The angle of attack of compressor blades in a gas turbine engine depends primarily on which of the following?

• A. TAS, RPM and Compressor Outlet Pressure
• B. Ambient Temperature
• C. Fuel Flow
• D. Compressor Vibration

Answer: (A)

The angle of attack for compressor blades is set by the velocity triangle the blade sees: the relative flow direction is the combination of the axial flow into the compressor and the blade’s own rotation. The axial component of that flow comes from the aircraft’s true airspeed, while the tangential component comes from how fast the blade is spinning (RPM). The pressure rise across the stage, represented by the compressor outlet pressure, reshapes how the flow accelerates or slows through the stage, which in turn alters the axial component of the relative flow and thus the incidence angle on the blade.

So, increasing TAS increases the axial flow into the compressor, changing the relative flow direction; increasing RPM raises the blade’s tangential speed, also altering the relative flow; and the compressor outlet pressure (the stage pressure rise) changes how the flow deflects through the stage, all of which primarily determine the angle of attack the blade experiences. Ambient temperature, fuel flow, or compressor vibration don’t directly set this flow geometry.