When we consider the physical properties of the air, the most noticeable features are its low density and its compressibility, and a student new to the subject might be excused for doubting whether the air were not too light and yielding to support an aeroplane in flight. The air is indeed very compressible, but this property of the air becomes important only when the speed of motion becomes a fairly large fraction of the speed of sound, and in practice its influence is felt only in the behaviour of airscrews with high tip speeds.

A more important property of the air is its viscosity or internal friction, and this property is of fundamental importance in flight, either by its direct frictional action or by its indirect action in determining the type of flow round a body. As an example of this indirect action we may consider the lift of a wing.