Variable pitch vane for a turbomachine fan having a stiffness gradient in the root

Abstract

Disclosed is a variable pitch vane for a turbomachine fan comprising a blade (60) and a root (62). The root (62) comprises a bulb (72) and an upright support (74) connecting the bulb (72) to the blade (60), the bulb (72) being connected to the upright support (74) by a neck (82) defining a local minimum of the cross-section of the root (62) along a plane orthogonal to the pitch axis. At least one section of the root (62) including the neck (82) and at least a portion of the upright support (74) comprises a surface layer and an inner layer between the pitch axis and the surface layer, the inner layer having a stiffness strictly greater than the stiffness of the surface layer.

Description

[0001] DESCRIPTION

[0002] TITLE: VARIABLE PITCH BLADE FOR TURBOMACHINE FAN WITH STIFFNESS GRADIENT IN THE ROOT

[0003] FIELD OF THE INVENTION

[0004] The present invention relates generally to the field of variable-pitch blades for turbomachine fans, of the type comprising a blade capable of extending into an air flow and a root configured to be inserted into a cell of a fan hub. It relates more particularly to the field of variable-pitch blades made of composite material.

[0005] A preferred field of application of the invention is that of turbojets with unducted fans (better known under the English names "propfan", "open rotor" and "unducted single fan"). However, the invention also applies to turbojets with ducted fans and to turboprops with one or more pusher propellers.

[0006] BACKGROUND

[0007] One of the avenues currently being explored to improve the specific consumption of civil aircraft engines is the development of unducted fan turbojets, such as that described in document FR 3 080 322 A1. These turbojets comprise a conventional turboshaft gas generator, one or more turbine stages of which drive one or more unducted fans extending outside the engine nacelle.

[0008] The advantage of these unducted fan engines is that the fan diameter is not limited by the presence of a shroud, so it is possible to design an engine with a high bypass ratio, and therefore reduced fuel consumption. Thus, in this type of engine, the fan blades can have a large span.

[0009] Furthermore, these engines generally include a mechanism for modifying the angular position of these blades (called the pitch angle) in order to adapt the thrust generated by the fan according to the different phases of flight. In order to facilitate this pivoting of the blades and reduce the size of said blades at the hub, their root most often extends over only part of the chord length of the blade.

[0010] In use, the blades equipping such turbomachines are subjected to numerous forces, including in particular the so-called 1 P forces (also called 1 P loads). These forces are cyclic forces resulting from the difference between the direction of incidence of the air flow, which is not guided by the fairing, and the axis of rotation of the fan (engine axis itself positioned with respect to the aircraft axis relative to the air flow). They induce a bending load on the blade, in particular in the interface zone of the blade with the fan disc, which is very significant. Due to the cyclic nature of these forces (the 1 P load of a blade changes each time a blade passes from one position to the diametrically opposite position), they induce accelerated fatigue of the blade. There is therefore a tendency to very strongly limit the admissible 1 P forces to guarantee the service life of the blades.

[0011] Currently, these blades are generally made of metallic material. While blades made of metallic material have good mechanical strength, they nevertheless have the disadvantage of having a relatively high mass. In order to reduce this mass, it is desirable to be able to manufacture these blades composed at least in part of a composite material structure comprising a fiber reinforcement densified by a polymer matrix. However, conventional composite material blade architectures do not allow for the combination of resistance to aerodynamic forces, in particular 1P forces, to which these blades would be subjected, aerodynamic performance of the blades and limitation of the size at the hub (translated by a given geometry).

[0012] STATEMENT OF THE INVENTION

[0013] One objective of the invention is to improve the resistance of fan blades to the aerodynamic forces to which they are subjected, for example to 1 P forces, without degrading their aerodynamic performance. Another objective is to lighten fan blades without degrading their resistance to the aerodynamic forces to which they are subjected, for example to 1 P forces, or their aerodynamic performance.

[0014] To this end, the invention relates, according to a first aspect, to a variable-pitch blade for a turbomachine fan, comprising an aerodynamically profiled blade and a root configured to be inserted into a cell of a fan hub, the blade being able to pivot relative to a frame of the fan hub around a pitch axis, the root having a peripheral skin surface and comprising a bulb and a stilt connecting the bulb to the blade, the bulb being connected to the stilt by a neck defining a local minimum of the section of the root along a plane orthogonal to the pitch axis, in which at least one section of the root comprises a surface layer delimiting at least partly the skin surface and an internal layer comprised between the pitch axis and the surface layer, the surface layer having a first stiffness and the internal layer having a second stiffness strictly greater than the first stiffness,the section of the foot including the collar and at least part of the stilt.,

[0015] According to particular embodiments of the invention, the variable-pitch blade has one or more of the following characteristics, taken in isolation or in any technically possible combination(s): the first stiffness comprises a longitudinal stiffness of the surface layer and the second stiffness comprises a longitudinal stiffness of the inner layer; the stilt flares from the neck towards the blade; the root section extends over at least 30% of the height of the root, measured parallel to the pitch axis; the blade has a proximal end connecting to the root, a free distal end, a leading edge, a trailing edge and a chord connecting the leading edge to the trailing edge, and the bulb has a maximum radius, measured in a direction perpendicular to the pitch axis, less than 50%, preferably less than 25%, of the length of the chord of the blade at its proximal end;the blade has at its proximal end a maximum thickness, the maximum radius of the bulb being greater than said maximum thickness; the bulb has a shape of revolution around the wedging axis; the surface layer and the internal layer each extend over the entire height of the root section; the internal layer is constituted by a central layer extending from the wedging axis towards the surface layer; the root section comprises an intermediate layer interposed between the surface layer and the central layer, the intermediate layer having a third stiffness strictly greater than the first stiffness and strictly less than the second stiffness; the third stiffness comprises a longitudinal stiffness of the intermediate layer; the stiffness of the root section increases progressively from the surface layer to the internal layer;the foot section comprises a central layer extending from the wedging axis towards the surface layer, the internal layer being constituted by an intermediate layer interposed between the surface layer and the central layer; the stiffness of the foot section decreases between the intermediate layer and the wedging axis; the first stiffness is less than or equal to 50%, preferably less than or equal to 25%, of the second stiffness; the surface layer has at each point a thickness, measured in a direction perpendicular to the wedging axis and passing through said point, of between 1 and 25% of the radius of the foot section measured in this same direction; the surface layer is made of composite material; the internal layer is constituted by a metal structure, for example steel or titanium;the surface layer is made of a woven composite and the inner layer is made of a laminate of unidirectional plies whose fibers are oriented substantially parallel to the pitch axis; the blade comprises a composite material structure obtained by three-dimensional weaving of warp strands, oriented substantially parallel to the pitch axis, and weft strands, at least one of the surface layer and the inner layer being made of the composite material structure; the composite material structure has a first warp-to-weft ratio in the surface layer and a second warp-to-weft ratio, greater than the first warp-to-weft ratio, in the inner layer; and the warp strands have lower weaving in the inner layer than in the surface layer.;

[0016] The invention also relates, according to a second aspect, to a turbomachine fan comprising a fan hub and a plurality of variable-pitch blades as defined above.

[0017] The invention also relates, according to a third aspect, to a turbomachine comprising such a fan.

[0018] According to a particular embodiment of the invention, the turbomachine has the following characteristic: the turbomachine is a turbomachine with an unducted fan.

[0019] Finally, according to a fourth aspect, the invention relates to an aircraft comprising such a turbomachine. BRIEF DESCRIPTION OF THE FIGURES

[0020] Other characteristics and advantages of the invention will appear on reading the following description, given solely by way of example and with reference to the appended drawings, in which: Figure 1 is a top view of an aircraft according to an exemplary embodiment of the invention, Figure 2 is a simplified view in partial longitudinal section of a turbomachine of the aircraft of Figure 1, Figure 3 is a simplified view in partial longitudinal section of a part of a fan of the turbomachine of Figure 2, Figure 4 is a perspective view of a root of the blade of Figure 3, and Figures 5 to 11 are simplified views in longitudinal section of different variants of a section of the blade root of Figure 3.

[0021] DETAILED DESCRIPTION

[0022] The aircraft 10 shown in Figure 1 includes turbomachines 12 to propel it.

[0023] In the example shown, the aircraft 10 is an airplane. This aircraft comprises, in a conventional manner, a fuselage 14, a tailplane 16 and two wings 18. The turbomachines 12 are here two in number and are each housed under a respective wing 18. As a variant (not shown), the turbomachines 12 are arranged along the fuselage 14, for example near the tailplane 16. As a further variant (also not shown), the aircraft 10 comprises a single turbomachine 12 or at least three turbomachines 12.

[0024] One of the turbomachines 12 is shown in Figure 2.

[0025] As visible in this Figure, the turbomachine 12 is elongated along a longitudinal axis X. It typically has angular symmetry around said longitudinal axis X, that is to say that there is at least one angle for which the turbomachine is invariant by rotation around the longitudinal axis X.

[0026] Here and hereinafter, the terms “interior” and “exterior”, “internal” and “external”, as well as their variations, are understood with reference to the X axis, an element described as “interior” or “internal” being oriented towards the X axis while an “exterior” or “external” element is oriented opposite the X axis. The turbomachine 12 comprises, in a conventional manner, a nacelle 20, an internal vein 22 for circulating an air flow through the nacelle 20, a combustion chamber 24 housed in the vein 22, an engine body 26 and a gas exhaust nozzle 28.

[0027] In the following, the terms “upstream” and “downstream” are understood to refer to a direction of flow of an air flow through the vein 22.

[0028] The engine body 26 comprises a compressor 30, a turbine 32 and a transmission shaft 34 coupling the turbine 32 to the compressor 30 for driving the compressor 30 by the turbine 32. The compressor 30 is arranged upstream of the combustion chamber 24 and supplies the combustion chamber 24 with compressed air. The turbine 32 is arranged downstream of the combustion chamber 24 and receives the exhaust gases leaving the combustion chamber 24.

[0029] The transmission shaft 34 has the longitudinal axis X as its axis of rotation.

[0030] The transmission shaft 34 is guided in rotation relative to the nacelle 20 by means of bearings (not shown).

[0031] In the example shown, the turbomachine 12 is a multi-body turbomachine, in particular a double-body turbomachine, comprising a low-pressure body 40 in addition to the engine body 26. The engine body 26 then constitutes a high-pressure body, the compressor 30 being a high-pressure compressor, the turbine 32 being a high-pressure turbine and the transmission shaft 34 being a high-pressure shaft.

[0032] The low pressure body 40 comprises a low pressure compressor 42, a low pressure turbine 44 and a low pressure shaft 46 coupling the low pressure turbine 44 to the low pressure compressor 42 for driving the low pressure compressor 42 by the low pressure turbine 44.

[0033] The low pressure compressor 42 is arranged upstream of the high pressure compressor 30 and supplies the latter with compressed air. The low pressure turbine 44 is arranged downstream of the high pressure turbine 32 and receives the exhaust gases leaving the latter.

[0034] The low pressure shaft 46 is guided in rotation relative to the nacelle 20 by means of bearings (not shown).

[0035] The low pressure shaft 46 is coaxial with the high pressure shaft 34. It therefore also has the longitudinal axis X as its axis of rotation. In particular, the low pressure shaft 46 extends inside the high pressure shaft 34.

[0036] The turbomachine 12 also comprises a fan 50 for driving the air flow in an external circulation vein 52 surrounding the nacelle 20. A primary air flow A (hot) is thus distinguished, consisting of the portion of the air flow driven in the internal circulation vein 22, and a secondary air flow B (cold), consisting of the portion of the air flow driven in the external circulation vein 52. The fan 50 comprises a fan rotor 54. This fan rotor 54 is rotatably mounted relative to the nacelle 20 around the longitudinal axis X. It comprises a hub 55 (Figure 3) provided with fan blades 56 extending substantially radially outward from the hub 55. These blades 56, when they are rotated, drive the air flow in the external circulation vein 52.

[0037] The fan rotor 54 is driven in rotation by the low-pressure turbine 44, via the low-pressure shaft 46. In the example shown, this drive is direct, that is to say that the fan rotor 54 is integral in rotation with the low-pressure shaft 46. In a variant (not shown), this drive is done via a reduction gear allowing the fan rotor 54 to rotate at a speed lower than that of the low-pressure shaft 46.

[0038] In the example shown, the fan 50 also comprises a fan stator 58 comprising fixed blades 59 arranged at the periphery of the nacelle 20, in the external circulation vein 52, along a plane orthogonal to the longitudinal axis X. This fan stator 58 is here arranged downstream of the fan rotor 54. As a variant (not shown), the fan 50 comprises, instead of the fan stator 58, a counter-rotating fan rotor.

[0039] Advantageously, the fan 50 is, as shown, unducted, that is to say that the external circulation vein 52 has no peripheral delimitation. The turbomachine 12 is then constituted, as shown, by a turbojet engine with an unducted fan or, alternatively, by a turboprop. As a variant (not shown), the external circulation vein 52 is defined between the nacelle 20 and a fan casing surrounding the fan 50; the turbomachine 12 is then typically constituted by a turbojet engine with a high bypass ratio, the bypass ratio being defined as the ratio of the flow rate of the secondary flow B (cold) to the flow rate of the primary flow A (hot).

[0040] In the example shown, the turbomachine 12 is in particular of the “puller” type, that is to say that the fan 50 is arranged upstream of the internal circulation vein 22 and also drives the air flow in the latter. In a variant (not shown), the turbomachine is of the “pusher” type, that is to say that the fan 50 is placed around the downstream half of the nacelle 20.

[0041] One of the blades 56 of the fan rotor 54 is illustrated schematically in Figure 3. As visible in this Figure, it is elongated in a direction of elongation Y which is substantially radial, that is to say perpendicular to the longitudinal axis X. In what follows, the term "height" will denote a distance along the elongation axis Y.

[0042] The blade 56 comprises a blade 60 with an aerodynamic profile and a root 62. The blade 60 extends radially outside a casing 63 of the hub 55 which internally delimits the external circulation vein 52. The blade 60 is thus able to extend into the air flow circulating in said vein 52. It is shaped so as to generate lift when it is moved in said air flow.

[0043] The blade 60 has a proximal end 64 for connection to the root 62, a free distal end 65, a leading edge 66, a trailing edge 67 and a chord (not shown), orthogonal to the elongation axis Y, connecting the leading edge 66 to the trailing edge 67.

[0044] The blade 60 also has a lower surface 68 (Figure 2) and an upper surface 69 (Figure 2). In the following, the term “thickness” will be used to denote a distance in a plane normal to the elongation axis Y and along an axis extending between the lower surface 68 and the upper surface 69.

[0045] Referring to Figure 4, the foot 62 has a peripheral skin surface 70. It also includes a bulb 72 and a stilt 74.

[0046] The bulb 72 constitutes a radially internal part of the root 62. It delimits an internal end 76 of the blade 56, that is to say the end of the blade 56 closest to the axis X. It extends from said internal end 76 to a joining surface 78 connecting the bulb 72 to the stilt 74. This joining surface 78 is typically discoidal.

[0047] The bulb 72 flares from the joining surface 78 opposite the blade 60, thus delimiting a bearing surface 80 oriented towards the blade 60. The bearing surface 80 constitutes a part of the skin surface 70.

[0048] The bulb 72 has a maximum radius, measured in a direction perpendicular to the elongation axis, less than 50%, preferably less than 25%, of the length of the chord of the blade 60 at its proximal end 64. This maximum radius is typically greater than the maximum thickness of the blade 60 at its proximal end 64.

[0049] It preferably has a shape of revolution around the elongation axis Y.

[0050] The stilt 74 constitutes a radially external part of the foot 62. It connects the bulb 72 to the blade 60. It extends from the blade 60 to the functional surface 78.

[0051] The stilt 74 flares from the function surface 78 towards the blade 60. Thus, the junction surface 78 constitutes a neck 82 defining a local minimum of the section of the foot 62 along a plane orthogonal to the elongation axis Y.

[0052] The flaring of the stilt 74 from the neck 82 towards the blade 60 is in particular visible in a plane parallel to the elongation axis Y and to the chord of the blade 60 at its proximal end 64. In a plane parallel to the elongation axis Y and orthogonal to the chord of the blade 60 at its proximal end 64, the stilt 74 narrows from the neck 82 to a minimum thickness 84 before flaring towards the blade 60. Returning to Figure 3, the hub 55 comprises for each blade 56 an attachment piece 88, arranged at the blade root, with which the blade 56 is secured. This attachment piece 88 delimits a cell 90 in which the root 62 of the blade 56 is inserted. This cell 90 opens radially outside the part attachment part 88 through an orifice 92. The attachment part 88 delimits, at the periphery of this orifice 92, a bearing surface 94 oriented towards the bottom of the cell 90 opposite the orifice 92.This bearing surface 94 cooperates with the support surface 80 of the bulb 72 to retain the foot 62 in the cell 90.

[0053] The blades 56 of the fan rotor 54 are variable-pitch, that is to say that each blade 56 is pivotally mounted relative to a frame 96 of the hub 55 around a specific pitch axis C. This pitch axis C extends in the direction of elongation Y of the blade 56. It is perpendicular to the longitudinal axis X.

[0054] For this purpose, each attachment part 88 is rotatably mounted relative to the hub 55 around the wedging axis C. More precisely, the attachment part 88 is rotatably mounted inside a housing 98 provided in the frame 96 of the hub 55 by means of balls 99 or other rolling elements.

[0055] The fan 50 further comprises a pitch change mechanism 100 for adjusting the pitch angle of each blade 56 about its pivot axis P so as to adapt the performance of the turbomachine 12 to the different flight phases. This pitch change mechanism 100 comprises an actuator 102 comprising a fixed part 104 secured to the frame 96 and a movable part 106 movable in translation along the longitudinal axis X relative to the fixed part 104 between a retracted position and a deployed position. It also comprises a connecting system 108 connecting the movable part 106 to the attachment part 88 so as to convert the translation of the movable part 106 along the longitudinal axis X into a rotation of the attachment part 88 and, thereby, of the blade 56 about the pitch axis C.This connection system 108 is here formed of an annular slide 110 mounted integral with the movable part 106 and a pin 112 mounted integral with the attachment part 88 and capable of sliding in the slide 110 and rotating relative to the slide 110.

[0056] With reference to Figures 5 to 10, each blade 56 comprises a composite material structure 120. As visible in said Figures, the root 62 is at least partially formed by said structure 120. This structure 120 comprises a fibrous reinforcement obtained by three-dimensional weaving and a matrix in which the fibrous reinforcement is embedded.

[0057] The fiber reinforcement is typically formed from a single-piece fiber preform with varying thickness comprising warp strands 122 (i.e., strands extending along the elongation axis Y of the blade 56) and weft strands 124 (i.e., strands extending along the chord of the blade 56), these strands 122, 124 comprising, for example, carbon, glass, basalt, and / or aramid fibers. Said fiber preform is advantageously obtained by three-dimensional or multi-layer weaving, i.e., the warp strands 122 follow sinuous paths in order to bind together weft strands 124 belonging to different weft strand layers 124, it being noted that said three-dimensional weaving may include 2D surface weaves. Different three-dimensional weave weaves may be used, such as interlock, multi-satin or multi-voile weaves, for example, as described in particular in WO 2006 / 136755.

[0058] During weaving, tension is applied to the warp strands 122 and the weft strands 124 in order to give them a predetermined differential stiffness and therefore a respective weave. By weave of a strand, we mean here the difference between the length of a given strand when it is perfectly straight and the actual length (in the fiber reinforcement) of this strand due to the interlacing that it achieves in order to bond with the other strands, and thus defining what is commonly called the woven weave of the fiber reinforcement. Weave is generally expressed as a percentage and thus characterizes the waviness of the strand. In a manner known per se, when a given strand is straight, its weave is equal to 0%; the more wavy the strand, the higher its weave.

[0059] The matrix is ​​typically a polymer matrix, for example epoxy, bismaleimide or polyimide. The blade 56 is then formed by molding using a vacuum resin injection process of the RTM (for “Resin Transfer Molding”) type, or VÀRTM (for Vacuum Resin Transfer Molding).

[0060] As seen in Figure 5, the foot 62 comprises a section 126 comprising a surface layer 130 delimiting at least in part the skin surface 70 of the foot 62 and a central core layer 132 extending from the wedging axis C towards the surface layer 130, the surface layer 130 having a first stiffness and the central layer 132 having a second stiffness strictly greater than the first stiffness. In the example shown in Figure 5, the foot 62 also comprises an intermediate layer 134 interposed between the surface layer 130 and the central layer 132, the intermediate layer 134 having a third stiffness strictly greater than the first stiffness and strictly less than the second stiffness.The surface layer 130, the central layer 132 and the intermediate layer 134 each extend over the entire height of the section 126, that is to say that each of said layers 130, 132, 134 extends from a lower end (not referenced) to an upper end (not referenced) of said section 126, said lower and upper ends of the section 126 delimiting the section 126 along the elongation axis Y.

[0061] The stiffness is understood here and hereinafter as comprising at least the longitudinal stiffness, i.e. measured orthogonally to the chord of the blade 56 and substantially parallel to the skin surface 70. Advantageously, the stiffness also comprises the transverse stiffness, i.e. measured orthogonally to the direction of elongation Y and substantially parallel to the skin surface 70, the stiffness then being compared direction by direction (i.e. the phrase “the stiffness of layer A is greater than the stiffness of layer B” is understood to mean that the longitudinal stiffness of layer A is greater than the longitudinal stiffness of layer B and that the transverse stiffness of layer A is greater than the transverse stiffness of layer B).This stiffness is typically measured by cutting a standardized specimen from the relevant layer 130, 132 or 134 and measuring the stiffness of this specimen by means of standardized tests, the shape of the cut specimen and the tests carried out to determine its stiffness being the same for each of the layers 130, 132 and 134.

[0062] The first stiffness is less than or equal to 50%, preferably less than or equal to 25%, of the second stiffness. The third stiffness is less than or equal to 66%, preferably less than or equal to 50%, of the second stiffness. Advantageously, the stiffness is substantially halved at each layer change; thus, typically, the third stiffness is substantially equal to 50% of the second stiffness and the first stiffness is substantially equal to 25% of the second stiffness.

[0063] Preferably, the stiffness varies continuously at the interface between the layers 130, 132, 134 and inside the intermediate layer 134, so that the stiffness of the section 126 increases progressively from the surface layer 130 to the central layer 132. Advantageously, the stiffness also varies continuously inside the surface layer 130 and the central layer 132, so that the stiffness of the section 126 increases progressively from the wedging axis C to the skin surface 70 of the foot 62.

[0064] The section 126 includes the neck 82 and at least the inner portion of the stilt 74, i.e. the portion of the stilt 74 closest to the bulb 72. It extends over at least 30% of the height of the foot 62, for example between 30 and 60% of the height of the foot.

[0065] Preferably, the section 126 includes more than 30% of the height of the stilt 74. Optionally, the section 126 extends over 100% of the height of the stilt 74, that is to say that the stilt 74 is entirely included in the section 126, which then extends to the blade 60.

[0066] Advantageously, the section 126 also extends into the bulb 72 and includes at least the outer portion of the bulb 72, i.e. the portion of the bulb 72 closest to the stilt 74. It then typically includes at least 30% of the height of the bulb 72.

[0067] The surface layer 130 has at each point a thickness, measured in a direction perpendicular to the wedging axis C and passing through said point, of between 1 and 25% of the radius of the section 126 measured in this same direction. The central layer 132, for its part, has at each point a thickness, measured in a direction perpendicular to the wedging axis C and passing through said point, of between 20 and 40% of the radius of the section 126 measured in this same direction.

[0068] At least one of the layers 130, 132, 134 is constituted by the composite material structure 120. In the examples of Figures 5 to 9, the surface layer 130 is constituted by the composite material structure 120. In the examples of Figures 5, 6 and 7, the central layer 132 and, where appropriate, the intermediate layer 134 are also constituted by the composite material structure 120.

[0069] In the exemplary embodiment of Figure 5, the difference in stiffness between the layers 130, 132, 134 is obtained by a difference in the warp-weft ratio, this ratio increasing as the stiffness increases. Thus, the warp-weft ratio in the central layer 132 is greater than the warp-weft ratio in the surface layer 130. Furthermore, the warp-weft ratio in the intermediate layer 134 is, where appropriate, between the warp-weft ratio in the central layer 132 and the warp-weft ratio in the surface layer 130 and is preferably close to an average between the warp-weft ratio in the central layer 132 and the warp-weft ratio in the surface layer 130.

[0070] The central layer 132 and, where appropriate, the intermediate layer 134 thus have, relative to the surface layer 130, excess warp strands 122. Advantageously, these excess warp strands 122 gradually mix with the weft strands 124 of the surface layer 130 from an upper end of the section 126 in order to limit the gradients of change of property in the composite material structure 120 (stiffness and breaking strength) which would be likely to weaken the blade 56.

[0071] In the embodiment of Figure 6, the difference in stiffness between the layers 130, 132, 134 is achieved by a difference in the damping of the warp strands 122, the damping being lower as the stiffness increases. The damping of the warp strands 122 is thus lower in the central layer 132 than in the surface layer 130. Furthermore, the damping of the warp strands 122 in the intermediate layer 134 is, where appropriate, greater than in the central layer 132 and lower than in the surface layer 130.

[0072] This difference in wetting is obtained here by inserting unidirectional warp strands 136 into the central layer 132, i.e. warp strands 122 having a wetting of 0%, the surface layer 130 being free of such unidirectional warp strands 136. These unidirectional warp strands 136 are typically free warp strands, i.e. they are not interlaced with weft strands 124. The central layer 132 is here made up of said unidirectional warp strands 136. Advantageously, the unidirectional warp strands 136 of the central layer 132 gradually mix with the weft strands 124 of the surface layer 130 and, where appropriate, of the intermediate layer 134 from an upper end of the section 126 in order to limit the gradients of change of property in the composite material structure 120 (stiffness and breaking strength) which would be likely to weaken the blade 56.

[0073] Alternatively (not shown), the difference in stiffness is achieved by applying a different tension to the warp strands 122 and / or the weft strands 124 in the loom used to make the fiber reinforcement so that the tension experienced by the warp strands 122 at the core (in the central layer 132) is greater than the tension experienced by the warp strands 122 near the skin (in the surface layer 130) and / or the tension experienced by the weft strands 124 at the core is less than that experienced by the weft strands 124 near the skin. Changing the tension applied to the weft strands 124 has a direct impact on the tension experienced by the warp strands 122 near their interface with the weft strands 124 and therefore their stiffness and stiffness.For example, the difference in tension experienced by the warp strands 122 is achieved by increasing the tension applied to the warp strands 122 in the core layer 132 and / or by reducing the tension applied to the warp strands 122 in the surface layer 130 and, if applicable, in the intermediate layer 134. Alternatively or optionally, the difference in tension experienced by the warp strands 122 is achieved by reducing the tension applied to the weft strands 124 in the core layer 132 and / or by increasing the tension applied to the weft strands 124 in the surface layer 130 and, if applicable, in the intermediate layer 134.

[0074] The variation in tension applied by the loom to the warp strands 122 and / or weft strands 124 can be obtained by any suitable means, the principle being to exert a take-up tension directly at the outlet of the bobbin on which the strand is wound. In a manner known per se, this tension can be applied by a spring system pulling each warp strand 122, or using weights positioned between the outlet of the warp strand 122 from the bobbin and the eyelets of the heddles of the loom. Furthermore, bobbins are commercially available for controlling the applied tension.Finally, the tension applied to the weft strands 124 can be managed in a similar manner as for the warp strands 122, and / or by using a clamp which catches the end of the weft strand 124 and pulls it through the shed (interlacing of the warps), then releases the weft strand 124 once the beating of the loom has moved on to the next sequence. These means of applying tension to a strand (of warp 122 or weft 124) being known per se, they will not be detailed further here.When the tension difference is obtained by applying a different tension to the warp strands 122 of the central layer 132 and to the warp strands 122 of the surface layer 130 and, where appropriate, of the intermediate layer 134, the warp strands 122 of the surface layer 130 and, where appropriate, of the intermediate layer 134 are advantageously progressively removed from the fiber reinforcement from an upper end of the section 126 and replaced by as many warp strands 122 undergoing a tension equivalent to that of the warp strands 122 of the central layer 132, in order to limit the property change gradients in the composite material structure 120 (stiffness and breaking strength) which would be likely to weaken the blade 56.

[0075] In the embodiment of Figure 7, the difference in stiffness between the layers 130, 132 is obtained by a difference in the nature of the fibers making up the warp strands 122 and / or weft strands 124 of the central layer 132 and those making up the warp strands 122 and / or weft strands 124 of the surface layer 130 and, where appropriate, of the intermediate layer 134. The warp strands 122 and / or weft strands 124 of the surface layer 130 are thus typically made of fibers of a first material, those of the central layer 132 are made of fibers of a second material, and those of the intermediate layer 134, where appropriate, are made of a third material, the first material having a modulus of elasticity lower than that of the second material and the third material having, where appropriate, a modulus of elasticity between those of the first and second materials and preferably close to the average of said elastic moduli.For example, the first material has a modulus of elasticity between 150 and 190 GPa and is typically made of glass or basalt, the second material has a modulus of elasticity between 240 and 350 GPa and is typically made of carbon, and the third material has a modulus of elasticity between 195 and 235 GPa and is typically made of carbon.

[0076] Advantageously, the warp strands 122 of the surface layer 130 and, where appropriate, of the intermediate layer 134 are progressively removed from the fiber reinforcement from an upper end of the section 126 and replaced by as many warp strands 122 of second material, in order to limit the gradients of change of property in the composite material structure 120 (stiffness and breaking strength) which would be likely to weaken the blade 56.

[0077] In the examples of Figures 8 and 9, only the surface layer 130 is made up of the composite material structure 120. The central layer 132 is made up of another stiffer structure. The intermediate layer 134 does not exist.

[0078] Thus, in the exemplary embodiment of Figure 8, the central layer 132 is made up of a laminate 140 of unidirectional plies 142 whose fibers are oriented substantially parallel to the wedging axis C. In the exemplary embodiment of Figure 9, the central layer 132 is made up of a structure 144 made of metal, for example steel or titanium.

[0079] Advantageously, the thickness of the laminate 140 or of the metal structure 144 decreases progressively from an upper end of the section 126, this loss of thickness being compensated by the progressive introduction of additional warp 122 and weft 124 strands into the composite material structure 120, in order to limit the gradients of change of property in the blade 56 (stiffness and breaking strength) which would be likely to weaken the blade 56.

[0080] In the example of Figure 10, only the central layer 132 is made up of the composite material structure 120. The surface layer 130 is made up of another, more flexible structure. The intermediate layer 134 does not exist.

[0081] Thus, in the example shown, the surface layer 130 is made up of a laminate 150 of added plies 152. These added plies 152 are preferably multidirectional plies, for example bidirectional. Each added ply 152 is typically made up of a 2D woven sheet or a non-crimp fabric sheet (better known by the acronym NCF, from the English “non-crimp fabric”).

[0082] The added plies 152 are preferably inclined plies whose fibers are oriented at an angle of between 5 and 95°, advantageously between 20 and 60°, for example substantially equal to 45°, relative to the wedging axis C. As a variant or as an option, the fibers of the added plies 152 have a lower modulus of elasticity than that of the fibers constituting the warp 122 and weft 124 strands of the composite material structure 120.

[0083] Advantageously, the thickness of the laminate 150 decreases progressively from an upper end of the section 126, this loss of thickness being compensated by the progressive introduction of additional warp 122 and weft 124 strands into the composite material structure 120, in order to limit the gradients of change of property in the blade 56 (stiffness and breaking strength) which would be likely to weaken the blade 56.

[0084] It will be noted that these different embodiments can be combined with each other. Thus, in embodiments of the invention not shown: the section 126 comprises the surface layer 130, the central layer 132 and the intermediate layer 134, o the central layer 132 being made up of: the laminate 140 or the metal structure 144, and o the central layer 132 and the intermediate layer 134 being made up of the composite material structure 120, o the intermediate layer 134 having:

[0085] ■ a warp-to-weft ratio greater than that of the surface layer 130 and / or

[0086] ■ warp strands 122 having a lower embouchure than that of the warp strands 122 of the surface layer 130 and / or

[0087] ■ warp strands 122 or weft strands 124 composed of fibers of a material having a modulus of elasticity greater than that of the material composing the fibers constituting the warp strands 122 or weft strands 124 of the surface layer 130; the section 126 comprises the surface layer 130, the central layer 132 and the intermediate layer 134, all three consisting of the composite material structure 120, o the central layer 132 having:

[0088] ■ a warp-to-weft ratio greater than that of the intermediate layer 134 and / or

[0089] ■ warp strands 122 having a lower embouchure than that of the warp strands 122 of the intermediate layer 134 and / or

[0090] ■ warp 122 or weft 124 strands composed of fibers of a material having a modulus of elasticity greater than that of the material composing the fibers constituting the warp 122 or weft 124 strands of the intermediate layer 134, and o the intermediate layer 134 having:

[0091] ■ a warp-to-weft ratio greater than that of the surface layer 130 and / or

[0092] ■ warp strands 122 having a lower embouchure than that of the warp strands 122 of the surface layer 130 and / or

[0093] ■ warp strands 122 or weft strands 124 composed of fibers of a material having a modulus of elasticity greater than that of the material composing the fibers constituting the warp strands 122 or weft strands 124 of the surface layer 130; the section 126 comprises the surface layer 130, the central layer 132 and the intermediate layer 134, o the surface layer 130 being made of the laminate 150, o the intermediate layer 134 being made of the composite material structure 120, and o the central layer 132 being made of:

[0094] ■ of the structure in composite material 120 and having:

[0095] • a warp-to-weft ratio greater than that of the intermediate layer 134 and / or

[0096] • warp strands 122 having a lower embouchure than that of the warp strands 122 of the intermediate layer 134 and / or

[0097] • warp 122 or weft 124 strands composed of fibers of a material having a modulus of elasticity greater than that of the material composing the fibers constituting the warp 122 or weft 124 strands of the intermediate layer 134, or

[0098] ■ laminate 140, or

[0099] ■ of the metal structure 144; the section 126 comprises only the surface layer 130 and the central layer 132, both made of the composite material structure 120, the central layer 132 having a warp-to-weft ratio greater than that of the surface layer 130 and / or the warp strands 122 having a lower wefting than that of the warp strands 122 of the surface layer 130 and / or the warp strands 122 or weft strands 124 composed of fibers of a material having a modulus of elasticity greater than that of the material composing the fibers constituting the warp strands 122 or weft strands 124 of the surface layer 130.

[0100] Although, in the embodiments described above, it is the central layer 132 which has the highest stiffness, the invention is not limited to this single embodiment. In general, it is appropriate for the layer having the highest stiffness to be an internal layer between the wedging axis C and the surface layer 130.

[0101] Thus, in another embodiment shown in Figure 11, the layer having the highest stiffness is the intermediate layer 134.

[0102] The stiffness of the intermediate layer 134 is then strictly greater than the first stiffness (of the surface layer 130) and the second stiffness (of the central layer 132). The second stiffness is preferably greater than or equal to the first stiffness. Typically, the first stiffness is less than or equal to 50%, preferably less than or equal to 25%, of the stiffness of the intermediate layer 134, and the second stiffness is less than or equal to 66%, preferably less than or equal to 50%, of the stiffness of the intermediate layer 134.

[0103] Advantageously, the stiffness varies continuously at the interface between the layers 130, 132, 134 and inside the surface layer 130 and the central layer 132, so that the stiffness of the section 126 increases progressively from the wedging axis C to the intermediate layer 134, then decreases, still progressively, from the intermediate layer 134 to the skin surface 70 of the foot 62.

[0104] In the example shown in Figure 11, the higher stiffness of the intermediate layer 134 is obtained by a warp-to-weft ratio of the composite material structure 120 inside the intermediate layer 134 higher than the warp-to-weft ratios found in the surface layer 130 and in the central layer 132. Alternatively (not shown), this higher stiffness is obtained by: the use, in the intermediate layer 134, of warp strands 122 having a lower wefting than that of the warp strands 122 of the surface layer 130 and of the central layer 132 and / or the use, in the intermediate layer 134, of warp strands 122 or weft strands 124 composed of fibers of a material having a modulus of elasticity higher than that of the material composing the fibers constituting the warp strands 122 or weft strands 124 of the surface layer 130 and of the central layer 132.

[0105] Thanks to the embodiments described above, the neck 82 and the base of the stilt 74 are more flexible in the skin and stiffer in the core, which reduces the concentrations of the 1 P loading on the skin of the root 62. Due to the greater stiffness of the core, the stresses due to the 1 P loading are better distributed in the depth of the root 62, which relieves the skin. The stresses being better distributed, the resistance of the blade 56 to the 1 P forces is increased, without modification of its shape and therefore of its aerodynamic characteristics. It is thus possible to produce a blade 56 at least partially from a composite material having a resistance to the 1 P loading sufficient to equip this blade 56 on a turbojet with an unducted fan such as the turbomachine 12.It is thus possible to lighten the blades of a turbojet with an unducted fan, without degrading their resistance to 1 P forces or their aerodynamic performance, by simply replacing these blades with blades such as blade 56.

Claims

CLAIMS 1. Variable-pitch vane (56) for a turbomachine fan, comprising a blade (60) with an aerodynamic profile and a root (62) configured to be inserted into a cell (88) of a fan hub (55), the vane (56) being able to pivot relative to a frame (96) of the fan hub (55) about a pitch axis (C), the root (62) having a peripheral skin surface (70) and comprising a bulb (72) and a stilt (74) connecting the bulb (72) to the blade (60), the bulb (72) being connected to the stilt (74) by a neck (82) defining a local minimum of the section of the root (62) along a plane orthogonal to the pitch axis (C), in which at least one section (126) of the root (62) comprises a surface layer (130) delimiting at least for part the skin surface (70) and an internal layer (132, 134) between the wedging axis (C) and the surface layer (130), the surface layer (130) having a first stiffness and the internal layer (132,134) having a second stiffness strictly greater than the first stiffness, the section (126) of the foot (62) including the neck (82) and at least part of the stilt (74)., 2. Variable pitch vane (56) according to claim 1, wherein the first stiffness is less than or equal to 50%, preferably less than or equal to 25%, of the second stiffness.

3. Variable-pitch blade (56) according to claim 1 or 2, in which the surface layer (130) has at each point a thickness, measured in a direction perpendicular to the pitch axis (C) and passing through said point, of between 1 and 25% of the radius of the section (126) of the root (62) measured in this same direction.

4. Variable pitch blade (56) according to any one of the preceding claims, comprising a composite material structure (120) obtained by three-dimensional weaving of warp strands (122), oriented substantially parallel to the pitch axis (C), and weft strands (124), at least one of the surface layer (130) and the internal layer (132, 134) being constituted by the composite material structure (120).

5. Variable pitch vane (56) according to any one of the preceding claims, in which the inner layer (132, 134) is constituted by a metal structure, for example steel or titanium.

6. Variable pitch vane (56) according to any one of claims 1 to 4, in which the surface layer (130) is made of a woven composite and the inner layer (132, 134) is made of a laminate (140) of unidirectional plies (142) whose fibers are oriented substantially parallel to the pitch axis (C).

7. Variable pitch vane (56) according to claim 4, wherein the composite material structure (120) has a first warp-weft ratio in the surface layer (130) and a second warp-weft ratio, greater than the first warp-weft ratio, in the inner layer (132, 134).

8. A variable pitch vane (56) according to claim 4, wherein the warp strands (122) have lower fogging in the inner layer (132, 134) than in the surface layer (130).

9. Variable pitch vane (56) according to any one of the preceding claims, in which the surface layer (130) and the internal layer (132, 134) each extend over the entire height of the section (126) of the root (62).

10. Variable pitch vane (56) according to any one of the preceding claims, in which the inner layer (132, 134) is constituted by a central layer (132) extending from the pitch axis (C) towards the surface layer (130).

11. Variable-pitch blade (56) according to claim 10, in which the section (126) of the root (62) comprises an intermediate layer (134) interposed between the surface layer (130) and the central layer (132), the intermediate layer (134) having a third stiffness strictly greater than the first stiffness and strictly less than the second stiffness.

12. Variable pitch blade (56) according to any one of claims 1 to 10, in which the stiffness of the section (126) of the root (62) increases progressively from the surface layer (130) to the internal layer (132).

13. A turbomachine fan (50), comprising a fan hub (55) and a plurality of variable-pitch blades (56) according to any one of the preceding claims.

14. Turbomachine (12) comprising at least one fan (50) according to claim 15. Aircraft (10) comprising at least one turbomachine (12) according to claim 14.

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