Dissymmetry of lift

The amount of lift generated by an airfoil is proportional to the square of its airspeed (velocity).

As the blades rotate, the speed of the blade-tips relative to the air remains constant.

Blade flapping is the primary means of countering dissymmetry of lift.

Conversely, the retreating blade flaps down, develops a higher angle of attack due to a change in relative wind vectors, and generates more lift.

Dissymmetry of lift is also countered by cyclic feathering, i.e. a change to the angle of incidence of the rotor blades as they rotate around the hub.

There is a limit to which angle of attack changes can counter dissymmetry of lift.

At low airspeeds, blade flapping compensates for dissymmetry of lift.

As airspeed increases, typically above 70 knots, these systems allow a more level fuselage attitude which reduces stresses on the rotor driving mechanisms.