A helicopter is transitioning from a hover into forward flight. As airspeed increases, the advancing main rotor blade generates more lift than the retreating blade at the same blade angle. What causes this dissymmetry of lift?
- AGround effect is stronger on the advancing side of the rotor disc
- BThe Coriolis effect slows the advancing blade and speeds up the retreating blade
- CThe advancing blade has a higher relative wind velocity (rotational speed plus aircraft speed) than the retreating blade
- DBlade twist automatically increases pitch on the advancing side during forward flight
Show answer & explanationAnswer & explanation
Correct answer: C. The advancing blade has a higher relative wind velocity (rotational speed plus aircraft speed) than the retreating blade
In forward flight, the advancing blade's speed is the sum of rotational velocity plus aircraft forward speed, while the retreating blade's speed is rotational velocity minus aircraft speed. This produces more lift on the advancing side, which flapping compensates for through blade flap-up/flap-down (Coriolis is unrelated to this, and blade twist is a fixed design feature, not a dynamic response).
Why the other options are wrong
- A. Ground effect is unrelated to which side of the disc is advancing or retreating.
- B. Coriolis effect relates to blade flapping and lead-lag, not the root cause of dissymmetry of lift.
- D. Blade twist is a fixed manufacturing feature, not a dynamic in-flight compensation for dissymmetry.
Dissymmetry of Lift
The unequal lift generated between the advancing and retreating halves of the rotor disc during forward flight, caused by differing relative wind speeds.
- Advancing blade speed = rotational speed + aircraft speed
- Retreating blade speed = rotational speed − aircraft speed
- Compensated primarily by blade flapping (flap-up on advancing side)
Memory trick: Advancing blade adds speed, retreating blade subtracts — flapping fixes the fight.