This is a subject which during recent years has aroused considerable interest in all industries and particularly in the aircraft industry and a large amount of investigation work has been carried out. In some industries, both light and heavy, the growth of welding as a “ production tool ” has been phenomenal, and the same growth is at the present time occurring in the structural steel work and shipbuilding industries. If one views this change which is taking place it will be observed that the change is entirely at the expense of riveting and bolting, and before proceeding I will roughly summarise the general disadvantages of these two processes.

During the last fifty years much experimental and theoretical work has been carried out on the subject of the failure of struts, i.e., rods under axial compression. Most of this work has been devoted to the case of struts pin-jointed at both ends and only a small part examined the behaviour of rigidly encastred struts. From these investigations we know that in case of very slender struts the critical load for rigid encastrement is the fourfold of that for pin-jointed ends. This difference becomes less with decreasing slenderness of the strut, but even for slenderness ratios usually attained in aircraft framework it is considerable, if high tensile steel is used. In actual framework, compression members being more or less rigidly joined to nodes which may twist elastically, they represent a case between both forementioned extremes and may best be regarded as elastically encastred. Since according to above statements the load which may be sustained by a strut obviously depends a great deal upon end constraint, it is of great importance for design purposes to determine exactly the effect of elastic encastrement.

The theory of the aerofoil has now been studied to such an extent that, from this province, it is hardly possible to expect further material improvement in its aerodynamical qualities : profiles differing but little from an inverse of a parabola (Joukovski profile) would appear to be the theoretical ideal. Subsequent important progress in that respect may be sought only in another direction, viz., in the application of a series of supplementary contrivances having a marked influence on the properties of the flow around the aerofoil. Here we are referring to such devices as the sucking away of the boundary layer (Absaugeflügel), or the insertion of appliances on the aerofoil itself. Nevertheless, up to the present, only one of the very earliest attempts in this direction, namely, the slotted wing, has developed sufficiently to be in any way widely adopted in contemporary aircraft construction.