This is a great occasion; I feel quite unworthy to do justice to this commemoration of a great man and a revolutionary event and to become one of the illustrious band who have so graced this occasion in the past. For the more one reads about the Wrights' achievements, the more one is impressed by them as men of stature—of great character, determination and intellectual strength with a most endearing modesty. They had too, such a sense of fun and there is a golden thread of family affection running all through the record. They were men of vocation just as their father, Bishop Milton Wright, and there is no doubt of the influence of the family ties on what they did. One senses how much home meant to them and how much they all turned to Katharine, the womanly stabilising element. A comment of Wilbur's in one of his letters to George Spratt commenting on a technical argument they had been having, is very revealing. "It is a characteristic of all our family to be able to see the weak points of anything", a vivid hint of most lively family discussions, and of course, this most rare combination of brothers must have been a fundamental element in all their work.

Since the mythical dawn of man's attempt to emulate birds and fly, and some believe this started with Icarus, man has been frustrated by his apparent inability to predict or prevent occasional disaster arising from failure of his mechanical aids to flight. From ancient Icarus with his ultimate and disastrous thermal structural failure, to the present and future aircraft the same problem although now in a more sophisticated and complex form, confronts the aircraft designer and engineer of today.

This paper is not intended to be a comprehensive treatise on Aircraft Accident Investigation as a whole because this is a subject of many facets and problems, but rather is it intended to draw attention in basic and elementary form to some types of mechanical failures or inadequate design criteria which have caused accidents to aircraft in the past, the methods used in their diagnosis and, what is perhaps more important, some lessons that have been learnt from them.

It is a great privilege to have been invited to give the 1963 Trenchard Memorial Lecture, and I welcome the opportunity to pay tribute to the memory of a great soldier and airman. Lord Trenchard's deep understanding of the meaning of air power, his single-minded insistence on its proper use, and his dedicated leadership of the Royal Air Force during its formative years, will continue to inspire successive generations of airmen. It is appropriate that the Royal Aeronautical Society's Trenchard Memorial Lecture should be given here at Henlow before an audience of young airmen; for it will be their responsibility to master the new techniques on which will depend the continued evolution of air power.

It is probably not necessary for an economist to justify his temerity in addressing the Air Law Group of this Society on Licensing since I am here by invitation. But it is worth noting that a great deal of air law is, in fact, concerned with the economic regulation of the industry: the law is endeavouring to give effect to conclusions about how best to arrive at certain economic results.

I shall not discuss any details of the law relating to air transport licensing: I shall concern myself with the broad economic issues which underlie the law. I shall start from first principles and ask “Why should we regulate air transport?” and, if there are good reasons for so doing, “what are the essentials of a regulatory system?”.

The existing hypersonic test facilities at the Royal Aircraft Establishment are described. In particular the Hypersonic Tunnel and the Shock Tunnel are considered in some detail from the aspects of design, performance and use, with a view to seeing what lessons can be learned from their construction and operation. The reasons which led to the building of the present facilities are compared with modern ideas for tackling hypersonic research. Consideration of the problems which will require experimental investigation in the future suggest that additional tunnels will be required which, like the Hypersonic Tunnel, will run steadily for periods of seconds rather than for much shorter times. The air supply and power requirements of such tunnels are given and the recent developments in heater design which make their realisation possible are reviewed.

At the very high flight speeds of ballistic missiles, re-entering satellites and meteorites, the air in contact with the body may attain very high temperatures and may even become ionised. For example, the air in the stagnation region of a low-drag ballistic missile re-entering the Earth's atmosphere can reach a temperature of 8000°K and the electron density may be as high as 1016 per c.c. In any experimental work on high enthalpy flow it therefore becomes necessary to have techniques for measuring these high temperatures and high electron densities. The aim of the present paper is to describe techniques for measuring these quantities.

Morgan has described a spatial instability in the flow through screens or grids of small open-area ratio. Head and Rechenberg and others have observed large span-wise variations in the thickness and shear stress of nominally two-dimensional boundary layers on flat plates and aerofoils in wind tunnels. It now appears that these spanwise variations are caused by the instability of flow through the screens. The jets of air issuing from the pores of the screen attempt to entrain more air by the usual mixing processes, but can only entrain it from each other, so that groups of jets coalesce in rather random (steady) patterns determined by small irregularities in the weave. The resulting variations in axial velocity are virtually eliminated by the wind tunnel contraction, but variations in flow direction are not so greatly reduced: a theoretical analysis shows that the observed variations of boundary-layer thickness, which often reach ± 10 per cent of the mean, can be produced by directional variations in the working section of the order of ± 1/20 deg, with a spanwise wavelength of the same order as the boundary-layer thickness.

The need for simultaneous use of more than one type of camera to record Schlieren images of aerodynamic phenomena has led a recent investigator to employ a system in which an additional image was reflected from the Schlieren stop when it intercepted a deflected beam. This note describes an alternative arrangement capable of producing at least four images, although in its present form it is only designed to produce three.

This note illustrates the correlation of the pressure distributions measured on three NACA 16-series aerofoils at subsonic and transonic speeds. The transonic similarity parameters proposed by Spreiter

are used for the subsonic, transonic and supersonic types of flow illustrated in Fig. 1.

The German metallurgists were the first to discover how to make the light alloy of aluminium which they called duralumin. I understand it is a mixture of 97 per cent aluminium and 3 per cent copper, but it may have a very small percentage of rarer metals. Whereas aluminium has a yield point of about nine tons per square inch, duralumin has a yield point of twenty-five tons per square inch.

Before and during the First World War the German aircraft constructors used duralumin in the construction of Zeppelin rigid airships, aeroplanes and seaplanes and flying boats. Towards the end of the First World War, I began to fear that the Germans would invent all-metal aircraft with highly streamlined forms which would increase the speed of their aircraft and that at very high speed with their bombing aircraft, they would fly at almost house top level, drop a time bomb on our works and be at a safe distance before the explosion occurred.

The distribution of stress in a flat plate having a circular cut-out and subjected to known forces at its outer boundary is of considerable practical importance. Assuming that (i) the plate is thin, so that the problem is one of plane stress, and (ii) the material of which it is formed is isotropic and obeys Hooke's law, exact or approximate solutions have been obtained for a limited number of particular cases. Most of these solutions relate to a plate which is infinite in extent in one or more directions. The case of a plate of finite width and infinite length having a circular hole on the axis of symmetry was discussed by Howland, and by Howland and Stevenson. Wang gave an approximate solution for a perforated shear web, and Mindlin investigated the stress distribution around a hole near the edge of a semi-infinite plate under uniform tension.