Variable pitch bucket for turbine fan with stiffness gradient in root

Abstract

A variable pitch bucket for a turbine fan includes a blade (60) and a root (62). The root portion (62) comprises a spherical portion (72) and a shank portion (74) connecting the spherical portion (72) to the blade (60), the spherical portion (72) being connected to the shank portion (74) by a neck portion (82) defining a local minimum of a cross-section of the root portion (62) along a plane orthogonal to the pitch axis. At least one section of the root (62) comprising at least a portion of the neck (82) and the shank (74) comprises a surface layer and an inner layer located between the pitch axis and the surface layer, the inner layer having a stiffness strictly greater than the stiffness of the surface layer.

Description

Technical Field

[0001] The present invention generally relates to the field of variable pitch blades for turbomachine fans of the type comprising a blade capable of extending in an air flow and a root configured to be inserted into a cavity of a fan hub. The present invention more particularly relates to the field of variable pitch blades made of composite materials.

[0002] A preferred field of application of the invention is the field of turbojets with unducted fans (better known as propeller fans, open rotors and unducted single fans). However, the invention is also applicable to turbojets with ducted fans and turboprops with one or more propulsion propellers. Background Art

[0003] One of the approaches currently being explored to improve the specific consumption of civil aircraft engines consists in developing turbojets with unducted fans, such as those described in document FR 3 080 322 A1. These turbojets comprise a conventional turbine engine gas generator, one or more turbine stages of which drive one or more unducted fans extending outside the nacelle.

[0004] The advantage of these unducted fan engines is that the diameter of the fan is not limited by the presence of the cowling, so it is possible to design engines with a high bypass ratio, which reduces fuel consumption. Therefore, in this type of engine, the fan blades can have a large span.

[0005] Furthermore, these engines generally include a mechanism for modifying the angular position of these blades, known as the pitch angle, so as to adjust the thrust generated by the fan according to the different flight phases. In order to facilitate this pivoting of the blades and to reduce the bulk of said blades at the hub, the root of the blade generally extends only over a portion of the blade chord length.

[0006] In use, the blades equipped with such a turbine are subjected to various forces, including in particular forces known as 1P forces (also called 1P loads). These forces are cyclic forces caused by the difference between the direction of incidence of the air flow not guided by the fairing and the axis of rotation of the fan (the engine axis itself positioned with respect to the air flow relative to the aircraft axis). These forces generate very large bending loads on the blades, especially in the interface area between the blades and the fan disk. Due to the cyclic nature of these forces (each time the blade moves from one position to a diametrically opposite position, the 1P load of the blade changes), these forces can lead to accelerated fatigue of the blade. Therefore, in order to ensure the service life of the blades, there is a strong tendency to limit the permissible 1P forces.

[0007] At present, these blades are usually made of metal materials. Although blades made of metal materials have good mechanical resistance, they still have the disadvantage of relatively large mass. In order to reduce this mass, it is desirable to be able to make these blades at least partially composed of a composite structure, which includes fiber reinforcements densified by a polymer matrix. However, the conventional architecture of composite blades does not allow to combine resistance to the aerodynamic forces to which these blades will be subjected (especially IP forces), the aerodynamic performance of the blades and the limitation of the hub volume (converted by a given geometry). Summary of the invention

[0008] One object of the present invention is to increase the resistance of a fan blade to aerodynamic forces (such as 1P force) without reducing its aerodynamic performance. Another object is to reduce the weight of a fan blade without reducing its resistance to aerodynamic forces (such as 1P force) or its aerodynamic performance.

[0009] To this end, according to a first aspect, the present invention relates to a variable pitch blade for a turbine fan, the variable pitch blade comprising an aerodynamically shaped blade and a root configured to be inserted into a cavity of a fan hub, the blade being capable of pivoting about a pitch axis relative to a frame of the fan hub, the root having a peripheral skin surface and comprising a bulbous portion and a shank portion connecting the bulbous portion to the blade, the bulbous portion being connected to the shank portion by a neck, the neck defining a local minimum of a cross-section of the root along a plane orthogonal to the pitch axis, wherein at least one section of the root comprises a surface layer at least partially defining the skin surface and an inner layer between the pitch axis and the surface layer, the surface layer having a first stiffness and the inner layer having a second stiffness strictly greater than the first stiffness, the section of the root comprising at least a portion of the shank portion and the neck.

[0010] According to a particular embodiment of the invention, the variable pitch blade has one or more of the following features, which are adopted individually or in any technically possible combination:

[0011] - the first stiffness comprises the longitudinal stiffness of the surface layer, and the second stiffness comprises the longitudinal stiffness of the inner layer;

[0012] -The petiole flares from the neck to the blade;

[0013] - the section of the root extends over at least 30% of the height of the root measured parallel to the pitch axis;

[0014] - the blade has a proximal portion connected to the root, a free distal portion, a leading edge, a trailing edge, and a chord connecting the leading edge to the trailing edge, the bulb having a maximum radius measured in a direction perpendicular to the pitch axis, which is less than 50% of the length of the chord of the blade at the proximal portion of the blade, preferably less than 25% of the length of the chord of the blade at the proximal portion of the blade;

[0015] - the blade has a maximum thickness at the proximal end of the blade, the maximum radius of the bulb being greater than said maximum thickness;

[0016] - the bulbous portion has a shape of revolution about the pitch axis;

[0017] - the surface layer and the inner layer each extend over the entire height of the section of the root;

[0018] - The inner layer consists of a central layer extending from the pitch axis to the surface layer;

[0019] - the section of the root comprises an intermediate layer between the surface layer and the central layer, the intermediate layer having a third stiffness strictly greater than the first stiffness and strictly lower than the second stiffness;

[0020] - The third stiffness includes the longitudinal stiffness of the middle layer;

[0021] - The stiffness of this section of the root increases gradually from the surface layer to the inner layer;

[0022] - the section of the root comprises a central layer extending from the pitch axis to the surface layer, the inner layer consisting of an intermediate layer between the surface layer and the central layer;

[0023] - Between the middle layer and the pitch axis, the stiffness of the section of the root decreases;

[0024] - the first stiffness is less than or equal to 50% of the second stiffness, preferably less than or equal to 25% of the second stiffness;

[0025] - the surface layer has at each point a thickness measured in a direction perpendicular to the pitch axis and passing through said point, which thickness is comprised between 1% and 25% of the radius of that section of the root measured in this same direction;

[0026] - The surface layer is made of composite material;

[0027] - the inner layer consists of a metal structure, for example of steel or titanium;

[0028] - the surface layer consists of a woven composite material and the inner layer consists of a laminate of unidirectional plies whose fibers are oriented substantially parallel to the pitch axis;

[0029] - the blade comprises a composite material structure obtained by three-dimensional weaving of warp strands and weft strands, the warp strands being oriented substantially parallel to the pitch axis, at least one of the surface layer and the inner layer consisting of the composite material structure;

[0030] - the composite structure has a first warp-weft ratio in the surface layer and a second warp-weft ratio in the inner layer that is greater than the first warp-weft ratio; and

[0031] - The take-up rate of the warp strands in the inner layer is lower than that in the surface layer.

[0032] According to a second aspect, the present invention also relates to a turbomachine fan comprising a fan hub and a plurality of variable pitch blades as described above.

[0033] According to a third aspect, the invention also relates to a turbomachine comprising such a fan.

[0034] According to a particular embodiment of the invention, the turbine has the following features:

[0035] -The turbine is a turbine with an unducted fan.

[0036] Finally, according to a fourth aspect, the invention relates to an aircraft comprising such a turbine. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Other characteristics and advantages of the invention will emerge on reading the following description given purely by way of example and with reference to the accompanying drawings, in which:

[0038] - Figure 1 is a top view of an aircraft according to an exemplary embodiment of the present invention,

[0039] - Figure 2 yes Figure 1 A simplified view of a partial longitudinal section of a turbine of an aircraft,

[0040] - Figure 3 yes Figure 2 Simplified view of a partial longitudinal section of a part of the fan of a turbine,

[0041] - Figure 4 yes Figure 3 A perspective view of the root of the blade, and

[0042] - Figures 5 to 11 yes Figure 3 Simplified longitudinal cross-sectional views of different variants of a section of a blade root. DETAILED DESCRIPTION

[0043] Figure 1 The aircraft 10 shown in FIG. 1 includes a turbine 12 for propelling the aircraft 10 .

[0044] In the example shown, the aircraft 10 is an airplane. The aircraft generally comprises a fuselage 14, a tail 16 and two wings 18. The turbines 12 are here two and each is housed under a respective wing 18. As a variant (not shown), the turbines 12 are arranged along the fuselage 14, for example in the vicinity of the tail 16. As another variant (also not shown), the aircraft 10 comprises a single turbine 12 or at least three turbines 12.

[0045] Figure 2 One of the turbines 12 is shown in FIG.

[0046] As shown in this figure, the turbine 12 is elongated along a longitudinal axis X. The turbine generally has an angular symmetry about said longitudinal axis X, that is to say there is at least one angle for which a rotation about the longitudinal axis X does not vary the turbine.

[0047] Here and below, the terms "inner" and "outer", "inner" and "outer" and variations thereof are to be understood with reference to the axis X, elements described as "inner" or "inner" being oriented toward the axis X, while "outer" or "outer" elements are oriented opposite to the axis X.

[0048] The turbomachine 12 generally includes a nacelle 20 , an internal flow path 22 for circulating air through the nacelle 20 , a combustion chamber 24 housed in the flow path 22 , an engine body 26 , and an exhaust nozzle 28 .

[0049] In the following, the terms “upstream” and “downstream” are to be understood with reference to the flow direction of the air flow through the flow path 22 .

[0050] The engine body 26 includes a compressor 30, a turbine 32, and a drive shaft 34 that couples the turbine 32 to the compressor 30 to drive the compressor 30 through the turbine 32. The compressor 30 is arranged upstream of the combustion chamber 24 and supplies compressed air to the combustion chamber 24. The turbine 32 is arranged downstream of the combustion chamber 24 and receives exhaust gas leaving the combustion chamber 24.

[0051] The transmission shaft 34 has a longitudinal axis X as a rotation axis.

[0052] The transmission shaft 34 is guided in rotation relative to the nacelle 20 by means of bearings (not shown).

[0053] In the example shown, the turbomachine 12 is a multi-shaft turbomachine, in particular a twin-shaft 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 is a high-pressure compressor, the turbine 32 is a high-pressure turbine, and the drive shaft 34 is a high-pressure shaft.

[0054] The low-pressure body 40 includes 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 to drive the low-pressure compressor 42 through the low-pressure turbine 44 .

[0055] The low-pressure compressor 42 is arranged upstream of the high-pressure compressor 30, and supplies compressed air to the high-pressure compressor 30. The low-pressure turbine 44 is arranged downstream of the high-pressure turbine 32, and receives exhaust gas leaving the high-pressure turbine 32.

[0056] The low pressure shaft 46 is guided for rotation relative to the nacelle 20 via bearings (not shown).

[0057] The low-pressure shaft 46 is coaxial with the high-pressure shaft 34 . Therefore, the low-pressure shaft 46 also has a longitudinal axis X as a rotation axis. In particular, the low-pressure shaft 46 extends inside the high-pressure shaft 34 .

[0058] The turbomachine 12 further comprises a fan 50 for driving an air flow in an outflow flow path 52 surrounding the nacelle 20. Thus, a distinction is made between a primary (hot) air flow A consisting of the portion of the air flow driven in the inflow flow path 22 and a secondary (cold) air flow B consisting of the portion of the air flow driven in the outflow flow path 52.

[0059] The fan 50 comprises a fan rotor 54. The fan rotor 54 is mounted to rotate relative to the nacelle 20 about the longitudinal axis X. The fan rotor 54 comprises a hub 55 ( Figure 3 ), the hub 55 is provided with fan blades 56 extending substantially radially outward from the hub 55. These blades 56 drive the air flow in the outflow flow path 52 when rotating.

[0060] 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, the fan rotor 54 is rotationally fixed to the low-pressure shaft 46. As a variant (not shown), this drive is carried out via a speed reducer which causes the fan rotor 54 to rotate at a speed lower than that of the low-pressure shaft 46.

[0061] In the example shown, the fan 50 also comprises a fan stator 58 comprising blades 59 arranged at the periphery of the nacelle 20 in the outflow flow path 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 a counter-rotating fan rotor instead of a fan stator 58.

[0062] Advantageously, as shown, the fan 50 is an unducted fan, that is to say, the outflow flow path 52 has no peripheral limit. The turbomachine 12 is then constituted by a turbojet engine with an unducted fan, as shown, or, as a variant, by a turboprop engine. As a variant (not shown), the outflow flow path 52 is defined between the nacelle 20 and a fan casing surrounding the fan 50; the turbomachine 12 is then generally constituted by a turbojet engine with a high bypass ratio, the bypass ratio being defined as the ratio of the flow of the secondary (cold) flow B to the flow of the primary (hot) flow A.

[0063] In the example shown, the turbine 12 is in particular of the “puller” type, that is to say, the fan 50 is arranged upstream of the internal flow path 22 and also drives the air flow in the internal flow path 22. As a variant (not shown), the turbine is of the “pusher” type, that is to say, the fan 50 is arranged around the downstream half of the nacelle 20.

[0064] Figure 3 Schematically one of the blades 56 of the fan rotor 54 is shown in FIG. As shown in the figure, the blade 56 is elongated along a substantially radial direction of elongation Y, that is to say, perpendicular to the longitudinal axis X. In the following, "height" refers to the distance along the elongation axis Y.

[0065] Bucket 56 includes an aerodynamically shaped blade 60 and a root 62 .

[0066] The blades 60 extend radially outside of a housing 63 of the hub 55 which internally delimits the outer flow path 52. The blades 60 are thus able to extend into the air flow circulating in said flow path 52. The blades 60 are shaped to generate lift when they move in said air flow.

[0067] The blade 60 has a proximal end 64 connected 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 axis of elongation Y connecting the leading edge 66 to the trailing edge 67 .

[0068] The blade 60 also has a soffit 68 ( Figure 2 ) and Arch 69( Figure 2 ). In the following, “thickness” refers to the distance in a plane perpendicular to the axis of elongation Y and along the axis extending between the soffit 68 and the arch dorsum 69.

[0069] Reference Figure 4 The root portion 62 has a peripheral skin surface 70. The root portion 62 also includes a bulb portion 72 and a handle portion 74.

[0070] The bulb 72 constitutes the radially inner portion of the root 62. The bulb 72 delimits an inner end 76 of the blade 56, i.e. the end of the blade 56 closest to the axis X. The bulb 72 extends from said inner end 76 to a joining surface 78 connecting the bulb 72 to the shank 74. This joining surface 78 is generally disc-shaped.

[0071] The bulb 72 flares out from the engagement surface 78 opposite the blade 60 so as to define a support surface 80 oriented towards the blade 60 . The support surface 80 constitutes a part of the skin surface 70 .

[0072] The bulbous portion 72 has a maximum radius measured in a direction perpendicular to the axis of elongation that is less than 50% of the length of the chord of the blade 60 at the proximal end 64, preferably less than 25% of the length of the chord of the blade 60 at the proximal end 64. The maximum radius is typically greater than the maximum thickness of the blade 60 at the proximal end 64.

[0073] The bulbous portion 72 preferably has a shape of revolution about the axis Y of elongation.

[0074] The shank 74 constitutes a radially outer portion of the root 62. The shank 74 connects the bulb 72 to the blade 60. The shank 74 extends from the blade 60 to an engagement surface 78.

[0075] The shank 74 flares from the engagement surface 78 towards the blade 60. The engagement surface 78 thus constitutes a neck 82 defining a local minimum of the cross section of the root 62 along a plane orthogonal to the axis Y of elongation.

[0076] The flaring of the shank 74 from the neck 82 toward the blade 60 is particularly evident in a plane parallel to the axis of elongation Y and the chord of the blade 60 at its proximal end 64. In a plane parallel to the axis of elongation Y and orthogonal to the chord of the blade 60 at its proximal end 64, the shank 74 narrows from the neck 82 to a minimum thickness 84 before flaring toward the blade 60.

[0077] Back to Figure 3 For each blade 56, the hub 55 comprises a fastener 88, which is arranged at the blade root and to which the blade 56 is fixed. The fastener 88 defines a cavity 90, into which the root 62 of the blade 56 is inserted. The cavity 90 opens radially to the outside of the fastener 88 through a hole 92. The fastener 88 defines a bearing surface 94 at the periphery of the hole 92, which is oriented toward the bottom of the cavity 90 opposite to the hole 92. The bearing surface 94 cooperates with the support surface 80 of the bulb 72 to keep the root 62 in the cavity 90.

[0078] The blades 56 of the fan rotor 54 are variable pitch blades, that is to say each blade 56 is pivotally mounted relative to the frame 96 of the hub 55 about a specific pitch axis C. This pitch axis C extends in the direction of elongation Y of the blade 56. This pitch axis is perpendicular to the longitudinal axis X.

[0079] To this end, each fastener 88 is mounted rotatably relative to the hub 55 about the pitch axis C. More specifically, the fastener 88 is mounted rotatably in a housing 98 arranged in a frame 96 of the hub 55 by means of balls 99 or other rolling elements.

[0080] The fan 50 also includes 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 turbine 12 to different flight phases. The pitch change mechanism 100 includes an actuator 102, which includes a fixed portion 104 fixed to the frame 96 and a movable portion 106 that can be moved in translation along the longitudinal axis X relative to the fixed portion 104 between a retracted position and a deployed position. The pitch change mechanism 100 also includes a connection system 108 that connects the movable portion 106 to the fastener 88 so as to convert the translation of the movable portion 106 along the longitudinal axis X into a rotation of the fastener 88 about the pitch axis C, and thus into a rotation of the blade 56 about the pitch axis C. This connection system 108 is formed here by an annular slide 110 which is fixedly mounted to the movable part 106 , and a pin 112 which is fixedly mounted to the fastener 88 and which is able to slide in the slide 110 and to rotate relative to the slide 110 .

[0081] Reference Figures 5 to 10 , each blade 56 comprises a composite material structure 120. As shown in said figures, the root 62 is at least partially formed by said structure 120. This structure 120 comprises a fiber reinforcement obtained by three-dimensional weaving and a matrix in which the fiber reinforcement is embedded.

[0082] The fiber reinforcement is generally formed by a single fiber preform of variable thickness, comprising warp strands 122 (i.e., strands extending along the axis of elongation 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. The fiber preform is advantageously obtained by three-dimensional weaving or multi-layer weaving, that is to say, the warp strands 122 follow a serpentine path so as to interconnect the weft strands 124 belonging to different weft strand layers 124, it being noted that the three-dimensional weaving may include a surface 2D weaving. Different three-dimensional weaves may be used, such as interlocking, multi-satin or multi-plain weaves, for example, as described in particular in document WO 2006 / 136755.

[0083] During the weaving process, tension is applied to the warp strands 122 and the weft strands 124 in order to give them predetermined different stiffnesses and thus give them corresponding take-up rates. The take-up rate of a strand refers here to the difference between the length of a given strand when it is completely straight and the actual length of this strand (in the fiber reinforcement) (due to the entanglement of the strands in order to connect with other strands, thus defining the so-called weaving weave of the fiber reinforcement). The take-up rate is usually expressed as a percentage and therefore characterizes the curling of the strand. In a manner known per se, when a given strand is straight, its take-up rate is equal to 0%; the more the strand is curled, the higher its take-up rate.

[0084] The matrix is ​​usually a polymer matrix, such as epoxy resin, bismaleimide or polyimide. The blades 56 are then formed by molding by a vacuum resin injection method of the RTM (Resin Transfer Molding) or VARTM (Vacuum Resin Transfer Molding) type.

[0085] like Figure 5 As shown, the root 62 includes a section 126 including a surface layer 130 at least partially defining the skin surface 70 of the root 62 and a core center layer 132 extending from the pitch axis C toward the surface layer 130, the surface layer 130 having a first stiffness, and the center layer 132 having a second stiffness strictly greater than the first stiffness. Figure 5 In the example shown, the root 62 also includes an intermediate layer 134 between the surface layer 130 and the central layer 132, the intermediate layer 134 having a third stiffness that is strictly greater than the first stiffness and strictly lower 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 segment 126, that is, each of the layers 130, 132, 134 extends from a lower end (not marked) to an upper end (not marked) of the segment 126, the lower end and the upper end of the segment 126 delimiting the segment 126 along the elongation axis Y.

[0086] Here and hereinafter, stiffness is understood to include at least the longitudinal stiffness, that is to say the stiffness measured orthogonal to the chord of the blade 56 and substantially parallel to the skin surface 70. Advantageously, the stiffness also includes the transverse stiffness, that is to say the stiffness measured orthogonal to the direction of elongation Y and substantially parallel to the skin surface 70, and the stiffness is then compared direction by direction (that is to say, the sentence "the stiffness of layer A is greater than the stiffness of layer B" means that the longitudinal stiffness of layer A is greater than the longitudinal stiffness of layer B, and the transverse stiffness of layer A is greater than the transverse stiffness of layer B). This stiffness is usually measured by cutting a standardized specimen from the relevant layer 130, 132 or 134 and measuring the stiffness of this specimen by a standardized test, the shape of the specimen cut and the test performed to determine its stiffness being the same for each of the layers 130, 132 and 134.

[0087] The first stiffness is less than or equal to 50% of the second stiffness, preferably less than or equal to 25% of the second stiffness. The third stiffness is less than or equal to 66% of the second stiffness, 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.

[0088] Preferably, the stiffness varies continuously at the interfaces between the layers 130, 132, 134 and within the intermediate layer 134, so that the stiffness of the section 126 gradually increases from the surface layer 130 to the central layer 132. Advantageously, the stiffness also varies continuously within the surface layer 130 and the central layer 132, so that the stiffness of the section 126 gradually increases from the pitch axis C to the skin surface 70 of the root 62.

[0089] Segment 126 includes neck 82 and at least an inner portion of shank 74, that is, the portion of shank 74 closest to bulb 72. Segment 126 extends over at least 30% of the height of root 62, such as between 30% and 60% of the height of the root.

[0090] Preferably, segment 126 comprises more than 30% of the height of shank 74. Optionally, segment 126 extends over 100% of the height of shank 74, that is, shank 74 is completely contained within segment 126, which then extends to blade 60.

[0091] Advantageously, the section 126 also extends in the bulb 72 and comprises at least the outer portion of the bulb 72, that is to say the portion of the bulb 72 closest to the shank 74. The section 126 then typically comprises at least 30% of the height of the bulb 72.

[0092] 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, which is between 1% and 25% of the radius of the segment 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 pitch axis C and passing through said point, which is between 20% and 40% of the radius of the segment 126 measured in this same direction.

[0093] At least one of the layers 130, 132, 134 is comprised of the composite material structure 120. Figures 5 to 9 In the example of FIG. 1 , the surface layer 130 is composed of the composite material structure 120 . Figure 5 , Figure 6 and Figure 7 In the example shown, the central layer 132 and (where appropriate) the intermediate layer 134 are also composed of the composite structure 120.

[0094] exist Figure 5 In the exemplary embodiment of the present invention, the difference in stiffness between the layers 130, 132, 134 is obtained by the difference in the longitude and latitude ratio, which ratio increases as the stiffness increases. Therefore, the longitude and latitude ratio of the center layer 132 is greater than the longitude and latitude ratio of the surface layer 130. In addition, the longitude and latitude ratio in the middle layer 134 is between the longitude and latitude ratio of the center layer 132 and the longitude and latitude ratio of the surface layer 130, where appropriate, and is preferably close to the average value between the longitude and latitude ratio of the center layer 132 and the longitude and latitude ratio of the surface layer 130.

[0095] Thus, the central layer 132 and, where appropriate, the intermediate layers 134 have a surplus of warp strands 122 relative to the surface layers 130. Advantageously, these surplus warp strands 122 gradually blend with the weft strands 124 of the surface layers 130 from the upper end of the segment 126 to limit the gradients of performance variations (stiffness and breaking strength) in the composite structure 120 that could weaken the blade 56.

[0096] exist Figure 6 In an exemplary embodiment of the present invention, the difference in stiffness between the layers 130, 132, 134 is obtained by the difference in the take-up rate of the warp strands 122, and the take-up rate decreases as the stiffness increases. Therefore, the take-up rate of the warp strands 122 in the center layer 132 is lower than the take-up rate in the surface layer 130. In addition, where appropriate, the take-up rate of the warp strands 122 in the middle layer 134 is greater than the take-up rate of the warp strands 122 in the center layer 132 and lower than the take-up rate of the warp strands 122 in the surface layer 130.

[0097] Here, this take-up difference is obtained by inserting unidirectional warp strands 136 (that is, warp strands 122 with a take-up of 0%) into the central layer 132, and the surface layer 130 does not have such unidirectional warp strands 136. These unidirectional warp strands 136 are usually free warp strands, that is, they are not entangled with the weft strands 124. Here, the central layer 132 is composed of the unidirectional warp strands 136.

[0098] Advantageously, the unidirectional warp strands 136 of the center layer 132 gradually blend with the weft strands 124 of the surface layer 130 and (where appropriate) the intermediate layer 134 from the upper end of the section 126 to limit performance variation gradients (stiffness and breaking strength) in the composite structure 120 that could weaken the blade 56.

[0099] As a variant (not shown), the take-up difference is obtained by applying different tensions to the warp strands 122 and / or the weft strands 124 in the weaving machine used to produce the fiber reinforcement, so that the tension to which the core warp strands 122 (in the central layer 132) are subjected is greater than the tension to which the warp strands 122 are subjected near the skin (in the surface layer 130), and / or the tension to which the core weft strands 124 are subjected is lower than the tension to which the weft strands 124 are subjected near the skin. The change in the tension applied to the weft strands 124 does have a direct effect on the tension to which the warp strands 122 are subjected near their interface with the weft strands 124, and therefore also on the take-up and stiffness of the warp strands 122. For example, the difference in tension to which the warp strands 122 are subjected is obtained by increasing the tension applied to the warp strands 122 in the center layer 132 and / or by reducing the tension applied to the warp strands 122 in the surface layer 130 and (where appropriate) the middle layer 134. As a variant or alternatively, the difference in tension to which the warp strands 122 are subjected is obtained by reducing the tension applied to the weft strands 124 in the center layer 132 and / or by increasing the tension applied to the weft strands 124 in the surface layer 130 and (where appropriate) the middle layer 134.

[0100] The variation of tension applied to the warp strands 122 and / or the weft strands 124 by the loom can be obtained by any suitable means, the principle being to apply a restoring tension directly at the outlet of the bobbin on which the strands are wound. In a manner known per se, the tension can be applied by pulling a spring system of each warp strand 122, or by using a counterweight located between the outlet of the warp strands 122 from the bobbin and the eyelet of the loom heald. In addition, bobbins are commercially available for controlling the tension applied. Finally, the tension applied to the weft strands 124 can be managed in a manner similar to the warp strands 122, and / or by using a clamp that clamps the end of the weft strands 124 and pulls it through the shed (interweaving of the warp threads), and then releases the weft strands 124 after the beat of the loom enters the next sequence. These devices for applying tension to the (warp 122 or weft 124) strands are known per se and will not be described in detail here.

[0101] When a tension difference is obtained by applying different tensions to the warp strands 122 of the central layer 132 and to the warp strands 122 of the surface layer 130 and (where appropriate) the intermediate layer 134, it is advantageous to gradually remove the warp strands 122 of the surface layer 130 and (where appropriate) the intermediate layer 134 from the fiber reinforcement starting from the upper end of the section 126 and to replace them with as many warp strands 122 as possible that are subjected to a tension equal to that of the warp strands 122 of the central layer 132, so as to limit performance variation gradients (stiffness and breaking strength) in the composite structure 120 that could weaken the blade 56.

[0102] exist Figure 7 In an exemplary embodiment of the present invention, the difference in stiffness between the layers 130, 132 is obtained by the difference in the properties of the fibers constituting the warp strands 122 and / or weft strands 124 of the central layer 132 and the fibers constituting the warp strands 122 and / or weft strands 124 of the surface layer 130 and (where appropriate) the intermediate layer 134. Therefore, the warp strands 122 and / or weft strands 124 of the surface layer 130 are generally composed of fibers of a first material, the warp strands 122 and / or weft strands 124 of the central layer 132 are composed of fibers of a second material, and (where appropriate) the warp strands 122 and / or weft strands 124 of the intermediate layer 134 are composed of a third material, the elastic modulus of the first material being lower than the elastic modulus of the second material, and the third material having an elastic modulus, where appropriate, being between the elastic moduli of the first material and the second material, preferably close to the average value of the elastic moduli. For example, the first material has an elastic modulus between 150 GPa and 190 GPa and is typically composed of glass or basalt, the second material has an elastic modulus between 240 GPa and 350 GPa and is typically composed of carbon, and the third material has an elastic modulus between 195 GPa and 235 GPa and is typically composed of carbon.

[0103] Advantageously, the warp strands 122 of the surface layer 130 and (where appropriate) the intermediate layer 134 are gradually removed from the fiber reinforcement from the upper end of the section 126 and replaced with as many warp strands 122 of the second material as possible to limit the performance variation gradients (stiffness and breaking strength) in the composite structure 120 that could weaken the blade 56.

[0104] exist Figure 8 and Fig. 9 In the example of FIG. 1 , only the surface layer 130 is composed of the composite material structure 120. The center layer 132, for its part, is composed of another, more rigid structure. The intermediate layer 134 is not present.

[0105] Therefore, in Figure 8 In the exemplary embodiment of , the center layer 132 is composed of a laminate 140 of unidirectional plies 142 whose fibers are oriented substantially parallel to the pitch axis C. Fig. 9 In the exemplary embodiment of FIG. 1 , the center layer 132 is comprised of a structure 144 made of a metal, such as steel or titanium.

[0106] Advantageously, the thickness of the laminate 140 or metal structure 144 gradually decreases from the upper end of the section 126, and this thickness loss is compensated by gradually introducing additional warp strands 122 and weft strands 124 into the composite structure 120 to limit the performance variation gradients (stiffness and breaking strength) in the blade 56 that may weaken the blade 56.

[0107] exist Fig.10 In the example of FIG. 1 , only the center layer 132 is composed of the composite material structure 120. The surface layer 130, for its part, is composed of another, more flexible structure. The intermediate layer 134 is not present.

[0108] Thus, in the example shown, the surface layer 130 is composed of a laminate 150 of additional plies 152. These additional plies 152 are preferably multidirectional plies, for example bidirectional plies. Each additional ply 152 is typically composed of a 2D woven web or a non-crimp fabric web (better known by the abbreviation NCF).

[0109] The additional ply 152 is preferably an inclined ply, the fibers of which are oriented at an angle between 5° and 95°, advantageously between 20° and 60°, for example at an angle substantially equal to 45°, relative to the pitch axis C. As a variant or alternatively, the fibers of the additional ply 152 have a lower modulus of elasticity than the fibers of the warp strands 122 and weft strands 124 constituting the composite structure 120.

[0110] Advantageously, the thickness of the laminate 150 tapers from the upper end of the section 126 , this loss in thickness being compensated by gradually introducing additional warp strands 122 and weft strands 124 into the composite structure 120 to limit performance variation gradients (stiffness and breaking strength) in the blade 56 that could weaken the blade 56 .

[0111] It should be noted that these different exemplary embodiments can be combined with each other. Therefore, in an embodiment of the present invention not shown:

[0112] - the segment 126 comprises a surface layer 130 , a core layer 132 and an intermediate layer 134 ,

[0113] ○The central layer 132 is composed of the following parts:

[0114] ■Laminate 140, or

[0115] ■Metal structure 144, and

[0116] o The center layer 132 and the middle layer 134 are composed of the composite material structure 120,

[0117] ○ The middle layer 134 has:

[0118] ■ a latitude and longitude ratio greater than that of the surface layer 130, and / or

[0119] ■ a warp strand 122 having a lower take-up rate than the warp strand 122 of the surface layer 130, and / or

[0120] ■ the warp strands 122 or weft strands 124 are composed of fibers of a material having a larger elastic modulus than the elastic modulus of the fibers of the warp strands 122 or weft strands 124 constituting the surface layer 130;

[0121] The segment 126 comprises a surface layer 130 , a core layer 132 and an intermediate layer 134 , all three layers being composed of the composite material structure 120 ,

[0122] o The core layer 132 has:

[0123] ■ a latitude and longitude ratio greater than that of the intermediate layer 134, and / or

[0124] ■ a warp strand 122 having a lower take-up rate than the warp strands 122 of the intermediate layer 134, and / or

[0125] ■ the warp strands 122 or weft strands 124 are composed of fibers of a material having a modulus of elasticity greater than the modulus of elasticity of the fibers of the warp strands 122 or weft strands 124 constituting the intermediate layer 134; and

[0126] ○ The middle layer 134 has:

[0127] ■ a latitude and longitude ratio greater than that of the surface layer 130, and / or

[0128] ■ a warp strand 122 having a lower take-up rate than the warp strand 122 of the surface layer 130, and / or

[0129] ■ the warp strands 122 or weft strands 124 are composed of fibers of a material having a larger elastic modulus than the elastic modulus of the fibers of the warp strands 122 or weft strands 124 constituting the surface layer 130;

[0130] - the segment 126 comprises a surface layer 130 , a core layer 132 and an intermediate layer 134 ,

[0131] ○ The surface layer 130 is composed of a laminate 150,

[0132] o The middle layer 134 is composed of the composite material structure 120, and

[0133] ○The central layer 132 is composed of the following parts:

[0134] A composite material structure 120 having:

[0135] a warp-to-weft ratio greater than that of the intermediate layer 134, and / or

[0136] Warp strands 122 having a lower take-up rate than the warp strands 122 of the intermediate layer 134, and / or

[0137] The warp strands 122 or weft strands 124 are composed of fibers of a material having a modulus of elasticity greater than the modulus of elasticity of the fibers of the warp strands 122 or weft strands 124 constituting the intermediate layer 134;

[0138] ■Laminate 140, or

[0139] ■Metal structure 144;

[0140] - Section 126 includes only a surface layer 130 and a center layer 132, both of which are composed of a composite material structure 120, the center layer 132 having a warp-to-weft ratio greater than the warp-to-weft ratio of the surface layer 130, and / or warp strands 122 having a take-up rate lower than the take-up rate of the warp strands 122 of the surface layer 130, and / or warp strands 122 or weft strands 124 composed of fibers of a material having an elastic modulus greater than the elastic modulus of the fibers constituting the warp strands 122 or weft strands 124 of the surface layer 130.

[0141] Although in the above exemplary embodiment, the center layer 132 has the highest stiffness, the present invention is not limited to this embodiment. Generally, the layer with the highest stiffness is suitable as the inner layer between the pitch axis C and the surface layer 130.

[0142] Therefore, in Fig.11In another embodiment shown, the layer with the highest stiffness is the middle layer 134 .

[0143] Thus, the stiffness of the middle layer 134 is strictly greater than the first stiffness (of the surface layer 130) and the second stiffness (of the center 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% of the stiffness of the middle layer 134, preferably less than or equal to 25% of the stiffness of the middle layer 134, and the second stiffness is less than or equal to 66% of the stiffness of the middle layer 134, preferably less than or equal to 50% of the stiffness of the middle layer 134.

[0144] Advantageously, the stiffness varies continuously at the interfaces between the layers 130 , 132 , 134 and within the surface layer 130 and the center layer 132 , so that the stiffness of the section 126 gradually increases from the pitch axis C to the middle layer 134 and then decreases (still gradually) from the middle layer 134 to the skin surface 70 of the root 62 .

[0145] exist Fig.11 In the example shown, the higher stiffness of the middle layer 134 is obtained by making the warp and weft ratio of the composite structure 120 inside the middle layer 134 higher than the warp and weft ratio in the surface layer 130 and the central layer 132. As a variant (not shown), this higher stiffness is obtained in the following way:

[0146] - using warp strands 122 in the middle layer 134 having a take-up rate lower than the take-up rate of the warp strands 122 in the surface layer 130 and the center layer 132; and / or

[0147] - In the intermediate layer 134 , the warp strands 122 or the weft strands 124 composed of fibers of a material having an elastic modulus greater than that of the fibers constituting the warp strands 122 or the weft strands 124 of the surface layer 130 and the center layer 132 are used.

[0148] As a result of the exemplary embodiments described above, the base of the neck 82 and the shank 74 are more flexible in the skin and more rigid in the core, which reduces the concentration of 1P loads on the skin at the root 62. Due to the greater stiffness of the core, the stress caused by the 1P load is better distributed over the depth of the root 62, which reduces the burden on the skin. The stress is better distributed, and the resistance of the blade 56 to the 1P force is increased without changing its shape and, therefore, its aerodynamic characteristics. Therefore, it is possible to produce a blade 56 made at least partially of a composite material, which has sufficient resistance to 1P loads so that the blade 56 can be equipped on a turbojet engine with an unducted fan (such as the turbine 12). Therefore, the blades of a turbojet engine with an unducted fan can be lightened by simply replacing these blades with blades such as the blade 56, without reducing the resistance of the blade to the 1P force or the aerodynamic performance of the blade.

Claims

1. A variable pitch blade (56) for a turbomachine fan, comprising an aerodynamically shaped blade (60) and a root (62) configured to be inserted into a cavity (88) of a fan hub (55), the blade (56) being pivotable 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 shank (74) connecting the bulb (72) to the blade (60), the bulb (72) being connected to the shank (74) by a neck (82), the neck defining a local minimum of a cross section of the root (62) along a plane orthogonal to the pitch axis (C), in, At least one section (126) of the root (62) includes a surface layer (130) that at least partially defines the skin surface (70) and an inner layer (132, 134) between the pitch axis (C) and the surface layer (130), the surface layer (130) having a first stiffness, and the inner layer (132, 134) having a second stiffness that is strictly greater than the first stiffness, and the section (126) of the root (62) includes at least a portion of the handle (74) and the neck (82).

2. The variable pitch blade (56) according to claim 1, wherein: The first stiffness is less than or equal to 50% of the second stiffness, preferably less than or equal to 25% of the second stiffness.

3. The variable pitch blade (56) according to claim 1 or 2, wherein: 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, said thickness being between 1% and 25% of the radius of the section (126) of the root (62) measured in the same direction.

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

5. A variable pitch blade (56) according to any one of the preceding claims, wherein: The inner layer (132, 134) is composed of a metal structure, such as a steel or titanium structure.

6. The variable pitch blade (56) according to any one of claims 1 to 4, wherein: The surface layer (130) is composed of a woven composite material and the inner layer (132, 134) is composed of a laminate (140) of unidirectional plies (142) having fibers oriented substantially parallel to the pitch axis (C).

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

8. The variable pitch blade (56) according to claim 4, wherein: The take-up rate of the warp strands (122) in the inner layer (132, 134) is lower than the take-up rate in the surface layer (130).

9. A variable pitch blade (56) according to any one of the preceding claims, wherein: The surface layer (130) and the inner layers (132, 134) each extend over the entire height of the section (126) of the root (62).

10. A variable pitch blade (56) according to any one of the preceding claims, wherein: The inner layer (132, 134) consists of a central layer (132) extending from the pitch axis (C) to the surface layer (130).

11. The variable pitch blade (56) according to claim 10, wherein: The section (126) of the root (62) includes an intermediate layer (134) between the surface layer (130) and the central layer (132), the intermediate layer (134) having a third stiffness that is strictly greater than the first stiffness and strictly lower than the second stiffness.

12. The variable pitch blade (56) according to any one of claims 1 to 10, wherein: The stiffness of the section (126) of the root (62) gradually increases from the surface layer (130) to the inner layer (132).

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

14. A turbomachine (12) comprising at least one fan (50) according to claim 13.

15. An aircraft (10) comprising at least one turbine (12) according to claim 14.

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