The 805th Lecture to be read before the Royal Aeronautical Society was given on 19th October 1950 at the Institution of Civil Engineers, Great George Street, London. Major G. P. Bulman, C.B.E., F.R.Ae.S., President of the Society, presided during the first part of the meeting and introduced the Lecturer, Mr. R. Hills, B.A., A.F.R.Ae.S., who had been engaged for some years on low and high speed tunnel work and was in charge of the No. 2 eleven and a half foot tunnel at the Royal Aircraft Establishment, Farnborough.

Throughout the world helicopters are being operated in a variety of roles today and, whereas ten years ago, there was not one type which had got beyond the stage of being an inventor's plaything, several types are now suitable for regular military and civil operations. At present, so far as is known to the author, public transport services using helicopters are confined to charters and scheduled mail services, with the exception of a scheduled passenger service which has been run by the British European Airways Corporation as an operational experiment since June 1950. The factors, both technical and economic, which have so far prevented the large scale introduction of public transport helicopter services, are being progressively overcome and in due course operators will find that manufacturers have made available suitable aircraft. The operators' task will then be to fly these aircraft in a safe and regular manner and give the public the best service possible.

Measurements were made of the downwash effects behind two finite wings 3.1 percent, thick, having square and 20° raked tips respectively. The tests were conducted at a Mach number of 1.45 and a Reynolds number of 1.2 millions by traversing a yawmeter 1.62 chords behind the trailing edge of the finite wings.

In general, a maximum downwash of the order of ½° per degree of wing incidence was observed in that portion of the tip Mach cone behind the wing, and a maximum upwash of similar magnitude was observed in that part of the tip Mach cone situated outboard of the wing.

Thus it is apparent that these effects are large enough to affect the lift on any surface situated in the tip Mach cone behind a finite wing. In particular, placing the rear surface in the downwash region behind a finite wing, will tend to reduce the overall lift while placing it in the upwash region will tend to magnifiy the variations of lift initiated by the finite wing.