Fuselage, Wings and Stabilizing Surfaces — Page 28, Lesson 33

Let’s start with the materials that make up the airframe, because everything else—the fuselage, the wings, the tail—is built from these choices. Modern aircraft are constructed mainly of aluminium and its alloys, with smaller amounts of steel and titanium for the major structural components. Composites are used extensively for the more lightly loaded structures. But on the latest aircraft, we now see modern composites used for the empennage—that’s the tail unit—plus cabin floor panels, flying control surfaces, engine cowlings, and fairings. Each material is chosen for its particular properties. The four properties we weigh are fatigue strength, wear resistance, strength-to-weight ratio, and fire resistance. Fatigue strength is how well the material survives repeated loading cycles without cracking. Wear resistance is how it stands up to abrasion. Strength-to-weight ratio is exactly what it sounds like—how much strength you get per unit of weight. Fire resistance is how it behaves under heat. Aluminium and its alloys are the most widely used metals for structural use, because of a good strength-to-weight ratio. Within that family, the ‘duralumin’ type alloys predominate, because of their good fatigue resistance. Duralumin is a copper-based aluminium alloy. Its weakness is poor corrosion resistance—unless it is clad with pure aluminium, and that clad product is called Alclad. It also has good thermal and electrical conductivity, but it is difficult to weld. Steel and its alloys are used only where strength is vital and weight penalties can be ignored. So you won’t find steel everywhere—only where you absolutely need the strength and can afford the weight. Titanium is much lighter than steel, and it can be used where fire protection is required—for example, firewalls. It has good strength, and it retains that strength and its corrosion resistance up to temperatures of 400°C. That temperature limit is the key figure to remember for titanium. Magnesium alloys are also used, and their principal advantage is their weight. That gives an excellent strength-to-weight ratio—aluminium is one and a half times heavier than magnesium. But the elastic properties of magnesium are not very satisfactory, so its use in primary structures is limited. Primary structures are the load-bearing ones, like the main wing spars and the fuselage frames. Now, composite materials. A composite is made of at least two elements, to produce a material with properties that are different from those of the original elements. So you take two or more distinct materials, combine them, and the result behaves differently—and usually better—than either one alone. That’s the whole principle behind composites. So the picture is this: aluminium alloys for the bulk of the structure, duralumin for fatigue-critical parts, steel where strength is paramount, titanium where you need fire protection and light weight, magnesium where weight is king but you accept limited elastic behaviour, and composites for the lightly loaded structures and increasingly for the empennage and control surfaces on modern aircraft.