FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
A pilot applies an aft cyclic input that produces a downward force on the tip-path plane at the 3 o'clock position of a main rotor that rotates counterclockwise when viewed from above. Due to gyroscopic precession, at what position on the rotor disc will the maximum resulting displacement occur?
- A12 o'clock
- B9 o'clock
- C6 o'clock
- D3 o'clock, where the force was applied
Show answer & explanationAnswer & explanation
Correct answer: A. 12 o'clock
Gyroscopic precession causes the resultant force to be displaced 90° in the direction of rotation from the point where the force was applied. For a counterclockwise rotor, 90° later in rotation from the 3 o'clock position is the 12 o'clock position, so maximum displacement occurs there.
Why the other options are wrong
- B. 9 o'clock is 180° from the applied force, not 90° in the rotation direction.
- C. 6 o'clock is 90° opposite the direction of rotation, not correct.
- D. The force is applied here, but the effect is displaced 90° in rotation direction, not felt here.
Gyroscopic Precession
A property of rotating bodies where an applied force is manifested 90° later in the direction of rotation.
- Effect occurs 90° in the direction of rotation from the applied force
- Explains rotor disc tilt response to cyclic and mast movement
- Critical to understanding two-bladed semi-rigid rotor dynamics
Memory trick: Push now, feel it 90° later — precession plays a delay game.