Both the authors have been members of a small Helicopter Research Team at the Royal Aircraft Establishment, whose main activities in recent years have already been described in papers presented at the V/STOL Loading Action Symposia held at Buffalo and Southampton in 1963 and 1965 respectively. It is a matter of regret that the size of this team, and indeed of corresponding teams elsewhere in Britain, has been such that significant reports could not be made more frequently. However, progress in research has been rapid elsewhere, particularly in the USA, and there has been a noticeable trend recently for changes in helicopter design. Hence, the Rotorcraft Section Committee requested a general survey of current rotorcraft aerodynamics. This is the object of the present paper, in which the findings of many workers in Western countries have been included. Even so, there will be omissions because the authors will not comment on subjects with which they have had no first-hand experience: rotor blade flutter and divergence is a notable omission on which there have been advances recently by the Massachusetts Institute of Technology and Southampton University.

All of us know the history of the helicopter, but certain facets of this history need to be repeated in the framework of the development of the rigid rotor helicopter. It was in 1919, I believe, that Juan de la Cierva, shocked from the death of his brother in a spin accident, reasoned that the best way to build an aeroplane was to build one that made use of the spin as a fundamental. He put a bearing between the spinning wings and the fuselage so that the wings could spin without spinning the fuselage. This was a great advance in the state of the art and created an aeroplane with some rather good low-speed characteristics.