Jeremy S. Robinson
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.
Margaret Mulherne
An aircraft that could take off, climb through the atmosphere into space at hypersonic speeds and survive a scorching descent to land at its destination like a conventional airplane might be seen as the ultimate test of structural materials. To power itself into orbit on its own store of fuels the craft would need a structure built of substances at once lightweight and very stiff and strong, able to cope with great aerodynamic stresses. Materials in some parts of the airframe would need to retain their strength at a temperature of more than 1,000 degrees Celsius; materials in the engines would have to remain strong at still higher temperatures. The class of materials from which most of its airframe and parts of its engine would be made are known as composites.
The need for lightweight, stiffness and strength combined has led many designers of military and commercial aircraft, sports equipment and cars to turn to composites for many components. Composites that meet the added requirement of resistance to high temperatures are found in rocket-motor components and missile nose cones. Wherever advancing technology has created a need for combinations of properties which no single material can provide, composites are becoming the material of choice. Most of us, in our daily lives, have used something made from, or incorporating, composites - skis, golf clubs, sail boards, court racquets, sailing dingeys and yachts, and yet few have a realistic picture of their capabilities and the opportunities they present for innovative design.
Engineering designers are frequently admonished for their lack of knowledge and interest in the full range of materials options available for engineering components. In the designers defence it can be said that they have learned to mistrust the often inflated claims of the material developers. Each year a new crop of wonder materials seems to emerge from the laboratory, but for these the challenges of manufacturing have yet to be addressed and will in fact prove to be a significant obstacle to widespread industrial adoption. The truth is that it can take many years of development and testing for a new material to reach the stage where it can be adopted by the engineering industries. Composite materials fall into this category. The problem that has dogged engineering composites since their earliest use is the difficulty in adopting automated manufacturing processes and the cost penalty that this imposes.
An equally difficult challenge is to educate designers into a new way of thinking: 'How best can we use the properties and processing opportunities offered by these materials in a complete new design?' Rather than: 'How can we adapt our existing design to make some limited use of the material's capabilities?'
Composites are now the natural material for military aircraft, racing yachts and racing cars to the point that there has to be a good reason to revert to traditional materials. In other industries composites are increasingly being seen to offer cost savings in addition to performance benefits, drawing more manufacturers to these materials.
At UL I work in the composites manufacturing research unit, where I manage a BRITE/EURAM EC project. We're designing manufacturing techniques for thermoplastic and thermoset composites. These are mainly glass and carbon fibre reinforced plastics and they are currently used in the structural components of military and commercial aircraft and also in high performance sports equipment such as Formula 1 racing cars and the composite structured Lotus Sports bicycle on which Chris Boardman won the gold medal at the Barcelona Olympics. Thermoplastics and thermosets are usually supplied in 'prepreg' form. This implies that the fibres have been impregnated with the appropriate polymer. In general the thickness of a prepreg ply is such that when eight plies are consolidated, a laminate of 1mm thick is produced. My work involves designing moulds for thermoplastic composites. When a thermoplastic fibre-reinforced laminate is formed in a mould at a temperature above the melting point, cooled and removed from the mould, it assumes a shape which can be significantly different from the shape of the mould. I predict the thermal distortions and use them in the design of moulds in which parts to close dimensional tolerances can be made.
In this project there are six partners, including European Aerospace companies like Dornier of Germany, Casa of Spain and a number of European Universities. There are meetings every six months where each of the partners discuss their progress and the difficulties encountered. The meetings are held in turn at each of the partner's work establishments, and this provides opportunities to see other activities that are carried out in the composite design area.
I studied mechanical engineering and the area I'm working in now, composite engineering, is a branch of this. If you have an interest in the practical and science subjects, maths, physics and chemistry etc.,you should consider mechanical or aeronautical engineering when filling out your CAO form.
Ms. Margaret Mulherne is Project Manager for Brite-Euram project "Diaphram Form of Thermoplastics Composites". She is pursuing a Doctorate in "Mould Design for Advanced Composites".
