The influence of structural materials

The introduction of new materials has opened up a range of possibilities for the design of more efficient aircraft, and even new types of aircraft. Man-powered flight would probably not have been possible using traditional materials.

Since the First World War, aluminium alloys have been almost universally used as the primary structural material, even for supersonic aircraft capable of Mach numbers up to about 2.2, such as Concorde. However, for sustained flight at Mach numbers above about 2.5, the effects of kinetic heating render conventional aluminium alloys unsuitable. Instead titanium and steel alloys may be employed. Unfortunately, their use presents something of an economic barrier. Apart from the higher cost of these materials, the fabrication tech­niques required tend to be more expensive. It is this economic barrier, rather than any purely aerodynamic problem, that has limited the maximum Mach number to around 2.5 for all but a handful of experimental aircraft. Rare exceptions are the MiG-25 combat aircraft, which is capable of Mach 3, and the even faster specialised Lockheed SR-71 reconnaissance aircraft.

Since the 1950s, gradually increasing use has been made of fibre reinforced materials. Originally, glass fibres were used, but a major advance came with the introduction of carbon (graphite) fibres. Carbon fibres can be produced in a number of forms, and can be optimised either for high strength, or for high stiffness (high modulus). It is the high stiffness of carbon fibres that make them a particularly attractive alternative to metals in aircraft construction. Boron fibres show even better properties, but are less cost effective than carbon fibres, and have only been used in experimental or highly specialised applications.

Although fibre reinforced or composite materials can have a higher strength – to-weight or stiffness-to-weight ratio than metals, they cannot simply be used as a direct replacement. The main problem is that they do not deform plastically like metals, and cannot be joined by conventional types of bolts or rivets, since this causes local cracking. The general adoption of fibre reinforced materials

was, therefore, slowed down by the need to develop suitable fastenings and construction techniques. Increasing use of composites is now being made, particularly in military combat aircraft and helicopters. The Beech Starship (Fig. 4.10) was one of the first civil transport aircraft designed for large-scale production, to use composites for its primary structure.

In addition to high strength and stiffness, fibre reinforced materials have some other important special properties. By aligning the fibres in particular patterns within a structure, it is possible to control the relationship between bending and torsional stiffness. This technique is one of the methods that can be used to reduce the tendency to structural divergence of forward-swept wings, and gives us another example of the way in which the development of materials can influence aerodynamic design judgements.

The use of moulded composite structures has also made it economically practical to produce complex aerodynamically optimised shapes, even for light aircraft.

Because fibre reinforced materials are built-up, rather than being cut or bent out of solid block and sheet, they can be produced in much more complex, ‘organic’ forms, with continuous variations in thickness, curvature and stiff­ness. Such structures begin to resemble the highly efficient optimised shapes found in the bones of birds.

Composite structures were initially restricted to smaller components such as control surfaces, but more recent aircraft employ composite materials for the main structural components, as on the Airbus A350XWB shown in Fig. 14.6, and also the A400 heavy lifter, and the Boeing Dreamliner. The weight saving allows for significant improvement in fuel consumption, or enhanced payload capacity.

Further discussion of aircraft structural design is beyond the intended scope of this book, but Megson (2007) gives a good introduction.

Recommended further reading

Megson, T. H. G., Aircraft structures for engineering students, 4th edn, Butterworth – Heinemann, 2007, ISBN 9780750667395. A well-respected standard undergraduate textbook. Includes examples. Solutions manual available.

Stinton, Darrol, The anatomy of the aeroplane, 2nd edn, Blackwell Science, Oxford, 1998, ISBN 0632040297. A classic introduction to aircraft design.

Wilkinson, R., Aircraft structures and systems, 2nd edn, Mechaero Publishing, St Albans, UK, 2001, ISBN 095407341X. A good easily read introductory text with a non-mathematical approach.

Conclusion

This concludes our introduction to the subject of aircraft flight. We have tried to include all of the important basic principles, and one or two items of inter­est. Inevitably we will have omitted something important, but the references given in this book should lead you to most of the missing information.