Flight control system and method for a rotary wing aircraft, enabling it to maintain either track or heading depending on its forward speed

A flight control method for a rotary wing aircraft, the aircraft following a track Tsol relative to the ground with a forward speed Va, a longitudinal direction X extending from the rear of the aircraft to the front of the aircraft, an elevation direction Z extending upwards perpendicularly to the longitudinal direction X, and a transverse direction Y extending from left to right perpendicularly to the longitudinal and elevation directions X and Z, the aircraft comprising:

• at least one rotary wing having a plurality of main blades of collective pitch and cyclic pitch that are variable about respective pitch axes, the aircraft being capable of performing movements in rotation about the directions X, Y, Z and of performing movements in translation along the directions X, Y, Z;
• an autopilot that generates control signals in predefined modes of operations and in application of flight setpoints, the control signals being capable of causing the aircraft to perform the movements in rotation and/or in translation relative to the directions X, Y, Z;
• and
• flight controls having at least one control member provided jointly with a plurality of movement axes A, B, C, D;
• the method comprising the following steps:
• applying a first mode of operation of the control members and of the autopilot when the longitudinal speed UX of the aircraft is greater than a first threshold speed Vthresh1, the longitudinal speed UX being a projection of the forward speed Va onto the longitudinal direction X, the autopilot then enabling the aircraft to fly while maintaining track relative to the ground, the flight setpoints of the autopilot being a ground course angle TKsol, the forward speed Va, a flight path angle P, and a heading Ψ;
• and
• applying a second mode of operation for the control members and the autopilot when the longitudinal speed UX is less than a second threshold speed Vthresh2, the first threshold speed Vthresh1 being greater than the second threshold speed Vthresh2, the autopilot then enabling the aircraft to fly while maintaining heading, the flight setpoints of the autopilot being the longitudinal speed UX, a lateral speed VY, a vertical speed WZ, and the heading Ψ.