It is now well known among aircraft engineers that the compressibility of the air has an increasingly important effect on the aerodynamic forces as the flight speed rises and approaches the speed of sound. As a result of the development of the gas turbine and other improvements, aircraft speeds have risen very rapidly during the last few years, and compressibility effects are, therefore, of great importance in many new aircraft designs. Unfortunately, the designer is faced with very great difficulties in attempting to predict the behaviour of a new aircraft flying at high speeds. The main reason for this is simply that there is very little systematic knowledge of air flow at high speeds past wings and bodies. A further difficulty arises because many of the methods and ideas which have proved so useful in the design of low speed aircraft may have to be changed completely when high speeds are considered. To mention only one example, it is well known that at low speeds a separation of the boundary layer at the rear of an aerofoil causes an increase of drag, but is not so well known that a separation of the same kind at supersonic speeds causes a reduction of drag (for a given incidence). Because there may be differences as important as this between high and low speeds it is not enough that the designer should merely modify his present methods and ideas to allow for compressibility; he must regard the design problems of high speed flight as completely new ones, and acquire a new scientific background to deal with them. It is important that the designer of high speed aircraft should have a sound knowledge of the fundamental principles of air flow at high speeds. Unfortunately, much of the information which is available on this subject is scattered among a large number of books and reports, and is not easily accessible. Thus there is a great need for a book giving a concise introduction to the subject, to enable the aircraft designer to read and understand the current reports dealing with recent developments, and to provide the scientific background which is so necessary for good design.
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