Abstract:
A system for controlling pitch of a propeller blade of a turboshank engine, including two automatic locking members making it possible, depending on their controlled position, to lock rotation of a blade root in both directions of rotation, or to allow the rotation of the blade root support in either of the two directions.

Description:
TECHNICAL FIELD 
     The present invention generally concerns a system for controlling propeller blade pitch in an aircraft turboshaft engine. 
     The invention also concerns a propeller equipped with such a control system, as well as a method for steering said system. 
     The invention applies to all types of propeller, simple propellers or contra-rotating propellers, preferably for turbojet engines or turbo-props. 
     BACKGROUND OF THE INVENTION 
     A turbo-prop can be equipped with a pitch controlling system associated with each of the blades of its propeller, so as to adapt the orientation of the blades to the speed of the aircraft. 
     Such a system is designed such that the incidence of the blade remains fixed when the control system is not actuated, and must therefore make it possible to resist the action of the torque generated by the aerodynamic and centrifugal forces exerted on said blade during rotation of the propeller. More generally, to keep its pitch, it is considered that the blade must be locked in both directions of rotation along its own axis. To do this, the system is generally equipped with a locking mechanism, usually of the type ensuring contact/friction between two parts, such as a disc brake. To control the incidence of the blade, it is therefore necessary to perform a preliminary step for unblocking the incidence of the blade, by breaking the contact between the two parts of the locking mechanism. 
     Of course, the presence of the locking mechanism greatly complexifies the design of the control system, which creates drawbacks in terms of mass, reliability and bulk. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention therefore aims to at least partially resolve the abovementioned drawbacks, relative to the embodiments of the prior art. 
     To that end, the invention first concerns a system for controlling the propeller blade pitch in a turboshaft engine, comprising:
         an annular part for housing the root of said blade, the rotation of which is intended to ensure setting of the incidence of said blade, said annular part defining a first track having at least first and second notches circumferentially spaced apart from each other and each delimited by a first stop surface in a first direction of the circumferential direction and by a second stop surface in a second direction of the circumferential direction, opposite said first direction;   a second substantially annular track, concentric to said first track and positioned opposite and outside in relation thereto, said second stop surface of the first notch and said first stop surface of the second notch also each being oriented towards said second track;   a member for actuating the annular part arranged between the first and second tracks, said actuating member having a first stop surface in the first direction of the circumferential direction as well as a second stop surface in the second direction of the circumferential direction;   a first locking member provided between the first and second tracks, housed in said first notch and opposite said second stop surface of the actuating member; and   a second locking member provided between the first and second tracks, housed in said second notch and opposite said first stop surface of the actuating member;       

     said first and second locking members being able to occupy, during the operation of the turboshaft engine:
         a normal over-center position, in which said first locking member is on one hand in contact with the second stop surface of the first notch, at a distance from said first stop surface of said notch and from the second stop surface of the actuating member, and on the other hand in contact with the second track, position in which a first spring placed between said first stop surface of the notch and said first locking member causes a first contact stress of said second stop surface of the notch on the first locking member, and creating a first reaction stress of said second track on said first locking member, the first contact stress and the first reaction stress ensuring a first over-center of the first and second tracks, making them integral in rotation in the first direction,       

     and in which said second locking member is on one hand in contact with the first stop surface of the second notch, at a distance from said second stop surface of said notch and from the first stop surface of the actuating member, and on the other hand in contact with the second track, position in which a second spring placed between said second stop surface of the notch and said second locking member causes a second contact stress of said first stop surface of the notch on the locking member, and creating a second reaction stress of said second track on said second locking member, the second contact stress and the second reaction stress ensuring a second over-center of the first and second tracks, making them integral in rotation in said second direction; and
         an unlocking position in said first direction, in which the first locking member is in contact with the second stop surface of the actuating member, this unlocking position being ensured by the application of a first actuating torque in said first direction on said actuating member, of a value making it possible to exert, on the first locking member with said second stop surface of the actuating member, a first unlocking stress opposing the force of the first spring, and sufficing to make said first bearing stress null, and thereby to break said first over-center, said unlocking position in the first direction allowing the rotational movement, in relation to the second track, in said first direction of the actuating torque, of the assembly including said first track, said first and second locking members and said actuating member; and   an unlocking position in said second direction, in which the second locking member is in contact with the first stop surface of the actuating member, this unlocking position being ensured by the application of a second actuating torque in said second direction on said actuating member, of a value making it possible to exert, on the second locking member with said first stop surface of the actuating member, a second unlocking stress opposing the force of the second spring, and sufficing to make said second bearing stress null, and thereby to break said second over-center, said unlocking position in said second direction allowing the rotational movement, in relation to the second track, in said second direction of the actuating torque, of the assembly including said first track, said first and second locking members and said actuating member.       

     The invention is remarkable in that it allows automatic locking of the blade when no actuating torque is supplied to the actuating member, due to the over-center created by the first and second tracks, by the first and second locking members occupying their normal over-center position. 
     However, when it is necessary to change the pitch of the blade, a torque is applied with an appropriate value and direction on the actuating member, this torque making it possible both to unlock the system by placing the locking members in one of the two unlocking positions, and to drive the rotation of the first track in the desired direction, ensuring the setting of the blade&#39;s incidence. It should be noted that the unlocking and the rotation of the first track are caused simultaneously or practically simultaneously. 
     Thus, the pitch control system according to the invention has a simplified design in relation to those encountered in the prior art, since a single and same control makes it possible to ensure the unlocking and movement of the blade&#39;s incidence. No separate locking mechanism is therefore required, as was the case before, which creates advantages in terms of mass, reliability and bulk. 
     Lastly, the system according to the invention also procures high precision in the setting of the blade associated with this system. 
     Preferably, said first and second stop surfaces of the actuating member are also each oriented towards said first track. Because of this, the rotation of the first track ensuring the setting of the blade is easier, since the reaction stress of the second track on the locking member in contact with the actuating member is greatly reduced, if not eliminated. The resistance to the rotational movement of the first track is indeed decreased. 
     Preferably, the system comprises elastic return means coupled to said actuating member, and making it possible, when the latter is not subjected to said first actuating torque or said second actuating relative to said first track, so as to automatically bring said first and second locking members back into the normal over-center position. 
     Preferably, the system comprises an actuating engine controlling the rotation of said actuating member. It is therefore this engine that is intended to deliver the actuating torque causing the movement of the locking members in their unlocking position, as well as the rotation of the first track in relation to the second track. 
     Preferably, said first and second locking members are rollers. One alternative consists of providing that said first and second locking members are balls. In each of these cases, rolling members are therefore provided, which advantageously limits the friction in relation to that encountered in locking mechanisms of the disc brake type of the prior art. 
     Preferably, said first and second locking members form a pair of locking members, and the system is equipped with a plurality of pairs of locking members circumferentially spaced apart from each other. This allows a more homegenous distribution, in the circumferential direction, of the stresses ensuring the over-center of the first and second tracks. Moreover, in the normal over-center position, each locking member must therefore support compression stresses of lower intensity than those encountered in the single pair solution, which makes it possible in particular to improve the system&#39;s reliability. 
     The invention also concerns a propeller for an aircraft turboshaft engine comprising a pitch control system as described above, associated with each of the blades. 
     The invention also concerns an aircraft turbojet engine, comprising at least one propeller as described above. 
     The turbomachine preferably comprises a system of contra-rotating propellers, with each of its two propellers designed in the manner previously described, this turbomachine preferably being a turbo-prop, but alternatively able to be a turbojet engine. Naturally, in the latter case, the system of propellers is intended to form the fan of the turbojet engine. 
     Lastly, the invention also concerns a method for controlling a system for controlling propeller blade pitch in a turboshaft engine, as described above. According to this method, when an incidence change is required, a suitable actuating torque is applied to said actuating member. 
     Other advantages and features of the invention will appear in the non-limiting detailed description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This description will be done in light of the appended drawings, which: 
         FIG. 1  is a diagrammatic longitudinal half-section view of a propeller part for an aircraft turboshaft engine, according to one preferred embodiment of the present invention; 
         FIG. 2  is a perspective view of a retaining ring of the propeller blades of  FIG. 1 ; 
         FIG. 3  is a detailed view of a system for controlling the pitch of a propeller blade of  FIG. 1 , in transverse half-section, and also corresponding to a half-section view along line III-III of  FIG. 4 ; 
         FIG. 4  is a detailed view of the system for controlling the pitch of a propeller blade along line IV-IV of  FIG. 3 , with the locking members of the system occupying their normal over-center position; 
         FIG. 5   a  shows a view similar to that of  FIG. 4 , with the locking members of the system occupying their unlocking position in the first direction, adopted during setting of the blade aiming to reduce the incidence thereof; 
         FIG. 5   b  shows a view similar to that of  FIG. 5   a , with the locking members of the module occupying their unlocking position in the second direction, adopted during setting of the blade aiming to increase the incidence thereof; and 
         FIG. 6  is a perspective view of part of a setting system according to another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows part of a propeller  1  of a turboshaft engine according to one preferred embodiment of the present invention, for example belonging to a system of contra-rotating propellers. 
     The X axis corresponds to the longitudinal direction of the propeller  1 , this direction also corresponding to the longitudinal direction of the turbo-prop intended to integrate such a propeller  1 . The Y axis corresponds to the transverse direction of the propeller  1 , and the Z axis to the vertical direction or the height, these three axes being orthogonal to each other. 
     The propeller  1  comprises a stator or a casing  2 , centered on a longitudinal axis  4 , parallel to the X axis. This stator  2  is intended in a known manner to be integral with other casings of the turbomachine. 
     Moreover, a primary air flow direction through the propeller  1  is diagrammatically shown by arrow  10  parallel to the X axis, this primary air flow direction also serving as reference for the terms “upstream” and “downstream” used below. 
     For information, in the case of a system of contra-rotating propellers, the two propellers (only one being shown) are intended to rotate in opposite directions around the axis  4  on which they are centered, the rotations occurring in relation to the stator  2 , which remains immobile. 
     The propeller  1  comprises a drive shaft  16  centered on the axis  4 , and intended to be driven in rotation by a mechanical transmission device (not shown), for example forming an epicyclic gear train, itself driven by the turbine of the turbomachine. In the case of a pair of contra-rotating propellers, it is also possible for the latter parts to be driven directly by a contra-rotating turbine. 
     The hollow shaft  16  fixedly supports, at its downstream end, a rotor  18  housing, at its outer radial end, i.e. at its circumferential crown, propeller blades  6 . More precisely, as shown in  FIG. 2 , the rotor  18  is equipped with a retaining ring of the blades  19 , centered on the axis  4 , and having a plurality of circumferentially spaced housings  21 , each intended to receive a blade root, and being an integral part of a system for controlling the pitch of said blade. 
     The pitch control system  26 , diagrammed in  FIG. 1 , makes it possible to move the blade  6  to which it is connected between a minimal incidence position and a maximal incidence position, in relation to the axis  4 . The movement of the blade  6  between these two positions is done by pivoting thereof on itself, i.e. around its main axis  24 , which also corresponds to the axis of the housing of the retaining ring in which the blade root  23  is inserted. Of course, each of the blades  6  of the propeller is equipped with its own pitch control system  26 , the latter being steered preferably simultaneously so that each blade has, at any moment, the same incidence. 
     A pitch control system  26 , according to one preferred embodiment of the present invention, will now be described in reference to  FIGS. 3 and 4 . 
     It first comprises an annular part  52  for housing the root of the blade, centered on the axis  24 , and having a central portion inserted freely rotating in the housing  21  of the retaining ring  19 . Moreover, bearings  55 ′, such as tapered roller bearings, are provided between the two parts, so as to facilitate the relative rotation between them, along the axis  24 . 
     For example, the central portion of the part  52 , also called pivot, itself has an inner bore  53  in which the blade root  23  is housed, integral therewith in rotation along the axis  24 . Thus, the rotation of the part  52  is intended to ensure the rotation of the blade  6  along its axis  24 , and therefore intended to ensure the setting of the incidence of said blade. 
     This part  52 , which preferably constitutes the inner part of the control system centered on the axis  24 , defines a first track  50  oriented radially outwardly, and substantially annular. It has first and second notches  54   a ,  54   b  radially open towards the outside and spaced apart from each other along a circumferential direction  55 . 
     The first notch  54   a  is delimited by a first stop surface B 1  in a first direction  55   a  of the circumferential direction  55 , and by a second stop surface B 2  in a second direction  55   b  of said direction, opposite the first direction. In cross-section orthogonal to the axis  24  as shown in  FIG. 4 , the surface B 1 , preferably planar, is preferably orthogonal to the local circumferential direction, i.e. the tangential direction. However, the surface B 2 , also preferably planar, is also oriented radially outwardly, its angle with the local circumferential direction preferably being between 5 and 85°, the value selected being chosen in particular as a function of the coefficient of friction of the materials present, to be able to create an over-center that will be described below. 
     The two surfaces B 1 , B 2  are spaced circumferentially away from each other by a notch bottom. 
     The second notch  54   b  is defined by a first stop surface B′ 1  in the first direction  55   a  of the circumferential direction  55 , and by a second stop surface B′ 2  in the second direction  55   b . In cross-section orthogonal to the axis  24  as shown in  FIG. 4 , the surface B′ 2 , preferably planar, is preferably orthogonal to the local circumferential direction. However, the surface B′ 1 , also preferably planar, is also oriented radially outwardly, its angle with the local circumferential direction preferably being between 5 and 85°, the value selected here also being chosen as a function of the coefficient of friction of the materials present, to be able to create an over-center. The two surfaces B′ 1 , B′ 2  are spaced circumferentially away from each other by a notch bottom. 
     The surfaces B 2  and B′ 1  are situated back to back on a same outward radial protrusion  57  of the annular part  52 , passed through by a radial symmetrical plane P passing through the axis  24 . The notches  54   a ,  54   b  as well as their surfaces B 1 , B 2 , B′ 1 , B′ 2  are indeed arranged on either side of the plane P, symmetrically in relation thereto, as shown in  FIG. 4 . As a result, in the second direction  55   b , successively there is the surface B 1 , the notch bottom  54   a , the surface B 2 , the surface B′ 1 , the notch bottom  54   b , then the surface B′ 2 . 
     The system  26  also includes a second substantially annular track  56 , also with axis  24  and arranged opposite and outwardly in relation to the first track  50 , creating an annular space between them. This track  56 , radially inwardly oriented, is provided on the retaining ring  19 , away from and concentrically to the housing of the blade root  21 . 
     Thus, the two surfaces B 2  and B′ 1  are each substantially oriented towards this second track  56 , due to their incline described above. 
     The system  26  also includes an actuating member  60  of the annular part  52 , arranged between the first and second tracks  50 ,  56 . This member  60  assumes the form of a lug integral with the outer radial end of a substantially annular plate  61 , also with axis  24 . It is preferably pivotably connected with the second track  56  on which it is preferably in contact, its plate  61  being connected to the rotor of an actuating engine  40 , in order to be able to set in rotation by the latter, along the axis  24 . To that end, it is noted that the engine  40  has a stator fastened on the rotor  18  of the propeller  1 . 
     The actuating member  60  has a first stop surface C 1  in the first direction  55   a , as well as a second stop surface C 2  in the second direction  55   b.    
     In cross-section orthogonal to the axis  24  as shown in  FIG. 4 , the surface C 2 , preferably planar, is also oriented radially inwardly and towards the first notch  54   a , its angle with the local circumferential direction preferably being between 5 and 85°. Likewise, the surface C 1 , preferably planar, is also oriented radially inwardly and towards the second notch  54   b , its angle with the local circumferential direction also preferably being between 5 and 85°. 
     In the normal position shown in  FIG. 4 , which will be explained below, the surfaces C 1  and C 2  are also arranged back to back symmetrically in relation to the plane P, also corresponding to the plane of symmetry of the lug  60 . 
     Moreover, a first locking member  64   a , preferably in the form of a roller, is provided between the first and second tracks, housed in the first notch  54   a  and opposite the second stop surface C 2  of the member  60 . In the same way, a second locking member  64   b , preferably in the form of a roller, is provided between the first and second tracks, housed in the second notch  54   b  and opposite the first stop surface C 1  of the member  60 . 
       FIG. 4  shows the control system  26  with the locking members  64   a ,  64   b  occupying, during the operation of the turboshaft engine, a normal over-center position. 
     In that position, the first locking member  64   a  is on one hand in contact with the second stop surface B 2 , away from the first stop surface B 1  and the notch bottom, and on the other hand in contact with the second track  56 . This position is in particular ensured by a first spring  59   a , placed between the first stop surface B 1  and the roller  64   a . This spring  59   a  then exerts an action r 1  on the roller  64   a  that tends to move the latter in the second direction  55   b , until it comes into contact with the stop surface B 2 . The roller  64   a  therefore being stopped in rotation in this second direction  55   b  by the second stop surface B 2 , the latter then exerts a first contact stress F 1  on the roller  64   a.    
     This stress F 1  creates a first reaction stress R 1  of the second track  56  on the roller  64   a . Thus, the first contact stress F 1  and the first reaction stress R 1  jointly ensure a first over-center of the first and second tracks, making them integral in rotation in the first direction  55   a . In that respect, during operation of the turboshaft engine causing the rotation of the propeller, an aerodynamic force is exerted on the blades, and creates a torque of a given direction on the part  52 , due to its mechanical connection to the blades. If the given direction corresponds to the first direction  55   a , the part  52  will advantageously remain immobile in rotation in relation to the retaining ring  19 , along the axis  24 , since the torque applied to said annular part  52  will only strengthen the over-center procured by the roller  64   a , by increasing the intensity of the stresses F 1  and R 1 . 
     Still in the normal over-center position, the second locking member  64   b  is on one hand in contact with the first stop surface B′ 1 , away from the second stop surface B′ 2  and the notch bottom, and on the other hand in contact with the second track  56 . This position is in particular ensured by a second spring  59   b , placed between the second stop surface B′ 2  and the roller  64   b . This spring  59   b  then exerts an action r 2  on the roller  64   b  that tends to move the latter in the first direction  55   a . The roller  64   b  being stopped in rotation in this first direction  55   a  by the first stop surface B′ 1 , the latter then exerts a second contact stress F 2  on the roller  64   b.    
     This stress F 2  creates a second reaction stress R 2  of the second track  56  on the roller  64   b . Thus, the second contact stress F 2  and the second reaction stress R 2  jointly ensure a second over-center of the first and second tracks, making them integral in rotation in the second direction  55   b . In this respect, during the operation of the turboshaft engine causing the propeller to rotate, if the direction given to the torque applied to the part  52 , resulting from the aerodynamic force exerted on the blades, corresponds to the second direction  55   b , the part  52  will advantageously remain immobile in rotation in relation to the retaining ring  19 , along the axis  24 , since the torque applied to said annular part  52  will only strengthen the over-center procured by the roller  64   b , by increasing the intensity of the stresses F 2  and R 2 . 
     This normal over-center position of the locking members  64   a ,  64   b  is kept while the engine  40  is not actuated, and prohibits any change to the incidence of the blade. 
     To vary the incidence of the blade, the system  26  must be steered in order to bring the rollers  64   a ,  64   b  into another so-called unlocking position in one or the other of the two directions  55   a ,  55   b.    
       FIG. 5   a  concerns the case where the incidence of the blade must be modified towards its minimal incidence position. 
     In this unlocking configuration in the first direction, the roller  64   a  is brought into contact with the second stop surface C 2 . This position is ensured by the application of a first actuating torque C in the first direction  55   a , on the actuating member  60 , and more specifically on the plate  61  via the engine  40 , driving the lug  60  to come into contact with the roller  64   a . This torque C is of a value making it possible to exert, on the first roller  64   a , with a second stop surface C 2 , a first unlocking stress F′ 1  opposing the force r 1  of the first spring, aiming to compress the latter so as preferably to bring the roller  64   a  into contact with the surface B 1 . Generally, the unlocking stress F′ 1  is sufficient to make the first bearing stress F 1  null. There is therefore a loss of contact between the roller  64   a  and the stop surface B 2 , such that the first over-center is broken. Possibly, due to the incline of the stop surface C 2  towards the first track  50 , the contact between the second track  56  and the roller  64   a  can also be broken, due to the tendency of that roller  64   a  to be raised by the stop surface C 2 . 
     This makes the reaction stress R 1  null, and ensures gripping of said roller  64   a  between the compressing spring  59   a  and the surface C 2 . In this respect, as mentioned above, the compression of the spring can be such that the roller comes into contact with the stop surface B 1 . In the case where the contact between the second track  56  and the roller  64   a  is not broken, it is preferably done such that the reaction stress R 1  is extremely low, allowing rolling and/or sliding between the two members. 
     This position makes it possible to set the part  52  in rotation in relation to the second track  56  of the retaining ring  19 , along the axis  24 , in the first direction  55   a . It is indeed the assembly including the first track  50 , the first and second locking members  64   a ,  64   b , and the actuating member  60  that are simultaneously moved in rotation, under the effect of said member  60  driven by the torque C, by bearing of the roller  64   a  on the stop surface B 1 , possibly via the spring  59   a . Moreover, it should be noted that this principle applies regardless of the direction of the torque applied to the annular part  52 , resulting from the aerodynamic force exerted on the blade. 
     Moreover, it should be noted that the second over-center does not create an obstacle to the rotation of the pivot  52  in the first direction  55   a , at least due to the fact that this rotation tends to eliminate the contact between the second roller  64   b  and the stop surface B′ 1 , making the stress F 2  null and therefore leading to breaking this second over-center. The roller  64   b  is then able to accompany the rotation of the pivot  52 , by rolling and/or sliding on the second track  56  while remaining in its second notch. 
     Thus, the actuating torque C simultaneously makes it possible to unlock the system  26 , and to cause the rotation of the pivot  52  in relation to the second track  56  of the retaining ring  19  of the rotor. This creates a variation of the pitch of the blade  6 , from its maximal incidence position to its minimal incidence position. 
     Once the engine  40  is stopped, the control system  26  is automatically brought back into its configuration ensuring the normal locking position of the rollers  64   a ,  64   b  via elastic return means coupled to the actuating member  60 , such as a spring (not shown). This spring in fact makes it possible to move said actuating member  60  in rotation in relation to the first track  50 , so as to break the contact between the roller  64   a  and the surface C 2 . Simultaneously, the spring  59   a  pushes the roller  64   a  back on the stop surface B 2 , again ensuring the first over-center. In the same way, the spring  59   b  pushes the roller  64   b  back on the stop surface B′ 1 , again ensuring the second over-center. 
     Thus, when the engine  40  is stopped, the pivot  52  keeps its angular position in relation to the second track, which ensures great pitch precision of the blade. 
       FIG. 5   b  concerns the case where the incidence of the blade must be modified towards its maximal incidence position. 
     In this unlocking configuration in the second direction  55   b , the roller  64   b  is brought into contact with the first stop surface C 1 . This position is ensured by the application of a second actuating torque C′ in the second direction  55   b , on the actuating member  60 , and more specifically on the plate  61  via the engine  40 , driving the lug  60  to come into contact with the roller  64   b . This torque C′ is of a value making it possible to exert, on the second roller  64   b , with the first stop surface C 1 , a second unlocking stress F′ 2  opposing the force r 2  of the second spring, aiming to compress the latter so as preferably to bring the roller  64   b  into contact with the surface B′ 2 . Generally, the unlocking stress F′ 2  is sufficient to make the second bearing stress F 2  null. There is therefore a loss of contact between the roller  64   b  and the stop surface B′ 1 , such that the second over-center is broken. 
     Possibly, due to the incline of the stop surface C 1  towards the first track  50 , the contact between the second track  56  and the roller  64   b  can also be broken, due to the tendency of that roller  64   b  to be raised by the stop surface C 1 . 
     This makes the reaction stress R 2  null, and ensures gripping of said roller  64   b  between the compressing spring  59   b  and the surface C 1 . In this respect, as mentioned above, the compression of the spring can be such that the roller  64   b  comes into contact with the stop surface B′ 2 . In the case where the contact between the second track  56  and the roller  64   b  is not broken, it is preferably done such that the reaction stress R 2  is extremely low, allowing rolling and/or sliding between the two members. 
     This position makes it possible to set the part  52  in rotation in relation to the second track  56  of the retaining ring  19 , along the axis  24 , in the second direction  55   b . It is indeed the assembly including the first track  50 , the first and second locking members  64   a ,  64   b , and the actuating member  60  that are simultaneously moved in rotation, under the effect of said member  60  driven by the torque C′, by bearing of the roller  64   b  on the stop surface B′ 2 , possibly via the spring  59   b . Here again, it should be noted that this principle applies regardless of the direction of the torque applied to the annular part  52 , resulting from the aerodynamic force exerted on the blade. 
     Moreover, it should be noted that the first over-center does not create an obstacle to the rotation of the pivot  52  in the second direction  55   b , at least due to the fact that this rotation tends to eliminate the contact between the first roller  64   a  and the stop surface B 2 , making the stress F 1  null and therefore leading to breaking this first over-center. The roller  64   a  is then able to accompany the rotation of the pivot  52 , by rolling and/or sliding on the second track  56  while remaining in its first notch. 
     Thus, the actuating torque C′ simultaneously makes it possible to unlock the system  26 , and to cause the rotation of the pivot  52  in relation to the second track  56  of the retaining ring  19  of the rotor. This creates a variation of the pitch of the blade  6 , from its minimal incidence position to its maximal incidence position. 
     Once the engine  40  is stopped, the control system  26  is automatically brought back into its configuration ensuring the normal locking position of the rollers  64   a ,  64   b  via elastic return means coupled to the actuating member  60 , as well as via springs  59   a ,  59   b , as described above. 
     According to one preferred embodiment of the present invention, several pairs of locking members  64   a ,  64   b  are provided, circumferentially spaced away from each other around the axis  24 , as shown in  FIG. 6 . Preferably, these members  64   a ,  64   b  are arranged alternating, with, for each pair, an actuating member arranged between the two members  64   a ,  64   b  as described for the preceding preferred embodiment. Furthermore, these members  60 , each in lug form, are integral with the plate  61  driven by the actuating engine (not shown in  FIG. 6 ). This configuration allows a more homegenous overall distribution, in the circumferential direction, of the stresses ensuring the over-center of the first and second tracks. 
     Moreover, it should be noted that although the locking members  64   a ,  64   b  are preferably rollers, and the stop surface C 1 , C 2  with which they cooperate are preferably substantially planar surfaces, one alternative embodiment may consist of providing that the locking members  64   a ,  64   b  are balls, and the surfaces C 1 , C 2  substantially spherical surfaces. 
     Of course, various changes can be made by a person skilled in the art to the invention just described, solely as non-limiting examples.