Document
N93-29721
DESIGN OF A TURBOFAN POWERED REGIONAL J
3¢-0
TRANSPORT AIRCRAFT
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PURDUE UNIVERSITY
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The majority of the market for small commercial transport aircraft is dominated by high-efliciency, propeller-driven aircraft of non-U.S, manufacture. During the past year senior student design teams at Purdue developed and then responded to a Request For Proposal (RFP) for a regional transport aircraft.
The RFP development identified promising world markets and their needs. The students responded by designing aircraft with ranges of up to 1500 n.tlt and passenger loads of 50 to 90. During the design project, special emphasis was placed upon keeping acquisition cost and direct operating costs at a low level while providing passengers with quality comfort levels. Twelve student teams worked for one semester developing their designs. This report describes several of the more successful designs and those that placed a high premium on innovation. The report also illustrates the depth of detail and analysis in these student efforts.
BACKGROUND: I'H]E REGIONAL AIRLINE INDUSTRY In general, world growth of regional trallic, including Asia, is expected to remain healthy and growing into the foreseeable future. The number of new units required to fill demand for The Federal Aviation Administration defines the regional new aircraft and replacements for older aircraft has been transport industry as "those air carriers that provide regularly scheduled passenger service and whose fleets are composed predicted to be as high as 6000 aircraft through 1998.
On the other hand, problems such as airport congestion have predominantly of aircraft having 60 seats or less." The regional occurred as an increasing amount of air traffic has been txans_rt industry's primary goal is to provide air transport from scheduled to converge at major hub airports in the United States small secondary airports to large metropolitan and international and in Europe. To ease this crowding, new regional routes have airports served by commercial air carriers. The market for aircraft been developed to bypass these hub-spoke combinations. As to perform this mission is dominated by high-efficiency, pro- peller-driven aircraft, with the bulk of the aircraft manufactured a result, the regional airlines both serve and compete with major air carriers.
by companies outside the United States.
The trend toward hub-bypass and point-to-point regional Since airline deregulation began in the late 1970s the differences between regional airlines with small aircraft and carrier operation has changed the original mission of regional airlines. This change requires new capabilities from the aircraft larger air carriers have become less distinct. In 1978 regional serving these missions. These new capabilities either are not airlines operated at a level of approximately 49,500 passengers per carrier. By 1988 this average had risen to 180,200, an increase met by existing aircraft or are not met efficiently. The current of 205%. This growth of the regional industry outpaced the average route length or stage length is 150 to 250 n.m. for other parts of the commercial airline industry. The Federal regional transport. These shorter routes are served primarily Aviation Administration (FAA) predicts that the number of by small capacity, propeller-driven aircraft. Some predictions revenue passenger miles on regional carriers will nearly double see the stage length increasing to over 300 rtm. with maximum between 1988 and the year 2000. ranges of over 1000 n.m. required on some routes. In this case, Areas of high growth are likely to be in Europe and Asia, the tur_fan engine becomes competitive.
and, to a lesser extent, the United States. However, in the U.S. In addition to efficiency, airlines must consider passenger convenience, comfort, and cabin noise levels. Regional airlines there is a greater acceptance of the regional airline industry (and their passengers) will demand faster, quieter aircraft with by the public and increasing numbers of commercial partnership more passengers on each flight so that they can serve markets agreements, called code-sharing, between small carriers and the major carriers. These agreements are essential to the survival efficiently and competitively. Passengers accustomed to the of regional airlines because their financial health is tied to the comfort, speed, and in-flight amenities of major air carriers will health of the major airline partner. In 1989, 43 of the 50 largest come to expect the same attributes on the regional routes. This regional companies participated in code-sharing agreements so-called seamless service between larger carriers and smaller
withm jor carriers. carriers will be a major criterion in the design of new regional
aircraft.
The European industry today resembles the U.S. industry immediately after deregulation. E_ traffic has had recent Finally, regional airlines must continue to be capable of op- increases near 17% per year. European airlines have not yet erating from small community airports. Many of the important begun the U.S. practice of code-sharing, but it is only a matter smaller airports have runway lengths of as little as 5000 ft. In of time before this occurs.
addition, the smaller communities have stringent noise require- 84 Proceedings of the NASA/USRA Advanced Design Program 7tb Summer Conference The final result of each team's work was a detailed, 100- ments. These FAR 36 noise requirements and the desire to keep page design report, an executive summary of this report, and passenger cabin noise at low levels will impose important con- a 25-page mid-term report that was evaluated by a team of straints on the designer.
technical writIng experts from the Thiokol Corporation. What follows is a summary of some of the data presented in these
OBJECrrVES
reports.
During the past year the mission of 12 Purdue senior student THE VALUE OF TIME AND THE COST OF SPEED design teams was to develop and respond to a Request For Proposal (RFP) for a regional transport mission. This RFP Time is money. Tune is of value to a passenger on a regional contained performance requirements chosen by individual teams transport, but it also costs money to acquire the speed neces._ry on the basis of their perception and analysis of the transport to save time. This cost is reflected in all of the empirical relations market as it will exist in 1995. Special emphasis was placed used to estimate aircraft cost.
upon designing to cost, a cost that includes aircraft acquisition cost and operational cost (Doc). Designs incorporating unusual Figure 1 shOWS the amount of time required to complete a trip as a function of airspeed. This so-called timescoping analysis features and creativity were encouraged. The result of this study shows a knee in the curve. At Mach numbers or airspeeds above was not only a perception of what a regional transport should this knee, there is very little change in the trip time as Mach look like, but also an idea of what students thought the most number increases. In general, the knee moves right to larger important markets would be.
Although the semester provides a 14-week work schedule, airspeeds when the range of the aircraft increases. For short- each team had only about 10 weeks to conceive and develop range aircraft, it is not important that the aircraft be extremely fast.
its design concept. The first four weeks of the semester were Because of the market factors that governed each group's used to develop market studies and to acquire special design design, the 12 teams independently arrived at the conclusion skills such as aircraft weight estimation, design sensitivity that it was unnecessary to have the aircraft travel extremely techniques, and other traditional techniques.
fast. In addition, because aircraft acquisition cost increases with cruise Mach number, cruise speeds were kept down so that
TEAM REQUmEMENTS
they ranged from Mach 0.70 to 0.82. These cruise Mach numbers can be compared to longer range aircraft that may cruise up Each design team was subject to stringent analytical, to Mach 0.90.
conceptual and reporting requirements for their design. It was required that extensive information on aerodynamic perfor- PASSENGER LOADS, RANGE, AND REQUIREMENTS mance be generated together with stability, control, and flying quality information. The structural loads, member layout, and Recent trends in the regional transport business have been weights and balance information were also required. Coupled directed toward development of aircraft with up to 100 seats with the weights information were the requirements for guarantees that the landing gear could support the ground loads and ranges up to 1500 n.ro_ As a result, the RFPs developed by the 12 design teams displayed a wide range of seating and and would meet minimum tip-over and takeoff clearance range objectives. Figure 2 shows this data for the 12 design requirements.
groups and compares it to two other aircraft now in service.
The ability to perform the required transport mission from takeoff to cruise to landing with required reserves was rigorously checked using analytical procedures that ranged from highly preliminary to extremely sophisticated. These checks used class- Time Scoping Results developed performance computer codes and, in many cases, 12" the Flight Performance and Optimization (FLOPS) code developed by NASA/Langley and modified at Purdue for use on the personal computer. To obtain performance data it was 10" necessary to have extensive engine data. Such data is usu'_y a closely held secret of engine manufacturers.
To remedy the problem of obtaining accurate engine data, two personal computer codes, ONX and OFFX, were used. These codes can match and generate crucial engine performance data such as fuel flow at various Mach numbers, altitudes and power 4" settings. These codes were used extensively by the USRA Teaching Assistant during the summer of 1990 and a videotape 2- i i • ! • t • i • | and set of &_signments were formulated for class use.
0 100 200 _,00 400 300 600 These codes were used to modify the engine cycle and inlet VELocrI'Y, knots temperatures as required to meet the specific missions of the Range = 1250 nmi design team aircraft. In some cases this required extensive redesign of the three engine designs that the students were Fig. 1. Trip time vs. airspeed.
given at the beginning of the class.
Purdue University 85
The smallest aircraft developed at Purdue has a passenger Profile
Market
Design Teams'
capacity of only 50 with a range of 800 to 900 n.m. (with reserves). The largest aircraft is designed to hold 90 passengers Passenger Number 12o and had a range of 1650 n.m., comparable to the Fokker 100.
Design groups identified the European and Asian markets as 11o Fokker 100 being more promising than the U.S. market. As a result, while BAe148-300 lOO they used FAR standards in their work, design teams also used the Association of European Airlines Requirements (AEA) as a standard.
• • While the AEA standards repeat many of the FAR requirements • • for safety, they also set minimum standards for passenger comfort in terms of such items as seat pitch. All 12 aircraft meet these 6o AEA standards and use AEA guidelines to calculate DOC. Let us now consider some of the designs generated by the design Canadair RJIO0 teams and their features. Note that all of these designs are 900 1000 1100 1200 1300 1400 1500 1600 required to carry a cockpit crew of two.
Range NMi THE WAG-78 Fig. 2. Design team passenger number vs. range.
The WAG-78 is a 78-passenger aircraft with a range of 1100 n.m. It is designed to cruise at M = 0.80 at 35,000 ft with an operational ceiling of 39,000 ft. The aircraft will take off from a runway longer than 5500 ft on a standard day in Denver. This design is a modification of a design that has appeared during the past ten years and is shown in Fig. 3.
The WAG-78 provides an example of a departure from conventional subsonic aircraft design because it uses the joined wing concept developed several years ago. This joined wing has a rear wing surface that acts both as a horizontal tail and as an external strut to stiffen and strengthen the wing. The takeoff gross weight (TOGW) of this aircraft is 54,900 liE) with an empty weight of 30,500 lb. Some weight savings were achieved because of the joined wing structural design.
The WAG-78, like all the other student designs, was powered by a redesigned General Electric TF 34 engine. This engine was resized and slightly redesigned to develop a thrust of 11,900 lb.
Two engines were used for this design to satisfy one engine inoperative (OEI) requirements and so that the engines would be capable of developing thrust levels sufficient to meet the Fig. 3. WAG-78 Joined W'mg Design.
takeoff requirements and OEI criteria. The thrust-to-weight ratio for this aircraft is rather large so that the aircraft can climb rapidly to its cruise altitude.
3OO 1
The designers of the WAG-78 were conservative in their estimates of the number of aircraft that they could market. They predicted that they would be able to sell 175 aircraft over an 11 year development and production cycle. This number did not include the 5 test aircraft that they chose for a development phase that was to last 3 to 5 years. This unusuMly large number of test aircraft were thought to be necessary because of the new joined-wing design feature that they proposed to use.
The WAG-78 designers estimated a development and testing -500" cost of $810 million and production costs of $2.262 billion.
-6OO A cash bucket analysis shown in Fig. 4 was used to estimate 0 2 4 6 8 10 12 YEAR the price of this aircraft to be $20 million if the cost of capital is 18%. Operating costs for an 1100-n.m. trip were estimated at $2060 to give a low 3.6 cents per revenue seat mile assuming -_" 18 MILUON _ 20 MILLION _ 22 MILLION' J a 66.7% load factor.
The WAG-78 design team compared their design to the BAe Fig. 4. Cash Bucket Price _ m 146-100 and the DeHaviland Dash 8-400 and found that the 86 Proceedings of the NASA/USRA Advanced Destgn Program 7tb Summer Conference WAG-78 cost 0.5 to 1 million dollars more than these aircraft.
for a T-tail design. Although the c.g. movement during flight On the other hand, it could be operated at a seat mile cost is minimal, the ARCA-60 requires a large tail volume to rotate of about I0% less than the BAe 146-100 and only slightly more the nose on takeoff from short runways. The extra cruise drag than the Dash 8. The Dash 8 is a turboprop aircraft and, in from this configuration was regarded by the design team to its latest stretched version, its range has been reduced to 800 n.m. be acceptable.
at a speed of 350 knots. The ARCA-60 has a predicted TOGW of 60,300 lb and a wing loading of 75 psf at takeoff. The wing quarter chord sweep THE ARCA-60 is 20.4 ° to help reduce torsional loads while maintaining aerodynamic efficiency. After extensive analysis, a taper ratio Design reviews with industrial representatives were held dur- of 0.2 was chosen so that the lift distribution approached that ing both semesters of design team activity. Design representatives of a minimum drag, elliptical spanwise lift distribution.
included a marketing authority, a propulsion and maintenance The thrust per engine was 9650 lb and is much lower than expert, and an airline pilot. The airlines represented included the WAG-78. With engine cost estimated at $2.4 million per Southwest Airlines, USAir and Northwest Airlines.
aircraft, the ARCA-60 is estimated to cost $19 million. This In all cases, the teams were encouraged to simplify their number is based on a production run of 300 and a cost of designs and to consider flight operations and maintenance. While capital of 10%. This latter cost is low compared to the 18% this advice was valuable, it also tended to discourage con- estimate of the WAG-78 team. The ARCA-60 program is estimated figuration innovation. As a result, aircraft external features to last for 20 years and to produce a profit of $819 miUion.
evolved to become somewhat traditional.
As shown in Fig. 6, the cabin crogs-section is designed for An excellent example of a well-conceived, traditional, DC- comfort. This feature is also present in the other 11 designs.
9-like design is the ARCA-60, shown in Fig. 5. This aircraft has seating for 60 passengers with an 1100-mile range and enough THE SRT-80 AIRCRAFt fuel to fly to an alternate airport 200 n.m. away and hold for 30 minutes. It has a maximum Mach number of 0.80 and cruises at 35,000 feet at M : 0.75. The SRT-80 design, shown in Fig. 7, is representative of several Extensive studies were done by the ARCA-60 aerodynarnicist designs produced during the project (note that this image is to obtain an efficient airfoil shape for low drag. These efforts produced by a mesh generation program and some distortion led to the choice of a NASA supercritical airfoil, the SC(2)- in engine placement will occur when the computer screen image 0412. An Euler code analysis of the section estimated the drag is printed). This aircraft resembles the 737/757 class of aircraft divergence Mach number of this section to be 0.75. This code with wing-mounted engines. This aircraft can cruise at Mach was used to accurately model the nonlinearities that occur in 0.80 and carries 80 passengers a distance of 1200 n.m. with transonic flow.
reserves. It has a wing loading of 55 psf to allow it to take The ARCA-60 wing was mounted low on the fuselage to allow off from 5500-ft runways at 2000 ft above sea level.
for aft mounting of the engines and easy storage of the landing The aircraft has a span of 94 fi and a length of 93 ft. The gear. Aft mounting of the engines resulted in the requirement wing itself has a dihedral angle of 5 ° for stability. At a design
Zs--S
-, ,%
131 ° Fig. 5. The ARCA-60 Aircraft. Fig. 6. Cabin Cross-section.
Purdue University 87
J
THE WOMBAT
T--
r 93fi. 17.5-fl: The last airplane to be reviewed is a blend of conventional design with a few unconventional features. This design, shown in Fig. 8, began as a design that closely resembled the BAe- 146 or the C141. The high wing was judged by the designers to be desirable because of its handling qualities during the landing in ground effect. W'mg mounted engines were used for or ease of access. Landing gear is stowed in a blister pod in the fuselage and meets tip-over criteria.
The Wombat has a wing span of 92.2 ft, a length of 105 ft and weighs 66,950 lb to give it a wing loading of 89 psf at takeoff. The Wombat is designed to carry 70 passengers, but will also be available in a stretch version that will carry 100 passengers. The projected cost is $22 milh'on.
The design team became concerned about cabin noise from the engines and overhead hydraulic lines as well as the potential for blade damage from an engine failure in flight. As a result, they moved the wing back instead of attaching the engines to the fuselage as a number of other design teams had done. This necessitated the addition of a canard to raise the nose at takeoff.
It also generated concern for the effects of the canard tip vortices on the engine intakes.
The aerodynamicist and the stability and control specialist cooperated to place the wing and canard properly to reduce trim drag in flight. The result was an optimized three-lifting- surface aircraft shown in Fig. 8.
Fig. 7. The SRT-80.
TOGW of 60,900 lb this aircraft will use 9400 lb of fuel to complete its mission. The engines on the SRT-80 are designed so that the integrated airframe and propulsion units will generate 101 seat miles (n.m.) per gallon of fuel.
The engines are modified versions of the GE TF 34 turbofan design They were sealed up to increase the thrust from each engine. The TF 34 was selected by the SRT-80 team because of its superior fuel efficiency. The propulsion specialist increased the bypass ratio from 6.23 to 7.0 to increase thrust by almost 7% and to decrease fuel consumption by over 4%.
Like most of the designs, the structure of the SRT-80 is composed primarily of aluminum, with small amounts of composites used in non-load-bearing structure. The structure is estimated to be 40.1% of the TOGW. Passengers and baggage are an additional 31.6% while the systems and equipment are 3.3%. The remaining weight is due to passengers and their baggage. Fig. 8. The Wombat (note difference in scales).
88 Proceedings of the NASA/USRA Advanced Design Program 7tb Summer Conference One agreement among the design teams was that the regional 11ae fuselage of this aircraft is to be constructed of Ara]l.
This composite material has an organic fiber material sandwiched transport market would grow. As a result, a successful design will have a good chance of returning a profit to its investors.
between layers of aluminum. This material should be safer and Because of the emphasis placed upon practicality and economy, deaden sound fi'om the engines better than conventional aluminum. most aircraft have a conventional _ce. In addition, most aircraft use minimal amounts of composite materials for construction and have conventional controls. On the other hand, CONCLUSION all groups embraced supercritical airfoil technology.
The emphasis upon cost and price of the aircraft required a model to predict these numbers. The teams developed such The Purdue design class considered the engineering/ models and the ability to judge the desirability of trading one economic task of designing a regional transport aircraft with technology against another. In the long run, it is the dear turbofan engines. Market considerations drove this design to relationship between market forces and engineering decisions passenger capabilities of about 70 passengers. As a result, one that will prove to be the most valuable aspect of this design of the three available engines, the GE TF 34, was the clear choice of the 12 teams that participated. experience.