What is meant by dissymmetry of lift produced by a helicopter rotor?

• A. The difference in lift between the two sides of the rotor disc when the helicopter is in forward flight, with the advancing blade producing less lift.
• B. The difference in lift between the two sides of the rotor disc during forward flight; the retreating blade produces more lift due to its weaker relative wind.
• C. The difference in lift between the two sides of the rotor disc when the helicopter is in forward flight; the advancing blade produces greater lift because forward speed adds to rotor speed.
• D. A condition where both blades experience equal lift in forward flight.

Answer: (C)

Dissymmetry of lift is the unequal lift generated by the rotor blades on opposite sides of the rotor disc during forward flight. As the helicopter moves forward, the blade on the advancing side is moving into the direction of travel, so its tip is sweeping through air that is moving faster relative to the blade. This increases its relative wind and makes that blade produce more lift. The blade on the retreating side is moving against the direction of travel, so its relative wind is slower, producing less lift. This difference tends to roll the helicopter and tilt the rotor disc.

Pilots cancel this tendency by using cyclic pitch to increase the angle of attack on the retreating blade and decrease it on the advancing blade, balancing the lift around the disk. The rotor system’s flapping action also helps reduce the effect. Dissymmetry of lift occurs only in forward flight; in hover, every part of the rotor disc experiences roughly the same relative wind, so lift is symmetric.