Abstract:
A blade for a turbine engine propeller, in particular a propfan engine, comprising a protruding part on the leading edge thereof, wherein said blade comprises means for controlling the position of the protruding part along the leading edge thereof.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a blade for a turbine-engine propeller, in particular of the unducted-fan type, and to a corresponding propeller and turbine engine. 
       PRIOR ART 
       [0002]    Although the present invention is particularly suited to unducted-fan turbine engines, the implementation thereof is however not limited to such an application. 
         [0003]    As is known, an unducted-fan turbine engine may comprise two coaxial contra-rotating external propellers, respectively upstream (front) and downstream (rear), which are each rotated by a turbine and extend substantially radially outside the nacelle of the turbine engine. Each propeller usually comprises a hub concentric with the longitudinal axis of the turbine engine, on which blades are fixed. 
         [0004]    The aerodynamic interaction between the upstream and downstream contra-rotating propellers of such an unducted-fan turbine engine causes very high operating acoustic levels. This is because the rotation of the upstream and downstream contra-rotating propellers causes, among other things, the formation of: 
         [0005]    wakes along the span of the blades, downstream thereof; 
         [0006]    main vortices at the free end of the blades. 
         [0007]    These aerodynamic disturbances downstream of the upstream propeller are partly the cause of the interaction aerodynamic noise when they strike the downstream propeller or pass close thereto. 
         [0008]    In particular, during phases of low-speed operation of an unducted-fan turbine engine (such as, when it is mounted on an aircraft, takeoff, the climbing phase, landing and approach), the dominant contribution of the radiated noise comes from the interaction lines associated with the downstream propeller that functions in the stream of the upstream propeller, passing through the vortex layers consisting of wakes and main vortices formed by the blades of the upstream propeller (also referred to as upstream blades). When a marginal vortex of upstream blades interacts with the blades of the downstream propeller (otherwise referred to as downstream blades), the interaction between downstream blade and marginal vortex dominates the acoustic spectrum radiated for the majority of the directivities. 
         [0009]    Thus, in order to reduce the undesirable noise emissions of such turbine engines and thus meet the acoustic certification criteria imposed by the aviation authorities, it is necessary to reduce the low-speed radiated noise by reducing the interaction between downstream blade and marginal vortex. 
         [0010]    Currently, the most widespread known solution—referred to as clipping—consists of reducing the diameter of the downstream propeller so as to make the main vortices generated by the upstream blades pass outside the downstream blades in order to limit the interaction of the latter with the main vortices. This generally involves an increase in the chord of the downstream blades in order to maintain the desired traction and the torque ratio between the upstream and downstream propellers. Such a solution may be pushed to the extreme by very highly loading the end of the upstream blades, so as to relieve the remainder of each of the upstream blades in order to reduce the impact of the wake of the upstream propeller on the downstream propeller, also giving rise to undesirable interaction noise. 
         [0011]    However, such a solution proves to be acceptably only for an isolated configuration of the turbine engine (that is to say without any external element connected thereto) and without incidence. In the presence of elements (strut, fuselage) or incidence, the contraction and the axisymmetry of the flow of air behind the upstream propeller are modified, so that the clipping carried out no longer prevents the interaction of the downstream blades and the main vortices generated by the upstream blades. A greater reduction in the height of the downstream blades (corresponding to significant clipping) involves an increase in the chord associated with the downstream blades so as to preserve the load, which degrades the efficiency of the associated turbine engine and is therefore not satisfactory. 
         [0012]    The applicant proposed another solution to this problem, in the prior application FR 2 980 818. This other solution consists of equipping each upstream blade with a single protrusion on its leading edge, this protrusion being situated in a predetermined location in order to locally disturb, when the propeller rotates, the distribution of the circulation around each blade, so as to form two independent main vortices downstream: 
         [0013]    a first natural vortex (or marginal vortex) forming at the free end of the blade; 
         [0014]    a second distinct forced vortex (or supplementary main vortex) taking place in the vicinity of the protrusion. 
         [0015]    The marginal and supplementary vortices are co-rotating (that is to say they have the same direction of rotation) and remain independent of each other as far as the downstream propeller. In this way a modification to the distribution of the circulation around the single local position is modified and the result is the formation of two vortices—of lower intensity than the single marginal vortex observed in the prior art—that do not merge together. 
         [0016]    However, the applicant found that this other solution is not entirely satisfactory since it is not effective irrespective of the operating conditions, that is to say the various flight phases (takeoff, cruising, landing, etc.). This is because the rotation speed of the upstream propeller, the speed of travel of the aircraft equipped with this propeller, and the pitch angle of the blades of this propeller for example, have an influence on the path of the vortices from the leading edges of the blades. In the solution proposed in the prior application FR 2 980 818, the position of the protrusion on each blade is fixed and determined for a single flight phase, preferably takeoff, in order to reduce the noise nuisance for people living near the airport. 
         [0017]    The object of the present invention is thus to remedy this drawback and in particular to substantially reduce the noise radiated by an unducted-fan turbine engine with dual contra-rotating propellers by weakening the interaction between downstream propeller and main vortices, whatever the operating conditions. 
       DISCLOSURE OF THE INVENTION 
       [0018]    The invention thus proposes a blade for a turbine-engine propeller, in particular of the unducted-fan type, comprising a protrusion on its leading edge, characterised in that it comprises means for adjusting the position of the protrusion along its leading edge. Preferably, the protrusion is configured in order, when the propeller rotates, to disturb the distribution of the circulation around the blade, so as to form two independent main vortices downstream. 
         [0019]    Conventionally, a turbine-engine propeller blade comprises a suction side and a pressure side that are connected together, upstream, by a leading edge and, downstream, by a trailing edge. Upstream and downstream refer to the flow of the gases across the propeller, the leading edge being a leading edge for the gases and the trailing edge being a trailing edge for the gases. The blade further comprises a bottom or radially inner end, referred to as the root, and a top or radially outer end, referred to as the tip, the radial orientations being defined with respect to the rotation axis of the propeller, which may be the longitudinal axis of the turbine engine. 
         [0020]    The invention is particularly advantageous since it makes it possible to adjust the position of the protrusion on the leading edge of the blade, in particular according to the operating conditions. It can thus be envisaged for the protrusion on each blade to be in a first position (for example low) during takeoff of the aircraft comprising a turbine engine equipped with blades according to the invention, for it to be in a second position (for example intermediate) during the cruising flight of the aircraft, for it to be in a third position (for example upper) during the landing of the aircraft, etc. The protrusion may adopt at least two different positions, preferably a plurality of different positions, along the leading edge of the blade. Naturally the protrusions on the blades of the same propeller are preferably in the same position for an operating condition. 
         [0021]    The invention is particularly suited to unducted-fan turbine engines, but its implementation is however not limited to such an application. It may for example be applied to a turbine-engine ducted fan in order to limit interactions between the vortex generated by this fan with the aerodynamic structures downstream thereof. It can also apply to a propeller of a turboprop engine to limit interactions between the main vortices generated by this propeller with the wings of the aircraft. 
         [0022]    According to one embodiment of the invention, the protrusion is situated at the end of a finger that is guided in translation in a groove extending along part of the leading edge of the blade. The adjustment means are thus of the slide type, the groove forming a slide along which the protrusion can translate. 
         [0023]    Advantageously, the protrusion and the part of the leading edge comprising the groove are covered with a flexible membrane fixed to the blade. This membrane may be elastically deformable. This is designed so as to allow movement of the protrusion along the leading edge while locally preserving the aerodynamic surface quality of the profile of the blade. It provides in fact continuity of aerodynamic surface between the region of the leading edge in which the protrusion is situated and the rest of the leading edge, as well as between this region and the pressure face and the suction face of the blade. The membrane is preferably relatively thin (it has for example a thickness of between 1 mm and 5 mm). It may be produced for example from polymers covered with an erosion-resistant skin. 
         [0024]    The inventors found that the impact of the invention on the aerodynamic performance of the propeller is negligible. 
         [0025]    Advantageously, the blade comprises means for maintaining the volume contained between the membrane and the leading edge of the blade under vacuum. This enables the membrane best to follow the shape of the protrusion and of the leading edge of the blade. This makes it possible to preserve substantially the exact shape of the protrusion, whatever the position thereof on the leading edge. 
         [0026]    The blade preferably comprises means for lubricating the interface between the protrusion and the membrane. This facilitates the movement of the protrusion on the leading edge of the blade. 
         [0027]    According to one embodiment, the protrusion is connected to the piston of an actuator controlling the movement of the protrusion along the leading edge. The actuator may be a pneumatic or hydraulic actuator and is then connected to a source of pressurised fluid such as a gas (for example air) or oil. 
         [0028]    According to a variant embodiment, the protrusion is connected to an end of at least one cable, the opposite end of which is attached to a rotary shaft with a view to the coiling of the cable around said shaft. Said at least one cable may be guided by at least one pulley. 
         [0029]    According to another variant embodiment, the protrusion comprises a series of elements made from deformable material of the piezoelectric type. Each of said elements is preferably connected independently to electrical supply means. 
         [0030]    The present invention also relates to a propeller, in particular for an unducted-fan turbine engine, characterised in that it comprises a plurality of blades of the type specified above. 
         [0031]    The propeller preferably comprises a plurality of blades, wherein the protrusions on the blades are connected to the piston rod of a single actuator controlling the movement of these protrusions. 
         [0032]    The present invention also relates to a turbine engine, in particular of the unducted-fan type, characterised in that it comprises at least one propeller of the aforementioned type. 
         [0033]    The turbine engine may comprise a sensor, such as an acoustic receiver, mounted in the vicinity of the propeller and configured so as to transmit information to a computer, this computer being connected to means for actuating the means for adjusting the positions of the protrusions on the blades. 
         [0034]    Finally, the present invention relates to a method for reducing noise emissions of a turbine engine of the aforementioned type, characterised in that the position of the protrusion along the leading edge of each blade is adjusted according to the operating conditions of the turbine engine, and in particular according to a set of operating parameters comprising for example the rotation speed of the propeller and the pitch angle of its blades. 
         [0035]    Optimum positions for the protrusions on the blades may be predetermined for a plurality of operating conditions. The method may then consist, for a given operating condition, of putting the protrusions on the blades in a corresponding predetermined optimum position. 
         [0036]    The method may comprise a step of detecting vortices generated by the propeller or of detection of noise generated by the interaction of these vortices with a surface situated downstream, and a step of adjusting the position of the protrusions on the blades according to the signal (such as a noise level) detected at the previous step. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0037]    The accompanying figures will give a clear understanding as to how the invention can be implemented. In these figures, identical references designate similar elements. 
           [0038]      FIG. 1  is a schematic view in longitudinal section of an unducted-fan turbine engine equipped with upstream blades, according to an embodiment in accordance with the invention. 
           [0039]      FIG. 2  is an enlarged schematic view in elevation of an upstream blade of an unducted fan according to the prior art. 
           [0040]      FIG. 3  is an enlarged schematic view in elevation of an upstream blade of an unducted fan according to the invention. 
           [0041]      FIGS. 4 and 5  show an embodiment of the upstream blade of  FIG. 3 , in accordance with the present invention. 
           [0042]      FIG. 6  is a schematic view of a turbine-engine propeller comprising blades according to a variant embodiment of the invention. 
           [0043]      FIG. 7  shows a variant embodiment of a blade according to the invention. 
           [0044]      FIGS. 8 and 9  show another variant embodiment of a blade according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]      FIG. 1  shows, schematically and by way of non-limitative example, an unducted-fan turbine engine  1  in accordance with the invention, which comprises, from upstream to downstream, in the direction of flow of the gases (represented by the arrow F) inside the turbine engine of longitudinal axis L-L, a compressor  2 , an annular combustion chamber  3 , a high-pressure turbine  4  and two low-pressure turbines  5  and  6  that are contra-rotating, that is to say which turn in two opposite directions about the longitudinal axis L-L. 
         [0046]    Each of the low-pressure turbines  5  and  6  is constrained to rotate with an external propeller  7 ,  8  extending radially outside the nacelle  9  of the turbine engine  1 , the nacelle  9  being substantially cylindrical and extending along the axis L-L around the compressor  2 , the combustion chamber  3  and the turbines  4 ,  5  and  6 . The combustion gases emerging from the turbines are expelled through an exhaust nozzle  10  in order to increase the thrust. 
         [0047]    The propellers  7  and  8  are disposed coaxially one behind the other and comprise a plurality of blades  11 A and  11 B equiangularly distributed around the longitudinal axis L-L. The blades  11 A and  11 B extend substantially radially and are of the variable pitch type, that is to say they can turn about their longitudinal axis so as to optimise their angular position according to the required operating conditions of the turbine engine  1 . Naturally, in a variant, the blades of the propellers could also be of the fixed pitch type. 
         [0048]    Each upstream  7  or downstream  8  propeller comprises a rotary hub  12 ,  13  supporting the blades  11 A,  11 B and disposed concentrically with the longitudinal axis L-L of the turbine engine  1 , perpendicular thereto. 
         [0049]    The upstream  11 A and downstream  11 B blades are each formed by a blade body  14  and a blade root  15 , mounted so as to rotate on the corresponding hub  12 ,  13 . 
         [0050]      FIG. 2  shows the prior art as described in the prior application FR 2 980 818. In this prior art, each blade  11 A of the upstream propeller  7  comprises a single protrusion  16  formed on the leading edge  17  of the blade  11 A in question. 
         [0051]    This protrusion  16  has a rounded form and is defined by the following parameters: 
         [0052]    a fixed position on the span h, which is between 0.75 H and 0.85 H, with H the height of the upstream blade  11 A; 
         [0053]    a height on the span d, which is between 0.05 H and 0.2 H; and 
         [0054]    a chord width I, which is between c/16 and c/8, with c the length of the local chord of the blade at the position on the span h of said protrusion  16 . 
         [0055]    The protrusion  16  provides a disturbance of the distribution of the circulation around the upstream blade  11 A, which causes two co-rotating main vortices: 
         [0056]    a first natural vortex (or marginal vortex) forming at the free end  18  of the upstream blade  11 A; 
         [0057]    a second distinct forced vortex (or supplementary main vortex) taking place in the vicinity of the single protrusion  16 . 
         [0058]    The protrusion  16  also causes the formation of contra-rotating auxiliary vortices (that is to say in an opposite direction to the two marginal and supplementary vortices) that are inserted between the two co-rotating main vortices, thus preventing merging thereof before impacting the downstream propeller  8 . 
         [0059]    In other words, when the upstream propeller  7  rotates, the protrusion  16  locally disturbs the distribution of the circulation around the upstream blade  11 A, so as to form two independent main vortices downstream and which remain as far as the downstream propeller  8 . 
         [0060]    This solution makes it possible to divide the acoustic source into two sources out of phase, which leads to a reduction in the interaction noise. 
         [0061]    The invention, the principle of which is shown schematically in  FIG. 3 , represents an improvement to this technology. 
         [0062]    The inventors have found that the target traction (for a relevant flight point, which represents the force necessary for moving the aircraft), in particular in the case of a pair of propellers, can be achieved by means of various combinations of parameters such as: the rotation speed of the propellers and the pitch angle of their blades. A different distribution of circulation around the upstream blade  11 A corresponds to each combination of parameters. It is therefore necessary to adapt the position of the protrusion  16  to the flight point in question in order to act as effectively as possible. Studies have shown that the position of the protrusion  16  is decisive in obtaining the required effect. This is because this protrusion  16  has the effect of influencing the generation of vortices by the leading edges of the blades. However, these depend, in terms of position and intensity, on the flight parameters, such as the speed of travel of the aircraft, the speed of rotation of the propellers and the angular pitch of the blades. The inventors have in fact found that the paths of the vortices of the leading edges on the suction faces of the blades depend on the flight configurations, and therefore that the position of the vortices varies according to the flight parameters. It would therefore be necessary for the position of the protrusions to be adapted to the position of the vortices in order to significantly reduce the interaction noise during the various flight phases. 
         [0063]    The solution proposed consists of a protrusion  16  located on the leading edge  17  of the blade  11 A, the positioning (arrows  19 ) of which along this leading edge, that is to say along the span h of the blade, can be adapted to the flight point (takeoff, flight over, cruising, approach, etc.). The solution thus meets the aforementioned requirement. 
         [0064]    For this purpose, the invention proposes equipping the blade  11 A with means for adjusting the position of the protrusion  16  along its leading edge  17 . 
         [0065]      FIGS. 3 and 4  show a non-exclusive embodiment of the invention in which the adjustment means are of the runner type. 
         [0066]    The protrusion  16  is here formed by a dome and is carried by a finger  20  that is guided in a groove  21  extending along part of the leading edge  17  of the blade  11 A. 
         [0067]    The protrusion  16  can be brought to, and held, in any position on the leading edge  17 , between two respectively bottom ( FIG. 3 ) and top ( FIG. 4 ) extreme positions. In the case where the optimum position of the protrusion  16  can vary between 0.75 H and 0.85 H (H being the height of the upstream blade  11 A) according to the operating positions, the bottom extreme position in  FIG. 3  is situated at 0.75 H and the top extreme position in  FIG. 4  is situated at 0.85 H. 
         [0068]    The protrusion  16  and the part of the blade  11 A extending around the groove  21  are covered with a membrane  22 , preferably flexible and thin, which is intended to follow the shape of the protrusion  16  and of the leading edge  17  in order to ensure continuity of aerodynamic surface between the protrusion  16  and the rest of the blade and to limit pressure drops in operation. The part of the membrane  22  covering the protrusion  16  defines a boss that reproduces, preferably as faithfully as possible, the shape and dimensions of the protrusion  16 . The movement of the protrusion  16  along the leading edge  17  causes a deformation, preferably elastic, of the membrane  22 . The boss defined by the membrane  22  then moves, following the protrusion  16 . 
         [0069]    As shown schematically by the drawings, the blade  11 A is preferably equipped with: 
         [0070]    firstly means  23  for lubricating the interface  24  between the protrusion  16  and the membrane  22 , for example by the injection of lubricating oil at this interface, in order to limit the friction forces between the protrusion  16  and the membrane  22  that may oppose the movement of the protrusion, 
         [0071]    and secondly means  25  for putting under vacuum the space contained between the membrane  22  and the protrusion  16 , and preferably also between the membrane and the part of the blade covered by the membrane  22 . These means  25  are for example means for aspirating gas intended to maintain a negative pressure in the aforementioned space, so that the membrane remains pressed against the protrusion and the blade. 
         [0072]    It will be understood that the blade  11 A is, in the example shown, at least partly hollow and comprises at least one internal cavity for housing the aforementioned means  23 ,  25 . 
         [0073]    The finger  20  is connected to actuation means that comprise, in the example shown, a control actuator  26  of the pneumatic or hydraulic type. The actuator  26  comprises a cylinder  27  secured to the blade  11 A and a piston rod  28  that is connected to the finger  20 . The protrusion  16  is moved from one position to another along the leading edge  17  of the blade  11 A by movement of the piston rod  28  relative to the cylinder  27  of the actuator  26 , the piston rod  28  being able to emerge from the cylinder  27  or retract into this cylinder  27 . 
         [0074]    Conventionally, the end of the piston rod  28  opposite to the finger  20  carries a disc  29  for separating two internal chambers, respectively front and rear, of the cylinder  27 . Each chamber is connected to means for supplying pressurised fluid (gas, oil, etc.) and discharging this fluid, in order to cause the movement of the piston rod  28  relative to the cylinder  27  and therefore the movement of the protrusion  16 . The supply and discharge means comprise here fluid conduits  30  that are intended to be connected to a pump  31  and to a fluid source  32  preferably situated outside the blade. 
         [0075]    The pump  31  is actuated by a computer  33  that thus controls the movement and position of the protrusion  16  on the blade  11 A. 
         [0076]    As shown in  FIGS. 3 and 4 , each blade  11 A of the propeller may be equipped with its own actuator  26 . In a variant and as shown in  FIG. 5 , a single actuator  34  makes it possible to control, for example by means of a linkage system, the movement of the protrusions  16  on all the blades  11 A of the propeller  7 , which are also at least partly hollow. This actuator  34  may be mounted in the nacelle  9  of the turbine engine  1 . 
         [0077]    The optimum position of the protrusion  16  on the leading edge  17  of a blade  11 A may: (i) either be defined upstream of the design by means of digital computations, recorded in the engine flight commands, and managed by the computer  33 , (ii) or be determined during the flight by means of the computer  33 . 
         [0078]    In the first case (i), the optimum positions of the protrusions  26  on the blades  11 A, which at a time t, must all be identical, are computed and predetermined according to the various flight points in order to optimise the required purpose, namely reducing the noise nuisances related to the interaction of the main vortices generated by the blades of the upstream propeller  7  with those of the downstream propeller  8 . It is considered that each flight point or each operating condition is defined by a set of a plurality of parameters, including the rotation speed of the propeller, the speed of travel of the aircraft equipped with this propeller, and the pitch angle of the blades of the propeller. Thus a pre-programmed position is available for each set of parameters. It will thus be understood that the computer  33  will control the movement of the protrusions  16  on the blades according to the current flight point. 
         [0079]    The other case (ii) may consist of equipping the turbine engine  1  with at least one sensor  35  such as a pressure sensor or an acoustic receiver. The computer  33  then comprises a control algorithm for adjusting the position of the protrusions  16  so as to minimise the acoustic signal perceived by the sensor  35 . The sensor  35  is preferably positioned close to the region of impact of the vortices, for example on one of the blades  11 B of the downstream propeller  8 , as shown in  FIG. 5 . 
         [0080]    The above description refers to an unducted-fan turbine engine. Although the invention is particularly suited to such a turbine engine, it is not limited to this application and can be applied to other types of turbine engine such as a turboprop engine or a ducted-fan turbine engine. 
         [0081]    In the case of a turboprop engine, the invention can be applied to the propeller of this turboprop engine so as to limit the noise nuisances related to the interaction of the main vortices generated by the propeller with the fuselage of the aircraft and/or with the nacelle of the turboprop engine. The aforementioned sensor  35  may thus be mounted on the fuselage of the aircraft or the nacelle of the turboprop engine. 
         [0082]    In the case of a ducted-fan turbine engine, the invention can be applied to the fan propeller so as to limit the noise nuisances related to the interaction of the main vortices generated by this propeller with the strut connecting the turbine engine to the aircraft. The aforementioned sensor  35  may thus be mounted on the strut. 
         [0083]      FIG. 7  is a view corresponding to  FIG. 3  and depicting a variant embodiment of the invention and more particularly a variant embodiment of the means for adjusting the position of the protrusion  16  along the leading edge  17  of the blade  11 A, which are here of the cable  40  type. 
         [0084]    The protrusion  16  is here formed by a dome and is carried by a finger  20  that is guided in a groove extending along a part of the leading edge  17  of the blade  11 A. 
         [0085]    The protrusion  16  may be brought into, and held, in any position on the leading edge  17 , between two respectively bottom and top extreme positions. In the case where the optimum position of the protrusion  16  may vary between 0.75 H and 0.85 H (H being the height of the upstream blade  11 A) according to the operating conditions, the bottom extreme position is preferably situated at 0.75 H and the top extreme position is preferably situated at 0.85 H. 
         [0086]    The protrusion  16  and the part of the blade  11 A extending around the groove are covered with a membrane  22 , preferably flexible and thin, which is intended to follow the form of the protrusion  16  and of the leading edge  17  in order to provide continuity of aerodynamic surface between the protrusion  16  and the rest of the blade and to limit pressure drops in operation. The part of the membrane  22  covering the protrusion  16  defines a boss that reproduces, preferably as faithfully as possible, the form and dimensions of the protrusion  16 . The movement of the protrusion  16  along the leading edge  17  causes a deformation, preferably elastic, of the membrane  22 . The boss defined by the membrane  22  then moves, following the protrusion  16 . 
         [0087]    As shown schematically in the drawings, the blade  11 A is equipped with at least one cable  40 , one end of which is attached to the finger  20  and the opposite end of which is attached to a rotary shaft  42  and is configured firstly so as to coil around the shaft when the latter turns in a first direction about its rotation axis, and secondly to uncoil when the shaft turns in a second, opposite direction. In the example shown, the coiling of the cable  40  around the shaft  42  causes a movement of the protrusion  16  towards the low position, and an uncoiling of the cable causes a movement of the protrusion towards the high position. The latter movement is made possible in operation by the centrifugal forces to which the protrusion  16  is subjected, related to the rotation of the propeller. This is because, in operation, the protrusion  16  is subjected to a continuous force, oriented towards the direction opposite to the tension of the cable  40 . 
         [0088]    It will be understood that the blade  11 A is, in the example shown, at least partially hollow and comprises at least one internal cavity housing the cable  40 . 
         [0089]    It is possible to mount, in the cavity of the blade  11 A, one or more pulleys  44  for guiding the cable, in order to facilitate the kinematics of the system and to optimise the forces exerted on the sliding connection between the protrusion  16  and the blade  11 A. 
         [0090]      FIGS. 8 and 9  are views corresponding to  FIG. 3  and showing another variant embodiment of the invention and more particularly a variant embodiment of the means for adjusting the position of the protrusion  16  along the leading edge  17  of the blade  11 A, which here have a deformable material of the piezoelectric type for example. Such a material is a material that deforms when it is subjected to an electric current. 
         [0091]    The protrusion  16  is here formed by a series of elements  46  produced in such a material and disposed alongside one another along a part of the leading edge  17  of the blade  11 A. 
         [0092]    The form of the protrusion  16  can be modified and may comprise a protrusion at any position on the leading edge  17 , between two extreme positions respectively top and bottom. In the case where the optimum position of the protrusion  16  can vary between 0.75 H and 0.85 H (H being the height of the upstream blade  11 A) according to the operating conditions, the bottom extreme part is preferably situated at 0.75 H and the top extreme part in  FIG. 8  is preferably situated at 0.85 H ( FIG. 9  shows an intermediate position). 
         [0093]    The elements  46  of the protrusion  16  are covered with a membrane  22 , preferably flexible and thin, which is intended to follow the form of the protrusion  16  and of the leading edge  17  in order to provide continuity of aerodynamic surface between the protrusion  16  and the rest of the blade and to limit the pressure drops in operation. The part of the membrane  22  covering the protrusion  16  defines a boss that reproduces, preferably as faithfully as possible, the form and dimensions of the protrusion  16 . The deformations of the protrusion  16  along the leading edge  17  cause a deformation, preferably elastic, of the membrane  22 . The boss defined by the membrane  22  then moves following the protrusion  16 . 
         [0094]    As shown schematically in the drawings, the blade  11 A is equipped with means  48  for electrical supply to the elements  46 , said means  48  being connected to the elements by electric wires  50 . Each element can be supplied independently so as to obtain the required form of the protrusion  16 . 
         [0095]    It will be understood that the blade  11 A is in the example shown at least partly hollow and comprises at least one internal cavity housing the wires  50  or even the supply means  48 .