A flight control system for a rotary-wing aircraft, said aircraft flying along a trajectory T sol relative to the ground at a ground course angle TK sol, a forward speed Va, a gradient P and a heaving ψ, a longitudinal direction X extending from the rear of said aircraft to the front of said aircraft, an elevation direction Z extending upwards perpendicularly to said longitudinal direction X and a transverse direction Y extending from left to right perpendicularly to said longitudinal direction X and elevation direction Z, - said aircraft comprising at least one rotary wing provided with a plurality of main blades, the collective pitch and the cyclic pitch of which are variable about a pitch axis, said aircraft being capable of performing movements in rotation about said directions X, Y, Z and in translation in said directions X, Y, Z, - said flight control system comprising o at least one control member (31, 32) provided jointly with a plurality of movement axes A, B, C, D, o an autopilot which generates control commands in accordance with predefined modes of operation and in accordance with flight setpoints, said control commands being capable of causing said movements in rotation and/or in translation relative to said directions X, Y, Z of said aircraft, characterised in that - at least one control member (31, 32) is movable relative to a first movement axis A in order to bring about a modification in said forward speed Va independently of said ground course angle TK sol and of said gradient P by a first action relative to said first movement axis A and by means of said autopilot, - said at least one control member (31, 32) is movable relative to a second movement axis B in order to bring about a modification in said ground course angle TK sol independently of said forward speed Va and of said gradient P by a second action relative to said second movement axis B and by means of said autopilot, and - said at least one control member (31, 32) is movable relative to a third movement axis C in order to bring about a modification in said gradient P independently of said ground course angle TK sol and of said forward speed Va by a third action relative to said third movement axis C and by means of said autopilot. - said aircraft comprising at least one rotary wing provided with a plurality of main blades, the collective pitch and the cyclic pitch of which are variable about a pitch axis, said aircraft being capable of performing movements in rotation about said directions X, Y, Z and in translation in said directions X, Y, Z, - said flight control system comprising o at least one control member (31, 32) provided jointly with a plurality of movement axes A, B, C, D, o an autopilot which generates control commands in accordance with predefined modes of operation and in accordance with flight setpoints, said control commands being capable of causing said movements in rotation and/or in translation relative to said directions X, Y, Z of said aircraft, o at least one control member (31, 32) provided jointly with a plurality of movement axes A, B, C, D, o an autopilot which generates control commands in accordance with predefined modes of operation and in accordance with flight setpoints, said control commands being capable of causing said movements in rotation and/or in translation relative to said directions X, Y, Z of said aircraft, - at least one control member (31, 32) is movable relative to a first movement axis A in order to bring about a modification in said forward speed Va independently of said ground course angle TK sol and of said gradient P by a first action relative to said first movement axis A and by means of said autopilot, - said at least one control member (31, 32) is movable relative to a second movement axis B in order to bring about a modification in said ground course angle TK sol independently of said forward speed Va and of said gradient P by a second action relative to said second movement axis B and by means of said autopilot, and - said at least one control member (31, 32) is movable relative to a third movement axis C in order to bring about a modification in said gradient P independently of said ground course angle TK sol and of said forward speed Va by a third action relative to said third movement axis C and by means of said autopilot.