The Nature of Liquids and Gases

Abstract

Before turning to flight itself, we need to discuss a number of fundamental ideas in order to lay down a scientific basis for aerodynamics. In this chapter we will learn about some of the physical properties of fluids, a term that includes both gases and liquids. This broad expression is used, as we will see later, because the motion of objects in air and in water obeys identical laws until their speed approaches the speed of sound. Occasionally this discussion—just like others to follow—may appear to be unrelated to the behavior of an airplane. Yet concepts such as pressure and phenomena such as the static behavior of water are relevant to the instrumentation used in wind tunnels and on airplanes.

It is true that nature begins by reasoning and ends by experience; but, nevertheless, we must take the opposite route. We must begin with experiment and try through it to discover the reason.

Leonardo da Vinci (1452–1519)