جمعه ۲۳ شهریور ۰۳ | ۲۰:۴۶ ۷ بازديد
5.2 Center Body Design
In Blended Wing Body configuration, both the fuselage (center body) and wings are
integrated with each other smoothly and acts as a single body. The center body is composed of
distinct and separate wing structures, though the wings are smoothly blended into it.
The center body or fuselage results in most of the drag of the airplane (25-50 percent),
therefore center body of aircraft is designed in a shape to have minimum possible drag. Various
drags which act on fuselage are [6]:
(A). Friction drag
(B). Profile drag
(C). Base drag
(D). Compressibility drag or wave drag
(E). Induced drag
In order to have minimum friction drag, minimum wetted area is required for a given volume,
which further depends upon the shape of the body. Effect of shape on wetted area can be observed in
Fig 19. Sphere is the best option for minimum friction drag but it’s not conducive to the streamlines
and thus increases drag. Flatted disc is the second best option for minimum friction drag [14]. Profile
and base drag is determined by the front and after body shape. To have minimum profile and base
drag, ideal streamline flow is required over nose and tail. The drag related with compressibility due
to high speed is called compressibility drag. The compressibility drag includes any variation of the
viscous and vortex drag with Mach number, shock-wave drag, and any drag due to shock-induced
separations. Compressibility drag can be reduced by increasing sweep angle.
Cylinder shape used in conventional airplanes has lesser frontal area that results in lesser
profile and base drag as compared to BWB configuration. But cylindrical fuselage has more
frictional drag due to more wetted area than BWB fuselage for same volume
During designing of fuselage, trade-off has to be made between various drags to get best
possible shape. For BWB 601 fuselage, sphere is flattened to streamlined disk, which is
integrated with wings to have minimum wetted area.
Figure 19: Effect of shape on wetted area [14]
The cabin has to be designed for internal pressure in addition to bending, shear and torsional
loads. It should be noted that disc shape cabin requires more strength for same internal pressure as
compared to conventional cylindrical; this is due to the fact that in a conventional cylindrical
fuselage, internal pressures are carried more efficiently in hoop stresses by a thin skin, whereas for
disc shape fuselage, internal pressure induces large bending stresses which require heavier structure.
Studies have been conducted by NASA and Boeing to address this structural issue [4]. They
investigated two concepts: Multi bubble fuselage structure and single strong shell (Fig 20 and Fig
21). Multi bubble structure consisted of cylindrical shells inside main disc for sustain internal
pressure loads and outer skin to support bending. Boeing argued with multi-bubble theory and raised
the issue that outer skin still needs to be designed to take internal pressure loads in case there is any
leakage in the inner bubble. As the outer skin has to be designed for internal
pressure, there is no point to build inner shells. Their research concluded to use single shell structure
strong enough able to withstands all the loads. The additional weight due to heavy structure should
not be problem as the aerodynamic gains from BWB configuration will outnumber this weight
increase. For BWB 601 cabin design, single shell approach will be used (Fig
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