Boeing Area Rule Aircraft
It is well known that as an aircraft approaches the high subsonic flight regime, there is a steep rise in aircraft aerodynamic drag. The onset of the rise in drag results from local regions of sonic or supersonic flow that occur on parts of the aircraft because of the contour of the aircraft surfaces; such regions of locally sonic or supersonic flow typically arise at flight Mach numbers of about 0.8 or so for many aircraft.
As the Mach number is increased beyond this threshold, the drag begins to rise at a steep rate. It is known that the onset of this drag rise can be delayed to a higher Mach number by careful design of the aircraft fuselage and wing. In particular, it is known that so-called area-ruling of the aircraft fuselage can be effective in delaying the onset of the transonic drag rise. In accordance with this technique, the fuselage in the vicinity of the fuselage-wing interface is contoured so as to locally reduce the fuselage cross-section to compensate for the cross-section of the wing.
Area-ruling of the fuselage of a passenger aircraft involves a number of design considerations, not the least of which is the desire to provide adequate space for the passengers so that they will not be cramped. Unfortunately, the desire to area-rule the fuselage in the vicinity of the wing is at odds not only with the need to maintain adequate passenger seating space but also with other design features in this part of the aircraft. For instance, traditionally the wing-fuselage intersection of a low-wing passenger transport aircraft includes a large fairing defining the lower aerodynamic surface of the fuselage in the area below the center portion of the wing that passes through the fuselage.
The fairing is needed in order to accommodate stowed landing gear, to house air conditioning units, for structural and aerodynamic reasons, and to protect the center fuel tank in the wing in the event of a landing with the landing gear not deployed. The fairing increases the fuselage cross-section at precisely the longitudinal station where it would be desirable to reduce the fuselage cross-section, i.e., at the wing-fuselage intersection. Consequently, at high subsonic flight Mach numbers (e.g., M=0.85 or above), the fairing contributes substantially toward overall aircraft drag.
On such aircraft, the area-ruling of other regions of the fuselage at the longitudinal stations corresponding to the wing's maximum cross-sectional area can be effective in lessening the deleterious impact of the fairing and the wing with respect to transonic drag. Area-ruling of the fuselage in the horizontal direction is not practical, however, because it leads to inefficiencies in the use of the space in the fuselage for passenger seating.
This goal is easier to state than to achieving, as is difficult in practice because of the many countervailing design constraints. One important constraint is the need to protect the center fuel tank of the aircraft in the event of a gear-up landing. In such a landing, the aircraft will essentially slide on its belly on the runway, thus bringing the center fuel tank into close proximity with the ground. There must be adequate structure between the ground and the tank to prevent the tank from rupturing. The fairing described above traditionally plays an important role in this regard. Thus, the problem becomes how to achieve a greater extent of area-ruling of the fuselage in the vicinity of the wing-fuselage intersection, in view of the traditionally required fairing and the need to maintain adequate passenger space.
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