Behaviour of aeroplane at shock stall

All this rather assumes that there is something to be feared about a shock stall, and that pilots try to avoid it. After all, there was a time when we looked upon the high incidence stall in the same way – something to be avoided at all costs. Now, however, it is practised by all pilots in the very early stages of learning. Much the same is happening to the shock stall – it is all a question of knowl­edge, and many aircraft currently cruise safely well into the transonic region.

By far the most important effect is a considerable change of longitudinal trim – usually, but not always, towards nose-down, and sometimes first one way then the other. Unfortunately the change of trim is made even worse by the very large forces required to move the controls, and the ineffectiveness of the trimmers. There is also likely to be buffeting, vibration of the ailerons, and pitching and yawing oscillations which may become uncontrollable, and which are variously described as snaking (yawing from side to side), por­poising (pitching up and down), and the Dutch roll (a combination of roll and yaw).

These effects can, though, be alleviated by the use of power controls and automatic stability augmentation systems. The best way of avoiding the diffi­culties is to keep an eye on the machmeter – if there is one – and, if the worst comes to the worst, to get into regions of higher temperature. The best ways of getting out of trouble are to stop going so fast – or to go faster! In a climb it is easy to stop going so fast – just throttle back. That is why the safest and best research work can often be done in climbing flight. In level flight it may not be quite so easy to lose speed, especially if the controls cannot be moved; and in a dive, which is where these troubles are most likely to occur, it will be even more difficult. It is essential therefore that all aircraft which are capable of these speeds, and which have undesirable characteristics in the transonic range, should have some kind of dive brake, or spoiler, which can safely be used at high Mach Numbers. We have had to go very fast on aeroplanes before the need for a brake was recognised! As to going faster; well, that will take us into the region of supersonic flight which will be dealt with in the next chapter.