The first part of this title is the name of the Royal Aeronautical Society's own book with which I was concerned, as an exercise in long term studies, stimulated by the Centenary Year. The second part was added especially for the lecture as it seemed a concise way of relating long term speculations with some practical restraints which have to be circumvented too, if aeronautics is to progress.

The rotor performance is computed for the two modes of VTOL flight, hover and cruise. In the hover mode the rotor must provide sufficient thrust to support the whole aircraft weight, whereas in the cruise mode only a small fraction of the thrust is required. On the other hand, the flow through the rotor is relatively small in hover, but large in cruise. These different conditions in the two modes of operation demand variable blade geometry in terms of blade twist, as well as aerofoil camber. The high thrust in the hover mode must be obtained with a minimum of blade area to avoid excessive drag in the cruise mode. To achieve this a fairly high tip speed must be combined with high lift coefficients. In the cruise mode on the other hand, tip speed and lift coefficients must be reduced considerably. This leads to a high advance ratio.

When asked to give the de Havilland Memorial Lecture I could have taken as my subject the general state of play in aerospace research and development, with special reference to what Farnborough and its outstations were up to. Geoffrey de Havilland was a Farnborough product—a fact of which we at the RAE are very proud—so such a theme might have been appropriate. But de Havilland was not only a gifted aircraft designer, he was also essentially a test pilot. The whole rather special atmosphere at Hatfield which he created and sustained—and which still survives his death—owes much to the fact that we had here in one man both a quite exceptional engineer and an experienced and highly critical pilot who was quite capable of evaluating his own products in the air. As a matter of interest, I have reproduced in Figs. 1(a), and 1(b), pages from one of his 1911 notebooks at Farnborough which have recently come to light; even in those early days the pilot/designer combination sings out.

The achievement of high propelling nozzle performance in supersonic flight is vital to the success of any aircraft designed to cruise supersonically for prolonged periods. For such projects, the payload fraction is small compared with subsonic aircraft, and the ratio of gross to net thrust is high. As a result, the penalty in range/payload performance for a shortfall in nozzle gross thrust efficiency is inevitably severe.

The purpose of this note is to outline the basic physical features of parametric oscillation or instability and to show how situations might arise in practice in aircraft structures in which these effects would be significant. In most situations, the appearance of parametric oscillations will be the exception rather than the rule because of the relatively stringent conditions for its existence as a substantial component of the response. Nevertheless it is felt that the dynamicist should be aware of the phenomenon and should have some knowledge of its nature both as an aid to its recognition when it does appear and as a guide to the sort of situations in which it might be expected.

Scientific and technological development over the past 20 years has been tremendous. It presents the military planner with a dazzling array of possible developments to meet a wide range of operational requirements. Unfortunately, in every country resources are limited and a choice between the various alternatives has to be made. An objective and, if possible, quantitative method of examining alternative proposals is required. This is what cost-effectiveness analysis attempts to do. It can be and is applied at any level. For example, the government may require a study examining the implications of the different allocations of national resources to defence, foreign aid, civil industrial research, social security benefits, etc.; the Ministry of Defence may require a study examining the balance between the forces on land, at sea or in the air (force structure) and the contribution each make in a number of scenarios; a single service may require a study of the resources that should be allocated to defensive or offensive action; within a single role a study may be required of the alternative weapons that could be deployed to meet a particular requirement. In this paper, only questions arising in the study of defence systems will be considered.

Most guided weapon projects are inherently very difficult to bring to a successful conclusion, and the management methods necessary to achieve the required performance and time-scale, and at the same time to maintain control of costs, have had to be developed and are still developing; the procedures in use are being continually improved and refined. The essential problem in the overall control of a programme is in matching the technical achievement at any time against the corresponding expenditure, hence the involvement of an R & D establishment. There is room for many views on how this objective might best be achieved; those expressed in this paper are the author's and do not necessarily reflect official Government policy.