Patent Publication Number: US-2023141180-A1

Title: Fan rotor with variable pitch blades and turbomachine equipped with such a rotor

Description:
FIELD OF THE INVENTION 
     The invention lies in the field of fan, ducted (fan) or unducted (propeller), rotors equipped with variable pitch blades. 
     The potential applications are the aeronautical propulsion industry, for example turboprops, variable pitch fans (VPF), unducted fans (unducted single fans or USF), or unducted rotors with two contra-rotating propellers (contra-rotating open rotor or CROR) but also the renewable energy industry (wind turbines). 
     The invention also relates to a turbomachine equipped with a fan rotor having variable pitch blades. 
     PRIOR ART 
     Already known from document FR 2 918 129 is a fan rotor comprising a rotor disc provided at its periphery with cells each intended to receive a blade root, with fixed pitch. This rotor comprises a wedge of 3D-woven composite, interposed between the blade root, also made of 3D-woven composite, and the bottom of the metal cell. During the forced mounting of the wedge, it is elastically deformed due to its longitudinal curvature and exerts a pressure below the blade root, which has the effect of pressing the blade root on the bearing surfaces of the cell. 
     The two roles of the wedge are to ensure the proper positioning of the blade in the disc and to damp the impact of the blade root on the bottom of the cell during a provoked shock, due for example to bird ingestion into the rotor. 
     Also known in the prior art is a fan rotor provided with variable pitch blades. Documents EP 3 010 799 or FR 3 017 163 describe for example a pivot comprising a blade support, provided with a housing intended to receive a blade root, this blade support being secured to a rotating support mounted radially on a propeller hub while being able to pivot around a radial axis of the rotor. 
     Document WO2012/156633 describes a fan rotor the blades of which do not have variable pitch and which consequently does not comprise fasteners mounted in rotation around a pitch axis. This document simply describes the use of a wedge of elastically deformable material, inserted into a cell for receiving a blade root. 
     Document FR 2 934 873 describe a fan rotor provided with cells for receiving a blade root in the form of a dovetail. A wedge is inserted between the blade root and the bottom of the cell. 
     Also known from document FR 2 881 174 is a fan rotor with fixed pitch blades in which the rotor disk is provided with a plurality of cells for receiving blade roots. A deformable wedge is inserted between the bottom of the cell and a blade root. 
     Document FR 3005683 describes a fan rotor with variable pitch blades, provided with a plurality of fasteners for receiving a blade root. The fastener comprises a groove for receiving the blade root and a wedge is inserted in the groove below the blade root. 
     None of these four documents describes or suggests the use of a prestressing rod. 
     The design of a fan blade involves several disciplines, the objects of which are generally antagonistic. It must allow obtaining optimal aerodynamic performance (i.e. supply thrust while maximizing efficiency) and guarantee the mechanical strength of the blade (i.e. withstand the mechanical stresses resulting from static and dynamic loads), while limiting the mass of the blade as well as its acoustic signature. In particular, the improvement of the aerodynamic performance of the fan tends toward an increase in the bypass ratio for a double flow engine, which translates into an increase in its outer diameter and therefore the span of its blades. 
     At the same time, in the previously mentioned architectures (VPF, USF, CROR and turboprop), the engine start is carried out with a very open pitch, called “feathered.” 
     In the appended  FIGS.  1  and  2   , which show respectively conventional blade operation and feathered operation, the blade A, the engine axis X, the propeller plane P, the pitch angle C and the angle of attack I can be seen. 
     Power is proportional to the product of speed and torque. But torque increases with the angle of attack I, which can be increased via the pitch C. Starting feathered allows consuming power through torque, which ensures the safety of the machine by guaranteeing low fan speeds. 
     A person skilled in the art knows that the resulting force (arrow F) on a blade profile is, to a first approximation, perpendicular to the chord of the blade and can be broken down into two components: thrust along the engine axis X and blade drag in the plane P of the propeller. Thus, with the increase in the pitch of the blades, the resulting force moves toward the plane P, which has the effect of increasing the drag of the aerodynamic profile and reducing thrust. 
     In  FIG.  2   , the thrust generated by the fan is zero, the torque is a maximum and the speed a minimum. 
     However, the angle of attack I becomes so large that the blades A then undergo a strongly separated turbulent aerodynamic flow which generates a strong vibration excitation. In particular, in blades with a large chord and large span which generate substantial drag, this aerodynamic force F is intense, even though the speed is not high. 
     For a variable pitch blade, assembled with a pivot like that described for example in documents EP 3 010 799 or FR 3 017 163, this aerodynamic force is so intense that it can cause solid-body movement of the blade root in its cell which are similar to “rolling-up,” see the rotations illustrated in the appended  FIG.  3   : roll i (rotation around the broaching axis of the blade), pitch ii (rotation around an axis perpendicular to the broaching axis and to the radial axis) and finally yaw iii (rotation around the radial axis). 
     In fact, when starting feathered, the reduced speed of the fan does not allow generating a sufficient centrifugal force to prevent these movements induced by the aerodynamic force. It follows that there is a risk of damage to the blade and to the wedge inserted between the blade root and the housing of the “wear by friction” type (fretting), and limitation of the lifetime of the blades of the fan. 
     For this reason, fastening by pivot, previously described, does not seem in this case to be a viable solution for variable pitch fan blades with a large chord and a large span. 
     DISCLOSURE OF THE INVENTION 
     The invention therefore has as its object to limit the rolling-up of the blade when starting the rotor feathered, and to avoid premature wear. 
     To this end, the invention relates to a fan rotor provided with variable pitch blades, this rotor comprising a rotor disc equipped at its periphery with a plurality of fasteners, each fastener being rotatably mounted relative to said rotor disc around a radial pitch axis and each fastener comprising a cell for receiving the root of one of said blades, an elongated wedge being arranged in each cell. 
     In conformity with the invention, the central portion of the blade root has a recess, said blade root being arranged in the cell so that its recess is oriented toward the bottom of the cell, a prestressing rod with at least one cam is also arranged in each cell, said wedge being made of an elastically deformable material, at least one longitudinal segment of this wedge has a transverse profile in the form of an arc the central region of which is curved so that it comprises a domed portion protruding in the direction of the concavity of said arc and a hollow in the direction of the convexity of the arc, said wedge is arranged in the cell so that is domed portion faces the recess of the blade root and its hollow is oriented toward the bottom of the cell, the bottom of said cell comprises as many retraction cavities as the rod has cams, said prestressing rod is interposed between the hollow of the central region of said wedge and the bottom of the cell so that said cam faces the corresponding retraction cavity and said prestressing rod is movable in rotation around its longitudinal axis so as to be able to be moved between a resting position, in which the cam is housed in the retraction cavity, and an armed position in which the cam exerts a radial pressure on the central region of the wedge so as to move said wedge in the direction of the blade root and so that the two lateral regions of said wedge, arranged on either side of the central region, are in contact with the lateral regions of the blade root and are remote from the bottom of the cell. 
     Due to the prestressing rod, it is possible, once this prestressing rod is in the armed position, to deform the wedge so that it then presses on the blade root while behaving like a spring. This allows compensating the loss of centrifugal force by the application of a large preload under the blade root. 
     Compared to a conventional broached fastener (without a prestressing rod), the invention allows limiting the premature wear of the blade during phases of starting when feathered. 
     Moreover, the system is advantageous in terms of mass, bulk and geometric tolerance. 
     According to other advantageous and non-limiting features of the invention, taken alone or in combination:
         there exists a function clearance between the recess of the blade root and the domed portion of the central region of the wedge located facing it, this clearance being greater in the resting position of the rod than when the rod is in the armed position;   the bottom of the cell is provided with a longitudinal groove intended to receive and to guide the prestressing rod and said retraction cavities are provided in this groove;   the wedge has a greater thickness in its central region that the thickness of its ends;   said wedge consists of several sections connected by narrower junctions;   a setting foil is arranged in the bottom of the cell of the fastener;   one of the ends of the prestressing rod, preferably its upstream end, has a shape such as cut-aways which allows its gripping and its driving in rotation around its longitudinal axis by a tool;   the prestressing rod has, in proximity to one of its ends, preferably its upstream end, a poka-yoke which extends radially relative to the longitudinal axis of said rod and which protrudes on the same side of the rod as the cams protrude;   an upstream axial retention lock of the blade root, formed from a plate, is inserted into two upstream slots provided at the upstream end of the flanks of said cell for receiving the blade root, these two upstream slots being arranged in a V, and a rod lock is fastened to said upstream axial retention lock, this rod lock being provided with an opening for receiving one of the ends of the prestressing rod, preferably its upstream end, and a notch for receiving the poka-yoke of said rod, so as to block said prestressing rod in the armed position;   a downstream axial retention lock of the blade root, formed from a plate, is inserted into two downstream slots provided at the downstream end of the flanks of said cell for receiving the blade root, these two downstream slots being arranged in a V, this downstream lock being drilled with an orifice for receiving the downstream end of said wedge and this downstream retention lock has on its inner face a surface forming an axial abutment for the downstream end of said stressing rod;   said wedge is made of a 3D-woven composite material;   the blades are made of 3D-woven composite material.       

     The invention also relates to a turbomachine equipped with a fan rotor with variable pitch blades as mentioned previously. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       Other features, objects and advantages of the invention will be revealed by the description that follows, which is purely illustrative and not limiting, and which must be read with reference to the appended drawings in which: 
         FIG.  1    is a schematic showing the operation of a blade in the conventional mode. 
         FIG.  2    is a schematic showing the operation of a blade in the feathered mode. 
         FIG.  3    is a schematic perspective view of the different rotations that a blade root of the prior art can undergo, housed in a rotor disc cell. 
         FIG.  4 A  is a schematic perspective view of an unducted fan equipped with two contra-rotating rotors in conformity with the invention. 
         FIG.  4 B  is a perspective and partial section view of a portion of the rotor disk of the fan of  FIG.  4 A . 
         FIG.  5    is a perspective view of a cell for receiving the blade root. 
         FIG.  6    is a perspective view of the cell of  FIG.  5   , provided with a setting foil. 
         FIG.  7    is a perspective view of the cell of  FIG.  5   , provided with a setting foil and in which is arranged the prestressing rod, the latter being in the resting position. 
         FIG.  8    is a view similar to  FIG.  7   , but in which the prestressing rod is in the armed position. 
         FIG.  9    is a perspective view of the wedge in conformity with a first embodiment of the invention, and of the prestressing rod located in the resting position. 
         FIG.  10    is a perspective view of the wedge in conformity with a second embodiment of the invention, and of the prestressing rod located in the armed position. 
         FIG.  11    is a schematic view in transverse section of the prestressing rod, of the wedge and of the blade root, arranged inside a cell of a rotor disc fastener, the rod being in the resting position. 
         FIG.  12    is a schematic view in transverse section of the prestressing rod, of the wedge and of the blade root, arranged inside a cell of a rotor disc fastener, the rod being in an intermediate position between its resting position and its armed position. 
         FIG.  13    is a schematic section view of the prestressing rod, of the wedge and of the blade root, arranged inside a cell of a rotor disk fastener, the rod being in the armed position. 
         FIG.  14    is a longitudinal section view of the prestressing rod, of the wedge, of a portion of the blade root and of the cell, of the upstream and downstream retention locks and of the rod lock taken along a section plane passing through the axis X 2 -X′ 2  of the rod  3 . 
         FIG.  15    is a perspective view of the downstream axial retention lock. 
         FIG.  16 A  is a perspective view of the upstream axial retention lock. 
         FIG.  16 B  is a perspective view of the upstream retention lock, taken along a viewing angle different from that of  FIG.  16 A . 
         FIG.  17    is a perspective view of the rod lock. 
         FIG.  18    is a perspective view of the rod lock and of the prestressing rod retained by it. 
         FIG.  19    is a perspective view of the upstream end of the blade fastener, of the prestressing rod, of the upstream retention lock and of the rod lock, the latter two not being assembled. 
         FIG.  20    is a perspective view of the upstream end of the blade fastener, of the prestressing rod, of the upstream retention lock and of the rod lock, the latter two being assembled. 
         FIG.  21    is a view similar to  FIG.  13    showing the load paths in the case of normal operation of the rotor. 
         FIG.  22    is a view similar to  FIG.  13    showing the load paths in the case where the blade root is subjected to roll. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG.  4 A , an example of an unducted fan  1  can be seen, comprising two fan rotors  11 ,  12  conforming to the invention. They are mounted on a nacelle  13 , itself intended to be fastened to the fuselage of an airplane. Each rotor  11 ,  12  comprises a rotor disc  140  (or hub) to which is attached an outer casing  14  mounted in rotation relative to the nacelle  13 , and a plurality of blades  15  attached to the disc. The disc  140  is shown in dotted lines in  FIG.  4 A  because the outer casing  14  hides it, but it is more visible in  FIG.  4 B , in which the casing  14  is not shown. 
     In the present application, upstream and downstream are defined with respect to the normal direction of flow of gas in the rotor  11 ,  12 . Moreover, its axis of rotation is called the axis X of the rotor. The “axial” direction corresponds to the direction of the axis X and a “radial” direction is a direction perpendicular to this axis and passing through it. Moreover, the “circumferential” direction corresponds to a direction perpendicular to the axis X and not passing through it. Unless the contrary is stated, inner and outer, respectively, are used with reference to a radial direction so that the inner portion or face of an element is closer to axis X than the outer portion or face of the same element. 
     Moreover, the rotor  11 ,  12  comprises a fastener  16  for each blade  15 . Each fastener  16  is mounted in rotation relative to the rotor disc  140 , around a radial pitch axis Y. 
     More precisely, the fastener  16  is mounted in rotation inside a housing provided in the rotor disc, by means of balls or other rolling elements. The fastener  16  is also known by the name of “pivot” in the literature. Document FR 2 943 312 can be referred to on the subject of this pivoting assembly. 
     As appears more clearly in  FIG.  5   , each fastener  16  comprises a cell  17  for receiving a blade  15  root  150 , this root having for example the shape of a dovetail. It is more clearly visible in  FIGS.  7  to  13    in particular. 
     The fastener  16  comprises two flanks  161  and  162 , which define between them the upper radial opening  170  of the cell  17 , opposite to the bottom  171  of the cell. The two flanks  161  and  162  are inclined toward one another and form bearing surfaces. 
     Each flank  161 ,  162  is provided with a downstream groove  1611 , respectively  1621  and with an upstream groove  1612 , respectively  1622 . The two downstream grooves  1611 ,  1621 , arranged at the downstream end of the cell  17  face one another and are arranged in a V. the same is true for the two upstream grooves  1612  and  1622  (see  FIG.  5   ). 
     The cell  17  extends in an axial direction between an access on the side of the leading edge of the blade and an access on the side of the trailing edge of the blade. It is by one of these two opposite accesses that a blade root can be engaged in the cell  17 , by sliding. 
     The invention will be better understood by describing explicitly in more detail the relation which exists between the different forces which act on a blade with a pivoting fastener. On the one hand, the centrifugal force exerted on the blade  15  is oriented in the radial direction and its value is proportional to the square of the speed of rotation of the rotor. This force therefore depends strongly on the engine speed. On the other hand, the centrifugal force presses the blade root  150  on the bearing surfaces  161 ,  162  of the cell  17 , which ensure its retention. In other words, each bearing surface of the cell generates reaction force on the blade root  150  which is directed along the normal to the contact surface and the resultant of these forces opposes the centrifugal force. It is deduced that the value of the reaction forces at the bearing surfaces  161 ,  162  is directly linked to the centrifugal force. However, these reaction forces also play another very important role because they oppose the moment of the aerodynamic forces which cause the rolling-up of the blade  15 . Consequently, in the case of starting when feathered, characterized by a reduced engine speed and a turbulent aerodynamic flow, the low centrifugal force induces reaction forces at the bearing surfaces  161 ,  162 , which are insufficient to oppose the moment of the intense aerodynamic forces, which causes the rolling-up of the blade  15 . 
     The invention consists of compensating the low centrifugal force by the application of a large preload under the blade root  150 . To this end, a wedge  2  and a prestressing rod  3  with at least one cam are installed between the blade root  150  and the bottom  171  of the cell. 
     The wedge  2  will now be described by referring to  FIGS.  9  to  13   . 
     This wedge  2  has a generally elongated shape. It has a rectilinear profile along its longitudinal direction shown schematically by the axis X 1 -X′ 1  in  FIGS.  9  and  10   . 
     The wedge  2  consists of a single block of elastically deformable material. This material can for example be aluminum. However, advantageously, this material is a 3D (in three dimensions) woven composite material, for example of the “interlock weave” type. What is meant here by “interlock weave” is a 3D weave pattern in which each layer of warp yarns links several layers of weft yarns, with all the yarns of the same warp column having the same movement in the plane of the weave. 
     Also preferably, this material is anisotropic and its stiffness in the broaching direction, i.e. along the axis X 1 -X′ 1 , is greater than the stiffness in the direction which transmits the forces of the cams to the blade root (circumferential direction Z—see  FIG.  11   ). 
     The wedge  2  comprises at least one longitudinal section. In the exemplary embodiment shown in  FIGS.  9  and  10   , this wedge  2  comprises three longitudinal sections  20   a ,  20   b ,  20   c  (their number could be different), connected by narrower junctions  21  of the same material, the assembly forming a single part. 
     As appears more clearly in  FIG.  11   , each longitudinal section  20   a ,  20   b ,  20   c  has a transverse profile in the form of an arc of which the central region  22  is curved, so that it has a domed portion  221 , protruding in the direction of the concavity of said arc and it has a hollow  220  in the direction of the concavity of the arc. The two lateral regions  23 ,  24  of the wedge are arranged at the two ends of the arc. 
     The wedge  2  is arranged in each cell  17  so as to be interposed between the blade root  150  and the prestressing rod  3 , and so that its two lateral regions  23 ,  24  are in contact with the corresponding lateral regions  151 ,  152  of the blade root  150 , while being remote from the bottom  171  of the cell  17  and its central region  22  is facing the central region  153  of said blade root  150 . 
     The wedge  2  has a downstream end  25  and an upstream end  26  (see  FIG.  9   ). 
     The prestressing rod  3  with at least one cam will now be descried by referring to  FIGS.  7  to  11   . 
     The rod  3  is cylindrical and has two ends, respectively upstream  31  and downstream  32 . 
     It has a longitudinal axis X 2 -X′ 2 . It comprises at least one cam  33 , preferably formed in a single piece with the rest of the rod. This cam  33  protrudes over a portion of the circumference of the rod. The rod  3  can comprise more than one cam  33 , two for example (see  FIG.  10   ) or three (see  FIGS.  7  to  9   ), distributed over its length, or even more than three. 
     Preferably, the prestressing rod  3  is made of steel or of a titanium alloy. 
     Advantageously, the bottom  171  of the cell is prepared and has a longitudinal groove  172 , (preferably machined, see  FIG.  5   ) to receive the rod  3 . This groove  172  has as many retraction cavities  173  as the rod  3  has cams  33 , each retraction cavity  173  being intended to receive a cam  33 . 
     This groove  172  is intended to guide and to support the rod  3  in its regions with no cam. It plays the role of a lower half-bearing when the rod  3  is in the groove. 
     Once placed in the groove, the rod  3  can be manipulated in rotation so as to make it pivot around its longitudinal axis X 2 -X′ 2 , by means of a tool. To this end, one of its ends, preferably its upstream end  31 , advantageously has cut-aways facilitating its gripping (see  FIG.  9   ). 
     Advantageously, the prestressing rod  3  can comprise a poka-yoke  34 , preferably arranged in proximity to the upstream end  31  (see for example  FIGS.  9  and  18   ). In  FIG.  10   , the section is such that the poka-yoke is not visible. 
     The poka-yoke  34  extends radially relative to the longitudinal axis X 2 -X′ 2  of the rod  3  and it is oriented so as to protrude on the same side of the rod  3  as the cams  33  do. 
     Thus, when the poka-yoke  34  extends downward in  FIG.  9    (i.e. in the radially inward direction toward the center of the rotor disc  140 ), the rod  3  is in the position called the “rest” position, and when the poka-yoke extends upward in  FIG.  18    (i.e. in the radially outward direction toward the outside of the rotor disk  140 ), the rod  3  is in the position called the “armed” position. 
     According to a variant embodiment shown schematically in  FIG.  10   , oblong holes  25  are provide in the central regions  22  of at least certain sections  20   a ,  20   b ,  20   c , in order to further improve the flexibility and the deformability of the different portions of the wedge  2 . These oblong holes  25  also allow lightening the wedge. They are positioned at the places which are not in contact with the cams  33  of the prestressing rod  3 . 
     Advantageously, and as can be seen better in  FIG.  6   , a metal setting foil  4  is placed in the bottom of the cell  17 , in order to protect it from friction with the rod  3 . This setting foil  4  also extends to the bearing surfaces  161 ,  162  of the cell in order to protect the fastener  16  from friction with the blade root  150 . The setting foil  4  is preferably perforated around the retraction cavities  173  which are not subject to friction (see the perforations labeled  40 ) and around the grooves  1611 ,  1621 ,  1612 ,  1622 . 
     The setting foil  4  is preferably made of stainless steel and preferably has a thickness of a few tenths of a millimeter. 
     Other protective elements can also be present to protect the parts made of composite materials. Impregnated fabrics (or wearstrips) specially designed to resist friction, can for example be installed on the bearing surfaces of the blade root  150 , in the regions in contact with the cell and on the wedge  2  in regions in contact with the cams  33  or the blade root  150 . 
     Advantageously, and as can be seen in  FIGS.  11  to  13   , the shape of the transverse section of the base of the blade root  150  can have a central recess  154  (shaped like a bone head) in order to adapt itself optimally to the presence of the wedge  2  and the prestressing rod  3  with cams. This particular form allows clearing space for integrating the wedge and the rod below the blade root, while limiting the boring of the cell  17 . 
     However, this is not the only function of this bone head shape, which also serves as an abutment for the cam  2 , by cooperation of shapes, in the event of uncontrolled rolling-up of the blade (bird ingestion for example) as will be explained subsequently. 
     The axial retention of the blade root  150 , of the wedge  2  and of the rod  3  are ensured by a downstream axial retention lock  5 , an upstream axial retention lock  6  and the locking of the rod  3  in its armed position is ensured by a rod lock  7 . These three locks are preferably made of metal. 
     One exemplary embodiment of the downstream axial retention lock  5  is shown in  FIG.  15   . 
     This lock  5  comprises a plate  50  with a pentagonal shape with two lateral edges  51 ,  52 , called “locking” edges, connected by a radially outer edge  53  and two radially inner edges  54 ,  55 . The plate  50  has an inner face  56  on which is arranged a honeycomb damping part  57 . Finally, the plate  50  is drilled with an orifice  58  arranged between the two edges  54 ,  55  lower than the honeycomb  57 . 
     The plate  50  is dimensioned so that its two lateral edges  51 ,  52  can be inserted respectively into the downstream grooves  1611  and  1621  of the fastener  16 , this in a radial direction from inside to outside, i.e. from bottom to top in  FIGS.  5  to  8   . 
     The tip of the lock  5  arranged between its two edges  54 ,  55 , below the orifice  58 , is labeled  59 . One exemplary embodiment of the upstream axial retention lock  6  is shown in  FIGS.  16 A and  16 B . This lock  6  comprises a plate  60  with a hexagonal shape with two lateral edges  61 ,  62 , called “locking” edges, connected by a radially outer edge  63 , and two inner lateral edges  64 ,  65  inclined toward one another and connected by a radially inner edge  66 , opposite to the edge  63 . The plate  60  has an inner face  601 , on which is arranged a honeycomb damping part  67  and an opposite outer face  602 , which is continued outward by a perpendicular wing  68 , drilled with at least one orifice  680 , here with two. 
     The plate  60  is dimensioned so that its two lateral locking edges  61 ,  62  inclined in a V, can be inserted respectively into the upstream grooves  1612 ,  1622  of the fastener  16 , this in a radial direction, from inside to outside, i.e. from bottom to top in  FIGS.  5  to  8   . In this position, the honeycomb  67  is in abutment against the blade root  150  and the lock  6  blocks this root axially. 
     The rod lock  7  will now be described in connection with  FIGS.  17  and  18   . This lock  7  has the general shape of a jumper. It comprises a U shaped portion  70  delimiting a through opening  71  in the lower portion of the U. This U shaped portion is blocked at its upper portion by a plate  72  which extends beyond the U on either side so as to provide two wings  720 . Each wing  720  is drilled with an orifice  721 . 
     A notch  73  is provided in the central portion of the plate  72 , at its junction with the inner face  700  of the U shaped portion  70 , i.e. the face intended to be turned toward the cell  17  when the rod lock  7  is in place (see  FIG.  14   ). 
     In  FIGS.  14  and  20   , it can be seen that the opening  71  is configured to receive the upstream end  31  of the rod  3  so that the periphery of this end (and for example its cut-aways) are no longer accessible and cannot be manipulated in rotation. 
     In  FIGS.  14  and  18   , it can be seen that the notch  73  is configured to receive the radial poka-yoke  34  when the prestressing rod  3  is in the armed position, i.e. when the cams  33  press against the wedge  2 . 
     The rod lock  7  can be assembled with the upstream retention lock  6  by applying the plate  72  against the wing  68 , by aligning the orifices  721  with the orifices  680  and by inserting assembly members  8 , such as screws and nuts, into these orifices (see  FIG.  20   ). 
     The assembly sequence of the blade root  150  into the cell  17  of the fastener  16  follows in succession the following steps: 
     Installing the metal setting foil  4  in the bottom  171  of the cell  17  of the pivoting fastener  16 , 
     Installing the prestressing rod  3  in the bottom of the cell  17  in its rest position, with the cams  33  housed in the retraction cavities  173  (see  FIG.  7   ), 
     Installing the downstream retention lock  5  in the downstream guide slots  1611 ,  1621 , the end  32  of the rod  3  coming into abutment against the tip  59  of the lock  5  (see  FIG.  10   ), Inserting the blade root  150  inside the cell  17  of the fastener  16  by sliding until it is in abutment on the honeycomb  57  of the downstream retention lock  5 , 
     Installing the upstream retention lock  6  in the upstream guide slots  1612 ,  1622 , the honeycomb  67  coming into abutment against the blade root  150 , 
     Introducing the wedge  2  between the blade root  150  and the prestressing rod  3 , the downstream end  25  of the wedge  2  penetrating into the orifice  58  of the downstream retention lock  5  (see  FIG.  14   ). The situation shown in  FIG.  11    is then present. The assembly of the wedge  2  then occurs without effort because a clearance J (see  FIG.  11   ) exists between the face of the rod  3  not having a cam  33  and the bottom of the recess  154  of the blade root  150  and the wedge  2  is rectilinear along its longitudinal axis X 1 -X′ 1 . 
     Applying a torque (arrow G,  FIG.  12   ) to the upstream end  31  of the prestressing rod  3  with a tool to accomplish a rotation of 180° and thus engaging the cams  33  in the hollows  220  of the central region  22  of the wedge  2 . During this rotation, the geometry of the cams  33  generates a hard point felt by the operator, during which the wedge  2  is slightly overstressed (see  FIG.  12   ), without ever entering into contact with the central recess  154  of the blade root  150 . There exists a clearance J 1  between the bottom of the recess  154  and the domed portion  221  (see  FIG.  12   ) which is suited to allowing this transitory deformation of the wedge  2 . Once this hard point is passed, the continuation of the rotation brings the rod  3  into a new equilibrium position (the armed position) shown in  FIG.  13   . The value of the clearance J 1  varies depending on the orientation of the cam  33 . The geometry of the cams  33  is symmetrical, and has a hard point on either side of this armed equilibrium position, preventing the system from being able to disarm itself, in one direction or in the other, without the force supplied by the operator. In the position of  FIG.  13   , the cams  33  are supported on the wedge  2  and induce its deformation. By way of comparison, in  FIGS.  12  and  13   , the dotted line arranged under the cam  2  shows its shape when it is not armed. 
     Installing the rod lock  7  and bolting on the upstream retention lock by means of bolts  8  (see  FIGS.  14  and  20   ). 
     Another advantage of the rod lock  7  resides in the fact that the notch  73  and the U shaped portion  70  surrounding the poka-yoke of the prestressing rod, preventing its assembly if the prestressing rod  3  is not armed. As can be seen in  FIG.  19   , when the poka-yoke  34  is oriented downward (rod  3  at rest), the U shaped portion  70  abuts against it, the orifices  721  not being able to be aligned with the orifices  680  and assembly being impossible. This allows preventing the assembly of the blade without preload of the root  150 . 
     When all the parts are assembled and armed: 
     the downstream retention lock  5  serves as a downstream abutment of the prestressing rod  3  and of the wedge  2 , 
     the wedge  2 , armed by the prestressing rod  3 , pushes the upstream  6  and downstream  5  retention locks into their respective V grooves of the fastener  16 , 
     the poka-yoke  34  of the prestressing rod  3  serves as an upstream abutment for the wedge  2 , 
     the rod lock  7  serves as an upstream abutment for the prestressing rod  3 , locks the arming of the rod  3  and prevents access to the control for rotating the prestressing rod  3 . 
     The disassembly of the blade is easily carried out by performing these steps in the reverse order. 
     When the system is armed, the function of the rod with cams  3  is to impose a radial outward movement on the center  22  of the wedge  2  the ends  23 ,  24  of which are supported by the blade root  150 . The wedge  2  then behaves as a spring because the resulting radial force depends on its structural stiffness. By analogy, the wedge  2  can also be seen, in its transverse section, as a beam in three-point bending. The central force exerted by the cams  33  on the wedge  2  is equal to the sum of the forces exerted by the ends  23 ,  24  of the wedge on the blade root  150 . The force transmitted to the blade root depends on the bending in the center of the wedge  2 , i.e., the radial outward movement, imposed by the cam  33  when the system is armed. Using the same analogy, the internal stresses in the wedge  2  are considered to be a maximum in its center and at the surface. Consequently, an advantage of this system is the possibility of adjusting the preload force on the blade root  150  by acting either on the geometry of the cam  33 , the geometry of the wedge  2  or the material of the wedge  2 . 
     However, the wedge  2  must resist internal pressures due to its bending. It is therefore preferable that it have a minimum thickness, in particular at its center, but also without being too stiff. Thus, advantageously and to satisfy this compromise, the thickness of the wedge  2  decreases from the center (thickness E 1  in  FIG.  13   ) to the ends (thickness E 2 ), with E 1  greater than E 2 . 
     Then it is important not to lose the assembly preload due to the centrifugal force which will move the blade root radially outward (upward in  FIGS.  11  to  13   ) and consequently relax the deformation level of the wedge  2 . This implies that the radially outward movement imposed by the cams  33  is large compared to the radially outward movement of the blade root  150  in the starting regime. To achieve the targeted preload force, it is therefore preferable to reduce the stiffness of the wedge  2 , for example by reducing its stiffness E 1 , and increasing the movement imposed by the cams  33  by increasing their dimensions. 
     In  FIG.  21   , the dotted arrows show the force paths when the prestressing rod  3  is in the armed position and the rotor is in normal operation. The forces transit from the center  22  of the wedge  2  to its ends  23 ,  24  in contact with the blade root. The functional clearance J 1  between the domed portion  221  of the wedge  22  and the recess  154  of the blade root  150  allows the wedge  2  to play its role as a spring. This allows retaining the preload when the blade  15  moves under the influence of a centrifugal force. 
       FIG.  22    shows the situation when the prestressing rod  3  is armed but the preload becomes insufficient to counteract the roll-up of the blade root  150 , due to too great an aerodynamic force or a bird ingestion into the rotor blades for example. In this case, not only does the blade root  150  roll up (arrow H) but it descends into the cell  17 . The surfaces in contact between the wedge  2  and the blade root  150  approach the surfaces in contact between the wedge  2  and the cams  33 . The force paths (dotted lines) shorten and it is the compression of the wedge  2  in its thickness that directly opposes the rolling-up of the blade. The wedge  2  acts as an abutment. The value of the clearance J 1  decreases. 
     Finally, it will be noted that the different sections  20   a ,  20   b ,  20   c  of the wedge  2  react separately to the movement of the blade  15 , which allows accompanying as well as possible the movement of each of the portions of the blade root  150 . 
     It should be noted that the clearance J 1 , present between the recess  154  of the blade root  150  and the domed portion  221  of the central region  22  of the wedge  2  arranged facing it, is greater in the resting position of the rod  3  than when the rod is in the armed position. 
     In the preceding description, the blades  15  are made of a 3D-woven composite material on the general principle of known blades made of composite having a woven preform impregnated with a resin, the root of which is however adapted to conform to the provisions for the root described in the corresponding passage of the preceding description.