Abstract:
A gas turbine engine including at least a first composite part configured for mounting in a second metal part of the engine, the first composite part including an interface surface configured to be in surface contact with the second metal part, the engine including a metal protection removably mounted on the first composite part and configured to cover the interface surface.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to gas turbine engines in particular for use in the aeronautical field. 
     Description of the Related Art 
     A gas turbine engine intended to be fitted on an aircraft conventionally includes a plurality of parts which are connected to one another by fixings of different types such as flanges. In some configurations, two parts of the engine can be made to slide against one another along their contact surfaces due to thermal expansion. By way of example, a low-pressure distributor of a gas turbine engine includes a plurality of metal blades which are mounted in a fixed manner in the metal housing of the engine. To this end, each blade includes a lower platform and an upper platform which are shaped so as to be fixed respectively to a lower housing element and to an upper housing element of the engine. During the operation of the engine the platforms of the blade and the housing elements expand, which results in sliding between the platform and the housing element thereof in which it is fixed. The surface contact of two parts is currently designated “fretting” by the person skilled in the art. The more substantial the fretting is, the more the fatigue resistance of the parts is reduced. 
     In the engines according to the prior art, the fretting has no drawbacks given that the parts in contact are both metal and thus have comparable expansion factors. 
     In order to reduce the mass of an engine, it was proposed to replace the metal blades by blades made of composite material having fibres embedded in a matrix. When a platform of a blade made of composite material is in surface contact with a metal housing element, the matrix of the platform deteriorates and can cause oxidation of the fibres of the composite material. The service life and the performance of the blades made of composite material may then be impaired. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to eliminate at least some of these drawbacks, the invention proposes to protect the contact surfaces of the platforms made of composite material by covering them with a metal protection. The origin of the invention lies in the fixing of a distributor blade made of composite material in a metal housing but it is applicable to the fixing of any part made of composite material which has a metal part and is subject to “fretting”. 
     To this end, the invention relates to a gas turbine engine including at least a first composite part arranged for mounting in a second metal part of the engine, the first composite part including an interface surface intended to be in surface contact with the second metal part, the engine including a metal protection removably mounted on the first composite part and adapted to cover said interface surface. 
     The metal protection advantageously makes it possible to protect the matrix of the composite part, which improves the service life of the assembly. The second metal part is solely in surface contact with the metal protection, which limits the damage due to fretting. 
     The metal protection preferably has a shape complementary to the composite part so as to be able to form a metal skin on the composite part. 
     The first composite part preferably includes a means for radial locking of the metal protection. Thus the metal protection is integral with the composite part whilst being free to expand. 
     According to a preferred aspect of the invention, the radial locking means is presented in the form of an annular groove. 
     Preferably, the metal protection includes an annular lip adapted to co-operate with the annular groove. Thus the metal protection is locked radially with the composite part by co-operation of the annular groove with the annular lip. 
     According to an aspect of the invention, the metal protection includes an annular groove of which the cavity is turned towards the second metal part. 
     Preferably, the metal protection being formed by deformation of a flat metal sheet, the annular groove is formed facing the annular lip on the metal sheet. The metal protection advantageously originates from a flat metal sheet, which reduces the cost of manufacture of the protection. The deformation of the metal sheet in order to form the annular lip on a face of the sheet results in the formation of an annular groove on the opposite face of the metal sheet. 
     The metal sheet preferably has a constant thickness, which reduces the cost of manufacture of the protection. 
     Also preferably, the engine includes a sealing joint in the annular groove of the metal protection so as to provide sealing between the metal protection and the metal part. The sealing joint is preferably toric. 
     Preferably, the first composite part is a stator blade and the second metal part is a housing of the engine. The composite stator blade, covered with a metal protection, is advantageously fixed in the metal housing of the engine, which avoids any damage to the blade by fretting. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be better understood when reading the following description given solely by way of example and with reference to the appended drawings, in which: 
         FIG. 1  shows a sectional view of a blade of a low-pressure distributor of a gas turbine engine according to the invention; and 
         FIG. 2  shows a view close to the upstream contact zone between the lower platform of the blade and a lower housing element of the gas turbine engine of  FIG. 1 . 
     
    
    
     With reference to  FIG. 1 , a gas turbine engine for aircraft extends axially along an axis X. The terms “upstream” and “downstream” are defined below in relation to direction of the gases in the engine, the axis X being oriented from the upstream to the downstream direction in  FIG. 1 . The low-pressure distributor of the engine includes a plurality of stator blades  1  which are mounted in the metal housing of the engine  2 . 
     DETAILED DESCRIPTION OF THE INVENTION 
     In this example the blade  1  consists of composite material formed from fibres embedded in a matrix. In this example, the composite material includes glass fibres embedded in a ceramic matrix (CMC). As shown in  FIG. 1 , the blade  1  includes an upper fixing platform  11  and a lower fixing platform  12  which are made of composite material. Each platform  11 ,  12  includes an upstream leg  13  and a downstream leg  14  which are fixed to the housing  2  of the engine. The housing  2  includes an upper housing element  21  and a lower housing element  22  to receive respectively the upper fixing platform  11  and the lower fixing platform  12 . To hold the upstream and downstream legs  13 ,  14  of each platform  11 ,  12 , each housing element  21 ,  22  includes an upstream flange  23  and a downstream flange  24  arranged to be in surface contact with the upstream and downstream legs  13 ,  14  respectively as shown in  FIG. 1 . 
     Each platform  11 ,  12  is preferably produced in a loose texture. A loose texture is characterised by a separation, in its thickness, of the layer of fibres forming the platform in order to form two distinct layers, known as loose texture legs, which diverge radially from one another whilst remaining connected by the ceramic matrix. 
     In order to prevent the legs  13 ,  14  made of ceramic material of the blade  1  from being damaged by surface contact with the flanges  23 ,  24  of the housing  2 , the legs  13 ,  14  are covered by a metal protection  3  as shown in  FIGS. 1 and 2 . 
     With reference to  FIG. 2  showing the upstream flange  23  of the lower housing element  22  in contact with the upstream leg  13  of the lower platform  12 , the upstream metal flange  23  includes a radial annular surface S 1  intended to come into contact with a radial annular surface S 2  (or interface surface S 2 ) of the upstream leg  13  made of composite material. In order to limit the stresses relating to the differential expansions and protect the upstream leg  13  made of composite material, the contact surface S 2  of the upstream leg  13  is covered by a metal protection  3  known to the person skilled in the art under the name “foil”. This metal protection  3  is removable from the upstream leg  13  to enable it to be replaced in case of wear. 
     The surface contact between the metal protection  3  and the metal flange  2  does not present any drawbacks. The two metal materials are not prone to deteriorate by friction against one another by “fretting” given that the coefficients of expansion thereof are comparable. Furthermore, only the contact zones are protected, which do not have an adverse effect on the mass of the blade  1 . 
     The metal protection  3  is mounted integral with the composite leg  13  in order to form a skin at the level of its contact surface S 2  with the metal flange  23 . In other words, the metal flange  23  is no longer in contact with the composite leg  13  but with the metal protection  3 , which eliminates any risk of wear and deterioration of the composite material of the upstream leg  13  by friction with the metal flange  23 . 
     As shown in  FIGS. 1 and 2 , the upstream leg  13  of the lower platform  12  extends radially towards the axis of the engine. The metal protection  3  envelops the radial end of the upstream leg  13  in such a way as to cover the upstream surface S 2 , intended to be in contact with the metal upstream flange  23 , and the downstream surface S 3  thereof. In this example, the metal protection  3  is circumferential and has an axial U-shaped section, the upstream leg  13  being accommodated in the cavity of the U. In other words, the metal protection  3  includes an upstream branch  31 , a bottom  32  and a downstream branch  33 , the branches  31 ,  33  being resilient to enable the mounting of the protection  3 . In this example, with reference to  FIG. 2 , the downstream branch  33  is longer than the upstream branch  31 . The form of the metal protection  3  is advantageously defined to co-operate, by complementarily of forms, with the upstream leg  13  in order to form an integral connection which compensates for the thermal expansions. 
     The upstream leg  13  also includes a means for radial locking of the protection  3 . In this example, the locking means is in the form of a circumferential annular groove  15  arranged in the upstream contact surface S 2  of the upstream leg  13  as shown in  FIG. 2 . The annular groove  15  of the upstream leg  13  is arranged to receive a circumferential annular lip  34  which is formed in the upstream branch  31  of the metal protection  3  and which extends axially downstream to penetrate into the cavity of the annular groove  15 . Thus the protection  3  is firmly positioned on the upstream surface S 2  of the upstream leg  13 . 
     Once the protection  3  is placed on the composite leg  13 , the protection  3  cannot move radially relative to the composite leg  2  because of the co-operation of the lip  34  in the groove  15 , rotation of the metal protection  3  relative to the composite leg  13  being nevertheless still possible. An annular groove  15  is very advantageous as a means of locking the protection, given that it allows thermal expansion of the protection  3  relative to the upstream leg  13 . Of course other locking means could also be suitable. 
     Preferably, the metal protection  3  is made of a flat metal sheet which preferably has a constant thickness. The sheet is deformed to form a three-dimensional protection. Such a metal protection  3  advantageously has a low manufacturing cost. 
     As shown in  FIG. 2 , the protection  3  has on the upstream face of its upstream branch  31  an annular groove  35  of which the cavity is turned towards the upstream flange  23 . In this example, the annular groove  35  is facing the lip  34 . During the three-dimensional deformation of the sheet of constant thickness to form the protection  3 , the formation of the annular lip  34  on a face of the sheet results in the formation of a groove  35  on the opposite face. 
     In other words, with reference to  FIG. 2 , the upstream surface of the protection  3  which is in contact with the upstream flange  23  is not flat but has an annular groove  35  of which the cavity extends axially downstream. Advantageously, a toric sealing joint  4  is fitted in the annular groove  35  of the composite leg  13 . As shown in  FIG. 2 , the joint  4  is fitted between the upstream flange  23  and the protection  3  in order to ensure sealing between the upstream surface of the protection  3  and the downstream surface S 1  of the upstream flange  23 . This is particularly advantageous in order to form a sealed volume between the lower housing element  22  and the lower platform  12  of the blade  1 , for example, when the blade  1  includes a ventilation tube  5  opening into the said volume ( FIG. 1 ). 
     Previously an annular housing element  21 ,  22  was shown which may be in one piece or divided into sectors radially. For a housing element  21 ,  22  which is divided into sectors, the metal flanges  23 ,  24  are also divided into sectors. To improve the sealing of the contact between a flange divided into sectors  23 ,  24  and a metal protection  3 , the protection  3  is preferably divided into sectors in such a way that the interfaces of the sectors of the protection  3  are offset relative to the interfaces of the sectors of the flange divided into sectors  24 . In other words, a radial interface of a flange sector  23 ,  24  is not aligned axially with a radial interface of a sector of the protection  3  to prevent the circulation of an axial airflow. This is particularly advantageous in order to form a sealed volume between the lower housing element  22  and the lower platform  12  of the blade  1 , for example, when the blade  1  includes a ventilation tube  5  opening into the said volume ( FIG. 1 ).