Stability and control problems

In the previous chapter we considered some of the problems of stability and control at transonic speeds. Some of these still apply in the supersonic region, and control surfaces should be of the all-moving slab type, and fully power-operated. But whereas in the transonic range this applied mostly to the tail, in the supersonic range it can be applied also in all-moving wing tips to replace conventional ailerons, and even to an all-moving fin and rudder. Since the main plane may be nearly as small as the tail plane, it, too, may be movable to give pitch; in fact in some missiles it is not easy to decide which is the main plane and which the tail plane.

But there are also new problems at supersonic speeds because the inertia forces are so great that it is practically impossible to provide the inherent natural stability of the kind that is associated with such devices as dihedral and fin area. In order to be effective against the inertia forces the surfaces would have to be so large that the cost in weight and drag would be prohibitive – and this applies particularly at great heights where there is so little air density.

Another difficulty, which is especially applicable to military aircraft, is that pilot and crew have so much to do in looking after the equipment that they must be relieved as far as possible of flying the aeroplane.

Of course we have long been familiar with the automatic pilot, but the modern conception is very different from this – nothing more nor less, in fact, than synthetic stability and automatic control. Is the pilot then necessary at all? Strictly speaking, probably not, the aircraft can be controlled from the ground like a guided missile. But pilots can still do some things that instru­ments cannot, they can monitor the automatic systems, tell them what to do, investigate any failure and, if necessary, take over control.