Real gas effects are among the major complications of hypersonic flow and this is illustrated by examples of the temperatures and pressures attained when a flow is brought to rest. Once dissociation appears, these depend to a marked extent on the type of compression.

In designing for the alleviation of aerodynamic heating there are two distinct cases. The first is the uncontrolled re-entry of a freely falling body, for which it is best to have a shape with a high pressure drag. The second is sustained flight, for which a shape with low pressure drag could be better, radiation giving appreciable control of surface temperature.

Low pressure drag accords with design for aerodynamic efficiency and there is scope for research on three-dimensional lifting shapes. There is some discussion of this and also of the philosophy of securing maximum amounts of separated flow.

Finally, there is continual emphasis on the need for experimental research, since hypersonic flow fields may differ considerably from those that would be expected by extrapolation of conventional supersonic experience.

Some of the aerodynamic problems that arise in flight at hypersonic speeds can be studied experimentally with wind tunnels and measuring techniques that do not differ in principle from those used for research on supersonic flow. If, however, it is required to simulate the very high temperatures of hypersonic flight, it is usually necessary to use heaters and tunnels of unconventional design, frequently having a very short running time, or to make tests with a model launched or propelled at high velocity.

The short running times sometimes involve the use of measuring techniques that differ considerably from those of conventional wind tunnel practice. Also, in the presence of high temperature, real gas effects, it is sometimes necessary to measure additional quantities in order to define the state of the gas. The paper contains a brief introductory review of these topics, and of the extent to which experimental test facilities can reproduce the conditions of full-scale hypersonic flight.

The object of this paper is to give a brief account of the development of the study of hypersonic flow over the past three decades, and to stress some of the ways in which hypersonic flow differs from supersonic flow. In some of the early work on the subject a wrong emphasis was given to the study of flows with attached shock waves and to the nature of the shock boundary layer interaction problem, and it is only quite recently, mainly as a result of experimental work, that it has become possible to obtain a balanced picture of the important features of hypersonic flow fields. In the present paper real gas effects are not taken into account; in general, the inclusion of these effects will not markedly alter the picture given.

When a body moves through air at very high speed at such a height that the air can be considered as a continuum, the distinction between sharp and blunt noses with their attached or detached bow shocks loses its significance, since, in practical cases, the bow wave is always detached and fairly strong. In practice, all bodies behave as blunt shapes with a smaller or larger subsonic region near the nose where the entropy and the corresponding loss of total head change from streamline to streamline due to the curvature of the bow shock. These entropy gradients determine the behaviour of the hypersonic flow fields to a large extent. Even in regions where viscosity effects are small they give rise to gradients of the velocity and shear layers with a lower velocity and a higher entropy near the surface than would occur in their absence. Thus one can expect to gain some relief in the heating problems arising on the surface of the body. On the other hand, one would lose farther downstream on long slender shapes as more and more air of lower entropy is entrained into the boundary layer so that the heat transfer to the surface goes up again. Both these flow regions will be discussed here for the simple case of a body of axial symmetry at zero incidence. Finally, some remarks on the flow field past a lifting body will be made. Recently, a great deal of information on these subjects has appeared in a number of reviewing papers so that little can be added. The numerical results on the subsonic flow regions in Section 2 have not been published before.

On the face of it boundary layer problems at hypersonic speed present enormous difficulties. However some aspects have turned out to be somewhat simpler than was visualised some years ago, and with the aid of certain simplifying assumptions many problems have been solved, at least to a good design approximation, and inaccuracies in the results are due not so much to the approximations involved, but to our lack of knowledge of the physical properties of air such as viscosity and Lewis number.

Having agreed with Dr. Shercliff that I would discuss dissociation and its applications, after which he would speak about ionisation and its applications, it seemed appropriate that I should repeat the lecture I gave to the International Congress of the Aeronautical Sciences on “Dynamics of a Dissociating Gas”—although with apologies to those few in the audience who may have come back from Madrid with memories of anything except Flamenco and Paella and Goya and Sangria.

Professor Lighthill's paper has discussed the dissociative behaviour of air at the high temperatures produced when it is compressed in a shock or heated by friction in a boundary layer under hypersonic flight conditions. Just as molecules may dissociate into atoms, so may these molecules and atoms split further into charged ions and electrons. The equilibrium state of this ionisation process is amenable to the same thermodynamical or statistical-mechanical treatment as in the case of dissociation, the Law of Mass Action in this context being referred to as the Saha equation. Consider the simplest case where a monatomic gas is singly ionised, a fraction α of the atoms having lost one electron, so becoming singly charged positive ions.

The intention of this paper is to point out certain aerodynamic problems that may be of future interest, by trying to argue in general terms about their possible context. Perhaps the most distinctive feature of any project analysis of a vehicle designed to carry a man into an orbit in space and return him safely to the ground, is the need to limit the accelerations and retardations of his passage, and the temperature of his environment. There are of course innumerable other considerations which have to do with his comfort and safety, but it is his inability to withstand excessive heat or high g which distinguishes man from more rugged but less versatile instruments.

During the discussion the point was made by a speaker from the Industry that he had come to the meeting to learn; that many of the audience of 200 agreed with him was reflected in the discussion, for, of the 23 who participated 19 were from the research establishments and universities.

Of the million acres occupied for farming in New Zealand approximately million acres or 66 per cent are too steep to be cultivated by implements. The productive capacity of much of this country can be improved by oversowing with clovers and grass seed, and the application of superphosphate, but there was little opportunity to do this on an extensive scale other than spreading laboriously by hand until aircraft were used for the purpose.

Although a great deal has been said and written in the past eight years or so about engine noise, previous discussion has been very largely concerned with the physical aspects of jet noise and with the possibility of suppressing it by means of special propelling nozzles and ground mufflers. There is however another, and in general a better, way of dealing with jet noise, namely by using engines of low specific thrust, i.e. of low exhaust velocity and one purpose of the paper is to examine the possibilities of such engines. This leads to a consideration of blade noise from fans and turbines for as jet noise is reduced this tends to become the dominant factor. But first of all it is necessary to re-examine the nature of the noise problem.

Two-Dimensional Stress concentration factors may be obtained more quickly and simply than the corresponding three-dimensional factors, either by experiment or mathematical analysis. It would be convenient to obtain information, for varying geometry in the two-dimensional case of a particular type of stress raiser, e.g. a shoulder, groove or hole, and use this either to predict the three-dimensional stress concentration factors or to extend the range of existing three-dimensional results. Clearly a comparison is only possible if the three-dimensional stress raiser embodies a plane of symmetry (which gives the geometry of the similar two-dimensional stress raiser), and if the loading conditions can be reproduced in both the two- and three-dimensional cases. The latter requirement restricts the correlation to the stress concentration factors obtained in tension and in bending. The three-dimensional torsional loading system has no plane of symmetry which can be simulated in two dimensions.