Refer to figure.
The inlet is where air enters the system. Its function is to convert ram-air pressure (from forward movement) into static pressure, ready for the compressor, for which inlet ducts should be as straight and smooth as possible, producing a minimal boundary layer, and delivering the air to the compressor with an even pressure distribution.

The amount of air a gas turbine engine needs is around 10 times that a reciprocating engine needs. The inlet is designed to conduct incoming air to the compressor with minimum energy loss from drag or ram pressure loss; the airflow into the compressor should be free of turbulence for maximum efficiency. Proper design contributes materially to performance by increasing compressor discharge pressure to duct inlet pressure ratio.

The amount of air passing through the engine depends on:

• The speed of the compressor.
• The forward speed of the aircraft.
• The density of the ambient air.

Bell mouth air intakes are aerodynamically very efficient and used where there is little ram pressure available to force air into the engine, as with stationary or slow flight in a helicopter - they have a convergent profile, and the rounded shoulders offer little air resistance. However, the complete shape produces some drag. Hot oil, electric mats, or engine bleed air may heat them.