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For subsonic aircraft like commercial jets, the inlet has the job of making sure the correct airflow is coming into the engine. Correct airflow is important for keeping the engine's compressor stage working. Too little or too much airflow will cause the compressors to stall and therefore no combustion happens. The inlet must keep airflow constant throughout the different stages of flight, including take-off, landing and banking.

The way an inlet ensures correct airflow into the engine is by slowing down the airflow and increasing the pressure. The inlet has a large volume which will decrease the speed and therefore will decrease the pressure. An inlet will work differently during low speed flight and high speed flight:

  • Low speed flight – During low speed flight including take-off and landing, the aircraft isn’t going fast enough for sufficient airflow to enter the engine. All engines have an operating design where they need certain airflow to work. To get enough airflow into the engine at low speeds, the engine's fan will suck in air. Air will be sucked in from a large area in front of the inlet. The streamlines of air will converge together as they get closer to the inlet. However, there is a location on the inlet where air does not move. At low speed flight, this stagnation point is on the outside of the inlet. This forces oncoming air to enter the inlet and not go around the engines nacelle. This provides the engine with enough air velocity at low speeds to operate efficiently.

  • High speed flight – During high speed flight the aircraft is going fast enough for sufficient airflow to enter the engine. However, allowing too much airflow will cause the compressors to stall. The inlet isn’t relying on the fan to suck in air from all directions. Air directly enters the inlet because the aircraft's velocity is fast enough. So to stop too much airflow entering the engines, the stagnation point moves further inside the inlet. This means that the desired airflow enters the engine, and the unwanted airflow flows around the nacelle.

Is this understanding of subsonic inlets correct?

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Your understanding is correct.

However, you attribute too much activity to what is essentially a passive device. The inlet only guides the air such that it flows uniformly, without separation and at the desired angle towards the fan and the compressor. The pressure field resulting from this flow is doing the acceleration rsp. deceleration, and the pressure itself is a result of the compressor swallowing just as much air as it needs.

Converging streamlines indicate a pressure drop and acceleration, and diverging streamlines a pressure increase and deceleration. In the static condition, the engine creates suction inside the intake which causes air from all around to flow towards the intake. At high speed it cannot swallow more than a fraction of the air flowing towards it and partially blocks the flow through the nacelle. As a consequence, air gets pushed to the sides and the flow into the intake is decelerated.

It is really the engine, not the intake, that controls how much air is ingested. The intake only makes sure that this happens in an orderly fashion without separation and vorticity. Different flight speeds only mean that the pressure ahead of the compressor changes, while the speed of the air entering it is quite constant over flight speed.

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