Monocoque vs. Semimonocoque Fuselage
There are two main approaches to the construction of the outer shell of an aircraft fuselage: monocoque and semimonocoque. Monocoque systems were employed in early aviation, starting in 1918, but semimonocoque methods are the most widely seen approach in modern aviation. The main differences between the two approaches is in their designated support structures.
Monocoque is a French term that means “single shell”. Similar to the concept of the surface of an aluminum can, the fuselage is wrapped in a layer of stressed aluminum alloy. The monocoque system does not have a second support structure behind its outer layer, so it is completely dependent on the construction of the outer layer to distribute stress and load. To add strength to the skin of the fuselage, it is sometimes fortified with stiffeners. This approach becomes very strong, as twisting and bending stresses can be distributed through the external skin. However, this method is not tolerant to any type of surface corrosion or deformation. When stiffeners are added, it increases the weight of the monocoque. As such, monocoque approaches have been widely phased out in favor of semimonocoque structures. Automobiles still use the monocoque approach, and some smaller aircraft, such as helicopters. This is because a one-layer system allows for more space in the interior of an aircraft.
Semimonocoque methods utilize part of monocoque technology, as the name suggests, but add a partial “inner skeleton”. It is essentially a dual layered construction. A semimonocoque fuselage uses a substructure that consists of bulkheads and formers which reinforce an outer skin layer. The second layer of support reinforces the outermost layer by assuming part of the applied load, therefore stress is evenly distributed across the fuselage. Due to the extra layer of support, the fuselage shape is able to stay intact, and has better longevity than a monocoque approach.