Monocoque

Monocoque (/ˈmɒnəkɒk, -koʊk/ MON-ə-ko(h)k), also called structural skin, is a structural system in which loads are supported by an object's external skin, in a manner similar to an egg shell. The word monocoque is a French term for "single shell".

First used for boats, a true monocoque carries both tensile and compressive forces within the skin and can be recognised by the absence of a load-carrying internal frame. Few metal aircraft other than those with milled skins can strictly be regarded as pure monocoques, as they use a metal shell or sheeting reinforced with frames riveted to the skin, but most wooden aircraft are described as monocoques, even though they also incorporate frames.

By contrast, a semi-monocoque is a hybrid combining a tensile stressed skin and a compressive structure made up of longerons and ribs or frames. Other semi-monocoques, not to be confused with true monocoques, include vehicle unibodies, which tend to be composites, and inflatable shells or balloon tanks, both of which are pressure stabilised.

Early aircraft were constructed using frames, typically of wood or steel tubing, which could then be covered (or skinned) with fabric such as Irish linen or cotton. The fabric made a minor structural contribution in tension but none in compression and was there for aerodynamic reasons only. By considering the structure as a whole and not just the sum of its parts, monocoque construction integrated the skin and frame into a single load-bearing shell with significant improvements to strength and weight.

To make the shell, thin strips of wood were laminated into a three dimensional shape; a technique adopted from boat hull construction. One of the earliest examples was the Deperdussin Monocoque racer in 1912, which used a laminated fuselage made up of three layers of glued poplar veneer, which provided both the external skin and the main load-bearing structure. This also produced a smoother surface and reduced drag so effectively that it was able to win most of the races it was entered into.

This style of construction was further developed in Germany by LFG Roland using the patented Wickelrumpf (wrapped hull) form later licensed by them to Pfalz Flugzeugwerke who used it on several fighter aircraft. Each half of the fuselage shell was formed over a male mold using two layers of plywood strips with fabric wrapping between them. The early plywood used was prone to damage from moisture and delamination.

While all-metal aircraft such as the Junkers J 1 had appeared as early as 1915, these were not monocoques but added a metal skin to an underlying framework.

The first metal monocoques were built by Claudius Dornier, while working for Zeppelin-Lindau. He had to overcome a number of problems, not least was the quality of aluminium alloys strong enough to use as structural materials, which frequently formed layers instead of presenting a uniform material. After failed attempts with several large flying boats in which a few components were monocoques, he built the Zeppelin-Lindau V1 to test out a monocoque fuselage. Although it crashed, he learned a lot from its construction. The Dornier-Zeppelin D.I was built in 1918 and although too late for operational service during the war was the first all metal monocoque aircraft to enter production.