ABSTRACT
All airbreathing engines installed in an aircraft must be provided with an air intake and a ducting system (which is also known as an inlet or diffuser) to diffuse the air from free-stream velocity to a lower velocity acceptable for further processing by other engine components [1]. The inlet component is designed to capture the exact amount of air required to accomplish the diffusion with maximum static pressure rise and minimum total pressure loss, to deliver the air with tolerable flow distortion (as uniform as possible), and to contribute the least possible external drag to the system. For a gas turbine engine (turbojet, turbofan, and turboprop), the airflow entering subsonic compressors or fans must be of low Mach number, of the order 0.4–0.5 or less, even if the aircraft speed is supersonic. For a ramjet also, the inlet reduces the speed to a subsonic value to have a subsonic combustion. Thus the entrance duct usually acts as a diffuser.
