Aircraft Structures Design

Fuselage structures can be made light, yet strong and rigid, 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 additional support structure underneath to provide the strength and rigidity needed to operate effectively. In many modern aircraft, the deck and part of the frame are made from a single piece of material. The outer skin thus hides a complex piece of structure that must be strong, rigid and reliable.

Connecting rods, braces, beams and souls.

The structure of most airframe components is made up of four main types of structural elements. Loops are elements subject purely to tension (traction). Since the tension won’t cause the tie to bend, it doesn’t need to be stiff, although it often is. The ties can be made with rigid elements, such as tubes, or simply with wire, such as the bracing wires of a biplane.

The struts support compression loads. Because compressive loads can cause the member to buckle, the design of a strut is less simple than that of a tie. If overloaded, struts will fail in one of two ways: a long, skinny strut will buckle; a short thick strut will collapse from cracking or crushing as the material it is made of is overloaded. A medium strut can do either, or even both, depending on its dimensions and other factors. Tubes make excellent struts, because the material is uniformly loaded, so the strength-to-weight ratio is high in compression.

Beams carry loads at an angle (often at right angles) to their length, so they are loaded primarily in bending. Many of the main parts of an fuselage 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. The nets are thin sheets that carry shear loads in the plane of the material. The ribs and the skin itself are cut nets. Thin sheets are ideal for resisting shear, especially if they are supported to resist buckling.

You may be under the impression that each part of a fuselage is a brace, strut, beam or web, but it is not. Some elements, such as wing spars, act almost entirely as one type of member, but others act as different members for different loads. For example, the fuselage skin may be subjected to both tensile and shear loads simultaneously. Pure bending loads almost never exist alone; they are almost always related to a shear load. Therefore, a beam will normally carry both bending and shear loads.

The goal of aircraft structural design.

By carefully blending these members and ensuring that each part of each member carries its share of the loads, the aircraft’s structural design will achieve the greatest strength with the least weight, and thus the best operating efficiency and maximum safety. The designer’s goal is to ensure that each part of each structure supports reasonable stress so that the capacity of each part of the structure is used effectively. Only by doing this can the weight of an airframe be reduced to the minimum possible while still providing adequate strength.

There are many uses for struts in an airframe, including floor supports on a transport aircraft, landing gear legs, drive jacks of all kinds, and pushrods to operate flight controls. Struts also frequently act as ties, when the load they support is reversed; again, acting cats are typical examples of this.