The Function of the air intake of a gas turbine engine is to deliver whatever air mass flow is required, with the best recovery of ram stagnation pressure, over the desired range of flight speeds and altitudes.

Although it is generally shown in other forms, the performance of an air intake for supersonic flight can be represented on charts very similar to those of a rotating compressor. In Fig. 1 the ratio between ambient (static) pressure and stagnation pressure at the diffuser of a typical intake is shown as a function of corrected inlet and outlet air mass flow (which are themselves functions of the local flow Mach numbers), for a series of flight Mach numbers.