FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
As a main rotor blade flaps upward during flight, its center of mass moves closer to the axis of rotation, causing the blade to momentarily speed up in its plane of rotation. This tendency, and the hinge used to accommodate it, are best described as which of the following?
- AGyroscopic precession, accommodated by the feathering hinge
- BTranslational lift, accommodated by the flapping hinge
- CRetreating blade stall, accommodated by blade twist
- DCoriolis effect, accommodated by the lead-lag (drag) hinge
Show answer & explanationAnswer & explanation
Correct answer: D. Coriolis effect, accommodated by the lead-lag (drag) hinge
The Coriolis effect describes the tendency of a rotor blade to accelerate or decelerate in the plane of rotation as its center of mass moves closer to or farther from the axis of rotation during flapping (conservation of angular momentum). Fully articulated rotor systems use a lead-lag (drag) hinge to allow the blade to move fore and aft in response to this effect.
Why the other options are wrong
- A. Gyroscopic precession involves force displacement 90° in rotation direction, unrelated to this description.
- B. Translational lift concerns airspeed effects on rotor efficiency, not blade flapping dynamics.
- C. Retreating blade stall is an aerodynamic stall condition, unrelated to mass distribution changes.
Coriolis Effect
The tendency of a rotor blade to speed up or slow down in its plane of rotation as its center of mass moves closer to or farther from the axis of rotation during flapping, per conservation of angular momentum.
- Blade flaps up → mass moves inward → blade speeds up (leads)
- Blade flaps down → mass moves outward → blade slows down (lags)
- Accommodated in fully articulated rotor systems by the lead-lag (drag) hinge
Memory trick: Like a spinning skater pulling in arms — blade speeds up as mass moves inward.