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Gliding is a kind of low speed flying, so we shall ignore compressiblility effects where applicable: a note will be made when this happens.
compressibility
Three types of pressure are used in the physics of flying. This section introduces the concepts, which will be used subsequently in later sections.
Pressure is a potential. It measures the ability of some gas to do useful mechanical work. Compressed gas can be used to move a piston and start an engine, for example, because that gas is at high pressure. Thus, a gas at some pressure also has some energy associated with it. Understanding this part will show that some definitions are not invented in random.
=== Definitions ===
==== Incompressible flow ====
A '''flow''' is '''incompressible''' if the fluid density does not change for the problem of interest. If a flow is incompressible, it does not imply that the fluid density cannot be changed in general. A flow that is not incompressible is '''compressible'''. We shall treat the air flow around a glider as incompressible: this is an '''assumption''', but this can be justified by solving the compressible problem and show that the density change is absolutely tiny.
==== Static pressure ====
