The first aircraft had two wings made of light weight wood frames with cloth skins, held apart by wires and struts. The upper wing and the struts provided compression support while the lower wing and the wires supported tension loads.
In the 1920s, metal began to be used for aircraft structure. A metal wing is a box structure with the skins comprising the top and bottom, with front and back formed by I-beams called spars, interior fore-aft stiffeners called ribs, and in-out stiffeners called stringers. In level flight, the lower skin is in tension while the upper skin is in compression. For this reason, this design is referred to as stresssed skin construction. During turbulence, upper and lower skins can experience both tension and compression. This box structure is able to support the above-mentioned moments, making single wing aircraft possible. The elimination of the struts and wires so dramatically reduced air drag that aircraft were able to fly twice as fast as before with the same engine.
While steel was used for a few aircraft in Germany in the 1930s, the metal of choice was, and still is, aluminum. Figure 19-1 shows an aluminum aircraft fuselage skin subassembly.