Invariably the texts in elasticity and strength of materials derive the stability criteria for struts by first setting up the beam bending moment equation. Together with suitable boundary conditions this equation is then solved as an eigenvalue problem for non-zero solutions. It is indicated here that in general this procedure is too limited, both physically and mathematically, and that in some cases the correct approach yields additional results which normally would not become evident by employing the classical method.

During the past few years a new series of low drag aerofoils has been developed which represents a radical departure from earlier practice. The changes envisaged are much greater than those which accompanied the general change-over from the biplane to the monoplane, and give rise to many problems whose solution requires considerable theoretical and experimental work. An important feature of the new sections is the precision in design and manufacture which is essential for their success. This has given renewed interest to the investigation of many of the detailed problems of air flow and calls for parallel improvements in manufacturing technique so as to achieve the high standard of surface finish required.

The purpose of this paper is to give a brief account of the theoretical basis of the design and application of the modified profiles as aircraft wing sections. It deals with the design of aerofoils for the subsonic range only, or, to be more precise, for flight at speeds below the critical Mach Number at which shock waves are first formed. The critical value usually lies in the range 0.6 to 0.8, depending on the wing shape and incidence, as will be described in more detail later.

The strengths of the metals at present available to industry are of especial importance to the aeronautical engineer who is also in a position to appreciate the need for greatly improved materials, the absence of which often places restriction on much needed developments. Although the materials of the future may become available by the somewhat fortuitous development methods at present employed, it is undeniable that greatly accelerated developments would result if a correct understanding was obtained of the fundamental characteristics of the cohesion and fracture of metals, of which the former belongs to the field of the atomic physicist.

It has been found possible, for the first time, to show that failure under static and fatigue stressing is associated with changes in the crystalline structure which are identical. These changes are (1) a dislocation of the initially perfect grains into large components which vary in orientation from that of the internal grain by amounts up to about 2°,(2) the formation of “crystallites,” approximately 10-4 to 10-5 cm. in size, whose orientation varies widely from that of the original grains, and (3) the presence of severe internal stresses in the crystallites. At fracture, whatever the type of applied stressing, the whole of the specimen behaves to the X-ray beam as a medium of crystallites showing marked lattice distortion and oriented completely at random. X-ray diffraction methods are shown to distinguish clearly between the effects of the application of safe and unsafe ranges of stress; the first method that has been successful in this respect.

In order to show the relationship between the new work described and previous work dealing with the use of X-rays in studying the deformation characteristics of metals, a preliminary section of the paper deals with cold-rolling and drawing. A survey is also presented of the present position regarding strength and atomic structure, together with references to various theories regarding the imperfections of crystals as encountered in practice. An introductory section describes briefly the atomic structure of metals, as revealed by X-rays.