Swept wings

Swept wings originated in designs for subsonic aeroplanes and were first used practically on the Boeing B 52 bomber. The sweep back was about 35°. It is now clear that, for optimum performance of a wing, there is a direct link between the speed of the aeroplane and the sweep back. The sweep back increases to about 60° at Mach 2 and more as the speed increases again. But there seem to be some limit on the further increase in sweep back that may be the result of the need to mount munitions externally on the wings. It is as if Mach 2 is a high enough speed for combat aeroplanes.

 

The use of sweep back eliminates the formation of a shock wave at the leading edge of the wing and then the wing of a supersonic aeroplane becomes just another wing in the range of wings for speeds from say Mach 0.5 to Mach 2.

 

We must see how this comes about. In figure 14-23 I have drawn a part of the leading edge of a wing having an angle of sweep back of  and moving at a speed of . This speed can have components along the wing, in green, and at right angles to the leading edge in red. We know that a shock wave can form at the leading edge if the, red, normal component of the velocity is greater than . It follows that if the angle of sweep back  is chosen so that the normal component to the leading edge of the speed of the aeroplane is less than  there will be no shock  wave formed at the leading edge.

 

This means that the graph 14-1 of  versus M based on  can be drawn for the condition when the normal component =  and this graph will give the minimum value of  for any value of M if an oblique shock wave is to be avoided. It more or less matches the angles used on existing aeroplanes.

 

This means that wing sections can have rounded leading edges and be developed for a compromise between supersonic and subsonic performance.