Modern aircraft are complex machines designed to move people or payload large distances at high speed. The structures of aircraft are made from a variety of different materials. These materials are chosen by considering the density (mass/volume - units Kgm-3) and the mechanical properties, like strength (units - MNm-2) or stiffness (units - GNm-2). The best materials for aircraft are those with high specific properties (mechanical property/ density).Table 1 indicates some properties of potential aircraft materials. The light metals, aluminium and titanium are popular aircraft materials, as are composite materials like glass or carbon fibre reinforced plastic.
Figure 1 shows the actual proportion of structural materials used in the Boeing 747, which first flew in 1969, and the latest Boeing 777 which will have its first flight in 1994. Aluminium alloys constitute by far the biggest proportion of structural mass of most modern aircraft, with steels, titanium alloys and structural composites all accounting for approximately 10%. The wood sitka spruce was listed as a candidate material in Table 1, and early aircraft designs (including the first aircraft to make a manned powered flight - The Wright Flyer) did consist mainly of wooden structural members (spruce or bamboo), glued or screwed together to form a frame, which was then covered in canvas. The wings were supported by wooden compression struts and steel tension wires. Most of the aircraft built until the early 1920s used this 'stick and stringer' type of construction. Wooden aircraft were extremely successful, but as aircraft became bigger, serious problems due to fungal rot forced designers to consider metallic aircraft.
The discovery by a German metallurgist in 1911, that aluminium alloyed with copper could be made stronger than mild steel paved the way for aluminium framed and skinned aircraft. The use of these Duralumin alloys as they became known, enabled some of the aerodynamic forces to be carried by the stressed skin of the wings and fuselage. This resulted in very efficient airframes. Most of the aircraft in World War II were aluminium alloy stressed skin designs. Remarkably, the early aluminium alloys developed in the 1930s and 1940s are still used extensively today. Figure 2 shows the method of construction of the Airbus A340 fuselage. You can see the frames, stringers and skins. All these components are fabricated from aluminium alloys. A new series of aluminium alloys have recently been developed by Material Scientists which contain the element lithium. These alloys are lighter and stiffer than existing alloys, and are now finding use on the latest aircraft designs, see Figure 3.
Titanium has a density approximately twice that of aluminium, but when alloyed with other elements, can exhibit very high mechanical properties. These properties make it especially useful for high load bearing applications. An example would be the pylon structure that holds the engines onto the wings of civil airliners. The reason titanium alloys are not used more extensively on airframes is due to cost. Titanium alloys cost up to 10 times more than aluminium alloys.
Even though steel has a high density compared to aluminium and titanium, it can be alloyed and heat treated to produce ultra-high mechanical properties. This is useful for applications like the landing gear of aircraft, which must be very strong, but not take up too much space.
Structural composite materials are finding increasing use on modern aircraft because of their very attractive low density and high mechanical properties. Composites generally consist of a plastic matrix of epoxy resin, reinforced by many fine fibres of either carbon, boron, glass or Kevlar. Structural composites are replacing aluminium alloys on <%-3>airframes, and most modern aircraft have composite vertical and horizontal stabilisers, rudders, ailerons and engine fairings. Military aircraft use much greater proportions of composite material, and Figure 3 shows the Eurofighter 2000. This aircraft, which is due to fly in early 1994, has a composite fuselage and wing.
The next generation of supersonic transport aircraft will fly 200 passengers at Mach 2.8+ (Concorde flies 100 passengers at Mach 2.2). When aircraft fly this fast for long periods of time, friction from the air passing over the aircraft heats up the outer surfaces of the fuselage and wings. For example, an aircraft flying at Mach 3 will experience a temperature increase of ~415ºC from this kinetic heating. The maximum temperature of the outer surface will be 357ºC. One of the reasons these aircraft have not yet been built, is that the low density materials required to operate at these high temperatures have not been developed. Material Scientists are currently investigating advanced polymeric and metallic composites, to provide materials that are light, strong, and capable of withstanding elevated temperatures for over 100,000 hours of service.
Dr Jeremy Robinson is a Lecturer in Metallurgy and Materials. He has research interests in aerospace metallic materials.
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