Aspect ratio

We have so far only considered aerofoils from the point of view of their cross- section, and we must now consider the effect of the plan form. Suppose we have a rectangular wing of 12 m2 plan area; it could be of 6 m span and 2 m chord, or 8 m span and 1.5 m chord, or even 16 m span and 0.75 m chord. In each case the cross-sectional shape may be the same although, of course, to a different scale, depending on the chord. Now according to the conclusions at which we have already arrived, the lift and drag are both proportional to the area of the wing, and therefore since all of these wings have the same area they should all have the same lift and drag. Experiments, however, show that this is not exactly true and indicate a definite, though small, advantage to the wings with larger spans, both from the point of view of lift and lift/drag ratio.

The ratio span/chord is called aspect ratio (Fig. 3.26), and the aspect ratios of those wings which we have mentioned are therefore 3, 5.33 and 21.33 respectively, and the last one, with its ‘high aspect ratio’, gives the best results (at any rate at subsonic speeds which is what we are concerned with in this chapter). Why? It is a long story, and some of it is beyond the scope of this book; but the reader has the right to ask for some sort of explanation of one of the most interesting and, in some ways, one of the most important, prob­lems of flight. So here goes!