Spring system varying stiffness with applied force for use in a torque dependent rotor of a rotary wing aircraft

A cyclic pitch control system for controlling a blade pitch angle of a thrust generating rotor assembly, the cyclic pitch control system comprising:

• the rotor assembly, wherein the rotor assembly comprises a rotor torque assembly comprising a rotor shaft rotationally connected to a rotor blade assembly comprising rotor blades configured to tilt about a rotor blade axis;
• a mechanism configured to transfer changes in a rotational offset between a rotational position of the rotor torque assembly and the rotor blade assembly into changes in rotor blade pitch;
• at least one spring member connected to both the rotor torque assembly and the rotor blade assembly and configured to transfer torque between the rotor torque assembly and the rotor blade assembly;
• and
• a spring system comprising a spring element configured to interact with the at least one spring member to non-linearly vary a stiffness of the at least one spring member in response to a torque applied to the at least one spring member;
• wherein the at least one spring member and/or moments of inertia associated with the rotor blade assembly and the rotor torque assembly are configured to provide a rotational resonance frequency range of the rotor torque assembly at least partially within a range of rotational frequencies of operation for the rotor assembly, the resonance frequency range being a range of frequencies for which, for a given cyclic change in a torque driving the rotor torque assembly, a corresponding cyclic change in rotational speed of the rotor torque assembly increases.