Aerodynamics is the foundation on which we build aircraft, as did the Wright Brothers. In their day, it was something of a mystery. It has become a profession in itself—a Science and an Art.

Its parent science—the mechanics of fluids—is one of the most difficult branches of the general science of mechanics, which is itself possibly not one of the easier departments of science. The simplest of our flying machines is aerodynamically excessively complex. We have always been prepared to design and build aircraft to meet extraordinarily severe conditions. There is only one yardstick by which we can expect to be judged—results. We need all our skill—and skill is art, though not the whole of it.

In the earlier days of flying, when aircraft speeds were low, it was a relatively easy matter to resort to a parachute to escape from an aircraft in trouble but, as aircraft speeds increased, escape by means of a parachute was more a matter of good fortune than of personal effort. Early in 1944, with the introduction of jet aircraft into the Royal Air Force, it became apparent that some assisted means of escape would have to be provided to enable air crews to escape from fast jet aircraft in an emergency. It was early in 1944, therefore, that I was invited by the Ministry of Aircraft Production to investigate the practicability of providing, in fighter aircraft, a means of safe escape for the pilot in abandoning the aircraft.

The most powerful theoretical tool in the solution of the aerodynamic problems of aircraft is the theory of small perturbations, which states that if a wing is thin (or a body slender), and if the incidence is small, then in inviscid flow the fluid velocity at any point can be treated as a small perturbation from the stream velocity. The backbone of our knowledge of the aerodynamics of aircraft is provided by this theory, to which the effects of thick wings and large incidences, and the effect of viscosity, introducing as it does the concept of boundary layers, can be added as additional or correction effects. It is known that at subsonic and again at supersonic speeds, the theory of small perturbations is a linear theory; that is, the assumptions implicit in it lead to a linear partial differential equation for the velocity potential, with linear boundary conditions.

Few aeronautical engineers would disagree that the development of new propulsive systems has been the main influence responsible for the achievement of supersonic flight. This is not the same thing as claiming that sustained flight at these speeds, over fairly long ranges and with reasonable economy, would ever be possible without correspondingly large contributions from the aerodynamicist, the structural engineer, and the designers of various equipment. However, if we had been limited for all time to the piston engine/ propeller power plant almost universally employed juring the first 40 years of heavier-than-air flight, it is unlikely that we should ever have exceeded a Mach number of one.

The 991st Lecture to be given before the Society, “ High Speed Test Flying ” by Lt.-Colonel Charles E. Yeager, U.S.A.F., was given at the Institution of Civil Engineers, Great George Street, London, S.W.I, on 19th April 1956. Mr. N. E. Rowe, C.B.E., B.Sc, F.I.A.S., F.R.Ae.S., President of the Society, presided for the first part of the meeting and Sir Sydney Camm, C.B.E., F.R.Ae.S., Past President, for the second part.

Opening the meeting Mr. Rowe regretted that he was unable to stay for the Discussion but he had particularly wanted to hear Colonel Yeager who was a world figure in the subject on which he was going to address them. This was the concluding lecture of the normal session this year (apart from the Wilbur Wright Lecture) and he felt that they could not possibly have anything better than the occasion which they were going to enjoy that evening.

The lecturer had had a very full life although he was still young and he would touch on a few of the high spots.

Colonel Yeager joined the Army Air Corps as a private and became an aircraft mechanic— so he knew something about the works before he went into the office—and he received his commission as a Second Lieutenant in 1942. He had a distinguished War career and he finished as a group leader. He was an escapist—he was shot down on his twelfth mission, smuggled by the French Underground into Spain, was put into prison; they forgot to take away his escaping kit, he sawed his way out by a highly tempered saw and got back to England and continued with the job. For his work in the Second World War he was awarded many distinctions including the Silver Star with one oak leaf cluster, the Distinguished Flying Cross with oak leaf cluster, the Bronze Star, the Air Medal with seven oak leaf clusters and the Purple Heart. Colonel Yeager was transferred to Wright Paterson Field and in January 1950 he was re-assigned to Air Research Development Command, Flight Test Center, at Edwards Air Force Base, which some of them visited at Muroc last year when they visited America.

Colonel Yeager's first great world-shattering achievement was to pass through the sonic barrier, October 1947. This was done on the Bell X-l which was taken into flight on the B-29 and did its great flying after reaching high altitude. He actually took off in this aircraft under its own nower using rockets and did a very astonishing climb at supersonic speeds, achieving a rate climb of 13,000 ft./min. He thought that in itself was an astonishing feat.

For these records, Colonel Yeager was presented with the Mackay Trophy by the Chief of Staff of the United States Air Force, the Collier Trophy by the President of the United States and he was voted the "Airman of the Year" for 1951 by the Civil Air Patrol Cadets of fourteen nations.

President Eisenhower awarded Colonel Yeager the Harmon Trophy at the White House. The Junior Chamber of Commerce of the United States named him one of the ten outstanding young men of 1954. Since February 1955 he had been with the 50th Fighter Bomber Wing in Germany and it was said, and he was sure it was absolutely true, that he was glad to be back working with a tactical organisation because he enjoyed the team spirit.

This was a brief record of tremendous achievement over a comparatively short flying life.

I have chosen for my address the subject of Co-ordination of Aeronautical Research in the British Commonwealth, not only because this is a very important and vital matter, but because at this time there are being held in Melbourne meetings between aeronautical research workers from Australia, New Zealand, South Africa and the United Kingdom in certain fields of research. Simultaneously, in Canada similar meetings are being held between aeronautical research workers from Canada, United Kingdom and Australia.

Aeronautical research, like human flight, is of comparatively recent origin as far as serious work is concerned. In Australia we can be proud to have had one of the pioneers—Lawrence Hargreaves—while the Wright Brothers based the design of their aircraft on experiments with aerofoils carried out in a wind tunnel of their own design.

Three of the primary considerations involved in the design of an aeroplane are safety, weight and serviceaability. The structure weight being a major item in the design which can influence its efficiency as a flying machine, the considerations of safety and serviceability have always to be studied with respect to their effect on the weight of the structure. The history of the development of aircraft structural design indicates clearly the tremendous amout of work which has been devoted to the development of higher strength materials and more accurate stressing and testing methods, always with the aim of ensuring adequate safety and reliability consistent with efficiency of design, construction, and operation.

The importance of maintenance of aircraft in airline operations is emphasised by the fact that, in British European Airways at least, 25 to 30 per cent, of the total direct cost of operating an aircraft is absorbed by maintenance costs. This applies fairly generally to all aircraft in B.E.A.'s fleet, and is also applicable to most of the world's airlines: in the U.S. the proportion is slightly lower, which may be due not to relatively lower maintenance costs, but to relatively higher crew costs there.

Throughout the animal kingdom powered flight is a surprisingly common attribute and whenever it occurs both the sustenation and the propulsion are provided by the same structural and aerodynamic mechanism. There is, in fact, but a single aerodynamic reaction which is divided, for convenience, into thrust and lift. In earlier times this convention was presumably not recognised, for the ornithopter appears to have formed for some centuries the principal technical basis of man's aspiration to heavier-than-air flight. Of the exceptions, Leonardo da Vinci's helicopter is now, of course, an accepted vehicle, but this lecture is concerned mainly with fixed-wing aircraft for which it is significant that success came only with the aeroplane, or, as Lachmann has recently put it, with the powered glider.

Existing methods of estimating the blade loadings on a helicopter rotor in forward flight have suffered from the necessity of using false simplifying assumptions in order to make possible an analytical approach. The following list gives the most usual of these assumptions:

1. (i) Blades have no twist or taper, or are twisted and tapered in a particular way.

2. (ii) The coning angle is small enough to neglect the resulting changes in velocities, angles, and so on.

3. (iii) The disc is horizontal.

4. (iv) The components of velocity parallel to the blade length may be neglected, and forces are calculated for and act with reference to the velocities normal to the blade length.

5. (v) There is no variation in downwash over the disc, or the variation is capable of simple expression.

6. (vi) The blades extend from hub, and tip losses are neglected.

7. (vii) The blades have pure flapping hinges and the analogy between flapping and feathering enables loadings calculated for flapping blades to be referred to a “hingeless” rotor system.

At approximately 14.00 hours on an overcast December day in 1954, persons in the vicinity of Idlewild Airport, New York, were startled by a violent explosion followed by the signs of an intense fire. Investigation revealed that an incoming European aircraft, a DC-6B, had crashed into the pier supporting the approach lights to Runway 4, had been demolished by the impact and had carried 26 persons to their deaths in the icy waters of Jamaica Bay. An inquiry into the accident was made by the Civil Aeronautics Board and their findings published. The probable cause of the accident was determined as an erratic approach which resulted in a descent too low to avoid striking the pier, but a significant rider was added to the effect that a contributing factor was pilot fatigue due to the particular and difficult circumstances of the flight.

The prospect of sustained supersonic flight has, in recent years, greatly stimulated research on the many problems involving temperature and heat transfer which must be solved before this prospect becomes a reality. Aerodynamicists have been investigating the nature and intensity of the heat loading. Metallurgists have intensified their research on the properties of conventional materials, and they have been joined by physicists, chemists, and engineers in the quest for new materials of construction. Research on relevant structural problems has begun with a number of theoretical and experimental studies of structural components under thermal loading, and laboratories are now being prepared for the full-scale simulation of the thermal conditions predicted in supersonic flight.

Supersonic speeds of flight have brought thermal problems due to the kinetic heating of skin materials. These materials, generally metallic, suffer loss of strength and stiffness with increase in temperature and a knowledge of changes in value of these properties at elevated temperatures is a pre-requisite to a design study. More particularly for stress offices the compressive stress-strain curves are required from which are derived tangent- and secant-moduli used to predict buckling in components. A fixture has therefore been developed for testing under edgewise compression sheet materials up to temperatures of 400 °C.

The main factors determining the shape of a supersonic aeroplane or missile are aerodynamic in origin. However, as always, a compromise with structural considerations must be achieved, zero wing thickness for supersonic aircraft being just as unattainable as infinite aspect ratio for subsonic ones.

This compromise is more far-reaching in the supersonic aircraft because of the much greater importance of aeroelasticity.

In addition, there are entirely new problems which arise from the high structural temperatures attained in supersonic flight.

From the earliest days, it has been an accepted article of faith that every aircraft flying outside its own country should carry some guarantee of its safety, conforming to an international standard, if not controlled internationally. No aim has been more assiduously pursued. But, after nearly forty years of the international regulation of aviation, there is no international standard of airworthiness with which aircraft are compelled to comply. The questions that now arise are whether the objective is ever likely to be achieved, and whether, in fact, international standards are necessary to safety.

Some purely aerodynamical phenomena, which might profitably be investigated by means of electrical analogue computors, are described. No attempt has been made to furnish a complete catalogue of problems but rather to present current issues, the solution of which would aid the development of practical aerodynamics.

Consideration has only been given to those questions which can be solved more suitably by an analogue computor rather than by a digital machine, although the feasibility of partially solving certain of them by the use of an analogue computor, and completing the work with a digital machine, is mentioned.

In scope, the survey includes two and three dimensional aerofoils as well as interference and a number of special problems.

The relative merits of the various forms of analogue computors are not extensively discussed.

The “ China-Film ” technique was developed by Murphy and Phinney from the “ China Clay” technique for indicating transition originated by Richards and Burstall. It consists in applying a suspension of china clay in a volatile liquid to a surface subsequently subjected to air flow. Not only is transition indicated, but when all the liquid has evaporated, the china clay remains deposited on the surface in patterns which may be interpreted to yield information on the direction of air flow near the surface and the incidence of shocks thereon.

Experiments were carried out, at supersonic and subsonic air speeds, to determine the process by which the patterns form and to establish the aerodynamic meaning of the “ flow lines ” of the pattern. The influence of the viscosity of the carrier liquid was also studied. Indications of flow direction were found to be unreliable near a trailing edge, on a vertical surface at low speeds and in regions where the surface shear stress is only of the same order of magnitude as the product of pressure gradient and film thickness.

The range of frequency for which aerial systems for aircraft are required is immense: the highest used frequency is over one hundred thousand times the lowest. Very differing techniques are required throughout this range and this lecture will be chiefly concerned with the principles on which these techniques are based, rather than on individual aerial systems.

One method of classification of aerials is by their operating frequency or wavelength range.