Patent Publication Number: US-10316669-B2

Title: Reinforcement for the leading edge of a turbine engine blade

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of French Patent Application 1660479, filed Oct. 28, 2016, the contents of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a turbine engine blade, and more particularly to a reinforcement for the leading edge of such blade. Blade means here both the moving blades and the fixed blades of turbine engines. 
     BACKGROUND 
     In order to increase the resistance of blades to FOD (foreign object damage) in the airflow, that is to say to foreign bodies such as birds and hailstones, they comprise a leading-edge reinforcement, the role of which is to protect the leading edge from damage during impact with an FOD and to distribute the impact force over a large surface area of the blade. 
     A reinforcement for the blade leading edge conventionally comprises a suction-face fin at least partially covering the aerodynamic suction-face surface of the blade and a pressure-face fin at least partially covering the aerodynamic pressure-phase surface of the blade, these two fins being joined by a leading edge of the reinforcement. 
     When the blade is able to move with respect to the axis of the turbine engine, it turns its pressure-face surface to the front, that is to say the air comes into contact on the pressure-face surface, thus creating an overpressure on the pressure-face surface and a negative pressure on its suction-face surface. 
     The impact of an FOD on the leading-edge reinforcement has a tendency to cause a detachment of the upper portion of the pressure-face fin. Beyond a certain mass of FODs, the force of the impacts is greater on the reinforcement, which is also causes a detachment of the upper portion of the suction-face fin. The overpressure generated on the pressure-face tends to limit the detachment of the pressure-face fin to the pressure face. On the other hand, the combination of centrifugal force, greater at the blade tip than at the root, with the negative pressure generated on the suction face, tends to promote the detachment of the suction-face fin. 
     When the blade is a fan blade mounted in an external fairing carrying an internal abradable layer facing the blades, the detachment of the suction-face fin causes damage to the internal abradable layer. This is because the suction-face fin projects from the suction face of the blade and penetrates the internal abradable layer, which creates a furrow in the internal abradable layer. It is then necessary to immobilise the turbine engine in order to replace both the blade the leading-edge reinforcement of which has detached and the internal abradable layer. Such a mobilisation gives rise to a high cost resulting from the lack of operation of the turbine engine, which it is important to reduce or even eliminate. 
     SUMMARY 
     The aim of the invention is in particular to afford a simple, effective and economical solution to this problem. 
     To this end, the invention proposes, firstly, a turbine engine blade extending along a longitudinal axis, comprising an aerodynamic surface that extends in a first direction between a leading edge and a trailing edge, and in a second direction substantially perpendicular to the first direction between a root and tip of the blade, and a leading-edge reinforcement comprising a fin partly covering the aerodynamic surface of the blade, characterised in that the fin has a radially outer edge arranged in the vicinity of the tip of the blade and extending between the leading edge and the trailing edge, this radially outer edge comprising an upstream point fitting flush with the tip of the blade at the leading edge and a downstream point distant from the tip of the blade. 
     The spacing of the downstream point from the top edge of the suction-face fin makes it possible to limit the penetration of the fin in the internal abradable layer of the turbine engine, in the event of detachment of the downstream point of the blade, since it is then distant from the abradable part because of its distance during the mounting of the blade tip. 
     In a particular embodiment of the invention, the upstream point is situated at the upstream end of the top edge, that is to say at the leading edge of the blade, and the downstream point is situated at the downstream end of the radially outer edge of the fin. 
     In the reference frame of the turbine engine, it can thus be considered that the downstream point is radially spaced towards the inside of the blade tip. 
     Advantageously, the aerodynamic surface is a suction-face surface, and the fin is a suction-face fin, the suction-face part of the reinforcement being more particularly subject to detachment, a detachment increased in particular by the centrifugal force for a moving blade. 
     Advantageously, the radially outer edge of the fin comprises an intermediate point situated between the upstream point and the downstream point and defining with the upstream point a first portion of the radially outer edge, fitting flush with the tip of the blade and, with the downstream point, a second portion of the radially outer edge spacing progressively from the tip of the blade in the direction of the trailing point. 
     The separation into two portions offers a good compromise between limitation of penetration of the fin in the internal abradable layer in the event of detachment of the fin, and good distribution of the forces in the event of impact of an FOD on the leading-edge reinforcement. 
     The intermediate point can be arranged longitudinally at equal distances from the upstream point and downstream point. 
     This makes it possible to protect the blade over the entire height since the first portion fits flush with the tip of the blade. 
     Preferably, the second portion of the radially outer edge of the suction-face fin is curved and convex. This particular form facilitates manufacture of the reinforcement and also limits the creation of disturbances in the airflow. 
     Advantageously, the intermediate point and the trailing point are separated from each other by a distance, measured along a median longitudinal axis of the fin, comprised between 0 and sin α×L÷4 
     where:
         L is the length of the fin before optimisation, that is to say between the upstream point and the fictive extreme point corresponding to the symmetry of the upstream point with respect to the median axis substantially perpendicular to the longitudinal axis of the turbine engine, and passing at least through the centre of the tip of the fin, and   α is the angle measured between a line passing through the upstream point and the intermediate point of the radially outer edge and a tangent to the radially outer edge, parallel to the longitudinal axis and passing through the intermediate point.       

     This distance also offers a good compromise between limitation of penetration of the fin in the internal abradable layer in the event of detachment of the fin, and good distribution of the forces in the event of impact of an FOD on the reinforcement of the leading edge. 
     Preferably, the reinforcement comprises a pressure-face fin partly covering an aerodynamic pressure-face surface of the blade. 
     This pressure-face fin also protects the aerodynamic pressure-face surface of the blade against FODs. 
     To provide good protection of the blade, the leading-edge reinforcement is produced from a metallic material. 
     The invention proposes, secondly, an assembly comprising a central disc on which a plurality of blades as previously described are mounted, said blades being evenly distributed around the periphery of the central disc, and extending substantially radially to the central disc. 
     The invention proposed, thirdly, a turbine engine comprising an assembly as previously described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be understood better and other details, features and advantages of the invention will emerge from a reading of the following description given by way of non-limitative example with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of a turbine engine comprising an assembly having a plurality of blades; 
         FIG. 2  is a perspective view of a blade according to the invention, in particular a fan blade, this blade carrying a leading-edge reinforcement limiting the degradation of the internal abradable layer of the turbine engine; 
         FIG. 3  is a view in cross section of the blade along the cross-sectional plane III-III in  FIG. 2 ; 
         FIG. 4  is a detail view of a top portion of a blade in accordance with the inset IV in  FIG. 2 , and 
         FIG. 5  is a detail view to an enlarged scale of the detail V in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a turbine engine  2  having an assembly  4  comprising a central disc  6  rotatable about a longitudinal axis A of the turbine engine  2 , and on which a plurality blades  8  are mounted. The blades  8  are evenly distributed around the periphery  6   a  of the central disc  6 , and extending substantially radially to the central disc  6 . In the present case, the assembly  4  is the fan of the turbine engine  2 , and the blades  8  are the fan blades. 
     Conventionally, the turbine engine  2  also comprises, from upstream to downstream, and downstream of the fan, a low-pressure compressor  10 , a high pressure compressor  12 , a combustion chamber  14 , a high-pressure turbine  16 , a low-pressure turbine  18  and an exhaust casing  20 . Furthermore, for attachment thereof to the aeroplane, the turbine engine  2  comprises attachment means  22 , in this case two, each carried by an intermediate fan casing  24  carrying an internal abradable layer  24   a  (visible in  FIG. 4 ), and a turbine casing  26 . 
     In the remainder of this description, the term radial means any direction substantially perpendicular to the axis A of the turbine engine  2 , the term upstream the side by means of which the air reaches a part of the turbine engine  2 , and the term downstream the side through which the air moves away from said part of the turbine engine  2 . The airflow direction is depicted in  FIG. 2  by the arrow F. 
     Blade  8  means here both the moving blades (for example the rotor blades) and the fixed blades (for example the stator blades) of the turbine engines  2 . 
     The blade  8 , illustrated in perspective in  FIG. 2  and in cross section in  FIG. 3 , comprises an aerodynamic suction-face surface  28  and an aerodynamic pressure-face surface  30  that extend in a first direction between a leading edge  8   a  and a trailing edge  8   b  of the blade  8 . The blade  8  of a fan being twisted, the first direction changes in a plane XY along the cross section taken in a radial direction along the axis Z, which forms with axes X and Y an orthonormal reference frame in  FIG. 2 . In a second direction substantially perpendicular to the first direction, the aerodynamic suction-face surface  28  and the aerodynamic pressure-face surface  30  extend between a root  8   c  and a tip  8   d  of the blade  8 . 
     The blade  8  also comprises a leading-edge reinforcement  32  comprising a suction-face fin  32   a  partly covering the aerodynamic suction-face surface  28  of the substantially radial blade  8 , and a pressure-face fin  32   b  partly covering the aerodynamic pressure-face surface  30  of the blade  8 . These two fins  32   a ,  32   b  have, as can be seen in  FIG. 3 , a cross section that becomes thinner from upstream to downstream. 
     The two fins  32   a ,  32   b  are joined by a leading edge  32   c  that covers the leading edge  8   a  of the blade  8  and, in cross section, has thickness greater than the maximum thickness of the fins  32   a ,  32   b.    
     As can be seen in  FIG. 2 , the reinforcement  32  of the leading edge  8   a  of the blade  8  extends substantially from the root  8   c  of the blade  8  as far as its tip  8   d.    
     The leading-edge reinforcement  32  is preferably produced from a high-strength metallic material, such as for example a titanium alloy. 
     The detail view in  FIG. 4  shows a particularity of the suction-face fin  32   a  of the leading-edge reinforcement  32 . Indeed, the suction-face fin  32   a  has a radially outer edge  34  (also referred to as the top edge) arranged in the vicinity of the tip  8   d  of the blade and which extends from the leading edge  8   a  to the trailing edge  8   b  ( FIG. 2 ). This radially outer edge  34  comprises an upstream point  34   a  that fits flush with the tip  8   d  of the blade  8  at the leading edge  8   a  and a downstream point  34   b  that is spaced from the tip  8   d  of the blade  8 . The term “upper” extends according to, the orientation in  FIG. 4 . In other words the radially outer edge  34  is disposed radially externally with respect to the axis A of the turbine engine  2 . 
     It should be understood that the upstream point  34   a  is arranged on the same side as the leading edge  8   a  of the blade  8  and the downstream point  34   b  is arranged on the same side as the trailing edge  8   b  of the blade  8  in the direction F of airflow ( FIG. 2 ) on the blade  8  from the leading edge  8   a  to the trailing edge  8   b.    
     Furthermore, the upper radially outer edge  34  of the suction-face fin  32   a  comprises an intermediate point  34   c  situated between the upstream point  34   a  and the downstream point  34   b  and defining with the upstream point  34   a  a first portion  36  of the radially outer edge, fitting flush with the tip  8   d  of the blade  8  and, with the downstream point  34   b , a second portion  38  of the upper edge moving away gradually from the tip  8   d  of the blade  8 . The connection of the first portion  36  of the radially outer edge  34  with the second portion  38  of the upper edge is substantially tangential. 
     According to one aspect, the intermediate point  34   c  is arranged at equal distances from the upstream point  34   a  and the downstream point  34   b , in an axial direction parallel to the longitudinal axis A. However, the intermediate point  34   c  could be closer to the upstream point  34   a  or to the downstream point  34   b.    
       FIG. 5  shows a fictive extreme point  34   e  corresponding to the symmetry of the upstream point  34   a  with respect to a median axis M substantially perpendicular to the axis A of the turbine engine  2 , and passing at least through the centre of the tip of the suction-face fin  32   a . This fictive extreme point  34   e  corresponds to an extreme point of the suction-face fin  32   a  before optimisation thereof. 
     Advantageously, this extreme point  34   e  makes it possible to define the gradual separation of the downstream point  34   b  with respect to the tip  8   d  of the blade  8 . 
     The spacing of second portion  38  of the radially outer edge  34  of the suction-face fin  32   a  is preferably curved and convex. In other words, the second portion  38  has substantially a curved shape that spaces continuously from the tip  8   d  of the blade  8  in the direction of the root  8   c  ( FIG. 2 ) thereof, and this from upstream to downstream. 
     However, according to variant embodiments not shown in the figures, the second portion  38  of the radially outer edge  34  of the suction-face fin  32   a  could be rectilinear or on the other hand comprise an alternation of protrusions and hollows. 
     According to a preferred embodiment shown in  FIG. 5 , the intermediate point  34   c  and the downstream point  34   b  are separated from each other by a distance H 1  measured along the longitudinal median axis M, that is to say in the radial direction Z, H 1  being between 0 and sin α×L÷4 
     where:
         L is the length of the fin  32   a  before optimisation, that is to say between the upstream point  34   a  and the fictive point  34   e , and   α is the angle measured between a line passing through the upstream point  34   a  and the intermediate point  34   c  on the radially outer edge  34  and a tangent T to said radially outer edge  34 , parallel to the longitudinal axis A of the turbine engine  2  and passing through the intermediate point  34   c.          

     The distance L, the tangent T and the angle α are illustrated in  FIG. 5 . 
     Thus, in the event of impact of an FOD on the leading-edge reinforcement  32 , if the suction-face fin  32   a  detaches, it will not come into contact with the internal abradable layer  24   a  carried by the intermediate fan casing  24 . Consequently it will be necessary only to repair the blade  8  that has been impacted (or the impacted blades  8 ), which is simpler, quicker and less expensive that complete immobilisation of the turbine engine  2  for replacing the impacted blade  8  (or impacted blades  8 ) of the intermediate fan casing  24  and its internal abradable layer  24   a.    
     For reasons of simplicity in manufacture of the reinforcement  32  of the leading edge, the pressure-face fin  32   b  also comprises a top edge having an upstream point fitting flush with the tip  8   d  of the blade  8  and a downstream point distant from the upstream point and spaced from the tip  8   d  of the blade  8 , that is to say radially distant internally. 
     The top edge of the pressure-face fin  32   b  may also comprise an intermediate point situated between the leading point and the trailing point and defining with the leading point a first portion of the top edge, fitting flush with the tip  8   d  of the blade  8  and, with the trailing point, a second portion of the top edge spacing gradually from the tip  8   d  of the blade  8  in the direction of the root  8   c.    
     However, the forms and dimensions of the portions of the pressure-face  32   b  are smaller compared with the forms and dimensions of the portions  36 ,  38  of the top edge  34  of the suction-face fin  32   a.    
     Thus an asymmetric reinforcement  32  will be obtained.