Compressor Stall & Surge

Every compressor blade is an aerofoil, and like any aerofoil it stalls if its angle of attack becomes too great. The angle of attack is set by the vector sum of the axial velocity of the air through the compressor and the rotational speed of the blade. Anything that reduces axial velocity while blade speed stays high, most obviously a rise in back pressure from the combustion section, increases the angle of attack toward the stalling value.

A compressor map plots pressure ratio against corrected mass flow. The working line is where the engine normally runs, and the surge line above it marks where the blades stall. The vertical gap between them is the surge margin. A rapid slam acceleration adds fuel faster than the compressor can accelerate the airflow, so the back pressure jumps and the operating point moves up toward the surge line.

Beyond that line the flow breaks down. Rotating stall is a localised cell of stalled blades travelling around the annulus, while a full surge is a complete momentary reversal of flow through the engine, heard as a loud bang and seen as an EGT rise, fluctuating N2 and thrust loss. The defences are variable stator vanes, which re-angle the air onto the rotor blades at low speed, bleed valves, which dump air from the middle of the compressor to restore axial velocity, multi-spool construction, and the FADEC acceleration schedule that limits how fast fuel may be added.

• Try this: Press Slam accelerate with the protections working. Watch the operating point move toward the surge line and then recover, and note what the VSVs and bleed valve do.
• Try this: Now Lock VSVs open and Close bleed valve, then slam accelerate again. Why does the same throttle movement now cause a surge?
• Try this: Hold a low N2 and raise the fuel flow slowly. Why is a compressor most vulnerable to surge at low speed?