飞机结构布局

12.1.2 The Function of the Fuselage

The fuselage structure must allow components such as lifting surfaces, engines, and landing gear to be mounted and offer adequate load paths to react the large loads these generate. Among amenities that complicate the fuselage design are the various openings that are required for easy access into and out of the volume. The openings must be carefully laid out in order to keep the number of highly stressed regions to a minimum. Since doors are usually not intended to transfer axial and shear loads (except in the case of pressurized vessels, where doors must be capable of transferring the out-of-plane pressurization loads) the openings must be reinforced to relieve stress concentrations with minimum amount of deformation of the structure. It is inevitable that each such opening (door or window) will increase stress concentration, which calls for localized reinforcement. These, in turn, increase the empty weight of the vehicle. For this reason, the designer should evaluate objectively whether a given opening into the fuselage is justifiable: is it necessary or is it just desirable? Some factors that will affect the design of the fuselage are: (1) If the airplane transports people, sufficient internal space must be given to each person. Larger transport aircraft should offer ample space for the passengers and cabin crew members to move around (for instance, to go to a lavatory, or exit in case of an emergency). (2) If the airplane is large, amenities (lavatories and galleys) must be provided for the occupants. Large passenger transport aircraft should have at least one lavatory per 50 passengers and one galley per 100 passengers. For instance, a typical 150-passenger Boeing 737 has two galleys (one in the front, the other in the back of the cabin) and three lavatories (one in the front, two in the back). (3) The cockpit should be ergonomically laid out, regardless of airplane size. This means primary instruments and controls should all be within reach of the pilot and not require him or her to lean in order to access them. (4) Windscreen shape and strength requirements will dictate the design of the forward part of the airplane and depend on airplane geometry and operational requirements (e.g. pressurization, bird strike, etc.). (5) Layout of emergency exits: for instance 14 CFR Part 121.291 requires all operators of passenger aircraft with seating capacity greater than 44 to demonstrate it can be completely evacuated in less than 90 seconds. (6) The layout of control, electrical, and other important systems. The fuselage structure should be expected to accommodate control cables, pushrods, pulleys, and wiring harnesses so they go around critical structural members and do not penetrate them. (7) The fuselage should be designed with compartments intended to carry baggage and freight that are easily accessible. If the aircraft is large, such compartments must be accessible from the outside. The fuselage must provide structure to allow baggage to be tied down so it will not shift in flight, possibly altering the CG. This structure should be stout enough to react emergency landing loads as well.

If landing gear loads are reacted by the fuselage (in contrast to the wing) this will require hoop frames in the area of the landing gear to be substantially reinforced. Typically, the main landing gear will then retract into special aerodynamically shaped housings on the bottom of the fuselage. An opening should be provided in the front part of the airplane to house the nose landing gear. The author is not aware of any instance that features a nose landing gear that retracts into a separate housing unit and not the fuselage itself. It is good practice to examine existing aircraft of similar configuration and study how the landing gear housing is designed when evaluating the pros and cons of a design direction. The fuselage must also provide structure to attach it to the wing. Commuters and similar passenger aircraft usually feature high or low wing configurations. Mewing commuters are practically unknown in modern times e the most recent one was the