FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
During forward flight, a rotor blade produces lift by accelerating air downward through the rotor disk. This downward-moving air, called induced flow, combines with the blade's rotational relative wind to form the resultant relative wind. What is the primary effect of induced flow on the blade's angle of attack?
- AIt decreases the angle of attack below the pitch angle
- BIt increases the angle of attack above the pitch angle
- CIt has no effect on the angle of attack
- DIt reverses the direction of the angle of attack
Show answer & explanationAnswer & explanation
Correct answer: A. It decreases the angle of attack below the pitch angle
Induced flow (downwash) tilts the resultant relative wind downward, which reduces the angle between the chord line and the relative wind, meaning the actual angle of attack is less than the blade's pitch angle.
Why the other options are wrong
- B. Induced flow reduces, not increases, the angle of attack.
- C. Induced flow has a direct and significant aerodynamic effect on angle of attack.
- D. Induced flow does not reverse the angle of attack sign, only reduces its magnitude.
Induced Flow (Downwash)
The downward movement of air through the rotor disk produced as a byproduct of lift generation, which reduces the blade's actual angle of attack.
- Combines with rotational relative wind to form resultant relative wind
- Reduces angle of attack below pitch angle
- Greater near hover, decreases with forward airspeed due to increased horizontal airflow
Memory trick: Downwash drags the wind down, dragging AOA down with it