A vibration reduction system including plural force generators and a controller, wherein each of the plural force generators comprises: a hub, which is rotatable about an axis thereof; an elongate member coupled to the hub such that the elongate member is rotatable with the hub and extends radially outwardly away from the hub and the axis along a radial dimension defined with respect to the axis; and a mass, which is movably disposed along the elongate member and is adjustable to multiple radial mass positions relative to the hub; and an actuator to drive a movement of the mass, wherein the controller is coupled to each of the force generators and is thus configured: to control the actuator to control a movement of the mass along the elongate member to achieve a desired amplitude of force generation for the respective force generator, and to drive a rotation of the hub about the axis for each force generator at at least one frequency that is in line with at least one frequency currently affecting the airframe to mitigate or reduce at least one of vibrations generated by rotor (151, 160) rotation and/or by aircraft damage; characterised in that: said plural force generators are disposed about an airframe of an aircraft; and each hub is fixed to the airframe at a given location. wherein each of the plural force generators comprises: a hub, which is rotatable about an axis thereof; an elongate member coupled to the hub such that the elongate member is rotatable with the hub and extends radially outwardly away from the hub and the axis along a radial dimension defined with respect to the axis; and a mass, which is movably disposed along the elongate member and is adjustable to multiple radial mass positions relative to the hub; and an actuator to drive a movement of the mass, a hub, which is rotatable about an axis thereof; an elongate member coupled to the hub such that the elongate member is rotatable with the hub and extends radially outwardly away from the hub and the axis along a radial dimension defined with respect to the axis; and a mass, which is movably disposed along the elongate member and is adjustable to multiple radial mass positions relative to the hub; and an actuator to drive a movement of the mass, wherein the controller is coupled to each of the force generators and is thus configured: to control the actuator to control a movement of the mass along the elongate member to achieve a desired amplitude of force generation for the respective force generator, and to drive a rotation of the hub about the axis for each force generator at at least one frequency that is in line with at least one frequency currently affecting the airframe to mitigate or reduce at least one of vibrations generated by rotor (151, 160) rotation and/or by aircraft damage; characterised in that: said plural force generators are disposed about an airframe of an aircraft; and each hub is fixed to the airframe at a given location. to control the actuator to control a movement of the mass along the elongate member to achieve a desired amplitude of force generation for the respective force generator, and to drive a rotation of the hub about the axis for each force generator at at least one frequency that is in line with at least one frequency currently affecting the airframe to mitigate or reduce at least one of vibrations generated by rotor (151, 160) rotation and/or by aircraft damage; characterised in that: said plural force generators are disposed about an airframe of an aircraft; and each hub is fixed to the airframe at a given location. said plural force generators are disposed about an airframe of an aircraft; and each hub is fixed to the airframe at a given location.