In the Realm of stability and control, we are at present travelling headlong into a region of new problems. Besides tidying up the outstanding items on the types of aircraft we are used to seeing flying about to-day, a not inconsiderable task when one considers the troubles we encounter when we undertake the design of a new so-called “conventional” aeroplane, we have to tackle two major lines of research and development. First, the peculiar problems associated with the stability and control of aircraft of large size, both military and civil, into which difficulties associated with high Mach numbers intrude themselves but little; and secondly, the even more extensive problems associated with the stability and control of aircraft, both large and small, which are to fly at very high Mach numbers, not only at their design cruising and diving speeds, but also in the extremely important slow speed conditions. Mr. M. B. Morgan of the R.A.E. has dealt with some aspects of the second series of problems, so I propose to concentrate on some of the problems involved in the first series, that is those primarily associated with the increase of aircraft size.

The rotary derivatives of a Delta wing pitching or rolling at supersonic speed are obtained in addition to the lift curve slope first given in an earlier investigation.

The method of achieving efficient and economical servicing with the maximum rate of utilisation of aircraft, is a problem which always confronts the aircraft operator, either a private owner flying his modest machine or a national airline with a variety of large and expensive aircraft. From the point of view of safety, their problems are the same.

Efficient servicing is closely related to safety in so far as the airworthiness aspect is concerned and, under present servicing methods, the direct cost of providing this aspect of safety may well amount to some 20 per cent, of operating costs; to this must be added the delays inseparable from major maintenance work which can reduce earning capacity as a result of the time taken to rectify defects. Invariably the cost to the private owner is higher than he had expected and, unless his aircraft is maintained by a competent servicing organisation, it can be subjected to frequent bouts of unserviceability—usually when he needs it most.

At a section of a cantilever distant x from its tip, let the depth of section be h and the bending moment M. Then if all the resistance to bending is concentrated in the flanges the loads in the flanges are ±M/h. If the flanges are inclined at a small angle α these flange loads have a component transverse to the cantilever axis of magnitude Mα/h. Now α=dh/dx and the shear load at section x is dM/dx. Therefore the shear load on the web of the cantilever is dM/dx-(M/h)dh/dx, and, if h/M is constant, the web is not sheared at all.