ABSTRACT
All air breathing engines installed in an aircraft must be provided with an air intake and a ducting system (which is also identified as inlet or diffuser) to diffuse the air from freestream 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, minimum total pressure loss, deliver the air with tolerable flow distortion (as uniform as possible), and contribute the least possible external drag to the system. For 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.
