Method of driving a main rotor of a rotorcraft in rotation in compliance with a speed of rotation setpoint of variable value

A method of driving rotation of at least one main rotor of substantially vertical axis of a rotorcraft by a power plant of the rotorcraft, said method comprising at least the following operations:

• a control unit applying a method of determining the value of a regulation setpoint (C) relating to a speed for driving said at least one main rotor in rotation, said method determining the value of the regulation setpoint (C) at least depending on the value of a physicochemical parameter of the ambient air outside the rotorcraft;
• and
• transmitting the previously-determined value of the regulation setpoint (C) to a regulator unit for regulating the operation of the power plant, the regulator unit causing said at least one main rotor to be driven in compliance with the transmitted value for the regulation setpoint (C);
• wherein the value of the regulation setpoint (C) is determined at least by applying a calculation rule that continuously generates progressive and continuous variation of a value for an initial setpoint (NRini) from which the regulation setpoint (C) is determined, the progressive and continuous variation in the value of the initial setpoint (NRini) being calculated depending on progressive and continuous variation in the value of the density (D) of the ambient air outside the rotorcraft.