A considerable research effort is now directed at the problem of vibrations excited in aircraft structures by airborne sound from the power plants. Both jet and propeller aircraft are involved, although perhaps for different reasons. With jet aircraft the problem is to make the structure strong enough to withstand the vibrations and the accompanying stress fluctuations; with propeller aircraft the problem is usually to prevent the transmission of sound through the structure into the passenger cabin.

The 825th lecture to be read before the Royal Aeronautical Society, “ Planning ami Production Methods Used in the Construction of the de Havilland Comet,” by H. Povey, A.F.R.Ae.S., was given at the Institution of Civil Engineers, Great George Street, London, S.W.I, on Thursday 12th April 1951. Major G. P. Bulman, the President of the Society, presided.

Opening the meeting the President said they met that evening in the shadow of a grevious loss sustained by aviation throughout the world, and by the Society, in the unexpected death of Mr. W. G. A. Perring on 8th April through heart failure. The shock to those of them who worked with him and knew him well was still so deep that one could not try to pay an adequate tribute to his great services.

The Classical Kirchhofif–Love Theory for the deflection of thin plates leads to fourth order Lagrange's differential equation,D△4 w — q = 0 for which a general solution is not always possible. Exact solutions are known so far only for a few special cases and, therefore, numerical solutions have often been tried. The advantage of numerical solution is that it can be applied easily to any plate plan form which is in marked contrast to the analytical method where, for mathematical reasons, definite restrictions have to be imposed on the geometrical shape of the plate. Among the various numerical methods, relaxation is the easiest, but when applied to solving a biharmonic equation, the process becomes extremely difficult and laborious as convergence is very slow and the unit relaxation operator cumbersome to deal with.

Our engineering habits form slowly, and once formed are slow to change. This is perhaps less true in aeronautics than in other branches of engineering. Nevertheless some aeronautical customs are deeply rooted, so much so that it is easy to confuse the superstructure of habit with the basic foundations. In this article I have reviewed some of the customary methods of specifying strength conditions and have tried to disentangle the underlying principles. Current changes in these methods emphasise the need of a proper appreciation of the principles, and a study of these principles suggests even more radical changes of methods in future.

The recent revival of interest in gliding in this country is my excuse for drawing attention to a feature in the wings of birds whose gliding depends primarily on ascending currents, mainly raptores (eagles, vultures, hawks, kites, etc.), especially in the tropics.