Abstract:
A control system ( 10 ) for controlling the pitch of a propeller ( 50 ), the system comprising a propeller shaft ( 60 ), a blade swivel device ( 20 ) having a rotary control element ( 22 ) suitable for placing the blades ( 52 ) in an angular position corresponding to a desired propeller pitch, and a transmission ( 12 ) presenting an outlet member coupled in rotation with the rotary control element ( 22 ) of the blade swivel device ( 20 ). 
     The transmission ( 12 ) includes a variable speed drive ( 70 ) having drive, control, and outlet rotors that are coupled in rotation respectively with the propeller shaft ( 60 ), with a control member, and with the outlet member of the transmission. 
     By means of the variable speed drive ( 70 ), the speed of rotation of the outlet member of the transmission is a predetermined function of the speeds of rotation not only of the propeller shaft, but also of the control member.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a propeller pitch control system. 
       TECHNOLOGICAL BACKGROUND 
       [0002]    In known manner, a propeller comprises a set of blades designed to be driven in rotation by a propeller shaft. With variable pitch propellers, each of the blades is configured to be capable of pivoting about its own longitudinal axis, which axis extends in a radial direction of the propeller. 
         [0003]    A propeller pitch control system is designed to vary the pitch of the propeller, i.e. to vary the angle formed by its blades relative to the direction of the propeller axis, which is also the direction of the propeller shaft. 
         [0004]    One such system is described by way of example in Document U.S. Pat. No. 8,167,553, having a FIG. 2 that is reproduced herewith as  FIG. 1 . 
         [0005]    That document thus presents a propeller pitch control system  10  arranged in such a manner as to control the pitch of the propeller  50  of an airplane engine. 
         [0006]    The propeller  50  has blades  52  mounted radially around a propeller shaft  60 . 
         [0007]    The propeller shaft  60  is a hollow shaft, with its end closed by a wall  62 . 
         [0008]    In the proximity of this end, in the periphery of the shaft  60 , there are provided radial openings  64  for fastening blades. A respective blade  52  is fastened in each radial opening  64 . 
         [0009]    The blades  52  are fastened in such a manner as to be capable of pivoting about their respective longitudinal axes Z (which axes are radial for the propeller  50 ), via ball bearings  54 . 
         [0010]    The propeller pitch control system  10  includes a blade swivel device  20  for swiveling the blades. 
         [0011]    The device  20  includes a rotary control element constituted by a wormscrew  22  arranged on the axis X of the propeller shaft  60 . The wormscrew  22  is held in position inside the shaft  60  by ball bearings  21 , which position is stationary other than the possibility of rotating about its axis X. 
         [0012]    The blade swivel device  20  also has a nut  26  that is arranged on the wormscrew  22 . 
         [0013]    The nut  26  is connected to the propeller shaft so as to rotate permanently at exactly the same speed of rotation as the propeller shaft. In this embodiment, this connection is constituted by a retaining finger  23 . 
         [0014]    In addition, the nut  26  is connected to the blade so that the axial position of the nut (along the propeller axis X) determines the angular position of the blades. 
         [0015]    For this purpose, each blade has a blade attachment finger  56 . Each of these attachment fingers  56  extends radially from the root of the blade of which it forms a part along a radial attachment axis Z′. 
         [0016]    Each attachment axis Z′ is offset from the axis of the radial opening  64  for fastening the blade that is designed to receive the blade  52  of which the attachment finger  56  forms a part. Consequently, movement of the attachment finger  56  along the axial direction causes the blade  52  of which the finger  56  forms a part to turn about its pitch axis Z. 
         [0017]    In order to actuate the fingers  56 , the nut  26  has notches  28 , with each notch  28  being designed to receive a blade attachment finger  56 . 
         [0018]    More precisely, the root of each of the blades  52  presents an off-center wrist-pin  55  that extends perpendicularly to the axis Z of the blade. The end of the wrist-pin  55  opposed to the axis Z presents a blade attachment finger  56  that projects from the wrist-pin along the radial direction Z′ towards the propeller axis X. For each blade  52 , the nut  26  has a notch  28  designed to receive the blade attachment finger  56  in question. When the nut  26  moves axially along the propeller axis X under the effect of the wormscrew  22  rotating, the attachment fingers  56  all move by the same amount. Under the effect of this movement, each of the blades  52  pivots about its pivot axis Z. This pivoting movement places the blades  52  in the desired angular position. 
         [0019]    While the propeller is rotating, and in order to ensure that the propeller pitch remains constant, the speed of rotation of the wormscrew  22  thus needs to be exactly equal to the speed of rotation of the propeller  50 , so as to avoid any movement of the nut  26  along the screw  22 . 
         [0020]    Conversely, in order to change the pitch of the propeller, there must be a difference between the speeds of rotation of the propeller and of the wormscrew  22 . 
         [0021]    The direction in which the pitch of the propeller changes thus depends on the relationship between the speed of rotation of the wormscrew  22  and the speed of rotation of the propeller shaft  60 . 
         [0022]    When the wormscrew  22  is driven to rotate at a speed faster than that of the shaft  60 , the nut  26  moves in a first axial direction so as to vary the pitch of the propeller in a first direction; conversely, when the wormscrew  22  is driven in rotation at a speed less than that of the shaft  60 , the nut moves in a second axial direction so as to cause the pitch of the propeller  50  to vary in the opposite direction. 
         [0023]    The wormscrew  22  associated with the nut  26  thus constitutes a blade swivel device  20  enabling the blades  52  to be placed in a desired angular position, so that they occupy a position corresponding to the desired propeller pitch. The angular position of the wormscrew (more precisely its angular position relative to the propeller shaft  60 ) determines the pitch angle imposed on the blades  52 . 
         [0024]    In order to actuate the blade swivel device  20 , the propeller pitch control system  10  also presents a transmission  30  that is driven by a motor  40 . The motor  40  is an electric motor with a rotor  42  including in particular a shaft on which a gearwheel  44  is mounted. 
         [0025]    The transmission  30  is constituted by an epicyclic geartrain. It transmits the torque transmitted via the gearwheel  44  to the wormscrew  22  (rotary control element of the blade swivel device  20 ). 
         [0026]    The transmission  30  is constituted essentially by a ring  32  having two sets of teeth  321  and  322 , which ring constitutes its inlet member, and by a single planet wheel  34  that constitutes its outlet member  34 . The ring  32  is supported by the propeller shaft  60  by means of two ball bearings  33 . The set of teeth  321  is an outside set of teeth meshing with the gearwheel  44 : the gearwheel  44  of the rotor of the motor  40  drives the ring  32  via this set of teeth  321 . The set of teeth  322  is an inside set of teeth meshing with the planet wheel  34 . The planet wheel  34  is supported by the shaft  60 ; its axis  341  thus rotates together with the shaft  60 . 
         [0027]    The end of the wormscrew  22  that is remote from the wall  62  is constituted by a gearwheel  36 . The teeth of this gearwheel mesh with the teeth of the planet wheel  34 . 
         [0028]    Thus, the rotation of the rotor  42  of the motor  40  is transmitted to the wormscrew  22  by means of the transmission  30 . This transmission involves a transmission ratio R, i.e. the speed of rotation of the wormscrew  22  about the axis X is equal to the speed of rotation of the ring  32  multiplied by the coefficient R. 
         [0029]    The term “transmission ratio” is used herein to mean the ratio of the speeds of rotation respectively of the member driven by the outlet member of the transmission and of the inlet member of the transmission. 
         [0030]    The propeller pitch control system  10  is configured in such a manner that when the inlet member  32  of the transmission  30  is driven at an appropriate speed of rotation (which is a function of the speed of rotation of the shaft  60  and of the desired pitch), the transmission  30  drives the rotor  22  of the blade swivel device  20  at a speed of rotation such that the blade swivel device  20  places the blades  52  in the desired angular position. 
         [0031]    The propeller pitch control system  10  serves to vary the pitch of the propeller  50  and thus to modify the power demand on the engine of the airplane. 
         [0032]    Nevertheless, that system presents a drawback. 
         [0033]    Specifically, it requires an electric motor to operate continuously in flight. Specifically, the motor  40  is operating at all times in order to drive the rotation of the wormscrew  22  and ensure that it has a speed of rotation that is equal to the speed of rotation of the propeller shaft  60 , or that is at least close to that speed. As a result of operating continuously in that way, the control system  10  presents a high level of energy consumption and it presents wear that is relatively fast. 
       SUMMARY OF THE INVENTION 
       [0034]    The object of the invention is to remedy that drawback and to propose a propeller pitch control system of the type described in the introduction but that presents reduced energy consumption and low wear. 
         [0035]    This object is achieved by means of a control system for controlling the pitch of a propeller for a propeller having blades designed to be driven in rotation by a propeller shaft, each blade being configured to pivot about a substantially radial axis of the blade, the system comprising:
       the propeller shaft;   a blade swivel device comprising a rotary control element suitable for placing the blades in an angular position corresponding to a desired propeller pitch as a function of rotation of said rotary control element relative to the propeller shaft; and   a transmission presenting an outlet member that is coupled to rotate with the rotary control element of the blade swivel device;       
 
         [0039]    wherein:
       the transmission includes a variable speed drive having a drive rotor, a control rotor, and an outlet rotor;   said drive, control, and outlet rotors are coupled to rotate respectively with a drive member that is the propeller shaft, with a control member, and with the outlet member of the transmission, and consequently the speeds of rotation of the drive, control, and outlet rotors are proportional respectively to the speeds of rotation of the drive, control, and transmission outlet members; and   the variable speed drive is configured to drive the outlet rotor in rotation at a speed that is a first predetermined function of the speeds of rotation of the drive and control rotors.       
 
         [0043]    As a result, the speed of rotation of the outlet member of the transmission is a second predetermined function of the speeds of rotation of the drive and control members. This second function is deduced from the first function by taking account of the multiplicative coefficients that exist respectively between the speeds of rotation of the drive, control, and outlet rotors and the speeds of rotation of the drive, control, and outlet members of the transmission. 
         [0044]    In this control system, the variable speed drive serves to drive the outlet member of the transmission, directly or indirectly. By means of the variable speed drive, the mechanical power delivered by the transmission no longer comes from the motor, or at least does not come only from the motor, but also comes at least in part from the propeller shaft. Consequently, compared with the above-described propeller pitch control system, the system of the invention advantageously enables energy consumption to be reduced. 
         [0045]    The main difference between the propeller pitch control system of the invention, as defined above, and the system proposed by Document U.S. Pat. No. 8,167,553 lies in using a variable speed drive having two inlets, namely the drive rotor and the control rotor, which are coupled respectively to the drive and to the control members of the transmission, instead of merely transmitting the torque produced by an external motor (the motor  40 ). 
         [0046]    The variable speed drive is a device comprising three rotors that are configured to be put into rotation relative to one another. The function of the variable speed drive is to drive the outlet rotor in such a manner that the speed of rotation of this rotor is a function (the “first” function) of the speeds of rotation of the two inlet rotors. 
         [0047]    The drive rotors and the outlet rotor are coupled respectively to the drive and control members of the transmission and to the outlet member of the transmission. Furthermore, each of these rotors is either itself one of the drive, control, or outlet members, as the case may be, of the transmission, or else it is connected to one of the drive, control, or outlet members, as the case may be, of the transmission. 
         [0048]    When two members are said herein to be coupled, that means that the respective speeds of rotation of the two coupled members at any instant are proportional to each other. This coupling may be implemented by any means (a gear, a geartrain, belts, . . . ), which means may in particular be mechanical, magnetic, etc. 
         [0049]    The control system is arranged in such a manner that the drive member of the transmission is the propeller shaft; the control member of the transmission is designed to be driven by a motor or some other means, e.g. an electric motor. Since the drive member of the transmission is the propeller shaft, the control system of the invention can avoid consuming energy stored in the energy supplies of the machine (e.g. electrical energy stored in storage batteries). 
         [0050]    The transmission is thus arranged in such a manner as to transmit the rotary motion of the propeller shaft to the rotary control element of the blade swivel device. 
         [0051]    Advantageously, the transmission presents a controllable transmission ratio because of the presence of the variable speed drive with two inlets: the transmission ratio of the transmission can be controlled by varying the speed of rotation of the control member of the transmission, which is coupled with the control rotor of the variable speed drive. 
         [0052]    In one embodiment, the blade swivel device is arranged in such a manner that when the speeds of rotation of the propeller shaft and of the rotary control element of the blade swivel device are equal, then the propeller pitch does not vary. Advantageously, this embodiment enables the transmission ratio of the transmission to vary over a range situated around the value 1. 
         [0053]    In this embodiment, the propeller pitch is controlled by the control system as follows. 
         [0054]    If it is desired to conserve a propeller pitch that is constant, then the control member of the transmission is driven in rotation in such a manner that the transmission ratio is equal to one. The transmission then drives the rotary control element of the blade swivel device in rotation at the same speed as the propeller shaft, so the propeller pitch remains unchanged. 
         [0055]    Conversely, by varying the transmission ratio so that it is greater than or less than one, action is taken on the rotary control element of the blade swivel device so as to cause the pitch of the propeller to change. 
         [0056]    In an embodiment, the outlet rotor is coupled to rotate with the outlet member via a gear ratio device. In particular, this device may present a ratio S lying in the range 0.2 to 5, and more preferably in the range 0.6 to 2. 
         [0057]    In an embodiment, the transmission is arranged so that when the speed of rotation of the control member is zero, the rotary control element of the blade swivel device is driven at a speed of rotation for which the propeller pitch does not vary. 
         [0058]    By means of this arrangement, when it is desired to keep a propeller pitch constant, there is no need to actuate the transmission control member; in other words, if this member is actuated by a motor, there is then no need to actuate the motor. Consequently, and advantageously in this embodiment, the transmission does not consume any power during stages in which the pitch of the propeller is kept constant (or at least does not consume any power drawn by the control member of the variable speed drive). 
         [0059]    When the blade swivel device is arranged so that the speeds of rotation of the propeller shaft and of the rotary control element of the blade swivel device are equal, the propeller pitch does not vary, with this property being obtained as follows. 
         [0060]    Under such circumstances, the transmission needs to be arranged so that its transmission ratio is equal to one when the speed of rotation of the control member is zero. Thus, the speed of rotation of the outlet member is equal to the speed of rotation of the drive member (i.e. the propeller shaft) when the speed of rotation of the control member is zero; under such conditions, the propeller pitch does not vary. 
         [0061]    In an embodiment, the transmission is arranged in such a manner that the speed of rotation of the outlet member is the sum of the speed of rotation of the drive member plus a value that is a function of the speed of rotation of the control member. This value may in particular be proportional to the speed of rotation of the control member. 
         [0062]    Preferably, the value that is a function of the speed of rotation of the control member is zero when the speed of rotation of the control member is zero: it then follows that the speed of rotation of the outlet member, and thus of the rotary control element controlling the changes of pitch of the propeller, is equal to the speed of rotation of the propeller shaft when the speed of rotation of the control member is zero. 
         [0063]    This property is naturally particularly advantageous when the blade swivel device is arranged in such a manner that when the speeds of rotation of the propeller shaft and of the rotary control element of the blade swivel device are equal, then the propeller pitch does not vary. 
         [0064]    Various embodiments can be adopted for the blade swivel device. 
         [0065]    In an embodiment, the rotary control element of the blade swivel device is a wormscrew, and the blade swivel device further includes a nut arranged on the wormscrew. 
         [0066]    This nut is configured to be connected to the blades in such a manner that the axial position of the nut determines the angular positions of the blades. The nut is designed to move axially along the wormscrew when the wormscrew is caused to rotate about its longitudinal axis. 
         [0067]    In order to swivel the blades, in a preferred embodiment, the nut includes notches, each notch being configured to receive a blade attachment finger extending radially from a blade root along a radial attachment axis, and each attachment axis being offset relative to a radial opening for fastening the blade that is arranged in the periphery of the propeller shaft. 
         [0068]    The blade swivel device may in particular be analogous to the device disclosed in above-mentioned U.S. Pat. No. 8,167,553. 
         [0069]    In another embodiment, said rotary control element of the blade swivel device is a pitch adjustment rotor including one radial actuator rod for each blade, each actuator rod being configured to engage a blade and to impart a desired angular position thereto. A blade swivel device of this type is disclosed by way of example in patent FR 2 992 376. 
         [0070]    In one possibility concerning the above-mentioned embodiment, the propeller shaft also has a radial finger for each of the blades, each radial finger being engaged with a blade so as to constrain said blade to pivot about the axis of the radial finger; and for each blade, the actuator rod and the radial finger associated with the blade are situated axially at a distance apart from each other, so that rotation of the pitch adjustment rotor relative to the propeller shaft imparts a corresponding rotation on the blades about the axes of said radial fingers. 
         [0071]    Various embodiments may also be adopted for the transmission. 
         [0072]    In an embodiment, the transmission is arranged in such a manner that the drive member and the member driven by the outlet member of the transmission necessarily rotate in the same direction (it is thus assumed implicitly that their axes of rotation are parallel). The transmission ratio of the transmission is thus positive. This arrangement is optional but it makes the transmission easier to make. 
         [0073]    In an embodiment, when the outlet rotor is coupled in rotation with the outlet member via a geartrain, the variable speed drive and the geartrain are preferably arranged in such a manner that when the speed of rotation of the control member is zero, the transmission ratio (S) of the variable speed drive is equal to the reciprocal of the transmission ratio (R) of the geartrain. This arrangement is naturally particularly advantageous when the blade swivel device is arranged in such a manner that when the speeds of rotation of the propeller shaft and of the rotary control element of the blade swivel device are equal, then the pitch of the propeller does not vary. 
         [0074]    In an embodiment, the variable speed drive comprises a rotary mechanical differential. An example of a rotary differential that is suitable for use is disclosed in Document U.S. Pat. No. 6,547,689. A rotary differential is a gear device having three coaxial rotors that are configured to be set into rotation relative to one another. The rotary differential is configured so that the speed of rotation of the outlet rotor is a function of the speeds of rotation of the two inlet rotors. 
         [0075]    In another embodiment, the variable speed drive comprises a magnetic gear. An example of a magnetic gear suitable for use is disclosed in Document US 2011/0037333. 
         [0076]    In this embodiment, the three rotors of the variable speed drive (drive, control, and outlet rotors) are arranged coaxially and they constitute an inner rotor, an intermediate rotor, and an outer rotor, in the same order or in some other order. The intermediate rotor has magnetic poles interposed between the inner and outer rotors. In the magnetic gear, the speeds of rotation of the three rotors are related by a predetermined relationship. 
         [0077]    By way of example, in the gear disclosed in Document US 2011/0037333, the speeds of rotation of the three rotors are related by the following relationship: 
         [0000]        Wi+GWo −(1+ G ) Wp= 0  (1)
 
         [0000]    where Wi, Wo, and Wp are the respective speeds of rotation of the inner, outer, and intermediate rotors. 
         [0078]    On the basis of the equation relating the speeds of rotation of the three rotors of the variable speed drive, in a propeller pitch control system of the invention having a magnetic gear, the speed of rotation of the outlet member of the transmission can be determined as a function of the speeds of rotation of the drive and control members. Where appropriate, it may also be necessary to have knowledge as well of the multiplicative coefficients relating respectively the speeds of rotation of the drive, control, and outlet rotors of the variable speed drive with respect respectively to the speeds of rotation of the drive, control, and outlet members of the transmission. 
         [0079]    When the variable speed drive is a magnetic gear, it results from equation (1) that the speed of rotation of the outlet member of the transmission is the sum of a first term proportional to the speed of rotation of the drive member plus a second term proportional to the speed of rotation of the control member. Under such circumstances, and advantageously, the second term (function of the speed of rotation of the control member) is zero when the speed of rotation of the control member is zero. 
         [0080]    The transmission may in particular be arranged in such a manner that the first term is equal to the speed of rotation of the drive member when—as in the example described—, the blade swivel device is arranged in such a manner that when the speeds of rotation of the propeller shaft and of the rotary control element of the blade swivel device are equal, the pitch of the propeller does not vary. 
         [0081]    The rotors of the magnetic gear may be arranged in other ways, in particular with respect to the drive and control members of the transmission. 
         [0082]    In an embodiment, said drive, control, and outlet rotors are coaxial, and of them, the control rotor, is the outer rotor. The term “outer” rotor is used herein to mean that one of the rotors of the variable speed drive that has the largest radius. 
         [0083]    In an embodiment, the variable speed drive comprises three coaxial rotors, an inner rotor constituting the drive rotor, and an intermediate rotor constituting the outlet rotor. 
         [0084]    In another embodiment, the variable speed drive comprises three coaxial rotors, an intermediate rotor constituting the drive rotor, and an inner rotor constituting the outlet rotor. 
         [0085]    As mentioned above, each of the rotors may be connected to the drive, control, or outlet member of the transmission, as appropriate, by various means. 
         [0086]    In an embodiment, the outlet rotor is coupled to rotate with the outlet member via an epicyclic geartrain having two planet gears meshing with each other, carried by the propeller shaft, and meshing respectively with the outlet rotor of the variable speed drive and with the outlet member of the transmission. In an embodiment, the variable speed drive and the geartrain are arranged in such a manner that when the speed of rotation of the control member is zero, the transmission ratio (S) of the magnetic gear is equal to the reciprocal of the transmission ratio of the geartrain. (The term “transmission ratio” is used herein to mean the ratio between the speed of rotation of a member driven by the outlet member and the speed of rotation of the inlet member of a transmission.) 
         [0087]    Consequently, the speed of rotation of the member driven by the outlet member of the transmission is equal to the speed of rotation of the propeller shaft when the speed of rotation of the control member is zero. 
         [0088]    The control system of the invention may be used for propellers in air or in water. It can be integrated in a machine, for instance a turbine engine, in particular an aeroengine. 
         [0089]    The invention also relates to a propeller engine having a propeller and a control system as defined above. The invention also relates to an aircraft or a watercraft, e.g. a surface vessel or a submarine, including one or more engines of this type. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0090]    The invention can be well understood and its advantages appear better on reading the following detailed description of embodiments given as non-limiting examples. The description refers to the accompanying drawings, in which: 
           [0091]      FIG. 1 , described above, is a diagrammatic fragmentary longitudinal section view of an airplane propeller engine showing a prior art propeller pitch control system as disclosed in Document U.S. Pat. No. 8,167,553; 
           [0092]      FIG. 2  is a diagrammatic fragmentary longitudinal section view of a propeller engine including a system for controlling the pitch of the blades of the propeller in a first embodiment of the invention; and 
           [0093]      FIG. 3  is a diagrammatic fragmentary longitudinal section view of a propeller engine including a system for controlling the pitch of the blades of the propeller, in a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0094]    In the figures, elements that are identical or similar are either given the same references or else they are given references that differ by a multiple of 100. 
         [0095]      FIG. 2  shows an airplane propeller engine  100  having a propeller pitch control system  10  representing a first embodiment of the invention. 
         [0096]    The system  10  presents a general arrangement that is quite similar to that of the system described with reference to  FIG. 1 ; thus, the system  10  is not described in detail and the details of the description below relate to characteristics associated with the present invention. 
         [0097]    In this embodiment, the system  10  has a blade swivel device  20  identical to that described with reference to  FIG. 1 , serving to modify the pitch of each of the blades by pivoting them about their longitudinal axes. Any blade swivel system performing the blade swivel function can be used in the context of the invention. 
         [0098]    The system  10  also includes a transmission  30  that is arranged differently from the transmission of the control system described with reference to  FIG. 1 . 
         [0099]    In this embodiment, the transmission  30  includes a variable speed drive  70  and a geartrain  80 . 
         [0100]    The variable speed drive is constituted by a magnetic gear made up of three coaxial rotors, namely an outer rotor Ro, an intermediate rotor Rp, and an inner rotor Ri. 
         [0101]    The outer rotor Ro, which is of radius greater than the rotors Ri and Rp, is the control rotor. It is formed by a portion of a cylindrical tube placed coaxially outside the propeller shaft  60 , and it carries an outside set of teeth  72 . This outside set of teeth  72  meshes with the teeth of the gearwheel  44  of the outlet rotor  42  of an electric motor  40 . The motor  40  can thus drive the rotor Ro in rotation. 
         [0102]    The intermediate rotor Rp is the outlet rotor. It is likewise formed by a portion of cylindrical tube placed coaxially outside the propeller shaft  60 . 
         [0103]    The inner rotor Ri is the drive rotor. It is formed by the propeller shaft, and more precisely by the portion of the propeller shaft  60  that is situated in register with the intermediate rotor Rp. 
         [0104]    Permanent magnets Mo and Mi are fastened respectively in the outer rotor Ro and the inner rotor Ri (shaft  60 ). Magnetic poles P are placed in the intermediate rotor Rp. Ball bearings  74  are arranged respectively between the shaft  60  and the intermediate rotor Rp, and between the intermediate rotor Rp and the outer rotor Ro, in order to allow relative rotation between them. 
         [0105]    The inner, outer, and intermediate rotors Ri, Ro, and Rp are configured so as to constitute a magnetic gear  70 . 
         [0106]    When the motor  40  is actuated, its gearwheel  44  rotates and causes the control rotor or outer rotor Ro to rotate at a speed Wo. 
         [0107]    The propeller shaft rotates at a certain speed Wi; it drives the propeller  50  at this speed. 
         [0108]    Under the effect of the rotation of the inner rotor Ri at a speed Wi, and of the outer rotor at a speed Wo, the outlet rotor Rp of the gear  70  is driven in rotation at a speed Wp. This speed Wp can be deduced from the speeds Wo and Wi using formula (1). 
         [0109]    The geartrain  80  mainly comprises two planet gears  82  and  84  arranged respectively about a shaft  821  and a shaft  841 . The ends of these shafts are placed in bores arranged in the propeller shaft  60 , which means that the shafts  821  and  841  are both driven in rotation by the shaft  60 . In this embodiment, the use of two planet gears  82  and  84  causes the member driven by the outlet member of the transmission, namely the wormscrew  22 , to rotate necessarily in the same direction as the drive member of the transmission, namely the propeller shaft  60 . 
         [0110]    The outlet rotor or intermediate rotor Rp has an inside set of teeth  76  meshing with the teeth of the planet wheel  82 . The same teeth mesh in turn with the teeth of the planet wheel  84 . The teeth of the planet wheel  84  mesh in turn with the teeth of the gearwheel  36  of the wormscrew  22 . 
         [0111]    Consequently, the planet wheel  84  of the geartrain  80  (outlet member of the transmission  30 ) transmits rotation from the outlet rotor Ri of the variable speed drive  70  (magnetic bearing  70 ) to the wormscrew  22 , which constitutes the rotary control element of the blade swivel device  20 . 
         [0112]    The geartrain  80  is designed to have a transmission ratio equal to R, which ratio is equal to the ratio of the speeds of rotation of the wormscrew  22  (member driven by the outlet member of the transmission  30 , i.e. the planet wheel  84 ) relative to the inlet member of the geartrain  80 , i.e. the planet wheel  82 . 
         [0113]    Let the transmission ratio of the variable speed drive (relative to its drive member) be written S: this is the ratio of the speeds of rotation of the planet wheel  82  (member driven by the outlet member of the variable speed drive) relative to the drive member, the propeller shaft  60 . 
         [0114]    The overall transmission ratio of the transmission  30 , including both the variable speed drive  70  and the geartrain  80 , i.e. equal to the ratio of the speed of rotation of the wormscrew  22  relative to the speed of rotation of the propeller shaft  60 , is equal to the product S×R. 
         [0115]    In the embodiment described, the transmission  30  is arranged in such a manner that when the control rotor Ro is stopped, and remains in a stationary position (speed of rotation Wo zero), then the transmission ratio S of the variable speed drive  70  and the transmission ratio R of the geartrain  80  are reciprocals of each other (S×R=1). 
         [0116]    Furthermore, the blade swivel device  20  is arranged in such a manner that when the speeds of rotation of the propeller shaft  60  and of the wormscrew  22  (rotary control element of the blade swivel device  20 ) are equal, then the propeller pitch does not vary. 
         [0117]    Consequently, when the motor  40  is stopped, the wormscrew  22  is driven by the transmission  30  at exactly the same speed as the propeller shaft  60 . Consequently, when the motor  40  is stopped, the propeller pitch remains constant. 
         [0118]    Conversely, when the motor  40  is caused to rotate, that leads to a variation in the transmission ratio S of the variable speed drive  70 . The product S×R is then no longer equal to 1, and consequently there is variation in the speed of rotation of the wormscrew  22 . The difference in speed of rotation between the propeller shaft  60  and the wormscrew  22  thus causes the nut  26  to move, and consequently leads to the blades  52  swiveling, and thus to a change in the pitch of the propeller  50 . 
         [0119]    A second embodiment of the invention is described below with reference to  FIG. 3 . 
         [0120]    This embodiment differs from the first embodiment in two respects: the blade swivel device  120  and the transmission  130 . 
         [0121]    In this embodiment, the blade swivel device is similar to that disclosed in Document FR 2 992 376. 
         [0122]    Thus, whereas in the blade swivel device described with reference to  FIG. 2 , the blades were caused to swivel by axial movement of the nut  26  along the propeller axis X, in contrast, in the present embodiment the blades are caused to swivel by a movement in rotation, namely directly by turning a shaft  122  having radial actuator rods  126 . The shaft  122  thus constitutes a pitch adjustment rotor in the meaning of the invention. 
         [0123]    In particular, the shaft  122  is constrained to move in rotation with the transmission outlet member (planet wheel  84 ); it is configured in such a manner that its angular position relative to the propeller  50  determines the angular positions of the blades  52 . 
         [0124]    For this purpose, each of the blades  52  presents a radial bore  164  in its root. The propeller shaft  160  presents radial drive fingers  156 . The radial fingers  156  are arranged in such a manner that each blade  52  can be engaged on a radial finger  156 . The blades  52  are held in position relative to the radial fingers  156  in the bores  164  by ball bearings  154 . These bearings enable the blades  52  to pivot about the axes of the radial fingers  156 . 
         [0125]    The pitch of the propeller  50  is adjusted by means of the pitch adjustment rotor, i.e. the shaft  122 . 
         [0126]    This shaft is held coaxially inside the propeller shaft  160  by ball bearings  21 . 
         [0127]    At its end remote from the wall  62 , the shaft presents a gearwheel  36  identical to the gearwheel of the first embodiment. This gearwheel  36  is coupled to rotate with the planet wheel  84 , which constitutes the outlet member of the transmission  30 . The shaft  122  thus constitutes the rotary control element of the blade swivel device  120 . 
         [0128]    In order to cause the angular orientation of the blades  52  to vary, the shaft  122  is configured as follows. 
         [0129]    For each of the blades  52 , it has a radial actuator rod  126 . Each of these rods  126  projects from the shank  125  of the shaft  122  and passes through an oblong hole  127  formed in the shaft  160  (one oblong hole for each rod  126 ; the circumferential extent of the oblong hole is determined as a function of the amplitude of the angular pivoting of the shaft  122 ); and it extends into a bore  124  formed in the root of the blade  52 . 
         [0130]    When the shaft  122  turns about its axis X, the rods  126  perform the same movement in rotation. 
         [0131]    Consequently, controlling the angular position of the shaft  122  serves to adjust the pitch of the propeller  50 , as follows. 
         [0132]    For the pitch of the propeller to remain constant, the shaft  122  must be driven in rotation at the same speed as the propeller shaft  160 . Under such circumstances, there is no relative movement of the rods  126  relative to the propeller  50 , i.e. relative to the blades  52 . 
         [0133]    Conversely, in order to change the pitch of the propeller, it suffices to reduce or increase the speed of rotation of the shaft  122  a little relative to that of the propeller  50  (i.e. relative to the shaft  160 ) such that an angular offset occurs between the shaft  160  and the shaft  122 . 
         [0134]    When this angular offset occurs, the rods  126  engage the root of the blades  52  and thus cause them to pivot about their respective pivot axes Z, which are the axes of the radial drive fingers  156 . Consequently, the angular offset of the shaft  122  relative to the propeller  50  causes the orientation of the blades  52  to be changed, i.e. changes the pitch of the propeller. 
         [0135]    Thus, the angular position of the shaft  122 , acting as a pitch adjustment rotor, relative to the propeller  50  determines the angular positions of the blades  52 . 
         [0136]    In this second embodiment, the transmission  130  has a variable speed drive  170  and a geartrain  180  that are arranged in a manner different from the first embodiment. 
         [0137]    As in the first embodiment, the variable speed drive  170  is constituted by a magnetic gear having three coaxial rotors, namely an outer rotor Ro, an intermediate rotor Rp, and an inner rotor Ri. Nevertheless, the functions of the inner and intermediate rotors are interchanged. 
         [0138]    The outer rotor Ro is the control rotor. It is formed by a portion of cylindrical tube placed axially outside the propeller shaft  60  and carrying an outside set of teeth  72 . These outside teeth  72  mesh with the teeth of the wheel  44  of the outlet rotor  42  of an electric motor  40 . Thus, the motor  40  serves to drive the rotor Ro in rotation. 
         [0139]    The inner rotor Ri is the outlet rotor. It is formed by a portion of cylindrical tube placed coaxially inside the propeller shaft  60 . 
         [0140]    The intermediate rotor Rp is the drive rotor. It is formed by the portion of the propeller shaft  160  that is situated in register with the inner and outer rotors Ri and Ro. 
         [0141]    Permanent magnets Mo and Mi are fastened respectively in the outer rotor Ro and the inner rotor Ri (the shaft  60 ). Magnetic poles P are placed in the intermediate rotor Rp. Ball bearings  74  are arranged respectively between the shaft  160  and the intermediate rotor Rp, and between the intermediate rotor Rp and the outer rotor Ro, in order to enable relative rotation therebetween. 
         [0142]    The inner, outer, and intermediate rotors Ri, Ro, and Rp are configured so as to constitute the magnetic gear  170 . 
         [0143]    When the motor  40  is actuated, its gearwheel  44  rotates and causes the control rotor or outer rotor Ro to rotate at a speed Wo. 
         [0144]    The propeller shaft rotates at a certain speed Wp; it drives the propeller  50  at that speed. 
         [0145]    Under the effect of the rotation of the intermediate rotor Rp at the speed Wp, and of the outer rotor at a speed Wo, the outlet rotor Ri of the magnetic gear drive  170  is driven in rotation at a speed Wi. This speed is deduced from the speeds Wo and Wp using the formula (1). 
         [0146]    The geartrain  180  mainly comprises two planet wheels  82  and  84  that are arranged about respective shafts  821  and  841  supported by the propeller shaft  160 . 
         [0147]    The difference between the geartrain  80  and the geartrain  180  lies in particular in the fact that the geartrain  180  is driven by the inner rotor Ri of the magnetic gear  170 , whereas the geartrain  80  is driven by the intermediate rotor of the magnetic gear  70 . 
         [0148]    The intermediate rotor Ri has an inside set of teeth  176  meshing with the teeth of the planet wheel  82 . These teeth mesh in turn with the teeth of the planet wheel  84 . The teeth of the planet wheel  84  mesh in turn with the teeth of the gearwheel  36  of the shaft  122 . 
         [0149]    Consequently, the planet wheel  84  of the geartrain  180  (outlet member of the transmission  130 ) transmits the rotation of the outlet rotor Ri of the variable speed drive  170  (the magnetic gear  170 ) to the shaft  122 , i.e. the rotary control element of the blade swivel device  120 . 
         [0150]    The geartrain  180  is designed to have a transmission ratio equal to R, which ratio is equal to the ratio of the speed of rotation of the shaft  122  relative to the speed of rotation of the planet wheel  82 . 
         [0151]    Once more, let the transmission ratio of the variable speed drive as determined between the planet wheel  82  driven by the outlet member Ri of the variable speed drive  170  and the drive member of the variable speed drive, namely the propeller shaft  60 , be written S. 
         [0152]    The overall transmission ratio combining both the variable speed drive  170  and the geartrain  180  as measured between the propeller shaft  60  and the shaft  122  is equal to the product S×R. 
         [0153]    As in the preceding embodiment, in this second embodiment, when the control rotor Ro is in a fixed position (speed of rotation Wo zero), the transmission ratios S of the variable speed drive  170  and R of the geartrain  180  are advantageously reciprocals of each other (S×R=1). 
         [0154]    Consequently, when the motor  40  is stopped, the shaft  122  is driven by the transmission  130  at the same speed as the propeller shaft  60 , and the propeller pitch remains constant. 
         [0155]    Conversely, when the motor  40  rotates, the shaft  122  is driven in rotation at a speed that is slightly greater or slightly smaller than the speed of the propeller shaft  60 . As described above, this rotation leads to an angular offset of the shaft  122  (i.e. the fingers  126 ) relative to the propeller shaft  160  (radial fingers  156 ). This angular offset causes the pitch of the propeller to change. 
         [0156]    Although the present invention is described with reference to specific embodiments, it is clear that various modifications and changes may be undertaken on those embodiments without going beyond the general ambit of the invention as defined by the claims. In addition, individual characteristics of the various embodiments mentioned may be combined in additional embodiments. Consequently, the description and the drawings should be considered in a sense that is illustrative rather than restrictive.