Mr. J. D. Siddeley in a paper on the reliability of aircraft engines in the Journal of the Royal Aeronautical Society for March, 1925, says under the heading Reduction Gears: “ The inclusion of this component has been responsible for limiting seriously the degree of reliability of the power unit. Considerable progress has been made, but the difficulties have by no means been solved. A stage existed when this component controlled the period between inspection and overhauls on the power unit. Improvements in the method of manufacture, particularly as affecting tooth form, have been responsible for a very considerable increase in the degree of reliability, but it is suggested that it represents one of the chief factors controlling the standard of reliability of the power unit as a whole.”

This statement is sufficiently serious ; coming from so great an authority on aircraft engines it compels attention and investigation.

This is the first occasion on which I am able to make public the results of one of the pieces of research work on which I have been engaged during the past three years; it therefore seems to be an opportune moment for placing on record, side by side with the results, the forms of assistance to me which enabled those results to be obtained. I am anxious to do this as I have been ploughing a lonely furrow, and I would like to see the kind of help which I have received widely extended, so that the vast field of research may soon be scored by a number of single furrows; if the powers that be are far-sighted enough to give this help, I believe that from these furrows will spring a harvest of no small value, even when compared with the products of the great Government and trade efforts.

The term “Experimental Flying” is used to cover so many different subjects dealing with the development of practical aviation that I feel the title of this paper would be suitable for a volume of no small size, if I were to deal in detail with all it infers.

Students of Nature have come to believe that the sum total of energy cannot be changed; that is, in the principle of the conservation of energy. Bernoulli's theorem is a simple expression of the application of the principle to fluid motion and in this form the principle is probably one of the most useful conceptions that the engineer has as a guide in a subject which presents many difficult problems.

Stalled flying is a subject which at the present time is occupying the thought and time of aerodynamic experts all over the world. It is still more or less in the experimental stages, but its value to flying, and more especially commercial aeronautics, is now almost universally recognised.

It has hitherto been possible to fly an aeroplane at all angles of incidence from the no-lift angle up to the burble point of the aerofoil section employed for the wings. The burble point on the lift curve marks the change-over from streamline to turbulent critical airflow, and at this angle the aeroplane will stall. The machine sinks, and what is more dangerous, becomes unstable laterally so that the pilot loses power over its orientation. A spin then usually ensues.

It may be as well to state here that the only way of defining the change-over point from normal to stalled conditions is by means of the stalling angle.