Airframe structures can be made light, yet strong and stiff, and the aircraft skin contributes to the overall strength of the structure.
All airframes, whatever the aircraft, are designed using the same principles. The smooth exterior provides a streamlined shape, with extra supporting structure underneath to provide the strength and stiffness needed to operate effectively. In many modern aircraft, the covering and part of the framework are made from a single piece of material. The outer skin, then, hides a complex piece of structure that must be strong, stiff and reliable.
Struts, ties, beams and webs.
The structure of most airframe components is made up of four main types of structural member. Ties are members subject purely to tension (pulling). Because tension will not cause the tie to buckle, it does not need to be rigid, although it often is. Ties can be made from rigid items, such as tubes, or simply from wire, like the bracing wires on a biplane.
Struts carry compression loads. Because compressive loads can cause the member to buckle, the design of a strut is less simple than a tie. If overloaded, struts will fail in one of two ways: a long, thin strut will buckle; a short, thick strut will collapse by cracking or crushing, as the material from which it is made is overstressed. A medium strut may do either, or even both, depending on its dimensions and on other factors. Tubes make excellent struts, because the material is evenly loaded, so that the strength-to-weight ratio is high in compression.
Beams carry loads at an angle (often at right angles) to their length, and so are loaded primarily in bending. Many of the major parts of an airframe are beams, such as the main spars. The fuselage and wings themselves are structural members, and are beams, because they support the bending loads imposed by weight, inertia and aerodynamic loads. Webs are thin sheets carrying shear loads in the plane of the material. Ribs and the skin itself are shear webs. Thin sheets are ideal for carrying shear, especially if they are supported so that they resist buckling.
You may get the impression that each part of an airframe is either a tie or a strut or a beam or a web, but this is not so. Some items, such as wing spars, act almost entirely as one type of member, but others act as different members for different loads. For instance, the fuselage skin may be subjected to tensile and shear loads simultaneously. Pure bending loads almost never exist alone; they are almost always related to a shear load. So a beam will normally carry both bending and shear loads.
The aim of aircraft structural design
By carefully mixing these members, and making sure that each part of each member is taking its share of the loads, aircraft structural design will achieve the greatest strength with minimum weight, and so get the best operating efficiency and maximum safety. It is the designer’s aim to ensure that each part of each structure carries a reasonable stress, so that the capability of every part of the structure is used effectively. Only by doing this can the weight of an airframe be made as low as possible, while still providing adequate strength.
There are many uses of struts in an airframe, including the supports for the floor in transport aircraft, undercarriage legs, actuation jacks of all kinds and pushrods for operating flying controls. Struts also frequently act as ties, when the load they take is reversed; again, actuation jacks are typical examples of this.