Method for aiming to optimise the noise emitted by an auxiliary rotor of a rotorcraft, said rotorcraft extending longitudinally along a first anteroposterior plane separating a first side from a second side of the rotorcraft, said rotorcraft being provided with at least one main rotor, said rotorcraft being provided with an auxiliary rotor exerting a lateral thrust controllable in order to control the yaw movement of the rotorcraft, said lateral thrust being directed towards said second side in order to counter a torque generated by said main rotor on a fuselage of the rotorcraft, said rotorcraft comprising a power plant for setting in rotation the main rotor and the auxiliary rotor, said rotorcraft comprising a tail fin provided at least partly with a deflectable aerodynamic surface controllable in orientation and generating a transverse lift (111, 112) on extending in elevation, said aerodynamic surface having a zero deflection angle when said aerodynamic surface is present in a second plane, said aerodynamic surface having a trailing edge, characterised in that the deflection angle of said aerodynamic surface is controlled to direct its trailing edge towards said second side so that the aerodynamic surface has a negative deflection angle relative to the second plane or to direct its trailing edge towards said first side so that the aerodynamic surface has a positive deflection angle relative to the second plane, the aerodynamic surface having the function of causing the auxiliary rotor to tend towards at least one predetermined operating point seeking to optimise the performance of the rotorcraft and to minimise the noise generated by the auxiliary rotor, said control of the orientation of said aerodynamic surface being performed at least as a function of a current value of a speed parameter of the rotorcraft and of a current value of a power parameter of said power plant, said deflection angle being controlled by defining: - a first zone for which the deflection angle is at a maximum on reaching a positive threshold angle (δmax), the first zone being reached at a low forward speed, - a second zone for which the deflection angle is at a maximum on reaching the positive threshold angle (δmax), the second zone being reached at an intermediate forward speed and at a high power developed by the power plant, - a third zone for which the deflection angle is positioned at a medium orientation, the third zone being reached at a high forward speed and at a high power developed by the power plant, - a fourth zone for which the deflection angle is small on reaching a negative threshold value (δmin), the fourth zone being reached at a high forward speed and at a low power developed by the power plant. - a first zone for which the deflection angle is at a maximum on reaching a positive threshold angle (δmax), the first zone being reached at a low forward speed, - a second zone for which the deflection angle is at a maximum on reaching the positive threshold angle (δmax), the second zone being reached at an intermediate forward speed and at a high power developed by the power plant, - a third zone for which the deflection angle is positioned at a medium orientation, the third zone being reached at a high forward speed and at a high power developed by the power plant, - a fourth zone for which the deflection angle is small on reaching a negative threshold value (δmin), the fourth zone being reached at a high forward speed and at a low power developed by the power plant.