Until about ten years ago the highest speeds achieved by aircraft, even in dives, rarely exceeded half the speed of sound. Under these conditions the air round the aircraft behaved very much as if it were incompressible, and the forces acting on the aircraft could be derived on the basis of laws governing the aerodynamics of incompressible fluids.

During the 1939-45 War great advances were made in the performance of aircraft and eventually speeds of over three-quarters of the speed of sound were being reached, even in level flight. At these speeds the air no longer behaves as an incompressible fluid; the aerodynamic laws involved become much more complicated and the aircraft designer is faced with a mass of new problems, involving many strange and unexpected effects.

The purpose of this lecture is to discuss the contribution which research in flight can make towards the elucidation of these problems, with special reference to work done at the Royal Aircraft Establishment during the past few years.