Effects of shock waves – the shock stall

It is clear from schlieren photographs that there is a sudden and considerable increase in density of air at the shock wave, but there is also, as has been stated, a rise in pressure (and incidentally of temperature), and a decrease in speed. Most important of all perhaps is the breakaway of the flow from the surface, though it is sometimes argued whether this causes the shock wave or the shock wave causes it. Whichever way it is the result is the same.

As is rather to be expected all this adds up to a sudden and considerable increase in drag – it may be as much as a ten times increase. This is accompa­nied, if it is an aerofoil, by a loss of lift and often, due to a completely changed pressure distribution, to a change in position of the centre of pressure and pitching moment, which in turn may upset the balance of the aeroplane. At the same time the turbulent airflow behind the shock wave is apt to cause severe buffeting, especially if this flow strikes some other part of the aeroplane such as the tail plane. One can hardly avoid saying – very like a stall. Yes, so like the stall that it is called just that – a shock stall.

But the similarity must not lead us to forget the essential difference – no it isn’t the speed, we have already made it clear, or tried very hard to make it clear, that the ordinary stall can occur at any speed; the essential difference is that the ‘ordinary stall’ occurs at a large angle of attack and, to avoid confu­sion, we shall in future call it the high-incidence stall to distinguish it from the shock stall which is more likely to occur at small angles of attack.

From what has already been said the reader will probably have realised that the formation of shock waves is not a phenomenon that occurs on the wings alone; it may apply to any part of the aeroplane. Even the shock stall, which may first become noticeable owing to the sudden increase of drag and onset of buffeting – it is sometimes called the buffet boundary – may be caused by the formation of shock waves on such parts as the body or engine intakes, rather than on the wings (Fig. 11B, overleaf).