The construction of aircraft fuselages evolved from the early wood truss structural arrangements, to monocoque shell structures, to the current semimonocoque shell structures.

2.5.1 Truss Structure #

The main drawback of truss structure is its lack of a streamlined shape. In this method, lengths of tubing called longerons are welded in place to form a well-braced framework. Vertical and horizontal struts are welded to the longerons, giving the structure a square or rectangular shape when viewed from the end. Additional struts resist stress from any direction. Stringers and bulkheads (formers) are added to shape the fuselage and support the covering.

As technology progressed, designers began to enclose the truss members to streamline the airplane and improve performance. This was originally done with cloth fabric, which eventually gave way to lightweight metals such as aluminum. In some cases, the outside skin can support all or a major portion of the flight loads. Most modern aircraft use a form of this stressed-skin structure known as monocoque or semimonocoque construction. [Figure 2-14]

2.5.2 Monocoque #

Monocoque construction uses stressed skin to support almost all loads, much like an aluminum beverage can. Although very strong, monocoque construction is not highly tolerant to deformation of the surface. For example, an aluminum can supports considerable forces at its ends, but if its side is deformed slightly while supporting a load, it collapses easily.

Because most twisting and bending stresses are carried by the external skin rather than an open framework, the need for internal bracing was eliminated or reduced, saving weight and maximizing space. One notable, innovative use of monocoque construction was by Jack Northrop, who in 1918 devised a new way to build the monocoque fuselage of the Lockheed S-1 Racer using two molded plywood half-shells glued together around wooden hoops or stringers. The two halves were each less than a quarter inch thick. Although used in the early aviation period, monocoque construction would not reemerge for several decades due to the complexities involved. Everyday examples can be found in automobile manufacturing, where the unibody is standard.

2.5.3 Semimonocoque #

Semimonocoque construction (partial, or one-half) uses a substructure to which the airplane’s skin is attached. The substructure—bulkheads and/or formers of various sizes plus stringers—reinforces the stressed skin by taking some of the bending stress from the fuselage. The main fuselage section also includes wing attachment points and a firewall. On single-engine airplanes, the engine is usually attached to the front of the fuselage; a fireproof partition between the rear of the engine and the flight deck or cabin protects occupants from accidental engine fires. This partition, called a firewall, is usually made of heat-resistant material such as stainless steel. A newer emerging process is the integration of composites, or aircraft made entirely of composites.