The influence of aspect ratio

The amount of lift generated depends on the circulatory strength of the bound vortex, and on its length, which in turn depends on the span of the wing. A given amount of lift can be generated either by a short strong bound vortex, or a long weaker one. The longer weaker bound vortex will produce weaker trailing vortices, and as the downwash produced by the trailing vortices is responsible for the trailing vortex (induced) drag, the longer wing will produce less drag.

The longer the wing is, the weaker is the bound vortex required. For a given wing section and angle of attack, the strength of the bound vortex depends on the wing chord, so for a given amount of lift, the chord required reduces as the wing span is increased. Thus, wings designed to minimise trailing vortex (induced) drag, have a long span, with a small chord: in other words, the aspect ratio is high.

For a given wing section shape, any reduction in chord produces a corres­ponding reduction in depth. Therefore, as the aspect ratio is increased, it becomes more difficult to maintain adequate strength and stiffness.

Competition gliders or sailplanes often have wings with an extremely high aspect ratio, but for both structural and aerodynamic reasons, low aspect ratio wings are more suitable for very manoeuvrable aircraft such as the Hawk trainer shown in Fig. 9.2.

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Fig. 2.9 High aspect ratio and large wing area were used on the Lockheed TR-1, which was designed for long range and endurance (Photo courtesy of Lockheed California Co.)

Because high aspect ratio wings have a good ratio of lift to drag, they are used on aircraft intended for long range or endurance. The aircraft shown in Fig. 2.9 is a good example. It is noticeable that long-range, high-endurance sea-birds also have high aspect ratio wings. The albatross has an aspect ratio of around 18. However, very low aspect ratio wings, such as those of Concorde, produce less drag in supersonic flight, as will be explained in later chapters.