NIFE Engines 3-3-1 Compressor Stalls

Question 1

Describe the angle of attack of compressor blades

Answer 1

The angle between the relative wind and the rotor blade’s chord line

Question 2

Describe a compressor stall

Answer 2

Occurs when airflow over an airfoil breaks away causing the airflow to lose lift due to the excessive AOA.

Mainly the result of airflow distortion or mechanical malfunctions.

Question 3

State four airflow distortions that can cause a stall

Answer 3

1. Abrupt change in aircraft attitude
2. Encountering air turbulence
3. Deficiency of air velocity or volume caused by atmospheric conditions
4. Rapid throttle movement

Question 4

State the four mechanical malfunctions that can lead to a compressor stall

Answer 4

1. Variable inlet guide vane and stator vane
2. Fuel control unit (FCU)
3. Foreign object damage
4. Variable exhaust nozzle

Question 5

Describe how variable inlet guide vane and stator vane can lead to a compressor stall

Answer 5

Failure to change the AOA will cause too much or too little airflow at low engine speeds

Question 6

Describe how fuel control unit can lead to a compressor stall

Answer 6

Too much or too little fuel is added to the burner. If over-rich, causes excessive burner pressure and a back-flow of air into the compressor, leading to a stall. If too lean, engine may flame-out

Question 7

Describe how foreign object damage can lead to a compressor stall

Answer 7

When an object damages the blades of the compressor. Deformation will change the aerodynamic properties, potentially causing a stall

Question 8

Describe how variable exhaust nozzles can lead to a compressor stall

Answer 8

Typically needed when an afterburner is used. It is closed at subsonic speeds and opened to allow exhaust gasses to expand properly when using the afterburner. If it fails to open, excessive back pressure could lead to a compressor stall

Question 9

Describe appropriate pilot actions during a compressor stall

Answer 9

1. Reduce attitude of the aircraft to reduce inlet AOA
2. Reduce PCL to just below stall threshold to allow engine to catch up with inlet airflow
3. Wait for engine indications to return to normal

Question 10

State four engine design features that can be incorporated into a gas turbine engine to minimize the likelihood of a compressor stall

Answer 10

1. Variable inlet guide vanes
2. Dual/twin/split-spool compressor
3. Bleed valves
4. Variable exhaust nozzle

Question 11

Describe how variable inlet guide vanes minimize the likelihood of a compressor stall

Answer 11

Changes the AOA at low engine speeds. Maintains the velocity of the air within limits for low airflow conditions. also permits high airflow with a minimum of restriction.

Question 12

Describe how dual/split/twin-spool compressors minimize the likelihood of a compressor stall

Answer 12

Allows the front rotor to turn at a slower RPM than the rear rotor. Allows the front rotor to turn without being choked by the low airflow

Question 13

Describe how bleed valves minimize the likelihood of a compressor stall

Answer 13

Installed near the middle or rear of the compressor to “bleed” air and increase airflow in the front of the compressor at low engine RPMs

Question 14

Describe how variable exhaust nozzles minimize the likelihood of a compressor stall

Answer 14

Used to unload the pressure during afterburner operation