Ideal intake conditions:
For air to flow smoothly through a compressor, its velocity should be about 0.5 mach at the compressor inlet; this includes aircraft flying faster than the speed of sound. Hence intakes are designed to decelerate the free stream airflow to this condition over the range of aircraft speeds. Intakes should also convert the kinetic energy into pressure energy without undue shock or energy loss. This means that the ideal compressor inlet pressure should be the same as the total head pressure at the intake lip.
The magnitude of the losses occurring in an intake during ram compression are measured by means of the intake efficiency. Typical optimum efficiencies of some common types of intake, at subsonic speeds assuming straight-through flow, are:
a Turbo-jet engine Pitot 99 to 96% Wing root 95 to 87% Side 89 to 80%
b Turbo-prop engine Annular 82 to 74% (DART)
In cases where the direction of flow of the air is reversed within the intake, these values are reduced by about 10%.