Final answer:
The dissymmetry of lift in helicopters refers to the unequal distribution of lift between the advancing and retreating rotor blades. Helicopters with two sets of lifting blades rotate them in opposite directions to balance the lift distribution and prevent rolling.
Step-by-step explanation:
The dissymmetry of lift in helicopters refers to the unequal distribution of lift between the advancing and retreating rotor blades. This phenomenon occurs because the advancing blade on one side of the rotor disc has a higher relative velocity and generates more lift than the retreating blade on the opposite side. As a result, the helicopter tends to roll in the direction of the retreating blade. To counteract this, helicopters with two sets of lifting blades rotate them in opposite directions. This helps to balance the lift distribution and prevent the helicopter from rolling uncontrollably.