15.3 and 15.4 Inlets and compressors — stall, surge and airflow control

Scope

• 15.3 Inlet — A 2, B1 2: inlet ducts; effects of various inlet configurations; ice protection.
• 15.4 Compressors — A 1, B1 2: axial and centrifugal types; construction, operation and applications; fan balancing; causes and effects of stall and surge; airflow control (bleed valves, VIGVs, VSVs, rotating stator blades); compressor ratio.

Inlet functions

The inlet must deliver air to the compressor face with minimum pressure loss, uniform (undistorted) flow and at a speed the compressor can accept (around Mach 0.4–0.5 at the face), at all flight conditions including crosswinds and high angle of attack. Inlet distortion is a primary cause of compressor stall.

Inlet types

Inlet	Shape	Use
Subsonic (pitot) inlet	Divergent duct — slows the air, raising static pressure (ram recovery)	Airliner and business-jet nacelles
Supersonic inlet	Convergent–divergent, often with variable ramps/spikes producing shock waves that slow the air to subsonic before the compressor	Supersonic aircraft
Bellmouth	Rounded convergent entry, maximum efficiency at zero forward speed	Helicopters, engine test cells
Divided / plenum	Split entrance or plenum chamber feeding the compressor	Some military and turboprop installations
Turboprop / turboshaft inlets	Often with an inertial particle separator or inlet screens/filters	Protects against sand, ice and FOD

Fan (A) flange — the inlet cowl bolts to the engine's front flange on turbofans. Acoustic lining in the inlet reduces fan noise.