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Semimonocoque and monocoque structure

Seminonocoque and monocoque fuselage structure.



Truss-type fuselage structure.

Truss-type fuselage structure.



The monocoque wooden shell of the S-1 Loughead sport biplane of 1919 was formed in this concrete mold.

The monocoque wooden shell of the S-1 Loughead sport biplane of 1919 was formed in this concrete mold.



The fuselage of Loughead Aircraft's S-1, designed by Jack Northrop, was molded plywood.

The fuselage of Loughead Aircraft's S-1, designed by Jack Northrop, was molded plywood.



Monocoque and Truss-Type Construction

Monocoque is a French word meaning "single shell." It describes a type of construction used on a plane's fuselage in which wooden hoops are shaped over a curved form and then glued. Braces usually run the length of the fuselage (semi-monocoque). Strips of plywood are glued over this form. Most twisting and bending stresses are carried by the external skin rather than by an open framework, eliminating the need for internal bracing and resulting in a more streamlined airplane than with a truss-type fuselage. The first wood monocoque structure was designed by the Swiss Ruchonnet and applied to a Deperdussin monoplane racer by Louis Béchereau in 1912. This term is sometimes used interchangeably with "stressed-skin," which was originally meant to apply to the structure of wings and tail-surfaces that were laid over metal spars.

In 1918, Jack Northrop devised a new way to construct a monocoque fuselage for the Lockheed S-1 racer. The technique called for two molded plywood half-shells that were glued together around wooden hoops or stringers. But to construct the half shells, rather than gluing many strips of plywood over a form, three sets of spruce strips were soaked with glue and laid in a semi-circular concrete mold that looked like a bathtub. This process reduced the time needed from several days down to about 20 minutes. Then, under a tightly clamped lid, a rubber balloon was inflated in the cavity to press the plywood against the mold. Twenty-four hours later, the smooth half-shell was ready to be joined to another to create the fuselage. The two halves were each less than a quarter inch thick.

The truss structure, on the other hand, is not a streamlined shape. In this construction 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 and give the structure a square or rectangular shape when viewed from the end. Additional struts are needed to resist stress that can come from any direction. Stringers and bulkheads, or formers, are added to shape the fuselage and support the covering.