Flight at supersonic speeds

Introduction

As explained in the previous chapter subsonic, transonic and supersonic flight merge one into the other, and it is not easy to define where one ends and the other begins. But the change from subsonic to transonic does at least involve some outward and visible signs – in the laboratory there is the formation of shock waves, and in flight there is the shock stall with its varying effects according to the type of aircraft – whereas the change from transonic to super­sonic is not accompanied by any such signs whether in the laboratory or the air, so the dividing line between the two is even more vague. In general we can only say that supersonic flight begins when the flow over all parts of the aero­plane becomes supersonic. But at what Mach Number does that happen? Does it in fact ever happen? Are there not always likely to be one or two stagnation points? And what about the boundary layer where the flow near the surface is certainly subsonic? Perhaps after all it is better to say at about Ml.2, or 1.5, or maybe 2? Figure 12A (overleaf) illustrates an aircraft designed for super­sonic speeds.

Supersonic shock pattern

We have already had a look at the supersonic shock pattern in Fig. 11.12e, and except that the angles of shock waves become rather more acute as the speed increases there is very little change in this pattern over the supersonic range of speeds. To see why the angles of the shock waves change, we must understand the meaning and significance of the Mach Angle.