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.

In preparing this lecture I found it necessary, when considering recent changes in aircraft design, constantly to ask myself the question “Is this particular change due to airworthiness requirements or does it arise from reasons other than airworthiness?” In fact, one must consider whether requirements have any influence at all on the development of trends in design. Should design be likened to a runaway horse with requirements following some way behind, or are requirements a strict rein by which the pace and direction of design is controlled?

My own answer to these questions is that the pace at which development takes place is not influenced at all by airworthiness requirements, but that requirements help to prevent design from running off the path of safety. To carry the analogy a little farther, the faster the development is, the more difficult it is to keep it running on the path of safety.

From the correspondence columns of the JOURNAL it is obvious that there is a great deal of interest in the present-day standard of maintenance design and that the general consensus of opinion is that it is not up to a high enough standard.

In order to stimulate still further the interest in design for maintenance the following series of questions are asked. These questions are the thoughts that run through the mind of a maintenance engineer when he is introduced to a new item of equipment. The perfect maintenance design is extremely rare, i.e. when all the questions can be answered in a satisfactory manner.

It is easy to produce what seems to be an efficient design on paper but the conditions of a well-lit and well-heated drawing office are different from an unheated hangar early in the morning of a winter day.

The importance of maintenance being considered right through all the design stages is even more vital these days, when the number of aircraft being manufactured is less than during the war years and the economic side is just as important as the performance figures. In fact, the words “economy” and “maintenance” are bound together. It is not possible to issue modifications in order to correct faulty maintenance design and still keep a high percentage of aircraft flying.