A mechanic explains to a student pilot that the main rotor blade's pitch angle (measured between the chord line and a reference plane) is not the same as its angle of attack (measured between the chord line and relative wind). During a steady hover in still air, why would these two angles typically be nearly equal?
- ABecause dissymmetry of lift causes the angles to average out to be equal across the disk
- BBecause blade twist eliminates any difference between pitch angle and angle of attack at all rotor stations
- CBecause the induced flow through the rotor disk is zero during a hover
- DBecause in a hover, the relative wind is essentially straight up through the rotor disk, making the reference plane and relative wind nearly parallel
Show answer & explanationAnswer & explanation
Correct answer: D. Because in a hover, the relative wind is essentially straight up through the rotor disk, making the reference plane and relative wind nearly parallel
Pitch angle is a mechanical/geometric angle referenced to the rotor's plane of rotation, while angle of attack is aerodynamic, referenced to the actual relative wind, which includes downward induced flow. In a hover, induced flow (downwash) is relatively small compared to blade rotational velocity and is roughly perpendicular, so pitch angle and angle of attack are close but not identical—AOA is always slightly less than pitch angle due to induced flow bending the relative wind downward.
Why the other options are wrong
- A. Dissymmetry of lift only occurs in forward flight, not during a hover, and does not average pitch angle and AOA.
- B. Blade twist changes pitch angle progressively along the blade span but does not eliminate the general difference between pitch angle and angle of attack.
- C. Induced flow (downwash) is never zero in a hover; it's actually most significant in a hover relative to blade speed, which is why AOA is somewhat less than pitch angle.
Pitch Angle vs. Angle of Attack
Pitch angle is the geometric angle between the blade chord line and the rotor's plane of rotation, while angle of attack is the aerodynamic angle between the chord line and the actual relative wind, which is affected by induced flow (downwash).
- Pitch angle is controlled directly by the pilot via collective/cyclic
- Angle of attack also depends on induced flow, so it can differ from pitch angle
- In forward flight, dissymmetry of lift and blade flapping further separate the two angles
Memory trick: Pitch angle is what you set; angle of attack is what the wind actually sees.