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Turbine Engine Operational Considerations (Part Two) Compressor Stalls

by Flight Learnings

in Aircraft Systems

Compressor Stalls

Compressor blades are small airfoils and are subject to the same aerodynamic principles that apply to any airfoil. A compressor blade has an angle of attack which is a result of inlet air velocity and the compressor’s rotational velocity. These two forces combine to form a vector, which defines the airfoil’s actual angle of attack to the approaching inlet air.

A compressor stall is an imbalance between the two vector quantities, inlet velocity and compressor rotational speed. Compressor stalls occur when the compressor blades’ angle of attack exceeds the critical angle of attack. At this point, smooth airflow is interrupted and turbulence is created with pressure fluctuations. Compressor stalls cause air flowing in the compressor to slow down and stagnate, sometimes reversing direction. [Figure 6-28]

compressor stalls

Figure 6-28. Comparison of normal and distorted airflow into the compressor section.

Compressor stalls can be transient and intermittent or steady and severe. Indications of a transient/intermittent stall are usually an intermittent “bang” as backfire and flow reversal take place. If the stall develops and becomes steady, strong vibration and a loud roar may develop from the continuous flow reversal. Often, the flight deck gauges do not show a mild or transient stall, but they do indicate a developed stall. Typical instrument indications include fluctuations in rpm and an increase in exhaust gas temperature. Most transient stalls are not harmful to the engine and often correct themselves after one or two pulsations. The possibility of severe engine damage from a steady state stall is immediate. Recovery must be accomplished by quickly reducing power, decreasing the aircraft’s angle of attack, and increasing airspeed.

Although all gas turbine engines are subject to compressor stalls, most models have systems that inhibit them. One system uses a variable inlet guide vane (VIGV) and variable stator vanes, which direct the incoming air into the rotor blades at an appropriate angle. To prevent air pressure stalls, operate the aircraft within the parameters established by the manufacturer. If a compressor stall does develop, follow the procedures recommended in the AFM/POH.

1 tejas February 10, 2011 at 2:53 am

the compressor stalls at low ac speeds and high rpm…
then why does it not stall during takeoff lineup at 100 % rpm and zero speed??

2 mike May 4, 2011 at 5:30 am

No stall at take off as various airflow control devices, such as variable stator vanes, variable bleed valves or triple spool engine configuration ensures that the correct amount of air is flowing through the engine to prevent stall. If these devices fail or are incorrectly rigged, then stalls can occur.

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