Blended-Wing-Body (BWB) Fuselage Structural Design for Weight Reduction
Alexander Bell Drive, Suite 500, Reston VA 20191-4344
AIAA 2005-2349: Tracking No: 25578
BLENDED-WING-BODY (BWB) FUSELAGE STRCTURAL DESIGN
FOR WEIGHT REDUCTION
V. Mukhopadhyay*
NASA Langley Research Center, Hampton, VA
Abstract
Structural analysis and design of efficient pressurized fuselage configurations for the advanced Blended-Wing-Body (BWB) flight vehicle is a challenging problem. Unlike a conventional cylindrical pressurized fuselage, stress level in a box type BWB fuselage is an order of magnitude higher, because internal pressure primarily results in bending stress instead of skin-membrane stress. In addition, resulting deformation of aerodynamic surface could significantly affect performance advantages provided by lifting body. The pressurized composite conformal multi-lobe tanks of X-33 type space vehicle also suffered from similar problem. In the earlier BWB design studies, Vaulted Ribbed Shell (VLRS), Flat Ribbed Shell (FRS); Vaulted shell Honeycomb Core (VLHC) and Flat sandwich shell Honeycomb Core (FLHC) concepts were studied. The flat and vaulted ribbed shell concepts were found most efficient. In a recent study, a set of composite sandwich panel and cross-ribbed panel were analyzed. Optimal values of rib and skin thickness, rib spacing, and panel depth were obtained for minimal weight under stress and buckling constraints. In addition, a set of efficient multi-bubble fuselage (MBF) configuration concept was developed. The special geometric configuration of this concept allows for balancing internal cabin pressure load efficiently, through membrane stress in inner-stiffened shell and inter-cabin walls, while the outer-ribbed shell prevents buckling due to external resultant compressive loads. The initial results from these approximate finite element analyses indicate progressively lower maximum stresses and deflections compared to the earlier study. However, a relative comparison of the FEM weight per unit floor area of the segment unit indicates that the unit weights are still relatively higher that the conventional B777 type cylindrical or A380 type elliptic fuselage design. Due to the manufacturing concern associated with multi-bubble fuselage, a Y braced box-type fuselage alternative with special resin-film injected (RFI) stitched carbon composite with foam-core was designed by Boeing under a NASA research contract for the 480 passenger version. It is shown that this configuration can be improved to a modified multi-bubble fuselage which has better stress distribution, for same material and dimension.
I. Introduction
Structural analysis and design of efficient pressurized fuselage configurations for the advanced Blended-Wing-Body (BWB) flight vehicle has been a challenging problem for many years with no clear solution. Unlike a conventional cylindrical pressurized fuselage, stress level in a non-cylindrical shoebox type BWB fuselage is an order of magnitude higher, because internal pressure primarily results in bending stress instead of skin-membrane stress. The pressurized composite conformal multi-lobe tanks of X-33 type space vehicle also suffered from similar problem. Thus, the primary objective is to specially design the primary highly loaded structure and geometry such that the load path leads to mostly membrane stress, thus minimizing the overall structural weight of the vehicle, while satisfying stress, deflection and buckling safety factors, under the critical flight and ground loads. However, due to manufacturing consideration, large under-carriage retraction bay, pressure bearing main rear spar and additional flat pressure bulkheads, a non-traditional design approach and material are required, in order to increase bending moment of inertia, without paying significant weight penalty. The first and second-generation 800 passenger BWB concepts were developed at Boeing Phantom Works through NASA contract and internal research.Additional conceptual design of an efficient fuselage configuration concept was developed under NASA R&D funding.
In this paper, the lessons learned from the early structural design studies of the BWB 800 passenger version are summarized. Progresses towards recent 480 passenger version of BWB structural analysis are described. Use of rapid finite element analysis tools and results of several geometric configurations of the Y-braced fuselage
derivatives are presented. Design improvements are suggested.
II. Lessons learned from early design studies :
Unlike the traditional aircraft tubular fuselage, the high stress and weight problem associated with BWB pressurized cabin can be explained using the sketch in Figure 1. This figure illustrates a cylindrical and a square box fuselage under internal pressure p . In a cylindrical pressure vessel of radius R and skin thickness t , the pressure is resisted by uniform stretching resulting membrane stress is equal to p(R/t). In BWB box like fuselage, the nearly flat upper cabin wall resists the pressure by bending deformation. Let us model it as a simply supported beam or plate of length l , thickness t, then the maximum bending stress is equal to 0.75p(l/t)2. Assuming R is of same order as l , the bending stress is one order of magnitude higher. The problem is aggravated by the non-linear effect of compressive load acting on the deflected beam or plate. So in order to obtain an efficient structure, one must increase the bending stiffness using deep sandwich shell with light weight high-strength composite skin with composite deep stiffener. The alternative is to use a multi-bubble concept shown in the inset sketch. With proper design, the adjacent bubble membrane stress resultant is balanced by tension in the intra-cabin wall.
Non-Cylindrical Pressure Vessels
It is difficult to contain pressure in a shoebox!
)
typical: R/t = 1000
Based on the design load definition
developed in Ref. 1, a beam-column based sizing calculation, a set of coarse finite element model (FEM) were developed in Ref. 4. The vaulted shell radius was chosen so as to provide a minimum of 4 inch depth at the mid-arch. From this early FEM analysis, the vaulted ribbed shell and flat ribbed shell concepts appeared to be significantly better than the deep sandwich concept. The vaulted concept offered the advantages of cylindrical pressure vessel, but was not preferred due to manufacturing complexity. The deep sandwich concept was eliminated due to weight and maintenance considerations. The pressure bearing front and rear spar were modeled as deep sandwich honeycomb flat shell and contributed significantly to the overall weight. The elastic modulus and allowable stress properties of an orthotropic resin-film injected (RFI) stitched carbon composite material were used. The initial sizing was based on the composite material allowable stress, but no optimization or buckling analysis was performed. Figure 1. High bending stress associated with a non-cylindrical pressure vessel.
The first and second generation 800
passenger BWB concepts were developed at Boeing Phantom Works (formerly McDonnell Douglas) through NASA contract and internal research (Ref. 1-3). A conceptual design of an efficient fuselage configuration concept was developed under NASA R&D funding and research findings were reported in Refs. 4 and 5. In Ref. 4, an isolated fuselage bay-3 of the
early 800 passenger version of the BWB design
was analyzed. A schematic diagram of the vehicle platform and the bay-3 cross section of two fuselage concepts are shown in Figure 2. Four structural concepts were studied, namely, Vaulted Ribbed Shell, Flat Ribbed Shell; Vaulted shell with Light and Heavy Honeycomb Core; Flat sandwich shell with Light and Heavy Honeycomb Core. A relative comparison of bay-3 weight and component non-optimal weight breakdown were made.
150
150
175 R
10
4
FLAT SHELL CONCEPT
VAULTED SHELL
CONCEPT
Figure 2. BWB fuselage bay-3 section, flat ribbed and vaulted ribbed shell concepts of first generation BWB 800 passenger version.
From the isolated bay-3 FEM analysis
results, a comparison of weight components of these four concepts is shown in Figure 3. From this study, the double skin vaulted ribbed shell concept appeared to have the least FEM structural weight. Hence, a coarse finite element model of the vehicle super structure was developed and analyzed using the double skin vaulted ribbed shell concept for the pressurized fuselage section and the outer wing.
The vehicle element von-Mises stress distribution and deformation at the critical 2.5 g pull-up load case are shown in Figure 4.
Figure 3. Weight component comparison of bay-3 fuselage concepts.
52,000psi
26,000250
Load case 3:
18.6 psi internal cabin
pressure on inner vaulted skin,
490,000 lb elliptic aero load at 2.5g pull-up maneuver
54,000psi
27,000
400Load case 3:
18.6 psi internal cabin
pressure on outer flat skin,490,000 lb elliptic aero load at 2.5g pull-up maneuver
Figure 4. Von-Mises stress and deflection of double skin ribbed vaulted shell fuselage and primary wing structure.
In this FEM analysis, the stress levels at
the wing-fuselage junction and wing rear spar were well above the allowable limit and needed to be resized and redesigned. The pressure bearing front and rear spar also needed to be modeled properly. Moreover, no sizing study or bucking analysis was performed. The FEM weight of this configuration of half the vehicle structure was 29200 lbs (13245 kg). The actual as built weight of the airframe with all the fasteners, doublers, are usually twice the FEM weight. In later design, the front pressure bearing spar was eliminated and the wing leading edge curved surface was designed to bear the cabin internal pressure load.
Based on the full vehicle FEM analysis
from Ref. 1-3, a set of critical design loads were developed for detailed design and optimization of the most highly loaded central section fuselage
panel. These load definition was used in Ref. 5, for the following three structural analyses models and design study.
(a) Idealized beam-column model stress and bucking analysis:
(b) Top surface panel plate model FEM analysis and optimization of rectangular composite plates: (c) Multi-bubble model concept FEM analysis of a representative section of the BWB fuselage:
III. a) Idealized beam-column model
An idealized beam-column model stress
and bucking analysis including the nonlinear effect of bending deflection was performed using closed form solution. Analysis equations and sample results of this idealized deep sandwich with aluminum skin are shown in Figure 5.
P Pcr
Figure 5. Idealized beam-column analysis with nonlinear effect of bending deflection.
Figure 6. Top surface ribbed and vaulted plate panel models at a highly loaded fuselage section.
b) Top surface panel plate models
The top surface panel models at a highly loaded fuselage section are shown in Figure 6. A
rectangular composite sandwich plate model, a cross-ribbed flat panel, a vaulted ribbed panel, and a catenary’s supported panel were analyzed in Ref.
5. A summary of FEM analysis and optimization results are presented in Table 1.
Concept Initial Optimized
depth skin t p/pcr wt/area depth skin P/Pcr wt/area
max disp.
material meter meter kg/sq.m meter meter kg/sqm. meter
2D HC Beam
0.150.006
0.4
42.4
0.2640.00450.18 37.90.03AL 2D HC Beam 0.15240.0064 0.42 25.550.17780.00510.37 21.30.048SRFI
flat HC 0.1370.003 0.5 15.30.2010.00460.66 24.10.0086SRFI flat ribbed 0.1520.003 0.5 13.50.1840.00580.66 25.60.0064SRFI flat HC+cat 0.1670.003 0.5 15.40.2010.00460.71 24.20.009SRFI vaulted HC 0.20.003 0.5 170.170.00330.66 25.40.0112SRFI
Table 1. Summary of beam column and plate analysis and optimization results.
c) Multi-bubble models
The multi-bubble fuselage configuration concept was developed for balancing internal cabin pressure load efficiently, through membrane stress in inner-stiffened shell and inter-cabin walls. An outer-ribbed shell was designed to prevent buckling due to external resultant compressive loads. For comparison purposes, FEM based structural analysis results were also presented for conventional B777 type cylindrical fuselage section and A380 type elliptic section fuselage. Figure 7 shows a schematic view of the three vehicle fuselage sections.
Figure 7. Conceptual Fuselage section analyzed for B777, A380 and BWB 800 class vehicles.
Double-bubble design:
As explained earlier, and from the design
results described in Ref. 5, the rectangular composite sandwich plate and cross-ribbed panel designs were found to be structurally inefficient to carry the internal cabin design pressure and compressive load simultaneously. Based on the lessons learned from these analyses, 3-floor load-
balanced multi-bubble stiffened-shell pressure
vessel concepts were developed. Diameters of the
cylindrical segments were almost same as a typical
B777 cylindrical fuselage. In this design shown in Figure 8, the two merging bubble-sections meet with the inter-cabin vertical wall at an angle, so that surface in-plane membrane forces are in self-equilibrium. Thus in an ideal case, the resulting membrane stresses on the cylindrical section skin are balanced by tension in the inter-cabin walls. This geometrical arrangement could reduce undue bending at these joints, thereby preserving the advantage of a cylindrical section fuselage, under internal cabin
pressure.
1.90 E+8
1.43 E+8
9.57 E+7
4.79 E+7
3.14 E+5
Von-Mises nodal stress,Pascals (0.000145 psi)
Figure 8. Double-bubble concept: Nodal Von-Mises stress in due to 127530 Pascal (18.6 psi ) internal pressure.
Triple-bubble design:
This special geometry was extended to a triple-bubble 3-floor multi-aisle fuselage, shown in Figure 9. These force-balanced double and triple bubble configurations were extended to a four-bubble 3-floor concept with additional outer
stiffened double panels, at top and bottom of the fuselage, as shown in Figure 10. In this and subsequent FEM models, cylindrical skin segments were stiffened with ring stiffeners, which are generally typical for commercial transport aircrafts.
Von-Mises nodal stress,Pascals (0.000145 psi)
1.81E+8
1.35E+8
9.03E+7
4.52E+7
0.0
Figure 9. Triple bubble force balanced three floor fuselage configurations: Nodal Von-Mises stress due to 127530 Pascal (18.6 psi ) internal pressure
These outer panels initially consisted of
two stiffened shells supported by the cylindrical inner fuselage sections at midpoint. These outer shells were not connected to the inter-cabin vertical walls directly at top. The outer shell was added to provide bending and buckling stiffness to span-wise bending loads that were not considered in the double- and triple-bubble concepts. This model was subjected to the standard 12.3 psi (84835 Pascal) internal cabin pressure as load case 1. For load case 2, top stiffened panels were subjected to 164350 kg/meter span-wise compressive loads. Equal and opposite tensile loads were applied at bottom panels, to represent an equivalent maximum estimated bending moment.
Four-bubble-design:
The four-bubble configuration, shown in
Figure 10 was analyzed with internal design pressure loads, as well as with estimated equivalent compressive loads on top panels (and equal tensile load on bottom panels) due to fuselage bending, in order to obtain acceptable stress, deflection and buckling stability safety factor.
Five-bubble-design:
In the five-bubble fuselage concept,
shown in Figure 11, the radius of inner cabin vaulted ceiling was reduced from 3.875m to 3.75 meters. Radius of the outer cabin was reduced to 3.248 meters in order to get approximate
membrane stress equilibrium at outboard joints. Additional span-wise running tie-rods were also used at the top and bottom of the cabin. Since only half the fuselage was modeled, symmetric clamped boundary conditions were assumed at the plane of
symmetry.
Von-Mises
2.50 E+8
1.88 E+8
1.24 E+8
6.22 E+7
0.0
element stress, Pascals (0.000145 psi)
Fig. 10 Element nodal Von-Mises stress at top due to 84835 Pascal (12.3 psi) internal cabin pressure and 164350 kg/meter (9200 lbs/in) span-wise compressive load on top panel and equal tensile load at bottom panel.
Von-Mises element stress, Pascals (0.000145 psi)
1.75 E+8
1.30 E+8
8.73 E+7
4.36 E+7
0.0
Fig. 11 Five-bubble force-balanced stiffened fuselage concept with vented double outer skin: Element nodal Von-Mises stress at top surface due to 84835 Pascal (12.3 psi) internal cabin pressure and 164350 kg/meter (9200 lbs/in) span-wise compressive load on top panel and equal tensile load at bottom panel.
IV. Unit weight comparison.
Initial results of the redesigned multi-bubble fuselage appear to be significantly better compared to the flat shell design. Figure 11 shows Von-Mises stress distribution at element nodes computed on top surface due to combined internal cabin pressure and span-wise compressive load on top panel and equal tensile load at bottom panel. These stresses were well within allowable limits and about 25% lower than the four-bubble design with about 10% increase in unit weight/floor area.
10l b /s q f t f l o o r a r e
a
4-Bubble-19.3m
2-Bubble-11.6m
Elliptic
Figure 12. Relative Finite Element Model weight of fuselage configurations per unit floor area: (Ref. 4: FHHC- flat sandwich shell with heavy honeycomb core; VHHC- vaulted shell with heavy honeycomb core; VLRS- vaulted ribbed shell; HHC- heavy honeycomb core flat shell).
Figure 12 shows a relative non-optimal
FEM weight comparison from Ref. 5. These
relative unit weights were computed as ratio of
total finite element weight of the structure divided
by floor areas inside the fuselage section. First
three concepts labeled BWB bay-3 weights of
vaulted shell with heavy honeycomb (VHHC); flat
sandwich shell with heavy honeycomb core
(FHHC); and vaulted ribbed shell (VLRS) also
included side walls, front and rear pressurized spar
of similar construction 4. It should be noted that
these previous designs were not optimized or
analyzed for buckling stability, although sizing was
done using 2-D nonlinear beam-column analysis
similar to that described earlier in the paper.
V. BWB Derivative Vehicle Sizing
The weight comparison shown in Figure 12 provided a trend of fuselage FEM weight per unit floor area. These FEM models contained beam
elements for stringers and stiffeners and shell elements for skin. Since these FEM models used for the multi-bubble concept were too detailed to extend into a full vehicle FEM, Bradley (Ref. 6) recently developed a simple methodology for sizing a conceptual derivative BWB vehicle using an approximate equivalent thickness property with plate finite element analysis. Equivalent plate thickness was obtained from the bending stiffness of skin-stringer combination and equivalent isotropic plate.
Figure 13 Equivalent flat plate thickness and elastic property estimation scheme (Ref. 6).
The equivalent plate thickness and
Young’s modulus were obtained as shown in Figure 13. The extensional stiffness, k, and bending stiffness, E*I, were equated for the skin/stringer arrangement and the equivalent flat
plate. The figure also shows the equations used to
determine the thickness and modulus of elasticity
of the equivalent flat plate. Although this scaling
did not use standard non-dimensional parameters
for plate bending and buckling equations, this
simplification enabled sizing of a series of scaled
FEM model of 200 to 480-passenger version
vehicles with design variables such as Cabin Area
(5 models), take-off gross weight (to provide
aerodynamic load distribution), and Skin/stringer
depth and spacing arrangement modeled with
equivalent flat plate. However this study did not
take advantage of the multi-bubble geometric
configuration and its stress balancing property,
since it was intended for projecting a weight trend
based on the number of passengers, with 480
passenger version as a baseline. Moreover,
material modulus of elasticity property scaling resulted in under estimation of the weight. The weight regression equations had to be scaled using
baseline vehicle weight data.
VI. Y braced box fuselage In a recent study of a 480 passenger version of BWB by Boeing under a NASA research contract (Ref. 7), box-type Y braced fuselage alternative was designed and analyzed with special resin-film injected (RFI) stitched carbon composite skin with foam-core. The Y brace reduces the bending at the joint of the roof and cabin walls. The stitched foam-core with RFI skin provides higher bending stiffness without
adding significant weight penalty. The design also
considered span-wise I-section and J-section
stiffeners with RFI-foam construction.
Figure 14. Box fuselage scheme with Y-brace for
480 passenger version BWB vehicle.
A schematic view of this 3 bay Y braced fuselage concept is shown in Figure 14. An initial finite element analysis of the fuselage indicated excessive stress at the joint where the pressurized section ends and non-pressurized section begins. To alleviate this stress, additional Y brace were
added as shown in Figure 15a and 15b.
Figure 15a. Modified fuselage design with additional
Y brace at wing-fuselage junction.
Figure15b. Factor of safety distribution of fuselage slice with AL6061 material, 24 inch (0.61 m) frame spacing, 0.25 inch (0.0063m) thick skin, at 9.3 psi (62760 Pascal) internal pressure and 1 psi (6896 Pascal) floor load.
Figures 16a and 16b show the modified
fuselage designed with the Y brace replace by
vaulted shell. A large number of rapid FEM analyses were done with solid and ribbed shell finite element model for different configuration for stress,
deflection and safety margin check using SolidWorks and Cosmos/DesignStar FEM software. Each FEM model represent a slice of fuselage with 24 inch (0.61 meter) frame spacing, 0.25 inch (0.0063m) skin and frame with AL6061 material, with about 26,000 degrees of freedom. The fuselage is subjected to 9.2 psi (62760 Pascal) internal pressure load and 1 psi (6896 Pascal) floor load. The FEM model is inertia balanced to satisfy free
body boundary condition.
Figure 16a Modified fuselage designed with Y brace
replaced by vaulted shell.
Figure 16b Factor of safety distribution on modified fuselage designed with Y brace replaced by vaulted shell, AL6061 material, 24 inch (0.61 m) frame spacing, 0.25 inch (0.0063m) thick skin, at 9.3 psi (62760 Pascal) internal pressure and 1 psi (6896 Pascal) floor load.
Modeling of sub-structure with stitched
composite skin and foam core is necessary for future detailed analysis, as an alternative to multi-bubble aluminum skin-stringer construction. Detailed full vehicle finite element analysis is presently being conducted at Boeing. The Boeing design report also proposed, for phase-II, fabrication of a series of test components and tests in order to determine the elastic properties of test
coupons for future finite element analysis and to establish the failure modes of stitched RFI composite construction of vehicle components. Conclusions Design and analysis of efficient structural concepts for pressurized fuselage design of blended-wing-body type flight vehicles were presented. Vaulted shell and special multi-bubble geometric configuration are efficient in distributing the stress due to internal pressure load for these non-conventional flight vehicles. Due to manufacturing concern, Y braced box type fuselage design using
special composite material construction is a possible practical alternative. It is shown that this configuration can be improved to a modified vaulted shell partial multi-bubble type fuselage which has better stress distribution, for same material and dimension. Efficient design of non-cylindrical pressurized structure is vital for non-conventional Aeronautics and Exploration Mission vehicles. Advanced Geometric configuration for stress balancing and Composite Material fabrication techniques are essential. Structural weight penalty can be severe. For a successful design, it is necessary to develop efficient rapid geometric and structural layout, FEM analysis and optimization tools.
Acknowledgement
The author wishes to thank Dr. Robert Liebeck, Daniel Hansen and Alexander Velicki of Boeing Company and Kenneth Bradley of US Air Force for their contribution and insight. The work was funded by Efficient Aerodynamics and Shape Integration (EASI) project, Conceptual Design Shop (CDS) sub-project. The finding support by EASI Project manager Jim Pittman and EASI/CDS sub-project Manager Craig Nickol are gratefully acknowledged.
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