The Gliding and Soaring Flight movement is as old as History. The observation of bird flight must have shown the men of former centuries in the same way as our own generation, that apart from power flight with wing beats there must be another flight possibility, enabling the use of the energy in the movement of air masses for flight without the expenditure of other power. The experiments undertaken in these times on the solution of the flight problem have come down to us only through myths and sagas, and we cannot differentiate between truth and imagination in these stories.

Experimental research, which can certainly be considered as the foundation of modern physics, has also in the realm of aerodynamics laid the basis for modernaeronautics. In this gliding and soaring flight plays the rôle of full–scale experiments, not as an end in itself, but as a proving-ground and last station before the invention of power flight.

An accurate knowledge of the strength of materials is more essential in aircraft construction than in any other branch of engineering, as every aircraft component has to be reduced in weight to a minimum compatible with safety. Equally essential in aircraft construction is the use of every available refinement in calculation, although it is not possible as yet to estimate with sufficient accuracy from calculations alone the strength of many of the components. At every stage hv aircraft construction, mechanical tests are therefore made, first to determine the qualities of the materials, then to verify the strength of each part and finally to. check the strength of the complete machine.

Owing to the wide range of my subject and to the limited time available, I propose to deal briefly with some of the aspects of mechanical testing of special importance in aircraft research and development, describing more particularly some of the test apparatus and methods of testing I have had opportunities of developing.

Apart from the subject of straight normal flight and all the problems of stability and performance concerned with it, more thought has been given to the spin than to any other form of motion of an aeroplane. The reasons for this are not far to seek. The spin is the evolution which, until the recent introduction of certain well known methods of lateral control and stabilisation, invariably followed the loss of control at speeds below the stall, and its occurrence near the ground has been responsible for the majority of fatal flying accidents.

It is the only form of stable uncontrolled motion which is exhibited by an aeroplane other than that which it may possess in straight flight. This stability has in a number of cases proved extremely dangerous, if not fatal, as in prolonged spins on certain aeroplanes it has been found impossible to recover.

The problem of the helicopter is one which has intrigued inventors for centuries, and which has perhaps attracted more attention and effort than any other phase of design of heavier-than-air craft.

Even as far back as the fifteenth century we find Leonardo da Vinci devoting several years of his life to studying- bird and mechanical flight, and he has left many sketches showing- flying machines. Of particular interest to us is a design showing- an aircraft consisting of a sustaining screw driven about a vertical axis.

The first helicopter actually to fly was a small model, little more than a toy, which was shown before the French Academie des Sciences in 1784 by Launay and Bienvenue, and since that time literally thousands of schemes have been proposed by inventors all over the world. The patent files of all countries are full of helicopter specifications, the greater part of them being based on faulty physical principles, and obviously impracticable.

Petroleum as it occurs in Nature is a very complex mixture of hydrocarbons. Imagine a mixture of petrol, paraffin, gas oil, fuel oil and lubricating oil, together with a certain amount of water and other foreign matter, and you have an idea of crude petroleum as it is found in Nature.

It is interesting to note that practically the sole use of petroleum for some time after the first well was drilled was as a burning oil. The object of the refiner was therefore to produce as large as possible a percentage of paraffin from his crude. The lighter fractions were regarded as a necessary evil, and it became a very difficult problem to dispose of them satisfactorily. It was, of course, necessary to keep the flash point of the paraffin high enough to avoid the danger of explosion of the lamps in which it was used. Petroleum inspectors were employed to ensure that the paraffin on sale was not “adulterated” with the then valueless light petroleum spirit. No market other than as a source for lighting gas was found for this light spirit until the advent of the motor car.

In aeronautical engineering the strength-weight ratio is so important, apart from the fact that the consequences of structural failure are generally more disastrous, that it becomes essential to investigate the loads and stresses in the various parts of the structures far more accurately and in much greater detail in order that smaller factors of safety may be used.

It is often possible, in the case of structures of a low order of redundancy which have to be designed to meet several different systems of loading, to select by inspection those members which may be considered to be the redundant ones, the elimination of which would result in an increase of the design loads in most of the remaining members. If the members are designed to such increased loads many members of the structure will be overstrong and unnecessarily heavy. The excess of strength of the various members of the complete redundant structure will almost certainly differ, so that the weight of the whole will be increased above the essential in a greater proportion than the strength.

Any theoretical attempt to evaluate the forces and couples experienced by an aircraft when in flight by a mathematical analysis of the pressures exerted by the air when in motion about the various parts, leads to what have, so far, proved insuperable difficulties. It involves the integration of the equations of motion of a real fluid, and, except in a few very special cases, these have been insoluble. The actual motion of a fluid is affected by a number of its properties, and, in general, accounts would have to be taken of its density, viscosity, and at high speeds its compressibility. In certain circumstances the effect of these last two can be neglected and the classical theory of hydrodynamics dealing with the motion of a non-viscous, incompressible or perfect fluid can be applied. A further simplification consists in supposing that the motion is irrotational, that is, any small portion of the fluid at a point has no angular velocity about its centre of gravity.