JOURNAL OF AIRCRAFT
Vol. 41, No. 1, January–February 2004
Design of the Blended Wing Body Su bsonic Transport
R. H. Liebeck
¤
The Bo eing Company, Huntington Beach, California 92647
The Boeing Blended-Wing–Body (BWB) airplane concept represents a potential breakthrough in subsonic trans-
port ef ciency. Work began on this concept via a study to demonstrate feasibility and begin development of this
new class of airplane. In this initial study, 800-passenger BWB and conventionalcon gu ration airplanes were sized
and compared for a 7000-n mile design range. Both airplanes were based on en gine and structural (composite)
technology for a 2010 entry into service. Results showed remarkable performance improvements of the BWB over
the conventio nal baseline, including a 15% reduction in takeoff weight and a 27% reduction in fuel burn per seat
mile. Subsequent in-house studies at Boeing have yielded the development of a family of BWB transports ranging
from 200 to 600 passengers with a high level of parts commonality and manufacturing ef ciency. Studies have
also demonstrated that the BWB is readily adaptable to cruise Mach numbers as high as 0.95. The performance
improvement of the latest Boeing BWBs over conventional subsonic transports based on equivalent technology has
increased beyond the predictions of the early NASA-sponsored studies.
I. Introduction
I
T is appropriate to begin with a reference to the Wright Flyer
itself, d esigned and rst own in 1903. A short 44 years later,
the swept-wing Boeing B-47 took ight. A comparisonof these two
airplanes shows a remarkable engineeringaccompli shmentwithin a
period of slightly more than four decades . Embodied in the B-47 are
most of the fundamental design features of a mod ern subsonic jet
transport: swep t wing and empennage and podded engines hung on
pylons beneath and forward o f the wing. The Airbus A330, designed
44 years after the B-47, appears to be essentially equivalent, as
shown in Fig. 1.
Thus, in 1988, when NASA Langley Research Center’s Dennis
Bushnell asked the question: “Is there a renaissance for the long-
haul t ransport?” there was cause for re ection. In response, a brief
preliminary design study was conducted at McDonnell Douglas to
create and evaluate alternate con gurations.A preliminary co n gu-
ration concept,shown in Fig. 2, was the result. Here, the pressurized
passengercompartmentconsistedof adjacent paralleltubes, a lateral
extension of the double-bubble concept. Comparison with a con-
ventional con guration airplane size d for the same design mission
indicated that the ble nded con guration was signi cantly lighter,
had a higher lift to drag ratio, and had a substantially lower fuel
burn.
This paper is intended to chronicle the technical development of
the Blended-Wing–Body (BWB) concept. Development is broken
into three somewhat distinct phases: formulation, initial develop-
ment and feasib ility,and, nally, a descriptionof the current Boe ing
BWB baseline airplane.
II. Formulation of the BWB Concept
The performance potential implied b y the blended con guration
provided the incentive for NASA Langley Research Center to fund
a small study at McDonnell Douglas to develop and compare ad-
vanced technologysubsonictransportsfor the designmission of 800
passengersand a 7000-n mile range at a Mach number of 0.85. Com-
posite structure and advanced technology turbofans were utilized.
De ning the pressurize dpa ssengercabin for a very large airplane
offers two challenges.First, the s quare-cubelaw shows that the cabin
Received 9 June 2002; revision received 19 December 2002; accepted
for publication 10 January 2003. Copyright
c°
2003 by the American In-
stitute of Aeronautics and Astronautics, Inc. All rights reserved. Copies of
this paper may be made for personal or internal use, on condition that th e
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¤
Boeing Senior Technical Fellow, Phantom Works. Fellow AIAA.
surface area per passengeravailable for emergencyegress decrease s
with increasing passenger count. Second, cabin pressure loads are
most ef ciently taken in hoop tension. Thus, t he early study began
with an attempt to use circular cylinders for the fuselage pressure
vessel, as shown in Fig. 3, along with the correspondi ng rst cut
at the airplane geo metry. The engines are buried in the wing root,
and it was intended that passengers could egress from the sides of
both the upper and lower levels. Clearly, the concept was headed
back to a conventional tube and wing con guration. Therefore, it
was d ecided to abandon the requirement for taking pressure l oads
in hoop tension and to assume that an alternate ef cient structural
concept could be developed. Removal of th is constra int became
pivotal for the development of the BWB.
Passenger cabin de nition became the origin of the design, with
the hoop tension structural requirement deleted. Three canonical
forms shown in Fig. 4a, each sized to hold 800 passengers, were
considered. The sphere has minimum surface area; however, it is
not streamlined.Two canonicalstreamlined optionsincludethe con-
ventional cylinder and a disk, both of which have nearly equivalent
surface area. Next, each of these fuselages is placed on a wing that
has a total surface area of 15,000 ft
2
. Now the effective masking of
the wing by the disk fuselage results in a reduction of total aerody-
namic wetted area of 7000 f t
2
compared to the cyli ndrical fuselage
plus wing geometry, as shown in Fi g. 4b. Next, adding engines
(Fig. 4c) provides a difference in total wetted area of 10,200 ft
2
.
(Weight and balance re quire that the engines be located aft on the
disk con guration.) Finally, adding the required control surfaces
to each con guration as shown in Fig. 4d results in a to tal wetted
area difference of 14,300 ft
2
, or a reduction of 33%. Because the
cruise lift to drag ratio is related to the wetted area aspect ratio,
b
2
=
S
wet
, the BWB con guration implied a substantial improvement
in aerodynamic ef ciency.
The disk fuselagecon guration sketched in Fig. 4d has been used
to describe the germination of the BWB conc ept. Synergy of the ba-
sic disciplines is strong. The fuselage is also a wing, an in let fo r the
engines, and a pitch c ontrol surface. Verticals provide directional
stability, control, and act as winglets to increase the effective as -
pect ratio. Blending and smoothing the disk fus elage into the wing
achieved tr ansformation of th e sketch into a realistic airplane con-
guration. In addition, a nose bullet was added to offer c ockpit
visibility. This also provides additional effective wing chord at the
centerline to offset compressibility drag due to the unsweeping of
the isobars at the pl ane of symmetry.
Modern supercriticalairfoils with aft camber and divergent trail-
ing edges were assumed for the outer wing, whereas the c enterbody
was to be based on a re exed airfoil for pitch trim. A properspanload
implies a relatively low lift coef cient due to the very large center-
body chords.Therefore, airfoil LW102A was designed for
c
l
D
0:25
10
LIEBECK 11
Fig. 1 Aircraft design evolution, the rst and second 44 years.
Fig. 2 Early blended con guration concept.
Fig. 3 Early con guration with cylindrical pressure vessel and engines
buried in the wing root.
a) Effect of body type on surface area
b) Effect of wing/body integration on surface area
c) Effect of engine installation on surface area
d) Effect of controls integration on surface area
Fig. 4 Genesis of the BWB concept.
and
c
mc=4
D C
0:03 at
M D
0:7 using the method of Ref. 1. The re-
sulting airfoil section is shown in Fig. 5, along with a planform
indicating how pitch trim is accomplished via centerbody re ex,
whereas the outboard wing carries a proper spanload all of the way
to the wingtip. Blending of th is centerbo dy airfoil with the out-
board su percritical sections yielded an aerodynamic con guration
with a nearly elliptic spanload. At this early stage of BWB devel-
opment, the structurally rigid centerbody was regarded as offering
free wingspan. Outer wing geometry was essentially taken from a
12 LIEBECK
Fig. 5 Original centerbody a irfoil LW109A and planform showing pitch trim effector.
Fig. 6 First-generation BWB.
conventionaltransport and bolted to the side of the centerbody.The
result was a wingspan of 349 ft, a trapezoidal aspect ratio of 12, and
a longitudinal static margin of
¡
15%, implying a requirement for a
y-by-wire control system.
The aft engine location, dictated by balance requirements, of-
fered the opportun ity for swallowing the boundary layer from that
portion of the centerbody upstream of the inlet, a somewhat unique
advantage of the BWB con gura tion. In p rinciple, bo undary-layer
swallowing can provide impro ved propulsiveef ciency by re ducing
the ram drag, and this was the motivation for the wide “mail-slot”
inlet sketched in Fig. 6. However, this assumed that such an inlet
could be designed to provide uniform ow and e f cient pressure
recovery at the fan face of the engi ne(s).
Two structural concepts (Fig. 7) were considered for the center-
body pressure vessel. Both required tha t the cabin be composed of
longitudinal compartments to prov ide for wing ribs 150 in. apart to
LIEBECK 13
Fig. 7 Centerbody pressure vessel structural concepts.
Fig. 8 Flight control system architecture of the rst-generation BWB.
carry the pressureload.The rst concept used a thin,arched pressure
vessel above and below each cabin, where the pressure vessel skin
takes the load in tensi on and is independentof the wing skin. A thick
sandwich structure for both the upper and lower wing surfaces was
the basis for the second concept. In this ca se, both cabin pressure
loads and wing bendingload s are taken by the sandwichstructure.A
potentialsafety issue exists with the separate arched pressure vessel
concept. If a rupture wer e to occ ur in the thin arched skin, the cabin
pressure would have to be borne by the wing skin, which must in
turn be sized to carry the pr essure load. Thus, once the wing s kin
is sized by this condition, in principle there is no ne ed for the inner
pressure vessel. Consequently,the thick sandwich concept was cho-
sen for the c enterbody structure. A three- view of the original BWB
is given in Fig. 6, and a description of the pa ckaging of the interior
is also shown there. Passengersare carried in both single and double
deck c abins, and the cargo is carried aft of the passengercabin. As a
tailless con guration, the BWB is a challenge for ight mecha nics,
and the early control system architecture is shown in the isometric
view in Fig. 8. A complete de scription of original BWB study is
given in Ref. 2. Future generations of BWB designs would begin
to address constraints not observed by this initial c oncept, but the
basic character of the aircraft persists to this day.
III. BWB Design Constraints
As an integra ted airplane con guration, the BWB must satisfy a
unique set of design requirements. Included are the following:
A. Volume
Passengers,cargo, an d systems must be packagedwithin the wing
itself. Th is leads to a requirement for the maximum thickn ess-to-
chord ratio on the order of 17%, a value that is much higher than is
typically associate d with transonic airfoils.
B. Cruise Deck An gle
Because the passenger cabin is packag ed within the centerbody,
the centerbody airfoils mu st be designed to gene rate the necessary
lift at an angle of attack c onsistent with cabin deck angle require-
ments (typically less than 3 deg). Taken by itself, this requirement
suggests the use of positive aft camber on the centerbody airfoils.
C. Trim
A BWB con guration is c onsidered trimmed (at the nominal
cruise condition) when the aerodynamic center of pressure is co in-
cident with the center of gravity, and all of the trai ling-edgecontrol
surfaces are faired. Positive static stability requires that the nose-
down pitching moment be minimized. This limits the use of positive
aft ca mber and con icts with the precedingdeck angle requirement.
D. Landing Approach Speed and Attitude
BWB trailin g-edge con trol surfaces cannot be u sed as aps be-
cause the airplane has no tail to trim the resul ting pitching moments.
Trailing-edge surface de ection is set by trim requirements, rather
than maximum lift. Therefore, the maximum lift coef cient of a
BWB will be lower than that of a conventional con guration, and,
hence, the wing loading of a BWB will be lower. Also, because
there are no aps, the BWB’s maximum lift coef cient will occu r
at a rel atively large angle of attack, a nd the ight attitude during
approach is correspondinglyhigh.
E. Buffet and Stall
The BWB planfo rmcauses the outboard wing to be highlyloaded.
This puts pressureon the wing designerto increaseboth the outboard
14 LIEBECK
wing cho rd and washout, which degrades cruise performance. A
leading-edgeslat is requiredoutboard for low-speed stall protection.
These issues apply to a conventional con guration, bu t they are
exacerbated by the BWB planform.
F. Power for Control Surface Actuation
Tailless con gurations have short moment arms for pitch and
directional control , and, therefore, multiple, large, rapidly moving
controlsurfaces are required.Trailing-edgedevicesand winglet rud-
ders are called on to perform a host of duties, incl uding basic trim,
control, pitch stability augmentation,and wing load alleviation.Be-
cause some of the control surfaces can perform multiple functions
(e.g., outboardelevon/drag rudder offers pitch, roll, and yaw author-
ity), control surface allocation become s a criti cal issue. The mere
size of the inboard control surfacesimplies a constrainton the airfoil
design to minimize hinge moments. Hinge moments are related to
the scale of the co ntrol surface as follows: The area increasesas the
square of the scale, and, in turn, the moment increaseswith the cube
of the scale. Once the hyd raulic system is size d to meet the maxi-
mum hinge moment, the power requirement becomes a function of
rate at which a control surface is moved.
If the BWB is designed with a negative static ma rgin (u nstable),
it will require active ight contro l with a high bandwidth, and the
control system power required may be prohibitive. Alternatively,
designing the airplane to be stable at cruise requires front-loaded
airfoils, washout, and limited (if any) aft camber. This implies a
higher angle of attack, which, in turn, threatens the deck angle
constraint.
G. Man ufacturing
The aerodynamic solution to the design constraintsjust listed can
readily result i n a complex three-dimensional shape that would be
dif cult and expensive to produce. Therefore, the aerodynamicist
must strive for smooth, simply curved surfaces that at the same time
satisfy the challenging set of c onstraints just described.
IV. Initi al Development a nd Feasibility
A NASA/industr y/university team was formed in 1994 to con-
duct a three-year st udy to demonstrate the technical and commer-
Fig. 9 Second-generation BWB.
cial feasibilit y of the BWB concept. McDonnell Douglas was the
Program Manager, and the team members included NASA Langley
Research Center, NASA John H. Glenn Research Center at Lewis
Field, Stanford Universi ty, the University of Southern California,
the University of Florida, and Clark-Atlanta University. The orig-
inal 800-passenger 7000-n mile design mission was reta ined. This
work is summarized in Ref. 3.
A. Con guration De nition and Sizing
This study began with a re ned sizing of the initial BWB con g-
uration (Fig. 6), where minimum takeoff gross weigh t (TOGW) was
set as the gure of merit . Primary constraints inc luded an 11,000-
ft takeoff eld length, 15 0-kn approach speed, low-speed trimmed
C
L max
of 1.7, and a cruise Mach number of 0.85. Initial cruise al-
titude (ICA) was all owed to vary to obtain minimum TOGW, but
with the requirement that the ICA be at least 35,000 ft. Thi s yie lded
a trapezoidal wing of aspect ratio of 10, with a corresponding sp an
of 280 ft and an area of 7840 ft
2
. The resulting trapezoidal wing
loading was on the ord er of 100 lb/ft
2
, substantially lower than th e
150 lb/ft
2
typical of modern subsonic transports. An explanation
offered was that a signi cant portion of the trapezoidal wing is in
effect hidden by the centerbody, and, therefore, the cost of trape-
zoidalwing area on airplanedrag is reduced.This in turnallowed the
airplane to optimize with a larger trapezoidal area to increase span
with a relatively low cost on weight. A three-view and isomet ric of
the resulting second-generationBWB is given in Fig. 9.
The dou ble-deck BWB interior was con gured with 10 150-in.
wide passenger cabin bays, as shown in Fig. 10, with cargo com-
partments located outboard of the passenger bays and fuel in the
wing, outboardof the cargo.Considerationsandconstraintsincluded
weight and balance, maximum offset of the passengers from the ve-
hicle centerline (ride quality) and the external area of the cabin.
Because this is the surface area of the pressure vessel, the extent
of this area has a signi cant effect on the structural weight o f the
centerbody. The cabin partitions are in fact, wing ribs that are part
of the primary structure. Windows we re located in t he leading edge
on both decks, and the galleysand lavatories were located aft to help
provide the passengers with an unobstructed forward view. Egress
was v ia the main cabin doors in the leading ed ge, and through aft
doors in the rear spar.
