A particular interest attaches to the study of the wings of insects, because, in some respects, they act more after the manner of human inventions for aerial locomotion than do the wings of any other flying creature. Though man has never been successful in equipping himself with wings, and has largely given up the hope of acquiring them, he has succeeded in making machines that will fly; and, whether his craft is lighter than air or heavier than air, its driving mechanism is a set of rotating blades, the nearest counterparts of which in the animal world are the rapidly-whirring wings of certain insects.

The internal motjon of a fluid assumes one or other of two distinct forms; either the elements follow one another in a direct and simple manner, or else they wander about in erratic paths as they progress downstream. In the first kind of motion, called “ regular ” or “ streamline,” the components of the velocity at any selected point are either constant, or vary in a definite manner directly related to the boundary motions. In the second, known as “ turbulent,” these components continually change in an irregular way, which cannot apparently be connected with the regular boundary motions.

The air flows past unseen as one flies. As it flows the invisible meridians and parallels pass with it, sharp but arbitrary demarcations imposed mathematically by man upon a sphere which he learned to measure long after he began to move intelligently about its surface; geographically non-existent hair lines which stubble the ends of certain radii projecting to the surface from the centre of our globe. But these same lines are cartographically displayed upon our maps, where we designate them graticules. And so the air navigator, resolving his position in the air above the surface of the earth as a mathematically unique point, knows both cartographically and geographically exactly where he is.

The importance of healthy relations between the sporting and the scientific sides of gliding is very great; and the fact increases the responsibility of my task this evening as well as the honour that the Council of the Aeronautical Society have done me in asking me to undertake it. The history of gliding on the continent shows that at times when financial or other difficulties have given cause for anxiety, some development suggested on the scientific side has rekindled enthusiasm and given the movement a new vitality. Change is as essentially connected with a healthy sport as with a living organism. Nobody can visit the Wasserkuppe without being impressed by the zeal which prevails there; it is almost religious in its fervour, and although some of the neophytes of the hierarchy may be mere gymnasts fired with the team spirit, its bishops and its missionaries are inspired by their determination to advance knowledge.

A regular air service between Amsterdam and Batavia has now been in existence for more than two years, a fact which to-day is generally known. The great and mutual interests which bind Netherland with its colonies—interests of a cultural, commercial and also of a governmental nature—have always placed in the foreground the necessity of bridging over as rapidly as possible the enormous distance separating both territories.

The lecturer has been concerned with commercial air survey operations since their inception after the war, more on the organisation, finance and business side, rather than on the technical. It is therefore from the angle seen from the directing side that this paper is submitted. As certain technical aspects will be of interest to the Institution, they are referred to, in order that attention may be drawn to them. Through the courtesy of the firms responsible for manufacture, certain apparatus is exhibited which includes an air camera, the automatic pilot, and the contouring stereoscope. Certain technical experts have kindly promised to be available to answer questions or to demonstrate apparatus, and certain individuals and firms have been good enough to lend photographs and slides. Acknowledgment for such assistance will be made in the reply to the discussion.

The spinning nose dive has confronted the aeroplane designer with two main problems. The first is to evolve an unspinnable aeroplane, and the second to provide controls of sufficient power to ensure an immediate recovery from any spin into which an aeroplane can be forced. At the present time there seems little prospect of achieving a general solution of the first class of problem, although interesting particular solutions have been evolved from time to time. It seems now to be generally accepted that the unspinnable aeroplane would be desirable for every purpose, but it is evident that it must be achieved without sacrifice of performance or manoeuvrability. Moreover, it must be remembered that an imperfect solution of the first class of problem, such as can sometimes be achieved by limiting the power of the control surfaces, may be worse than useless; for the pilot may be deprived of the power to regain normal flight quickly if his aeroplane falls into a spin in unusual circumstances. It is, in fact, very unfortunate that many devices which point the way to the evolution of the unspinnable aeroplane also add to the difficulty of recovery from a spin. It is not my purpose to consider the methods which have been devised to prevent the incipient spin, which occurs when a conventional aeroplane is stalled, but to confine my attention to the problems of ensuring recovery from established spins. For the views expressed I must, of course, accept personal responsibility.

The position to which aircraft construction has proceeded is due in a large measure to the intense metallurgical investigations of recent years. Amongst the materials that have arisen out of this are the light alloys of aluminium, and in particular, that known by the general name “duralumin.” This alloy plays the part of main structural material in many of our present-day aircraft. The object of this paper is to give a brief general account of duralumin with reference to aircraft construction.