Resonant mode damping system of a rotor drive train of an engine and method

A control system for increasing the damping of a resonant mode of a rotor drive train is provided. A control loop (102) is introduced in parallel with the free turbine speed governing control loop (110). The control loop (102) receives a feedback signal from the free turbine and rotor drive train, the feedback signal representative of the free turbine speed. The control loop (102) extracts the resonant component from the feedback signal to generate a resonant signal containing the resonant component and estimates a derivative of the free turbine speed from the resonant signal. The control loop (102) then outputs a request for damping the resonant mode on the basis of the derivative. A narrow-band reject filter (128) is introduced in the free turbine speed governing loop (110) to filter out the resonant component from the feedback signal, thereby ensuring that the free turbine speed governing loop (110) does not interfere with the control loop (102).