Helicopter Aerodynamics Dissymmetry of lift Dissymmetry of lift is the unequal lift produced by the advancing and retreating halves of a helicopter rotor disc during forward flight. The Cause: Airspeed Differential In a hover, all rotor blades move at the same speed relative to the air, creating uniform lift. However, as the helicopter moves forward: Advancing Blade: Moves in the same direction as the helicopter. Its total airspeed is the sum of the blade's rotational speed and the helicopter’s