JOURNAL OF AIRCRAFT Vol.41, No. 1, January–February 2004

Design of the Blended Wing BodySubsonic Transport

R.H.Liebeck ¤ TheBoeing Company, Huntington Beach, California 92647

The BoeingBlended-Wing– Body (BWB) airplaneconcept represents apotentialbreakthrough in subsonic trans- portefŽ ciency. Work began on this concept via a studyto demonstrate feasibility and begin development of this new class ofairplane. In this initial study, 800-passenger BWB and conventional conŽ guration airplanes were sized andcompared for a 7000-nmile design range. Both airplanes were basedon engine and structural (composite) technologyfor a 2010entry into service. Results showedremarkable performance improvements of the BWBover the conventionalbaseline, including a 15%reduction in takeoff weight and a 27%reduction in fuel burn per seat mile.Subsequent in-house studies at Boeinghave yielded the developmentof afamilyof BWB transportsranging from200 to 600 passengers with a highlevel of parts commonality and manufacturing efŽ ciency. Studies have alsodemonstrated that the BWBis readilyadaptable to cruise Machnumbers as highas 0.95.The performance improvementof the latestBoeing BWBs overconventional subsonic transports based on equivalent technology has increased beyondthe predictionsof the earlyNASA-sponsored studies.

I.Introduction surfacearea per passenger available for emergency egress decreases Tisappropriate to begin with a referenceto the Wright Flyer withincreasing passenger count. Second, cabin pressure loads are I itself,designed and Ž rst ownin 1903. A short44 yearslater, mostefŽ ciently taken in hoop tension. Thus, the early study began theswept-wing Boeing B-47 took  ight.A comparisonof thesetwo withan attemptto usecircular cylinders for the fuselage pressure airplanesshows a remarkableengineering accomplishment within a vessel,as shown in Fig. 3, along with the corresponding Ž rstcut periodof slightly more than four decades. Embodied in the B-47 are attheairplane geometry. The engines are buried in the wing root, mostof thefundamental design features of a modernsubsonic jet andit was intended that passengers could egress from the sides of transport:swept wing and empennage and podded engines hung on boththe upper and lower levels. Clearly, the concept was headed pylonsbeneath and forwardof thewing. The Airbus A330, designed backto a conventionaltube and wing conŽ guration. Therefore, it 44years after the B-47, appears to be essentially equivalent, as wasdecided to abandonthe requirement for taking pressure loads shownin Fig. 1. inhoop tension and to assumethat an alternateefŽ cient structural Thus,in 1988, when NASA LangleyResearch Center’ s Dennis conceptcould be developed. Removal of this constraint became Bushnellasked the question: “ Istherea renaissancefor the long- pivotalfor thedevelopment of theBWB. haultransport?” there was cause for re ection. In response, a brief Passengercabin deŽ nition became the origin of thedesign, with preliminarydesign study was conducted at McDonnellDouglas to thehoop tension structural requirement deleted. Three canonical createand evaluate alternate conŽ gurations. A preliminaryconŽ gu- formsshown in Fig. 4a, each sized to hold 800 passengers, were rationconcept, shown in Fig. 2, was the result. Here, the pressurized considered.The sphere has minimum surface area; however, it is passengercompartment consisted of adjacentparallel tubes, a lateral notstreamlined. T wocanonical streamlined options include the con- extensionof the double-bubble concept. Comparison with a con- ventionalcylinder and a disk,both of whichhave nearly equivalent ventionalconŽ guration airplane sized for the same design mission surfacearea. Next, each of these fuselages is placed on awingthat 2 indicatedthat the blended conŽ guration was signiŽ cantly lighter, hasa totalsurface area of 15,000ft .Now theeffective masking of hada higherlift to drag ratio, and had a substantiallylower fuel thewing by the disk fuselage results in a reductionof totalaerody- 2 burn. namicwetted area of 7000ft comparedto the cylindrical fuselage Thispaper is intended to chroniclethe technical development of pluswing geometry, as shown in Fig. 4b. Next, adding engines 2 theBlended-Wing– Body (BWB) concept.Development is broken (Fig.4c) provides a differencein total wetted area of 10,200 ft . intothree somewhat distinct phases: formulation, initial develop- (Weightand balance require that the engines be located aft on the mentand feasibility, and, Ž nally,a descriptionof thecurrent Boeing diskconŽ guration.) Finally, adding the required control surfaces BWB baselineairplane. toeach conŽ guration as shownin Fig. 4d results in atotalwetted areadifference of 14,300 ft 2,ora reductionof 33%. Because the II.Formulation of the BWBConcept cruiselift to drag ratio is related to the wetted area aspect ratio, b2=S ,theBWB conŽguration implied a substantialimprovement Theperformance potential implied by theblended conŽ guration wet inaerodynamic efŽ ciency. providedthe incentive for NASA LangleyResearch Center to fund Thedisk fuselage conŽ guration sketched in Fig. 4d hasbeenused asmallstudy at McDonnell Douglas to developand compare ad- todescribe the germination of theBWB concept.Synergy of theba- vancedtechnology subsonic transports for the design mission of 800 sicdisciplines is strong. The fuselage is alsoa wing,an inletfor the passengersand a7000-nmile range at aMachnumber of 0.85.Com- engines,and a pitchcontrol surface. V erticalsprovide directional positestructure and advanced technology turbofans were utilized. stability,control, and act as winglets to increase the effective as- DeŽning the pressurized passenger cabin for a verylarge airplane pectratio. Blending and smoothing the disk fuselage into the wing offerstwo challenges. First, the square-cube law shows that the cabin achievedtransformation of thesketch into a realisticairplane con- Žguration.In addition, a nosebullet was added to offer cockpit visibility.This also provides additional effective wing chord at the Received 9June2002; revision received 19December 2002;accepted centerlineto offset compressibility drag due to theunsweeping of forpublication 10 January2003. Copyright c 2003by theAmerican In- theisobars at theplane of symmetry. stituteof Aeronautics and Astronautics, Inc. °All rights reserved. Copies of thispaper may bemade forpersonal or internal use, on condition that the Modernsupercritical airfoils with aft camber and divergent trail- copierpay the $10.00 per-copy fee totheCopyright Clearance Center,Inc., ingedges were assumed for the outer wing, whereas the centerbody 222Rosewood Drive, Danvers, MA 01923;include the code 0021-8669/ 04 wasto be based on areexed airfoil for pitch trim. A properspanload $10.00in correspondence with the CCC. impliesa relativelylow lift coefŽ cient due to thevery large center- BoeingSenior Technical Fellow, Phantom Works. Fellow AIAA. bodychords. Therefore, airfoil L W102Awas designed for c 0:25 ¤ l D 10 LIEBECK 11

a)Effect ofbodytype on surface area Fig.1 Aircraft designevolution, the Žrst andsecond 44 years.

b)Effect ofwing/ bodyintegration on surface area

c) Effect ofengineinstallation on surface area Fig.2 Earlyblended conŽ guration concept.

d)Effect ofcontrols integration on surface area Fig.4 Genesis ofthe BWBconcept.

and cmc=4 0:03 at M 0:7usingthe method of Ref.1. There- sultingairfoil D C section is Dshown in Fig. 5, along with a planform indicatinghow pitch trim is accomplished via centerbody re ex, whereasthe outboard wing carries a properspanload all of the way tothe wingtip. Blending of this centerbody airfoil with the out- boardsupercritical sections yielded an aerodynamicconŽ guration witha nearlyelliptic spanload. At thisearly stage of BWB devel- Fig.3 EarlyconŽ guration with cylindrical pressure vessel andengines opment,the structurally rigid centerbody was regarded as offering buriedin the wingroot. freewingspan. Outer wing geometry was essentially taken from a 12 LIEBECK

Fig.5 Originalcenterbody airfoil L W109Aand planform showing pitch trim effector .

Fig.6 First-generationBWB.

conventionaltransport and boltedto theside of thecenterbody. The swallowingcan provide improved propulsive efŽ ciency by reducing resultwas a wingspanof 349 ft,a trapezoidalaspect ratio of 12,and theram drag, and this was the motivation for the wide “ mail-slot” alongitudinalstatic margin of 15%,implying a requirementfor a inletsketched in Fig. 6. However,this assumed that such an inlet y-by-wirecontrol system. ¡ couldbe designed to provide uniform  owand efŽ cient pressure Theaft engine location, dictated by balance requirements, of- recoveryat thefan face of theengine(s). feredthe opportunity for swallowing the boundary layer from that Twostructural concepts (Fig. 7) wereconsidered for the center- portionof the centerbody upstream of theinlet, a somewhatunique bodypressure vessel. Both required that the cabin be composed of advantageof theBWB conŽguration. In principle, boundary-layer longitudinalcompartments to provide for wingribs 150 in.apart to LIEBECK 13

Fig.7 Centerbodypressure vessel structuralconcepts.

III.BWB DesignConstraints Asan integrated airplane conŽ guration, the BWB mustsatisfy a uniqueset of designrequirements. Included are the following:

A. Volume Passengers,cargo, and systems must be packaged within the wing itself.This leads to a requirementfor the maximum thickness-to- chordratio on the order of 17%, a valuethat is muchhigher than is typicallyassociated with transonic airfoils.

B.Cruise Deck Angle Becausethe passenger cabin is packaged within the centerbody, thecenterbody airfoils must be designedto generatethe necessary liftat an angleof attackconsistent with cabin deck angle require- ments(typically less than 3 deg).T akenby itself,this requirement Fig.8 Flightcontrol system architecture ofthe Žrst-generationBWB. suggeststhe use of positiveaft camber on the centerbodyairfoils.

C. Trim carrythe pressure load. The Ž rstconcept used a thin,arched pressure ABWB conŽguration is considered trimmed (at the nominal vesselabove and below each cabin, where the pressure vessel skin cruisecondition) when the aerodynamic center of pressure is coin- takesthe load in tension and is independentof thewing skin. A thick cidentwith the center of gravity, and all of thetrailing-edge control sandwichstructure for both the upperand lowerwing surfaces was surfacesare faired. Positive static stability requires that the nose- thebasis for the second concept. In thiscase, both cabin pressure downpitching moment be minimized.This limits the use of positive loadsand wing bending loads are taken by thesandwich structure. A aftcamber and con icts with the preceding deck angle requirement. potentialsafety issue exists with the separatearched pressure vessel concept.If a rupturewere to occurin the thin arched skin, the cabin pressurewould have to beborneby thewing skin, which must in D.Landing Approach Speed and Attitude turnbe sized to carry the pressure load. Thus, once the wing skin BWBtrailing-edgecontrol surfaces cannot be used as apsbe- issizedby thiscondition, in principle there is noneedfor the inner causethe airplane has no tailto trimthe resulting pitching moments. pressurevessel. Consequently, the thick sandwich concept was cho- Trailing-edgesurface de ection is set by trimrequirements, rather senfor the centerbody structure. A three-viewof theoriginal BWB thanmaximum lift. Therefore, the maximum lift coefŽ cient of a isgivenin Fig. 6, and a descriptionof thepackaging of theinterior BWB willbe lower than that of a conventionalconŽ guration, and, isalsoshown there. Passengers are carried in both single and double hence,the wing loading of a BWBwillbe lower. Also, because deckcabins, and the cargo is carriedaft of thepassenger cabin. As a thereare no aps,the BWB’ s maximumlift coefŽ cient will occur taillessconŽ guration, the BWB isachallengefor  ightmechanics, ata relativelylarge angle of attack, and the  ightattitude during andthe early control system architecture is shown in the isometric approachis correspondinglyhigh. viewin Fig. 8. A completedescription of original BWB studyis givenin Ref. 2. Futuregenerations of BWB designswould begin E.Buffet andStall toaddressconstraints not observed by thisinitial concept, but the TheBWB planformcauses the outboard wing to be highly loaded. basiccharacter of theaircraft persists to this day. Thisputs pressure on thewing designer to increaseboth the outboard 14 LIEBECK wingchord and washout, which degrades cruise performance. A cialfeasibility of theBWB concept.McDonnell Douglas was the leading-edgeslat is requiredoutboard for low-speedstall protection. ProgramManager, and the team members included NASA Langley Theseissues apply to a conventionalconŽ guration, but they are ResearchCenter, NASA JohnH. GlennResearch Center at Lewis exacerbatedby theBWB planform. Field,Stanford University, the University of Southern California, theUniversity of Florida,and Clark-Atlanta University. The orig- F.Power forControl Surface Actuation inal800-passenger 7000-n mile design mission was retained. This TaillessconŽ gurations have short moment arms for pitch and workis summarized in Ref. 3. directionalcontrol, and, therefore, multiple, large, rapidly moving controlsurfaces are required. Trailing-edge devices and wingletrud- A.ConŽguration DeŽ nition and Sizing dersare called on to perform a hostof duties,including basic trim, Thisstudy began with a reŽned sizing of theinitialBWB conŽg- control,pitch stability augmentation, and wingload alleviation. Be- uration(Fig. 6), whereminimum takeoff gross weight (TOGW) was causesome of thecontrol surfaces can perform multiple functions setas the Ž gureof merit. Primary constraints included an 11,000- (e.g.,outboard elevon/ dragrudder offers pitch, roll, and yaw author- fttakeoff Ž eldlength, 150-kn approach speed, low-speed trimmed ity),control surface allocation becomes a criticalissue. The mere CL max of1.7,and a cruiseMach number of 0.85. Initial cruise al- sizeof the inboard control surfaces implies a constrainton theairfoil titude(ICA) was allowed to vary to obtainminimum TOGW ,but designto minimizehinge moments. Hinge moments are related to withthe requirement that the ICA beatleast 35,000 ft. This yielded thescale of thecontrol surface as follows:The area increases as the atrapezoidalwing of aspectratio of 10,with a correspondingspan squareof thescale, and, in turn, the moment increases with the cube of280 ft and an area of 7840 ft 2.Theresulting trapezoidal wing ofthescale. Once the hydraulic system is sized to meetthe maxi- loadingwas on the order of 100lb/ ft 2,substantiallylower than the mumhinge moment, the powerrequirement becomes a functionof 150 lb/ft2 typicalof modern subsonic transports. An explanation rateat whicha controlsurface is moved. offeredwas that a signiŽcant portion of the trapezoidal wing is in Ifthe BWB isdesigned with a negativestatic margin (unstable), effecthidden by the centerbody, and, therefore, the cost of trape- itwillrequire active  ightcontrol with a highbandwidth, and the zoidalwing area on airplanedrag is reduced. This in turn allowed the controlsystem power required may be prohibitive. Alternatively, airplaneto optimizewith a largertrapezoidal area to increase span designingthe airplane to be stable at cruise requires front-loaded witha relativelylow cost on weight.A three-viewand isometric of airfoils,washout, and limited (if any) aft camber. This implies a theresulting second-generation BWB isgiven in Fig. 9. higherangle of attack, which, in turn, threatens the deck angle Thedouble-deck BWB interiorwas conŽ gured with 10 150-in. constraint. widepassenger cabin bays, as shown in Fig. 10, with cargo com- partmentslocated outboard of the passenger bays and fuel in the G.Manufacturing wing,outboard of thecargo. Considerations and constraintsincluded Theaerodynamic solution to thedesign constraints just listed can weightand balance,maximum offset of the passengers from the ve- readilyresult in acomplexthree-dimensional shape that would be hiclecenterline (ride quality) and the external area of the cabin. difŽcult and expensive to produce. Therefore, the aerodynamicist Becausethis is the surface area of the pressure vessel, the extent muststrive for smooth, simply curved surfaces that at thesame time ofthisarea has a signiŽcant effect on thestructural weight of the satisfythe challenging set of constraints just described. centerbody.The cabin partitions are in fact, wing ribs that are part oftheprimary structure. Windows were located in the leading edge onbothdecks, and the galleys and lavatorieswere located aft to help IV.InitialDevelopment and Feasibility providethe passengers with an unobstructed forward view. Egress ANASA/industry/universityteam was formed in 1994 to con- wasvia the main cabin doors in the leading edge, and through aft ducta three-yearstudy to demonstrate the technical and commer- doorsin therear spar.

Fig.9 Second-generationBWB. LIEBECK 15

Fig.10 Interior arrangementof passengercabin. Fig.12 Navier– Stokes computed upper surface pressure distributions.

Fig.13 BWB inthe NASA LaRCNTF . Fig.11 Section lift coefŽ cient andthickness-to-chord ratio variation with span. C.Wind-TunnelT ests Transonicand low-speed wind-tunnel tests of the BWB con- B.Aerodynamics Žguration(Fig. 9) were conducted at NASA LangleyResearch Someinsight into the aerodynamic design of the BWB ispro- Center(LaRC) in the National Transonic Facility (NTF), andthis videdin Fig. 11, where the trade between wing chord, thickness, and representedan invaluable opportunity to test at close to the full- liftcoefŽ cient is shown.The outboard wing is moderately loaded, scaleReynolds number. Figure 13 shows the BWB modelmounted similarto a conventionalconŽ guration, where drag is minimized inthe tunnel, and NTF resultsare compared with CFD predic- witha balancebetween the wetted area and shockstrength. Moving tionsin Fig. 14. Excellent agreement for lift, drag, and pitch- inboard,the centerbody, with its very large chord, calls for cor- ingmoment, as well as wing pressure distributions, is shown, in- respondinglylower section lift coefŽ cients to maintain an elliptic cludingup to and beyond buffet onset. A primaryobjective of spanload.The low section lift requirement allows the very thick thetest was to establish the effectiveness of the current state-of- airfoilsfor packaging the passenger compartment and trailing-edge the-artCFD methodsfor predicting the aerodynamic characteris- reex for pitch trim. ticsof aBWBairplane.The remarkable agreement indicated that Navier–Stokes computational  uiddynamics (CFD) methodol- CFD couldbe reliably utilized for the aerodynamic design and ogyin boththe inverse design and direct solution modes was em- analysis. ployedto deŽ ne the Ž nalBWB geometry.A solutionshowing the Alow-speedtest of a powered4% scaleBWB wasconducted in theNASA LaRC 14 22footwind tunnel (Fig. 15). Results veriŽ ed pressuredistribution at the midcruise condition is shown in Fig. 12. £ Thetypical shock on theoutboard wing is smeared into a compres- trimmedCL max estimates,showed favorable stall characteristics, and sionwave on the centerbody. The  owpattern on the centerbody showedexcellent control power through stall. Power effects were remainedessentially invariant with angle of attack, and  owsepa- foundto be muchsmaller than expected. rationis initiated in thekink region between the outboard wing and thecenterbody. Outer wing  owremains attached, providing lateral D.Stability and Control controlinto the stall regime. Similarly, the  owover the centerbody Duringdevelopment of thesecond-generation BWB, itwas as- remainsattached and provides a nearlyconstant  owenvironment sumedand accepted that the airplane would be staticallyunstable to forthe engine inlets. This  owbehavior is a consequenceof signiŽ - achievehigh cruise efŽ ciency ( L=D).Balanceof the airplane was cantlateral  owon the centerbody that provides a three-dimensional achievedby sliding the wing fore and aft on the centerbody, much reliefof compressibility effects. However, the relief on thecenter- likethe procedure for a conventionalconŽ guration. However, this bodyis tradedfor a transonicallystressed  owenvironment in the wasclearly a morecomplex process due totheintegrated nature of kinkregion. This is the ideal spanwise location for the stall to begin, theBWB. Thelow effective wing loading meant that trailing-edge froma ightmechanics point of view:The ailerons remain effective, apswould not be required, but a leading-edgeslat on the out- andpitch-up is avoided. boardwing is required for the same reason as that on aconventional 16 LIEBECK

Fig.16 Elevon effectiveness inpitch.

Fig.14 Comparison of CFD predictionswith NTF windtunnel results.

Fig.17 Y awcontrol.

Fig.15 Powered BWB modelin the NASA LaRC14 22foottunnel forlow-speed test. £ airplane.The simple-hinged trailing-edge control surfaces function Fig.18 Flight control testbed builtby StanfordUniversity. aselevons.Flight-critical stability augmentation and envelope pro- tectionwas considered a requirement. Theoutboard elevons are the primary pitch and roll controls be- E.FlightDemonstrator causethey have the largest lever arm about the center of gravity. Low-speed ightmechanics were explored with a 6%scale  ight Figure16 shows the pitch authority of the individual elevons, as controltestbed (Fig. 18), built at Stanford University under NASA wellas the locus of their effective centers of pressure. Note that sponsorship.Called the BWB-17, the airplane had a 17-ftwingspan, theyyield relatively short lever arms about the center of gravity, weighed120 lb andwaspowered by two 35-cm 3 two-strokeengines andeven shorter lever arms about the landing gear for takeoff rota- withpropellers. The model was dynamically scaled to match the tion.However, total control power is substantial due tothefull span ightcharacteristics of the full-scale BWB. Stabilityaugmentation ofelevons.The winglets with rudders provide primary directional wasprovided by an onboardcomputer, which also recorded  ight- stabilityand control. For the low-speed engine-out condition, the testparameters. The Ž rst ightof the BWB-17 took place on 29 outboardelevons become split drag rudders, similar to those on the July1997, at El MirageDry Lake in California.Excellent handling B-2,as shown in Fig.17. qualitieswere demonstrated within the normal  ightenvelope. LIEBECK 17

Fig.19 Comparison of aerodynamic,inertial, and cabin pressure loads.

F.Propulsion didnot prove practical, boundary-layer diverters were assumed to Theaft engine location on theBWB offersthe opportunity for in- bethedefault. gestionof theboundary layer generated on the centerbody forward oftheinlets. In principle,boundary-layer ingestion (BLI) can im- provethe propulsive efŽ ciency by reducing ram drag. This assumes G.Structure thatan inletcan be designedthat provides proper pressure recovery Theunique element of theBWB structureis thecenterbody. As anduniform  owat the fan face of the engine. Alternatively, the thepassenger cabin, it mustcarry the pressure load in bending,and boundarylayer can be diverted around the sides of the inlets, but asa wingit must carry the wing bending load. A comparisonof thisimplies dumping low-energy air into an already transonically thestructural loading of a BWB withthat of aconventionalconŽ g- stressedpressure recovery region. Simply mounting the engines on urationis givenin Fig. 19. Peak wing bending moment and shear pylonsis anotheroption, but increased wetted area and weight plus forthe BWB isonthe order of one-halfof thatof theconventional nosedownthrust moment are detractors from this installation. conŽguration. The primary challenge was to developa centerbody NASA-sponsoredstudies of the BLI conceptwere conducted at structuralconcept to absorb the cabinpressure load. Unlike a wing, theUniversity of SouthernCalifornia (USC) andatStanfordUniver- whichrarely experiences its design load (typically via a 2.5- g gust), sity.At USC, awind-tunnelsimulation was created with an upstream thepassenger cabin sees its design pressure load on every  ight. atplateto generatethe boundary layer and variousduct geometries, Thus,on the basis of fatigue alone, the centerbody should be built leadingto astationrepresenting the fan face of theengine, where fromcomposites due to their comparative immunity to fatigue. the owquality was evaluated. Results indicated that proper con- Theoverall structural concept selected for this NASA-sponsored Žgurationsof vortexgenerators could provide a reasonablyuniform studyis shown in Fig. 20. Outboard wing structure is essentially owat thefan face with acceptable pressure recovery. These results conventionaland was assumed to be composite. The centerbody wereutilized at Stanford University to help guide a theoreticalmul- structuralshell was based on twocandidate concepts: a 5-in.thick tidisciplinaryoptimization study of the BWB engineinlet concept. sandwich,or a skinplus 5-in. deep hat-section stringers. A global Navier–Stokes based CFD wasused to represent the centerbody Žniteelement model was analyzed for the combined pressure and andinlet  owŽeld, and engine performance was modeled as afunc- wingbending loads on thecenterbody. Cabin skin de ection due to tionof the owquality at the fan face. The optimizer indicated that atwotimes pressure load is shown in Fig. 21. minimumfuel burn was obtained with the engine swallowing the boundarylayer, as opposed to divertingthe boundary layer around the inlet.4;5 Theaft engine location of theBWB allowsfor several installa- H.Safetyand Environmental tionoptions; however, integration affects all of thebasic disciplines. TheBWB offersseveral inherent safety features that are unique Uniquelyfor a BWB, thereis noexplicitpenalty for the centerline tothe conŽ guration. An uncontained engine failure can not impact engineof athree-engineinstallation. Candidate installation concepts thepressure vessel, fuel tanks, or systems.The pressure vessel itself includepodded with pylon, upper or lower surface inlet with S-duct, isunusually robust because its structure has been sized to carry both BLI, ordiverter;and, Ž nally,the engine count itself. Airplanes were thepressure loads and wing bending loads, and, consequently, its sizedfor 12 different combinations with appropriate gains and losses crashworthinessshould be substantial. forinlet recovery and distortion, wetted area drag (including the ad- Environmentally,the BWB naturallyoffers a lowacoustic signa- justmentfor BLI), weight, and thrust moment. The Ž gureof merit ture,before any speciŽ c acoustictreatment. The centerbody shields wasthe TOGW.Additionalconsiderations included ditching, emer- forwardradiated fan nose, and engine exhaust noise is not reected gencyegress, foreign object damage (FOD), noise,reverse thrust, fromthe lower surface of thewing. Airframe noise is reducedby andmaintainability. Lower surface inlets were discarded on theba- theabsence of aslotted aptrailing-edge high-lift system. Engine sisof FOD andditching. A three-engineconŽ guration with upper emissionsare reduced in direct proportion to the reduced fuel burn surfaceBLI inletsand S-ducts to theengines was selected. If BLI perseat mile described hereafter. 18 LIEBECK

Fig.20 Structural layout of second-generationBWB.

Table1 Performancecomparison of the second-generation BWBwiththe conventionalbaseline airplane

Model BWB Conventional Passengers 800 800 Range,n mile 7,000 7,000 MTOGW, lb 823,000 970,000 OEW, lb 412,000 470,000 Fuelburned, lb 213,000 294,000 L=D at cruise 23 19 Thrust,total lb 3 61,600 4 63,600 £ £

Fig.21 Finite element modelsolution showing exaggerated cabin skin Table2 Designrequirements andobjectives for the deection at twotimes pressure. BoeingBWB-450 baseline

Parameter Value Payload468 passengers baggage, three-class arrangementC Designrange 7750 n mile Crew Standardtwo-man crew Reserves Internationalreserve fuel Fuelequal to 5% ofblock fuel 200n mile diversionto alternate airport One-halfhour hold at 1500ft at holdingspeed Constraints11,000-ft Ž eldlength 140-knapproach speed 2.7± segment climbgradient 300-ft/minexcess powerat topof climb

andits fuel burn per seatmile is 27% lower. Given that the conŽ gu- rationwas the only technical difference in these two airplanes, the potentialfor the BWB conceptwas regarded as remarkable. V.BoeingBWB-450 Baseline Airplane Thethree-year study just described demonstrated the feasibility andperformance potential of theBWB. Basedon theseresults and predictions,it wasdecided to initiatea Boeingpreliminary design Fig.22 Conventional baseline conŽ guration. studyof a BWB transport.The 800-passenger 7000-n mile design missionof thefeasibility studies was deemed inappropriate for the I.Performance in-houseevaluation of the BWB. Comparisonswith existing air- Aproperevaluation of theBWB conceptrequired that a conven- planesand airplanes of other preliminary design studies would not tionalsubsonic transport be sized to thesame design mission, em- bepossible, and a payloadof 800 passengers was simply beyond ployingthe same composite structure technology and the same class marketforecast data. ofadvanced technology engines. A two-viewof the conventional baselineis shownin Fig.22, and Table1 comparesthe performance A.DesignRequirements andObjectives oftheBWB withthe baseline. In addition to thesigniŽ cant reduc- Thedesign mission selected for the baseline BWB isgiven in tionin weight,the BWB requiresone less 60,000-lb-class engine, Table2. Althoughdistinct from existing airplanes, this speciŽ cation LIEBECK 19

Fig.23 Boeing BWB-450 baseline.

Fig.25 Centerbody interior cross sections.

C.MultidisciplinaryDesign Optimization Fig.24 Three-class interiorarrangement. Asdescribed, the BWB isan integratedconŽ guration where the interactionof thebasic disciplines is unusually strong. Conventional designintuition and approach are challenged, if not overwhelmed, whenfaced with sizing and optimizing the BWB airplane.The methodof Ref.6, apragmaticand functional multidisciplinary air- offeredthe opportunity for some comparison of theresulting BWB planedesign optimization code, was adopted. This work hasevolved withthe B747, A340, and the then-pending A3XX. Initialspeci- intoa Boeingproprietary code called WingMOD. In the caseof the Žcationof 450 passengers (hence, the designation BWB-450) was BWB, theairplane is deŽ ned by an initial planform and a stack considerednominal, and the Ž nalpassenger count would be es- ofairfoilswhose section characteristics, for example, moment co- tablishedas the airplane was conŽ gured and sized. Also, although efŽ cient cmac anddrag coefŽ cient cd ,areknown as a functionof somewhatignored in the earlier studies, airport compatibility re- thickness-to-chordratio t=c,sectionlift coefŽ cient cl and Mach quirementswere enforced for the baseline BWB, inparticular, the number.WingMOD then models the airplane with a vortex–lattice wingspanlimit of 262 ft (80m). codeand monocoque beam analysis, coupled to givestatic aeroe- lasticloads. The model is trimmed at several  ightconditions to obtainload and induced drag data. ProŽ le and compressibilitydrag B.ConŽguration of the BoeingBWB-450 Baseline arethen evaluated at stationsacross the span, based on the airfoil Perthe requirements just listed and the optimization procedure sectionproperties and the vortex– lattice solution. Structural weight describedhereafter, the baseline BWB shownin Fig. 23 was created. iscalculated from the maximum elastic loads encountered through MinimumTOGW wasthe objective function. Trapezoidal aspect arangeof  ightconditions, including maneuver and vertical and ratiois 7.55, down substantially from the earlier BWBs, andthe lateralgusts. The structure is sized based on bending strength and wingspanof 249 ft Ž tseasily within the 80-m box for Class VI bucklingstability considerations. Maximum lift is evaluated by the airports.Passenger count is 478,based on three-class international useof a criticalsection method that declares the wing to be atits rules.Figure 24 showsthe interior arrangement, and Fig. 25 shows maximumuseable lift when any spanwise airfoil section reaches its representativecross sections of thecenterbody. The entire passenger maximumlift coefŽ cient. cabinis on theupper deck, and cargo is carried on thelower deck, Figure26 showsa smallportion of anexample WingMOD so- similarto conventionaltransports. All of thepayload is located ahead lutionfor the baseline BWB-450. The procedure begins with the oftherear spar. Crashworthiness contributed to this arrangement. manualdeŽ nition of abaselinedesign (not to beconfused with the 20 LIEBECK

Fig.26 Example WingMOD solution for the BWB baseline.

Fig.27 Comparison of centerbodyproŽ les ofthe second-generation BWBwiththe Boeingbaseline BWB. term“ baselineBWB” ). Subjectto the mission deŽ nition and con- straints(e.g., range, takeoff Ž eldlength, approach speed, interior volume,etc.), WingMOD provides the deŽ nition of theminimum TOGW conŽguration that meets the mission while satisfying all Fig.28 Planform comparisons of the second-generationBWB with the constraints.Put another way, the optimized airplane design is closed BoeingBaseline BWB: ——,BoeingBaseline and –––,secondgenera- andmeets all design mission requirements with minimum TOGW . tion(scaled).

D.Aerodynamics Aerodynamicdesign of the BWB-450 was coupled with andthe second-generation BWB. Ifthe buffet CL isdeŽ ned at the WingMODto obtainthe Ž nalaerodynamic deŽ nition (outer mold break in the CL vs C M curve,the improvement of thenew planform line).DeŽ nition of the airfoil stack was a keyelement to this ap- isapparent. Compared to the earlier design, there is almost twice the proach.A newclass of transonic airfoils for the centerbody was de- marginbetween midcruise CL , 1.3 g–buffet,and buffet itself. Cen- signedbased on constraintsof cross-sectionalarea required to hold terbodychords were increased to reduce their thickness-to-chord passengers,baggage, and cargo properly. The new airfoils tightly andafterbody closure angles. Although this increased wetted area, packagethe payload without a dragpenalty. More signiŽ cantly, the theincreased friction drag was more than offset by a reduction newairfoils smoothed and  attenedthe geometry to simplifyman- inpressure drag. Inboard elevon effectiveness was also improved. ufacture.Figure 27 shows a comparisonof the centerbody proŽ le Aerodynamicdesign of the BWB isdiscussed in more detail in ofthesecond-generation BWB withthe Boeing BWB-450. Ref. 7. Theplanform also underwent signiŽ cant change from the second- generationBWB, asshownin Fig.28, which also gives the com- E.Stabilityand Control parisonwhere both planforms are scaled to the same wingspan. Theplanform, airfoil stack, and twist distribution of theBWB-450 Airfoilchords have been increased on boththe outer wing and the resolvesthe longitudinal trim problem with more efŽ ciency than centerbody.Buffet onset level and characteristics primarily drove most ying-wingairplanes. Historically,  yingwings have been outboardchord increase. Figure 29 comparesthe lift curves ( CL vs trimmedby sweeping the wing and downloading the wingtips. ®)andlift vs pitchingmoment curves ( CL vs C M )forthe BWB-450 Whereasthis approach allows the wingtipsto functionallyserve as a LIEBECK 21

Fig.30 Centerbody structural concept.

fore,podded engines on pylonsbecame the selected installation for thebaseline BWB.

G.Structure TheBWB structureis divided into two main components: the centerbodyand the outer wings. The structure of the outer wings issimilar to that of a conventionaltransport. The centerbody is subdividedinto the forward pressure vessel and the unpressurized afterbody.Development of thestructure for the centerbody and its pressurevessel was approached by deŽ ning and comparing several concepts.W eightand cost were the primary Ž guresof merit. One ofthemost viable concepts was based on askin/stringerouter sur- facestructure where the stringers are on theorder of 5– 6 in.deep. Theinternal ribs have Y braceswhere they meet the skin,to reduce thebending moment on the skin created by the internal pressure. (Thiscould be regarded as a structuralanalog to the earlier con- ceptof an arched pressure membrane.) As shown in Fig. 30, the Fig.29 Comparison of lift and moment curves ofthe second- generationBWB withthe BoeingBaseline BWB. completecenterbody pressure vessel is composedof theupper and lowersurface panels, the rounded leading edge (which also func- tionsas the front spar), the rear main spar, the outer ribs (which horizontaltail, it imposes a signiŽcant induced drag penalty. The ef- mustalso carry the cabin pressure load in bending), and the internal fectiveaerodynamic wingspan is less than the physical span, and this ribs(which carry the cabin pressure load in tension).The cabin  oor penaltyis aprimaryreason that  ying-wingairplanes have failed to simplysupports the payload and does not carry wing bending loads. liveup totheir performance potential. As described earlier, the Ž rst- Finiteelement analyses have been used to developand verify this andsecond-generation BWB wereallowed to have signiŽ cantly neg- structuralconcept and its weight. The Ž nalresult is an unusually ativestatic margins to preserve a near-ellipticspanload. The BWB- ruggedpassenger cabin that weighs little more than a conventional 450hasbeentrimmed by acarefuldistribution of spanloadcoupled fuselage. witha judiciousapplication of wingwashout. The result is aying- Studiesto date have assumed composite material for the major- wingairplane that is trimmed at a stablecenter of gravity ( 5% ityof the BWB primarystructure. The outer wings could readily staticmargin) with all control surfaces faired, and with no inducedC befabricatedfrom aluminum with the typical 20% weight penalty. dragpenalty. Setting this design condition at the midcruise point However,as mentioned earlier, the weight penalty for using alu- resultsin atrimdrag of onecount at start of cruise (high CL ), and minumfor the centerbody structure would be larger. The design aone-halfcount of trimdrag at endof cruise (lower C L ). cabinpressure load is experienced on every  ight,and, thus, fa- tiguebecomes the design condition. Because cabin pressure loads F.Propulsion aretaken in bending, the margin required for aluminum could be Thesecond-generation BWB assumedboundary-layer ingestion prohibitive,whereas composites are essentially immune to fatigue forboth the engine installation and the performance estimate. For and,hence, would suffer no penalty. theBWB-450 it wasdecided to reduce the technology risk by exam- Figure31 showsa comparisonof thestructural weight fractions iningthe performance of bothboundary-layer diverters and simple ofaBWB anda conventionalconŽ guration, both sized for the same poddedengines on pylons.Navier– Stokes based CFD wasused to missionand both assuming the same composite structure technol- evaluatethese options. T otheextent they were studied, the diverters ogy.Although the centerbody structure of theBWB isheavier than showedan unacceptable drag increase due to thelow energy of the thatof a conventionalfuselage, the weight (OEW) ofthecomplete divertedboundary layer, plus its interaction with the pressurerecov- conŽguration of the BWB ismarkedly lighter. eryregion of theaft centerbody. Alternatively, the initial modeling ofthepodded engines on pylonsindicated that the increase in wetted H.Performance areawas only 4% comparedto thediverted conŽ guration. The thrust Aperformancecomparison of the Boeing BWB-450 with the moment,although undesirable, was deemed acceptable. A thorough AirbusA380-700 is given in Fig. 32. Both airplanes are compared CFD-baseddesign and analysis study showed that an interference- fora payloadof approximately 480 passengers and a rangeof freepodded engine and pyloninstallation could be achieved,and the 8700n mile.(A380 data are from an Airbus brochure.) Probably netdrag penalty was simply due to thewetted area increase. There- themost striking result is the BWB’ s 32%lowerfuel burn per seat. 22 LIEBECK

Fig.31 Comparison of structuralweight fractions from a BWBanda Fig.33 Interior volumecomparison of the BWB-450with the A380- conventionalconŽ guration. 700.

VI. Unique Opportunities andChallenges ofthe BWBConŽguration Creationof the original BWB wasmotivated by a searchfor anairplaneconŽ guration that could offer improved efŽ ciency over theclassic tube and wing. T akeoffweight and fuel burn were the primaryŽ guresof merit,and theBWB concepthas shownsubstan- tialreductions in thesetwo performance parameters, as described earlier.However, the BWB conŽguration offers some unique oppor- tunitiesthat were neither envisioned nor plannedduring its original creationin 1993. Three of theseare described hereafter.

A.ManufacturingPart Count TheBWB issimply a bigwing with an integrated fuselage and no empennage,save the winglets/ verticals.There are no complexwing– fuselageand fuselage– empennage joints of highlyloaded structures at90degto one another, and there are no Ž llets.All trailing-edge controlsurfaces are simple hinged with no track motion, and there areno spoilers.This manifests a substantialreduction (on the order of30%)in thenumber of parts when compared to a conventional conŽguration. A similarreduction in manufacturing recurring cost is implied.

B.Familyand Growth Reference2, whichdescribes the early development of theBWB, containsthe remark; “ Anychange such as wing area or cabin volume impliesa completereconŽ guration. Stretching is not in the vocabu- lary.”Thatwas the thought at thetime. As development progressed, itwas discovered that the BWB conceptcould be ideal for a family ofairplaneswith the potential for substantial commonality among itsmembers. Here stretching takes place laterally (spanwise), as Fig.32 Performance comparison of the BWB-450with the A380-700. opposedto longitudinally.Passenger capacity can be increased by addinga centralbay to the centerbody and vice versa. Wing area andspan automatically increase or decreaseappropriately with pas- sengercapacity, a qualitynot offeredby thelongitudinal stretching Bothairplanes are using equivalent technology engines of simi- ofaconventionalairplane. larthrust levels; however, the A380-700 requires four, whereas the Toachievethis growth capability, the aerodynamic outer mold BWB-450requires three. The primary structure of the A380-700 linesof all of the family members must remain smooth and pro- isaluminum, with the exception of the outer wing panels, which videproper aerodynamic performance. In addition, all of the air- areunderstood to be composite.The BWB-450 primary structure planesmust be trimmedand balanced.Geometrically, this has been isessentiallyall composite. A comparisonof the BWB-450 cabin achievedby essentiallydeŽ ning the centerbody as a ruledsurface volumewith that of theA380-700 is shownin Fig.33. inthe spanwise direction. In turn, this allows the deŽ nition of sev- eralairplanes ranging, for example,from 250 to 550 passengers, as shownin Fig. 34. Centerbody cabins are composed of combinations I.Environment oftwoor moredistinct cabins (shown in green, yellow, and orange). TheBoeing BWB-450 offers the potential for a signiŽcant re- Theouter wing panels and nose sections (shown in blue)are of iden- ductionin environmental emissions and noise. Lower total installed ticalgeometry for allfamily members. Distinct to eachairplane are thrustand lower fuel burn imply an equivalentreduction in engine thetransition section aft of thenose, the aft centerbody, and engines emissions,under the assumption of thesame engine technology. As (shownin gray). Nose gear and outer main gear could be common discussedearlier, the forward-radiated fan noise is shieldedby the forall family members, with a centermain gear of varyingcapacity vastcenterbody, and engine exhaust noise is not re ected by the addedwhere required. lowersurface of the wing. The lower thrust loading itself implies Arepresentativeset of theairplane family has been examined in lowernoise. There are no slotted trailing-edge  aps,so a major depthto establish the potential for commonality. A commonpart sourceof airframe noise is eliminated. Thus, before any speciŽ c numberfor the entire outer wing was the goal. Fuel volume of the acoustictreatment, the BWB offersa signiŽcant reduction in noise. outerwing is adequate for all members of thefamily. Navier– Stokes LIEBECK 23

Fig.34 Commonality of aBWB family.

Fig.36 Cross-sectional area variations, S(x) vs x.

Fig.35 Cabin cross-sectional growth from 250 to 450 passengers. analysesof several of the members of this example family demon- stratedproper aerodynamic performance. The airplanes are trimmed Fig.37 BWB planform, ML/D and MP/D variationwith Mach andbalanced. Finite element modeling was used to quantify the ef- number. fectof commonalityon the structure. The proposed commonality wasfeasible, but at a costof increased OEW forthe smaller air- planes.If the common part number requirement is relaxed to permit askingauge change, the OEW penaltyis substantiallyreduced. Commonalityextends naturally to theinterior, once the commit- mentto the centerbody growth concept is made. In principle, the cabincross section is the same for all of the airplanes, as shown inFig.35. This implies common galleys, lavatories, bag racks,and seats.Substantial maintenance and life-cycle cost savings are im- pliedfor the airline customer. Putsimply, commonality is a constraint,and almost any constraint imposedon an airplane is manifested by an increase in weight. However,the BWB conceptappears to offer the opportunity for anunusuallevel of commonalitywhile the aerodynamic efŽ ciency ismaintainedvia the natural variation of wingarea and span with weight.This implies signiŽ cant reductions in partcount and learning curvepenalties in manufacturing.Enhanced responsiveness to  eet- Fig.38 Comparison of BWB-250 conŽ guration with Mach number . mixrequirements is also implied. It remainsto evaluatethoroughly thetrade between airplane cost and performance offered by the BWB familyconcept. niŽcant distinction. Increased Mach number is accommodatedby anincreasein sweep and chord, which results in a correspondingin- C. Speed creasein weight. Some of thisweight increase is dueto the increase Figure36 showsa comparisonof theBWB-450 cross-sectional ofinstalled engine SFC withMach number. The classic aerody- areavariation S.x/ withthat of theclassical minimum wave drag due namicparameter M L=D isplottedas a functionof thecruise Mach tothevolume of theSears– Haak body. Also shown is the variation numberin Fig. 37. A moremeaningful graph is givenby thevaria- foran MD-11.It can be observedthat the BWB isnaturally area- tionof theparameter M P=D,alsoshown in Fig. 37. ( P isthe design ruled,and, hence, a highercruise Mach number should be achiev- payloadweight.) M P=D includesthe effect of airplaneweight itself, ablewithout a changein thebasic conŽ guration geometry. Figure 37 because M P=D .M L=D/ .P=W /.Isometricviews of BWBs givesthe results of aWingMOD-basedstudy for the effect of the designedfor M D0:85,0.90£and 0.95 are given in Fig. 38. designcruise Mach number on BWBperformanceand weight. All Thesepreliminary D results suggest that 0.90 could be the best ofthedesigns are closed, trimmed, and balanced for the same de- cruiseMach number. However, the economic value of speed must signmission. V ariationbetween planforms appears slight; however, beestablishedbefore selecting a designcruise Mach number. For acomparisonbetween the M 0:85and0.95 designs shows a sig- example,airplane utilization varies directly with speed, and for some D 24 LIEBECK longer-rangemissions, a slightincrease in speed could eliminate Table3 Issues andareas of risk (fromDouglas therequirement for a secondcrew. The question then becomes, how Aircraft Co.,1955) muchof anincrease in TOGW andfuel burn can be offsetby such Complex ightcontrol architecture andallocation, issues?Resolution remains to befound. ² withsever hydraulicrequirements Large auxiliarypower requirements D.Passenger Acceptance, Ride Quality,and Emergency Egress ² New class ofengineinstallation ² Theunique interior conŽ guration of theBWB offersboth opportu- Flightbehavior beyond stall ² High oorangleon take offand approach to landing nitiesand challenges.V erticalwalls of the passenger cabins provide ² amorespacious environment, similar to a railroadcar rather than the Acceptance bythe yingpublic ² Performance at longrange curvedwalls of aconventionalairplane. At thesame time, the low ² Experienceand data base fornew class of capacityof eachcabin (approximately 100 passengers) provides an ² conŽguration limited to military aircraft intimacynot available in wide-body conventional transports. How- ever,although there is a windowin each main cabin door, there are nowindowsin the cabin walls. As a surrogatefor windows, a at screendisplay connected to an arrayof digitalvideo cameras will makeevery seat a windowseat. Some example interior renderings areshown in Fig. 39. Ridequality has been a concerndue to the lateral offset of the passengersfrom the center of gravity. This has been addressed by comparisonof theresults from piloted  ightsimulator tests of the BWB-450and a B747-400using the same pilots and  ightproŽ le. Oneof themore severe cases studied was a takeoff,go-around, and landingin moderate turbulence with a 35-kncrosswind. Lateral and verticalrms g levelswere comparable for the “ worst”seats in both airplanes;however, the frequencycontent tended to be lowerfor the BWB. Gustload alleviation was not used on either airplane. Emergencyegress becomes a signiŽcant challenge when pas- sengercapacity exceeds 400. This is simply a consequenceof the square-cubelaw: Capacity increases with the cube of the length scale,whereas surface area for egress increases with the square of thelength scale. The BWB conŽguration lends itself particularly wellto resolvingthis problem. There is a maincabin door directly inthe front of each aisle, and an emergency exit through the aft pressurebulkhead at the back of eachaisle. In addition,there are fourcross aisles, as shown in Fig. 40.

Fig.40 Cabin egress owpatterns.

Thus,from virtually any location in the cabin, a passengerwill havea directview of one ormoreexits. Unlike a conventionaltrans- port,a 90-degturn will not be required to reach a doorfrom the aisle.Because there is no upperdeck, the problems with long slides, slideinterference, and overwing exits do not exist. Ultimately, this newclass of interiorconŽ guration will require a newset of emer- gencyevacuation criteria coordinated with the Federal Aviation Administration. VII. Summary Developmentof theBWB hasprogressed steadily over the past sevenyears. Once-apparent “ show-stoppers”have been reduced to technicalchallenges, or, in most cases, proper solutions. From adis- tance,the Boeing BWB-450 baseline airplane shows little distinc- tionfrom the Ž rst-generationBWB developedunder NASA spon- sorshipin 1993. The intent of thispaper has been to chronicle the engineeringwork that has brought the airplane to the state it is in today.T able3 presentsa listof issuesand areas of risk.They could readilyapply to the BWB. However,they are, in fact,extracted from DouglasAircraft Company memoranda written in the1950s regard- ingthe challenge of moving from the DC-7 to the DC-8. Hopefully, ourindustry will press on, just as Douglasand Boeing did 50 years Fig.39 Interior concepts forthe BWB. ago. LIEBECK 25

Acknowledgments RobertSeplak, Leonel Serrano, William Small, Norbert Smith, Thecreation and initial development of theBlended-Wing– Body DarrelTenney, Ben Tigner, William V argo,Sean W akayama, (BWB) conceptwas stimulated and funded by NASA LangleyRe- WilliamW atson,Jennifer Whitlock, Matthew Wilks, Karen searchCenter, primarily under contract NAS1-20275. NASA in- Willcox,Kenneth Williams. volvementwith the BWB hascontinuedwith their design and fabri- References cationof theBWB Low-SpeedV ehicle ightdemonstrator. NASA ’s initiativeand involvement with the BWB hasbeen critical to the suc- 1Liebeck,R. H.,“ Designof SubsonicAirfoils for High Lift,” Journal of Aircraft,Vol.15, Sept. 1978, pp. 549– 561. cessof theconcept. Their participation is gratefullyacknowledged. 2 TheBWB existstoday because of the contributions of an array of Liebeck,R. H.,Page, M. A.,Rawdon,B. K.,Scott, P .W.,andWright, R.A.,“ Conceptsfor Advanced Subsonic Transports,” NASA CR4624,Sept. veryspecial engineers. Following is a partiallist (partial, because it 1994. seemsinevitable that a keyindividual will have been overlooked) of 3Liebeck,R. H.,Page, M. A.,andRawdon, B. K.,“ Blended-Wing-Body thosewhose contributions have been fundamental to the develop- SubsonicCommercial Transport,”AIAA Paper98-0438, Jan. 1998. mentof the BWB: JohnAllen, Amer Anabtawi, Robert Bird, Ron 4Anabtawi,A., “ ExperimentalInvestigation of Boundary Layer Ingestion Blackwelder,Derrell Brown, George Busby, Jerry Callaghan, Peter intoDiffusing Inlets,” Ph.D. Dissertation, Aerospace EngineeringDept., Camacho,Douglas Cameron, Louis Feiner, Douglas Friedman, Univ.of SouthernCalifornia, Aug. 1999. RonaldFox, Richard Gilmore, Raquel Girvin, Antonio Gonzales, 5Rodriguez,D. L.,“AMultidisciplinaryOptimization Method for De- ArthurHawley, Ronald Kawai, Ilan Kroo, David Kwok, Peter signingBoundary Layer IngestingInlets,” Ph.D. Dissertation, Dept. of Lissaman,Roger L yon,Jacob Markish, Robert McKinley, Mark Aeronauticsand Astronautics, Stanford Univ., Stanford, CA, Jan. 2001. Meyer,Joshua Nelson, Alan Okazaki, W ayneOliver, Mark Page, 6Wakayama, S.,“Blended-Wing-BodyOptimization Problem Setup,” DharPatel, Wilfred Pearce, Regina Pelkman, Chris Porter, Mark AIAA Paper2000-4740, Sept. 2000. Potsdam,Norman Princen, Blaine Rawdon, Melvin Rice, William 7Roman,D., Allen, J. B.,andLiebeck, R. H.,“ AerodynamicDesign of the Rickard,David Rodriguez, Dino Roman, Kevin Roughen, George Blended-Wing-BodySubsonic Transport,” AIAA Paper2000-4335, Aug. Rowland,Peter Rumsey, Debbie Runyan, Matt Salcius, Paul Scott, 2000.