The Third British Commonwealth and Empire Lecture was delivered before the Society by Mr. James T. Bain on Thursday, 30th October 1947 at the Institution of Civil Engineers, Great George Street, London, S.W.1. The chair was taken by Dr. H. Roxbee Cox, F.R.Ae.S., President of the Society.

The importance of the effects of compressibility on the performance and behaviour of high speed aircraft has been appreciated for a great many years, but in this country it was not until 1937 that serious attention was given to the design of a wind tunnel that would enable the study of compressibility to be undertaken by tests on models of reasonable scale. Prior to 1937 the work which had gone on had established the characteristics of a few aerofoils and simple shapes; at Farnborough the work mainly related to the testing of high tip speed propellers, while at the National Physical Laboratory tests had been made of a two-dimensional character on aerofoils, these tests being made in a small wind tunnel driven on the injector principle, by air discharged from a high pressure reservoir.

It is becoming recognised that before civil aircraft can operate at high density, the most important problem to be solved is that of traffic control. Methods exist by which a single aircraft can navigate from a distant point to an airport and let down to a safe landing under bad weather conditions. Difficulties arise when several aircraft are involved at once and long delays can occur in the neighbourhood of airports carrying high density traffic. In this paper an analysis is made of the traffic problem in an attempt to clarify some of the factors involved.