A turbofan engine's high-pressure compressor incorporates variable stator vanes (VSVs) in its forward stages. The primary purpose of these vanes is to
- Areduce the rotational speed sensor error between N1 and N2
- Bprovide bleed air directly to the aircraft cabin pressurization system
- Cmaintain the correct airflow angle into the following rotor stage across the engine's speed range, preventing compressor stall
- Dincrease exhaust gas temperature during takeoff to maximize thrust output
Show answer & explanationAnswer & explanation
Correct answer: C. maintain the correct airflow angle into the following rotor stage across the engine's speed range, preventing compressor stall
Variable stator vanes automatically change their angle with engine speed to keep the airflow striking the following rotor blades at the correct angle of attack, which prevents aerodynamic stall of the compressor blades at low rpm where airflow velocity and compressor geometry are mismatched. This is one of several methods (along with bleed valves and multiple spools) used to prevent compressor surge/stall.
Why the other options are wrong
- A. VSVs have no function related to speed sensor calibration between spools.
- B. Bleed air for cabin pressurization is tapped from compressor stages, not provided by stator vane movement.
- D. VSVs are an anti-stall device, not a means of controlling EGT for thrust.
Variable Stator Vanes (VSV)
Adjustable compressor stator vanes that change angle with engine speed to maintain proper airflow angle into rotor blades, preventing compressor stall.
- Controlled by fuel control/FADEC based on N2 speed and temperature
- Work with bleed valves and multi-spool design to prevent stall
- Most critical at low engine speeds/high angles of attack
Memory trick: 'VSV Vanes Steer the Vortex' — angling airflow to keep it smooth.