Aerodynamics/ACEE: Aircraft energy efficiency

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Aerodynamics/ACEE

years. and to wholly rew classes of transports designed

Aircraft Energy Efficiency

specifically *, be fuel-efficient.

Several NASA research centers divide the workload

Aerodynamic concepts that point toward a new gen- of the ACEE program. Langley Research Center, eration of energy-efficient air transports are part of the Hampton. Virginia. is responsible for technology pro- Aircraft Energy Efficiency (ACEE) program managed by grams in aerodynamics. and in materials and structures.

the National Aeronautics and Space Administration Wind-tunne l 'esting is shared by Langley and the Ames The basic goa! of the program is to make possible the Research „enter, Moffett Field. California. In-flight re-

most efficient u! z ? of energy for aircraft propulsion and

sean:; I is conducted by the Dryden Flight Research

lift That, in turn, will reduce energy costs in air transport Center. Edwards California. Propulsion research is at

operations. the traditional home for such work. Lewis Research

ACEE is a ten-year planned p rogram, developed in Center, Cleveland. Ohio.

response to a request from the United States Senate Overall. the t;. oad purpose of the ACEE program is to

Committee on Aeronautical and Space Sciences It

provide an inventory of technology that can be used by

looks simultaneously at near-term and far-term prob-

the major manufacturers of transports and engines in lems. It attempts to develop expedient solutions that the United States It will help them to develop near-term can be applied to today s generation of transport aircraft derivative airliners that extend their currer: product and engines. to their derivatives expected in a few lines. to develop families of new designs for the near Figure 1: Three basic wind-tunnel test programs characterized the Energy-Efficient Transport i EET I portion of NASA s ACEE project. Integration of the propulsion system into the

airframe

for optimum efficiency was one of these. and typical tests included determining the interference effects of the long-duct

nacelle on the wing From that series of tests came another to

develop and test an optimum pylon mounting for the nacelle Advanced aerodynamic testing .ncluded evaluatin g winglets and wingtips on a model of the Boeing 747. and the use of active control systems for wing load alleviation Active con- trols also were tested first in model form on high-aspect-ratio wings, and later .n free flight on a drone aircraft launched from a'JASA-operated Boeing B-52D bomber loaned by the USAF

ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH

Figure 2: Under contract in the t.CEE program, the Lockheed TnStar prototy pe was modified with an active control system for maneuver load contro,. gust load alleviation, and suppression of the elastic mode of wing response to accelerations This in-flight photograph shows the TriStar as modified with outboard ailerons a flying stabilizer. and a boom with gust sensors The outboard ailerons later were incorporated into an improved model of the Lockheed transport now in airline service term, and perhaps to develop radically different aircraft for the far term It is significant that NASA had been studying the prob l em of energy-efficient aircraft some years before the fuel c: isis focused particular znd concentrated at- tention on it The Advanced Transport Technology pro- gram. initiated during the early 1970s. had as one goal the determination of the effect of a number of different new technologies on fuel consumption. It also is inter- esting to note that some of the same technologies under study then—supercritical a erodynamics, compos- ite structural materials. active control systems. and quiet propulsion—are foundation stones for the current

or

ACES program A major portion of the ACES program consists of funded studies placed with the major constructors of Figure 3: NASA s DAST program — Drones' or Aerodynamic U S air transports and engines They have the facilities and Structural Testing—utilizes a modified Teledyne Ryan and the test aircraft to do the work most efficiently.

Firebee drone as a research vehicle for free-flight evaluation Additionally, studies have been done by commercial of advanced technology Typical of the tests conducted in the airlines, to provide their valuable input from the real DAST program were these three Tt.e in-flight verification of world of day-to-day operations with jet transports of an advanced control system using ne v technology. the first flight of a reseai ch wing. with a supercritical airfoil and other contemporary design advanced aerodynamic features. and the development of a second research wing, using active controls to alleviate fluffier and other aerodynamic phenomena.

Energy-Efficient Transport

One of six major technology programs that comprise ACEE. the Energy-Efficient Transport (EET) is a planned commercial operators of air transports Direct operating sequence of analysis, experiment and flight research cost IDOC). re nge. and aircraft gross weight Since fuel leading to advanced concia ots for derivative and new air costs currently make up more than half of the total transport aircraft It focus-, - %n new technology in direct operating cost, the efficient use of energy can aerodynamics. propulsion, . • ttrol systems and materi- have a major impact on the level of airline DOC. By drag als and structures reduction• and by improvements in the ratio of lift to The efficient use of energy translates into three drag gains in range are achieved Finally. through new factors of design and performance that are important to materials and structural concepts, coupled with active control technology, airframe size and weight can be

ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH

High Aspect Ratio Low Sweep,

Winglets

Supercritical Wing

Propl.I Ision /Airframe

Integratioll

Relaxed

Static

Digital Avionics

Stability

_ Advanced

Long Duct ^'

^'

Design

Maneuver/Gust

Nacelle

Loads Control

Conventional

Design

High Lift

Devices

Figure 4: This pl:;n view compares the layout geometries of a contemporary transport and one using advanced technolos, rncl investigated under NASA s ACEE program, in the studies of Energy-Efficient Transports ( EET) Aerodynamics structures.

propulsion and control sys+ems are integrated into a single airrrame of advanced layout It features a supercritical wing with a h1g*1 aspect ratio and low sweep. with high-lift devices. winglets and maneuver gust load control The propulsion system is integrated into the ainrame with great care. and uses the long-duct nacelle concept Rclaxed static stability and digital avionics systems are part of the active controls designed for the EET The structure of such an aircraft would include a number of components made from composite materials reduced That makes for a lower initial purchase cost, and structures in the context of the overall ACEE pro- and for a greater fraction of the gross weight available gram and of the EET. This publication

is concerned only

for payload with the technc logv of aerodynamics 1 A hypothetical energy-efficient transport would com- There are two broad aerodynamic studies under EET bine a number of technological advances in several The first addresses the eternal problem of drag reduc- aeronautical disciplines. It would feature a supercritical tion, a traditional task at NASA and at the National tips and wing of high aspect ratio, with wmglets at its Advisory Committee for Aeronautics. NASA s pre-1958 high-life devices on Q3 leading and trailing edges. a sys- predecessor The second considers one specific means tem of active controls to handle relaxed stability require- for drag reduction, laminar flow control (LFC), as a ments, other active controls to moderate maneuver and long-range project looking toward a future transport guat loadings, and a meticulous integration of the air- design of wholly new configuration frame and the propulsion system Its overall technology would be interdisc: plenary In- tegration of propulsion system and airframe. for exam-

New Aerodynamic Technology

ple, would involve applying advances in aerodynamics.

The supercritical wing. a NASA development dating propulsion, and materials and structures I Other publi- back to the mid-1960s. uses an unconventional airfoil

cations in this NASA Facts describe the work of

series

shape to control the flow to avoid a sudden increase in

the agency in propulsion. guwance and control, and drag At very high flight speeds. flows that are curved NASA in propulsion. guidance and control. and material.

ORIGINAL PAGE IS

OF POOR QUALITY

above the surface of a wing can accelerate to a critical speed at which a shock wave forms That shock wave, which can be seen under certain atmospheric condi- tions as a small ghostly shape seeming to dance on the wing, causes a sudden, dramatic drag Increase In an earlier time. that shock wave was part of wha t was called the sonic barrier", a theoretical h!ndrance to high-speed flight The barrier turned out to Ue as ethereal as the visual presence of the shock wave It was conquered by brute force, by howling rocket mo- tcrs at first, and later by screaming let engines. The trick now is to subdue it quietly That s where supercritical aerodynamics comes In Supercritical reters to tt e fact that outstanding airfoil efficiency is achieved at speed higher than the critical speed at which a shoe wave would form. destroying efficiency on a more conventional airfoil Super, ritical aerod y namic technology has been ap- plied to several recent aircraft, including a new busi- ness aircraft The backlog of experience with this and other types of aircraft has validated NASA s early claims This semispan wind-tunnel model s;)own under test Figure 5: for the efficiency of the new airfoil family.

during the NASA EET program was used to evaluate the It indicates that the application of supercritical aero- long-duct nacelle concept This nacelle design provides dynamics to a new wing design for aircraft similar to greater mixing length for the air which bypasses the let contemporary wide-bodied lets would reduce the fuel flow pattern. and that engine via the outer annulus of the burned for a given trip by 10 to 15 percent. It would do which goes through the hot core of the engine and Is this by permitting an increased wing aspect ratio, a exhausted at high speed and high temperature Better mixing factor that relates the wing span to a multiple of the due to the longer dimension available oroduces greater efficiency and a lowered noise level aerodynamic chord. or fore-and-aft average measure- ment of the wing Wing aspect ratio is an Important term in the classical Breguet equation for determining theo- retical still-air range It affects the induceo drag. or the drag due to lift. term in the equation. an Increase in aspect ratio produces a decrease in drag due to lift.

There are collateral benefits from supercritical tech- nology. Generally, the use of the new NASA airfoil permits a thicker wing without Incurring an additional drag penalty In fact, compared to a conventional wing, the supercritical wing may not only have less drag, but also be thicker That extra thickness provides more useful Internal volume for the storage of fuel It also provides a greater depth for wing structure. which results in a lighter and more efficient structural design Figure 6: 1 his at list s concept shows a McDonnell Douglas In sum, then. a supercritical wing design can pay off DC- t U modified with two of the features developed under in increased aircraft efficiency in three ways By reduc- NASA s EET program Winglets long-duct nacelles and ing the drag of the wing, by Increasing the Internal volume for fuel storage. and by Increasing the structural efficiency of the wing, t hereby permitting a lighter the upper Nature being what it is. the flow fields try to overall structure adjust to reduce that pressure differential to zero They do this by moving air from the bottom of the wing to the top. from the high pressure area to the low That motion.

Vortex Drag Reduction

combined with the normal slipstream, produces the tip vortex flow Wmglets. which look like small lib sails mounted at A wingle.t. carefully resigned as a lifting surface.

and above the wingtips. came from some of the same reduces the cross-flow on the wing, and modulates the considerations of drag reduction that sparked devel- tip flow field to a weaker state The trailing vortex is opment of the NASA supercritical wing One source of reduced in strength. this reduced strength subtracts drag on a wing is the trailing vortex. a twisting, small drag from the lifting wing, improving its ratio of lift to scale whirlwind that traiis from the tip and contains drag enough energy to upset an airplane flown by an unwary An aerodynamicist would say that the induced drag pilot through the invisible twister term has been reduced Induced drag is the reference That tiny tornado starts because a lifting wing has to the component of drag due to the generation of lift, it different pressures above and below Its surface Posi- is that generation of lift that creates the vortex flow in tive pressur- on the underside, negative pressure on the first place l .1L PA, , _ PLAC K AND ' ► ' HJE PHJTOGRAPH

Surface Go

Bugs

Dirt

Surface Cc

Slots

Porous

Perforated

Suction Dis

Corrosion

Erosion

Clogging

— on p L)rI ii ustraiet 5omic" ^)i the

Figure 7: This sketk I oI ,in ui5tailaIIon of Llnunar - flow con I. tLI C) a t\ picai filiee engIIwo t "IF ^i problems of the system Bugs and dirt ca i contaminate the surface. corrosion. erosion and clogging can affect the air flow through the tiny slots Manufacturing irregularities can reduce the effectiveness of the system The g neral concept shown here L,.;ls for a supercritical wing with either slots. pertcrations, or a porous surface through which boundar y layer air is sucked During the latter half of 1979. flight tests began on a takeoff and landing The extension of flaps or other

high-lift devices also causes an increase in drag

Boeing KC-135A tanker operated by the U S Air Force

Typically, an airplane making a descent ;or a landing

Strategic Air Command It was modified by the addition

will extend flaps partially at first, then once or perhaps

of winglets. and the purpose of the flight research was

twice again before finally touching down Each of those

to evaluate the contribution of the winglets to reduced

fuel consumption Some NASA estimates anticipate a flap increments is matched by an increase in power, to

balance the added drag of the lifting surface That

tLIE! saving of about five percent

power is used at low altitudes, where let engines burn

the most fuel Consequently, any way to reduce thrust

at the lower altitudes seems worth consideration

High Lift and Active Controls

NASA traditionally has studied the widest variety of

Nigh-lift devices offer yet another way of saving

high-lift systems. both passive and active That work

incremental fuel during two phases of flight Takeoff,

continues, but with a different emphasis The applica-

climb and approach descent Contemporary transports

tion of high-lift devices to the leading and trailing edges

use wing flaps. sometimes coupled with leading-edge

of a su percritical wing

lift systems, to increase the available lift coefficient for

One such experiment combined leading-edge slats

with a partial-span, two-segment slotted flap For take-

ORiGifNAL PAGE I5

OF POOR QUALITY

off, a hypothetical let transport built around a supercriti- cal wing and high-lift systems of this type could generate a lift coefficient of about 2.3 Cut rent wide-bodied transports with conventional wing designs generate takeoff lift coefficients between 1.7 and 2 2, at best Active control systems differ from conventional air- craft controls in that they are automatic, and respond to some external stimulus. rather than to the command of the pilot Conventional controls are used to make the airplane do something Climb, turn, descend Active controls work to keep the airplane from doing some- thing, generally detrimental to its efficiency.

For example, active controls can be used to damper.

the response of an aircraft to violent turbulence. or to keep the wings from fluttering dangerously in very high-speed flight Such moderations of the aircraft s usual behavior can lead to a longer life for the aircraft structure, and a lighter structure That, in turn. reduces the amount of power required to fly the airplane, and therefore re- duces the amount of fuel burned In their first concepts. active controls were planned to be used on rudders and elevators of an aircraft The use Figure 8: Part of NASA laminar -flow control stzuftes under the of an active system would hermit smaller. and therefore ACES program called tot the design. construction and testing lighter, control surfaces to be built L ignter aircraft of typical wing sections that were candidates for incorporation require less power to maintain performance. and burn small portion of a leading-edge m ark LFC system This unit is a less fuel The smaller surfaces also create less drag.

structure with a porous surface Designed and built by the less drag equals less fuel Douglas Aircraft Company , this component was scheduled for More retrent applications ideas for active controls wtnd-tunnel evaluation to a test of the characteristics of LFC have centered on using them to prevent phenomena that cause unusual or peak stress?s on the wing Exam ples include the use of active control systems to

suppress flutter, or to redistribute wing loading dur ing Laminar Flow Control (LFC)

turning flight, where unsymmetrical loadings are I onui.i art au plane is a mixture of tic' .iii stream ar created laminar, or smooth, and turbulent. or rough. flow The ( Because the use of active controls involves interdis- turbulent areas produce skin-friction drag that is about ciplinary technologies in aerodynamics, guidance and half of the total drag One obvious way to reduce control, and materials and structures, they have been airplane drag —and therefore to improve fuel consump- considered in greater detail in NF-95, Guidance and tion — is to create laminar flow over the largest possible Control ACEE portion of the air-immersed surface of the craft In the first generation of let transport designs. the Flow becomes turbulent when some triggering action engines were slung underneath the wing on pylon occurs in the boundary layer, a thin. slow-moving lam- mounts. whose shapes were chosen using the best ina of air lying close to the wing or fusela ge or tail available information As alternate engines were se- surface It the tx)undary layer can be removed, there is lected by different airline customers, engine pvlons and no trigger ing, and therefore no turbulence mounts were modified slightly to accommodate the new Simple aerodynamic tricks can create some areas of powerplants laminar flow where they would not normally exist The When the second generation of let trar.sport ,; came use of compressed air, bled from a jet engine and along, the pylon mount system was well-established ducted over wing hailing-edge flaps, prOluces a local and was used for the newer aircraft as well But it wasn t boundary-layer control But to create large areas of necessarily the most efficient way of mounting an laminar flow requires the removal of the boundary layer engine to a wing in an aerodynamic sense over most of an aircraft s wing and tail surfaces So as part of its ACE program. and as part of the ATT LFC concepts generally have focussed on removing program Lwtore that. NASA has been systematically the boundary laver by suction Internal pumping sys- investigating nacelle design, position and mounting terns suck the boundary layer through slots or other techniques on a number of wind-tunnel models tested porous surfaces in the wings and tail at speeds corresponding to cruise conditions for the Laminar-flow control systems traditionally have been full-scale planes The tests have pointed out the inter- considered as wing installations there is a major advan- ference effects that exist in the airstream around the tage to be gained by maintaining boundary layer lami- juncture of the nacelle and the wing nar flow there But in the drive to reduce drag. no NASA studies are deriving ways of reducing the aircraft component can be ignored and the drag con- interference drag caused by the wing-nacelle intersec- tion Wind-tunnel tests have been made. and evaluated both by NASA and by the aircraft manufacturers for possible future use

07 1 I 1

3INAL PYIE

6 BLACK AND

WHITE PHOTOGRAPH

through longitudinal slots on the fuselage in a manner much like suction systems for maintaining wing laminar flow The converse, blowing air out through slots tan- gential to the fuselage sur f ace. appears also to offer some promise. Both these types—and in fact any LFC system—must be considered in light of the power they consume to maintain the lower drag. It does little good to save drag. and therefore power, it an equivalent amount of energy is expended sucking or blowing air through sots.

The availability of lightweight. powerful energy sources ouch as the gas turbine sparked another round of interest in LFC about 20 years ago It culminated in flight research on the X-21 A aircraft, a pair of Douglas WB-66Ds modified by the Northrop Corporation to test their LFC system under an Air Force contract. The limited flight program proved the feasibility of the bask Figure 9: As part of the subsystems development in NASA s system, but was terminated before the program had larunar-flow control protect (LFC). typical components were been exploited fully.

built and evaluated for possible future incorporation in a The potential for fuel savings from LFC is enormous complete research LFC aircraft One such component is this by comparison to the few percentage points picked up specimen of a leading edge structure developed by Lockheed, here and there from other aerodynamic or propulsion shown here undergoing a structural proof-load test

system developments For a typical long-range com-

mercial transport. LFC could reduce the fuel burned by

20 to 40 percent

Previous LFC systems, using similar concepts, have

often foundered on something as simple as a dirty wing

surface Dust, mud, even smashed insects can negate

the suction system over an area large enough to be a drawback. Other problems have been encountered with suction distribution, and with the manufacturing irregu- larities that triggered drag e% , en in the absence of the boundary layer. Finally, ti1F shape of the aircraft itself.

and the type and location of its engines, can affect the performance of the ideal LFC system LFC involves an inte rdisciplinary approach in two primary fields Aerodynamics. and materials and struc- tures The aerodynamic concept of LFC has been Figure 10: The tail insignia showing a soaring eagle with known for many years. but only recently have light- outstretched wings identifies—if identification is needed -a weight. strong and rigid materials been developed that winglet installation on an Air Force KC-135A The winglets are could make practical LFC systems a reality (Some of about 12 feet high, with a 6-foot root chord and a 2-loot tip the approaches in the materials and structures field are chord They are expected to improve the cruise efficiency of described in NF-1 17. Materialsand Structures/ACEE .l the modified tanker aircraft by about seven percent Since The NASA LFC program is three-phased The first, defense req uirements keep a I^rge tanker fleet a:rborne

now completed, provided systems definition and con-

around the clock substantial fuel savings could be achieved

cept selection The second, now underway. involves

by retrofitting the winglets to the entire fleet of KC- 135As operated by the Strategic Air Command One early estimate. subsystem development and evaluation In the third. a based on 1975 utilization rates. indicates that 68.000 gallons

research aircraft could be modified to integrate the

Of fuel could be saved per year per aircraft

selected LFC subsystems. and could be flown in a

reasearch program in a simulated airline operational

environment tributed by turbulent flow over the fuselage is also under study

Laminar flow is maintained over only a very short Aerodynamics by Computer

pertion of the fuselage now, and is triggered into Much of the LFC aerodynamic study was done by turbulence in its rush over a curved canopy or a computerized analysis, one of NASA s most powerful windshield In some current transports, it is possible to investigatory tools Three complete computer codes walk from the most forward pan of the cabin toward the were developed during Phase I of the program The first tail and to be aware, from the sudden change in interior two—an existing transonic wing code and a modified noise level, of the point whore laminar flow has changed boundary-layer code— provided the analysis capability to turbulent to define wing surface pressure distributions and boi ind- NASA scientists have been investigating ways to maintain the laminar flow over greater distances They include a suction system. pulling boundary-layer air in

PHOTOGRAPH

i bers between 0 8 and 0 9, the high subsonic range where any future transport using LFC may be expected to cruise.

NASA set up parallel investigations to consider the best way to design and build a wing for the real world of airline service. where maintenance and repair are among the design criteria The first steps toward that goal are being taken in the investigation of several different wing surface system concepts, using both slotted surfaces and suction areas made of porous materials Sample wing surface panels and related suction systems are being designed and will be test-d in Phase II of the LFC program.

contamination Surface problems were investigated in flight research, using a modified Lockheed JetStar light transport The bask problem was to keep the leading edge of the wing as clean as possible The tests used Figure 11: A major objective of the NASA ACES program was to get advanced technology into the hands of the manufactur- sample wing skin panels of very highly polished alumi- ers of transport aircraft as rapidly as possible In turn. the num and of Teflon, with and without in-flight washing manufacturers would incorporate that technology in their from a system of water spray nozzles mounted under- existing or near-term derivative aircraft, and plan for its use in neath the leading edge of the wing The tests showed future new designs One manufacturer who did lust that is that washed Teflon tape was an effective way of kee^r Lockheed Georgia Company, whose Advanced TriStar proto- ing the surface contamination low enough to negate its type is shown in flight It features extended winghps, visible effect on the LFC slots. These tests are continuing as because they are a lighter shade of gray. and outboard new concepts evolve ailerons. also a different color from the rest of the wing Both In Phase 11, two promising leading edge systems these features improved the aerodynamic efficiency of the consisting of suction surface and ductinq, insect re- TriStar, and both have been incorporated in its near-term moval protection, and de-icing, will be developed and derivative. the L-101 1-500 TnStar series now in airline service tested in iIrght on the JetStar aircraft In the first phase work the tests and development were done on small pieces. components of possible systems In the second phase, components and subsys- ary-layer characteristics in the presence of an LFC tems are being designed and constructed for evaluation system The third, a code that described boundary-layer in both full-scale and sub-scale tests stability, led to knowledge of the wing suction flow that From this work could come a final selection of one would he required to maintain laminar flow over a LFC system to be designed, fabricated and qualified by desired wing design flight research on a suitable aircraft, such as a modifica- Parallel to these analyses of flow fields was the tion of a commercial transport It could first fly a series careful evaluation of advanced supercritical shockless of experiments to prove the concept. and then could airfoil sections. The chosen section has a most unusual begin tests in a simulated airline environment shape. for those eyes accustomed to more conven- As one mayor source of guidance for the progi am.

tional airfoils The upper surface is a smooth, large- NASA awarded study contracts to each of the three radius arc, rounding into a leading edge The lower major air transport manufacturers in the United States surface is concave near both leading and trailing edges, Their fob was to select a design mission, and then to and convex between define a baseline configuration for an LFC transport that One of these airfoils was selected for thorough analy- could become operational with commercial airlines in sis and a wind-tunnel program to verity the theoretical the future These contracts also provide valuable inputs calculations The test wing section is being built as if it leading to consideration of alternatives in aerodynam- were a section of a ty p.cal advanced transport wing with ics. propulsion systems. materials and structures. and all an LFC system It has a seven-foot chord. a 35-degree the rest of the facets of a commercially viable transport sweep, and a 13.5 percent thickness ratio It is being The goal of the LFC program is to develop a techni- equipped with full-span suction slots on both upper and cally practical and economically attractive system lower surfaces, to maintain laminar flow over the wing It NASA will generate a technology base in this area of spans the test section of the transonic pressure tunnel laminar flow control systems that will be available to at the Langley Research Center, and will be tested at industry for the design of a new generation of transports nearly full-scale Reynolds number and at Mach num- ORIGINAL °AGE

BLACK AND WHITE PHOTOGRAPH