I must begin by explaining how I come to be giving this lecture. The experimental work with which I shall deal has, for the most part, been done at the N.P.L. and the R.A.E., under the general direction of the Aeronautical Research Committee. The way in which I come to be connected with the work is that I am a member of this Committee and am Chairman of a small panel that was created, some three years ago, by the Committee, to deal with this and other work relating to control and stability. The experiments that I shall describe and the methods of dealing with the results that I shall employ are, therefore, the results of the combined work of a considerable number of people. I can thus claim no special ownership of any of the ideas that I shall use, except in so far as I belong to the panel that has been working upon them. On the other hand I am giving the lecture as a private person, so that any views I express are personal ones and in no sense official.

It was suggested by Lanchester some years ago that an aerofoil immersed in a moving fluid gave rise to two trailing vortices of opposite hand, one situated near each of the extremities of span. The existence of the vortex pair has since been demonstrated in a variety of ways, of which perhaps the most striking is that due to Caldwell and Fales, who used the condensation of water vapour in a high-speed wind channel to obtain photographs. The vortex pair constitutes so important a feature in the stream behind most aerofoils that measurements capable of numerical analysis seemed desirable, and an account is given in the present Paper of some work to this end which appears capable of straightforward interpretation.

It may be that the residual motion sufficiently far behind any aerofoil is a vortex pair. Similar experiments to those about to be described, however, which deal with the motion only for a few chords back, have been made with several other aerofoils of a variety of shapes having long, graded wing tips, and with some of these no distinct vortex was found. With others apparently more than one pair could exist in certain circumstances, but opportunity has not occurred to probe deeply into these interesting cases. Attention is confined to two aerofoils representative of considerable variation in a class having uniform section and angle of incidence with blunt, or square-ended, wing tips. The general nature of the results is common even with aerofoils outside this class, but the foregoing remarks will serve to indicate that exceptions are known to exist.

Mr. Glauert's paper “Theoretical Relationships for the Lilt and Drag of an Aerofoil Structure ” strikes a bold note in asking us to discuss the fundamental principles of Prandtl's aerofoil theory rather than its detailed application. His suggestion is a particularly good one, however difficult the task. Justification of the geeneral truth of the suggestions brought forward by Lanchester, and so ably developed bv Prandtl and his colleagues and others, have frequently been afforded by the application of the theory, and similar support only awaits the seeking. But, to achieve its greatest utility, the theory should enable us to extrapolate experimental knowledge with confidence; an example is provided by the question as to what limit, if any, exists to the camber of wings on economic grounds and apart from burble considerations. An approximate tally—rough, in some cases—of the theory with observation over a limited range is inadequate to establish it sufficiently for such a purpose. The theory may indicate with truth the existence of, say, a limit to a certain development, but its numerical calculation of the limit may yet be widely wrong.

It is a fact of common experience that a body in motion relative to a gas or liquid is subject to a resultant force, and it is customary to resolve this force into two components, the drag opposing the relative motion and the lift at right-angles to the direction of this motion. In general the drag is the pre-dominant component, but the class of bodies known as aerofoils and used for the construction of aeroplane wings, is such that the lift is considerably in excess of the drag. The present discussion relates solely to this class of bodies whose essential characteristic is the production of a large lift correlated with a relatively small drag. It is a matter of very considerable importance to develop a theory which will explain the origin of the forces experienced by an aerofoil, and will provide a method of calculating the characteristics of any aerofoil structure from a knowledge of its geometrical form and of the physical properties of the fluid through which it moves. It is proposed to discuss the behaviour of aerofoils in two and in three dimensions, and in particular to discuss the vortex theory of lift and induced drag. The general nature of the flow pattern on which the theory is based has been described by Lanchester, but the mathematical development of the thcorv is due to Prandtl and his colleagues.