Noise mitigation system for an unducted propulsive rotor of an aircraft

A noise mitigation system for an unducted propulsive rotor of an aircraft, wherein the system comprises:

• a cyclic pitch mechanism arranged to apply a cyclic pitch schedule to rotor blades of the unducted propulsive rotor, the phase of the cyclic pitch schedule being adjustable and the blade pitch of the cyclic pitch schedule being below the average value thereof over a first azimuthal interval with respect to the rotor axis of the unducted propulsive rotor;
• a positioning and attitude system arranged to determine the position and attitude of the unducted propulsive rotor;
• a memory storing the position of a ground point;
• a processor arranged to carry out the steps of:
• (i) receiving input data corresponding to the position and attitude of the unducted propulsive rotor and the position of the ground point from the positioning and attitude system and the memory respectively;
• (ii) receiving input data corresponding to the velocity of the aircraft;
• (iii) determining from the input data that the ground point will enter the rotor plane of the unducted propulsive rotor after a time period greater than or equal to the reaction time of the system and calculate the radial and azimuthal coordinates of the position in the rotor plane at which the ground point will enter the rotor plane with respect to the rotor axis of the unducted propulsive rotor;
• and
• (iv) outputting a control signal to the cyclic pitch mechanism as necessary to adjust the phase of the cyclic pitch schedule such that the first azimuthal interval includes the azimuthal coordinate determined in step (iii).