TM 1-410. Technical Manual, Airplane Structures 1941: Section 2 - Fuselages

SECTION II: FUSELAGES

5. General.-Fuselages of the various types of airplanes have much in common from the standpoint of outline and general design. They vary principally in size and in arrangement of the different compartments. In the case of single-engine airplanes the power plant is mounted on the nose of the fuselage, while on the multiengine airplanes nacelles are used for this purpose. Detail design varies with manufacturer and requirements of the service for which intended. In some cases the fuselage extends laterally to include stub wings which are really a part of the fuselage and built integral with it, as shown in figure 10. On the amphibian-type airplanes the boat hull replaces the f uselage.

6. Structural features.-a. Fuselages of most military airplanes are of all-metal construction assembled in a modification of the monocoque design. The monocoque design relies largely on the strength of the skin or shell (covering) to carry the various loads. This design may be divided into three classes-monocoque, semimonocoque, and reinforced shell - and different portions of the same fuselage may belong to any one of these classes. The monocoque has as its only reinforcement vertical rings, station webs and bulkheads; the semimonocoque in addition to these has the skin reinforced by longitudinal members, that is, stringers, and longerons, but has no diagonal web members; and the reinforced shell has the skin reinforced by a complete framework of structural members.

The cross-sectional shape is derived from bulkheads, station webs, rings, etc., and longitudinal contour is developed with longerons, formers, and stringers as shown in figure 11. The skin (covering) which is riveted to all these members carries primarily the shear load,

FIGURE 10-Fuselage assembly.

and together with longitudinal members, the loads of tension and bending stresses. Station webs are built-up assemblies located at intervals to carry concentrated loads and at points where fittings are used to attach external parts such as wings, landing gear, engine mounts, etc. Formers and stringers may be single pieces or built-up sections.

b. The metal in general use for fuselage construction is aluminum alloy, principally one or the other of the two alloys commercially known as 17ST and 24ST. These are about three times lighter than steel and after being heat-treated have a strength approximately equal to that of mild steel. For some uses, generally surface covering, this alloy is made in sheets with a thin covering of pure aluminum on both sides. In this form it is known commonly by the trade name "Alclad." The pure aluminum serves as a protective coating to the base metal and need not be painted to protect the surface from corrosion.

c. Nacelles may be constructed as complete separate units, streamlined by cowling and installed on the airplane above, below, or, in the case of biplanes, between the wings. As a general rule, however, they are built either into or as an integral part of the leading edge of the wing structure as shown in figure 12. Welded tubular steel framework construction is extensively used, although they are sometimes of a semimonocoque construction similar to that of the fuselage. In some of the larger types of airplanes this latter construction is employed to permit access of crew personnel to the engine for the purpose of making adjustments, repairs, or replacement of accessories during flight.

FIGURE 11-Fuselage structure assembly.

d. A firewall is used to separate the engine compartment from the main structure of the airplane in order to localize any fire that might originate around the engine. On single-engine airplanes, it is mounted on the main fuselage structure and on multiengine airplanes it forms a part of each nacelle structure. The firewall is made of comparatively thick material, usually terneplate or stainless steel. All openings for conduits, tubing, controls, etc., are closed as nearly as possible by stuffing glands, gaskets, or closely fitted sections of fiber composition. Fiber is less likely to cut or damage such lines through vibration than the metal edges of openings in the firewall.

7. Inspection and maintenance .-Since inspection and maintenance procedure of fuselage and wing structure is so similar, details for both are given in paragraph 13.