The compressor blade angle is determined primarily by which combination of factors?

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

Answer: (A)

The blade angle of a compressor stage is set by how much energy needs to be transferred to the air and how the air is entering the compressor. This depends on the speed of the air flow into the engine (true airspeed), the rotational speed of the compressor (RPM), and the pressure rise the compressor must achieve (compressor outlet pressure).

If you fly faster (higher TAS), the incoming air has more momentum, so the relative flow direction changes and the blades must be angled to keep the flow attached and transfer energy efficiently. If the compressor spins faster (higher RPM), the air is impelled more vigorously, changing the energy imparted per pass; the blade angle must adjust to achieve the desired pressure rise at that speed. The required outlet pressure tells you how much energy per unit mass must be added; the blade geometry is chosen to deliver that energy transfer under the current RPM and inlet conditions.

Ambient temperature, engine oil temperature, and fuel flow influence density, lubrication, or overall engine behavior, but they do not primarily determine the blade angle in the same direct way as TAS, RPM, and the required outlet pressure.