In the early days of aero-engines the test equipment was limited to little more than a brake or dynamometer, a fuel flow meter and a few thermometers, and the development of these engines in the direction of better performance and reliability, to a large extent, was an art, built up of experience aided by instinct, of deducing the happenings inside an engine from the few meagre measurements obtainable.

Nevertheless, the aero-engine made healthy progress, became more complicated in the process, and introduced many more problems.

The title of Prototype Testing is taken here to refer to the flight tests carried out on a new aircraft by its constructor. It embraces a wide field of subjects and in a short paper, it cannot be covered in any way comprehensively. I propose therefore to deal mainly with very general aspects of the subject.

The paper does not attempt to embrace out of the ordinary types with special problems peculiar to themselves—an extreme example of which is probably the Brabazon I. Most of my experience on this subject has been concerned with small aircraft, many of them single-seaters, and consequently some of my statements may apply more particularly to this type of aircraft, although many will probably apply equally to other types of aircraft.

It is a curious reflection on the progress of science in our time, that a term which has only come into common use in aeronautics in the last decade commemorates the name of a man who carried out his researches some fifteen years before the Wright Brothers first flew in 1903. The obscurity surrounding Ernst Mach, whose name is given to the ratio of the speed of flow of a gas to the speed of sound under the same conditions, is ill-deserved, for there can be little doubt that he was one of the outstanding scientific personalities of the nineteeth century; this distinction arises not so much from his experimental work, which was extremely valuable, but more from his original conceptions on the philosophy of science which helped to lay the foundation for modern developments in physics.