Compressors
Compressors tests the difference between axial and centrifugal stages, how pressure ratio builds, and why surge, stall and bleed valves matter. Helicopter engines commonly use a centrifugal or mixed axial-centrifugal compressor, often with two stages.
Core rules
- Pressure ratio multiplies through stages: two stages at 3.5 each give about 12.25 overall before losses.
- Centrifugal stages give a high pressure rise per stage but are limited by tip speed and diffuser losses.
- Axial stages give high mass flow with smaller diameter but need more stages and are more surge-sensitive.
- Variable inlet guide vanes and bleed valves schedule airflow during acceleration and starting.
- Fouling raises compressor discharge temperature, reduces surge margin and can increase fuel flow for the same torque.
- Surge is a global flow reversal or unstable oscillation; rotating stall is a local stalled cell. Both can cause bang, TGT rise and power loss.
Worked example
A two-stage compressor has stage pressure ratios of 3.5 and 3.2. Overall pressure ratio = 3.5 × 3.2 = 11.2, before interstage and intake losses. If fouling reduces each stage to 3.3 and 3.0, the overall ratio becomes 9.9, about 12% lower. The engine control may schedule more fuel to recover power, which shows up as higher TGT and fuel flow at the same torque.
Traps
- Adding stage pressure ratios instead of multiplying them.
- Confusing surge with a simple power loss.
- Assuming bleed valves stay open at cruise; they are scheduled.
- Ignoring inlet guide vane rigging after maintenance.
- Washing a compressor without the approved procedure.