A rotary aircraft has a fuselage with wings and a rotor. The blades of the rotor are twistable about a pitch axis to vary collective pitch. A collective pitch shaft moves in an advancing direction to increase the collective pitch. Weights are mounted to the blades for outward movement along the blades in response to an increase in rotational speed of the blades. A linkage between each of the weights and the collective pitch shaft moves the collective pitch shaft in the advancing direction in response to an increase in rotational speed. A spring acting through a cam mechanism exerts a non linear force in opposition to the outward movement of the blades.