nasa and general aviation

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NASA SP-485

NASAandGeneralAvhtion

Jeffrey L. Ethel1

Scientific and Technical Information Branch 1986

National Aeronautics and Space AdministrationWashington, DC


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Library ofCongress Cataloging-in-PublicationData

EthelI, Jeffrey L.NASA and general aviation.

(NASA SP ;485)1. Private flying--United States. 2. United States.

National Aeronautics and Space Administration. I. Title.

11. Series.TL721.4.EM 1986 629.133' 340422 85-31931

For sale by the Superintendent of Documents, U.S. GovernmentPrinting Office,Washington, D.C. 20402


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CONTENTS

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

1 WhatIsGeneral Aviation? . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Aerodynamic Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Internal Combustion Engines . . . . . . . . . . . . . . . . . . . . . . . . 214 Turbine Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5 Commuter Aviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

General Aviation Utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . 737 Safety Improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Electronics and Avionics. . . . . . . . . . . . . . . . . . . . . . . . . .

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ABOUT THE AUTHOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

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9 TheFuture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

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FOREWORDIn 1915 a far-seeing United States Congress created the Na -

tional Advisory Committee fo r Aeronaut ics (NACA). Its purpose:improve and develop American aviation, which was then in its in-fancy.

Today NACAs successor, the National Aeronautics and SpaceAdministration (NASA), has a much broader mission. But as itsna me implies, NASA is still very much involved with aeronautics.Yet until now, the agencys important role in general aviationhas never been fully told.

1 find this rather astonishing, fo r in sheer numbers of planes,pilots, and air operations, general aviation is by far the largestsingle compon ent ofall U.S. aviation

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civil and military.NASA and GeneralAviation should do much to correct any

possible misconception about the fine working relationship be -tween the agency and th e aircraft industry. Though perhaps notgenerally known, the industry continues to rely heavily o n thebasic technology that NASA and its predecessor have developedduring decades of research. It is this traditional government-industry relationship that propelled America to a position ofleadership in world aviation and space.

But as the au thor clearly points out, our leadership in so me keytechnical areas is eroding. This is especially true in general avia-tion. While it is the biggest segmen t ofthe total aircraft industry, itstill gets the smallest share of any federal funding budget ed foraerospace 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 a t a distinct disadvantage. Ifwe wish to com pet e effectivelyin world markets, the n we must dedicate the talent and resourcesnecessary to advance ou r technology.

The selection ofJeffEthell to tell this enlightening story is com -mendable. Few full-time authors have the skills to communicatetechnical subj ects in other than highly complex terms. JeffEthell,an experienced pilot and an accomplished writer, is such a profes-sional.


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PREFACEGeneral aviation remains the single most misunderstood se ctor

of aeronautics in the United States. Airline companies make thenews on a regular basis and travel on t he major carriers is taken forgranted, yet they comprise but 10% of all US. nonmilitary flightoperations. The remainder belongs t o general aviation, one ofthemajor keys to American business success.

This bookwill give yo u a detailed look at how general aviationfunctions and how NASA helps keep it o n the cutting edge oftechnology in airfoils, airframes, commuter travel, environmentalconcerns, engines, propellers, air traffic control, agriculturaldevelopment, electronics, and safety. No doubt som e of you willbe surprised to find that there is so much activity in general avia-tion. This is not simply the weekend pilot who wants to tak e a spin ,but also the businessman who relies on aircraft to get him wherehe is going when he wants to go, the corporation that wantssophisticated jet transportation for its executives, the family thatwants t o travel where no major airline terminal exists, th e farmerwho wishes to have his crops protected, the mission pilot whotransport s spiritual and physical aid over dense jungle terrain, themedivac team that requires rapid transportation for an accidentvictim or transplant recipient. And the list of benefits goes on.

If there was ever a time when NASA's aid was pivotal to generalaviation's survival, it is now. Come along with me, as we take alook into a fascinating and seldom seen world through the eyes ofNASA and its numerous programs.

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CHAPTER 1WHAT IS GENERAL AVIATION?

hough few people ar e aware of it, general aviation, all civilflying activity except airline operations, accounts fo r 98 per-

815 000 pilots. It is by far the most active and t he mos t diverse ofall aviation operations. When combining t he amo unt ofairline andgeneral aviation flying, the airlines account for only 10percent ofthe total hours an d operations flown. And the majority, around 80percent by some estimates, of general aviation flying is forbusiness purposes, serving all ofthe nations 14 746 airports whilethe airlines reach only 733 (or 5 percent). The NationalAeronautics and Space Administration (NASA) includes air taxi

and commuter aircraft under its general aviation research pro-

gra ms while excluding rotorcraft.Though the public conception of general aviation is that of

private owners flying for pleasure, the op posite is true, particularlyin these days ofless discretionary income. It includes every kind ofpiloted airborne vehicle from ultralight powered hang gliders andballoons t o airline-size corporate jets.

Not only does the business community rely very heavily ongeneral aviation to function, but t he U.S. balance oftrade is heavi-ly dependent o n the export ofaircraft. In 1979, civil aircraft salesresulted in a net positive contribution of $10 billion t o our tra debalance. In the past 20 years, the U.S. share of total exports ofthemajor industrialized nations has dropp2d by almost one -third,while U.S. civil aircraft export sales have increased ninefold, andnow sustain over 60 percent of the total civil aircraft work force.

Tcent of the nations 215 000 aircraft and 96 percent of its

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NAS A and General Aviation

More than 90 percent of all general aviation aircraft in th e worldoriginated in U.S.factories, and the market is growing substantial-ly as foreign commuter airlines are seeking new replacement air -craft. Foreign aircraft manufacture rs are producing new turbopropaircraft to me et the need, threatening the U.S. lead in the field.

In the past, NASAs input to general aviation aircraft technologyhas lagged behind military types and commercial airliners. Themajority oftodays fleet was built on the aeronautical science tha twas driving the aircraft industry before an d during World War 11.With the exception ofso me new conce pts developed by innovativeexperimental aircraft hom e builders, and applications ofadvancedaeronautical technology in a new generation of business aircraft,todays general aviation aircraft would not have looked out ofplace in the 1950s. As a matter of fact, several designs thatoriginated in th e late forties and early fifties are still being produc -ed a nd have performance equal to ma ny newer designs.

From its inception in 1915, the National Advisory Committeefor Aeronautics (NACA) and now NASA have been involved inaeronautical research a nd therefore have been a part of the prog-ress in general aviation (GA). For example, most designs useNACA airfoils. Recently, more than a dozen con tempo rary produc-tion and proto type GA aircraft have been designed with the aid ofnew technology gained from NASA programs of the past decade,

ranging from twin- jet business aircraft to single

-engine trainers.Since the beginnings of NACA, the goal has been to conduct

research that industry can use in building better products; butwithout the talent a nd engineering in GA compa nies , there wouldbe no American lead in general aviation. Th e goal at NASA is topreserve and advance that lead. American leadership in the field,however, is on the wane.

NASAs basic goals for general aviation are continuing im-provements in efficiency, safety, environmental compatib ility, andutility, broken down into research dealing with aerodynamics,structures, propulsion, and avionics. NASAs Langley ResearchCenter, in Hampton, Virginia, is responsible for the majority of

thos e technology programs in general aviation aeronautics and forflight research. Much of the flying is done out of Wallops FlightCenter, Virginia. Wind tunnel testing has been conducted primari-

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WhatIs General Aviation?

ly by Langley. In addition, the unique facilities of the AmesResearch Center at Moffett Field, California, primarily the40 x 80 -foot wind tunnel, are used for specialized GA research.Propulsion research is done at the Lewis Research Center inCleveland, Ohio.

These NASA centers have a battery of research facilities which asingle aircraft company could never afford, ranging from com -puters to h uge wind tunnels that can test a full-size airplane a s if itwere in flight. Each one of these research tools can produceresults that might not be obtainable in any other feasible oreconomical way. An initial NASA approach to a problem often isanalytical. T he next s tep may be the testing ofa small wind tunnelmodel, or of more elaborate models flown by remote control,either in an adapted wind tunnel or outdoors. Pilotedsimulators

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versatile ground-based machines that duplicate keycharacteristics of the full-scale aircraft-may be used. Finally, theidea may be tested in free flight on a full-scale airplane flown byNASAs research pilots.

Though NASA has some unique facilities and personnel, GAprograms frequently involve industry an d universities since NASAcannot do the work alone. lndustry has to translate research intopractical reality, and if the result will not sell in the marketplace, ithas t o be scrapped. Since the beginnings of aeronautics, univer-sities have been a primary source of fundamental researchcapabilities. Without the help and leadership offered in both

cam ps , NASAs research would not be reflected in a final product.As NASAs Dr. Walter B. Olstad said before Congress, Ou r reasonfor existence is solely to provide technological services to ourcustomers.

The continuing growth of general aviation and commuter airservice reflects their increasingly important roles in busine ss andin the transportation system. A major reason for this growth hasbeen the restructuring of truck a nd regional airline routes whichhas reduced air service to man y smaller communities. Since theU.S. has fallen behind in comm uter aircraft markets both at hom eand abroad, many c ommuter airlines have had to pur chase foreignaircraft to me et 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

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NASA and General Aviation

national determination in several countries to capture greatershares of markets once dominated by the U.S.

A major factor in the fut ure success of the American commute rand general aviation industry is the development of technological -ly superior next-generation aircraft that a re safer, more reliable,more comfortable, and more economical to operate than those ofthe competition. Unfortunately, industry does not have thetechnical personnel, facilities, or resources to generate the highlevels of technology needed. Th e availability of this technology intime to influence new aircraft in the next 10 years is, therefore,heavily depe ndent on NASAs programs.

NASAs 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, empha si s will cont inue to beplaced on safety and energy efficiency. Further, research isneeded on stall/spin phenomena of both high- and low-wing air-craft as well as less conventional configurations such as canard(forward) winged aircraft. One promising development hasresulted in the segmented, drooped -wing leading edge that canreduce the abrcptness and severity of the roll-off at the stall andthereby prevent unrecoverable flat spins. Long-range goals are toevolve design me thods tha t industry needs for safer aircraft, em -phasizing simulation an d analytical technique s.

Other aerodynami cs research is aime d at increased energy effi-ciency, including work on cooling drag reduction, enginelairframeintegration improvements, and further development of naturallaminar flow airfoils and aircraft technology. Smooth contoursand surfaces, achieved with composite materials, are needed topromo te laminar flow. Composites themselves are the subject ofagreat deal of research d ue to potential weight reductions of u p to30 percent, reduced dr ag, and more favorable structural integra-tion.

Propulsion research will concentrate on achieving greater fuelefficiency, reduced weight, lower maintenance, and greaterreliability. Work will continue on spark-ignition reciprocatingengines, but improvements are required on supercharged two-strok e cycle diesel engines and stratified charge rotary combus -tion engines. Turbos upercharger research will remain a n integralpart ofthe program. Basic research will conti nue into ignition an d

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synthesis techniques to improve aidground communications, ad -vanced displays, advanced terminal a rea syst ems evaluation, an dways of better interpreting and using weather dat a in flight.

An extensive crashworthiness program is leading toward im-proved crash safety and occupant survivability, covering threebasic areas. One is energy-absorbing structures for seats andfuselage floors to limit the loads imposed on the occupants in acrash. Another is the development ofstructural analyses to predictlarge 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 to second-generation load-limiting seatsand 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 t he crash-activation ofemergency locatortransmitt ers (ELT), which general aviation aircraft must carry, inorder to recommend improvements.

In agricultural spraying, research has been conducted to im-prove dispersal an d distribution ofchemicalsor dust applied fromaircraft. A primary part

ofthe research involved determining the

interactions between the aircraft wake turbulence and the dis-persed materials and how a modified aircraft might improve swathcontrol. Wind tunnel and flight tests were conducted andtheoretical prediction meth ods were developed that are now beingused by other government agencies, universities, and industry.

The challenges of general aviation will remain the subject of in-ten se research a t NASA, within industry, and within the ac ade miccommunity as its utility increases. The Commuter and GeneralAviation Research and Technology Program of NASA's Office ofAeronautics and Space Technology provides a glimpse into thefuture of aviation's most active arena.

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CHAPTER2

AERODYNAMIC EFFICIENCY

A the heart of NASAs general aviation research efforts liesthe desire to improve aircraft efficiency: in other words, to,explo re areas that will allow aircraft t o fly farther andfaster on less fuel with improved passenger comfort and safety.Basic to this goal is research dealing with high lift/drag airfoils,supercritical aerodynamics, natural laminar flow, composite st ruc -tures, and drag reduction.

LaminarFlow

As fuel prices climbed in the early 1970s, fuel efficiency tookon ever

-increasing importance and research efforts were inten

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sified to reduce aerodynamic drag. As a result, NASA has madesignificant progress in reducing parasite (zero-lift) drag and in-duced (caused by lift) drag. The drag reduction challenge is t oreduce skin friction, which accoun ts for about 30 to 50 percent ofan aircrafts drag at cruise. NASAs approach in reducing drag hasbeen multifaceted, including use of both laminar and turbulentboundary layer control. Keeping the boundary layer (the air layerclosest to the skin) on a surface laminar, or thin and smoot h, canreduce skin friction by as much as 90 percent. Very smooth andcontrolled pressure gradients c an delay the boundary layer transi -tion to turbulent flow and produce significant regions of natural

laminar flow (NLF).For years, designers of general aviation aircraft, particularly oflight single- and twin-engine types, have used the four -digit

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The Cessna Citation 111 uses the NASA supercritical airfoil loobtain ahigh-aspecl-ralio wing. The company found 11 could use relatively [hick wing sections andslillachieveefficienl Mach0.8flight. The thick section minimizes Lhe weightofthe high-aspect-ratio wing (ratioofwing span to wi dthorchord), and provides a large volume

for fuel storage. (Cessna Aircraft)

series of airfoils developed by NACA as far back as the 1930s.Asa matter of fact, very few aircraft flying today, including Cessnasfrom the Airmaster to todays Skyhawk and Skylane, incorporateairfoil technology radically different from that used so successful-ly prior to and during World War I I . One of the majordevelopments of the late 1930swas the six-series of laminar flowairfoils which, though not totally successful in producing laminarflow using the existing construction methods, went a long waytoward improving the aerodynamic art.

Wings, stabilizers, propellers, and control surfaces use airfoilsthat can be improved to promote laminar flow (less drag). Amodern, properly cons tructed laminar flow airfoil is generally con-sidered on e that can achieve laminar flow over the first 70 percentof the upper and lower surfaces over a reasonable range of angle

ofattack and speed/altitude.With laminar flow the air particles move in smooth, parallel

(lamina ted) layers over the surface, each with a constant velocityand motion relative to its neighboring layers. Turbulent flow

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Aerodynamic Efficiency

results when these laminated layers break up, resulting in higherfriction drag. The problem of maintaining extensive laminar flowis mad e more difficult by rivets, dent s, bumps, joint overlaps,manufacturing imperfections, and even b ugs tha t have hit th e air-foil. However, the problems inherent in achieving laminar flow areconsidered worth solving to obtain improved cruise speed s whichmean less fuel burned for a fixed distance.

After th e four-digit series NACA developed the five-digit airfoilsin order to provide airfoils with be tte r maximum lift. They were ap -plied to such well-known products as t he Beech Bonanza, Baron,and King Air. But these four- and five-digit airfoils, while providingincreased improvement s in lift, promoted turbulent flow becauseof their shape.

NACA resea rchers knew this to be th e case even in the 1930ssolaminar flow was singled ou t for more intensive investigation. Thesix-series airfoils, when achieving laminar flow, can reduce dragup to 50 percent over the older series but t he airfoil surface mustbe very smoo th (minimum waviness). In addition, due to the s harpnose radius th e six series does not have very high maximum liftcapability. The use of bonding and composi te materials (smoothsurfaces) in the last few years has opene d the d oors to achieve ex-tensive natural laminar flow.

In the 1950s,NACA left low-speed airfoil research to concen -trate on transonic and supersonic designs. Out ofthis research the

so -called supercritical airfoil, optimized for drag reduction a t highsubsonic cruise, had camber located near the trailing edge withimproved lift over drag (UD) at higher cruise lift coefficients. In theearly 1970s, a low-speed derivative of supercritical airfoiltechnology was developed for us e on general aviation aircraft.Though t he resulting GA(W)-l was not a true supercritical airfoil, itpaved 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 ofrough surfaces, but it did want a better turbulent flow airfoil. Inthis regard, NASA developed an airfoil with higher maximumlift- the GA(W)- I , now known as the LS(1)-0417-and improvedUD at climb

-out speeds. This was particularly beneficial for im

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proved performance (safety) on twin-engine aircraft with oneengine out.

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NASA andGeneralAviation

Both the Beech Skipp er and the Piper Tomahawk light trainerswere designed and built in the mid-1970s using the new airfoil.The result for both ofthe se aircraft was a very low stall spe ed , idealfor traffic pattern training. The tradeoff came with decreased effi-ciency at normal cruise speeds, but that was not t o be the primarypurpose of either aircraft since they would be used primarily fortraining near airports. Due to the aft camb er of the wing and theresulting high pitching mome nt , flap deflection was limited to pro-vide drag for approach angle control rather than to decrease stallspeed.

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

With improved construction techniques that could eliminatemost of the surface roughness problems which restricted laminarflow, NASA developed a new series of airfoils in the mid-1970sthat would combine the high maximum lift of the LS series withthe low drag of the six series. With the NLF(1)-0416 andNLF( 1)-0215F,natural laminar flow bec ame a realistic possibility.Even if extensive laminar flow does not occu r, high lift still existsand th e drag will be no higher than on a turbulent flow airfoil ofthesa me thickness.

The NLF(1)-0215F, flight tested in glove form on a T-34C,uses aflap that c an b e deflected upward 10 degre es at cruise, much likehigh-performance jet aircraft, t o achieve the best UD for both thelow- and high-speed regimes. This will be particularly beneficialfor the new generation ofGA high-performance, single-engine air-craft designed to cruise at 300 miles per hour at 25 000 feet. Anadvanced NLF supercritical airfoil was also flight tested on amodified F-111 over a range of wing leading-edge sweep anglesfrom 10 to 2 6 degrees at Mach numbe rs from 0.8 to 0.85. The ex-periment also showed that significant laminar flow could beachieved at off-design cruise conditions of Mach number andleading-edge sweep. This is particularly important for commuter

and business jet transport aircraft (see chapter 5).Cessna believes tha t 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

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visualization prog ram o n a 210 wing using chemical spray in theWichita St at e University wind tunnel to verify NLF poten tial . Witha 375-horsepower turbocharged engine, the 210 can reach over30 000 feet where high true airspeeds a re obtained.

NASA conducted a study of the benefits of cruise design op-timization for high-performance, single-engine airplanes with acruise speed of 300 knots and a cruise range of 1300 nauticalmiles 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 majorachievement would be the potential for 200 to 400 percent greaterfuel efficiency than is achieved by current twin-engine airplanes

capable of similar cruise speeds, payloads, and ranges.NASA conduc ted flight tests t o seejust how much laminar flow

was being achieved on eight different airplanes with smooth,thick-skinned, bonded, milled aluminum, or composi te wing sur -faces. Tests were conducted on smooth portions of the CessnaP210 and Beech 24R. Extensive tests also were conducted on theBellanca Skyrocket 11, Lear 28/29, and three Rutan Aircraft Fac -tory designs, the Long-EZ, VariEze, and Biplane Racer, whichutilize modern construction materials a nd techniques to provideaerodynamic surfaces without significant roughness andwaviness. Previous flight experiments involving laminar flowmeasurements were limited to either airfoil gloves or specially

prepared (filled a nd sanded) wing sections. Only sailpl anes wereachieving significant laminar flow without modification. For thelater tests, n o preparation of the aircraft was allowed; they wereproduction quality straight o ut of the factory.

The results were more than encouraging. There were extensiveareas ofNLF on all the aircraft test ed, making laminar flow a verypractical possibility on modern production aircraft. Even with pro -peller slipstream effects it was found that the laminar boundarylayers were not destroyed completely, changing previous conclu-sions. Thus, NLF airfoils may provide drag reduction benefits,even on multiengine configurations with wing-mounted tractorengines. On swept wings, particularly in the high -speed cruise

regime, spanwise flow contamination at cruise was found not to b eas much a concern for laminar flow. Laminar flow on winglets ap -pears very promising as well.

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Insect impact contamination has always been a problem inachieving laminar flow, but t he insect debris pattern collected on aNACA six series airfoil revealed only one-fourth ofthe strikes to beof sufficient height and in a position t o cause transition. A co m-bination of the newer NLF airfoil geometri es and mission profilescould minimize th e sensitivity even more. Natural laminar flow isno longer out of reach for nonspecialized aircraft; it can beachieved not only o n current production aircraft (up to 70 percentin s ome cases) but to a n even greater extent on future designswithout great expense .

Computers and Airfoil Design

The use of computers is one of the important factors in thedesign ofairfoils to reach the projected efficiencies. NACA used t oproduce catalogs of airfoils that designers would ponder over,test , retest, an d finally apply to an airframe. It could take up to on eweek using a mechanical calculator to come up with a singlepressure distribution; the s am e calculation on a modern c omput ertakes one second. A designer ca n load in cruise speed, stall speed ,weight, lift coefficient at various speeds, and other aerodynamicdata, then custom design a unique airfoil from NASA-developed

codes. The structure can be optimized rapidly, with less emphasison percentage thickness of the wing, and the airfoil will moreclosely suit the purpose. New GA aircraft such as the MooneyM301 have derivative airfoils developed from NASA code s. Perfor-mance verification of t he comple ted des igns can then be tested inmodel form in NASA wind tunnels o r can b e teste d full scal e in th e30x 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 aminicomputer. Though t he program is limited t o som e degree, itis certainly a star t in placing airfoil design at the fingertips ofmostpeople around the world who have access to a minicomputer thatwill program in BASIC on an 8 K core.

An airfoil design institute has been established by NASA atOhio State University using a NASA-maintained data base. Anycustomer can obtain an optimized airfoil section, drasticallyreducing the time needed t o advance the state ofthe art. This can

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The black painted wing on this Bellanca Skyrocket I1 allowedNASA/OSU research-ers lo visualize chemicals applied to measure natural laminar flow (NLF). Theair-crafl, conslructed of fiberglass composiles, proued lo be extremely smoolhaerodynamically, with laminar flow extending from lhe wing leading edge for ap-

proximately lhe first50percent ofthe wing's surface. This greally aduanced NA SANL F research.

have a significant effect on reducing costs in the general aviationindustry.

Without a stable NASA research base, many US. companieswould find themselves unable to p roduce aircraft competitive withthe world market. A good example is the Learjet business jet,

which was designed and developed from a Swiss fighter in the ear-

ly 1960'sby a grou p of less than 100 engine ers with little wind tun-nel testing, limited computer facilities, and little specialized

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Using the NASA/Ohio State University airfoil program, Mooney Aircraft incor-porated a natural laminar flow wing on its new pressurized 301. Cruise speeds of260mph (Lop speed301 mph) aL 25000feet wi ll give a 1 150statute mile rangeaL14.5miles per gallon. NASA' sparticipalion in computer and windLunnel Lestingcon-tributed Lo this breakthrough forasix-seal generalauiation aircraft. (Mooney Aircraft)

equipment necessary to develop a high-

performance aircraft. In-

stead, Bill Lear and his associates relied heavily on the basictechnology that NACA and NASA had developed over decades ofresearch. Learjet wings an d tails employed modification of NACAairfoil secti ons developed in the late 1930'sand early 1940's.Oneofthe first Learjets built was mounted in NASA Ames' 40 x 80-footwind tunnel for testing in 1966, yielding a great deal of data onthis first business jet. Th e wing flap design was developed directlyfrom th e experimental workof NACA.

Winglets

Though several Rutan designs have employed NASA-developedwinglets to excellent advantage, the Gates Learjet Model 28became the first production aircraft to fly with them. Now theModel 29 and 55 Longhorn a re flying with the winglets. Cruise fuel

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Among the engineering challenges incorporated in the Gulfstream Ill wereNASAwinglets and airfoil, along with composite primary structures. Drag improvement

was significant, resulting in better fuel consumption and higher performance.

(Gulfstream Aerospace)

flows on the Model 28/29 are around 26 percent lower for thesame payload/speed combination. Takeoff and landing perform-ance also has been significantly improved. These improvementsresulted from several factors: the aircraft can operate at higheraltitudes due to the wing extensions (6 feet plus winglets eachside) where fuel consumption is reduced, and drag has been re-duced by removing the tip tanks , increasing the aspect ratio, andadding the NASA-developed winglets incorporated on theGulfstream 111 a s well with grea t success.

The winglet itself reduces vortex drag by producing a forwardlift component somewhat similar to sails on a boat. At high liftcoefficients this effect more than offsets the drag du e to th ewinglet itself. Winglets also significantly increase the lifting sur -face or aspect ratio of the wing, though a penalty of additionalstructural weight must b e considered. While there is great poten -tial for winglets on many different designs, each winglet must betailored for each design in ord er to achieve the desired results.

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cNAS A and General Aviation

Gulfstream Aerospace carried use of NASA researchouer inlo their CommanderFanjet 2500wi th the possibilityofnalural laminar flow wings and winglets. Winglelsimprove wing span efficiency by positiue control of the tipvortex flow field, reducingdrag due to lift. The cockpit of lhe 1500also will incorporate much ofthe NASAsingle pilotIFR research. (Gulfstream Aerospace)

Composite MaterialsSince 1970, NASA has actively sponsored flight service pro -

grams with advanced composite materials-essentially wovencloth impregnated with fibers of carbon or other high-tensilestrength materials-particularly on commercial transports andhelicopters. Composite aircraft structures have the potential toreduce airframe st ructural weight by 20 to 30 percent, reduce fuelconsumption by 10 to 15 percent, and thus reduce directoperating costs (DOC). NASA, primarily through its AircraftEnergy Efficiency (ACEE) program, has te sted all t ypes 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 gene ral aviation can be significant, as shownby Rutan Aircraft Factory and another Rutan company, ScaledComposites. They have built and flown several aircraft that are

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basically total composite structures and that have been provenboth s afe and very fuel efficient. Not only are composit es extre me-ly smooth, lending themselves to laminar flow, but they can bemade stronger and lighter than most aluminum designs. Crash -worthiness remains a NASA concern since composites have beenlabeled as too brittle and unable t o absorb enough energy during acrash. However, new construction techniques have helped com -posites absorb the impact ofa crash. NASA hopes t o crash com -posites in controlled tests, possibly with an all -compositehelicopter prototype.

One of the major problems in using composites centers aroundreducing production costs. Small GA comp anie s (at least in rela-

tion to the larger military and transport manufacturers) wouldhave to find major s ales oppor tuniti es in orde r to recoup retoolingcosts. In the far term, the ideal is that composite manufacture canbe automated far more than with metal construction techniques.

The Lear Fan madea radical deparlure frompast general auiation aircrafl since itwas builtalmostentirely ofcomposiles , promoting light weight and low drag. Itused

slightly more than200gallonsoffuel to lrauel 2000miles, while cruisinga1 400mph. (Lear Fan)

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In the near te rm, manufacture rs are looking for ways to eliminatelarge autoclaves required t o set u p the materials and go t o thermalstampi ng and forming. In other words, composit es could b e simp -ly heated and stamped out. Inherent in any transition to composit emanufacture is retraining of workers, a major expense regardlessof how efficient construction methods are.

The most aggressive use of composites and new NLF airfoilscan be seen in Bill Lears last design before his death, the LearAvia Learfan 2100, built almost entirely of graphite composites.The only metal com pone nts other than t he engi ne are the landinggear, propeller hub, and many small structural fittings. Even the

propeller blades are made of Kevlar-epoxy. With a customcomputer-designed airfoil and all -composite const ruction , it has a

maximum cruise speed of around 360 knots. Unfortunately, thecompany folded after being unable to obtain FAA certification.

Two other all-composite aircraft that do appear to be headingfor production are the Beech Starship and the AVTEK 400. Bothare twin-turboprop designs with canard surfaces and high cruisespeeds. Fuel econ omy and efficient payloads are major benefits ofthes e aircraft which are radical depart ures from the GA norm, verypossibly the sha pe of the future.

Gates Learjet (not related to Learfan) is looking into its next-generation product as suitable for secondary composite struc -

tures. Current Learjets utilize fairings that ar e composit e or com -posite based with new fiberglass/Nomex core materials. Learjet isalso looking at primary composite use on forward swept wings,but the cost of building th e wings is still too high for the few unitsthat would be produced. The compan y is looking for ways to bringthe price down since compos ites are the only avenue to creating awing stiff enough to resist the strong torsional/bending momentsinherent in forward swept wing designs.

Cessna Aircraft Company incorporated a substantial amount ofNASA research in its new Citation I l l busines s jet, which features aswept, 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 to delaying the drag riseshock wave effect, but the sm ooth bonded surface promotes s om elaminar flow. Kevlar, graphite, Nomex core, and fiberglass com -posites are being used for flap sections, spoilers, engin e nacelles,

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Aerodynarnic Efficiency

Gales Learjel is among the companies fhat dependonNASAfor advanced research.Gates builds several versions or the Learjel, including the Model 55 Longhorn, sonamed because ituses NASA winglets. (Gales Learjel)

seat structures, and s om e fairings and doors. Both t he CanadairChallenger and th e Mitsubishi Diamond 1 have supercritical, high-aspect -ratio wings as well, reflecting NASA's pervasive influenceon aerodynamics regardless ofcountry of origin.

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CHAPTER3

INTERNAL COMBUSTIONENGINES

rom the early days of aviation the argument betweenaerodynamicists and engine specialists over what drivesF aircraft design has never been satisfactorily resolved. Does

aerodynamic function determine needed power or does availablepower determine aerodynamic form? Though the de bate will mostlikely never be resolved, eng ine development has usually openedthe door for advances in aerodynamics, for without power aircraftcann ot fly. Needless to s ay, there are exceptions to this rule, par-ticularly in soaring and aircraft that b ecom e gliders like the SpaceShuttle.

Overall, there have been no major design changes in generalaviation aircraft engines since World War 11. The air-cooled, inter-nal combustion engine fueled by high-octane gasoline continuesto power many ofthe new designs rolling offthe industry's produc-tion line. Major changes are needed t o increase fuel efficiency andutility, from the smallest trainers to the largest multiengine cor -porate transports and comm uter airliners.

While general aviation manufacturers have been seeking im -proved safety and improved air traffic control, the heart of theirsurvival as companies lies with energy efficiency. If aviation gas(avgas) becomes unavailable or unreasonably expensive, thenthe se aircraft will have to be adapted t o use autogas, diesel, or jetfuel. Based on relative Btu content and production costs pergallon, the kerosene/diesel-type fuels also offer an inherenteconomic advantage of 20 percent or more over gasoline. With1985 avgas prices over $2.00 a gallon in the U.S., over $5.00 a

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gallon in Europe, and just plain unavailable in many parts ofAfrica and th e Middle East, t he writing is on the wall.

Environmental Concerns

The heart of NASAs general aviation propulsion research canbe traced t o an initial concern with meeting s tandar ds set by theEnvironmental Protection Agency (EPA) in the early 1970s. Anexhaust emissions reduction program was initiated in 1973 be-tween th e Federal Aviation Administration and NASA. A year laterTeledyne Continental Motors an d Avco Lycoming were under con -tract for emission -reduction testing through a number ofavenues.The 1974 fuel crunch then changed priorities to fuel economy,sending industry and NASA off a springboard that led to the ad -vanced engines discussed in chapter 4.

Even though most of NASAs current engine research isdevoted to improving efficiency, noise and exhaust emissions ingenera l aviation power plant s remain a conce rn, particularly si ncethey are perceived by the public a s major annoyances. There areabout 14 100 suburban airports in the U.S., most of which arelocated in small communiti es with no buffer zones and with peo-ple living nearby. Therefore, general aviation has the potential fo rgreater communit y reaction t o noise and pollution than commer -cial and large transport aircraft. In addition to the communitynoise, passengers and crew experience a great deal of noise andvibration, particularly in the smaller aircraft.

The exhaust emis sions reduction program continued with bothTeledyne Continental and Avco Lycoming. These enginemanufacturers successfully pursued program goals even thoughthe EPA lowered its emission standards in 1979.

Teledyne Continental Motors investigated and developed threeaircraft piston engine concepts to reduce emission of hydrocar-bons and carbon monoxide while simultaneously improving fueleconomy by improved fuel injection, improved cooling cylinderhead, a nd exhaust air injection. Variable ignition timing also wasexplored. After investigating each sys tem and incorporating theminto a 10-520 engine, the company conducted test flights with anew Cessna 210 Centurion single-engine aircraft.

The 210 handled and flew well with the prototype engine; the

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Internal CombustionEngines

only adverse result was backfiring in the exhaust after rapid throt-tle closing, a problem easily solved by shutting offair injection atlower than normal manifold pressures. For a 328.8-statute -mileflight, the prototype engine with standard fixed magneto timinggot 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 airinjection alone with no improvement in fuel economy or by usingonly the Simmonds improved fuel injection system to provide aleaned fuel schedule . The use ofexhaust air injection in combina-tion with exhaust port liners reduced exhaust valve stem

temperatures to levels below that of the baseline engine, whichcould result in longer valve guide life. Use of exhaust port linersalone can reduce cooling air requirements by at least 11 percentor a 1.5percent increase in propulsive power. A fixed ignition tim-ing of 27 degrees BTC (5 degrees over standard) provided a testbed fuel economy improvement of 2 percent in cruise but thiscould not be substantiated in flight testing.

The basic purpose of the contract was met by improving fuelmetering for better fuel-air ratio control, reducing heat transferfrom exhaust gases to cylinder heads, and oxidation of exhaustpollutants, even though the hardware used may not represent themost cost-effective mean s of obtaining the se results. The fuel in -

jection system could prove too expensive for production, and ther-

mal barrier coatings or improved exhaust port design might pro-duc e benefits similar to those obtained with port liners at less cost.A similar argument could be made for oxidation of exhaustpollutants 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 hecompany did not include flight testing as part of the program, itused three different engines fo r 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, andvarious other spark plug configurations. Test results revealed tha tnone of these resulted in a consistent, significant improvement

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over the stand ard ignition system. Neither th e spark duration norspar k plug tip locations were considered to be limiting factors forimproving performance or emissions.

Ultrasonic fuel atomization gave a significant improvement incylinder to cylinder mixture distribution at on e engine conditionbut there was no improvement in total engine emissions, fueleconomy, or performance. As a matter offact, a3 to 5 percent lossin rated full throttle engine performance resulted from the addi-tional manifold restriction of the ultrasonic unit.

Opti mum valve timing seque nces for each specific engine co n-dition, rather than only at rated power output, revealed little im-

provement over the standard high-

speed valve timing alreadyemployed in th e companys engine s even though improvements inengine performance up t o 13 percent at certain off-rated condi-tions were obtained. Over the ranges tested and at constant fuelair ratio, valve timing did not influence carbon monoxide emiss ionlevels. Additional testing, conducted to evaluate the effect of in-duction system tuning on th ese results, showed that performanceimprovements of the same magnitude could be accomplishedwith standard valve timing and a revised induction system. Con-sidering the magni tudes ofthe overall performance improvementsand d ue to th e greatly increased complexity of the variable valvetiming system, induction system tuning was determined a more

viable concept for improving overall engine performance.

Noise

NASAs general aviation noise research has been conductedthrough 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 do notproduce a s much noise as turboprops and piston-engine GA air-craft.

Prop-driven light aircraft have been a source of NASA researchfor many years. One of t he earliest programs to deve lop a quiet

aircraft led to a five-bladed p rop tes t bed with extensive muffling;th e 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.

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Internal Combustion Engines

Propeller noise prediction, comparin g m easured a nd calculatednoise, was quantified using a Twin Otter. Sound pressure level wasunderstood as a function of frequency expressed in multiples ofthe blade passage frequency. A microphone mounted o n a boomon the aircraft's wing was used t o take acoustic data. Wind tunneltests were also conducted.

A Rockwell 500Bwas experimented on to obtain data b ut it wasnever flown. With source noise left to other departments, theacoustics team worked o n sidewall design and treatment as theirprimary area of research. Working with Rockwell, noise wasmeasured outside and inside the standard aircraft. Varioustreatments were made to lower the noise level, but it was still

unclear as to what improved noise levels. The present push is todiscover why noise does what it does rather than how.

AEROACOUSTICAIRFOILS

CIRCULATION-CONTROL3PUSHERS

ELASTIC PITCH

CHANGE

PROPLETS

JL!BJ-BWIES

ADV.ANALYSIS

p1,--'--IIINTEGRATED

BLADE/SP lNNERCOMPOSITESlduanced general aviation propeller concepts

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Advanced general aviationand high-speed iurboprop propellers offer cruise efficien-cy trends that could increase fuel economies.

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

cabin.Research efforts have been centered around prediction ofstruc-

turally transmitted noise and development of noise controlmethods involving control of both noise radiated from panels toth e aircraft interior as well as noise transmitted through the e nginemounting vibration isolators. Fuselage sidewall transmission is

very important for those aircraft with wing-mounted propellersoperating close to t he fuselage sidewall. An Aero Co mmand er 680was modified with 15 pounds of asphalt type, glue on mass. Theresults indicated that even a modest amount of added mass may

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InternalCombustionEngines

reduce interior noise by 4 to 15 decibels depending on the fre-quency of the noise.NASAs overall priorities have reflected the industrys needs by

addressing energy efficiency directly, both by improving fuel con -sumption and looking into burning less costly, readily availablefuture fuels. Lewis Research Center scientists an d engineers havecentered their research on improvements to existing engines, ad-vanced intermittent internal combustion engines, rotary powerplants based on th e Wankel engine, stratified cha rge power plznts,cooling drag improvements, and advanced propellers. The newgeneration of more efficient turbine and turboprop engines iscovered in chapter 4.

Ideally, the advanced intermittent internal combustion enginesshould be able to use any ofa number of fuels, primarily jet fuelsince aviation gasoline conti nues to be a very small portion of thefuel produced by the major oil companies. These engines shouldburn less fuel more cleanly in a power plant of lower weight.

The rotary and two-stroke, stratified charge engines a re beingtested 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 m ade by designing the

fins to optimize size and spac ing, coupled with cowlings redesign-ed for optimum aerodynamic efficiency. Th e end result ha s beenless drag and maximum cooling.

Improving thePiston Engine

While turbine engine development has been actively pursuedsince the 1940s, piston engine research as a whole has beenfrozen since World War I1 with no major advances over the air-cooled rec iprocating engine. With work progressing on the GATE(general aviation turbine engine) program, NASA decided to open

the doors for the first time in 40 years on near-term improvementof conventional air-cooled spark-ignition piston engines and onfuture alternative engine systems based on all new spark-ignition

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NASA and General Auiation

piston engines, lightweight diesels, and rotary combustionengines.

NASA's conventional piston engi ne research involves applyingexisting technology to improve fuel economy by 20 percentthrough leaner operation, drag reduction, and flight at highaltitudes where fuel ec onomy is improved. Reduction of exhaustemissions, improved cooling and installation drag, improved fuelinjection systems, and advanced turbochargers are part of theseefforts as well.

Under a NASA contract Teledyne Continental Motors beganresearch and development of methods to improve fuel economy

and reduce the exhaust emissions of its aircraft piston engines.Four concep ts emerg ed permitting leaner operation an d reducedemissions of hydrocarbons and carbon monoxide : (1 ) a timed, air-density-compensated fuel injection sys tem to replace the familiarlow-pressure continuous-flow sys tem; (2) a t hermal barrier exhaustport liner for improved cylinder head cooling; ( 3 ) air injection,when combined with t he exha ust port liners, that reduces exhaustvalve stem tempera ture s below those of th e baseline engine whileincreasing oxidation in the exhaust; and (4)variable spark timingto mainta in best power spark timing over a broader engine revolu-tion-pe r-minute operating range.

Compared with the standard 10-520 engine, the version with

these four concepts met EPA standards along with a 10 percentimprovement in high-performance cruise fuel economy. After be-ing tested, the TSIO-520BE version now being installed in thePiper Malibu utilized many of these improvements along with dualAiResearch turbochargers run from separa te exhaust manifolds. Acommon exhaust manifold would have been subject to pressurewaves from one bank pulsing into the ot her, causing detonation.Divorcing the exhaust for the turbochargers has allowed leanermixtures without exceeding allowable peak exhaust temper atures.Aftercoolers also helped since induction temperatures werelowered from 300" F to 115' F, further reducing the deton ationproblem. Not only were exhaust emissions down, but the turbos

an d slower prop revolutions per minut e are natural noise reducers.effort was launched at Lewis and Ames to

develop and demonstrate performance and economy improve-ment ofpiston eng ine aircraft via reduced cooling and installation

In 1979 a joi

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InfernalCombustionEngines

drag. Contemporary engine cooling and installation designs ar ebased in part on technology and d ata developed for radial engine sin World War 11. This data base is not ad equate for precise designof an en gine installation using the comm on 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 twinwas mounted in the Ames 40 x 80 -foot wind tunnel for full-scaletests. A cooling drag penalty of 13percent of total airplane cruisedrag was found, defining a baseline for horizontally opposeddesigns 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 penaltyby at least 50 percent. Next, a propeller driven by an electricmotor and various cooling inlet openings were tested. The pro-peller slipstream reduced flow separation over the aft part of thenacelle and at the inlets, leading to a marked reduction in totaldrag . When t he inlet area was reduced, dra g increased d ue to inletspillage, 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 improvementwas around 20 percent for the production (large) inlet and even

mor e for the smaller inlets. These improvements were partially aresult of pressure rise related t o propeller slipstream, but the ma -jor effect was the reduction in the amo un t offlow separation insidethe recontoured inlet at higher angles of attack.

Next, an actual engine was installed in the nacelle with threecooling air inlet sizes. Tests were run over a freestream velocityrange from 50 to 150knots, an angle-of-attack range from 0 to 10degree s and a cowl-flap deflection range from 0 to 30 degrees.

Tests were also run using exits for the cooling air, located on t hesides of the nacelle instead of the usual cowl flap exit under thenacelle. The exit most forward of the wing resulted in least dragbut with a lower flow rate than that with the cowl flap 30 -degree

configuration.If

cowl flaps could be eliminated, cooling dragcould be reduced 7 percent, and it was found through surfacepressure measur ements o n the nacelle exterior that the pressure

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An electric molor-driven propeller was used to delerminefinal results in the coolingdrag tests. Important changes have been recommended in nacelle shapes and sizes.

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

Further, inlet designs providing higher pressure recovery anduse of improved cooling exits integrated into a lower drag nacelleproved beneficial. By adding a diffuser to the inlet, pressure

recoveries of up to 95 percent were de monstra ted. That improve-ment alone could eliminate most of the need for a cowl flap andthereby save up to 3 percent of total aircraft drag. The high drag is

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mainly a result of the current blunt shape and sh arp corners of thefront of t he nacelle, which cause an increase in boundar y layerthickness, resulting in flow separation around th e inlet and behindth e nacelle.

Thoug h this first cooling drag program is now over, the door hasonly just opened since a computer simulation code is needed tofind optimum external nacelle shapes and inlet geometry. ThenNASA can begin to conduct computer simulations of new design sand t o what extent they can reduce that typical 13percent coolingdrag on GA aircraft.

Studying the combustion process itself has been of major in-terest to both NASA and industry. Combustion-diagnostic in-

strumentation has been designed at Lewis to determine on a percycle, per cylinder basis, real -time measurements of the indicatedmea n effective pressure and percent m ass of char ge burned a s afunction ofcrank angle. These s ystems are being used by both theaircraft and automotive industries. Ionization probes , placed in thecylinder head to measur e flame position an d thickness as a func-tion of crank angle, have proven valuable. Laser Dopplervelocimetry (LDV) meas ure men ts of the velocities and turbulencelevels for cold flow within the combustion chamber have beendeveloped through a grant to Carnegie-Mellon University. Aunique 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 instudying the role of turbulence and gas motions in combustionchambers. A principal goal is to formulate a general mass ofcharge burned equation that includes engine air-fuel ratio, spe ed,and torque. Out of this has c ome development of a theory andmultidimensional compu ter cod e for future engine design and im-provement.

Research also is being conducted t o improve the inlet-port fuelinjection system by extending the lean limit, requiring a morecomplet e understanding of the relationship of the fuel-air mixturepreparation before induction into the combustion chamber andoverall engine performance. Tho ugh pas t investigations have sup -ported a well-mixed, homogeneous charge for lean operation,General Motors researchers have found that a wetted intake

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cha rge with fuel droplets and possibly with bulk stratification maybe optimum for lean combustion.

Spray nozzles have been investigated to study the physical s ta teof the fuel-air mixture through laser particle field measurementsof different injectors. Lewis has also been conducting manifold-flow visualization tests with the cylinder head of a TSIO-360engine. High-speed pho tos taken through fiber optics have provenexcellent in this diagnostic program, leading to hot performanceand emission tests.

Another important series of investigations has been conductedunder the high-altitude turbocharger technology program. Si nce

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 powerfrom a given engine displacement and it can maintain that powerfrom sea level to high altitudes. NASA initiated a program todevelop a family of advanced but cost-effective turbochargers a p -plicable to a spectrum of conventional and alternative engines,emphasizing near-term improved spark-ignition engines as thebase I ine.

Analysis through verification testing was planned. GarrettAiResearch and Avco Lycoming have been heavily involved indesign studies for advanced turbochargers on future engines. With

a Lycoming engine currently flying in the high-altitude MooneyM30 and a Continental mounted in the high-altitude Piper Malibu,

both using Garrett turbochargers, the development of turboc harg-ing appears to have entered a new era.

Advanced Concepts

Although current aircraft engines operate at high levels ofeffi-ciency and reliability, changing requirements in terms of fueleconomy, fuel availability, and environmental concerns havebrought about ideas for significantly improved or completely newtypes ofengines for future aircraft. NASA has addr essed the i ssue

through a series ofconceptual design study contracts with enginemanufacturers that should lead to radical departures from40 -year-old design philosophies.

Over the past 50 years the spark-ignition aircraft piston engine

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has proven so safe and reliable that major design chan ges haveco me slowly. Presently, thi s desig n serves as prime mover for 93percent ofthe nearly 200 000 active aircraft in the general aviationfleet. Rising fuel prices coupled with the possibility of reduced fuelavailability have added impetus to the search for advanced pistonengines that can preserve the increasing utility of this vital seg -ment of the U.S. transportation system.

Fuel availability surfaced as one of the mor e important factorsinvolved with future engines. NASA did a thorough survey of thepast, present, and future of the energy industry, not only of thetechnical aspects of development of primary energy sources, bu talso of the econ omic, social, and political trends that might affectchoice of a future fuel. Assuming the technology to exploit suchresources as oil shale and coal, the study concluded thatpetroleum-based fuels would be around for a long time to come.

There were two prospects identified for advanced engine fuel:continued use of 1OOLL avgas and kerosene-based commercial jetfuel. Low-lead 100octane avgas, for use in the near term, dictatesuse of a homoge neous ch arge combustion system similar to thatused today. For the far term th e move away from specialized avia-tion gasoline will have t o be ma de since it is less than 1 percent ofall the gasoline produced in the United States. Future use ofjetfuel suggests a stratified char ge combustion system.

Stratified Charge Engines

The term stratified charge refers to two levels of fuel richness ina combustion chamber. A lesser charge of a rich mixture is ig -nited, which the n fires the remainder ofa charge that is too lean toignite easily. A longer technical description has fuel injectedacross an ignition source tangentially into a rapidly swirling airmass (the rotary does this by its inherent geometry). With im -mediate ignition assured by a positive ignition source, the fuelthen proceeds to burn smoothly at a rate controlled by the fuel in-

jection. There is then no ignition delay period and no question ofknocking 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 anadiabatic engine. The ultimate limit for rapid heterogeneous com-

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NASA and General Auiation

bustion is expected to occur when the fuel as well as the air andcombustion chambe r surfaces are heated t o a highly combustibletemperature. This eliminates the physical part ofthe ignition pro-cess (droplet evaporation) so that the combustion rate is then con -trolled by mixing processes.

The two advanced engi nes decided on were similar with the e x-ception offuel and combustion system. The combustion chamberin both the moderate -risk an d high-risk engines was redesigned. Inthe 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 chargesystem injected jet fuel at high pressure directly into the combus -

tion chambe r just before the piston reached top de ad center.The new design was labeled HTCC or high-turbulence combus -

tion chamber. A bathtub -shaped recess was cut into the firedeck inthe exhaust valve area. A specially shaped pass age leading fromthe intake valve area to a corner of the bathtub produced an ex -tremely strong air swirl motion when the piston neared top deadcenter. This motion, combined with the HTCCs high compressionratio, resulted in high flame spee ds. In effect, it beca me poss ibleto ignite and burn mixtures that otherwise would be too lean tosuppo rt stable combustion. T he rapid burn and high initial co m -pression of the HTCC resulted in increased peak firing pressures.With the HTCC chamber, NASA demonstrated the detonation-freeoperation of a ho mogene ous c harge, 6-cylinder engine at a c om -pression ratio of 12:l compared with 8.5:1 for a standard engine.This increase in compress ion ratio had the effect of improving fueleconomy at cruise powers by 7 percent. Teledyne Continentals520-cubic-inch engine with the HTCC proved quite capable ofeven better performance figures with further research, leadingeventually to a stratified charge version which would burn jet fuel,liquid propane gas, or alcohol.

Power can be taken in several ways from the waste exhaustgases of an internal combustion engine, among th em turbocharg -ing, as discussed earlier, and turbocompounding. NASAs ad -vanced engines have been earmarked for both systems. Turbo-compounding is not a new idea since it was used in the3000-horsepower engines ofthe post-World War I I era but t o applyit to 350-horsepower engines consti tutes advanced technology.

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EXHA

STANDARD HTCC

8.k1COMPRESSIONRATIO

HIGH

SWIRL-

1201COMPRESSION 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 andpass through a power turbine, which transmi ts power back into theengine crankshaft through a s peed reduction unit. Th e gases thencarry their remaining energy to a turbocharger, maki ng it possibleto extract one horsepower for every pound of weight added,enhancing efficiency.

Both the moderate-risk an d high-risk engines will be adap ted toelectronic control of all operational sys tems. This means that thepresent three levers now used to control engine revolutions perminute, manifold pressure, and fuel mixture will be combined intoa 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,thu s enhancing safety.

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NASA andGeneral Auiation

Advanced materials have been earmarked in both engines fo rweight reduction an d increased durability. The baseline TSIO-550represented the current level of technology with a weight of585pounds, while the moderate-risk 420-cubic-inch engine camedown to 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 ofthese meta ls in conjunction with 10 pounds ofad -vanced materials.

The high-risk engine, also 420 cubic inches, used only 80pounds of steel, primarily in the crankshaft, reduction gears,cylinders, and exhaust valves. Aluminum was reduced somewhatand a total of 119 pounds ofadvanced material s was incorporatedfor an engine weight of 405 pounds, a 31 percent improvementover the present-day engine. In this engine the grea test part of theadvanced material weight was titanium, with a small amount ofreinforced plastic and ceramics.

All three engines were rated at 350 horsepower, cruising at2 5 000 feet at 250 horsepower. Service ceilings of the advancedengines were increased to 35 000 feet compared with 25 000 feetfor the current engine. The t ime between overhaul (TBO), 140 0hours on t he current engine, was increased to 2000 hours for th eadvanced engine. To get an idea of the fuel economy im -provements obtained, compare the power wasted in the exha ust ofthe three engines. The current technology engine dumped theequivalent of319 horsepower out the exhaust a t maximum cruisepower. For the moderate-risk engi ne this loss was reduced by 33percent to 214 horsepower and by 51 percent to only 156horsepower 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 acurrent single-engine aircraft designed for the high-risk engine.The present technology eng ine resulted in a range of51 8 nauticalmiles while the moderate -risk engine achieved 8 14 nautical miles,an increase in efficiency of 32 percent. With the high-risk engine,efficiency increased by 49 percent.

Intermit tent Combust ion EnginesWhen the Envi ronmental Protection Agency first told th e FAA it

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tion engines, NASA was tasked to do the research starting in1973. By the time Avco Lycoming and Teledyne Continental,along with in-house NASA studies, came up with alternative GAeng ine possibilities in 1 97 5 the fuel crunch had hit. Fuel econ omyand alternate fuels became the drivers for developing the enginetechnology.

By 197 7 advanced air-cooled, spark-ignition engines, diesels,rotaries, an d external combustion engines were being developedfor testing, although the latter was dropped since it was veryheavy. Th e central focus of this i ntermittent combustion (IC) classof engines was use ofjet ker osene a nd similar fuels with high effi-ciency. By 1982 NASA dropped further aircraft gasoline engineresearch in favor ofa horse race between turbine and IC engines tosee which would come out as the most efficient. The contest hasyet to be decided, if it has to be decided a t all, since both types ofengines have excellent futures.

Under NASA contracts, Teledyne Continental worked o n a n ad -vanced spark-ignition engine of conventional configuration;Teledyne General Products came up with a compact two -strokeradial piston diesel; and Curtiss-Wright analyzed a lightweightliquid -cooled, stratified charge rotary eng ine.All the iC candidates delivered improved performance when

compared with current production gasoline engines and ahypothetical, highly advanced but unregenerated turboprop. Therotary a nd t he d iesel, very close in mission fuel and aircraft weightsavings, provided the best overall performance with dramaticfuel/weight savings ofabout 43 percent. The baseline engine usedavgas while th e oth ers were jet kerosene burners. Considering thedifferences in energy content , density, and c ost between keros eneand avg as, the savings can be extend ed by another 10 to 15 per-cent for a more realistic measurement of the e conom ic benefits.

The rotary eng ine emerge d a s the overall top -choice IC engine,with the diesel a strong second. As far a s performance, th e rotaryhad only a small advantage but predicted passenger comfortlevels (based on low vibration) emerged a s significant positive fac-tors in the overall evaluation, which only the turboprop couldmatch.

Rotary and diesel engin es are also being considered for growthto supply both the GA and co mmuter markets . Both engines have

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been labeled adiabatic/turbocompound or ATC in reference to th emos t desirable versions of these engines , which use ceramics toblock most of the heat transfer that would otherwise g o into thecoolant and a compounding turbine to recover a portion of thethermal energy.

Automotive adiabatic (or noncooled) diesels have been undertest for so me t ime, particularly for the U.S. Army. Cummins DieselEngine has had an Army truck on the road with a 6-cylinderengine that has n o radiator, fan, water pum p and tank, expansiontank, water hoses, fan belt and pulleys, air scoops or openings,and related equipment. The ceramic cylinder liners, pistoncrowns, firedeck, and other internal insulated components were

able to contain the high-temperature combustion gases, blockoffmost of the heat transfer to the emp ty cooling jackets, and direct

the very hot exhaust gases to a turbocharger and compoundingturbine. Though too large for aircraft application, the newtechnology represents a major technical brea kthrough that can beincorporated into aircraft engine technology.

Rotary engines have finally become energy efficient. ToyoKogyo has overccme the early rotary auto engines excessive fuelconsumption with its Mazda cars. Curtiss-Wright, under U.S.Navy/Marine Corps sponsorship, developed a large stratifiedcharge multifuel rotary marine engine. Two fuel injectors, pilotand main, are located in the wasp -waist region of the trochoid

housing near the top center position. The pilot injector sprays in asmall amoun t offuel over a high-energy multiple discharge sparkplug, establishing an ignition tor ch, which continuously ignitesthe main fuel charge as it is injected. The motion of the rotor in thetrochoidal housing is such that the air motion in the injectorregion always proceeds in the downstream or leading direction.By tailoring the main injector flow to the instantaneous airflowrate past the injector station, a stationary flame front is estab-lished under the wasp-waist. The rotor merely pushes the airthrough the flame front by virtue of its inherent motion an dgeomet ry. The ignition torch is energetic enough to immediatelyignite 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

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U

- - _- . _ _ -- - I- - _ - -Principle of the Rotary Engine

1-4 Intake5-9 Compression

10-12 Power13-18 Exhaust

Rotary combustion engine basics

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InternalCombustionEngines

weighs under 1900 pounds an d, without turbocharging, develops1500 horsepower at only 3600 revolutions per minute. Based onC-W and NASA calculations, this power output could be morethan doubled by turbocharging plus a slight increase in rotationalspeed. A four-rotor engine of this type could very possibly develop4000 horsepower or more without weighing more than 2000pounds.

Of more immediate interest is the twin-rotor version that hascome to the end of a five-year development and preproductiontesting program for Marine Corps use in NATO. Curtiss-Wrightconducted a growth study of this engine for advanced an d highlyadvanced versions for com mut er aviation use. The technical risks

were found to be no greater and no less than those for the GArotary engine-

everything was just bigger.Taken together, the NASA GA engine studies, the Cummins

adiabatic diesel truck eng ine results, and the Curtiss-Wright workon stratified charge rotary engines were viewed as 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 ofaircraft diesel and rotary engines in the 800-to 2400-horsepower class.

Teledyne General Products completed a study of the 800- to2400-horsepower class of lightweight diesel commuter aircraftengines. While many key features remained from the GA engines,

these larger engines involved higher spee ds, higher loadings, andadiabatidceramic combustion ch amber technology similar to thatdeveloped by Cummins. These power plants combine thelightweight two-stroke radial design philosophy of the GA designswith Cummins -type adiabatic components. The insulated c ombus -tion chamber greatly reduces, and may even eliminate, thecoolant system head load, which should correspondingly reducethe cooling drag an d provide more energy to the turbocharger a ndcompounding turbine. This high -speed, high-pressure cycledem and s 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 commute r aircraft engine is a 90-degree X -8, two-stroke, piston -ported adiabatic turbocompoun d of 2000

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InternalCornbustion Engines

with the diesel work in th e sa me power range. Under a NASA con -tract, Curtiss-Wright extended its previous GA studies to the largerengine sizes, the only difference being that turbocompoundingwas allowed as an option. Engine definitions were based on con -ventional cooling and material temperatures were not increased,which , though inc lud ing many advanced mechanica ltechnologies, meant these engines have neither the benefits northe technical risks associated with adiabatic ceramics.

As this work was going on , NASA conduc ted a parallel i n-housestudy of the potential benefits ofadding adiabatic uncooled opera-tion t o th e Curtiss-Wright engine definitions. It was estimated thatby adding ceramic or other insulative trochoid liners and rotor/endhousing faces and deleting the conventional cooling system, thepower recovered in the compounding turbine could be increasedby 10 to 15 percent of engine total shaft power. The structuralweight added for ceramic/insulative components was generallyoffset by eliminating the conventional coolant system.

In the end, NASAs first attempt at an adiabatic turbocom-pounded rotary engi ne amounted to improving the C-W specificweights and cruise BSFCs by 10 percent each. Subse quent com-parative performance studies revealed that the conventionallycooled rotary is not competitive with a highly advanced turbopropfo r commuter aircraft. Looking at C-W and NASA studiestogether, the resulting adiabatic/turbocompound version of therotary engine retained all the known desirable features inherent torotary engines, with greatly extended durability a realisticpossibility.

In February 1984Curtiss-Wright, forced to close down the plantconducting rotary research, sold its rotary rights and interests toDeere & Co., which agreed t o carry on the research for NASA if asuitable aircraft engine partner could be found. By March 1985Avco Lycoming agreed to b ecom e a partner with Deere and bothcomp anies have since announced they plan to certify an d marketthe worlds first 350- to 400-horsepower Je t -A fuel-burning rotaryaircraft engine by 1990. During that time period Deere assumedthe NASA C-W contract to build a high -performance, multifuel

rotary test engine rig and successfully completed it with thepossibility of future work involving the testing of advanced co m -ponents and systems.

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As a result of the promise a nd ad vances in rotary research, inJune 1984 NASA decided to focus future general aviationresearch and testing efforts in that area. A typical 80-cubic-inchdisplacement engine developing 200 horsepower would weighabout 250 pound s, fit inside a 16-by-50-inchcylinder, and mat chthe BSFC oftodays highly developed avgas reciprocating eng ineswhile burning Jet-A kerosene-type fuels. A highly advanced ver-sion would have true multifuel capability, an overall fuel savingsofabout 50 percent, and nearly twice the power output for the samesize.

These developments in industry enthusiasm and commitment

have pushed GA rotary engines out of limbo and into productionreality. According to one of Lewis propulsion ma nage rs, While agreat deal re mains to be do ne, it appears that, with proper att en-tion to advanced technologies, this novel power plant could havethe most revolutionary impact on the aircraft engine businesssince the modern turbofan was introduced about 30 years ago.Just ly, and with pride, NASA can lay claim to 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 coolantheat rejection and cooling drag and more energy to the com-pounding turbine. On the other hand, as with the d iesels, 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 powerplant which, in the 2000-horsepower version, would fit into a2-foot diameter by 8-foot long cylinder. Engine weights are c om-parable, a s well as BSFCs if on e acce pts the ATC cycle rotary, butthe rotary is rated to cruise at 75 percent of maximum takeoffpower while the diesel was rated t o cruise continuously a t 100per-cent of its maximum power.

Aside from cruise-power applications and problematical dif-ferences in technological risk/credibility, the two engines appearto be e qual in potential benefits, leading to a horse race with gasturbine core engines.

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Internal Combus tion Engines

With thes e intriguing data in hand, NASA then did a seriesof in-house airplane/mission evaluation studies in which the presentdiesel and rotary engines were compared with each other and witha similarly advanc ed turboprop engine. For a 30-passenger, Mach0.6, 20 000-foot mission with a 2400-nautical-mile design range,both th e diesel and the rotary were sized considerably smaller thanthe turboprop because of the latter's considerably worse powerlapse rate from sea level up to 20 000 feet where engine sizing oc-curred. Also, with 100 percent diesel and 75 percent rotary cruisepower, the rotary ended up one-third larger than the diesel, with alarger base engine weight and corresponding weight differencesreflected th roughout the airplane.

Bearing th is difference in mind, both the diesel and rotary weresubstantially heavier than the turboprop while consuming con -siderably less fuel. All three airplanes turned o u t to be about thesame size but both the rotary and the diesel were at least com -petitive with the turboprop. The diesel showed a definite edge overthe rotary due almost entirely to its more aggressive ratingphilosophy.

In essence, both the diesel and the rotary give competitiveoverall performance but ar e more economical than the turboprop.Though many elements of the direct operating cost equationshave yet to b e addressed, these engines simply use less fuel. In anera of true fuel scarcity, which would be entirely different from to -day's high fuel price scenario, they could well make the differencebetween continuing operations or not.

The promise of both the diesel and the rotary is great but bothrequire four major new technology items to be completely suc -cessful, some of which are being addressed by NASA research.Despite varied applications, both require advancements in thetribology area, that is, low -friction, low-wear sealing elements andlubes t o survive in a h igh -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 condit ion at high speeds.

Both require at least partially insulated combustion chambercomponentry to approach the benefits of the ATC cycle. For thediesel a fairly straightforward extension ofthe Cummins approachmay do. For the rotary, however, there is no precedent at all, eventhough the basic intent is the s ame . Both require very fast fuel in-

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jection and stratified charge combustion systems, a substantialproblem despite reported progress in other areas. Cyclic combus -tion rates four to six times higher than state -of-the-art truckdiesels are needed.

Finally, both engines require advanced turbochargers and com -pounding concepts. The compoun ding system was not consideredcost-effective for the GA class of engines but is a unique require-ment for the larger engines. However, such a system ad ds weight,cost, and reliability concerns. An alternative approach is availablefor 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 itshould be possible to run the compressor discharge pressuresubstantially higher than the turbine inlet pressure. NASA has runin-house tests on a four -stroke diesel engine with over 10 percentimprovement to both power output and specific fuel consumption.

Engine/A irframe IntegrationIn order to help determine which of the four promising conc ept s

for new general aviation engines of the 1990s should be con -sidered for further research funding, NASA initiated the Advanced

Aviation Comparative Engine/Airframe Integration study withBeech and Cessna Aircraft Companies. Rotary, diesel, spark-ignition, and turboprop power plants were compar