AIR INLET DUCT
An engine's air inlet duct is normally considered an airframe part and made by aircraft manufacturer . During flight operation , it is very important to the engine performance . Engine thrust can be high only if the inlet duct supplies the engine with the required airflow at the highest posible pressure . The inlet duct has two engine functions and one aircraft function .
First : it must be able recover as much of the total pressure of the free air stream as posible and deliver this pressure to the front of the engine compressor .
Second : the duct must deliver air to the compressor under all flight conditions with a little turbulance .
Third : the aircraft is concerned , the duct must hold to a minimum of the drag.
The duct also usually has a diffusion section just ahead of the compressor to change the ram air velocity into higher static pressure at the face of the engine . This is called ram recovery . The inlet duct is built generally in the divergent shape (subsonic diffuser).