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

Abstract

A flight control method and system for a rotary wing aircraft. When the longitudinal speed U.sub.X of the aircraft is greater than a first threshold speed V.sub.thresh1, a first mode of operation of the method enables flight to be performed while maintaining track relative to the ground, the flight setpoints of an autopilot being a ground course angle TK.sub.sol, a forward speed Va, a flight path angle P, and a heading Ψ. When the longitudinal speed U.sub.X is less than a second threshold speed V.sub.thresh2, a second mode of operation enables flight to be performed while maintaining heading, the flight setpoints being the longitudinal speed U.sub.X, a lateral speed V.sub.Y, a vertical speed W.sub.Z, and the heading Ψ.

Claims

1. A flight control method for a rotary wing aircraft, the aircraft following a track T.sub.sol 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 U.sub.X of the aircraft is greater than a first threshold speed V.sub.thresh1, the longitudinal speed U.sub.X 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 TK.sub.sol, 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 U.sub.X is less than a second threshold speed V.sub.thresh2, the first threshold speed V.sub.thresh1 being greater than the second threshold speed V.sub.thresh2, the autopilot then enabling the aircraft to fly while maintaining heading, the flight setpoints of the autopilot being the longitudinal speed U.sub.X, a lateral speed V.sub.Y, a vertical speed W.sub.Z, and the heading Ψ.

2. A flight control method according to claim 1 for a rotary wing aircraft, wherein: the first mode of operation of the control members and of the autopilot remains engaged so long as the longitudinal speed U.sub.X is greater than or equal to the second threshold speed V.sub.thresh2; and the second mode of operation of the control members and of the autopilot remains engaged so long as the longitudinal speed U.sub.X is less than or equal to the first threshold speed V.sub.thresh1.

3. A flight control method according to claim 1 for a rotary wing aircraft, wherein during the first mode of operation of the control members and of the autopilot: in order to enable the aircraft to follow a new track T.sub.soln, it is possible by transparency to modify: the forward speed Va by a first action relative to a first movement axis A of a control member and by means of the autopilot; the ground course angle TK.sub.sol by a second action relative to a second movement axis B of a control member and by means of the autopilot; and/or the flight path angle P by a third action relative to a third movement axis C of a control member and by means of the autopilot; and the flight setpoints of the autopilot are aligned on the parameters of the new track T.sub.soln, the flight setpoints being a new ground course angle TK.sub.sol, a new forward speed Va.sub.n, a new flight path angle P.sub.n, and/or a new heading Ψ in order to enable the autopilot to follow the new track T.sub.soln.

4. A flight control method according to claim 3 for a rotary wing aircraft, wherein during the first mode of operation of the control members and of the autopilot: in order to cause the aircraft to follow a new track T.sub.soln, it is possible by transparency to modify the ground course angle TK.sub.sol relative to the ground by a fourth action relative to a fourth movement axis D of a control member and by means of the autopilot; and the flight setpoints of the autopilot are aligned on the new parameters of the new track T.sub.soln on a new ground course angle TK.sub.soln in order to enable the autopilot to follow the new track T.sub.soln.

5. A flight control method according to claim 1 for a rotary wing aircraft, wherein if the forward speed Va is less than a third threshold speed V.sub.thresh3, the flight path angle P is replaced by the vertical speed W.sub.Z as the flight setpoint of the autopilot in the first mode of operation of the control members and of the autopilot.

6. A flight control method according to claim 1 for a rotary wing aircraft, wherein the second mode of operation of the control members and of the autopilot: in order to enable the aircraft to follow a new track T.sub.soln, it is possible by transparency and independently to modify: the longitudinal speed U.sub.X by a first action relative to a first movement axis A of a control member and by means of the autopilot; the lateral speed V.sub.Y by a first action relative to a second movement axis B of a control member and by means of the autopilot; and/or the vertical speed W.sub.Z by a third action relative to a third movement axis C of a control member and by means of the autopilot; and the flight setpoints of the autopilot are aligned on: a new longitudinal speed U.sub.Xn of the aircraft if the new longitudinal speed U.sub.Xn is greater than a fourth threshold speed V.sub.thresh4; and a new lateral speed V.sub.Yn of the aircraft if the new lateral speed V.sub.Yn has an absolute value that is less than a fifth threshold speed V.sub.thresh5.

7. A flight control method according to claim 1 for a rotary wing aircraft, wherein in the second mode of operation of the control members and of the autopilot: in order to enable the aircraft to follow a new track T.sub.soln, it is possible by transparency and independently to modify: the longitudinal speed U.sub.X by a first action relative to a first movement axis A of a control member and by means of the autopilot; the lateral speed V.sub.Y by a second action relative to a second movement axis B of a control member and by means of the autopilot; and/or the vertical speed W.sub.Z by a third action relative to a third movement axis C of a control member and by means of the autopilot; and the flight setpoints of the autopilot are aligned on: a new longitudinal speed U.sub.Xn of the aircraft if the new longitudinal speed U.sub.Xn is greater than a fourth threshold speed V.sub.thresh4; and a new lateral speed V.sub.Yn of the aircraft after the pilot of the aircraft performs a specific action.

8. A flight control method according to claim 6 for a rotary wing aircraft, wherein in the second mode of operation of the control members and of the autopilot, the flight setpoints of the autopilot are aligned on a new vertical speed W.sub.Zn of the aircraft if the new vertical speed W.sub.Zn has an absolute value that is greater than a sixth threshold speed V.sub.thresh6.

9. A flight control method according to claim 6 for a rotary wing aircraft, wherein in the second mode of operation of the control members and of the autopilot: it is possible by transparency to modify the heading Ψ by a fourth action relative to a fourth movement axis D of a control member and by means of the autopilot, independently of the speed U.sub.X, V.sub.Y, W.sub.Z; and the flight setpoint of the autopilot corresponding to the heading Ψ of the aircraft is aligned on a new heading Ψ.sub.n.

10. A flight control method according to claim 6, for a rotary wing aircraft, wherein: if the new longitudinal speed U.sub.Xn has an absolute value that is less than a seventh threshold speed V.sub.thresh7, the flight setpoint corresponding to the longitudinal speed U.sub.X is zero; and if the new lateral speed V.sub.Yn has an absolute value that is less than the seventh threshold speed V.sub.thresh7, the flight setpoint corresponding to the lateral speed U.sub.Y is zero.

11. A flight control method according to claim 1 for a rotary wing aircraft, wherein the first and second threshold speeds V.sub.thresh1, V.sub.thresh2 are a function of the longitudinal speed of the relative wind to which the aircraft is subjected and of the lateral speed V.sub.Y.

12. A flight control method according to claim 1 for a rotary wing aircraft, wherein the aircraft has firstly a first control lever for controlling movements in rotation of the aircraft about the longitudinal and transverse directions X and Y, and secondly a second control lever for controlling movements of the aircraft in translation along the elevation direction Z, and a first control member is the first control lever, and a second control member is the second control lever, the first control member having the first movement axis A and the second movement axis B, the second control member having the third movement axis C.

13. A flight control method according to claim 12 for a rotary wing aircraft, wherein a violent action on the first control lever causes the autopilot to deactivate maintaining the flight path angle setpoint P, the first control lever then controlling movements in rotation of the aircraft about the longitudinal direction X, and the second control lever controlling movements in translation of the aircraft along the elevation direction Z.

14. A flight control method according to claim 1 for a rotary wing aircraft, wherein the aircraft has firstly a first control lever for controlling movements in rotation of the aircraft about the longitudinal and transverse directions X and Y, and secondly a second control lever enabling movements of the aircraft in translation to be controlled along the elevation direction Z, and a first control member is positioned on the first control lever and a second control member is positioned on the second control lever, the first control member having the first movement axis A and the second movement axis B, and the second control member having the third movement axis C.

15. A flight control method according to claim 14 for a rotary wing aircraft, wherein the second control member has a fourth movement axis D.

16. A flight control method according to claim 1 for a rotary wing aircraft, wherein the control members are calibrated and control precise movements of the aircraft.

17. A flight control method according to claim 1 for a rotary wing aircraft, wherein the flight setpoints of the autopilot can be varied in order to cause the aircraft to engage hovering flight towards a stop position S that is determined on applying the engagement of hovering flight for the aircraft.

18. A flight control system for a rotary wing aircraft, the aircraft following a track T.sub.sol 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; the flight control system comprising: at least one control member provided jointly with a plurality of movement axes A, B, C, D; and an autopilot that generates control signals in predefined modes of operation 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 wherein the flight control system performs the flight control method according to claim 1 for a rotary wing aircraft.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention and its advantages appear in greater detail from the context of the following description of embodiments given by way of illustration and with reference to the accompanying figures, in which:

(2) FIG. 1 shows an aircraft having flight control of the invention;

(3) FIG. 2 is a diagram showing the ground course angle and the heading of the aircraft;

(4) FIGS. 3 and 4 are two detail views of control levers of a rotary wing aircraft.

(5) Elements shown in more than one of the figures are given the same references in each of them.

DETAILED DESCRIPTION OF THE INVENTION

(6) In FIG. 1, there can be seen an aircraft 10 that has a main rotor 11 positioned above a fuselage 13 and an anti-torque tail rotor 12 positioned at the tail end of a tail boom 14. The aircraft 10 also has an instrument panel 5, a seat 20 on which a pilot of the aircraft 10 can sit, an autopilot 15, and manual control means made up in particular of two control levers 21 and 22, and of pedals 23.

(7) Furthermore, an X, Y, Z reference frame is attached to the aircraft 10, and more particularly to its center of gravity. The longitudinal direction X extends from the rear of the aircraft 10 to the front of the aircraft 10, the elevation direction Z extends upwards perpendicularly to the longitudinal direction X, and the transverse direction Y extends from left to right perpendicularly to the longitudinal and elevation directions X and Y.

(8) The longitudinal direction X is the roll axis of the aircraft 10, the transverse direction Y is its pitching axis, and the elevation direction Z is its yaw axis.

(9) The main rotor 11 has an axis of rotation that is substantially vertical, i.e. parallel to the elevation direction Z, and it is provided with three main blades 111, 112, 113 having collective pitch and cyclic pitch that are variable under the control of the control levers 21, 22 and of the autopilot 15. In similar manner, the tail rotor 12 has its axis of rotation substantially horizontal, i.e. parallel to the transverse direction Y, and it is provided with four secondary blades 121, 122, 123, 124 of collective pitch that is variable and controllable by means of the pedals 23 and of the autopilot 15.

(10) More precisely, the first control lever 21 is movable about the longitudinal and transverse directions X and Y and serves to control the cyclic pitch of the main blades 111, 112, 113 by means of a first control linkage 24. The second control lever 22 is movable about the transverse direction Y and controls the collective pitch of the main blades 111, 112, 113 by means of a second control linkage 25. Taking action on the first control lever 21 then serves to control movements in rotation of the aircraft 10 about the longitudinal and transverse directions X and Y, and taking action on the second control lever then serves to control movements in translation of the aircraft 10 along the elevation direction Z.

(11) Likewise, the pedals 23 control the collective pitch of the secondary blades 121, 122, 123, 124 via a third control linkage 26. Taking action on the pedals 23 then serves to control movements in rotation of the aircraft 10 about its yaw axis.

(12) The control linkages 24, 25, 26 serve to actuate the various blades and may for example be made up of connections that are entirely mechanical between the manual control means 21, 22, 23 and the blades. These control linkages 24, 25, 26 may also be made up of mechanical connections associated with hydraulic actuator means, or indeed electrical connections associated with such hydraulic actuator means.

(13) The autopilot 15 also serves to control the collective and cyclic pitches of the main blades 111, 112, 113, and also the collective pitch of the secondary blades 121, 122, 123, 124 by acting respectively on the same control linkages 24, 25, 26. The autopilot 15 then serves to control movements in rotation of the aircraft 10 about the longitudinal and transverse directions X and Y and movements in translation of the aircraft 10 along the elevation direction Z, and also movements in rotation of the aircraft 10 about its yaw axis.

(14) FIGS. 3 and 4 show in greater detail the respective grip zones of the first and second control levers 21 and 22. The grip zone of each control lever 21, 22 includes in particular a respective control member 31, 32 and a pushbutton 33. Each control member 31, 32 is movable about two specific movement axes A & B, C & D. A first control member 31 present on the first control lever 21 and shown in FIG. 3 is movable about two movement axes A and B. In similar manner, a second control member 32 present on the second control lever 22 and shown in FIG. 4 is movable about two movement axes C and D.

(15) A flight control system 1 is made up of manual control means 21, 22, 23, of the control members 31, 32, of the pushbutton 33, of the autopilot 15, and of the control linkages 24, 25, 26.

(16) The aircraft 10 can fly along a track T.sub.sol relative to the ground, this track T.sub.sol being determined relative to the ground and defined in a terrestrial geographical reference frame, e.g. determined by the cardinal points and the direction of terrestrial gravity.

(17) A flight of an aircraft 10 along the track T.sub.sol may be characterized using two types of characterization by using different parameters for the track T.sub.sol.

(18) In a first type of characterization, a flight of an aircraft 10 along the track T.sub.sol is characterized by a ground course angle TK.sub.sol between the direction of the track T.sub.sol and the direction of north in a horizontal plane of the terrestrial geographical reference frame, a forward speed Va of the aircraft 10, a flight path angle P formed by the angle between the direction of the track T.sub.sol and the horizontal orientation of the terrestrial reference frame, and a heading Ψ which is the angle formed between the direction of north and the projection of the longitudinal direction X of the aircraft 10 onto a horizontal plane of the terrestrial reference frame.

(19) The forward speed Va of the aircraft 10 is the speed of the aircraft 10 along the direction of the track T.sub.sol, and this speed may be defined relative to the ground or else relative to the air.

(20) In a second type of characterization, a flight of an aircraft 10 along the track T.sub.sol is characterized by a longitudinal speed U.sub.X along the longitudinal direction X, a lateral speed V.sub.Y along the transverse direction Y, and a vertical speed W.sub.Z along the elevation direction Z, and also by the heading Ψ.

(21) These longitudinal, lateral, and vertical speeds, U.sub.X, V.sub.Y, and W.sub.Z are respective components of the forward speed Va of the aircraft 10 along the three specified directions X, Y, Z of the aircraft 10.

(22) FIG. 2 shows a projection onto a horizontal plane of the terrestrial reference frame of a track T.sub.sol. The longitudinal and transverse directions X, Y of the aircraft 10 are also shown as are the directions N, W of a terrestrial geographical reference frame.

(23) The heading Ψ is thus shown between the longitudinal direction X of the aircraft 10 and the direction N of north. The course angle TK.sub.sol on the ground is shown between the direction of the track T.sub.sol and the direction N of north.

(24) It can be seen that the heading Ψ is different from the ground course angle TK.sub.sol. Consequently, the nose and the tail boom 14 of the aircraft 10, which are in alignment on the longitudinal direction X, are not in alignment with the track T.sub.sol. Likewise, the forward speed Va is in alignment with the track T.sub.sol and is not parallel to the longitudinal direction X.

(25) In addition, the longitudinal and lateral speeds U.sub.X and V.sub.Y are respective projections of the forward speed Va of the aircraft 10, and preferably of the forward speed of the aircraft 10 relative to the ground, onto the longitudinal and transverse directions X and Y. The vertical speed W.sub.Z and also the flight path angle P are not shown in FIG. 2, which lies in a horizontal plane of the terrestrial reference frame, and thus extends perpendicularly to the elevation direction Z.

(26) The aircraft 10 travels generally along a track T.sub.sol in order to reach a target on the ground, such as a landing ground. Nevertheless, the pilot may need to modify one or more parameters of the track T.sub.sol, e.g. in order to slow down, avoid an obstacle not listed in a database of the aircraft 10, or merely in order to change route. Such modifications are necessary in particular when performing visual flight and at low altitude, and as a function of the surroundings and/or of weather conditions.

(27) Nevertheless, depending on the flight conditions of the aircraft 10, and in particular on its longitudinal speed U.sub.X, the maneuvers performed by the pilot are different. When the aircraft 10 is traveling at a low longitudinal speed U.sub.X, the piloting of the aircraft 10 is generally performed by maintaining heading, with the pilot acting on the parameters of the track T.sub.sol in the second type of characterization. Under such circumstances, the pilot acts directly to control the longitudinal, lateral, and vertical speeds U.sub.X, V.sub.Y, and W.sub.Z, and also the heading Ψ of the aircraft 10, e.g. so as to move at very low longitudinal speeds U.sub.X and at low altitude close to buildings.

(28) In contrast, when the aircraft 10 is traveling at a faster longitudinal speed U.sub.X, the piloting of the aircraft 10 is generally performed by maintaining track, with the pilot acting on the parameters of the track T.sub.sol in the first type of characterization. Under such circumstances, the pilot prefers to control the forward speed Va of the aircraft 10 directly along the track T.sub.sol so as to slow down or accelerate the aircraft 10, and to control the ground course angle TK.sub.sol so as to modify said track T.sub.sol, and also to control the flight path angle P and possibly the heading Ψ.

(29) Nevertheless, travel of the aircraft 10 at a flight path angle P is meaningful only above a certain forward speed, which is of the order of 20 kt. When the forward speed Va of the aircraft 10 is less than a third threshold speed V.sub.thresh3, while in the speed range for maintaining track, the pilot of the aircraft 10 controls the vertical speed W.sub.Z of the aircraft 10 instead of the flight path angle P. The flight setpoint expressed in terms of flight path angle P is replaced by a flight setpoint expressed in terms of the vertical speed W.sub.Z of the aircraft 10.

(30) A flight control method by maintaining track or heading makes it possible to switch, as a function of the longitudinal speed U.sub.X of the aircraft 10, between a first mode of operation of the control members 31, 32 and of the autopilot 15 in the first type of characterization for the track T.sub.sol and a second mode of operation of the control members 31, 32 and of the autopilot 15 in the second type of characterization for the track T.sub.sol. This second mode of operation of the control members 31, 32 and of the autopilot 15 is thus used at low forward speed Va, with the first mode of operation being used at higher forward speeds Va.

(31) Thus, during this first mode of operation of the control members 31, 32 and of the autopilot 15, the autopilot 15 enables the aircraft 10 to fly with track being maintained relative to the ground, the flight setpoints of the autopilot 15 being the ground course angle TK.sub.sol, the forward speed Va, the flight path angle P or else the vertical speed W.sub.Z, where appropriate, and the heading Ψ. In contrast, in the second mode of operation of the control members 31, 32 and of the autopilot 15, the autopilot 15 enables the aircraft 10 to fly while maintaining heading, the flight setpoints of the autopilot 15 being the longitudinal speed U.sub.X, the lateral speed V.sub.Y, the vertical speed W.sub.Z, and the heading Ψ.

(32) The flight control system 1 enables this flight control method to be performed while maintaining track or while maintaining heading. This flight control method while maintaining track or maintaining heading is engaged by means of a button 33, e.g. by the pilot pressing the button 33 once only, or else by pressing it twice.

(33) The changeover between the first and second modes of operation of the control members 31, 32 and of the autopilot 15 is performed relative to a threshold with hysteresis using two threshold speeds V.sub.thresh1 and V.sub.thresh2, the first threshold speed V.sub.thresh1 being greater than the second threshold speed V.sub.thresh2.

(34) The first mode of operation of the control members 31, 32 and of the autopilot 15 is engaged as soon as the longitudinal speed U.sub.X exceeds the first threshold V.sub.thresh1 and it remains engaged so long as the longitudinal speed U.sub.X is greater than or equal to the second threshold speed V.sub.thresh2. Likewise, the second mode of operation of the control members 31, 32 and of the autopilot 15 is engaged as soon as the longitudinal speed U.sub.X becomes less than the second threshold speed V.sub.thresh2 and it remains engaged so long as the longitudinal speed U.sub.X is less than or equal to the first threshold speed V.sub.thresh1.

(35) The threshold speeds V.sub.thresh1 and V.sub.thresh2 may be a function of the flight conditions of the aircraft 10, essentially of the speed and the wind direction and also of the lateral speed V.sub.Y aircraft 10.

(36) In addition, during these two modes of operation of the control members 31, 32 and of the autopilot 15, piloting by transparency is possible in order to adjust the track T.sub.sol. The pilot can thus cause one or more parameters of the track T.sub.sol to be modified directly by using the control members 31, 32 and by means of the autopilot 15.

(37) In the first mode of operation of the control members 31, 32 and of the autopilot 15, this flight control method makes it possible to ensure that the track T.sub.sol is maintained by modifying the forward speed Va, the ground course angle TK.sub.sol, the flight path angle P or else the vertical speed W.sub.Z, where appropriate, and possibly also the heading Ψ by using the autopilot 15 acting on the various flight parameters.

(38) Likewise, in the second mode of operation of the control members 31, 32 and of the autopilot 15, the flight control method makes it possible to ensure that heading is maintained by modifying the longitudinal speed U.sub.X, the lateral speed V.sub.Y, and the vertical speed W.sub.Z, and the heading Ψ by means of the autopilot 15 acting on the various flight parameters.

(39) Each action of the pilot on one of the control members 31, 32 relative to a movement axis A, B, C, or D acts via the autopilot 15 to modify one of the parameters of the track T.sub.sol.

(40) Thus, while maintaining track, an action of the pilot on one of the control members 31, 32 relative to a movement axis A, B, C modifies respectively the forward speed Va, the ground course angle TK.sub.sol, or the flight path angle P or else the vertical speed W.sub.Z, as appropriate. Furthermore, the ground course angle TK.sub.sol can also be modified by the pilot acting on one of the control members 31, 32 relative to the movement axis D. The heading Ψ can be modified by the pilot acting on the pedals 23.

(41) In contrast, while maintaining heading, an action of the pilot on one of the control members 31, 32 relative to the movement axes A, B, C, D serves respectively to modify the longitudinal speed U.sub.X, the lateral speed V.sub.Y, the vertical speed W.sub.Z, or the heading Ψ.

(42) Naturally, the pilot can act simultaneously on one or two control members 31, 32 relative to a plurality of movement axes A, B, C, D in order to modify a plurality of parameters of the track T.sub.sol.

(43) The autopilot 15 takes account of the actions of the pilot on the control members 31, 32, modifies its flight setpoints as a function of these actions, and then generates control signals in order to modify the pitch of the main blades 111, 112, 113 of the main rotor 11 and possibly the pitch of the secondary blades 121, 122, 123, 124 of the tail rotor 12. The aircraft 10 then follows a new track T.sub.soln, for which one or more parameters have been modified as requested by the pilot, these modified parameters being the new flight setpoints of the autopilot 15.

(44) During each action of the pilot on one of the control members 31, 32, new flight setpoints of the autopilot 15 can be aligned with the new parameters for the track T.sub.soln, i.e. a new ground course angle TK.sub.soln, a new forward speed Va.sub.n, a new flight path angle P.sub.n or else a new vertical speed W.sub.Zn, as appropriate, and/or a new heading Ψ.sub.n in the first mode of operation of the control members 31, 32 and of the autopilot 15, and a new longitudinal speed U.sub.Xn, a new lateral speed V.sub.Yn, a new vertical speed W.sub.Zn, and/or a new heading Ψ.sub.n in the second mode of operation of the control members 31, 32 and of the autopilot 15.

(45) Nevertheless, in the context of maintaining heading, synchronization conditions may be taken into account in order to align these new flight setpoints as to avoid flight situations that are potentially dangerous for the aircraft 10 which is generally flying at low altitudes and close to buildings or to terrain.

(46) According to first synchronization conditions, these flight setpoints are aligned respectively and independently on the new longitudinal and lateral speeds U.sub.Xn and V.sub.Yn if the new longitudinal speed U.sub.Xn is greater than a fourth threshold speed V.sub.thresh4 and if the new lateral speed V.sub.Yn has an absolute value less than a fifth threshold speed V.sub.thresh5.

(47) According to second synchronization conditions, these flight setpoints are aligned respectively and independently on the new longitudinal speed U.sub.Xn if this new longitudinal speed U.sub.Xn is greater than a fourth threshold speed V.sub.thresh4 and on the new lateral speed V.sub.Yn after a specific action of a pilot of the aircraft 10. By way of example, this action of the pilot is pressing on a button for synchronizing the new lateral speed V.sub.Yn of the aircraft 10. According to these second synchronization conditions, the pilot decides whether the new lateral speed V.sub.Yn is to be one of the flight setpoints.

(48) In contrast, regardless of synchronization conditions, if the new longitudinal speed U.sub.Xn is less than a fourth threshold speed V.sub.thresh4, the flight setpoint corresponding to the longitudinal speed U.sub.X is aligned on the fourth threshold speed V.sub.thresh4.

(49) Likewise, if the new lateral speed V.sub.Yn has an absolute value greater than the fifth threshold speed V.sub.thresh5, the flight setpoint corresponding to the lateral speed V.sub.Y is aligned on this fifth threshold speed V.sub.thresh5.

(50) The flight setpoint corresponding to the vertical speed W.sub.Z of the aircraft 10 is generally zero when flying while maintaining heading. Such flight generally takes place at low altitude and the aircraft 10 travels in such surroundings in automatic flight at an altitude that is constant relative to the ground, i.e. with a vertical speed W.sub.Z that is zero. After the pilot has taken an action to generate a modification to this vertical speed W.sub.Z, the flight setpoint corresponding to the vertical speed W.sub.Z generally remains unchanged and thus zero.

(51) Nevertheless, if this action of the pilot causes the aircraft 10 to travel at a new vertical speed W.sub.Zn that is large and greater than a sixth threshold speed V.sub.thresh6, and if the pilot does not reduce this new vertical speed W.sub.Zn, it can be deduced that the pilot now seeks to travel with this new vertical speed W.sub.Zn. Under such circumstances, these flight setpoints may be aligned on the new vertical speed W.sub.Zn, which is greater than the sixth threshold speed V.sub.thresh6.

(52) In contrast, when the new vertical speed W.sub.Zn of the aircraft is negative, the flight setpoint corresponding to this vertical speed W.sub.Z may remain zero in order specifically to avoid a flight situation that is dangerous for the aircraft 10, or else it may be aligned on the new vertical speed W.sub.Zn when the new vertical speed W.sub.Zn lies within the following range of negative vertical speeds W.sub.Z: 0 ft/min to −500 ft/min.

(53) Furthermore, if an absolute value of the new longitudinal speed U.sub.Xn and/or of the new lateral speed V.sub.Yn is small and less than a seventh threshold speed V.sub.thresh7, the pilot seeks to maintain this new speed as zero and the corresponding flight setpoint needs to be zero.

(54) In contrast, the flight setpoints of the autopilot continue to be aligned on the new heading Ψ.sub.n.

(55) In addition, the first control lever 21 may be used as the first control member 31 and the second control lever 22 is used as the second control member 32.

(56) Nevertheless, such particular utilization of the control levers 21 and 22 is not suitable for performing urgently a sudden maneuver of the aircraft 10, e.g. for the purpose of avoiding an obstacle to be found on the track T.sub.sol or indeed close thereto. The first and second control levers 21, 22 then do not enable a vertical lateral movement of the aircraft 10 to be performed quickly.

(57) Maintaining the setpoint for the flight path angle P or for the vertical speed W.sub.Z, as appropriate, by means of the autopilot 15 is deactivated as soon as it is detected that the pilot is acting violently on the first control lever 21. Consequently, the pilot can control the longitudinal cyclic pitch so as to make the aircraft 10 move in rotation about the pitching axis, and the pilot may possibly act on the collective pitch in order to cause the aircraft 10 to move in translation along the elevation direction Z and thus perform the necessary avoidance maneuver.

(58) Regardless of whether track or heading is being maintained, the flight setpoints of the autopilot 15 are generally constant so long as the pilot does not act on a control member 31, 32. Nevertheless, these flight setpoints may be variable in the context of a particular mode of operation of the method for maintaining track or heading for the purpose of causing the aircraft 10 to hover at a determined stop position S on engaging this particular mode of operation.

(59) During this particular mode of operation, the pilot can act on each of the control members 31, 32 relative to the movement axes A, B, C, D so as to modify at least one parameter for the track T.sub.sol. Consequently, new flight setpoints of the autopilot are aligned, which new flight setpoints are also variable so as to enable the aircraft 10 to be caused to hover over a new stop position S.sub.n determined from the stop position S as determined when applying this particular mode of operation and from the actions of the pilot on the control members 31, 32.

(60) Naturally, the present invention may be subjected to numerous variations as to its implementation. Although several embodiments are described, it will readily be understood that it is not conceivable to identify exhaustively all possible implementations. It is naturally possible to envisage replacing any of the means described by equivalent means without going beyond the ambit of the present invention.

(61) In particular, the aircraft 10 with this flight control system 1 is not limited to the aircraft 10 shown in FIG. 1. By way of example, the aircraft 10 may have two main rotors or it may be a hybrid helicopter.

(62) Furthermore, the number of main blades 111, 112, 113 of a main rotor 11, and the number of secondary blades 121, 122, 123, 124 of a tail rotor 12 are not limited to the example aircraft 10 shown in FIG. 1. A main rotor 11 or a tail rotor 12 may have two, three, four, five, or even more than five blades.