Air Intake
Subsonic and supersonic intakes as diffusers, ram pressure recovery, distortion and ingestion hazards.
Air intake, summary notes
- 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.
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.
Air intake concept map
Air Intake
Air intake quiz
Air Intake, quiz
1. The function of a subsonic engine intake is to:
Deliver air to the compressor at the right velocity with minimum lossCompress the air to its final pressureBurn the fuel2. Intake icing is hazardous chiefly because:
Shed ice can damage the compressor and disturb airflowIt increases thrustIt cools the oil3. A subsonic (divergent) intake acts as a:
Diffuser, slowing the air and raising its pressureNozzle, speeding the air upHeat exchanger4. The main job of the intake is to deliver air to the compressor that is:
Subsonic, uniform and with minimum lossAs fast and turbulent as possibleSupersonic at all times5. Ram pressure recovery in the intake:
Increases with forward speedDecreases with forward speedIs unaffected by speed6. Supersonic intakes reduce the flow to subsonic using:
Shock waves and variable geometry (ramps/cones)A simple bellmouthExtra fuel7. Distorted or turbulent intake flow can cause:
Compressor surge/stallMore efficient combustionLower EGT8. Auxiliary/blow-in intake doors open to:
Admit extra air at high power/low speedDump fuelReverse thrust