Compressors and Airflow
Domain 3 - Compressors and Airflow
The exam tests axial and centrifugal compressor operation, stage matching, surge and stall, and the variable geometry used to protect surge margin.
Core rules
- Axial stage pressure ratio is typically 1.1–1.4. Overall pressure ratio is the product of stage ratios, allowing for interstage losses, not the sum.
- Diffusion in rotor and stator converts relative velocity into pressure. Too much diffusion separates the boundary layer and leads to rotating stall and surge. Surge is an axisymmetric breakdown with a loud bang and possible reverse flow; rotating stall is a local stalled cell that travels around the annulus.
- Surge margin is protected by bleed valves opened at low N2, variable inlet guide vanes and stators scheduled by the FADEC, casing treatments and active clearance control.
- Fouling raises compressor discharge temperature and reduces pressure ratio. Water washing restores performance. Tip clearance is a direct efficiency loss; light seal rubbing is normal, heavy rub is not.
- Centrifugal compressors are used on small engines and APUs: one stage gives pressure ratio 3–4 at lower mass flow.
Worked example
Eight identical stages each give a pressure ratio of 1.25. Overall ratio = 1.25^8 = 5.96. If fouling drops each stage to 1.22, the overall ratio becomes 1.22^8 = 4.91, a 17% loss, and the surge margin at the same N2 falls with it. The correct response is a compressor wash, not a fuel trim or a trim adjustment.
Traps
- Adding stage pressure ratios instead of multiplying them.
- Confusing surge, which is a whole-engine event, with rotating stall, which is local.
- Assuming more bleed always improves surge margin; bleed costs thrust and moves the operating line.