Air and airflow – subsonic speeds

Introduction – significance of the speed of sound

As was explained in Chapter 1, the remainder of the book will be concerned almost entirely with fluids in motion or, what comes to much the same thing, with motion through fluids. But it would be misleading even to start explaining the subject without a mention of the significance of the speed of sound.

The simple fact is that fluids behave quite differently when they move, or when bodies move through them, at speeds below and at speeds above the speed at which sound travels in that fluid. This virtually means that in order to understand modern flight we have to study two subjects – flight at speeds below that at which sound travels in air, and flight at speeds above that speed – in other words, flight at subsonic and flight at supersonic speeds.

To complicate things still further, the airflow at speeds near the speed of sound, transonic speeds, is complex enough to be yet a third subject in its own right. We shall cover these subjects as fully as we can, but we must not let our natural interest in supersonic flight tempt us to try to run before we can walk, and in the early chapters the emphasis will be on flight at subsonic speeds, though we shall point out from time to time where we may expect to find dif­ferences at supersonic speeds.

But first let us have a closer look at the fluid, air, with which we are most concerned.