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?