No one had to guess at the fuselage construction of the earliest aeroplanes. The spindly, strut-and-spar skeleton beneath the thin fabric covering was easy to detect. As aeronautical materials evolved, however, fuselage structure became more complex. Variations came and went, but three basic fuselage design themes have endured through most of aviation's history. In recent years, the advent of composite materials has added a fourth.
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In truss construction, long lengths of wood or welded steel tube form a basic fuselage framework. They are braced and supported at 90-degree angles by shorter cross-member struts. Bulkheads and lateral stringers are integrated to support the outer covering, which was originally fabric and later thin plywood or corrugated metal. The resultant structure was somewhat boxy. This lack of aerodynamic shape is evident in aircraft in the 1930s. Nevertheless, truss construction produced a sturdy framework that lent itself to existing materials such as wood and/or steel and could be easily mass-produced.
Monocoque construction uses the skin of the fuselage to bear the structural load with bulkheads at each end and forming rings at intervals to maintain the skin shape. Utilising semicircular strips of lightweight spruce glued together in a mould, this method enabled the first truly round or aerodynamically shaped fuselages. Because internal cross-members are eliminated, monocoque construction is much lighter. However, while it resists twisting and bending forces, the lack of internal support structure makes monocoque fuselages vulnerable to deformation and impact forces which can result in collapse of the structure.
A hybrid of truss and monocoque, in semi-monocoque construction panels of aerodynamically-curved skin are riveted on top of an internal structure consisting of bulkheads, stringers and followers to absorb the bending forces. The introduction of easily shaped aluminium as a skin material plus lightweight aluminium stringers and followers made semi-monocoque construction the preeminent design. Wing attachment points on the fuselage are reinforced, and in single engine aircraft the front bulkhead also serves as the engine mount and firewall.
Non-metallic composite materials like carbon fibre reinforced plastic -- CFRP -- are revolutionising aircraft fuselage structure. Already playing a major role in smaller general aviation aircraft like the best-selling Cirrus line, composite fuselage construction has now come to fruition in a big way with the Boeing 787. Large one-piece segments of the 787's all-composite fuselage are made by robotically wrapping carbon filament tape around a mandrel to form the fuselage's barrel shape, impregnating the unit with polymer resin and curing it in an enormous autoclave. The resultant fuselage segment is entirely self-supporting and requires no internal structural members. Construction is achieved without the thousands of fasteners required in aluminium construction and overall weight is reduced. Also, CFRP is more rigid than aluminium and not subject to metal fatigue over time. The Airbus A350, still in development at time of publication, will use individual panels of composite materials mounted on top of a metal airframe to form the fuselage.
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