The main body of a plane or helicopter, the fuselage, supports the engine, passengers and cargo. The fuselage must be able to withstand stress caused by the speed of flight, air pressure and the weight of its contents. There are two primary types of fuselage construction methods used in the aircraft industry, monocoque and welded truss. A third type of fuselage, semi-monocoque, is a hybrid of the two.
Monocoque fuselages use a rigid outer skin attached to a series of bulkheads that are supported by stringers. The rigid skin is considered "stressed' because it is inflexible and is typically made of wood or metal. The skin on a monocoque fuselage handles all the stress on the body of the fuselage, not the supporting bulkheads structure.
A welded truss fuselage uses metal tubes (longerons) along the whole length of the body welded to supporting metal tubes to form the frame. The frame absorbs the various stresses the body is exposed to, allowing the skin to be constructed of light, flexible, composite or rigid materials.
Semi-monocoque fuselages use additional longitudinal support members between the bulkheads to allow more of the stress on the craft to be dispersed through the frame than in monocoques. This allows for less rigid and lighter skins to be used, decreasing the weight of the fuselage.
Light, personal planes normally use a welded truss frame, as neither the weight of the cargo, the plane's speed, nor the maximum altitude that the plane reaches puts much stress on the fuselage. Helicopters, commercial aeroplanes and jets all experience much greater stresses in flight and use the monocoque or semi-monocoque fuselage constructions.