Dissymmetry of lift occurs because the advancing blade has more lift than the retreating blade.

• A. The Retreating Blade Has More Lift
• B. They Have Equal Lift
• C. Advancing Blade Has More Lift
• D. It Occurs Only in Hover

Answer: (C)

Dissymmetry of lift arises when a rotor is in forward flight because the advancing blade and the retreating blade through the disk experience different relative wind speeds. The advancing blade moves into the direction of travel, so its relative wind is stronger, which increases its effective angle of attack and generates more lift. The retreating blade, moving opposite to the direction of travel, sees a weaker relative wind and thus produces less lift. This uneven lift across the rotor disk would cause a roll if uncorrected, so the rotor system uses blade flapping and cyclic pitch to balance it—usually by increasing lift on the retreating side and reducing lift on the advancing side to keep the helicopter stable. In hover, forward speed is zero, so both blades see the same relative wind and the lift is effectively equal. So the advancing blade having more lift is the correct description of dissymmetry of lift.