Shock drag

The sudden extra drag which is such a marked feature of the shock stall has two main components. First the energy dissipated in the shock wave itself is reflected in additional drag (wave drag) on the aerofoil. Secondly, as we have seen, the shock wave may be accompanied by separation, or at any rate a thickening of and increase in turbulence level in the boundary layer. Either of these will modify both the pressure on the surface and the skin friction behind the shock wave.

So this shock drag may be considered as being made up of two parts, i. e. the wave-making resistance, or wave drag, and the drag caused by the thick turbulent boundary layer or region of separation which we will call boundary layer drag.

As has already been explained the shock wave and the thickened turbulent boundary layer or separation are like the chicken and the egg – we don’t know which comes first; what we do know is that when one comes so does the other. That is not to say that they are by any means the same thing, or that they have the same effects, or that a device which reduces one will necessarily reduce the other.