NASW-M+35

Design of a Turbofan Powered RegionalTransport Aircraftr

Final Report ,NASA/USRA Advanced Design Project O j 0

School of Aeronautics and AstronauticsPurdue University

West Lafayette, Indiana

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A b s t r a c t <M <?>The majority of the market for small commercial transport ° $ ^

a i rc ra f t is d o m i n a t e d by h igh-e f f i c i ency , p rope l l e r -d r iven ^ •£ ®aircraft of non-U.S. manufacture. During the past year senior <?• c ostudent design teams at Purdue developed and then responded to a z D °Request For Proposal (RFP) for a regional transport aircraft. The ,nRFP development identified promising world markets and their oneeds. The students responded by designing aircraft with ranges ^of up to 1500 nautical miles and passenger loads between 50 and 90. °During the design project, special emphasis was placed upon okeeping acquisition cost and direct operating costs at a low level i Qwhile providing passengers with quality comfort levels. Twelve ~ _,student teams worked for one semester developing their designs. c uThis final report describes several of the designs that placed a * < IT ohigh premium on innovation. ^ ̂ _

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Background - The regional national airports served by = 5"airline industry commercial air carriers. The < J*

market for aircraft to perform ^ ** a

The Federal Aviation this mission is dominated by °SSAdministration defines the re- h igh-e f f i c i ency , propeller- jf £gional transport industry as driven aircraft, with the bulk of % S "7"those air carriers that provide the aircraft manufactured by a S .-regularly scheduled passenger companies outside the United *~ §service and whose fleets are States. o < o>composed predominantly of - i n naircraft having 60 seats or Since airline deregulation o> o 3less." The regional transport began in the late 1970's the 7 cc windustry's primary goal is to differences between regional u Qprovide air transport from airlines with small aircraft and < J £small secondary airports to larger air carriers have become g g g-large metropolitan and inter- less distinct. In 1978, regional ^ °- ^

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https://ntrs.nasa.gov/search.jsp?R=19920011038 2018-08-28T17:18:18+00:00Z

airlines operated at a level ofapproximately 49,500 passen-gers per carrier. By 1988 thisaverage had risen to 180,200,an increase of 205%. Thisgrowth of the regional industryoutpaced the other parts of thecommercial airline industry.The Federal A v i a t i o nAdministration (FAA) predictsthat the number of revenuepassenger miles on regionalcarriers will nearly doublebetween 1988 and the year2000.

Areas of high growth arelikely to be in Europe and Asia,and, to a lesser extent, theUnited States. However, in theU.S. there is a greater accep-tance of the regional airline in-dustry by the public and in-creasing numbers of commer-cial partnership agreements,called code-sharing, betweensmall carriers and the majorcarriers. These agreements areessential to the survival ofregional airlines because theirfinancial health is tied to thehealth of the major airlinepartner. In 1989, 43 of the 50largest regional companiesparticipated in code-sharingagreements with major carri-ers.

The European industrytoday resembles the U.S. indus-try immediately after deregu-lation. European traffic has hadrecent growth increases near

17 per cent per year. Europeanairlines have not yet begun theU.S. practice of code-sharing,but it is only a matter of timebefore this occurs.

In general, world growthof regional traffic, includingAsia, is expected to remainhealthy and growing into theforeseeable future. The num-ber of new units required to filldemand for new aircraft andreplacements for older aircrafthas been predicted to be ashigh as 6000 aircraft through1998.

On the other hand, prob-lems such as airport congestionhave occurred as an increasingamount of air traffic has beenscheduled to converge at majorhub airports in the UnitedStates and in Europe. To easethis crowding, new regionalroutes have been developed tobypass these hub-spoke combi-nations. As a result, the re-gional airlines both serve andcompete with major air carri-ers.

The trend towards hub-bypass and point-to-point re-gional carrier operation haschanged the original mission ofregional airlines. This changerequires new capabilities fromthe aircraft serving these mis-sions. These new capabilitieseither are not met by existingaircraft or are not met effi-

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ciently. The current averageroute length or stage length isbetween 150 and 250 nauticalmiles for these regional trans-port routes.

These shorter routes areserved primarily by small ca-pacity, propeller-driven air-craft. Some predictions see thestage length increasing to over300 nautical miles with maxi-mum ranges of over 1000 nau-tical miles being required onsome routes. In this case, theturbofan engine becomes com-petitive.

In addition to efficiency,airlines must consider passen-ger convenience, comfort andcabin noise levels. Regionalairlines and their passengerswill demand faster, quieter air-craft with more passengers oneach flight so that they canserve markets efficiently andcompetitively. Passengers ac-customed to the comfort, speedand in-flight amenities of majorair carriers will come to expectthe same attributes on the re-gional routes. This so-calledseamless service betweenlarger carriers and smallercarriers will be a major crite-rion in the design of newregional aircraft.

Finally, regional airlinesmust continue to be capable ofoperating from small commu-nity airports. Many of the im-

portant smaller airports haverunway lengths of as little as5000 feet. In addition, thesmaller communities havestringent noise requirementsthat must be considered. TheseFAR 36 noise requirements andthe desire to keep passengercabin noise at low levels willimpose important constraintsupon the designer.

Object ives

During the past year themission of 12 Purdue seniorstudent design teams was todevelop and respond to aRequest For Proposal (RFP) fora regional transport mission.This RFP contained perfor-mance requirements chosen byindividual teams on the basis oftheir perception and analysis ofthe transport market as it willexist in 1995. Special emphasiswas placed upon designing tocost, a cost that includes air-craft acquisition cost and op-erational cost (DOC). Designsincorporating unusual featuresand creativity were encour-aged. The result of this studywas not only a perception ofwhat a regional transportshould look like, but also anidea of what students thoughtthe most important marketswould be.

Although the semesterprovides a 14 week workschedule, each team had only

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about ten weeks to conceiveand develop its design concept.This shortened time period wasdue to the fact that the firstfour weeks of the semesterwere used to develop marketstudies and to acquire specialdesign skills such as aircraftweight estimation, design sen-sitivity techniques and othertraditional techniques.

Team Requirements

Each design team wassubject to stringent analytical,conceptual and reporting re-quirements for their design. Itwas required that extensiveinformation on aerodynamicperformance be generated to-gether with stability, controland flying quality information.The structural loads, memberlayout and weights and balanceinformation were also required.Coupled together with theweights information were therequirements for guaranteesthat the landing gear couldsupport the ground loads andwould meet minimum tip-overand take-off clearance re-quirements.

The ability to perform therequired transport missionfrom take-off to cruise to

.landing with required reserveswas rigorously checked usinganalytical procedures thatranged from highly preliminaryto extremely sophisticated.

These checks used class-devel-oped performance computercodes and, in many cases, theFLOPS (Flight Performance andOptimization) code developedby NASA/Langley and modifiedat Purdue for use on the per-sonal computer. To obtainperformance data it was neces-sary to have extensive enginedata. Such data is usually aclosely held secret guarded byengine manufacturers.

To remedy the problemof obtaining accurate enginedata, two personal computercodes, ONX and OFFX, wereused. These codes can matchand generate crucial engineperformance data such as fuelflow at various Mach number,altitude and power settings.These codes were used exten-sively by the USRA TeachingAssistant during the Summer of1990 and a video tape and setof assignments were formu-lated for class use.

These codes were used tomodify the engine cycle andinlet temperatures as requiredto meet the specific missions ofthe design team aircraft. Insome cases this required ex-tensive re-design of the threeengine designs that the stu-dents were given at the begin-ning of the class.

The final result of eachteam's work was: a detailed

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100 page design report; an ex-ecutive summary of this report;and, a 25 page mid-term reportthat was evaluated by a teamof technical writing expertsfrom the Thiokol Corporation.What follows is a summary ofsome of the data presented inthese reports.

The value of time and thecost of speed

Time is money. Time isof value to a passenger on theregional transport, but it alsocosts money to acquire thespeed necessary to save time.This cost is reflected in all ofthe empirical relations used toestimate aircraft cost.

Figure 1 shows theamount of time required tocomplete a trip as a function ofairspeed. This so-called time-scoping analysis shows thatthere is a knee in this curve.At Mach numbers or airspeedsabove this knee, there is verylittle change in the trip time asMach number increases. Ingeneral, this knee moves rightto larger airspeeds when therange of the aircraft increases.For short range aircraft, it isnot important that the aircraftbe extremely fast.

Because of the marketfactors that governed eachgroup's design, the 12 teamsindependently arrived at the

conclusion that it was unneces-sary to have the aircraft travelextremely fast. In addition, be-cause aircraft acquisition costincreases with cruise Machnumber, cruise speeds werekept down so that they rangedbetween Mach numbers of 0.70and 0.82. These cruise Machnumbers can be compared tolonger range aircraft that maycruise up to Mach 0.90.

0 100 200 300 400 500 600 700

VELOCITY, knotsRange = 1250 nmi

Figure 1 - Trip time vs.airspeed

Passenger loads, range andrequ i remen t s

Recent trends in the re-gional transport business havebeen directed towards the de-velopment of aircraft with upto 100 seats and ranges up to1500 nautical miles (nmi.). Asa result, the RFP's developed bythe 12 design teams displayeda wide range of seating andrange objectives. Figure 2shows this data for the 12design groups and compares it

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to two other aircraft now inservice.

The smallest aircraft de-veloped at Purdue has a pas-senger capacity of only 50 pas-sengers with a range of be-tween 800 and 900 nauticalmiles (with reserves). Thelargest aircraft is designed tohold 90 passengers and had arange of 1650 nautical miles,comparable to the Fokker 100.

Design Teams' Market Profile

120

110

100

90

80

7O

60

60

40

Passenger Number

BMM6-300

Cwidtalr RJ1OO

600 900 1000 HOC J2OO MOO MOO 1SOO 1600 1700

Range NMi

Figure 2 - Design teampassenger number vs.

range

Design groups identifiedthe European and Asianmarkets as being morepromising than the U.S. market.As a result, while they usedFAR standards in their work,design teams also used the AEAor Association of EuropeanAirlines Requirements as astandard to meet.

While the AEA standardsrepeat many of the FAR

requirements for safety, theyalso set minimum standards forpassenger comfort in terms ofsuch items as seat pitch. All 12aircraft meet these AEAstandards and use AEAguidelines to calculate directoperating cost (DOC). Let usnow consider some of thedesigns generated by thedesign teams and theirfeatures. Note that all of these

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designs are required to carry acockpit crew of two.

The WAG-78

The WAG-78 is a 78 pas-senger aircraft with a range of1100 nautical miles. It is de-signed to cruise at M=0.80 at35,000 feet with an operationalceiling of 39,000 feet. The air-craft will take off from a run-way with a length greater than5500 feet on a standard day inDenver. This design is a modi-fication of a design that has ap-peared during the past tenyears and is shown in Figure 3.

Figure 3 - WAG-78 JoinedWing Design

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The WAG-78 provides anexample of a departure fromconventional subsonic aircraftdesign because it uses thejoined wing concept developedseveral years ago. This joinedwing has a rear wing surfacethat acts both as a horizontaltail and as an external strut tostiffen and strengthen thewing. The take-off grossweight of this aircraft is 54,900Ibs. with an empty weight of30,500 Ibs. Some weight sav-ings were achieved because ofthe joined wing structural de-sign.

The WAG-78, like all theother student designs, waspowered by a redesignedGeneral Electric TF 34 engine.This engine was resized andslightly redesigned to develop athrust of 11,900 Ib.

Two engines were usedfor this design to satisfy oneengine inoperative (OEI) re-quirements and so that theengines would be capable ofdeveloping thrust levels suffi-cient to meet the take-off re-quirements and one engine in-operative (OEI) criteria. Thethrust-to-weight ratio for thisaircraft is rather large so thatthe aircraft can climb rapidly toits cruise altitude.

The designers of theWAG-78 were conservative in

their estimates of the numberof aircraft that they could mar-ket. They predicted that theywould be able to sell 175 air-craft over an 11 year develop-ment and production cycle.This 175 number did not in-clude the 5 test aircraft thatthey chose for a developmentphase that was to last between3 and 5 years. This unusuallylarge number of test aircraftwere thought to be necessarybecause of the new joined-wingdesign feature that they pro-posed to use.

The WAG-78 designersestimated a Development andTesting Cost of $810 Millionand production costs of $2.262Billion. A cash bucket analysissuch as that shown in Figure 4was used to estimate the priceof this aircraft to be $20 Millionif the cost of capital is 18%.Operating costs for an 1100nmi. trip were estimated at$2060 to give a low 3.6 centsper revenue seat mile assuminga 66.7% load factor.

soo200100

I -100I -200

•300

-400

•500

•6006

YEAR10 12

|—1BMOHON-»-20MIUJON-«-22MajJON |

Figure 4 - Cash BucketPrice Analysis

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The WAG-78 design teamcompared their design to theBAe 146-100 and theDeHaviland Dash 8-400 andfound that the WAG-78 cost 0.5to 1 million dollars more thanthese other aircraft. On theother hand, it could be oper-ated at a seat mile cost of about10% less than the BAe 146-100and only slightly more than theDash 8. The Dash 8 is aturboprop aircraft and, in itslatest stretched version, itsrange has been reduced to 800nmi. at a speed of 350 knots.

The ARCA-60

Design reviews with in-dustrial representatives wereheld during both semesters ofdesign team activity. Designrepresentatives included amarketing authority, a propul-sion and maintenance expertand an airline pilot. The air-lines represented includedSouthwest Airlines, USAir andNorthwest Airlines.

In all cases, the teamswere encouraged to simplifytheir designs and to considerflight operations and mainte-nance. While this advice wasvaluable, it also tended to dis-courage configuration innova-tion. As a result, aircraft ex-ternal features evolved to be-come somewhat traditional.

An excellent example of awell-conceived, traditional, DC-9-like design is the ARCA-60,shown in Figure 5. This aircrafthas seating for 60 passengerswith an 1100 mile range andenough fuel to fly to analternate airport 200 nmi.away and hold for 30 minutes.It has a maximum Mach num-ber of 0.80 and cruises at35,000 feet at M=0.75.

Figure 5 - The ARCA-60Aircraft

Extensive studies weredone by the ARCA-60 aerody-namicist to obtain an efficientairfoil shape for low drag.These efforts led to the choiceof a NASA supercritical airfoil,the SC(2)-0412. An Euler codeanalysis of the section esti-mated the drag divergenceMach number of this section tobe 0.75. This code was neces-sary to accurately model thenonlinearities that occur intransonic flow.

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The ARCA-60 wing wasmounted low on the fuselage toallow for aft mounting of theengines and easy storage of thelanding gear. Aft mounting ofthe engines resulted in the re-quirement for a T-tail design.Although the e.g. movementduring flight is minimal, theARCA-60 requires a large tailvolume to rotate the nose ontake-off from short runways.The extra cruise drag from thisconfiguration was regarded bythe design team to be ac-ceptable.

The ARCA-60 has a pre-dicted TOGW of 60,300 Ibs. anda wing loading of 75 psf attake-off. The wing quarterchord sweep is 20.4 deg. tohelp reduce torsional loadswhile maintaining aerodynamicefficiency. After extensiveanalysis, a taper ratio of 0.2was chosen so that the liftdistribution approached that ofa minimum drag, ellipticalspanwise lift distribution.

The thrust per enginewas 9650 Ibs. and is muchlower than the WAG-78. Withengine cost estimated at $2.4Million per aircraft, the ARCA-60 is estimated to cost $19Million. This number is basedon a production run of 300 anda cost of capital of 10%. Thislatter cost is low compared tothe 18% estimate of the WAG-78 team. The ARCA-60

program is estimated to last for20 years and to produce aprofit of $819 Million.

As shown in Figure 6, thecabin cross-section is designedfor comfort. This feature ofthis design is also present inthe other 11 designs.

r-54'

Figure 6 - Cabin cross-section

The SRT-80 Aircraft

The SRT-80 design,shown in Figure 7, is represen-tative of several designs pro-duced during the project(notethat this image is produced bya mesh generation program andsome distortion in engineplacement will occur when thecomputer screen image isprinted). This aircraft resem-bles the 737/757 class of air-craft with wing mountedengines. This aircraft cancruise at Mach 0.80 and carries

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80 passengers a distance of1200 nmi. with reserves. Ithas a wing loading of 55 psf toallow it to take-off from 5500foot runways at 2000 feetabove sea level.

39 ft 108 ft

Figure 7 - The SRT-80

The aircraft has a span of94 feet and a length of 93 feet.The wing itself has a dihedralangle of 5 degree for stability.At a design TOGW of 60,900Ibs. this aircraft will use 9400Ibs. of fuel to complete its mis-sion. The engines on the SRT-80are designed so that the in-

tegrated airframe and propul-sion units will generate 101seat miles (nmi.) per gallon offuel.

The engines are modifiedversions of the GE TF 34 turbo-fan design They were scaledup to increase the thrust fromeach engine. The TF 34 wasselected by the SRT-80 teambecause of its superior fuel ef-ficiency. The propulsion spe-cialist increased the bypass ra-tio from 6.23 to 7.0 to increasethrust by almost 7% and to de-crease fuel consumption byover 4%.

Like most of the designs,the structure of the SRT-80 iscomposed primarily of alu-minum, with small amounts ofcomposites used in non-loadbearing structure. The struc-ture is estimated to comprise40.1% of the TOGW. Passengersand baggage comprise an addi-tional 31.6% while the systemsand equipment form 3.3%. Theremaining weight is due to pas-sengers and their baggage.

The Wombat

The last airplane to bereviewed is a blend of conven-tional design with a hint ofnonconventional features. Thisdesign, shown in Figure 8, be-gan as a design that closely re-sembled the BAe-146 or theC141. The high wing design

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was judged by the designers tobe desirable because of itshandling qualities during thelanding in the ground effect.Wing mounted engines werealso desired for ease of access.Landing gear are stowed in ablister pod in the fuselage andmeet tip-over criteria.

The Wombat has a wingspan of 92.2 feet, a length of105 feet and weighs 66,950 Ibs.to give it a wing loading of 89psf at take-off. The Wombat isdesigned to carry 70 passen-gers, but will also be availablein a stretch version that willcarry 100 passengers. The pro-jected cost is $22 Million.

Figure 8 - The Wombat(note difference in scales)

During the design processthe design team became con-cerned about cabin noise fromthe engines and overheadhydraulic lines as well as thepotential for blade damagefrom an engine failure in flight.As a result, they moved thewing back instead of attachingthe engines to the fuselage as anumber of other design teamshad done. This necessitated theaddition of a canard to raise thenose at take-off. It also gen-erated concern for the effectsof the canard tip vortices onthe engine intakes.

The aerodynamicist andthe stability and control spe-cialist cooperated to place thewing and canard properly toreduce trim drag in flight. Theresult was an optimized threesurface aircraft shown in Figure8.

The fuselage of this air-craft is to be constructed ofArall. This material has an or-ganic fiber material sand-wiched between layers of alu-minum to form a compositematerial. This material shouldbe safer and deaden soundfrom the engines better thanconventional aluminum.

Conclusion

The Purdue design classconsidered the engineer -ing/economic task of designing

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ing/economic task of designinga regional transport aircraftwith turbofan engines. Marketconsiderations drove thisdesign to passenger capabilitiesof near 70 passengers. As aresult, one of the threeavailable engines, the GE TF 34,was the clear choice of all 12teams that participated.

One aspect of commonagreement among the designteams was that the regionaltransport market should grow.As a result, a successful designwill have a good chance of re-turning a profit to its investors.Because of the emphasis placedupon practicality and economy,most aircraft have a conven-tional appearance. In addition,most aircraft use minimalamounts of composite materialsfor construction and have con-ventional controls. On theother hand, all groups em-braced supercritical airfoiltechnology.

The emphasis upon costand price of the aircraft re-quired a model to predict thesenumbers. The teams developedsuch models and the ability tojudge the desirability of tradingone technology against another.In the long run, it is the clearrelationship between the mar-ket forces and the engineeringdecisions that will prove to bethe most valuable aspect of thisdesign experience.

In addition to the ac-complishments of the class,several valuable pieces of soft-ware were developed by stu-dent Teaching Assistants. Themost valuable of these is aMacintosh package that com-putes the price of the aircraftsubject to financial informationsuch as the cost of capital. Inaddition, the direct operatingcosts are estimated using anAssociation of EuropeanAirlines package.

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