Section 15.3

Air Intake

Subsonic and supersonic intakes as diffusers, ram pressure recovery, distortion and ingestion hazards.

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Main ideas

The intake (inlet) delivers air to the compressor with minimum loss and as uniform a flow as possible, at the right velocity — the compressor needs subsonic, steady flow whatever the aircraft is doing.
A subsonic (pitot/divergent) intake acts as a diffuser: it slows the incoming air and raises its static pressure before the compressor — free 'ram' compression that grows with forward speed.
Supersonic intakes use converging-diverging geometry, shock waves, variable ramps/cones and spill/bleed doors to decelerate the flow to subsonic at the compressor face; the shock system does the initial pressure recovery.
The intake must avoid distortion, turbulence and separation, which the compressor sees as unsteady flow and which can trigger surge; icing and foreign-object/bird ingestion are key hazards addressed by anti-ice and inlet design.
At high power and low speed (take-off) the intake works hardest to feed the engine; auxiliary/blow-in doors on some designs open to admit extra air.
⚠ Exam trap: a divergent (subsonic) intake SLOWS the air and RAISES its pressure (it is a diffuser), the opposite of a nozzle; ram pressure recovery in the intake increases with forward speed.

Solved examples

At high forward speed, what happens to the pressure of the air between the intake lip and the compressor face of a subsonic engine, and why is it useful?
The divergent intake acts as a diffuser: the fast incoming air slows and its static pressure rises (ram recovery). This raises the pressure reaching the compressor 'for free', increasing mass flow and thrust as speed increases — one reason thrust recovers with airspeed.

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