The subsonic air intake is located in front of a civil aircraft jet engine. In other words, it is an inlet through which air flows into the engine compressor. In some supersonic aircraft, it is a very complex device, which needs a lot of efforts to control the airflow to the engine. The air intake shape, or form, depends on the engine arrangement and aircraft design. In order to function properly and efficiently, the intake must be designed and manufactured to supply the appropriate amount of airflow required by the aircraft engine, so that this flow be uniform when it enters the compressor.
Aircraft engine subsonic air intake
Having an almost circular cross-section, the subsonic air intake makes up the forward part of the engine nacelle in civil and military transport aircraft. An arrangement with the intake in the fuselage nose is no longer used, since it requires a long duct extending for the length of the fuselage, entailing great fluid dynamic losses. It also must be said that a nacelle intake causes 3% of total aircraft drag, contributing to aircraft weight, with both factors affecting payload and range. Thus, due to economic requirements, the engine intake must be a low drag, light-weight construction.
Today’s high subsonic cruise flight Mach numbers (Mach 0.78 to 0.85) force engineers to design engines featuring a relatively thin intake, where the external dimension of the intake is not much greater than the internal diameter. This will result in a small nose radius, with a relatively thin-lipped air intake. The external flow that surrounds the intake streamtube will effectively be prevented from developing undesirable excess velocities at the lip, thus minimizing the risk of flow separation with its corresponding increase in drag.
Crosswind is one source of impairing intake performance. On the windward side of the air intake, the crosswind components add to the flow at the lip, which results in a further increase of excess velocity there. Another source of alteration of the intake performance are the ground vortexes which can develop and be swallowed by the intake, acting to impair intake performance.
Below, diagram of air intake flowfield: a) aircraft at rest; b) static plus crosswind; c) low-speed flight.
