weekend pilot who wants to take a spin, but also the businessman who relies on aircraft to get him where he is going when he wants to go, the corporation that wants sophisticated jet transportation for its executives, the family that wants to travel where no major airline terminal exists, the farmer who wishes to have his crops protected, the mission pilot who transports spiritual and physical aid over dense jungle terrain, the medivac team that requires rapid transportation for an accident victim or transplant recipient. And the list of benefits goes on.

If there was ever a time when NASA's aid was pivotal t o general aviation's survival, it is now. Come along with me, a s w e take a look into a fascinating and seldom seen world through the eyes of NASA and its numerous programs.

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CHAPTER 1

WHAT I S GENERAL AVIATION?

hough few people are aware of it, general aviation, all civil flying activity except airline operations, accounts for 98 per-

T cent of the nation’s 215 000 aircraft and 96 percent of its

815 000 pilots. It is by far the most active and the most diverse of all aviation operations. When combining the amount of airline and general aviation flying, the airlines account for only 10 percent of the total hours and operations flown. And the majority, around 80 percent by s o m e estimates, of general aviation flying is for business purposes, serving all of the nation’s 1 4 746 airports while the airlines reach only 7 3 3 (or 5 percent). The National Aeronautics and Space Administration (NASA) includes air taxi and commuter aircraft under its general aviation research pro - grams while excluding rotorcraft.

Though the public conception of general aviation is that of private owners flying for pleasure, the opposite is true, particularly in these days of less discretionary income. It includes every kind of piloted airborne vehicle from ultralight powered hang gliders and balloons to airline - size corporate jets.

Not only does the business community rely very heavily o n general aviation to function, but the U.S. balance of trade is heavi - ly dependent on the export of aircraft. In 1979, civil aircraft sales resulted in a net positive contribution of $10 billion to our trade balance. In the past 20 years, the U.S. share of total exports of the major industrialized nations has dropp2d by almost one - third, while U.S. civil aircraft export sales have increased ninefold, and now sustain over 60 percent of the total civil aircraft work force.

NASA and General Aviation More than 90 percent of all general aviation aircraft in the world originated in U.S. factories, and the market is growing substantial - ly as foreign commuter airlines are seeking new replacement air - craft. Foreign aircraft manufacturers are producing new turboprop aircraft t o meet the need, threatening the U.S. lead in the field.

In the past, NASA’s input t o general aviation aircraft technology has lagged behind military types and commercial airliners. The majority of today’s fleet was built on the aeronautical science that was driving the aircraft industry before and during World War 11.

With the exception of s o m e new concepts developed by innovative experimental aircraft home builders, and applications of advanced aeronautical technology in a new generation of business aircraft, today’s general aviation aircraft would not have looked out of place in the 1950’s. As a matter of fact, several designs that originated in the late forties and early fifties are still being produc - ed and have performance equal t o many newer designs.

From its inception in 1915, the National Advisory Committee for Aeronautics (NACA) and now NASA have been involved in aeronautical research and therefore have been a part of the prog - ress in general aviation (GA). For example, most designs use NACA airfoils. Recently, more than a dozen contemporary produc - tion and prototype GA aircraft have been designed with the aid of new technology gained from NASA programs of the past decade, ranging from twin - jet business aircraft t o single - engine trainers.

Since the beginnings of NACA, the goal has been t o conduct research that industry can use in building better products; but without the talent and engineering in GA companies, there would be no American lead in general aviation. The goal at NASA is to preserve and advance that lead. American leadership in the field, however, is o n the wane.

NASA’s basic goals for general aviation are continuing im - provements in efficiency, safety, environmental compatibility, and utility, broken down into research dealing with aerodynamics, structures, propulsion, and avionics. NASA’s Langley Research Center, in Hampton, Virginia, is responsible for the majority of those technology programs in general aviation aeronautics and for flight research. Much of the flying is done out of Wallops Flight Center, Virginia. Wind tunnel testing has been conducted primari- What Is General Aviation?

ly by Langley. In addition, the unique facilities of the Ames Research Center a t Moffett Field, California, primarily the 40 x 80 - foot wind tunnel, are used for specialized GA research.

Propulsion research is done a t the Lewis Research Center in Cleveland, Ohio.

These NASA centers have a battery of research facilities which a single aircraft company could never afford, ranging from com - puters t o huge wind tunnels that can test a full - size airplane a s if it were in flight. Each one of these research tools can produce results that might not be obtainable in any other feasible or economical way. An initial NASA approach t o a problem often is analytical. The next step may b e the testing of a small wind tunnel model, or of more elaborate models flown by remote control, either in a n adapted wind tunnel or outdoors. Piloted simulators - versatile ground - based machines that duplicate key characteristics of the full - scale aircraft - may be used. Finally, the idea may be tested in free flight on a full - scale airplane flown by NASA’s research pilots.

Though NASA has s o m e unique facilities and personnel, GA programs frequently involve industry and universities since NASA cannot d o the work alone. lndustry has to translate research into practical reality, and if the result will not sell in the marketplace, it has t o be scrapped. Since the beginnings of aeronautics, univer - sities have been a primary source of fundamental research capabilities. Without the help and leadership offered in both camps, NASA’s research would not be reflected in a final product.

As NASA’s Dr. Walter B. Olstad said before Congress, “Our reason

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‘ for existence is solely to provide technological services to our customers.” The continuing growth of general aviation and commuter air service reflects their increasingly important roles in business and in the transportation system. A major reason for this growth has been the restructuring of truck and regional airline routes which has reduced air service t o many smaller communities. Since the U.S. has fallen behind in commuter aircraft markets both at home and abroad, many commuter airlines have had t o purchase foreign aircraft t o meet their needs. Foreign firms, backed financially by their governments, have been able to exploit new market oppor - tunities. Even greater foreign competition lies ahead, spurred by NASA and General Aviation national determination in several countries to capture greater shares of markets once dominated by the U.S.

A major factor in the future success of the American commuter and general aviation industry is the development of technological - ly superior next - generation aircraft that are safer, more reliable, more comfortable, and more economical to operate than those of the competition. Unfortunately, industry does not have the technical personnel, facilities, or resources t o generate the high levels of technology needed. The availability of this technology in time t o influence new aircraft in the next 10 years is, therefore, heavily dependent on NASA’s programs.

NASA’s current involvement with general aviation is an exten - sion of what has been taking place over the past several decades.

In aerodynamics and flight dynamics, emphasis will continue t o be placed on safety and energy efficiency. Further, research is needed on stall/spin phenomena of both high - and low - wing air - craft a s well as less conventional configurations such as canard (forward) winged aircraft. One promising development has resulted in the segmented, drooped - wing leading edge that can reduce the abrcptness and severity of the roll - off at the stall and thereby prevent unrecoverable flat spins. Long - range goals are t o evolve design methods that industry needs for safer aircraft, e m - phasizing simulation and analytical techniques.

Other aerodynamics research is aimed at increased energy effi - ciency, including work on cooling drag reduction, enginelairframe integration improvements, and further development of natural laminar flow airfoils and aircraft technology. Smooth contours and surfaces, achieved with composite materials, are needed t o promote laminar flow. Composites themselves are the subject of a great deal of research due t o potential weight reductions of up to 30 percent, reduced drag, and more favorable structural integra - tion.

Propulsion research will concentrate on achieving greater fuel efficiency, reduced weight, lower maintenance, and greater reliability. Work will continue on spark - ignition reciprocating engines, but improvements are required on supercharged two- stroke cycle diesel engines and stratified charge rotary combus - tion engines. Turbosupercharger research will remain a n integral part of the program. Basic research will continue into ignition and What Is General Aviation?

combustion chamber processes, engine cycle modeling, fuel in - jection, improved cooling, and cooling drag reduction. These ad - vanced engine concepts can lead t o improvements in fuel economy of one - third to one - half relative t o current engines.

Small turboprop engines are of particular interest for high - performance single - engine or light twin - engine aircraft, but addi - tional research must continue t o achieve increased fuel economy and initial cost reductions, Propeller research needs t o emphasize higher aerodynamic per - formance for greater energy efficiency using advanced blade plan - forms and airfoils and new concepts for reducing tip losses.

Reduced noise and increased performance will be studied while research into use of composites for aeroelastic benefits, lower blade weight, greater structural damage resistance, and improved fatigue life needs to continue. Propeller and nacelle integration studies should be extended to reduce installation and nacelle in - terference losses.

Ground and flight test of the demonstration advanced avionics system (DAAS) has been one of NASA’s efforts to integrate navigation and flight management, thus improving safety and utility. However, these systems must be reliable and affordable to find acceptance, making reliability, maintainability, and low cost major requirements. State - of - the - art electronics have been a ma - jor part of this program, which has an integrated data control center used for alphanumeric messages and serves a s the primary interface with the pilot. The lDCC can call up various functional checklists, execute specific functions such a s autocourse, serve a s a data entry point, and even provide a ground data link capability.

Other areas of avionics research include an integrated fluidic flight control system which will hold attitude, altitude, heading, and velocity, and provide glide slope instrument landing tracking.

Advanced synthetic voice response digital technology will be in - vestigated as a means of increasing general aviation communica - tion capacities. In addition, methods will be studied for using the azimuth and elevation measured by a microwave landing system (MLS) to fly curved paths without the need of expensive distance - measuring equipment .

Improving means for single - pilot instrument flight rule (IFR) operations is being investigated, including speech recognition and NASA and General Aviation synthesis techniques t o improve aidground communications, ad - vanced displays, advanced terminal area systems evaluation, and ways of better interpreting and using weather data in flight.

An extensive crashworthiness program is leading toward im - proved crash safety and occupant survivability, covering three basic areas. O n e is energy - absorbing structures for seats and fuselage floors t o limit the loads imposed on the occupants in a crash. Another is the development of structural analyses t o predict large deflection structural response on impact. A third area is con - trolled crash tests of full - scale aircraft, which has been very suc - cessful in studying structural deformation and impact loads.

Results of simulating crashes of 60 to 90 miles per hour on full - scale aircraft have led t o second - generation load - limiting seats and floors. Computer mathematical models for simulation of oc - cupant/seat/structural analysis have been a basic part of the pro - gram as well. In addition, the crashworthiness of the newer com - posite materials is being investigated.

NASA also has been assisting the Federal Aviation Administra - tion (FAA) in evaluating the crash - activation of emergency locator transmitters (ELT), which general aviation aircraft must carry, in order to recommend improvements.

In agricultural spraying, research has been conducted to im - prove dispersal and distribution of chemicals or dust applied from aircraft. A primary part of the research involved determining the interactions between the aircraft wake turbulence and the dis - persed materials and how a modified aircraft might improve swath control. Wind tunnel and flight tests were conducted and theoretical prediction methods were developed that are now being used by other government agencies, universities, and industry.

The challenges of general aviation will remain the subject of in - tense research a t NASA, within industry, and within the academic community as its utility increases. The Commuter and General Aviation Research and Technology Program of NASA's Office of Aeronautics and Space Technology provides a glimpse into the future of aviation's most active arena.

CHAPTER 2

AERODYNAMIC EFFICIENCY

the heart of NASA’s general aviation research efforts lies the desire t o improve aircraft efficiency: in other words, t o ,explore areas that will allow aircraft t o fly farther and

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faster on less fuel with improved passenger comfort and safety.

Basic t o this goal is research dealing with high lift/drag airfoils, supercritical aerodynamics, natural laminar flow, composite struc - tures, and drag reduction.

Laminar Flow

As fuel prices climbed in the early 1970’s, fuel efficiency took on ever - increasing importance and research efforts w e r e inten - sified t o reduce aerodynamic drag. As a result, NASA has made significant progress in reducing parasite (zero - lift) drag and in - duced (caused by lift) drag. The drag reduction challenge is t o reduce skin friction, which accounts for about 30 t o 50 percent of a n aircraft’s drag a t cruise. NASA’s approach in reducing drag has been multifaceted, including use of both laminar and turbulent boundary layer control. Keeping the boundary layer (the air layer closest t o the skin) o n a surface laminar, or thin and smooth, can reduce skin friction by a s much as 90 percent. Very smooth and controlled pressure gradients can delay the boundary layer transi - tion t o turbulent flow and produce significant regions of natural laminar flow (NLF).

For years, designers of general aviation aircraft, particularly of light single - and twin - engine types, have used the “four - digit” The Cessna Citation 111 uses the NASA supercritical airfoil lo obtain a high-aspecl- ralio wing. The company found 11 could use relatively [hick wing sections and slill achieve efficienl Mach 0.8 flight. The thick section minimizes Lhe weight ofthe high - aspect - ratio wing (ratio of wing span to width or chord), and provides a large volume for fuel storage. (Cessna Aircraft) series of airfoils developed by NACA a s far back a s the 1930’s. As a matter of fact, very few aircraft flying today, including Cessnas from the Airmaster to today’s Skyhawk and Skylane, incorporate airfoil technology radically different from that used so successful - ly prior to and during World War I I . One of the major developments of the late 1930’s was the six - series of laminar flow airfoils which, though not totally successful in producing laminar flow using the existing construction methods, went a long way toward improving the aerodynamic art.

Wings, stabilizers, propellers, and control surfaces use airfoils that can be improved to promote laminar flow (less drag). A modern, properly constructed laminar flow airfoil is generally con - sidered one that can achieve laminar flow over the first 70 percent of the upper and lower surfaces over a reasonable range of angle of attack and speed/altitude.

With laminar flow the air particles move in smooth, parallel (laminated) layers over the surface, each with a constant velocity and motion relative t o its neighboring layers. Turbulent flow Aerodynamic Efficiency results when these laminated layers break up, resulting in higher friction drag. The problem of maintaining extensive laminar flow is made more difficult by rivets, dents, bumps, joint overlaps, manufacturing imperfections, and even bugs that have hit the air - foil. However, the problems inherent in achieving laminar flow are considered worth solving t o obtain improved cruise speeds which mean less fuel burned for a fixed distance.

After the four - digit series NACA developed the five - digit airfoils in order to provide airfoils with better maximum lift. They were ap - plied to such well - known products a s the Beech Bonanza, Baron, and King Air. But these four - and five - digit airfoils, while providing increased improvements in lift, promoted turbulent flow because of their shape.

NACA researchers knew this to be the case even in the 1930’s so laminar flow was singled out for more intensive investigation. The six - series airfoils, when achieving laminar flow, can reduce drag up to 50 percent over the older series but the airfoil surface must be very smooth (minimum waviness). In addition, due to the sharp nose radius the six series does not have very high maximum lift capability. The use of bonding and composite materials (smooth surfaces) in the last few years has opened the doors to achieve ex - tensive natural laminar flow.

In the 1950’s, NACA left low - speed airfoil research to concen - trate on transonic and supersonic designs. Out of this research the so - called supercritical airfoil, optimized for drag reduction at high subsonic cruise, had camber located near the trailing edge with improved lift over drag (UD) at higher cruise lift coefficients. In the early 1970’s, a low - speed derivative of supercritical airfoil technology was developed for use o n general aviation aircraft.

Though the resulting GA(W)-l was not a true supercritical airfoil, it paved the way for general aviation manufacturers to obtain im - proved WD benefits. At that time, the GA industry was not op - timistic in achieving laminar flow because of the old problems of rough surfaces, but it did want a better turbulent flow airfoil. In this regard, NASA developed an airfoil with higher maximum lift - the GA(W)-I, now known a s the LS(1)-0417-and improved UD at climb - out speeds. This was particularly beneficial for im - proved performance (safety) on twin - engine aircraft with o n e engine out.

NASA and General Aviation Both the Beech Skipper and the Piper Tomahawk light trainers were designed and built in the mid - 1970’s using the new airfoil.

The result for both of these aircraft was a very low stall speed, ideal for traffic pattern training. T h e tradeoff came with decreased effi - ciency a t normal cruise speeds, but that was not t o be the primary purpose of either aircraft since they would b e used primarily for training near airports. Due t o the aft camber of the wing and the resulting high pitching moment, flap deflection was limited t o pro - vide drag for approach angle control rather than t o decrease stall speed.

NASA improved the GA(W)-l by moving the camber forward, thus increasing lift and decreasing the pitching moment at the cost of less docile stall characteristics. The GA(W)-2 or LS( 1)-0413 was a subsequent development.

With improved construction techniques that could eliminate most of the surface roughness problems which restricted laminar flow, NASA developed a new series of airfoils in the mid - 1970’s that would combine the high maximum lift of the L S series with the low drag of the six series. With the NLF(1)-0416 and NLF( 1)-0215F, natural laminar flow became a realistic possibility.

Even if extensive laminar flow does not occur, high lift still exists and the drag will be no higher than on a turbulent flow airfoil of the s a m e thickness.

The NLF(1)-0215F, flight tested in glove form on a T-34C, uses a flap that can be deflected upward 10 degrees at cruise, much like high - performance jet aircraft, t o achieve the best UD for both the low - and high - speed regimes. This will be particularly beneficial for the new generation of GA high - performance, single - engine air - craft designed t o cruise at 300 miles per hour at 25 000 feet. An advanced N L F supercritical airfoil was also flight tested on a modified F-111 over a range of wing leading - edge sweep angles from 10 t o 2 6 degrees at Mach numbers from 0.8 t o 0.85. The ex - periment also showed that significant laminar flow could be achieved a t off - design cruise conditions of Mach number and leading - edge sweep. This is particularly important for commuter and business jet transport aircraft (see chapter 5).

Cessna believes that its pressurized 2 10 is a prime candidate for benefiting from a wing that can achieve natural laminar flow. Col - laborating with NASA, Cessna has conducted a laminar flow Aerodynamic Efficiency visualization program o n a 210 wing using chemical spray in the Wichita State University wind tunnel t o verify NLF potential. With a 375 - horsepower turbocharged engine, the 210 can reach over 30 000 feet where high true airspeeds are obtained.

NASA conducted a study of the benefits of cruise design op - timization for high - performance, single - engine airplanes with a cruise speed of 300 knots and a cruise range of 1300 nautical miles with six passengers. The results revealed that these perfor - mance estimates could be reached with fuel efficiencies com - petitive with present - day, slower flying aircraft. But the major achievement would be the potential for 200 to 400 percent greater fuel efficiency than is achieved by current twin - engine airplanes capable of similar cruise speeds, payloads, and ranges.

NASA conducted flight tests t o see just how much laminar flow was being achieved on eight different airplanes with smooth, thick - skinned, bonded, milled aluminum, or composite wing sur - faces. Tests were conducted on smooth portions of the Cessna P210 and Beech 24R. Extensive tests also were conducted on the Bellanca Skyrocket 11, Lear 28/29, and three Rutan Aircraft Fac - tory designs, the Long - EZ, VariEze, and Biplane Racer, which utilize modern construction materials and techniques t o provide aerodynamic surfaces without significant roughness and waviness. Previous flight experiments involving laminar flow measurements were limited t o either airfoil gloves or specially prepared (filled and sanded) wing sections. Only sailplanes were achieving significant laminar flow without modification. For the later tests, no preparation of the aircraft was allowed; they w e r e “production quality” straight out of the factory.

The results were more than encouraging. There were extensive areas of N L F on all the aircraft tested, making laminar flow a very practical possibility on modern production aircraft. Even with pro - peller slipstream effects it was found that the laminar boundary layers were not destroyed completely, changing previous conclu - sions. Thus, N L F airfoils may provide drag reduction benefits, even on multiengine configurations with wing - mounted tractor engines. O n swept wings, particularly in the high - speed cruise regime, spanwise flow contamination at cruise was found not t o be as much a concern for laminar flow. Laminar flow on winglets ap - pears very promising as well.

1 1 NASA and General Aviation Insect impact contamination has always been a problem in achieving laminar flow, but the insect debris pattern collected o n a NACA six series airfoil revealed only one - fourth of the strikes t o be of sufficient height and in a position t o cause transition. A com - bination of the newer N L F airfoil geometries and mission profiles could minimize the sensitivity even more. Natural laminar flow is n o longer out of reach for nonspecialized aircraft; it can be achieved not only o n current production aircraft (up t o 70 percent in s o m e cases) but t o a n even greater extent o n future designs without great expense.

Computers and Airfoil Design

The use of computers is o n e of the important factors in the design of airfoils t o reach the projected efficiencies. NACA used t o produce catalogs of airfoils that designers would ponder over, test, retest, and finally apply to a n airframe. It could take up to o n e week using a mechanical calculator to c o m e up with a single pressure distribution; the same calculation on a modern computer takes one second. A designer can load in cruise speed, stall speed, weight, lift coefficient at various speeds, and other aerodynamic data, then custom design a unique airfoil from NASA - developed codes. The structure can be optimized rapidly, with less emphasis on percentage thickness of the wing, and the airfoil will more closely suit the purpose. New GA aircraft such as the Mooney M 3 0 1 have derivative airfoils developed from NASA codes. Perfor - mance verification of the completed designs can then be tested in model form in NASA wind tunnels or can be tested full scale in the 30 x 60 - foot tunnel at Langley or the 40 x 80 - foot tunnel a t Ames.

NASA has developed a low - speed airfoil design program for a minicomputer. Though the program is limited t o some degree, it is certainly a start in placing airfoil design at the fingertips of most people around the world who have access t o a minicomputer that will program in BASIC on an 8 K core.

An airfoil design institute has been established by NASA a t Ohio State University using a NASA - maintained data base. Any customer can obtain a n optimized airfoil section, drastically reducing the time needed t o advance the state of the art. This can The black painted wing on this Bellanca Skyrocket I1 allowed NASA/OSU research - ers lo visualize chemicals applied to measure natural laminar flow (NLF). The air- crafl, conslructed of fiberglass composiles, proued lo be extremely smoolh aerodynamically, with laminar flow extending from lhe wing leading edge for ap - proximately lhe first 50 percent o f the wing's surface. This greally aduanced NASA NLF research.

have a significant effect o n reducing costs in the general aviation industry.

Without a stable NASA research base, many US. companies would find themselves unable to produce aircraft competitive with the world market. A good example is the Learjet business jet, which was designed and developed from a Swiss fighter in the ear - ly 1960's by a group of less than 100 engineers with little wind tun - nel testing, limited computer facilities, and little specialized Using the NASA/Ohio State University airfoil program, Mooney Aircraft incor - porated a natural laminar flow wing on its new pressurized 301. Cruise speeds of 260 mph (Lop speed 301 mph) aL 25 000 feet will give a 1 150 statute mile range aL 14.5 miles per gallon. NASA's participalion in computer and wind Lunnel Lesting con - tributed L o this breakthrough for a six - seal general auiation aircraft. (Mooney Aircraft) equipment necessary t o develop a high - performance aircraft. In - stead, Bill Lear and his associates relied heavily on the basic technology that NACA and NASA had developed over decades of research. Learjet wings and tails employed modification of NACA airfoil sections developed in the late 1930's and early 1940's. O n e of the first Learjets built was mounted in NASA Ames' 40 x 80 - foot wind tunnel for testing in 1966, yielding a great deal of data on this first business jet. The wing flap design was developed directly from the experimental work of NACA.

Winglets

Though several Rutan designs have employed NASA - developed winglets to excellent advantage, the Gates Learjet Model 28 became the first production aircraft t o fly with them. Now the Model 29 and 55 Longhorn are flying with the winglets. Cruise fuel Among the engineering challenges incorporated in the Gulfstream Ill were NASA winglets and airfoil, along with composite primary structures. Drag improvement was significant, resulting in better fuel consumption and higher performance.

(Gulfstream Aerospace) flows o n the Model 28/29 are around 26 percent lower for the same payload/speed combination. Takeoff and landing perform - ance also has been significantly improved. These improvements resulted from several factors: the aircraft can operate a t higher altitudes d u e t o the wing extensions (6 feet plus winglets each side) where fuel consumption is reduced, and drag has been re - duced by removing the tip tanks, increasing the aspect ratio, and adding the NASA - developed winglets incorporated on the Gulfstream 111 a s well with great success.

The winglet itself reduces vortex drag by producing a forward lift component somewhat similar to sails on a boat. At high lift coefficients this effect more than offsets the drag due to the winglet itself. Winglets also significantly increase the lifting s u r - face or aspect ratio of the wing, though a penalty of additional structural weight must be considered. While there is great poten - tial for winglets on many different designs, each winglet must be tailored for each design in order to achieve the desired results.

NASA and General Aviation

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Gulfstream Aerospace carried use of NASA research ouer inlo their Commander Fanjet 2500 with the possibility of nalural laminar flow wings and winglets. Winglels improve wing span efficiency by positiue control of the tip vortex flow field, reducing drag due to lift. The cockpit of lhe 1500 also will incorporate much of the NASA single pilot IFR research. (Gulfstream Aerospace)

Composite Materials

Since 1970, NASA has actively sponsored flight service pro - grams with advanced composite materials - essentially woven cloth impregnated with fibers of carbon or other high - tensile strength materials - particularly on commercial transports and helicopters. Composite aircraft structures have the potential t o reduce airframe structural weight by 20 t o 30 percent, reduce fuel consumption by 10 to 15 percent, and thus reduce direct operating costs (DOC). NASA, primarily through its Aircraft Energy Efficiency (ACEE) program, has tested all types of com - posites from advanced fiberglass to graphite epoxy in real - world flight regimes, accumulating over 2.5 million total flight hours.

The applications to general aviation can be significant, a s shown by Rutan Aircraft Factory and another Rutan company, Scaled Composites. They have built and flown several aircraft that are Aerodynamic Efficiency basically total composite structures and that have been proven both safe and very fuel efficient. Not only are composites extreme - ly smooth, lending themselves t o laminar flow, but they can be made stronger and lighter than most aluminum designs. Crash - worthiness remains a NASA concern since composites have been labeled a s too brittle and unable t o absorb enough energy during a crash. However, new construction techniques have helped com - posites absorb the impact of a crash. NASA hopes to “crash” com - posites in controlled tests, possibly with an all - composite helicopter prototype.

O n e of the major problems in using composites centers around reducing production costs. Small GA companies (at least in rela - tion t o the larger military and transport manufacturers) would have to find major sales opportunities in order to recoup retooling costs. In the far term, the ideal is that composite manufacture can be automated far more than with metal construction techniques.

The Lear Fan made a radical deparlure from past general auiation aircrafl since it was built almost entirely ofcomposiles, promoting light weight and low drag. It used slightly more than 200 gallons of fuel to lrauel 2000 miles, while cruising a1 400 mph. (Lear Fan) NASA and General Aviation In the near term, manufacturers are looking for ways t o eliminate large autoclaves required to set up the materials and g o to thermal stamping and forming. In other words, composites could be simp - ly heated and stamped out. Inherent in any transition t o composite manufacture is retraining of workers, a major expense regardless of how efficient construction methods are.

The most aggressive use of composites and new NLF airfoils can b e seen in Bill Lear’s last design before his death, the Lear Avia Learfan 21 00, built almost entirely of graphite composites.

The only metal components other than the engine are the landing gear, propeller hub, and many small structural fittings. Even the propeller blades are made of Kevlar - epoxy. With a custom computer - designed airfoil and all - composite construction, it has a maximum cruise speed of around 360 knots. Unfortunately, the company folded after being unable t o obtain FAA certification.

Two other all-composite aircraft that d o appear to be heading for production are the Beech Starship and the AVTEK 400. Both are twin - turboprop designs with canard surfaces and high cruise speeds. Fuel economy and efficient payloads are major benefits of these aircraft which are radical departures from the GA norm, very possibly the shape of the future.

Gates Learjet (not related to Learfan) is looking into its next- generation product a s suitable for secondary composite struc - tures. Current Learjets utilize fairings that are composite or com - posite based with new fiberglass/Nomex core materials. Learjet is also looking a t primary composite use on forward swept wings, but the cost of building the wings is still too high for the few units that would be produced. The company is looking for ways to bring the price down since composites are the o n l y avenue t o creating a wing stiff enough to resist the strong torsional/bending moments inherent in forward swept wing designs.

Cessna Aircraft Company incorporated a substantial amount of NASA research in its new Citation I l l business jet, which features a swept, supercritical wing and bonded - riveted airframe construc - tion with selected use of composites. The wing is not only de - signed to push cruise speeds up due t o delaying the drag rise shock wave effect, but the smooth bonded surface promotes s o m e laminar flow. Kevlar, graphite, Nomex core, and fiberglass com - posites are being used for flap sections, spoilers, engine nacelles, A erody narn ic Efficiency Gales Learjel is among the companies fhat depend on NASA for advanced research.

Gates builds several versions or the Learjel, including the Model 55 Longhorn, so named because it uses NASA winglets. (Gales Learjel) seat structures, and s o m e fairings and doors. Both the Canadair Challenger and the Mitsubishi Diamond 1 have supercritical, high - aspect - ratio wings as well, reflecting NASA's pervasive influence on aerodynamics regardless of country of origin.

CHAPTER 3

INTERNAL COMBUSTION

ENGINES

rom the early days of aviation the argument between aerodynamicists and engine specialists over what drives

F aircraft design has never been satisfactorily resolved. Does

aerodynamic function determine needed power or does available power determine aerodynamic form? Though the debate will most likely never be resolved, engine development has usually opened the door for advances in aerodynamics, for without power aircraft cannot fly. Needless to say, there are exceptions to this rule, par - ticularly in soaring and aircraft that become gliders like the Space Shuttle.

Overall, there have been no major design changes in general aviation aircraft engines since World War 11. The air - cooled, inter - nal combustion engine fueled by high - octane gasoline continues t o power many of the n e w designs rolling off the industry's produc - tion line. Major changes are needed to increase fuel efficiency and utility, from the smallest trainers to the largest multiengine cor - porate transports and commuter airliners.

While general aviation manufacturers have been seeking im - proved safety and improved air traffic control, the heart of their survival a s companies lies with energy efficiency. If aviation gas (avgas) becomes unavailable or unreasonably expensive, then these aircraft will have to be adapted t o u s e autogas, diesel, or jet fuel. Based on relative Btu content and production costs per gallon, the kerosene/diesel-type fuels also offer a n inherent economic advantage of 20 percent or more over gasoline. With 1985 avgas prices over $2.00 a gallon in the U.S., over $5.00 a

PWECE%ING P A G E BLANK NOT'RLW

NASA and General Aviation gallon in Europe, and just plain unavailable in many parts of Africa and the Middle East, the writing is o n the wall.

Environmental Concerns

The heart of NASA’s general aviation propulsion research can be traced t o a n initial concern with meeting standards set by the Environmental Protection Agency (EPA) in the early 1970’s. An exhaust emissions reduction program was initiated in 1973 be - tween the Federal Aviation Administration and NASA. A year later Teledyne Continental Motors and Avco Lycoming were under con - tract for emission - reduction testing through a number of avenues.

The 1974 fuel crunch then changed priorities t o fuel economy, sending industry and NASA off a springboard that led t o the ad - vanced engines discussed in chapter 4.

Even though most of NASA’s current engine research is devoted t o improving efficiency, noise and exhaust emissions in general aviation power plants remain a concern, particularly since they are perceived by the public as major annoyances. There are about 14 100 suburban airports in the U.S., most of which are located in small communities with no buffer zones and with peo - ple living nearby. Therefore, general aviation has the potential for greater community reaction t o noise and pollution than commer - cial and large transport aircraft. In addition t o the community noise, passengers and crew experience a great deal of noise and vibration, particularly in the smaller aircraft.

The exhaust emissions reduction program continued with both Teledyne Continental and Avco Lycoming. These engine manufacturers successfully pursued program goals even though the EPA lowered its emission standards in 1979.

Teledyne Continental Motors investigated and developed three aircraft piston engine concepts t o reduce emission of hydrocar - bons and carbon monoxide while simultaneously improving fuel economy by improved fuel injection, improved cooling cylinder head, and exhaust air injection. Variable ignition timing also was explored. After investigating each system and incorporating them into a 10- 520 engine, the company conducted test flights with a new Cessna 2 10 Centurion single - engine aircraft.

The 210 handled and flew well with the prototype engine; the Internal Combustion Engines only adverse result was backfiring in the exhaust after rapid throt - tle closing, a problem easily solved by shutting off air injection at lower than normal manifold pressures. For a 328.8 - statute - mile flight, the prototype engine with standard fixed magneto timing got 6.3 percent better fuel economy over the baseline engine, primarily in climb, enroute climb, and approach modes.

The tests revealed that the EPA standards for carbon monoxide, hydrocarbons, and nitrogen oxide could be met using exhaust air injection alone with no improvement in fuel economy or by using only the Simmonds improved fuel injection system to provide a leaned fuel schedule. The use of exhaust air injection in combina - tion with exhaust port liners reduced exhaust valve stem temperatures t o levels below that of the baseline engine, which could result in longer valve guide life. Use of exhaust port liners alone can reduce cooling air requirements by at least 11 percent or a 1.5 percent increase in propulsive power. A fixed ignition tim - ing of 27 degrees BTC (5 degrees o v e r standard) provided a test bed fuel economy improvement of 2 percent in cruise but this could not be substantiated in flight testing.

The basic purpose of the contract was met by improving fuel metering for better fuel - air ratio control, reducing heat transfer from exhaust gases t o cylinder heads, and oxidation of exhaust pollutants, even though the hardware used may not represent the most cost - effective means of obtaining these results. The fuel in - jection system could prove too expensive for production, and ther - mal barrier coatings or improved exhaust port design might pro - duce benefits similar to those obtained with port liners at less cost.

A similar argument could be made for oxidation of exhaust pollutants by air injection.

Avco Lycoming approached the contract investigating high - energy multiple spark discharge and spark plug tip penetration, ultrasonic fuel vaporization, and variable valve timing. Since t h e company did not include flight testing as part of the program, it used three different engines for the bench tests, the TIGO - 541, 0 - 320, and 10- 360, respectively.

Several ignition system configurations were evaluated: capacitive discharge, multiple spark and staggered spark, and various other spark plug configurations. Test results revealed that none of these resulted in a consistent, significant improvement NASA and General Aviation over the standard ignition system. Neither the spark duration nor spark plug tip locations were considered to be limiting factors for improving performance or emissions.

Ultrasonic fuel atomization gave a significant improvement in cylinder to cylinder mixture distribution at o n e engine condition but there was no improvement in total engine emissions, fuel economy, or performance. As a matter of fact, a 3 to 5 percent loss in rated full throttle engine performance resulted from the addi - tional manifold restriction of the ultrasonic unit.

Optimum valve timing sequences for each specific engine con - dition, rather than only at rated power output, revealed little im - provement o v e r the standard high - speed valve timing already employed in the company’s engines even though improvements in engine performance up t o 13 percent at certain off - rated condi - tions were obtained. Over the ranges tested and at constant fuel air ratio, valve timing did not influence carbon monoxide emission levels. Additional testing, conducted t o evaluate the effect of in - duction system tuning on these results, showed that performance improvements of the s a m e magnitude could be accomplished with standard valve timing and a revised induction system. Con - sidering the magnitudes of the overall performance improvements and d u e to the greatly increased complexity of the variable valve timing system, induction system tuning was determined a more viable concept for improving overall engine performance.

Noise

NASA’s general aviation noise research has been conducted through three primary areas: noise prediction technology, pro - peller noise/performance optimization, and interior noise reduc - tion. The majority of the work has centered around propellers.

Turbine - powered aircraft with newer, quieter fan engines d o not produce a s much noise a s turboprops and piston - engine GA air - craft.

Prop - driven light aircraft have been a source of NASA research for many years. One of the earliest programs to develop a quiet aircraft led t o a five - bladed prop test bed with extensive muffling; the aircraft was extremely quiet but was far too heavy t o be effi - cient. In 1975 the NASA Acoustics Division was organized and in - terior noise was targeted for extensive research.

Internal Combustion Engines Propeller noise prediction, comparing measured and calculated noise, was quantified using a Twin Otter. Sound pressure level was understood a s a function of frequency expressed in multiples of the blade passage frequency. A microphone mounted o n a boom on the aircraft's wing was used to take acoustic data. Wind tunnel tests were also conducted.

A Rockwell 500B was experimented on to obtain data but it was never flown. With source noise left to other departments, the acoustics team worked on sidewall design and treatment as their primary area of research. Working with Rockwell, noise was measured outside and inside the standard aircraft. Various treatments were made to lower the noise level, but it was still unclear as t o what improved noise levels. The present push is to discover why noise does what it does rather than how.

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NASA has continued t o focus design methodologies on quiet, efficient GA propellers. NASA, EPA, and the Massachusetts In - stitute of Technology used a Cessna 172 t o demonstrate a 5 - decibel flyover noise reduction without loss of performance.

Computer codes were then created, with the most promising singled out for flight testing.

Ohio State University (OSU) took on the aeroacoustics propeller flight test program with a Beech Sundowner, testing five different propellers of small diameter (three two - bladed, one three - bladed, and o n e four - bladed). The ideal was t o keep performance while lowering noise. A 4.8 - decibel decrease in noise was achieved with negligible performance losses. Both the three - and four - bladed props were successful while questions remain o n the two - bladed versions.

Another flight test series directed by OSU was run at Lewis on an Aero Commander with three different props at 700 horsepower.

The props, built by Hartzell, Dowty Rotol, and Hank Borst, w e r e tested in the tunnel, then on one engine on the aircraft while measuring noise. While there was little difference in noise levels, the performance curve showed each prop t o be different in cruise.

Noise in the climb configuration, the worst case on the curve, has been singled out a s the primary area of testing.

The major effort in these programs has been reduction of prop diameter to lower tip speed, while shifting the aerodynamic load inboard with thinner, wider chord blades. Whether the noise reduction designs will be used by industry t o produce new pro - pellers will depend of whether noise reduction affects the owner/operator’s performance.

Interior noise reduction involves altering the characteristics of the sound path from the source to the observer, a s well as altering the characteristics of the noise source itself. Vibration remains a Internal Combustion Engines I f 9, '$-.I . " ' ?

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NASA and General Aviation Advanced general aviation and high - speed iurboprop propellers offer cruise efficien - cy trends that could increase fuel economies.

major source of interior noise in light aircraft, originating in the engine and transmitted through the support structure into the cabin.

Research efforts have been centered around prediction of struc - turally transmitted noise and development of noise control methods involving control of both noise radiated from panels t o the aircraft interior a s well a s noise transmitted through the engine mounting vibration isolators. Fuselage sidewall transmission is very important for those aircraft with wing - mounted propellers operating close t o the fuselage sidewall. An Aero Commander 680 was modified with 15 pounds of asphalt type, glue on mass. The results indicated that even a modest amount of added mass may Internal Combustion Engines reduce interior noise by 4 t o 15 decibels depending o n the fre - quency of the noise.

NASA’s overall priorities have reflected the industry’s needs by addressing energy efficiency directly, both by improving fuel con - sumption and looking into burning less costly, readily available future fuels. Lewis Research Center scientists and engineers have centered their research o n improvements t o existing engines, ad - vanced intermittent internal combustion engines, rotary power plants based on the Wankel engine, stratified charge power plznts, cooling drag improvements, and advanced propellers. The new generation of more efficient turbine and turboprop engines is covered in chapter 4.

Ideally, the advanced intermittent internal combustion engines should be able to use any of a number of fuels, primarily jet fuel since aviation gasoline continues t o be a very small portion of the fuel produced by the major oil companies. These engines should burn less fuel more cleanly in a power plant of lower weight.

The rotary and two - stroke, stratified charge engines are being tested by NASA in prototype form, showing significant perform - ance advances when compared with conventional production in - ternal combustion aircraft engines.

Air - cooled power plants dissipate much of their heat by radia - tion from multiple fins formed a s part of the cylinders. Research - ers found cooling improvements could be made by designing t h e fins t o optimize size and spacing, coupled with cowlings redesign - ed for optimum aerodynamic efficiency. The end result has been less drag and maximum cooling.

Improving the Piston Engine

While turbine engine development has been actively pursued since the 1940’s, piston engine research a s a whole has been frozen since World War I1 with no major advances over the air- cooled reciprocating engine. With work progressing o n the GATE (general aviation turbine engine) program, NASA decided t o open the doors for the first time in 40 years o n near - term improvement of conventional air - cooled spark - ignition piston engines and o n future alternative engine systems based o n all new spark - ignition NASA and General Auiation piston engines, lightweight diesels, and rotary combustion engines.

NASA's conventional piston engine research involves applying existing technology t o improve fuel economy by 20 percent through leaner operation, drag reduction, and flight at high altitudes where fuel economy is improved. Reduction of exhaust emissions, improved cooling and installation drag, improved fuel injection systems, and advanced turbochargers are part of these efforts as well.

Under a NASA contract Teledyne Continental Motors began research and development of methods t o improve fuel economy and reduce the exhaust emissions of its aircraft piston engines.

Four concepts emerged permitting leaner operation and reduced emissions of hydrocarbons and carbon monoxide: (1) a timed, air- density - compensated fuel injection system to replace the familiar low - pressure continuous - flow system; (2) a thermal barrier exhaust port liner for improved cylinder head cooling; ( 3 ) air injection, when combined with the exhaust port liners, that reduces exhaust valve stem temperatures below those of the baseline engine while increasing oxidation in the exhaust; and (4) variable spark timing t o maintain best power spark timing over a broader engine revolu - tion - per - minute operating range.

Compared with the standard 10 - 520 engine, the version with these four concepts met EPA standards along with a 10 percent improvement in high - performance cruise fuel economy. After be - ing tested, the TSIO-520BE version now being installed in the Piper Malibu utilized many of these improvements along with dual AiResearch turbochargers run from separate exhaust manifolds. A common exhaust manifold would have been subject t o pressure waves from one bank pulsing into the other, causing detonation.

Divorcing the exhaust for the turbochargers has allowed leaner mixtures without exceeding allowable peak exhaust temperatures.

Aftercoolers also helped since induction temperatures w e r e lowered from 300" F to 115' F, further reducing the detonation problem. Not only were exhaust emissions down, but the turbos and slower prop revolutions per minute are natural noise reducers.

In 1979 a joi effort was launched at Lewis and Ames t o develop and demonstrate performance and economy improve - ment of piston engine aircraft via reduced cooling and installation Infernal Combustion Engines drag. Contemporary engine cooling and installation designs are based in part on technology and data developed for radial engines in World War 11. This data base is not adequate for precise design of a n engine installation using the common GA horizontally op - posed engine.

Estimates were made showing that cooling drag for current de - signs ranges from 5 to 27 percent of the total airplane cruise drag.

A semispan wing/nacelle section from a Piper Seneca light twin w a s mounted in the Ames 40 x 80 - foot wind tunnel for full - scale tests. A cooling drag penalty of 13 percent of total airplane cruise drag was found, defining a baseline for horizontally opposed designs for the first time.

It was clear that an integrated approach to engine cooling, in - cluding reduced cylinder cooling requirements and improved in - ternal and external aerodynamics, could reduce this drag penalty by a t least 50 percent. Next, a propeller driven by a n electric motor and various cooling inlet openings were tested. The pro - peller slipstream reduced flow separation over the aft part of the nacelle and at the inlets, leading t o a marked reduction in total drag. When the inlet area was reduced, drag increased due to inlet spillage, particularly in the high angle - of - attack climb configura - tion. Inlet pressure recovery in cruise improved a s much a s 5 per - cent because of the slipstream effect. For climb, the improvement was around 20 percent for the production (large) inlet and even more for the smaller inlets. These improvements were partially a result of pressure rise related t o propeller slipstream, but the ma - jor effect was the reduction in the amount of flow separation inside the recontoured inlet at higher angles of attack.

Next, a n actual engine was installed in the nacelle with three cooling air inlet sizes. Tests w e r e run over a freestream velocity range from 50 t o 150 knots, an angle - of - attack range from 0 to 10 degrees and a cowl - flap deflection range from 0 to 30 degrees.

Tests were also run using exits for the cooling air, located on the sides of the nacelle instead of the usual cowl flap exit under the nacelle. The exit most forward of the wing resulted in least drag but with a lower flow rate than that with the cowl flap 30 - degree configuration. If cowl flaps could be eliminated, cooling drag could be reduced 7 percent, and it was found through surface pressure measurements on the nacelle exterior that the pressure A n electric molor - driven propeller was used to delermine final results in the cooling drag tests. Important changes have been recommended in nacelle shapes and sizes.

in the lower plenum could be reduced more by using the pressure field of the wing than by deflecting the cowl flap to 30 degrees.

Further, inlet designs providing higher pressure recovery and u s e of improved cooling exits integrated into a lower drag nacelle proved beneficial. By adding a diffuser to the inlet, pressure recoveries of up to 95 percent were demonstrated. That improve - ment alone could eliminate most of the need for a cowl flap and thereby save up to 3 percent of total aircraft drag. The high drag is Internal Combustion Engines mainly a result of the current blunt shape and sharp corners of the front of the nacelle, which cause a n increase in boundary layer thickness, resulting in flow separation around the inlet and behind the nacelle.

Though this first cooling drag program is now over, the door has only just opened since a computer simulation code is needed t o find optimum external nacelle shapes and inlet geometry. Then NASA can begin to conduct computer simulations of new designs and t o what extent they can reduce that typical 13 percent cooling drag on GA aircraft.

Studying the combustion process itself has been of major in - terest t o both NASA and industry. Combustion - diagnostic in - strumentation has been designed at Lewis to determine on a per cycle, per cylinder basis, real - time measurements of the indicated mean effective pressure and percent mass of charge burned a s a function of crank angle. These systems are being used by both the aircraft and automotive industries. Ionization probes, placed in the cylinder head t o measure flame position and thickness a s a func - tion of crank angle, have proven valuable. Laser Doppler velocimetry (LDV) measurements of the velocities and turbulence levels for cold flow within the combustion chamber have been developed through a grant t o Carnegie-Mellon University. A unique charge sampling system at Lewis measures the local fuel- air ratio within the combustion chamber at selected times in the cycle of an operating engine.

This type of instrumentation has been extremely valuable in studying the role of turbulence and gas motions in combustion chambers. A principal goal is t o formulate a general mass of charge burned equation that includes engine air - fuel ratio, speed, and torque. Out of this has c o m e development of a theory and multidimensional computer code for future engine design and im - provement.

Research also is being conducted t o improve the inlet - port fuel injection system by extending the lean limit, requiring a more complete understanding of the relationship of the fuel - air mixture preparation before induction into the combustion chamber and overall engine performance. Though past investigations have sup - ported a well - mixed, homogeneous charge for lean operation, General Motors researchers have found that a “wetted” intake NASA and General Aviation charge with fuel droplets and possibly with bulk stratification may be optimum for lean combustion.

Spray nozzles have been investigated t o study the physical state of the fuel - air mixture through laser particle field measurements of different injectors. Lewis has also been conducting manifold- flow visualization tests with the cylinder head of a TSIO - 360 engine. High - speed photos taken through fiber optics have proven excellent in this diagnostic program, leading t o hot performance and emission tests.

Another important series of investigations has been conducted under the high - altitude turbocharger technology program. Since aircraft are more efficient at higher altitudes, a great deal of in - dustry interest is directed toward higher altitude capability for air - craft engines of all sizes. Turbocharging can extract more power from a given engine displacement and it can maintain that power from sea level to high altitudes. NASA initiated a program to develop a family of advanced but cost - effective turbochargers ap - plicable t o a spectrum of conventional and alternative engines, emphasizing near - term improved spark - ignition engines a s the base I i ne.

Analysis through verification testing was planned. Garrett AiResearch and Avco Lycoming have been heavily involved in design studies for advanced turbochargers on future engines. With a Lycoming engine currently flying in the high - altitude Mooney M 3 0 and a Continental mounted in the high - altitude Piper Malibu, both using Garrett turbochargers, the development of turbocharg - ing appears t o have entered a new era.

Advanced Concepts

Although current aircraft engines operate a t high levels of effi - ciency and reliability, changing requirements in terms of fuel economy, fuel availability, and environmental concerns have brought about ideas for significantly improved or completely new types of engines for future aircraft. NASA has addressed the issue through a series of conceptual design study contracts with engine manufacturers that should lead t o radical departures from 40 - year - old design philosophies.

Over the past 50 years the spark - ignition aircraft piston engine Internal Combustion Engines has proven so safe and reliable that major design changes have c o m e slowly. Presently, this design serves as prime mover for 93 percent of the nearly 200 000 active aircraft in the general aviation fleet. Rising fuel prices coupled with the possibility of reduced fuel availability have added impetus to the search for advanced piston engines that can preserve the increasing utility of this vital seg - ment of the U.S. transportation system.

Fuel availability surfaced a s one of the more important factors involved with future engines. NASA did a thorough survey of the past, present, and future of the energy industry, not only of the technical aspects of development of primary energy sources, but also of the economic, social, and political trends that might affect choice of a future fuel. Assuming the technology t o exploit such resources a s oil shale and coal, the study concluded that petroleum - based fuels would be around for a long time t o come.

There were two prospects identified for advanced engine fuel: continued use of 1 OOLL avgas a n d kerosene - based commercial jet fuel. Low - lead 100 octane avgas, for use in the near term, dictates use of a homogeneous charge combustion system similar to that used today. For the far term the move away from specialized avia - tion gasoline will have t o b e made since it is less than 1 percent of all the gasoline produced in the United States. Future use of jet fuel suggests a stratified charge combustion system.

Stratified Charge Engines

The term stratified charge refers to two levels of fuel richness in a combustion chamber. A lesser charge of a rich mixture is ig - nited, which then fires the remainder of a charge that is too lean t o ignite easily. A longer technical description has fuel injected across a n ignition source tangentially into a rapidly swirling air mass (the rotary does this by its inherent geometry). With im - mediate ignition assured by a positive ignition source, the fuel then proceeds t o burn smoothly at a rate controlled by the fuel in - jection. There is then no ignition delay period and no question of knocking or detonation and there are no cetane or octane re - quirements. Spark plugs could very well be replaced with glow plugs, catalyst strips, or even the very hot internal surfaces of an adiabatic engine. The ultimate limit for rapid heterogeneous com- NASA and General Auiation bustion is expected t o occur when the fuel as well as the air and combustion chamber surfaces are heated t o a highly combustible temperature. This eliminates the physical part of the ignition pro - cess (droplet evaporation) so that the combustion rate is then con - trolled by mixing processes.

The two advanced engines decided on were similar with the ex - ception of fuel and combustion system. The combustion chamber in both the moderate - risk and high - risk engines was redesigned. In the case of the former, the system used a low - pressure fuel injec - tion system where gasoline was injected in the intake manifold just upstream of the intake valve. The high - risk stratified charge system injected jet fuel at high pressure directly into the combus - tion chamber just before the piston reached top dead center.

The new design was labeled HTCC or high - turbulence combus - tion chamber. A bathtub - shaped recess was cut into the firedeck in the exhaust valve area. A specially shaped passage leading from the intake valve area to a corner of the bathtub produced an ex - tremely strong air swirl motion when the piston neared top dead center. This motion, combined with the HTCC’s high compression ratio, resulted in high flame speeds. In effect, it became possible t o ignite and burn mixtures that otherwise would be too lean t o support stable combustion. The rapid burn and high initial com - pression of the HTCC resulted in increased peak firing pressures.

With the HTCC chamber, NASA demonstrated the detonation - free operation of a homogeneous charge, 6 - cylinder engine at a com - pression ratio of 12:l compared with 8.5:1 for a standard engine.

This increase in compression ratio had the effect of improving fuel economy at cruise powers by 7 percent. Teledyne Continental’s 520 - cubic - inch engine with the HTCC proved quite capable of even better performance figures with further research, leading eventually t o a stratified charge version which would burn jet fuel, liquid propane gas, or alcohol.

Power can be taken in several ways from the waste exhaust gases of an internal combustion engine, among them turbocharg - ing, a s discussed earlier, and turbocompounding. NASA’s ad - vanced engines have been earmarked for both systems. Turbo- compounding is not a new idea since it was used in the 3000 - horsepower engines of the post - World War I I era but to apply it t o 350 - horsepower engines constitutes advanced technology.

Internal Combustion Engines EXHA STANDARD HTCC H I G H SWIRL - 8 . k 1 COMPRESSION RATIO COMPRESSION RATIO Primary differences between a standard compression chamber and the high lur- bulence version are shown here. Compression ratio and lean fuel burn are significant - ly increased.

In these smaller engines exhaust gases leave the engine and pass through a power turbine, which transmits power back into the engine crankshaft through a speed reduction unit. The gases then carry their remaining energy t o a turbocharger, making it possible t o extract one horsepower for every pound of weight added, enhancing efficiency.

Both the moderate - risk and high - risk engines will be adapted t o electronic control of all operational systems. This means that the present three levers now used t o control engine revolutions per minute, manifold pressure, and fuel mixture will be combined into a single lever. With the increasing amount of single - pilot instru - ment flying and the ever - growing complexity of the air traffic con - trol system, this single power lever can reduce pilot work load, thus enhancing safety.

NASA and General Auiation Advanced materials have been earmarked in both engines for weight reduction and increased durability. The baseline TSIO - 550 represented the current level of technology with a weight of 585 pounds, while the moderate - risk 420 - cubic - inch engine came down t o 485 pounds, a weight reduction of 17 percent. The reduc - tion in use of steel and aluminum was brought about by the more judicious use of these metals in conjunction with 10 pounds of ad - vanced materials.

The high - risk engine, also 420 cubic inches, used only 80 pounds of steel, primarily in the crankshaft, reduction gears, cylinders, and exhaust valves. Aluminum was reduced somewhat and a total of 119 pounds of advanced materials was incorporated for an engine weight of 405 pounds, a 3 1 percent improvement over the present - day engine. In this engine the greatest part of the advanced material weight was titanium, with a small amount of reinforced plastic and ceramics.

All three engines were rated at 350 horsepower, cruising a t 2 5 000 feet at 250 horsepower. Service ceilings of the advanced engines were increased t o 35 000 feet compared with 25 000 feet for the current engine. The time between overhaul (TBO), 1400 hours on the current engine, was increased t o 2000 hours for the advanced engine. To get an idea of the fuel economy im - provements obtained, compare the power wasted in the exhaust of the three engines. The current technology engine dumped the equivalent of 319 horsepower out the exhaust a t maximum cruise power. For the moderate - risk engine this loss was reduced by 33 percent to 214 horsepower and by 5 1 percent to only 1 5 6 horsepower for the high - risk engine.

The bottom line is how all this results in improved airplane per - formance. All three engines were simulated for installation in a current single - engine aircraft designed for the high - risk engine.

The present technology engine resulted in a range of 5 1 8 nautical miles while the moderate - risk engine achieved 8 1 4 nautical miles, an increase in efficiency of 32 percent. With the high - risk engine, efficiency increased by 49 percent.

Intermit tent Com b us tion Engines

When the Environmental Protection Agency first told the FAA it wanted improved exhaust emissions standards for general avia- Internal Combustion Engines tion engines, NASA was tasked to d o the research starting in 1973. By the time Avco Lycoming and Teledyne Continental, along with in - house NASA studies, came up with alternative GA engine possibilities in 1975 the fuel crunch had hit. Fuel economy and alternate fuels became the drivers for developing the engine technology.

By 1977 advanced air - cooled, spark - ignition engines, diesels, rotaries, and external combustion engines were being developed for testing, although the latter was dropped since it was very heavy. The central focus of this intermittent combustion (IC) class of engines was use of jet kerosene and similar fuels with high effi - ciency. By 1982 NASA dropped further aircraft gasoline engine research in favor of a horse race between turbine and IC engines to see which would come out as the most efficient. The contest has yet to be decided, if it has to be decided at all, since both types of engines have excellent futures.

Under NASA contracts, Teledyne Continental worked on an ad - vanced spark - ignition engine of conventional configuration; Teledyne General Products came up with a compact two - stroke radial piston diesel; and Curtiss - Wright analyzed a lightweight liquid - cooled, stratified charge rotary engine.

All the iC candidates delivered improved performance when compared with current production gasoline engines and a hypothetical, highly advanced but unregenerated turboprop. The rotary and the diesel, very close in mission fuel and aircraft weight savings, provided the best overall performance with dramatic fuel/weight savings of about 43 percent. The baseline engine used avgas while the others were jet kerosene burners. Considering the differences in energy content, density, and cost between kerosene and avgas, the savings can be extended by another 10 to 15 per - cent for a more realistic measurement of the economic benefits.

The rotary engine emerged a s the overall top - choice IC engine, with the diesel a strong second. As far a s performance, the rotary had only a small advantage but predicted passenger comfort levels (based on low vibration) emerged a s significant positive fac - tors in the overall evaluation, which only the turboprop could match.

Rotary and diesel engines are also being considered for growth t o supply both the GA and commuter markets. Both engines have Internal Combustion Engines been labeled adiabatic/turbocompound or ATC in reference to the most desirable versions of these engines, which use ceramics t o block most of the heat transfer that would otherwise g o into the coolant and a compounding turbine t o recover a portion of the thermal energy.

Automotive adiabatic (or noncooled) diesels have been under test for s o m e time, particularly for the U.S. Army. Cummins Diesel Engine has had a n Army truck on the road with a 6 - cylinder engine that has no radiator, fan, water pump and tank, expansion tank, water hoses, fan belt and pulleys, air scoops or openings, and related equipment. The ceramic cylinder liners, piston crowns, firedeck, and other internal insulated components were able to contain the high - temperature combustion gases, block off most of the heat transfer to the empty cooling jackets, and direct the very hot exhaust gases t o a turbocharger and compounding turbine. Though too large for aircraft application, the new technology represents a major technical breakthrough that can be incorporated into aircraft engine technology.

Rotary engines have finally become energy efficient. Toyo Kogyo has overccme the early rotary auto engine’s excessive fuel consumption with its Mazda cars. Curtiss - Wright, under U.S.

Navy/Marine Corps sponsorship, developed a large stratified charge multifuel rotary marine engine. Two fuel injectors, pilot and main, are located in the “wasp - waist” region of the trochoid housing near the top center position. The pilot injector sprays in a small amount of fuel over a high - energy multiple discharge spark plug, establishing an ignition “torch,” which continuously ignites the main fuel charge a s it is injected. The motion of the rotor in the trochoidal housing is such that the air motion in the injector region always proceeds in the downstream or “leading” direction.

By tailoring the main injector flow to the instantaneous airflow rate past the injector station, a stationary flame front is estab - lished under the “wasp - waist.’’ The rotor merely pushes the air through the flame front by virtue of its inherent motion and geometry. The ignition torch is energetic enough to immediately ignite any fuel than can be pumped through a diesel - type injection system.

Curtiss - Wright (C - W) developed this engine further with two- rotor and four - rotor versions. The latter 1400 - cubic - inch engine U - - _ - . _ _ -- - I - - _ _ - - Principle of the Rotary Engine 1-4 Intake 5 - 9 Compression 10 - 12 Power 13 - 18 Exhaust Rotary combustion engine basics Internal Combustion Engines weighs under 1900 pounds and, without turbocharging, develops 1500 horsepower at only 3600 revolutions per minute. Based on C - W and NASA calculations, this power output could be more than doubled by turbocharging plus a slight increase in rotational speed. A four - rotor engine of this type could very possibly develop 4000 horsepower or more without weighing more than 2000 pounds.

Of more immediate interest is the twin - rotor version that has come to the end of a five - year development and preproduction testing program for Marine Corps use in NATO. Curtiss - Wright conducted a growth study of this engine for advanced and highly advanced versions for commuter aviation use. The technical risks w e r e found to be no greater and no less than those for the GA rotary engine - everything w a s just bigger.

Taken together, the NASA GA engine studies, the Cummins adiabatic diesel truck engine results, and the Curtiss - Wright work on stratified charge rotary engines w e r e viewed a s indicating ex - ceptionally good technical prospects for larger horsepower air - craft diesel and rotary engines. As a result, in 1981 NASA spon - sored more studies of aircraft diesel and rotary engines in the 800- to 2400 - horsepower class.

Teledyne General Products completed a study of the 800- t o 2400 - horsepower class of lightweight diesel commuter aircraft engines. While many key features remained from the GA engines, these larger engines involved higher speeds, higher loadings, and adiabatidceramic combustion chamber technology similar to that developed by Cummins. These power plants combine the lightweight two - stroke radial design philosophy of the GA designs with Cummins - type adiabatic components. The insulated combus - tion chamber greatly reduces, and may even eliminate, the coolant system head load, which should correspondingly reduce the cooling drag and provide more energy to the turbocharger and compounding turbine. This high - speed, high - pressure cycle demands substantial technology advancements, particularly in in - sulated combustion chamber components, high - speed, high - pressure fuel injection, high - performance turbocharger/turbocom- pounding, and advanced piston rings and lubes.

The resulting commuter aircraft engine is a 90 - degree X - 8 , two- stroke, piston - ported adiabatic turbocompound of 2000 Curtiss - Wright built the RC2- 75, which was test //own in a Cessna aircraft. This initiated the use of rotaries in general aviation. (Curliss- Wright) horsepower. Depending on whether advanced, lightweight com - posite materials are extensively used, the engine is projected to weigh 1350 t o 1550 pounds, fitting into a 2lh-by-3-by-5-foot box, making it a s compact as the rotary previously discussed. Notable is the 1 5 000 - foot cruising brake specific fuel consumption (BSFC) of 0.30 pound per brake - horsepower hour. The power plant is no larger than the Cummins truck diesel and probably no heavier, yet it develops 10 times the power, nearly equal BSFC, and all that on a full - time, 100 percent duty - cycle basis. Although the technical risks are great, the results are certainly worth pursu - ing.

Studies on stratified charge rotaries were conducted in parallel Internal Cornbustion Engines with the diesel work in the s a m e power range. Under a NASA con - tract, Curtiss - Wright extended its previous GA studies to the larger engine sizes, the only difference being that turbocompounding w a s allowed a s a n option. Engine definitions w e r e based on con - ventional cooling and material temperatures were not increased, which, t h o u g h including m a n y a d v a n c e d m e c h a n i c a l technologies, meant these engines have neither the benefits nor the technical risks associated with adiabatic ceramics.

As this work was going on, NASA conducted a parallel in - house study of the potential benefits of adding adiabatic uncooled opera - tion to the Curtiss - Wright engine definitions. It was estimated that by adding ceramic or other insulative trochoid liners and rotor/end housing faces and deleting the conventional cooling system, the power recovered in the compounding turbine could be increased by 10 t o 1 5 percent of engine total shaft power. The structural weight added for ceramic/insulative components was generally offset by eliminating the conventional coolant system.

In the end, NASA’s first attempt at an adiabatic turbocom- pounded rotary engine amounted t o improving the C - W specific weights and cruise BSFCs by 10 percent each. Subsequent com - parative performance studies revealed that the conventionally cooled rotary is not competitive with a highly advanced turboprop for commuter aircraft. Looking at C - W and NASA studies together, the resulting adiabatic/turbocompound version of the rotary engine retained all the known desirable features inherent t o rotary engines, with greatly extended durability a realistic possibility.

In February 1984 Curtiss - Wright, forced t o close down the plant conducting rotary research, sold its rotary rights and interests t o Deere & Co., which agreed t o carry on the research for NASA if a suitable aircraft engine partner could b e found. By March 1985 Avco Lycoming agreed t o become a partner with Deere and both companies have since announced they plan t o certify and market the world’s first 350 - to 400 - horsepower Jet - A fuel - burning rotary aircraft engine by 1990. During that time period Deere assumed the NASA C - W contract to build a high - performance, multifuel rotary test engine rig and successfully completed it with the possibility of future work involving the testing of advanced com - ponents and systems.

NASA and General Aviation As a result of the promise and advances in rotary research, in J u n e 1984 NASA decided t o focus future general aviation research and testing efforts in that area. A typical 80 - cubic - inch displacement engine developing 200 horsepower would weigh about 250 pounds, fit inside a 16-by-50-inch cylinder, and match the BSFC of today’s highly developed avgas reciprocating engines while burning Jet - A kerosene - type fuels. A highly advanced ver - sion would have true multifuel capability, an overall fuel savings of about 50 percent, and nearly twice the power output for the s a m e size.

These developments in industry enthusiasm and commitment have pushed GA rotary engines out of limbo and into production reality. According to one of Lewis’ propulsion managers, “While a great deal remains to be done, it appears that, with proper atten - tion t o advanced technologies, this novel power plant could have the most revolutionary impact on the aircraft engine business since the modern turbofan was introduced about 30 years ago.

Justly, and with pride, NASA can lay claim t o having had a signifi - cant and positive role in this process.”

Future Developments

As in the diesels, the use of insulated (possibly ceramic) com - bustion chamber components results in zero or minimal coolant heat rejection and cooling drag and more energy t o the com - pounding turbine. On the other hand, as with the diesels, the high - speed, high - pressure cycle demands significant technology ad - vancements, although with uncertainties higher than before. Com - pared with the previous diesel, this is a long, cigar - shaped power plant which, in the 2000 - horsepower version, would fit into a 2 - foot diameter by 8 - foot long cylinder. Engine weights are com - parable, a s well a s BSFCs if o n e accepts the ATC cycle rotary, but the rotary is rated to cruise a t 75 percent of maximum takeoff power while the diesel was rated t o cruise continuously a t 100 per - cent of its maximum power.

Aside from cruise - power applications and problematical dif - ferences in technological risk/credibility, the two engines appear t o be equal in potential benefits, leading t o a horse race with g a s turbine core engines.

Internal Combustion Engines With these intriguing data in hand, NASA then did a series of in - house airplane/mission evaluation studies in which the present diesel and rotary engines w e r e compared with each other and with a similarly advanced turboprop engine. For a 30-passenger, Mach 0.6, 20 000 - foot mission with a 2400 - nautical - mile design range, both the diesel and the rotary were sized considerably smaller than the turboprop because of the latter's considerably worse power lapse rate from sea level up to 20 000 feet where engine sizing oc - curred. Also, with 100 percent diesel and 75 percent rotary cruise power, the rotary ended up one - third larger than the diesel, with a larger base engine weight and corresponding weight differences reflected throughout the airplane.

Bearing this difference in mind, both the diesel and rotary were substantially heavier than the turboprop while consuming con - siderably less fuel. All three airplanes turned o u t to be about the s a m e size but both the rotary and the diesel were at least com - petitive with the turboprop. The diesel showed a definite edge over the rotary due almost entirely to its more aggressive rating philosophy.

In essence, both the diesel and the rotary give competitive overall performance but are more economical than the turboprop.

Though many elements of the direct operating cost equations have yet t o be addressed, these engines simply use less fuel. In an era of true fuel scarcity, which would be entirely different from to - day's high fuel price scenario, they could well make the difference between continuing operations or not.

The promise of both the diesel and the rotary is great but both require four major new technology items t o be completely suc - cessful, s o m e of which are being addressed by NASA research.

Despite varied applications, both require advancements in the tribology area, that is, low - friction, low - wear sealing elements and lubes t o survive in a high - speed, hot, high - pressure environment.

For the rotary in particular, apex seals are needed t o reduce con - tact force or a controlled clearance liftoff condition at high speeds.

Both require at least partially insulated combustion chamber componentry t o approach the benefits of the ATC cycle. For the diesel a fairly straightforward extension of the Cummins approach may do. For the rotary, however, there is no precedent at all, even though the basic intent is the same. Both require very fast fuel in- NASA and General Aviation jection and stratified charge combustion systems, a substantial problem despite reported progress in other areas. Cyclic combus - tion rates four t o six times higher than state - of - the - art truck diesels are needed.

Finally, both engines require advanced turbochargers and com - pounding concepts. The compounding system was not considered cost - effective for the GA class of engines but is a unique require - ment for the larger engines. However, such a system adds weight, cost, and reliability concerns. An alternative approach is available for four - stroke cycle engines with pressure or pneumatic com - pounding in which a “super turbocharger” concentrates any available excess exhaust energy into the form of higher com - pressor discharge pressures. With high component efficiencies it should be possible t o run the compressor discharge pressure substantially higher than the turbine inlet pressure. NASA has run in - house tests o n a four - stroke diesel engine with over 10 percent improvement t o both power output and specific fuel consumption.

Engine/A irframe Integration

In order to help determine which of the four promising concepts for new general aviation engines of the 1990’s should be con - sidered for further research funding, NASA initiated the Advanced Aviation Comparative Engine/Airframe Integration study with Beech and Cessna Aircraft Companies. Rotary, diesel, spark - ignition, and turboprop power plants w e r e compared against a conventional state - of - the - art piston engine as a baseline. Com - puter simulations of the performance of single - and twin - engine pressurized aircraft designs were used t o determine how the various characteristics of each engine interacted in the design process. Compariscns w e r e then made of how each performed relative t o the others when required t o fly a mission.

Evaluation of the results placed heavy emphasis o n low fuel consumption and direct operating cost and on high flight efficien - cy. Acquisition cost, noise, multifuel capability, and ease of in - stallation were also considered but not weighted a s heavily.

Cessna’s results indicated that the highly advanced rotary offers the best all - around performance and features for future general aviation aircraft. The diesel was rated only slightly lower, while the Internal Combustion Engines This illuslralion depicls a comparison o f [he same Cessna aircraft powered by nor - mal piston engines and rolaries under NASA's Advanced General Avialion Com - parative Engine/Airframe Integralion Sludy.

other engines, though showing worthwhile advances, did not ap - pear as promising. In particular, the turboprop was viewed primari - ly a s a viable replacement for the baseline engine, offering market appeal rather than large improvements in efficiency or cost.

Regardless of the assumptions made (drag level, weights, cost, etc.) and missions chosen, the results proved the s a m e while ad - vanced materials and aerodynamic features, combined with ad - vanced engines, offered substantial gains in performance, fuel burn, and cost.

Beech came up with basically the s a m e results. The highly ad - vanced rotary offered low weight and fuel consumption as well a s small size, making it ideal for aircraft use. The other advanced engines offered improvements that were only slightly less dramatic when compared t o the rotary.

Both manufacturers recommended that NASA consider NASA and General Aviation INOUCTIO# AIR FlLTEff STAIRTER-6CNERATOOR SMOCK M O U N T CMoLlN6 AIR H I T C A 8 l N COOLIN6 AIR CXIT ~ ~~ This is what a highly aduanced diesel engine installation could look like in a twin - enqine qeneral aviation aircraft. A s with the rotary, the frontal area is drastically - - reduced while performance gains are significant enough lo merit further inuestiga- tion.

developing the technologies required by all the advanced engines in common and allow t h e individual manufacturers to apply the technologies t o their designs rather than invest in the develop - ment of a single engine type. This could foster competition among the engine manufacturers and provide a wider range of engine op - tions for the 1990’s.

The pioneering engine programs mentioned here need to con - tinue, though they are often the first t o fall to the budget axe.

Developmental research could provide t h e next great breakthrough in leaving World War I I technology behind and reaching out t o the future.

CHAPTER 4

TURBINE ENGINES

hen NASA began to study general aviation potential, it started with turbine engines because of the large base of

W contemporary research being performed. In early 1977

four independent studies were contracted by NASA for a future small, general aviation turbine engine (GATE) ranging from 300 t o 600 horsepower with improvements in specific fuel consumption (SFC) of 20 percent and engine cost savings of 40 percent.

Generally the turbine engine has been accepted a s the most desirable type of power plant because of its very low vibration levels, high reliability, multifuel capability, better safety record, low weight, fewer exhaust emissions, less maintenance, and smaller installation losses. However, turbine engines have been limited t o above the 500 - horsepower range d u e to higher fuel con - sumption and a 3 t o 1 price differential when compared with low- horsepower reciprocating engines. The challenge, of course, is t o overcome the cost and fuel barriers without sacrificing the superior qualities. Current technology does not allow this.

NASA wanted to emphasize technologies that had high payoff and low risk, but which could be ready for production develop - ment by 1988, assuming sufficient funding. A market forecast was undertaken, considering all types of small aircraft and helicopters, then optimum engine configurations for each important mission were evaluated. Anticipating that the marketplace could not af - ford different optimum engines for each application, a n evaluation NASA and General Aviation NASA has conducted design studies, such as this one, for a propfan - powered ex - ecutive transport.

was m a d e of a single common core to be used in a family of engines. Lastly, the required research and technology program was defined.

As Garrett AiResearch, Detroit Diesel Allison, Teledyne Con - tinental, and Williams Research worked with a n d for NASA, all four companies came t o independent conclusions that turboprops instead of turbofans or turboshafts offered more promise. When trying t o lower cost of a turbine, efficiency suffered, but the challenge was either t o develop advanced technology that could improve performance without cost increases or t o design a cost - effective unit. In the end, three study teams pursued the low - cost turbine versus piston theme while the fourth concentrated o n a TurbineEngines high - performance, advanced turbine versus current turbine theme.

Design turbine - inlet temperatures were optimized at 2200 O F, or about 400" F above current small engine levels. Cost was about 40 percent cheaper since physical size was about 40 percent smaller and more cost - reducing technology was incorporated.

Fuel consumption was improved 15 percent due to the weight reduction, not cycle efficiency. As a matter of fact, the small airflows were not conducive to improved cycle efficiency and even resulted in increased fuel consumption. However, engine weight and cost savings also produced 15 to 20 percent improvements in airplane acquisition cost, operating cost, and total cost of owner - ship.

The four companies settled o n turboprops ranging from 335 to 565 horsepower, aimed primarily at the high - performance single- engine and twin - engine aircraft market. Engine cost im - provements were predicted through a number of technologies, in - cluding use of powdered - metal gears and laser hardening in the gearbox, composite material or die - cast aluminum gear cases, composite drive shafts, and full authority digital controls. The key elements in all the concepts involved the rotating machinery: high - performance centrifugal compressors using advanced analysis techniques, s o m e form of passive clearance control, and backward curvature. High - stage loadings without severe per - formance penalties were prevalent a s well as new manufacturing processes.

The common core concept for reducing engine cost was essen - tial to the project. Retaining parts commonality without sacrificing too much performance was the key because each of t h e diverse mission applications preferred a different optimum engine. For example, a 335 - horsepower engine grew 70 percent to a 565 - horsepower derivative with a 4 - inch extension, a 3 1 - pound weight increase, and a 54 percent increase in cost. At the s a m e time, the SFC was 10 percent lower because of the increased cycle temperature and pressure and component rematching. The price of commonality was a 2 percent SFC penalty for the basic core engine. However, the benefits w e r e a 7 percent lower cost and a 16 percent weight reduction.

GATE technology advances were theoretically applied to twin - NASA and General Aviation turboprop airplanes, resulting in cost reductions of 15 t o 25 per - cent and operational savings of 30 to 40 percent when compared t o their piston - powered counterparts. However, the benefits for high performance were only one - third t o one - half as much. When the GATE program ended around 1980 the door was open for large improvements in aircraft economies at the upper end of the reciprocating - power class. It also filled the gap between the relatively inexpensive reciprocating aircraft and the expensive tur - boprop aircraft, bringing the many other virtues of turbine engines t o a wider spectrum of users and applications.

When applied t o commuter aircraft, advanced turboprop engines and propellers offer 15 t o 20 percent fuel savings and 10 t o 15 percent reductions in direct operating costs compared with the new crop of 1500 - t o 2000 - horsepower engines currently in development. Unconventional engines could boost fuel savings to 40 percent. NASA sponsored a series of future advanced tur - boprop studies with Detroit Diesel Allison, General Electric, and Garrett Turbine Engine for engines ranging from 1500 horsepower for a 30-passenger, Mach 0.45 twin - engine airplane, t o 4800 horsepower for a 50-passenger, Mach 0.70 twin designed for both executive travel and commuter service. Component im - provements were earmarked in compressors, turbines, com - bustors, controls, gearboxes, shafts, bearings and seals, ac - cessories - virtually every major area.

The smaller commuter aircraft use GA - type propellers that are relatively simple with overall lower performance compared to the more sophisticated technology applied to larger commuter air - craft. These lower cost propellers are typically constructed with solid aluminum blades having circular shanks, which contribute to lower thrust efficiency. The newer, more sophisticated propellers utilize such weight - saving construction techniques as aluminum spar - fiberglass shell blades and such performance improvements a s airfoil shanks, advanced airfoils, and low activity factors.

Even with the relatively high propeller efficiency of 0.87, technology studies by Hamilton Standard, the McCauley Division of Cessna, and Purdue University have identified further oppor - tunities. Proplets or biblades could increase efficiency nearly 2 percent through reduced tip losses. Advanced materials could reduce fuel consumption about 1 percent through weight savings Turbine Engines or from the ability to use more blades while maintaining sufficient blade retention strength in the thinner root sections. This permits more efficient, lightly loaded blades with activity factors about 7 0 compared with currently limiting values of 90 t o 95. These blades could be constructed, for example, with a steel or metal matrix spar with a Kevlar or graphite shell. The shell could be load shar - ing, unlike current designs, and the blade specially tailored t o avoid aeroelastic flutter problems a t high propeller speeds.

Fatigue life and maintenance costs must be considered in a n overall evaluation of these new designs.

Another attractive concept is synchrophasing the left and right propellers t o within a technically challenging one degree in phase angle. Experimental evidence indicates that an 8 - decibel noise reduction potential exists for precision synchrophasing, which could eliminate large fuselage acoustic weight penalties for typical wing - mounted engine configurations. That amounts t o 800 pounds or more for a 30-passenger, Mach 0.45 airplane with Boe- ing 737 cabin noise level.

Advanced Gas Turbine

The problems involved in designing and building small turbine engines remain difficult t o solve since much turbine work is generic in nature. Small turbine research at NASA began in the 1960’s but the small rocket engine turbopumps and auxiliary power units involved w e r e nothing like the general aviation tur - bines. NASA’s small turbine research is concerned primarily with aerodynamic performance and turbine cooling in both radial and axial turbines. Though much of current advanced gas turbine research has been centered around automotive engines, the ap - plications t o general aviation are extensive.

The low - cost turbine work conducted from 1970 t o 1975 was directly applicable t o GA engines, leading t o an understanding of the influences of such things as rotor tip clearance, turbine and stator blade design, blade aerodynamics, and other factors affect - ing small turbines. Several analysis methods were formulated with computer codes for axial and radial turbines as well as channel flow analysis codes. Rotorcraft engine work also was initiated a t Lewis.

By the mid - 1970’s the term advanced gas turbine (AGT) sur- NASA and General AuiaLion faced in reference t o a very small high - work axial or radial turbine of very simple design with no cooling and extensive use of ceramic materials. An understanding of the cooling effects o n perform - ance had been studied for large turbines and much of this research was applied t o the newer AGT work.

By October 1979, NASA contracted with two engine companies t o build and test advanced gas turbines for automobiles with fallout applications for general aviation. Detroit Diesel Allison teamed with Pontiac while Garrett teamed with Ford t o develop turbine engines that would get over 42 miles per gallon on multi - ple fuels in a 3000 - pound car with exhaust emissions within federal standards set for 1985. Costs would have to be competitive along with reliability, low noise, and safety held within federal parameters.

This high - risk project had two evolving technologies that would significantly affect success: development of affordable ceramic components to permit higher gas turbine operating temperatures and development of small - engine - size aerodynamic components.

The AGT was chosen for development in part because of the Ceramic Applications in Turbine Engines (CATE) program, which was contracted under Lewis in July 1976. CATE applied ceramic components in an Allison GT 404 - 4 highway vehicle gas turbine engine, reducing fuel consumption by improving engine cycle effi - ciency due t o operation at higher temperatures, from 1900" t o 2070 " F. Since ceramics are brittle, NASA and Allison engineers had t o establish a level of understanding comparable t o that which five or six decades of experience with metals have afforded in - dustry.

This understanding grew from the ceramic material itself: the powders used, the process of generating complex shapes with ac - ceptable strength, quality control, nondestructive inspection of components, cost - effective machining techniques, assessment of thermal and chemical stability, and methods of handling the brit - tle components without damage. Nozzle vanes, turbine tip shrouds regenerator discs, and turbine blades were tested suc - cessfully in both the 1900" and 2070 " F engine configurations, paving the way for major industry/government research into large- scale use of ceramics. Use of these components can provide a tur - bine engine with 30 percent improvement in fuel economy over Turbine Engines projected 1985 spark - ignition engines and offer significant im - provement beyond that with further research. The Japanese are pursuing the use of ceramics in engine components with great vigor, Both Allison and Garrett faced major challenges in addition t o ceramics development. A small turbine poses unique technical problems not encountered in larger engines due t o high surface area t o volume ratios for liners and domes, very small fuel injec - tion passages subject to clogging and difficult to maintain, small passages which accentuate boundary layer, and end wall effects and complicated flow fields.

Engine applications also dictate combustor orientations. Large, advanced combustion systems are all of the axial, straight - through type while small combustors for rotorcraft, commuter, or general aviation aircraft can be reverse flow, axial straight - through or reverse flow axial and radial overflow or inflow. Since smaller engines have SFC levels well below those of larger engines, substantial improvement has to be made for the advanced cycle needs. In addition, small engine manufacturers are generally not a s well equipped to meet the challenges because of lack of compo - nent facilities, particularly high - temperature and pressure capabilities, and smaller technical staffs. NASA’s involvement proved crucial t o providing the impetus and management for AGT research.

Through 1983 both AGT engine projects and NASA’s own research program at Lewis had made significant contributions in component improvement, combustor development, advanced liners, fuel effects, fuel injectors, rotor clearances, aerodynamic flow, computer analysis, laser measurement of gas velocities, ceramics, and other areas. Both Allison and Garrett had tested portions of their engines with progress toward full - scale testing of the actual power plants. Through the 1980’s the AGT program will pave the way toward efficient small turbine engines for general aviation use.

Quiet, Clean General Aviation Turbofan

The fastest growing portion of general aviation is the turbofan- powered executive jet fleet. The worldwide fleet of around 5200 NASA and General Aviation executive jet aircraft is expected to expand, particularly in view of the fact that in spite of GA production line shutdowns in 1982 and 1983 the jet lines continued t o roll.

NASA's QCGAT (Quiet, Clean General Aviation Turbofan) pro - gram was initiated in April 1975 t o improve the environmental characteristics of civil aircraft near suburban airports. This was NASA's first application of large engine technology to small engines in the general aviation or small engine field, conducted in two phases.

The initial study phase examined the applicability of current large turbofan technology t o small engines, produced a preliminary design of the QCGAT engine, and developed re - quirements and program plan for the experimental phase. The ob - jective of the latter phase was to demonstrate that the application of large turbofan engine technology t o small, GA turbofans could result in less noise, lower emissions, and acceptable fuel con - sumption. Though the latter two goals were important, the pro - gram was primarily directed toward low noise. Among NASA pro - grams that led the way in QCGAT research were QCSEE, Quite Engine, Quiet Nacelle, Refan, and Clean Combustor.

During the study phase, which lasted six months, Garrett AiResearch, Avco Lycoming, and General Electric defined the engine. Then a competitive procurement was held with contracts going t o AiResearch and Avco Lycoming t o design, build, and test a n engine.

Each contractor approached the project by using an existing modern gas generator or engine core t o save development time and money. The engine was t o develop less than 5000 pounds of static thrust, and all rotating parts were t o be flight worthy. A boiler plate rather than a flight worthy nacelle was acceptable.

However, the internal aerodynamic contours and the acoustic treatment for the nacelle had t o be of flight design.

The emissions goals selected were the now - abandoned 1979 EPA standards for Class T 1 engines and NASA set its own noise goals. Since existing gas generators were being used, drastic reductions in fuel consumption could not be expected, though ef - ficiency was not t o suffer at the expense of reducing noise a n d pollution. Therefore, a fuel consumption goal equal t o or better Turbine Engines than existing engines was set. Each contractor synthesized a twin - engine aircraft for their engine t o get standardized noise calcula - tions. Both engines were then t o be delivered t o NASA Lewis for further experimental testing.

NASA’s noise goals during takeoff and approach, as well as sideline noise, were well below those set in Federal Aviation Regulation 36. As a matter of fact, four of the quietest jets flying, the Cessna Citation, Falcon 10, Learjet 36, and A300B, w e r e selected for comparison and the NASA goal was 8 t o 12 EPN decibels below any GA aircraft flying. In the range of aircraft gross weight used in the QCGAT program, the NASA goal was 16 t o 1 9 EPN decibels below the 1977 FAA rule.

These goals were set t o ensure the inclusion of existing low- noise technology in the QCGAT designs, resulting in aircraft noise levels that are perceived t o be 45 t o 55 percent less noisy than the levels of the quietest current business jets. Another way of il - lustrating the effect of achieving these goals is by using noise foot - print areas or the area below the aircraft which is subject t o a noise level greater than a given level during takeoff and landing. The footprint area for an aircraft using the QCGAT engines is predicted t o be one - tenth that of the quietest business jets. Essen - tially, achievement of the stringent QCGAT noise goals could eliminate noise a s a major constraint on the future growth of the turbofan - powered general aviation fleet. In addition, the aban - doned 1979 EPA emissions goals were kept for the QCGAT pro - gram.

Garrett AiResearch used the core of one of their Model TFE73 1- 3 engines, then adapted several unique components such a s fan, gearbox, combustor, low - pressure turbine, and associated structures. These components formed the basis for meeting the main program objective demonstrating the application of large turbofan engine design, emissions, and noise technology in small general aviation turbofans.

A workhorse nacelle incorporating interchangeable acoustic and hardwall duct liners showed that large engine attenuation technology could be applied t o small propulsion engines. The ap - plication of a mixer compound nozzle demonstrated both per - formance and noise advantages. Major noise reduction, beyond NASA and General Auiation that of an already quiet engine, was obtained, making the AiResearch QCGAT engine significantly quieter than any other business jet engine. The engine met design goals for thrust and emissions with slightly better reliability and performance than predicted.

Avco Lycoming met the primary QCGAT objectives with its engine and nacelle designs. They also found that large engine noise reduction technology could be successfully employed to the GA - size engine, demonstrating QCGAT acoustic goals with margin. The emissions goals were basically achieved with con - siderable margin for both carbon monoxide and unburned hydrocarbon (UHC) emissions and nitrogen oxide within 1 percent of the goal.

A fan module was developed around an existing turboshaft engine. The fan was designed using the latest in large engine noise control technology and a mixer was added to reduce the already low exhaust gas velocity. A nacelle incorporating sound treatment was provided. The bottom line came to significant noise control without a performance penalty.

Comparing the two engines, the AiResearch power plant was a higher thrust machine designed for an aircraft that cruises at high speed and altitude with long range. The Avco engine was a low- thrust unit designed for a n aircraft that cruises lower and slower at in termed ia te range.

The Avco engine was applied to a n advanced Beech design while the AiResearch engine was synthesized for a stretched ver - sion of the Learjet 35. A comparison of the two QCGAT aircraft was made with similar existing aircraft. The Avco/Beech perform - ed a similar mission to the Cessna Citation !, and e v e n though it was a much lighter aircraft, it had both a higher maximum payload capability and a lower fuel consumption at comparable cruise con - ditions. The AiResearch/Lear had a larger passenger or payload capability than the Learjet 35 along with lower fuel consumption at comparable cruise conditions.

The NASA QCGAT program objectives w e r e met, demon - strating that the application of large turbofan engine technology t o small GA turbofan engines can achieve low noise, low emis - sions, and acceptable fuel consumption.

Turbine Engines

Noise

High - speed turboprop noise also has been a part of the ad - vanced turboprop or propfan program. The new propeller con - figurations, involving very thin, multiple swept blades, provide lower noise and increased efficiency at high speeds. Research into advanced fuselage treatment for aircraft with high exterior fuselage noise levels is being conducted t o e n s u r e that future tur - boprop aircraft have an acceptable cabin environment.

Lockheed was contracted by NASA for propfan noise work. A monocoque section fuselage 1 5 feet long from a wrecked Swear- ingen M e t r o was used for extensive noise experiments. Double sidewalls were found to be significant noise reducers compared t o cdnventional aircraft fuselage sidewall construction.

More recently, Lord conducted a fuselage test with Swearingen to demonstrate active control of interior aircraft noise. A Metro I I fuselage was subjected t o a series of tests using real or simulated propeller noise. Then input and output transducers were mounted to reproduce the noise 180 degrees out of phase t o cancel o u t the noise. Basically, propeller noise is detected and loudspeakers under the fuselage floor produce phitse-reversed counter noise.

Though this research is still continuing, researchers believe there is good potential for the concept working since a 20 - decibel noise attenuation has been achieved with the possibility of 40 decibels in the 20- to 500 - Hertz range. This could very well lead to a reduced amount of passive acoustical materials for noise reduc - tion.

Propeller tip devices have been investigated a s well for both im - proving performance and reducing noise. Proplets, like winglets on aircraft wings, improve aerodynamic efficiency by reducing the total induced drag. An extension of t h e proplet concept that reduces structural loads is the bibladed propeller.

The proplet increases low - speed (130-knot) power and thrust for a 1 percent cruise efficiency improvement and a 4- to 5 - percent climb performance increase. With a baseline propeller noise of 97.6 decibels, a 10-degree swept back proplet dropped the noise level by 0.3 decibel. Sweeping the proplet back 20 degrees did not reduce the noise level any further while sweeping it forward resulted in increased noise. The unswept proplet had 0.4 decibel NASA and General Aviation Fairchild Aircraft and Lord have undertaken a NASA study for noise cancellation in a Metro commuter airliner, shown here. Noise inside and outside of turboprop com - muter transports remains a target for improvement. (Fairchild Aircraft) less noise than the corresponding unswept GA blade. Reducing the diameter of a propeller and adding a tip device t o maintain constant aerodynamic efficiency can result in noise reduction d u e t o reduced tip Mach number.

Turbine engine development will remain an important area of research and testing, though NASA has selected the rotary engine a s the major focus of future NASA efforts for general aviation.

CHAPTER 5

COMMUTER AVIATION

ith airline deregulation there has been a scramble for the more lucrative markets in air carrier operations. Out of

W this milieu, commuter or regional lines have emerged a s

potentially profitable and very active, feeding passengers from smaller cities to the major hub airports in aircraft seating 15 to 60 passengers. Commuter airline traffic grew from 4.3 million passengers and 43.5 million tons of cargo in 1970 to an estimated 15.5 million passengers and 500 million tons of cargo in 1980.

These smaller airlines conduct one - third of all scheduled airline flights. Many larger airlines abandoned short - haul routes because they were not able to operate larger jetliners at a profit. Commuter (regional) airlines have picked up these routes using smaller, lower performance, propeller - driven aircraft, but these aircraft represent a n older technology level much in need of redesign.

Small Transport Aircraft Technology

In 1978, NASA established a small transport aircraft technology (STAT) team t o study whether or not there are technical im - provements in commuter aircraft that would likely increase their public acceptance and use, and to examine the possibility that NASA’s aeronautical research and technology program could help U.S. commuter aircraft manufacturers develop superior future air - craft. This was particularly important since there was an apparent lack of US. - built commuter aircraft to meet market need. Com - panies in Spain, Brazil, Israel, Ireland, Britain, Holland, Canada, NASA and General Aviation IDENTIFY AND DEMONSTRATE THE COST - EFFECTIVE APPLICATION OF ADVANCED TECHNOLOGY TO ALLOW THE DEVELOPMENT OF SIGNIFICANTLY IMPROVED SMALL SHORT - HAUL TRANSPORT AIRCRAFT 4 ECONOMICS 4 EFFICIENCY 4 PERFORMANCE ENVIRONMENT PROPULSION STRUCTURES Small transport aircraft technology (STAT) France, Italy, and West Germany have aggressively pushed for - ward with new designs, often with direct government monetary support. On the whole, U.S. companies are finding it hard t o challenge this impressive foreign lead but at least six American firms are pressing ahead with development of new or modified commuter transport aircraft. Part of the reason for this is that the smaller manufacturers d o not have the large engineering depart - ments of the airline transport companies.

The STAT effort has been directed a t allowing the American air - craft industry to develop small future transports with significantly improved economics, performance, efficiency, and environmental compatibility. The airframe, engine, and propeller manufacturers participating in these studies were Beech, Cessna, Pilatus, General Dynamics - Convair, Lockheed - California, Garrett Turbine, General Electric, Detroit Diesel Allison, Hamilton Standard, and Mc- Cauley. Current technology aircraft w e r e used a s baselines with which t o compare advanced technologies t o determine the poten- This i s Cessna Aircraft's design study for an advanced technology small transport aircraft.

tial for improving aircraft performance and operational economics after 1985.

In examining commuterlregional airlines, NASA found that over three - fourths of the aircraft flown by local service airlines were jet powered with passenger capacities above 100 passengers. At stage lengths of less than 500 miles, the two- and three - engine B - 737, DC - 9, BAC-111, and B - 727 account for over 87 percent of U.S. jet transport fuel usage. This was tolerable when fuel was very cheap, but from 1973 to 1981 the average price of a gallon of jet fuel increased from 13 cents to more than $1.03. For aircraft such a s the B - 727 - 200 this drove fuel from 25 percent of the direct operating cost (DOC) to over 50 percent. M o r e efficient turboprop and piston commuter aircraft are ideally suited to take over these routes from the larger jets since they use 20 percent less fuel per NASA and General Aviation 1 1 a h Commuter Aviation seat mile, but advanced commuter aircraft could result in significantly greater savings.

STAT studies by Cessna, Convair, and Lockheed investigated new small transport aircraft designs with 19-, 30 - , and 50 - seat capacity with a 600 - nautical - mile range and optimized for minimum DOC over a 100-nautical-mile stage length. Additional design goals included a 4000 - foot field length and passenger com - fort levels equivalent to large jet transports.

Cessna’s 19- and 30 - passenger designs utilized structural bond - ing and composites in primary and secondary structures along with advanced engines, propellers, and aerodynamics. The result was aircraft that used around 40 percent less fuel (a 21 percent reduction in DOC) on a 100-nautical-mile trip.

All the Convair aircraft were designed for cruise at 2 5 0 knots and the 30 - passenger design incorporated a new high - lift, low- drag airfoil, composites, active controls, and improved propellers and engines. The design ended up 22 percent lighter with 51 per - cent less wing area, requiring 37 percent less horsepower and us - ing 31 percent less fuel (a 24 percent reduction in DOC) o n a 100-nautical-mile trip. With rear - mounted engines, cabin noise was predicted to be significantly lower.

Lockheed placed major emphasis on reducing airframe manufacturing costs using both aluminum and composite materials. For a 30 - passenger design with a n improved high - lift, low - drag wing, active controls, and propulsion system im - provements, the result was a 25 percent structural cost savings relative to conventional aluminum skin - stringer design. On a 100-nautical-mile trip, the rear - engine Lockheed design used 26 percent less fuel (a 16 percent reduction in DOC).

Beech applied advanced technology engines, propellers, sur - face coatings, and composites in the wing and empennage to o n e of their near - term 19-passenger designs with the s a m e fuselage and mission requirements. On a 100-nautical-mile trip the design used 34 percent less fuel (a 21 percent reduction in DOC) but with a 17 percent higher acquisition cost.

Pilatus Rritten - Norman also examined the application of ad - vanced technologies to its current 16-passenger Trislander but found little growth potential. The firm then proposed an advanced 19-passenger design with improved passenger accommodations, NASA and General Aviation 3 2 percent increase in cruise speed, 100 percent increase in range, lower external noise level, and 40 percent lower DOC per seat mile than the Trislander.

Though the above companies assumed rather than calculated improved propulsion, the validity of these assumptions was born out by the engine and propeller manufacturers.

Engines and Propellers

General Electric identified a dozen candidate advanced 1500- t o 2500 - shaft - horsepower turboprop technologies that could im - prove engine efficiency 1 4 percent and reduce engine weight relative t o the CT7, a 1600-shaft-horsepower-class engine still in development. GE managed the boost through increased compo - nent efficiencies and higher cycle pressures and temperatures - as high a s 2O:l and 2400' F. The compressor incorporated several aerodynamic improvements; there w e r e thermal barrier com - bustor coatings for increased durability, more effective turbine blade cooling, reduced turbine running clearances, and digital controls.

Detroit Diesel Allison studied both 2400- and 4800-shaft- horsepower engines suitable for 50 - passenger aircraft designed for Mach 0.45 and 0.70, respectively. In both cases the baseline engine was a scaled version of the 8000 - shaft - horsepower XT701 turboprop. Using DOC a s the central issue, Allison raised the com - pressor pressure ratio from 12.5 to 2 0 while maintaining the s a m e 2500' F turbine temperatures on both engines. An efficiency gain of 17 to 19 percent, a weight reduction of 1 3 t o 25 percent, re - duced costs of 16 to 19 percent, and a reduction in maintenance costs of 56 to 6 2 percent resulted. With a 100-nautical-mile mis - sion, 10 to 22 percent less fuel was used with 1 3 to 1 6 percent DOC savings.

Garrett examined power plants for 30- and 50 - passenger Mach 0.45 aircraft requiring 1800- to 2500 - shaft - horsepower engines.

Pressure ratio would be upped to 15:l or 20:1, which would in - crease efficiency 3 percent but also raise engine cost 20 percent.

With several other features and general aerodynamic improve - ment t o permit higher pressure ratios, a 1 3 to 1 4 percent engine Commuter Auiation efficiency improvement and a 2 2 to 32 percent weight reduction w a s realized relative to scaled versions of the newer 1000 - shaft - horsepower TPE331-11 engine. , Hamilton Standard defined advanced propeller technologies and associated benefits for the 30-passenger, Mach 0.45 Convair and the 50-passenger, Mach 0.7 Lockheed airplanes. The analysis concluded that for both aircraft, a six - bladed propeller with lightweight composite construction, advanced airfoils, tip prop- lets similar to winglets, and a n advanced precision synchrophaser are required to increase efficiency and lower cabin noise. The Mach 0.45 design employed straight but extremely narrow blades, whereas the Mach 0.7 design used wide, thin blades swept 45 degrees a t the tip. Both props would be 5 to 6 percent more effi - cient than the best of today’s propellers due t o advanced materials and construction techniques that permit narrower and thinner blades, and tip loss alleviation with proplets. For low - speed air - craft with tail - mounted engines these improvements resulted in a n 8 percent fuel savings (a 3 percent DOC reduction). For wing- mounted power plants, these values increased to 13 percent and 6 percent, respectively, because a large acoustic treatment weight penalty required in the baseline t o achieve a Boeing 727 cabin noise level was eliminated.

McCauley looked at propeller technology for a 19 - passenger, Mach 0.45 aircraft. Using advanced aerodynamics, materials, and structural concepts similar to Hamilton Standard, the projected ef - ficiency improvement would be 8 to 9 percent relative t o typical general aviation propellers. Fuel and DOC payoffs similar to the Hamilton Standard results were realized.

Teledyne Continental and Curtiss - Wright undertook studies on very advanced rotary and diesel engines a s alternative power plants to conventional turboprops. These intermittent combustion engines feature multifuel, stratified charge combustion systems, high - pressure ratio turbochargers (five to nine), high - speed fuel in - jection technology, and low heat - loss cylinders. Competitive fuel consumption was evident along with the capability of operating on jet fuel rather than avgas. More study is needed to gain a total assessment of these engines and of unconventional turboprop cycles (e.g., with regeneration).

NASA and General Aviation

Aerodynamics

With natural laminar flow airfoil glove tests o n a n F-111, signifi - cant laminar flow was evident a t off - design cruise conditions of Mach number and leading - edge sweep, the region of interest for small transports. Research is also being conducted t o develop air - foil d e s i g n s with improved m a x i m u m lift a n d c l i m b characteristics. A handbook resulted on upper - surface airfoil- contour modifications designed to improve the maximum lift coefficients, thus improving the stall characteristics of many small transport aircraft that today use NACA six - series airfoil sections.

Such modifications may be used for possible retrofit on existing aircraft or in the design of new aircraft.

To establish a n aerodynamic data base representative of current commuter aircraft technology, unpowered and powered small transport aircraft wind tunnel tests of a 15 percent scale model of the Swearingen Metro transport were conducted at Ames under a cooperative research agreement with Swearingen Aviation. Un - powered tests in the Ames 12 - foot pressure wind tunnel in - vestigated aircraft component drag buildup, while powered pro - peller tests investigated the effects of propeller slipstream on wing aerodynamic characteristics.

Most of NASA’s general aviation research can be applied t o small transports, including flight control systems, navigation and guidance systems, cockpit displays (see chapter 8), and other air - craft subsystems such as icing protection (see chapter 7). These systems are very important to the overall success of the aircraft design, including aircraft performance, passenger comfort, safety, and economics.

The overall conclusion of the STAT airframe, engine, and pro - peller studies is that very significant improvements in energy effi - ciency, operating cost, and passenger comfort are possible for future small transport aircraft through combined advances in all the aircraft disciplines. In many ways, the technologies studied de - pend o n each other for the maximum benefits.

A NASA ad hoc Advisory Subcommittee on Commuter Air Transport Technology comprised of government, university, and airline, airframe, engine, and propeller industry representatives recommended that NASA undertake a n aggressive research pro- Another advanced configuration small Lransport under wind tunnel test by NASA shows a canard (forward)wing, pusher props, advanced aerodynamics, and relaxed stability for active controls.

gram t o quickly bring the specialized small transport technologies to a state of readiness for commercial development by 1985.

The overall goal of this research would be the establishment of technology readiness enabling the following future commuter airplane improvements: Direct operating cost reductions of 20 percent Fuel savings of 35 percent Increased reliability and safety Boeing 727 level of passenger comfort Reduced maintenance Many other recommendations were made for further research not being done in the transport or general aviation areas. Engine and propeller technology needs emphasis since commuters fall between the larger transports and the smaller general aviation air - craft. A unique problem in commuter operations is the severe duty cycle (rapid changes from idle to takeoff power t o cruise to land - ing) imposed o n commuter engines. Ultimately, new engines designed specifically for this mission will have t o be built. Ride quality improvement through active control gust alleviation is another area of importance; not only would it provide more com - fort for the passengers, it would also reduce pilot work load and reduce wing structural fatigue. This is particularly significant for commuter aircraft since they operate routinely a t low altitudes and with lower wing loadings than large transport aircraft.

NASA and General Aviation The STAT studies explored all areas in which advanced technology application could improve passenger acceptance, safety, operating costs, and profitability with 19 - t o 50 - passenger aircraft. While available technology could improve future com - muter aircraft, the synergistic combinations of advances in aerodynamics, propulsion, configuration design, and aircraft systems could reduce fuel costs up to 40 percent, direct operating costs up to 24 percent, and aircraft acquisition cost by a s much a s 18 percent along with improvements in passenger comfort and convenience .

CHAPTER 6

GENERAL AVIATION UTILITY

n addition to the more revolutionary developments in NASA’s general aviation research, avenues are being investigated to in - crease the utility of the general aviation environment.

How can the usefulness of certain aspects of commercial and non - commercial GA flying be enhanced and developed? NASA’s ef - forts have centered around improvements for the air traffic control (ATC) system in which all aircraft operate, and research into agricultural applications and aerial spraying.

Air Traffic Control

The safe, efficient, and dependable flow of air traffic represents a major challenge for the ATC system. Moving traffic under instru - ment flight rules (IFR) is already a serious problem, and with air - craft operations projected t o double by the mid - 199O’s, airport congestion will reach an alarming condition. With the exception of small private fields, no new airports have been built in the U.S.

since 1970 because of cost, urban congestion, and a host of other factors. Clearly, maximum use must be made of our air space and existing airports.

The Federal Aviation Administration (FAA) has an extensive program for developing an improved ATC system. The two - way data link associated with the discrete address beacon system (DABS), scheduled t o become operational in the late 1980’s, will provide the vital missing element required for a quantum jump in advanced operating procedures. DABS will be capable of uplink- ing many services t o aircraft equipped with electronic displays and NASA and General Aviation appropriate input - output devices. Eventually, these services would augment voice communications with alphanumeric messages t o provide altitude assignment, weather data, and runway conditions.

DABS may be used t o uplink holding instructions, approach and departure clearances, and metering and space commands. It also may be used to transmit data for cockpit display of traffic informa - tion (CDTI). Ultimately, it may relay traffic - time - sequencing data needed for driving “time - boxes’’ in cockpit displays for precise space and time (4 - dimensional) navigation from takeoff t o landing.

Important to terminal area operations will be the navigational capability provided by the microwave landing system (MLS) slated for initial implementation in the mid - 1980’s. The MLS will permit high - precision guidance commands t o be generated for following trajectories optimized for energy efficiency, noise reduction, and airport capacity. A revolutionary possibility for increasing the volume of accurate navigation information is offered by the Navstar Global Position Satellite (GPS), which will provide posi - tion and velocity information over the entire Earth and a t all a I ti tudes.

As a counterpart of the FAA ground - system development effort, NASA is developing the advanced airborne system technology re - quired t o safely and efficiently operate aircraft in the future ATC system. These studies, involving applied human factors, include research in the areas of energy management procedures and displays, CDTI, and data - link applications for increased operating efficiency. Not only has a discrete, digital data link been looked a t for pilotlATC controller communication, but automatic aircraft vectoring via data link also is being explored. Potential benefits are reduced pilot technical error, reduced ATC controller work load, and a ground based area navigation capability.

N A S A has conducted time - based metering experiments in the Denver area where the FAA has developed and implemented a local flow management profile descent concept for arrivals into the terminal area. This provides fuel savings by matching aircraft arrival flow t o airport acceptance rate. Time control computations allow the pilot t o descend at his discretion from cruise altitude t o the metering fix and an idle - thrust configuration. NASA Ames developed and flight tested a n airborne descent argorithm de- General Aviation Ulilily signed t o improve the accuracy of delivering an airplane t o a me - tering fix a t a time designated by ATC. The Denver experiments demonstrated that arrival accuracy was improved from the current levels of 1 t o 2 minutes using guidance from the radar controllers t o more than 10 seconds with the airborne 4 - dimensional system.

Work load for both the pilots and controllers was also much less with the airborne algorithm.

The potential benefits and liabilities associated with displaying traffic information in the cockpit are being examined in a joint FAA/NASA program utilizing highly integrated simulators and aircraft in flight. The centra; issue involves the question of proper relationship, or “distributive management” role, of the flight crew t o the ATC controller when the crew is presented with a limited display of proximate traffic. Studies to date have defined informa - tion requirements and formats for a workable display, and con - siderable insight has been provided on pilot ability t o use the display in flight, such as self - spacing on another aircraft. Yet t o be determined are procedures, both operational and regulatory, for using the traffic display. Simulation experiments using CDTl have also been investisated for wake vortex avoidance (see chapter 7).

A erial Spraying

Though often viewed a s glamorous “crop dusting,” aerial ap - plication operations serve a major economic need in this country.

A fleet of approximately 8500 agricultural aircraft accounts for an estimated 10 percent of all US. agricultural production through the aerial application of fertilizers, seeds, and pesticides. Faced with serious economic and environmental pressures for im - provements in aircraft and dispersal systems, the aerial applica - tions industry asked NASA for help in solving those problems.

By 1977 a comprehensive, long - range aerial applications research plan was started by NASA to improve environmental safety, fuel efficiency, and aircraft productivity and safety. The NASA research effort centered around the following areas.

Drift of chemicals away from their target areas was the major in - dustry concern, both in the U.S. and in other countries, represent - ing not only pollution but direct economic loss. Spray behavior, wake aerodynamics, economic, weather, and biological factors are all in need of improved understanding t o solve the problem.

NASA and General Aviation Improved fuel efficiency, productivity, and safety are sought for the aircraft involved in this demanding line of commercial flying.

Agricultural aircraft must operate at high angles of attack with good stability, control, and handling. There is virtually no margin for error when flying so close t o the ground. These planes spend a s much a s 30 percent of total mission time in turns, where airspeeds are within a few miles per hour of stall speed. Good turning per - formance depends on low drag and a high safety level depends on reduced tendency for the wing t o drop or roll off at the stall. Wing droop leading - edge modifications from the NASA stall/spin pro - gram (see chapter 7) have been recommended a s simple retrofits t o current agricultural aircraft since stall/spin/spiraI/mush ac - cidents account for half of all deaths.

Flight tests with a Thrush agricultural aircraft were made using electronic swath guidance, resulting in f 1 meter accuracy for row crop spraying. A conventional course deviation indicator (CDI) used for navigation provided the pilot with steering information.

The key t o practical implementation requires the development of low - cost, highly reliable methods of getting the steering informa - tion t o the pilot.

Research on dispersal systems has been conducted in wind tun - nels and in flight tests seeking t o improve pattern uniformity and width. Industry has been highly interested in expanding the per - formance envelopes of dry material spreaders, sparked by the ex - cess power currently available in the larger, turbine - powered and high - horsepower piston agricultural aircraft. This excess power has the possibility of spreading fertilizer at rates as high a s 400 pounds per acre on swath widths a s high a s 80 feet and at speeds a s high as 120 miles per hour.

TOP (Right) Modifications investigated for the Thrush included leading - edge slats for high - lift improvement, ring cowl and wing - to - fuselage fairings for drag redustion, and improved wake characteristics. Also evaluated were wake modification concepts such as vortex attenuating splines and winglets, shown to have beneficial effects on wake - dispersal interaction.

BOTTOM (Right) After tunnel tests, the Thrush was modified to improve application efficiency. Winglets proved able to control the vortices swirling off the wing tips.

Moderation of the vortices improved the spray pattern and reduced drift. This was the end product of a long program that began with scale model tests in NASA Langley’s wake vortex research facility.

NASA and General Avialion New calibration techniques, such a s laser Doppler velocimetry, laser fluorosensing, biodetection, and others offer potential for improved documentation of drift and deposition of chemicals.

NASA has evaluated alternative spray accountancy techniques and documented their value for use by industry.

These areas of research involved a number of facilities and tests.

At the Langley Vortex Research Facility, small - scale models of agricultural aircraft w e r e used t o study aircraft interaction be - tween airplane wake and dispersed materials. An Ayres Thrush S2 R - 800 aircraft was mounted in the Langley full - scale 30 x 60 - foot wind tunnel t o determine its performance; then changes w e r e made t o study the effects of drag reduction devices, leading - edge high - lift devices, a turbine engine modification, and wake modification devices. Droplet size distributions were measured by laser spectrometer.

Flight testing at NASA Wallops Flight Center then provided a real - world environment in which the Thrush could validate previous tests. The aircraft was flown with modifications relating t o handling qualities, wake vortex modification, and spray distribution. Flying the Thrush for spray measurements proved t o be one of the more challenging aspects of the program. For the majority of the tests, the airplane was flown within a foot of the collection tray t o get accurate placement of the droplets and the tests were performed between the hours of midnight and 4:OO a.m., when air is the calmest. A series of approach and guidance lights was used to keep the pilot on target - it looked like something out of a science fiction movie.

Near - term results have been realized in drift reduction, stall departure safety, and dry materials dispersal improvements. As a result of this intensive program, computer codes are now available defining wake and spray pattern interaction. The overall agricultural applications data base available can make a great deal of difference t o the economic survival of a n important U.S. in - dustry.

7 8

CHAPTER 7

SAFETY IMPROVEMENTS

he quest for ever safer aircraft has been an overriding con - cern since the very beginnings of flight. From the forma -

T tion of NACA, safety research has attempted t o match the

incredible growth of aviation over the past 80 years, but it has often lagged behind. With NASA’s recent emphasis on general aviation, safety has been singled out for intensive study and im - provement in stalllspin prevention and recovery, crashworthiness, controls, wake vortex avoidance, and flight in icing conditions.

St a 1 1 /Spin

From man’s earliest attempts at flight, stall/spin accidents have plagued the development of virtually all types of aircraft. Even to - day, stall/spin accidents involving general aviation aircraft ac - count for more fatal and serious injuries than any other kind of ac - cident. During the past decade, more than 100000 Americans were involved in more than 39 000 light plane accidents, and near - ly one in four of those accidents are believed to have been due t o loss of control after the planes were either purposely or accidental - ly maneuvered into a stall or spin. Stalling is the sudden loss of lift that results when a wing exceeds its normal operating range.

Visualize air flowing smoothly over a wing in level flight. If the angle of the wing is increased more and more, the air begins t o deviate from smooth flow. The airstream breaks into turbulent ed - dies, and the wing - gently or abruptly - loses its lift. Spinning gets its name from the rotational pattern of an airplane - in a spin. A NASA and General Aviation spin may often follow a stall - the nose drops, and the aircraft begins t o rotate and lose altitude.

Stall/spin accidents are insidious, usually catching the pilot completely unaware. For example, after experiencing a n engine failure on takeoff, the pilot attempts t o turn back to the runway quickly before he or s h e runs out of altitude. Most pilots d o not realize that much higher speed is needed to make a rapid 180 - degree turn and avoid flow breakaway on the wing. Altitude loss during this turn can b e anywhere from 125 to 1000 feet depending on the type of aircraft, and most pilots have no idea how much they will lose in a 180 - degree turn for the aircraft they are flying.

Another classic stall/spin accident can occur when turning onto the final approach leg to an airport in a crosswind. The pilot perceives that for the normal bank angle, the turn rate is such that the aircraft will not b e lined up with the runway; thus, the pilot banks more steeply and pulls back on the wheel. Suddenly the low wing drops sharply, giving the pilot a shocking closeup view of the ground from a steep banked nosedown attitude. The pilot instinc - tively pulls back harder on the elevator control and the aircraft enters a deeper secondary stall from which there is insufficient altitude to recover.

Different airplanes have different stall and spin characteristics; o n e may be forgiving of ham - handedness, while another may seem to have a mind of its own, bent on destruction. J u s t about the time one general type of airplane is analyzed, tested, and understood, another comes along and presents a new set of problems.

NASA stalI/spin research programs g o back more than a half century. In 1930, NACA began operating its spin tunnel, a vertical, controlled airstream in which a small, dynamically similar model of a test airplane could be spun in free flight. Clockwork mechanisms activate the model’s control surfaces for pro - grammed recovery maneuvers. From that beginning, work grew t o include similar tests using a restraining rotary - spin balance t o measure the forces in the spin. Today work proceeds in this tun - nel, o n e of only two in the free world.

Other test approaches t o stall - spin phenomena have involved standard wind tunnel model tests, radio - controlled flying models, Sta/l/spin research airpianes simulation, and the actual aircraft tested in the full - scale wind tun - nel and in free flight. However, most of the research o n spin testing through the early 1970’s involved military aircraft. When aircraft speeds increased and swept wings entered the picture, general aviation research was left alone until 1973, when the NASA stall/spin program was expanded. The high GA accident rate demanded some specialized research but a t first the program was small. The initial goal was t o see if World War I I research was applicable t o current aircraft.

The only data available dealt with tail surface geometry, con - sidered t o be the governing factor in spin recovery. However, after a long and extensive series of detailed and rigorous tests, NASA reached the conclusion that tail geometry was not the complete answer to the total spin problem. The wing was a driving mechanism in the stall departure and the tendency t o enter a spin for typical GA aircraft, along with a number of other complex fac - tors.

NASA's spin tunnel is the only one of 11s kind in the free world. Scale weighted and balanced models cen be lhrown into lhc lunnel and spun in numerous configura- lions. Dala on spin characlenslics as well as spin recovery can be galhered before a full - scale lesl i s conducled. The lunnel has been a vilal goucrnmenl resource since long before World War I/.

In 1973 a series of flight test programs was initiated with t h e prototype Crumman American Yankee light single - engine trainer.

Different tails were fitted to test their results in spins and these flights revealed the complexity of the spin, which was basically the s a m e with each of the tail units. By 1976, with full funding, a developed spin and recovery program was under way, defining aerodynamics, emergency spin recovery systems (including tail parachutes), and automatic spin prevention systems.

Radio - controlled scale models of GA aircraft proved to be in - valuable tools in the program, particularly since they can enter flight regimes that are dangerous in piloted aircraft and then demonstrate recovery procedures without risking lives. One of the more successful early programs was the joint NASA/Beech YT- 34C scale spin tunnel and radio control model spin simulation tests that validated use of radio - controlled models in spin testing for GA aircraft.

Safety lmprouemenls Since many unswept wings have a tendency to stall asym - metrically (one wing first), thus creating a tendency to spin, wing leading - edge stall strips have been installed on many aircraft. The stall strip locations have often by cut - and - try methods resulted in almost a s many variations a s there are aircraft. The lack of design information regarding controlled - stall wing characteristics led NASA to study the effects of wing leading - edge modifications on staII/spin characteristics.

Development of differing drooped leading - edge wing exten - sions on test aircraft proved the point about the powerful effects of wing flow generating separation on spins. With s o m e full - span ex - tensions, even a relatively docile spinner could be made t o spin flat, fast, and viciously. But partial - span leading - edge extensions successively made three test airplanes considerably more spin- resistant and more easily recoverable.

Other design characteristics such a s high - wing versus low - wing, propeller slipstream, aft fuselage cross - sectional shape, and, still The initial spin Lesls with the Yankee were cenlered around the effects of tail design on spins. Several different lails were built and tesled on both the model and full - scale aircraft. The spin recouery parachute can be seen beneath the tail; 11 was used only for unrecouerable spins.

After extensiue testing, NASA researchers found that spins were primarily wing generated, and that the tail configuration played a minor role in comparison, The ma - j o r breakthrough came with leading - edge wing additions which not only prevented stalls from becoming uiolenl, but also actually made the airplane almost unspin- nable. These additions can be seen in red on [he outer leading edge of each wing.

Note the spin recouery mission markings under the cockpit.

not out of the picture, tail geometry, were found to have certain ef - fects on stall/spin. In some, but certainly not all, a high wing or a T - tail can reduce spin tendencies more effectively.

During flight tests on GA aircraft, a unique control augmenta - tion system of compact hydrogen peroxide thrusters located a t the wing tips was installed. Controlled by the pilot, these tiny rockets could b e used t o help recover from a spin, but they could also be fired in a pro - spin direction during a steady spin to generate much higher spin rates. As a result, the potential existence of flatter, more dangerous spin modes could be investigated and defined.

Another major area of research that has been generally limited to military aircraft is that of limiting pitch control power, now singled out for GA research. Such limitation can make it impossi - ble to stall an airplane by automatically reducing the elevator travel commanded by the pilot’s inputs. Mississippi State and Texas AGM universities have flown aircraft with pitch sensors elec - Safety Improoernents tronically hooked into the control system so that pitch and throttle are automatically activated or spoilers are deflected under the horizontal tail t o prevent a stall. Whether or not these systems will ever find their way into future GA aircraft will depend on the k e y issues of cost, reliability, maintenance, and system failure characteristics. However, with the increasing sophistication of GA aircraft, particularly light twins, and advances in microprocessors, stall proofing, including flight control following engine failure for twins, is on the horizon.

The spin characteristics of twin - engine aircraft have been in - vestigated in a joint NASA/Beech program involving the new generation light - twin Model 76 Duchess. This is an important step in looking at stall/spin in more complex aircraft. The program in - cluded spin tunnel and rotary balance testing with radio control model testing and flight testing, culminating in 150 spin manuevers. Using all the tools now standard for this kind of testing, the Model 76 (not approved for spins) was shown to b e One of the more interesting things about the Sundowner was the addition of thruster rockets lo the wing lips, which were used to speed up spins and make them deeper.

They also could be used for recovery.

a5 NASA and General Aviation C RP very docile in spins, even with o n e engine out. This has not only demonstrated that there has been progress in staII/spin technology, it also has demonstrated that there is a great deal more t o understand before the aircraft designer has the tools a t hand needed for confident design in the stall/spin arena.

The most important lesson t o be learned is that staIl/spin prob - lems revolve around continual dependence on all facets of technology rather than o n e area. The introduction of novel con - figurations such a s swept wings, wing - fuselage strakes, long pointed forebodies, T - tails, and many others requires extensive, time - consuming research and development. Unfortunately, the impact of future airplane design tends t o be dismissed if the cur - rent stalI/spin situation is under control. Repeatedly, the designer of a novel configuration finds a complete lack of data base for his or her use.

Advanced canard - type (forward horizontal tail and rear- A radio - controlled model almost ready lo be started /or a stal//spin test /light i s shown here.

mounted wing) GA designs appear to offer considerable promise for improved performance and inherent stalllspin resistance, par - ticularly Burt Rutan’s designs which have amassed a n impressive stall/spin safety record. NASA approached Rutan with the possibility of doing extensive tests o n his VariEze. After flying a scale model in the spin tunnel, it was found that properly loaded the plane is virtually impossible t o spin. However, the problem of wing rock a t low speeds with the nose high still remained after testing in the 30 x 60 - foot wind tunnel. Installation of the partial span leading - edge drooped modification mentioned earlier com - pletely solved the problem and it was recommended a s a change t o all VariEze owners. A s a result of this testing, NASA hopes t o overcome some of the problems with lack of data for designers of advanced configurations. Rutan has considered an advanced canard for the commuter market.

Historically, good stall/spin avoidance has chronically remained Seueral wing configurations were lesled on the model Yankee, tncludmg large winglets.

NASA and General Aviation a n elusive goal for designers of general aviation aircraft. During the past decade, the technology has rapidly accelerated to the point where improved design methods are available for significantly improved configurations. Rapid design methods and “stall proofing” remain paramount, hinging on control of stall pro - gression of the wing, development of a n acceptable means t o limit pitch control, and minimization of adverse cross - coupling effects by automatic means. Slowly the research emphasis has pro - gressed from stall/spin recovery t o prevention, and the future for safe general aviation flying has never looked brighter.

Crashworthiness

In 1972, NASA and the FAA embarked on a cooperative effort t o develop technology for improved crashworthiness and passenger survivability in general aviation aircraft with little or no increase in weight and acceptable cost. Since then, NASA has “crashed” dozens of GA aircraft by using the lunar excursion module (LEM) facility originally built for the Apollo program.

The LEM test rig at Langley was converted t o drop airframes on - t o a typical runway surface in a carefully controlled, extensively in - strumented environment. Photographic and other data are re - corded during the few milliseconds of the shattering impact. An - thropomorphic dummies harnessed in crew and passenger seats contain additional instrumentation to assess the loads imposed during a crash. Normal impact speeds of about 60 miles per hour are reached but a few tests have been m a d e at speeds up t o 100 miles per hour by using Falcon missile rocket motors.

The aircraft is suspended from the top of the gantry by two swing cables and is drawn back above the impact surface by a cable. The plane is then released from the pullback cable, permit - ting it t o swing like a pendulum into the ground. The swing cables are separated from the aircraft by explosive charges just prior to impact, thus approximating actual flight. The data umbilical re - mains attached as the crash takes place, but it is also separated by a n explosive charge before becoming taut during skid out. Flight path angle is adjusted from 0 ” to 60° by changing the length of the swing cable. The height of the aircraft above the ground at release determines the impact velocity.

Four Falcon air - to - air rocket engines were used lo accelerate this Navajo lo 145 k m per hour with a pitch angle o f - 30 degrees. Forgreatersurvival probability in an air - craft crash, the fuselage should remain intact. Researchers also invesligated seal m - tegrify and restraint systems, as well as seat design.

These tests emphasize development of the capability to predict the dynamic response of a n airplane’s structure during the impact.

How will it deflect and how will it break? How does it absorb energy or transfer the shock of impact t o seats and occupants? If these questions can be answered analytically, it will greatly reduce the design difficulties in future aircraft development programs.

This Navajo is impacting the concrele at 60 mph.

Current structural analyses d o not apply t o the special conditions of aircraft crashes, and NASA is endeavoring t o extend them so that they will.

Involved in these tests are seat, safety harness, and floor designs. A data base was developed so that the FAA could ade - quately define its dynamic seat testing. A computer program for modeling the crash behavior of seats and subfloor structures is now available t o industry.

An important focal point of the program is testing of crushable aircraft subfloor sections. These are designed t o crumple pro - gressively a t impact t o absorb s o m e of the energy of the crash and reduce the forces that the passengers feel. In s o m e of the NASA tests, the floor and the seats attached were undamaged, even though the subfloor was crushed.

The key t o this very dramatic testing is the ability to watch an aircraft in slow motion with extensive data input about what is ac - tually happening. It is o n e thing t o look a t an aircraft accident in

C -

NASA and General Aviafion reverse, so t o speak, through investigation; but this d o e s not pro - vide the opportunity t o follow it through without guesswork. Even the best crash investigators c o m e up with different results, and each sees something different when examining a NASA crash test.

NASA has discovered that little research h a s been done on cabin volume destruction. At least two - thirds of aircraft crashes resulting in death have left cabin volume essentially intact. What this says is that linear (forward) acceleration is not the real killer, but the load a s the aircraft impacts vertically. The bottom floor of the fuselage absorbs all of the load - there is twice a s much load vertically than horizontally. Yet, the human body is twice a s vulnerable t o G - forces (force of gravity multiplied) vertically than horizontally. Normal human G tolerances are 45 G s horizontally, 25 G s vertically, and 20 G s laterally. In other words, the spine is crushed. There is such a thing as a floor that is too strong (stiff).

This has been a major breakthrough in the program, pointing toward true crashworthiness.

In addition, the crush zone under the fuselage of a light twin distorts upwards and t o t h e side, forming a bow. This throws passengers violently into the sides of the aircraft. A new concept is a 2-inch-thick, reinforced floor plate with tuned structures underneath to absorb 20 t o 25 Gs, dissipating the force of the crash before t h e spine is crushed. A 12 - t o 18 - G tolerant seat is in the offing but a strong seat pan is needed a s reinforcement for the torso; the area beneath the seat itself has t o b e tuned as well.

Harnesses and lap belts need t o be placed where there is s o m e natural human strength - across the hip bones with t h e harness a t 0 " t o t h e back so that the passenger does not slide out from under it. O n e of the prime customers for crashworthy seats has been Wycliffe Bible Translators' mission flying arm, t h e Jungle Aviation and Radio Service (JAARS). Crashes on the far - flung mission fields have taken many lives in s o m e of the most demanding flying conditions. JAARS and NASA have been working together and have retrofitted several mission aircraft with better seats.

Several of these load - limiting seats have been tested in Langley's hydraulic loader, where ever - increasing loads can b e put o n any structure until it fails and then researchers can watch what happens a t that point. Several seats can take the loads, but when they fail they end up trapping the occupant's legs underneath.

Safety improvements CEILING ATTACHMENT LOAD LlMlTER WIRE BEFORE STROKING LOAD LIMITER AFTER OOR (a) CEILING SUPPORTED PASSENTER SEAT SEAT BACK TUBES LIMITER PATH INTO SEAT BACK LOAD OCCUPANT CG VERTICAL ST 30 cm (12 LIMITER (b) FLOOR SUPPORTED PASSENGER SEAT Floor supported and ceiling supported passenger seats haue been the subject o much research in the NASA crashworthiness program. Since humans can endure horizontal deceleration much better than vertical impact, seats have been redesigned accordingly. Crashworthy seats designed at NASA are currently flying in Jungle Aviation and Radio Service aircraft serving missionaries around the world.

NASA and General Aviation With every solution there are new problems t o solve since so little has been done previously in the area.

Controls and Human Factors

Though this sounds rather basic, controls are the heart of con - necting the pilot’s (or autopilot’s) wishes t o the airframe and power plant. NASA has conducted ongoing research into how human be - ings interact with aircraft controls. With the ever - increasing com - plexity of general aviation aircraft, particularly in the capability of their systems, pilots are in need of simple controls that require no diversion of effort. Not only does the pilot have t o fly, he or she must also navigate, monitor the cockpit displays, monitor the engines, and select the safest manner in which to perform the mis - sion, particularly in instrument flight rule (IFR) conditions (see chapter 8).

NASA’s aircraft energy efficiency program (ACEE), centered around large airline transports, has investigated a number of human factors areas that are beneficial t o general aviation. All air - craft systems interact, and design must take into account the ef - fect o n e change will have on other systems.

Modern control technology considers the effect of aeroelasticity t o reduce loads with highly flexible structures. Maneuver loads can be controlled on high - aspect - ratio wings to take advantage of im - proved aerodynamic performance. Gust loads also can be con - trolled t o reduce fatigue damage, improve ride quality, and reduce wing - bending moments. Weight penalties for flutter can be minimized or eliminated with active flutter suppression systems.

Successful application Gf these concepts requires improved methods for unsteady load prediction, integrated design tech - niques, and improved reliability of components and subsystems.

The heart of this ability t o move controls independently of the pilot’s input is the computer. Fly - by - wire systems represent leav - ing the old tube and cable control systems behind. In other words, the pilot or computer is connected to the control surfaces by elec - tronic wiring and not by direct controls. The electronic impulse moves the controls through a system that takes into account mean time between failures.

These active controls are already being flown on large transports such as the Lockheed L - 1011, but if they fail the pilot Safety Improvements can still control the aircraft. Full authority active control and per - formance benefits from such factors as relaxed static stability re - main in the future for GA aircraft, but the potential for great in - creases in fuel efficiency d u e t o minimizing weight and drag is very promising.

The revolution in digital electronics has dramatically affected the design and capabilities of emerging avionics systems because of the spectacular reduction in cost, size, and power required of digital electronic components. Although digital systems have been used for a number of years in navigation and other onboard applications, they are now emerging in safety - critical flight con - trols. The new generation of civil transports such a s the L-1011-500, DC - 9 Super 80, and the Boeing 757/767 family utilize digital flight control systems. In the far term, reliable microprocessors will increase safety and utility and further decrease the cost of avionics systems (see chapter 8).

In addition to the potential improvements in fuel efficiency and reduced operating costs offered by digital fly - by - wire flight control systems, the use of electromechanical actuators in lieu of hydraulic actuators with these systems offers additional benefits.

Though they have been under development for aircraft for many years, newer, smaller electric motors have made their use more realistic because of the reduced size, weight, and power re - quirements compared t o older motors. Replacing the hydraulic ac - tuators with electromechanical actuators could allow the removal of the entire hydraulic system (actuators, servos, pumps, line, and valves).

The major deterrent t o the widespread use of integrated avionics is systems reliability - a systems probability of failure of less than 1 0 - 9 a t o n e hour, three orders of magnitude better than the triplex systems of today, must be achieved. In order to reach these levels, two experimental fault - tolerant computer concepts are being developed at Langley. Both concepts use redundant computer elements, but each employs a different approach to fault detection and isolation (software voting versus hardware voting ) .

An advanced avionic or “all - electric’’ airplane, such a s the military F-16 fighter, requires careful integration of the airplane with the crew via advanced flight decks. Over the past several NASA and General Aviation years, numerous flight experiments have been conducted using the N A S A advanced transport operating systems (ATOPS) air - craft, a Boeing 737 - 100 equipped with a research cockpit located in the cabin area. From this experimental flight deck, the airplane has been flown using a fly - by - wire control system, electronic displays, and pilot - selectable automatic navigation, guidance, and control functions in simulated Category 111 (the most demanding IFR) conditions. Results from these tests have gone into the Boe- ing 757/767, but applications for general aviation aircraft also are being investigated.

The crew’s role in the future will continue to move toward one of manager of more automated flight systems. Advanced crew sta - tions will require additional research to assure proper integration, and toward this end NASA and Lockheed - Georgia are developing an advanced concepts simulator design that can be used as a start - ing point. A number of candidate cockpit layouts were examined looking a t both conventional and futuristic configurations. A desk - like console design has been chosen and a mockup has been made to look at initial placement of pilot controls, electronic displays, multifunction/multimode keyboards, touch panels, and voice recognition devices. Certainly the day of advanced human engineering for safer flying is near.

There are many other smaller programs being conducted in - volving control improvement which have nothing to do with elec - tronics. Improved ailerons, elevators, and rudders directly aid the pilot in controlling the flight path of an airplane.

Landings present a challenging task for general aviation pilots of small aircraft. Not only do many people find landings hard to master, but t h e approach a n d landing phase of operation represents a period of high accident risk. Overall, approximately 50 percent of all general aviation accidents involve landings, with the approach phase (overshoots, undershoots, or collision with ob - jects) being the primary cause. Glide path control certainly relates directly to these accidents.

NASA sponsored a study to evaluate the application of upper surface hinged - plate spoilers and lower surface dive brakes similar t o those used o n sailplanes to the wings of a Beech Musketeer.

Four different configurations were determined, and flights were made under a wide range of flight conditions with pilots ranging Safety Improvements from students t o advanced commercial pilots. The results were very encouraging for such a simple system.

Spoilers offered significant improvements in the aircraft’s per - formance and flying qualities for all elements of approach and landing. Spoiler deployment was linked t o power changes so that the throttle became the flight path controller. Touchdown ac - curacy and ease were much improved for all the pilots taking part.

With the results available t o industry, safer GA aircraft can b e designed.

Wake Vortex Hazards

An invisible danger t o small aircraft is continually present a t large airports. The wakes of heavy transports during landing or takeoff create horizontal whirlwinds that can upset a lighter air - craft flown into their trailing envelope. When this phenomenon was both experienced and identified at least 20 years ago, it became a matter of intense concern t o the FAA. It remains un - solved, and is one of the agency’s highest safety priorities.

NASA has conducted a variety of flight experiments t o try t o reduce the strength of the wake vortex and thereby make it less dangerous. None of the methods has proved t o be promising, and none has been proved practical by flight verification. This disap - pointing outlook has prompted NASA t o direct its studies toward fundamental wind tunnel and analytical investigations t o under - stand the mechanism of the wake vortex. Following that, it should be possible t o both predict the formation and dissipation of vor - tices and to control them. This work is of great importance t o GA aircraft owners and operators.

In addition, current restrictions on aircraft separation distance during approaches to airports are set because of the wake vortex hazard. Under IFR conditions, this restriction extends t o six miles or more for a light aircraft behind a large, wide - body transport.

Airport congestion, which severely limits the operation of all types of aircraft, will not be relieved until this restriction is removed.

NASA’s approach t o the problem is two - pronged - wake avoid - ance and wake attenuation.

Under visual flight rules (VFR), flight crews routinely reduce their in - trail separation behind other aircraft. They avoid the wake i u C w U .s v) Safefy Improvements vortex by piloting their aircraft along a slightly altered trajectory.

Simulation experiments have been undertaken whereby the lead aircraft is displayed on a n advanced cockpit heads - up display that also provides a computer - drawn runway symbol and other guidance information. Consecutive aircraft approaching the same runway will be on different flight paths having a different glide- slope angle and a different runway intercept. The challenge is to provide a display that will permit the crew t o maintain a prescribed separation and monitor the operation of the preceding aircraft for deviation from a nominal descent on its prescribed approach path.

Recent research in wake vortex attenuation has shown that alter - ing the span - wise loading (for example, retracting outboard flaps) generates systems of vortices that interact t o produce earlier dissipation of the vortex core. The use of spoilers to alter the load distribution a s well as to generate turbulence is also effective.

NASA Langley’s unique vortex research facility has proven in - valuable in trying t o solve this significant problem.

At a simulated distance of 2.6 nautical miles downstream of t h e airplane, the vortex pattern was seen t o be breaking up. This at - tenuation also was observed in flight tests with a Boeing 747. One of the more important results of this effort was the discovery that by oscillating the spoilers and ailerons in a maneuver that pro - duced a periodic aircraft roll, essentially total wake alleviation could be achieved. While this was obviously not operationally ac - ceptable both on the part of passenger comfort and consistent flight path, it did demonstrate that attenuation is possible.

king and Lightning

Since man’s f i r s t powered flights, weather has been a continual source of problems and has remained a major cause of accidents.

Improvement in IFR flying technique remains a major NASA con - cern (see chapter 8), but storm hazards and how to best operate aircraft in the vicinity of severe storms is of vital importance a s well. Two areas of particular concern are lightning and icing condi - tions.

Considerable uncertainty exists about t h e way composite materials react t o direct lightning strikes. Digital electronic con - trol and avionic systems also need t o be protected against possi- Until recently, there has been no in - depth research on lightning strikes and their ef - fects on aircraft in flight. This picture shows NASA flying an F- 106B fighter into the heart of thunderstorms. Hundreds of strikes have been recorded, adding invaluable data about what lightning does, where it comes from, and where it goes.

ble catastrophic effects. Almost all engineering data available on lightning characteristics have been obtained from cloud-to-cloud strokes t o instrumented towers. Characteristics of lightning a t flight altitudes have not been measured before.

NASA is currently obtaining new data by flying a heavily in - strumented F-l06B aircraft into thunderstorms, getting struck by lightning, and measuring current and the electric and magnetic flux rates of change t o 10 nanoseconds time resolution. Lightning strike patterns o n the aircraft have been carefully documented.

Several swept strokes across the wing mid - span have occurred, an unexpected phenomenon requiring further study. Numerous tran - sients have been measured a s a result of these strikes, with peak current recorded at 15 000 amps. In essense, a door has opened, NASA and General Aviation revealing a n area of research that is virtually untouched. Re - searchers have come t o realize they know almost nothing about lightning. Where it is potentially dangerous and where it is not re - mains t o be discovered and the results will prove vital t o all forms of aviation.

Over the last few years, an increasing need has surfaced for ad - vances in ice protection technology. The increasing cost of fuel has prompted aircraft designers to seek better ice protection systems t o save weight and fuel. During the 1940’s and 1950’s, both NACA and industry helped solve the icing problems for those aircraft that flew IFR, which included mainly commercial and military transports, a few general aviation aircraft, and no helicopters. Today, technological advances in avionics and flight controls make it possible for nearly all helicopters and GA aircraft t o be equipped to fly IFR.

Many of today’s GA aircraft are certified for flight into icing con - ditions, but they rely o n ice protection technology that is over 20 years old. The relatively small payload fraction and low power margins of these smaller aircraft mean that their ice protection systems must be lightweight and low in power consumption. Since small objects collect ice faster than large objects, all the serious problems of icing happen faster and are more serious on small, unprotected aircraft - drag rise, torque rise, power loss, lift deterioration, stall angle decrease, and stall speed increase.

In response t o this need, NASA has reestablished an icing research effort at the Lewis Research Center, centered around the reopened Icing Research Tunnel. The objectives of this research encompass improved icing forecast capability, more accurate ice detection instrumentation, low - cost protection systems, improved testing facilities and techniques, and widespread u s e of large, high - speed computers t o lower development and flight certifica - tion costs and methods. These include both short - term goals over the next 5 years and long - term goals extending over 10 years.

Since general aviation aircraft spend such large amounts of time in the low - altitude icing environment, development of im - proved icing protection systems is very important. Glycol fluid systems, electromagnetic impulse deicers, and icephobics are being investigated .

There is considerable interest in freezing - point depressant Safety Improvements GMZE ICING RIME ICING tice. min n tice -I X 0.01 ; Depickd here is a Comparison ofglazed and rimed icing in cruise condilions. Glaze icing has proved lo form in horned shapes compared with rime icing.

systems. Under a NASA grant, the University of Kansas has tested glycol fluid o n two modern GA airfoils in the Icing Research Tun - nel. The porous composite or stainless steel distributors are not only efficient, but they also can be used t o keep insect debris off laminar flow wings.

Electromagnetic impulse deicers offer a potential alternative to conventional hot - gas anti - icing systems. A capacitor is discharged through a coil of wire and the magnetic field induces eddy cur - rents in the airfoil skin, causing it to deflect rapidly and break the ice off. Though much research remains to be done, the initial results look promising.

lcephobics is the n a m e given t o any material that reduces the ability of ice t o stick to a surface. Besides reducing the adhesive bond of ice, an icephobic suitable for aircraft must also resist rain and sand erosion, must not be carried away by the shed ice, and must withstand exposure t o weather including the Sun’s heat and ultraviolet rays. A joint NASA/Air Force/Army program failed to produce a successful icephobic out of several attempts, but a cur - rent grant with Clarkson College of Technology is continuing the effort.

NASA and General Aviation Instrumentation to determine drop size and liquid water content in clouds has been tested in the icing tunnel, finding a direct cor - relation between droplet size and ice shape and drag on a NACA airfoil. Ice shape changed significantly and the resulting drag coef - ficient changed by a factor of five. NASA has funded Ideal Research t o develop a microwave instrument t o detect ice on the surface of a n aircraft component and t o measure the ice thickness and growth rate. Further development is needed to demonstrate that the instrument can distinguish between water and ice, because under glaze (clear) icing conditions both water and ice are present on the surface.

NASA experimental methods center around the Icing Research Tunnel, the largest icing wind tunnel in North America, with a 6 x 9 x 20 - foot long test section capable of 300 - mile - per - hour winds and temperatures down t o -30" F. Able t o operate year - round, the tunnel has 77 air atomizing water nozzles that produce a simulated icing cloud with variable liquid water content from sea level t o 3000 feet. The tunnel was built in 1944 and needs t o be rebuilt, but it is in continual use with a two - year backlog of test results. Full - scale components such a s airfoils and engine inlets, even propellers and engines can be tested in the main section.

There is a universal need for data o n aerodynamic degradation t o two - dimensional airfoils in icing. The data from the 1940's and 1950's often d o not agree with the results for new airfoils tested in the tunnel. O n e airfoil has a blunt leading edge that gives higher maximum lift coefficients and better stall characteristics than the older airfoils. Icing results were different. Other modern airfoils, such a s laminar flow control wings and supercritical wings, will be tested in the tunnel to see how ice affects their aerodynamic per - formance.

A full - scale general aviation wing with a NACA airfoil section was tested in the icing tunnel t o determine what rime (cloudy, air filled) and glaze (clear) ice would d o at cruise and climb condi - tions. Drag increases of about 130 percent were measured for a 20 - minute glaze icing encounter in cruise. The drag increased by approximately 40 percent for a 15 - minute rime icing encounter in the s a m e conditions. The NACA wing section w a s found t o be less sensitive t o rime and glaze ice accumulations in climb (higher angle of attack) conditions. Observed aft frost layer growths con- Ice has accumulated on this wing section in the icing tunnel. Tests have shown glaze and rime ice collect in different ways and often in bizzare patterns, which dislurb airflow even more than previously anticipated.

NASA and General Aviation tributed significantly t o the measured increase in section drag.

Comparisons of measured increases in section drag d u e t o primary ice accumulation with predictions of NACA drag correla - tion indicated agreement was a s good as that for the original data o n which the correlation was based. Agreement was less satisfac - tory for the higher liquid water content cases compared.

High - speed computers are now available t o model ice accretion and analyze the complex flow around airfoils with rough, irregular - ly shaped ice caps that can cause flow separation and reattach - ment. T o measure the physics of such an aerodynamic flow model, artificial ice shapes made of wood are being used in t h e tunnel without the icing cloud turned on. Drag values for both the real ice and wood replica turned out t o be very close for both rime and glaze ice. Verified icing scaling laws are also needed to permit accurate tests at actual facility conditions and to permit tests of small - scale aircraft models.

The long - term goal is t o use computers to predict the details of an aircraft icing encounter. As computer codes have speeded up so many other processes, such as airfoil design, they will be developed to predict changes in overall aircraft performance and aircraft handling characteristics due to ice buildup o n unprotected surfaces. Through 1980 there were virtually no icing analysis codes published and the increasing costs of icing flight tests pro - vide strong motivation to develop the codes.

Lewis has started a n icing research flight program using the NASA deHavilland Twin Otter. The twin - engine aircraft has been flown out of Lewis during the icing season from November through April; a major intention is to ensure that researchers con - ducting icing tests in the tunnel or developing computer codes have firsthand knowledge of how their results compare with flight test results in real icing conditions. During the flights, validation data for icing simulation will be conducted on the s a m e cylinders and airfoils being tested in the tunnel, instrumentation will be evaluated, icing cloud data will be recorded, and ice cloud forecasting will be investigated. NASA and Ohio State University are also conducting inflight icing experiments to measure lift and drag degradation o n the Twin Otter’s wings, a s well a s overall airplane performance loss. Flight results are being compared with tests in the tunnel of a Twin Otter wing section.

CHAPTER 8

ELECTRONICS AND AVIONICS

he general aviation pilot is faced with ever - increasing com - plexity as he or she uses the modern airspace system. It is

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becoming more difficult t o fly between major cities without the help of electronic wizardry in different forms. NASA has been involved in many research efforts geared toward helping the GA pilot cope with the demands of modern flying with improved safe - ty and efficiency.

Single Pilot instrument Flying

In 1978 NASA Langley began investigating the problems of single - pilot IFR (instrument flight rule) flying, or SPIFR for short.

Reflecting the trend of GA dominating U.S. airspace, IFR flying in 1981 involved 10.2 million airline, 4.6 million air taxi and com - muter, 3.9 million- military, and 18.5 million general aviation operations. By 1993 GA IFR operations alone are forecast t o in - crease t o 30.4 million.

A large portion, in some cases most, of the IFR operations in three of the categories (commuter, GA, military) involve a single crewman who is expected t o perform a s effectively a s the two- or three - person crew of airline transport class aircraft. It is generally thought by many that this level of effectiveness does not exist since a large number of SPIFR operations involve relatively inex - perienced pilots, often having limited equipment.

Although the GA accident rate has improved a s a whole, a re - cent analysis for GA SPIFR shows that single - pilot operations ac- NASA and General Aviation count for 79 percent of all accidents during IFR operations. About 50 percent of the single - engine accidents occur in the high work load landing phase of flight, and there are 10 times as many ac - cidents a t night. The findings also indicated that about half of the accidents are controlled collisions with the ground, a situation where the airplane is functioning normally and the pilot flies into the terrain because of lack of situational awareness. SPIFR ac - cidents based o n pilot error are predicted t o increase from about 150 a year t o 250 a year by 1993.

Under NASA’s SPIFR program a nationwide survey of 5000 cur - rently rated IFR pilots was conducted t o identify problem areas and possible solution concepts. Typical problems included timely weather information and dissemination, air traffic control (ATC) inflight demands and high cockpit work load, complex or ex - cessive ATC procedures, navigation chart format or content, in - terior noise, inadequate cockpit lighting, and maintaining recency of experience. These pilots listed their most common errors: not planning ahead, overconfidence in their ability t o cope with weather, exceeding their personal capabilities, misunderstanding ATC transmissions, and descending below mirimum descent altitudes. Specific solution concepts included better and more ac - curate weather information in the cockpit, better pilot interface with increased automation and display formats, and data link for enhanced information transfer.

An investigation into pilot interface with aircraft automation was undertaken. An autopilot complexity/benefit tradeoff study evaluated the relative benefit of various levels of state - of - the - art autopilot complexity. As it turned out, the more complex the auto - pilot, the more frequent the pilot blunders. The least errors oc - curred when only a wing leveler and heading - select system w e r e used. Researchers believe that many pilots lose awareness of their aircraft’s situation when they become “autopilot managers.” Often these pilots worry about subtle system failures instead of flying the aircraft. Poor pilot interface with automation was thought t o be the reason, a disturbing result since the pilots surveyed pre - ferred more automation t o help in their IFR flying.

Although it appears automation can reduce work load, too much automation requires too much pilot monitoring. There is a great need for more research into human factors, pilot training, Eleclronics and Avionics

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NASA and General Aviation and pilot interface with new technology. A second study used the s a m e simulator and flight scenario t o evaluate a n automatic ter - minal approach system (ATAS) concept, which automatically flew instrument approaches by using stored instrument approach data t o control the simulator’s autopilot and tune the radios. The ATAS also can execute a missed approach unless the pilot takes over t o land. This represents a level of automation well above the highest level evaluated during the first study. The results were en - couraging; there was lower pilot work load and fewer pilot blunders were committed with ATAS than with a low - level auto - pilot mode that in the first study had relatively few pilot blunders.

The reason for the improvement was a much better pilot interface with ATAS that enabled the pilot to maintain situational aware - ness during the automatic approaches.

Automated Pilot Advisory System

One of the more interesting systems tested t o help the pilot cope with entering an airport traffic area was the automated pilot advisory system (APAS). Its basic components are a primary radar system, tracking computer, weather sensors, a minicomputer, and a V H F radio transmitter. Data from the tracking computer and the weather sensors (wind, pressure, temperature, dew point) are fed into the minicomputer, which then organizes the data and selects from a memory of 64 prerecorded words and phrases to transmit the information t o pilots. The minicomputer also analyzes data from the wind sensor to advise pilots of the best landing runway from as many as six.

Using the automated voice system, APAS transmits a n airport advisory every two minutes, consisting of airport identification, Greenwich mean time, wind direction and speed, favored or active runway, altimeter setting, temperature, and dew point. At 20- sec - ond intervals, between the airport advisories, APAS transmits traf - fic advisories, calling traffic in the pattern first, then arriving and departing aircraft, and finally aircraft flying overhead above pat - tern altitude. While APAS can track u p t o 20 aircraft at a time, it issues advisories on a maximum of 10.

The APAS test facility was located at Manassas Airport, Virginia. The primary goal of the system is to meet the future growth of general aviation in the next decade, placing greater traf- _ __ - The Princeton Clniuersity auionics research Nauion has been at the heart of much of NASA single pilot IFR research. Many systems have been installed and tested in this aircraft for actual hands - on experience in the instrument flying enuironment.

fic demands on the uncontrolled (no control tower) airports. The FAA estimates that a tower costs between $1 million t o $5 million to build and between $250 000 and $500 000 yearly t o operate and maintain. NASA estimates it would cost $100 000 to install an APAS and that it would require little maintenance. Test results were quite favorable though there are still bugs to be worked out.

After detecting a n aircraft o n radar, APAS has a 7 - to 20 - second delay before it calls that aircraft a s traffic. In that time, the aircraft may have changed course. It cannot keep up with fast - moving jets and twins and there are problems with radar ground clutter. The radar also can pick up ground vehicles and mistake them for air - craft. However, NASA has found APAS to be 95 to 98 percent ac - curate, and elevating the radar a little can eliminate the clutter and confusion t o a large extent. The door is open to begin engineering a line unit should funding be available.

NASA and General Aviation

Digital €lectronics

The revolution in digital electronics has dramatically affected the design and capabilities of emerging avionics systems because of the spectacular reduction in cost, size, and power required of digital electronic components. System cost has decreased sharply with the transition from analog t o digital hardware. Data sensing, data processing, and data transfer have now been impressively enhanced and refined, all with reduced weight and number of con - nectors.

Digital fly - by - wire control systems, using electromechanical ac - tuators in lieu of hydraulic actuators, are a reality in military air - ‘ craft with applications for large transports and eventually GA air - craft. The advanced avionic or “all - electric” airplane is a very real possibility for the future of GA.

NASA has been studying GA digital optimal control autopilot designs. A joint NASA, Information and Control Systems, and Princeton University program designed and flight tested a multimode digital autopilot in the Navion. Heading command, altitude command, pitch/roll attitude command, and an Instru - ment Landing System (ILS) coupler mode autopilot were tested with overall good performance.

Controls

Controls w e r e also investigated under the SPIFR program. The University of Kansas assessed various nonconventional GA manual control devices. The study found that the conventional yoke is not the best aircraft control arrangement when consider - ing the pilot work load requirements of IFR flight. A side stick con - troller emerged a s the best configuration, having better two - axis integration of control inputs, fewer inadvertent inputs, increased instrument panel visibility, decreased pilot work load, and favorable control characteristics in general. Simulation and flight tests remain t o be conducted t o verify the findings.

Displays

Use of conventional primary displays in instrument landing ap - proaches has been studied both analytically and in the simulator.

Electronics and Avionics Pilots flying the simulator tended t o “chase the needles” or over - control a s they got closer t o touchdown, from 5 nautical miles out to 1.25 miles. During nonprecision instrument approaches (those without glideslope or vertical descent needles), conventional displays tended t o be disorienting, causing high pilot work load.

The development of cathode ray tube (CRT) and microprocessor technology has made it possible t o consider combining many sen - sor signals in o n e display. Improved performance and reduced pilot work load are definite possibilities.

Advanced symbology for GA terminal area displays was evaluated in flight tests using the Navion and the Wallops tracking radar. The display symbology was that previously used in NASA’s terminal configured vehicle program for commercial airlines, in - cluding such areas a s vehicle track - angle, flight - path angle, and a perspective representation of the runway. The question is whether or not this type of symbology, which takes up a significant amount of space o n an airliner’s instrument panel, can be fitted into the smaller GA aircraft panel. To investigate this, the advanced sym - bols w e r e selectively drawn on a CRT display along with the roll/pitch attitude and ILS localizer/glideslope deviation. In general, the symbology in both advanced displays permitted more precise and consistent tracking of the ILS localizer and glideslope signals than did the standard hardware display.

Man/Machine Interface

Since modern IFR flying has become extremely demanding, sometimes taxing the pilot’s limits due t o increased aircraft traffic and more sophisticated and complex ground control systems t o handle this traffic, GA users a s a whole consider it imperative that all the pilot’s sensory and manipulative skills be optimized in managing aircraft systems.

O n e revolutionary technology that can improve the pilot’s inter - face with these systems is computer - based voice recognition/syn- thesis. Princeton University tested this voice recognition equip - ment (VRE) in flight with its research Ryan Navion. A voice recognition module was linked to a digitally tunable VHF O m - nidirectional Radio Range (VOR) navigation receiver. After first “training” the module while taxiing out, the pilot tuned the NASA and General Aviation receiver in flight using brief spoken commands. Approximately 95 percent of the pilot’s spoken commands were properly interpreted by the VRE. A second pilot, for which the VRE was not trained, found that approximately 60 percent of the commands were properly interpreted.

An effort t o provide low - cost, timely, efficient weather data t o the pilot in flight is ongoing. Ohio State University studied taking digitized weather data generated by a ground - based weather radar and spherics equipment and displaying this information on a CRT in the cockpit. To take this weather data, which starts‘ a s a north- up map, and translate and rotate it so that an aircraft - centered, heading - up m a p can be provided will constitute a heavy burden for the aircraft computer, something OSU has been studying. Mitre Corporation has developed the initial hardware for installation in Langley simulators and the Cessna 402B SPIFR research aircraft.

A flight data console (FDC) has been developed with a CRT display and a set of controls for the pilot to send back acknowledgment signals.

Initial results have been very pleasing, since both a weather radar picture and basic weather data have been transmitted into the cockpit from ATC without voice communication. During flight tests in simulated IFR, the FDC “quiet cockpit” met with generally favorable comments from the pilots regarding reduced work load and error - free communications. The only problem concerned the lack of traffic information gained by monitoring ATC communica - tions with other aircraft since the FDC relies solely on the ATC data link.

A continuing effort is under way to evaluate advanced three - dimensional, computer - derived pictorial displays for enroute, ter - minal area, and final approach guidance. Simulator studies of what used t o be called Box in the S k y , now Follow - Me Box, have shown some improvements in SPIFR. This CRT display consists of a three - dimensional box that is located o n the desired enroute or instrument approach flight path. The box moves along the path ahead of the aircraft; all the pilot has t o d o is follow the box. Dur - ing the approach phase, the box serves a s a way point gate, and the pilot must fly through it to maintain proper glideslope and localizer position. Simulator results, which will have to be backed up with actual flight tests in the Cessna 402, have shown that Electronics and Avionics pilots were able t o fly instrument approaches with far less devia - tion than with the normal instrument landing system.

A flight test study of the system was conducted at Wallops using the Princeton Navion, verifying that the short, curved, descending, precisely controlled landing approach executed in the simulation study also could be performed in flight. Further studies t o refine the box are being undertaken for possible installation in the Cessna 402.

ILS CAPTURE AND TRACKING - BASELINE DISPLAY ALTITUDE 15 10 -300 ILS CAPTURE AND TRACKING - ADVANCED DISPLAY ALTITUDE PROFILE 300 ____..___.____ - .~ - (M) ---___ , I I CROSSTRACK IM) RANGE TO GO (Km) Sirnulalor studies were conducted Lhat compared how pilots performed flying a nor - mal inslrument landing system (ILS) wilh direclional needles versus how they did fly - ing advanced displays on cathode ray lubes (CR7). A s shown, the advanced displays significanlly enhanced accuracy.

NASA and General Aviation

Demonstration Advanced Avionics System

At the heart of NASA’s SPIFR program is the demonstration ad - vanced avionics system (DAAS) installed in the Cessna 402B. The purpose of the project is to define an avionics system for the future that would provide more information at a lower cost and a reduced work load.

In August 1978 a contract was awarded t o Honeywell through Ames, teamed with King Radio, for the design and fabrication of the DAAS, which would: (1) provide information crucial t o the design of integrated avionics for GA in the mid - 1980’s and beyond; (2) use data busing, distributed microprocessors, and shared electronic displays for enhancing reliability and pilot inter - face; and ( 3 ) enhance safety and reduce pilot work load for SPIFR operations. A series of 64 successful demonstration flights was made at Ames, ending in mid - 1982.

Many of the features now being incorporated into commercial airliners became a part of the DAAS system, including autopilot/flight director, navigatiodflight planning, flight warn - ing, weight and balance computation, Greenwich mean time clock, fuel totalizer, performance computations, air traffic control Mode S or discrete address beacon system message processing, built - in test, checklists and emergency procedures, and ground simulation.

DAAS is built around eight bus - connected microprocessors, seven of which are dedicated t o specific functions with the eighth a s a spare for two of the others. The integrated data control center (IDCC) is the primary pilot input device through a 10 - button keyboard linked to both CRTs and numerous push buttons. In ad - dition to the electronic displays, the D A A S instrument panel in - cludes a conventional attitude directodindicator and airspeed, altimeter, rate of climb, radio magnetic indicator, angle of attack, fuel, and engine instruments.

TOP (Right) The NASA Cessna 402B, stationed first at Ames and then at Langley, has been at the heart of advanced SPIFR research, particularly in connection with [he Demonstration Advanced Avionics System (DAAS).

BOTTOM (Right) Here is the DAAS installed in the Cessna 402 with both CRTs lighted and the systems turned on.

NASA and General Aviation Twelve push buttons at the top of the data display control the page format displayed t o the pilot, and a switch adjacent to the keyboard permits leafing forward or backward through multipage formats. Warning messages show up automatically on the bottom of the display t o indicate the reason for a caution or warning light.

The electronic horizontal situation indicator (EHSI), the CRT directly in front of the pilot, combines the features of a conven - tional HSI and Jeppesen navigation charts with additional alphanumeric and symbolic navigation data. It portrays the air - craft, way points and navigational aids, and course lines connect - ing way points in sequence. It also displays a course direction ar - row and course deviation dots. The map can be displayed in either a “north - up” or “heading - up” mode. “North - up” is normally used for reviewing a flight plan prior t o takeoff or for orientation.

“Heading - up,” which simulates the view out the windshield, nor - mally is used in flight. Map scales of 2, 8, or 40 nautical miles can be selected by the pilot for approach, cruise, and flight planning, respectively.

A heading scale, digital readout of heading, and heading select “bug” for autopilot or flight director are displayed a t the top of the EHSI, and a digital readout of selected heading is at the left below the scale. The left side displays the minimum descent altitude or decision height, active way point, selected course, distance and time t o the way point, and way point altitude for use in vertical navigation. A “dead reckoning” or manual navigation mode can take the place of an active way point. The right side of the EHSI displays radar altitudes, availability of the next way point, and a vertical track angle scale.

The bottom of the EHSl has a map scale readout and wind direc - tion arrow with digital velocity readout. The map can be slewed around t o view other portions. if a pilot loses the aircraft symbol on the m a p as a result of excessive slewing, the “map return” push button can bring things back t o normal.

The DAAS tests met the program goals and the 60 pilots who flew it generally thought the system w a s a good step forward in enhancing safety and reducing IFR pilot work load. Half said it was simpler t o use than conventional instrumentation but 22 percent said it was more complex. Training and currency on use of the Eleclronics and Avionics equipment were deemed mandatory, along with s o m e more human engineering.

Recommendations for more operationally oriented flight tests were made a s well as flight evaluations of the Follow-Me Box, ATAS, side arm controller, and other NASA projects. As a result, a second series of operational flight tests was initiated at Langley with attention t o the pilot/machine interface, pilot training and work load requirements, and the individual DAAS features. O n e of the major drawbacks for current development of the system is cost, primarily d u e t o the expensive CRT displays. Should cheaper flatboard hardware be developed, the DAAS could very well lead t o a major breakthrough in single - pilot IFR flying.

Fluidic Controls and Instruments

Another revolutionary development of the last 10 years has centered around fluidic controls and instruments. Electrofluidic instruments will leave the old gyroscopic instruments behind, which have a tendency to hang up or tumble.

O n e of NASA’s basic objectives in autopilot design was t o develop a n “aerodynamic aileron,” a device for use in autopilot and stabilization systems that could produce torques about the roll axis with no moving parts. An electrofluidic rate sensor package was constructed and calibrated, then sent to the Universi - ty of Kansas for tests. A laminar jet rate sensor, using ram air only, exhausts air through slots in the wing tips. Lift is increased by displacing wing tip vortex or decreased by disturbing flow pattern, providing proportional control of torque equivalent t o 7 degrees of aileron deflection using mechanical switching.

This led to the design of a GA autopilot that uses a n elec - trofluidic wing leveler and he ing hold with a highly reliable, low- cost sensor package. Heading reference w a s provided through a flux gate magnetometer.

A true airspeed sensor was developed as well with no moving parts, designed for low - cost fabrication and easy interface with digital systems. For very little cost this transducer can link up with a dead reckoning moving map to form a very cheap version of an inertial navigation system. Computer programming for the device is simple and state of the art, and microprocessors can be adapted / NASA and General Auiation Electronics and Avionics NASA and General Aviation ..-_ ..... :i u:: j 'I ' L . 1 . l . .

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Electronics and Avionics OF POUR QUALITY NASA and General Aviation Electronics and Avionics A true airspeed sensor was developed a s well with no moving parts, designed for low - cost fabrication and easy interface with digital systems. For very little cost this transducer can link up with a dead reckoning moving map to form a very cheap version of an inertial navigation system. Computer programming for the device is simple and state of the art, and microprocessors can be adapted to it for future use. The digital autopilot of the future now appears not a s complex a s once thought.

NASA has been involved in other fluidic studies and concepts a s well. A fluidic (Coanda effect) propeller was developed under a NASA contract with Bionetics Corporation. It may take the place of the standard variable pitch propeller. Fixed pitch, circulation - controlled propellers are those in which the necessary propeller aerodynamic changes caused by airplane speed changes could be obtained through changes in blown - jet mass flow rate.

Studies indicated that elliptical and supercritical circulation - controlled airfoils were aerodynamically feasible. The Coanda ef - fect is used by blowing at the trailing edge of the propeller t o vary the effective pitch, thus providing propulsion efficiencies com - parable to those of conventional variable pitch propellers without the attendant mechanical complexities of the variable pitch hub.

When comparing performance o n a 1600 - kilogram single - engine aircraft in computer studies, the supercritical, circulation - controlled prop with a single blowing plenum was equal t o stan - dard propellers at high - speed cruise. At low speed, the Coanda ef - fect prop exhibited a performance gain over a fixed pitch but lost performance when compared with a variable pitch type. More studies will have t o be conducted to see if it will be economical t o replace currently manufactured propellers with this revolutionary new design.

The meteoric improvements being made in electronics and avionics are finding their way into general aviation. Potential for increasing safety, utility, and comfort is wide open when coupled with the capability to make sophisticated equipment affordable t o industry and ultimately to GA users.

CHAPTER 9

THE FUTURE

he early 1980’s were certainly years of contrast for general aviation. The recession grounded aircraft and production

T lines were either slowed down or shut down entirely. To

visit Cessna, Beech, Piper, Mooney, or any of the other manufac - turers was quite a sobering experience, miles of half - completed aircraft silent in darkened and still buildings.

According t o many, those years were the worse since the 1973 - 75 recession. In the first 10 months of 1981 new aircraft sales fell 46 percent to 571. The single strongest sector of GA pro - duction involved new turboprops and corporate jets, although these lines were certainly slowed down. In spite of the slowdown, general aviation remains vigorously active.

NASA and its forerunner NACA have been central to keeping the United States in the forefront of aeronautics. Long before there was a “space agency,” NACA became a partner of the avia - tion community in bringing the U.S. to a position of world leader - ship in aviation. There are many reasons for this success. First, t h e government’s investment provides for the development and operation of major aeronautical ground and flight test equipment, which not even the largest of private companies could afford. Sec - ond, NACA was governed by a committee with membership from government, industry, and university sectors of the aviation com - munity. NASA continues t o maintain a strong Aeronautics Ad - visory Committee with similar representation, including programs designed to ensure responsiveness to community needs. Third, NASA and General Aviation NASA has no regulatory authority. It exists solely t o serve its customers: the Department of Defense; Federal Aviation Ad - ministration; thousands of private companies that build aircraft, engines, parts, and subsystems and that operate and service air - craft; and the universities that produce many skilled aeronautical scientists and engineers.

Aeronautics remains vital to the U.S. economy. Civil aircraft sales have grown more than eight times that of the overall gross national product. The export portion of these sales has become a major favorable component of the country’s trade balance. The ailing economy of the 1980’s has forced the GA industry t o slow down, opening the door t o foreign competition as never before.

NASA’s help is more vital than ever in preventing this key seg - ment of the U.S. balance of trade from deteriorating. It is in - teresting to note that the total sum expended on aeronautical research, facilities, and personnel by NASA and NACA over their combined 68 years of existence amounts t o less than half the dollar value of U.S. aeronautical exports in 1979 alone.

The U.S. enjoyed a clear and impressive leadership position in free world competition for the total $87 billion in civil aircraft sales during the 1970’s. For the much larger market of the 1980’s and 1990’s this leadership is by no means assured. Foreign manufacturers have concentrated effective efforts on the design and production of highly competitive air transports, commuter aircraft, and helicopters. European and South American competi - tion, along with developing capability in Japan, are already pro - ducing the technically equal or superior article t o U.S. products.

NASA remains a vital $4 billion national asset to keeping U.S.

aeronautics in the forefront.

Based on the government subsidies available t o foreign aircraft companies, U.S. manufacturers have asked for increased par - ticipation by NASA, particularly in general aviation and rotorcraft since research facilities are so limited.

Though much of the U.S. is sti!l a free market internally, aircraft are in an international environment where success may depend o n markets outside the U.S. and even outside Western Europe. The bottom line for the U.S. is unquestioned excellence at low manufacturing costs, a challenging combination.

General aviation remains vital to American business since 80 The Future percent of current and projected GA markets are for business and commercial applications. General aviation is the only mode of air travel available t o many businesses and small communities, a situation aggravated by airline deregulation. Future growth pro - jections for general aviation and commuter aviation total $80 to $90 billion through the end of the century.

Despite this attractive potential, there are several serious con - cerns such a s rising costs of operation, particularly due t o rising fuel costs, which reduce the utility and therefore the potential GA market. Avgas is being slowly dropped from production, leading t o rising cost and lack of availability. Energy efficiency and multifuel capability remain major areas of research.

In recent years, commuter airlines have had t o buy foreign air - craft because suitable U.S.-built products were not available.

Foreign competition in the higher performance turboprop and tur - bofan business aircraft has increased significantly. Safety will re - main in the forefront of research since accidents will have to be reduced to increase use of GA aircraft. When accidents d o occur, serious injuries and death will have t o be minimized.

General aviatiori’s future, though bright, may not involve the US. a s it has in the past if NASA’s research efforts are not bolstered. According to those companies involved with NASA, they would like more government involvement. As one GA com - pany president said, “ W e are now at the point where w e will work with NASA on a day - to - day basis in certain specialized areas. What this means is that aircraft manufacturers of our size whose poten - tial for basic and applied research, though improved, is still limited. Our future success in world markets, which we have dominated until now, will quite literally and directly depend on the success of NASA in performing the basic advanced aeronautical research which the agency and the industry can and should d o together.” In general, GA companies d o not believe that NASA’s role is t o develop actual aircraft hardware, but rather t o concentrate on ad - vanced concepts that industry can apply t o its product. Since re - cent U.S. research and development efforts have lagged behind those of other nations, once firm economic leads are evaporating.

Not too long a g o there were six U.S. manufacturers building business jets, an American innovation on the whole, with the NASA and General Aviation foreign competition coming from the aircraft industries of France, Germany, and England. Today there are only three U S . manufac - turers building “bizjets,” and they now face competition from companies in Canada, England, France, Israel, and Japan, all either owned or heavily subsidized by their respective govern - ments.

The United States is now entering a critical period in its economic history. If research and technology in aeronautics con - tinue t o be deemphasized, while at the s a m e time deemphasizing research and technology in agriculture, the U.S. will be undercut - ting the two areas that contribute most t o the nation’s balance of payments. As the GA manufacturer continued in commenting on NASA’s help and research, “It is almost inconceivable to m e that w e would be taking steps that literally kill the goose that lay the golden eggs in the economy of this country, and w e are talking about extraordinarily small amounts of money t o keep that goose not only alive but fat and laying very well.” NASA remains a vital link in America’s aeronautical leadership.

Nowhere is that more evident than in general aviation where com - panies depend on NASA to develop products that will compete in the world marketplace.

ABOUT THE AUTHOR

Jeffrey L. Ethell, a certified flight instructor and commercial pilot, is author of more than 24 aviation books and a well known free - lance writer for numerous aviation magazines, including Air Force Magazine, Air Progress, Air Classics, Aerospace America, and PopuIar Mechanics. H e is a member of the American Aviation Historical Society and the Warbirds Division of t h e Experimental Aircraft Association. In addition t o being a n experienced pilot of such World War I1 aircraft a s the P - 51 Mustang and the B - 25 Mitchell, Ethell has logged several hundred hours in modern mili - tary jet aircraft.