2000_4335

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

AOO-39894

AIAA-2000-4335

AERODYNAMIC DESIGN CHALLENGES OF THEBLENDED-WING-BODY SUBSONIC TRANSPORT

D. Roman*, J. B. Alien**, and R. H. Liebeck***The Boeing Company, Long Beach, California

ABSTRACT

The aerodynamic design of a Blended-Wing-Body(BWB) aircraft is substantially more complicatedthan that of a conventional wing. This paper providesan overview of unique design problems faced by theBWB wing designer, discusses the applicability ofNavier-Stokes analysis, and summarizes the progressmade to date.

BACKGROUND

Development of the Blended-Wing-Body (BWB)concept began with a NASA sponsored study tocreate a new, more efficient, configuration forsubsonic transport aircraft. The initial BWBapproach to the challenge sought to improve theaerodynamics by increasing wetted aspect ratio(b2/Swet). For the payload-range specification of 800passengers and 7000 nautical miles, the BWB-concept evolved from the streamlined-disk plus wingsketch shown in Figure 1, where it is compared with aconventional "tube and wing" configuration.

Figure 1

*Senior Principal Engineer/Scientist, Senior Member AIAA** Project Engineer*** Boeing Senior Technical Fellow, Fellow AIAACopyright © 2000 by The Boeing Company. Published by theAmerican Institute of Aeronautics and Astronautics, Inc. withpermission.

Here both fuselages are sized for the same payload.When joined to identical wings, the increase in wettedaspect ratio offered by the BWB is on the order of33%. Since the cruise lift-to-drag ratio is linearlyrelated to the square root of the wetted aspect ratio,the configuration offered a substantial improvementin aerodynamic efficiency.

Transformation of the conceptual BWB of Figure 1into a realistic aerodynamic configuration began byassuming a modern aft-cambered outer wing with areflexed centerbody airfoil for trim. Morespecifically, the outboard wing was based onsupercritical airfoils with divergent trailing edges.

The large centerbody chords called for a low liftcoefficient to achieve a reasonable spanload. Thusairfoil LW102A was designed for c, = 0.25 and cmc/4 =+ 0.03 at M = 0.7 using the method of Reference 1.The resulting section is shown in Figure 2.

0.0

0.01

MOTU.

Figure 2

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Blending of the LW102A centerbody airfoils with thesupercritical outboard wing sections yielded anaerodynamic configuration with a nearly ellipticspanload. At this early stage of BWB development,the structurally rigid centerbody was assumed to offer"free wingspan". The outer wing geometry wastherefore taken directly from a tube and wingtransport and "bolted" to the side of the centerbody.The result was a wingspan of 349 feet, a trapezoidalaspect ratio of 12, and a longitudinal static margin of- 15%, implying the requirement for a fly-by-wireflight control system. In response to the concern overthe unsweep of the isobars near the centerline, thecockpit section was extended forward. This alsoaccommodated clear view requirements for the flightcrew.

A 3-view of this "first" BWB is shown in Figure 3.(A description of this original BWB study is given inReference 2.) Future generations of BWB designswould begin to address the design constraints notobserved by this initial concept, but the basiccharacter of the aircraft persists to this day.

Figure 3

OVERVIEW OF BWB DESIGN CONSTRAINTS

Due to its integrated nature, a BWB configurationmust satisfy a unique set of design requirements. Theaerodynamic designer is faced with a host of new andchallenging constraints.

Volume

First and foremost is the volumetric requirement.Since there is no dedicated fuselage, the passengers,cargo, and systems must be enveloped within the

wing itself. This constraint can lead to a requirementfor maximum thickness-to-chord ratios as high as17% in the centerbody region, a value not usuallyassociated with transonic airfoils.

Deck Angle

Since the passenger cabin exists within the wingcenterbody, the centerline airfoils must be designed togenerate the necessary lift at angle of attacksconsistent with cabin deck angle requirements. Thisrequirement suggests incorporation of positive aftcamber in the centerline airfoils.

Clean Wing Trim

A BWB wing design is considered trimmed when (atthe nominal cruise condition) its center-of-pressurecorresponds with the specified center-of-gravitylocation, and each of the trailing edge controlsurfaces is faired. Trim at stable center-of-gravitylocations requires that the nose-down pitchingmoment be minimized. This restricts the use ofpositive aft camber and conflicts with the deck anglerequirement just mentioned.

Secondary Power for Control Surface Actuation

The effective tail arms for flying wing type aircraftare small. Tailless aircraft therefore incorporatemultiple, large, rapidly moving, trailing-edge controlsurfaces. Trailing edge devices and winglet ruddersare called upon to perform a host of duties includingtrim, longitudinal and lateral control, pitch stabilityaugmentation, directional control, and wing loadalleviation. Besides the daunting challenge of controlsurface allocation, the wing designer must ensure thatcontrol surfaces will perform their functionthroughout the flight envelope with no adverseperformance impact associated with their duties. Themere size of the inboard trailing edge devices requirescareful consideration of airfoil design to minimizehinge moments. The hinge moments are related tothe control surface size by the square/cube law, thatis, size increases by the square of the scale whereashinge moments increase by the cube of the scale.Once the hydraulic system is sized to meet themaximum hinge moments, the power required isrelated only to the rate at which the surfaces move.The secondary power required can easily exceed thatcurrently available from turbofan engines.

Because the BWB is both tailless and large, thecontrol surface hinge moments are substantial. If the

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(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)1 Sponsoring Organization.

aircraft is unstable and dependent upon active flightcontrols, the secondary power required may beprohibitive. It may be necessary to design the aircraftto be stable at cruise, though doing so threatensviolation of deck angle constraints. Trimming stableat cruise forces the wing to be washed out and/ortrailing edge camber to be reduced, both leading to aloss in lift and a required increase in angle of attackto cruise.

Landing Approach Speed

The BWB trailing edge surfaces cannot be usedeffectively as flaps; the aircraft has no tail to trim theresulting moments. Deflection of the trailing edgesurfaces will be determined by trim rather than bymaximum lift requirements. Since the maximum liftcoefficient of the aircraft will be substantially lessthan that of a conventional flapped design, the BWBwing loading must be substantially lower.

Buffet and Stall Characteristics

Because the outboard airfoils have chords muchshorter than the centerbody airfoils, the outboardairfoils must operate at higher lift coefficients if areasonable cruise spanload is to be achieved.Similarly, section lift builds up faster on the outboardwing than on the centerbody section. The outboardsections tend to separate first, and thus buffetcharacteristics can be poor. Addition of a slat to theoutboard wing can lead to substantial improvementfor the low-speed, slats-extended configuration.Resolution of the clean wing characteristics problemputs considerable pressure on the wing designer toincrease the outboard wing chords and washout. Bothattacks tend to degrade the cruise performance.

Landing Approach Attitude

Because the BWB high lift system consists of anoutboard slat but no flaps, the maximum liftcoefficient occurs at a large angle of attack. Theflight attitude during approach is correspondinglyhigh.

Propulsion / Airframe Integration

The engines are located on the upper surface near therear of the aircraft centerbody. Because of theinteractions among the wing, engines, and controlsurfaces, the design of this region is exceptionallycomplex.

Manufacturing

Aerodynamic solutions to other BWB designconstraints can result in complex three dimensionalshapes that would be extremely difficult andexpensive to manufacture. The aerodynamic designermust therefore strive for smooth, simply curvedsurfaces while attempting to resolve the conflictingconstraints summarized above.

Other constraints considered proprietary by theBoeing Company also significantly effect the wingdesign.

FIRST GENERATION CONFIGURATION

Because of the complexity of the task, theaerodynamic design of the BWB has been anevolutionary process. The initial wing design failedto meet most constraints. Within each subsequentdesign cycle, attention was directed at better meetinga single unresolved constraint, while preserving theprevious progress. Today's mature design is the resultof many iterations. It meets most (but not yet all) ofthe design requirements.

The first-generation BWB design study, againsponsored by NASA, used takeoff gross weight(rather than wetted aspect ratio) as the figure-of-merit. This resulted in a reduction of the wingspan to280 feet and a corresponding aspect ratio of 10. A 3-view is shown in Figure 4.

Figure 4

The outer wing panels are moderately loaded, andagain feature aft-cambered sections with divergent

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trailing edges. As in the first BWB, the very largechord of the centerbody called for a low section liftcoefficient, which in turn permitted a large thicknessand trailing edge reflex for pitch trim. Spanwisevariation of section lift coefficient and thickness-to-chord ratio is shown in Figure 5.

aircraft maintains power as well as pitch, roll, andyaw control throughout the upset.

This first generation BWB configuration study wassummarized in Reference 3.

NAVIER-STOKES ANALYSIS

Nano* Wing Chord

Figure 5

An example analysis result at the cruise condition isgiven in Figure 6. A typical shock is evident on theouter wing. The shock on the centerbody, however, isweak and can, in fact, almost be regarded as acompression wave. The centerbody shock pattern andstrength varies little with angle of attack.

Figure 6

Buffet onset begins at the outboard kink region.During a buffet upset, the flow into the enginesremains benign, while flow across the centerline andoutboard control surfaces remains attached. Thus the

Due to the unconventional nature of the inboard wingairfoils, handbook drag build-up methods based onflat plate friction and empirical form factors simplydo not apply. These large, thick airfoils have highclosure angles and operate at Reynolds Numbers aslarge as 300 million. They are not to be found on anydesign chart. Navier-Stokes analysis, however, iswell-suited for this configuration.

For preliminary design studies the engines andwinglets can be left out of the analysis. Gridding ofthe wing geometry then becomes a simple task usingreadily available gridding tools. Excellent accuracyis obtained with under one million grid points andsolutions can be computed with one to two days turnaround time on HP 9000/780 or Silicon GraphicsOctane workstations. With several workstationsavailable, a polar can be constructed in two days.

The NASA-developed Reynolds-averaged Navier-Stokes code CFL3D (Reference 4) with the Spalart-Allmaras one-equation turbulence model was selectedfor most of the BWB design and analysis work.CFL3D was chosen due to the appeal of its upwinddifferencing scheme. The advantages of upwinddifferencing are better shock capturing and avoidanceof excessive numerical dissipation encountered incentral differencing codes. As seen in Figure 7,CFL3D provided better drag estimates than otherpopular CFD codes at cruise conditions.

Grid Method•—&— -.....E. .... SW.

A Sid.•7 SW.

Stfl.

NTF I—I-CFUO i :OVERFLOW/Upwind r-TTLNS3D/OD Matte \ I

: .:

; '/^

ns .•• \: / i.-E : ; I

b / •••'•"' ; 1 i :

— •?/$*, — AC,. ---2m :

/Xi • "\ I, : !

' - . ' : • & . ! : :

- • ' . . : V . . ; . ! . . . ; . ....... . . . . . . . . . i

Figure 7

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Coupled to the CDISC inverse design capabilitydeveloped at NASA Langley, CFL3D proved to be anvaluable tool for BWB clean wing design. Themeasured data shown in Figures 7-10 were collectedat the National Transonic Facility (NTF) wind tunnelat the NASA Langley Research Center. (Refer toReference 5 for a detailed description of the test.)Data were collected at Reynolds Numbers from 3.5 to25 million (based on the trap wing MAC) at Machnumbers from 0.5 to 0.86. The model did notincorporate nacelles. Winglets were added for only asmall number of runs.

A primary objective of the test was to demonstrate theeffectiveness of current state of the art CFD methodsfor predicting the aerodynamic characteristics of aBWB configuration. The test results shown in Figure8 demonstrate that CFL3D provides a goodapproximation of the measured drag levels at thehigher Reynolds number.

Figure 8

The lift and pitching moment characteristics shown inFigure 9 are also well predicted up to and beyondbuffet onset. The pitching moment break is welldefined.

C, !

Since many of the BWB design refinements areguided by pressure distributions, it is encouraging tosee that they are accurately predicted from cruisethrough buffet as shown in Figure 10.

o NTF- CFL3D

INBD Mid-Cruisef

OUTBD

Buffet-Onset

Figure 9

1.0 0.0

Figure 10

While CFL3D has become the preferred tool forclean wing design efforts, the OVERFLOW code,Reference 6, is preferred for analysis of morecomplex configurations because of its overlappinggrid capability and our experience level with thecode. The propulsion-airframe integration effort has,therefore, relied heavily on OVERFLOW analysis.

CURRENT GENERATION CONFIGURATION

Over the past two years a great deal of attention hasbeen given to detailed aerodynamic linesdevelopment with particular emphasis on satisfyingthe varied design requirements mentioned previously.As the configuration evolved from an 850 passengeraircraft to a 450 passenger aircraft, many changes tothe planform and airfoil stack were incorporatedwhich provided significant improvements in aircraftaerodynamic performance.

Planform

The planform changes are apparent in Figure 11where the first and current generation configurations

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are scaled and overlaid. Both the outboard andcenterbody wing chords were increased. Theoutboard chords were increased to improve the buffetonset level and characteristics.

PLANFORM COMPARISON

PLANFORM COMPARISON

Figure 11

Buffet CL tends to be more a function of thestructure's response to aerodynamic separation ratherthan just an aerodynamic phenomenon. Forargument's sake, in the absence of any information onstructural response, it is assumed that buffet CL is atthe lift versus pitching moment curve break as notedin Figure 12.

PITCHING MOMENT COMPARISONM=0.85/35K FT CFL3DV6

————Current Ccocratia_ _ _ First Generation

-- C ... BRF.AK

<— MID. CRUISE

- MID. CRUISE

-M

Figure 12

It is here that any significant separation begins tooccur at the wing critical section. Post buffetcharacteristics can be qualitatively assessed by theseverity of the break. The new wing has acomparable to slightly less abrupt break, and istypical of other Boeing aircraft. It is evident from thefigure that there is a significant improvement in CLmargin between cruise and buffet. There is ampleprotection beyond the FAA required 1.3 g's to buffeton the new wing, whereas the first generation wingcould face some problems meeting this requirement atthe higher beginning-of-cruise CL'S. The lift curve inFigure 13 shows that the current wing has almost afull two degrees of protection from mid cruise tobuffet, almost twice that of the first generation wing.

LIFT CURVE COMPARISONM=0.85/35JC FT CFL3DV6

- Current Gencnuitn• First Generation

•- C v, BREAK

-MID.CRUISt

//<—. MID. CRUISE

l .fi 2.0 .VO 4.0

(dcg.)

Figure 13

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The centerbody chords were increased to reduce theirthickness-to-chord ratios and afterbody closureangles. Though a significant amount of wetted areawas added to the configuration, the increased frictiondrag was more than offset by the improvement inreduced pressure drag. The improvement to theinboard wing flowfield is evident in Figure 14, whichdepicts the results of the inboard wing design.

Inboard Sections Redesigned

Figure 14

Airfoil Stack

Several advances in airfoil technology and designtools allowed tailoring of the inboard sections,providing the aircraft profile with the cleaner, morestreamlined appearance shown in Figure 15.

A new class of airfoils that operate efficiently attransonic speeds was designed given constraints incross sectional area needed to effectively holdpassengers, baggage and cargo. The originalapproach was to use conventional transonic airfoilsand two-dimensional design methods. This lead toexcessive thickness-to-chord ratios and associatedcompressibility drag penalties. Scaling the chord toachieve the desired dimensional thickness but lowerthickness ratio lead to excessive wetted area andassociated parasite drag. A more efficient way topackage the interior was developed through a carefulcontouring of the upper and lower surfaces of theairfoils. Though of unorthodox contour, the newairfoils were incorporated into the design without adrag penalty. Indeed, the new centerbody airfoilsincreased the lift-to-drag ratio by 4%. Moresignificantly, the new airfoils smoothed and flattenedthe geometry to allow less expensive tooling andmanufacture.

All the chordwise pressure distributions on the wingwere extensively modified using a NASA Langleydeveloped inverse design capability, CDISC,Reference 7. Using CDISC coupled to CFL3D withspecified cabin constraints, pitching momentconstraints and spanload constrains allowed fortailoring of the pressures to achieve a smoothdistribution with weakened shocks and lessaggressive trailing edge pressure recoveries. CDISChowever was not able to trim the configuration byfront loading the airfoils in an effective, i.e. low drag,manner.

Trim

The planform and airfoil stack of the current BWBconfiguration resolves the longitudinal trim problem.The aircraft now trims far more efficiently thanprevious flying wing aircraft. Historically, flyingwings have been trimmed by sweeping the wings anddownloading the wingtips. While this approachallows the wingtips to serve the function of ahorizontal tail, it imposes a severe induced dragpenalty on the aircraft. The induced drag penalty isone of the major reasons that flying wing aircraft havefailed to live up to their performance potential. TheBWB wing, on the other hand, has been trimmed by acareful distribution of trailing edge camber coupledwith a judicious application of wing washout. Theresult is a flying wing aircraft, trimmed at a stablecenter-of-gravity, with all control surfaces faired,with no induced drag penalty.

Figure 15

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CFL3D analysis was used extensively here todetermine the effectiveness of various trailing edgedroop and wing washout schemes. It was determinedthat there is a unique combination of droop andwashout that moves the wing center-of-pressure to agiven location with minimum drag impact. This isshown in Figure 16 where the solid lines are estimatesderived from CFD sensitivities and solid symbols areactual cases analyzed in CFD with the estimateddroop and washout levels.

The above process assures trim with all controlsurfaces faired for only one cruise CL. The dragpenalty for trimming the aircraft throughout the cruiserange was assessed using CFD. Deflecting thetrailing edge devices a small amount will effectivelytrim the aircraft throughout the cruise CL range withminimal drag penalty. If the aircraft is designedtrimmed at mid cruise CL then the penalty as shownin Figure 17 is only 1.0 count at start-of-cruise(higher CL) to 0.5 counts at end of cruise (lower CL).

OPTIMUM TRIM RESULTSM=0.85 / 35k FT / CRUISE

WING TIP TWIST

A C.P. LOCATION(%MAC)

OPTIMUM TRIM RESULTSM=0.85/3Sk FT/CRUISE

ESTIMATED FROM SENSITIVITY ANALYSISSOLID SYMBOLS ARE COMPUTED RESULTS

4(UD)<1%

-IK .14

A C.P. LOCATION(%MAC)

CRUISE RANGE TRIM RESULTSM=0.85/35KFT

UNTRIMMED

CRUISE C L RANGE

CRUISE RANGE TRIM RESULTSM=0.85 / 3SK FT

i(CD)-ICOUNT

Figure 16 Figure 17

The close correlation attests to the accuracy andapplicability of the sensitivity analysis. As seen inthe figure the center-of-pressure can be movedsignificantly without incurring any trim drag penalty.Remarkably, each unique combination leads toidentically the same spanload.

SUMMARY

Compared to the first generation BWB wing design,today's design delays buffet onset, improves buffetand stall characteristics, allows the aircraft to be

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7

trimmed at a stable center-of-gravity location, reducesthe secondary power demand, and simplifies themanufacturing process. Significantly, theseimprovements have been incorporated into the designin conjuction with a 16% increase in lift-to-drag ratio.The improvement is evident in Figure 18 by the levelof lower surface pressure coefficients and thecomplete elimination of the lower surface shock.

COMPARISON OF CHORDWISE PRESSURE DISTRIBUTIONSM=0.85/35K FT/CRUISE

SYMBOL ALPHA L/D——— Cunent Generation 3.870 22.00............ First Generation i.no 18.96

Current GenerationUpper-surface Isobars

Figure 18

The new wing more effectively carries the lift withless negative pressure coefficients, leading tocompressibility drag reduction. Along with inboardchord extensions, the reduced inboard thicknessresulted in significantly smaller airfoil closure anglesand a more mild pressure recovery at the trailingedge. This is beneficial for engine installation,putting them in a less accelerated flow field.

The elimination of the divergent trailing edgesignificantly changes the character of the outboardwing. Even with significantly less camber theoutboard sections operate at comparable liftcoefficients with significantly reduced shock strength.

FUTURE WORK

Future aerodynamic design efforts will continue inseveral areas. First, while the wing meets most of thedesign constraints, it does not yet meet all of them.Deck angle and secondary power issues are yet to beresolved. Second, while substantial progress has

been made in integrating the nacelles, the goal of aninterference free installation has not yet beenachieved.

Early BWB concepts envisioned a "mail slot" inletthat would ingest the wing boundary layer. Althoughaerodynamically preferable, the engines sufferedexcessive performance penalties due to inletconditions. This evolved to isolated, submergedengines that ingested the boundary layer, and later toisolated engines that diverted it. These concepts wereunacceptable from both an aerodynamic and engineperformance perspective.

The current engine integration concept involvesmounting the engines on struts, locating the inletsoutside the wing boundary layer and aft of the highvelocity region of the wing. The limited CFD workthat has been done on this concept to date lendsconfidence that an interference free engineinstallation is possible.

Third, very little attention has been focused on thewing tip/winglet aerodynamic design. Winglets onthe BWB act as vertical tails with rudders. Thedesigner must assure that the rudder will function asneeded to control the aircraft throughout the flightand rudder deflection envelopes. Currently thewinglets offset their parasite drag giving a slight dragimprovement, but this can be improved with propertailoring of winglet planform, airfoil design, twist andwing tip/winglet fairing.

Additionally, with design requirements andconstraints well defined and with a good startingpoint, aerodynamic optimization methods coupled toCFD flow solvers can now be implemented to try toimprove the wing design. Some work has been doneto date with Antony Jameson's SYN88 optimizer(Reference 8) using Euler analysis. Initial resultsanalyzed in CFL3D are very promising and indicatethat as much as an additional 5% L/D improvementcan be realized.

ACKNOWLEDGMENTS

The original BWB concept was developed underNASA contract NAS1-18763. The NTF testing wasconducted in cooperation with NASA under contractnumber NAS 1-20268. The test was staffed and CFDanalysis in support of that test was performed by theSubsonic Transport Aerodynamic Technology Groupat The Boeing Phantom Works in Long Beach. The

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authors would like to thank all involved for theirefforts.

REFERENCES

1. Liebeck, R. EL, "Design of Subsonic Airfoils forHigh Lift", AIAA Journal of Aircraft, September1978.

2. Liebeck, R. H., Page, M. A., Rawdon, B. K.,Scott, P. W. and Wright, P. A., "Concepts forAdvanced Subsonic Transports", NASAContractor Report 4624, September 1994.

3. Liebeck, R. H., Page, M. A. and Rawdon, B. K,"Blended-Wing-Body Subsonic CommercialTransport", AIAA- 98-0438, January 1998.

4. http://fmad-www.larc.nasa.gov:80/~biedron/Cfl3dv6/cfl3dv6_home.htrnl#general

5. Pelkman, R. A., "Key Findings and Conclusionsfrom an NTF Wind Tunnel Test of an InitialBlended-Wing-Body Concept", NASA ContractNAS1-20268, Boeing Report No. CRAD-9402-TR-3985, August 1998.

6. http://hpcc.arc.nasa.gov/insights/vol5/overflow.htm

7. Campbell, R. L., "Efficient Viscous Design ofRealistic Aircraft Configurations", AIAA-98-2539, June 1998.

8. Jameson, A., "Re-Engineering the DesignProcess through Computation", AIAA-97-0641,January 1997.

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2000_4335