A single main rotor helicopter with a main rotor that turns counterclockwise (as viewed from above) is brought to a stationary hover with no wind. Without pilot correction, in which direction will the helicopter tend to drift, and what design feature is used to counteract it?
- AThe helicopter drifts right due to tail rotor thrust, corrected by tilting the main rotor mast slightly to the left or rigging the flight controls
- BThe helicopter drifts rearward due to retreating blade stall, corrected by increasing rotor RPM
- CThe helicopter drifts left due to tail rotor thrust, corrected by rigging the tail rotor to reduce its thrust
- DThe helicopter drifts forward due to translational lift, corrected by aft cyclic trim
Show answer & explanationAnswer & explanation
Correct answer: A. The helicopter drifts right due to tail rotor thrust, corrected by tilting the main rotor mast slightly to the left or rigging the flight controls
Translating tendency (tail rotor drift) results from the tail rotor's thrust needed to counteract main rotor torque; for a counterclockwise-rotating main rotor, tail rotor thrust acts to the right, causing the helicopter to drift right in a hover. Manufacturers counteract this by rigging the flight controls or tilting the main rotor mast slightly to the left so that some main rotor thrust opposes the tail rotor drift.
Why the other options are wrong
- B. Retreating blade stall is unrelated to hover drift and occurs at high forward airspeed.
- C. Direction of drift and correction method are reversed/incorrect.
- D. This describes translational lift, an unrelated phenomenon occurring in forward flight.
Translating Tendency (Tail Rotor Drift)
The tendency of a single-rotor helicopter to drift laterally in a hover due to tail rotor thrust counteracting main rotor torque.
- For a counterclockwise main rotor, drift is to the right
- Corrected by rigging or slightly tilting the main rotor mast/disc left
- Distinct from translational lift, which involves airspeed, not hover drift
Memory trick: Tail rotor pushes sideways, so the whole ship slides that way.