FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsHard
In a fully articulated main rotor system, which hinge allows the blade to move fore and aft within the plane of rotation, compensating for changes in blade tip speed caused by the Coriolis effect during flapping?
- ALead-lag (drag) hinge
- BFlapping hinge
- CPitch-change horn
- DFeathering hinge
Show answer & explanationAnswer & explanation
Correct answer: A. Lead-lag (drag) hinge
As a blade flaps up, its mass moves closer to the rotor axis, and by conservation of angular momentum it speeds up (Coriolis effect); as it flaps down, it moves farther out and slows. The lead-lag (drag) hinge allows the blade to move forward (lead) or aft (lag) in the plane of rotation to accommodate these speed changes without imposing excessive bending stress at the root.
Why the other options are wrong
- B. The flapping hinge allows up-and-down blade movement, which is the cause of, not the compensation for, the Coriolis effect.
- C. The pitch-change horn is a linkage component for changing blade pitch, not a hinge for in-plane movement.
- D. The feathering hinge changes blade pitch angle, unrelated to fore-aft movement in the rotational plane.
Lead-Lag (Drag) Hinge
A hinge on a fully articulated rotor blade root that permits the blade to move fore and aft in the plane of rotation, compensating for Coriolis-effect speed changes caused by flapping.
- Also called the drag hinge
- Compensates for Coriolis effect from flapping
- Prevents excessive bending stress at the blade root
- Often paired with a lead-lag damper to control blade oscillation
Memory trick: Lead-lag = the blade 'sprinting and jogging' in its own lane to match speed changes.