What is the angle-of-attack of a rotating propeller measured between?

• A. Between the blade chord or face and the relative airstream
• B. Between the blade leading edge and the relative wind
• C. Between the hub centerline and the air
• D. Between the blade trailing edge and the flow

Answer: (A)

Angle-of-attack for a rotating propeller is the angle between the blade’s chord line (the line from the blade’s leading edge to trailing edge, essentially the blade’s face) and the relative airstream that hits the blade. The relative wind is the resultant of the airplane’s forward speed and the blade’s tangential speed due to rotation, so the local flow direction at a blade section forms the AoA with that chord line. This angle governs how much lift and thrust the blade section can produce; as flight speed or RPM changes, the relative wind direction changes, altering the AoA along the blade. The other options don’t define AoA because it is not measured to the hub line, the leading edge, or the trailing edge with the flow.