Patent Publication Number: US-2012039710-A1

Title: Intermediate casing of aircraft turbomachine including structural connecting arms which perform separate mechanical and aerodynamic functions

Description:
TECHNICAL FIELD 
     The present invention relates in a general sense to the field of turbomachines with ducted fan for aircraft, and more specifically to the intermediate casings fitted to these turbomachines. 
     The invention preferably applies to turbomachines of the turbofan for aircraft type. 
     STATE OF THE PRIOR ART 
     In existing turbojets, of “ducted fan” design, there is generally a fan casing extended to the rear by an intermediate casing, which is attached to it securely. This intermediate casing includes a hub and an outer ferrule positioned concentrically, and connected to one another by structural connecting arms, distributed in the circumferential direction and habitually extending in the turbojet&#39;s radial direction. 
     The structural arms therefore have a high mechanical resistance allowing the efforts to be transmitted between the outer ferrule and the hub of the intermediate casing, which is generally located in line with a forward rolling bearing of the turbojet. In addition to the transfer of the efforts, these arms must be able to resist the projectiles likely to impact them. 
     These arms are habitually located downstream from multiple outlet guide vanes, also called OGVs, the function of which is to straighten the secondary airflow escaping from the fan, in order to limit its whirling. In such a case, the outlet guide vanes are located in the secondary annular channel of the turbojet and are supported by the fan casing, upstream from the structural arms. 
     To simplify the design of such a turbojet it has been proposed to incorporate the function of the outlet guide vanes within the structural connecting arms, so as to allow the former to be eliminated. To accomplish this each structural arm has an aerodynamic outer surface performing this role of straightener of the flow escaping from the fan. 
     Despite this simplification, such structural arms continue to have a substantial total mass, due to the fact that they generally consist of solid metal elements, which additionally leads to high material costs. In addition, since the aerodynamic outer surface of these solid metal elements must be machined precisely, production costs also reach high levels. 
     SUMMARY OF THE INVENTION 
     The purpose of the invention is therefore to provide at least partially a solution to the disadvantages mentioned above, compared with the embodiments of the prior art. 
     To accomplish this, a first object of the invention is a structural connecting arm for an intermediate casing of a turbomachine with ducted fan, where the arm is intended to connect a hub and an outer ferrule of this intermediate casing, and having an aerodynamic outer surface produced such that the arm also forms an outlet guide vane. 
     According to the invention, it includes multiple metal ties extending in the longitudinal direction of the arm, together with a shell made from composite material surrounding the said ties and forming the said aerodynamic outer surface. In addition, at least a part of an inner space demarcated by the shell and traversed by the ties is filled by a filling material forming a support of the said shell. 
     Thus, the invention is remarkable in that it involves an arm with dissociated elements in order to perform, respectively, the aerodynamic function of the flow straightener, and that of mechanical resistance. 
     Indeed, the mechanical resistance required to transfer the efforts between the outer ferrule and the hub of the intermediate casing, and for the resistance of the arm to the impacts of projectiles, is performed by the metal ties, whereas the aerodynamic function is performed by the shell of composite material, preferably of the type of a blend of glass and/or carbon fibres with a resin, for example of the epoxy resin type. 
     This results, firstly, in a gain in terms of total mass, particularly due to the presence of the composite material shell, the location of which within a structural part of a turbomachine is one of the original features of the present invention. The mass reduction has been assessed at between 25% and 35% compared to the known solution with solid metal elements, described above. 
     Moreover, the proposed solution advantageously leads to reduced materials and production costs compared to those found hitherto. 
     In addition, as mentioned above, at least a part of an inner space demarcated by the shell and traversed by the ties is filled by a filling material forming a support of the said shell. This enables the manufacture of the shell to be facilitated, and this shell can then be cast on this filling material, which acts as its support, and the latter may also possibly include the ties. 
     Each tie is preferably separated from the composite material shell by the said filling material. Consequently, in this case, it is arranged such that the ties are not in direct contact with the shell, for various reasons. The first lies in the desire to improve the support of the shell for its manufacture, incorporating a uniform support surface, consisting of an alternation between the filling material and the ties, which could subsequently lead to incipient cracks, or other faults. The second reason lies in the desire to obtain a structural arm the possible vibrations of the ties of which can be dampened by the filling material, and therefore not be transferred directly to the aerodynamic shell. The risks of floating of the latter are greatly and advantageously reduced thereby, with the positive consequences which this has for the thrust performance generated by the secondary flow. 
     To accomplish this, it is, for example, arranged such that each tie is sunk in the said filling material along the entire length of the shell, that is to say along the segment corresponding to the length of the shell, having a lateral surface which is completely covered by the filling material. 
     Each tie preferably extends, in the longitudinal direction of the arm, beyond the said shell, either side of the latter. Thus, the ends of the protruding ties can easily be used to assemble the arm on the outer ferrule and on the hub. 
     With this regard, it is arranged such that the said ties support at their radially outer ends means for attaching the arm on the outer ferrule of the intermediate casing, and support at their radially inner ends means for attaching the arm on the hub of the intermediate casing. 
     The said means for attaching the arm on the outer ferrule and the means for attaching the arm on the hub preferably each includes a bracket having holes for attaching the ties, and attachment holes for assembly on the outer ferrule and the hub, respectively. 
     Another object of the invention is an intermediate casing of a turbomachine with ducted fan, including multiple structural connecting arms such as the one described above, connecting the hub and the outer ferrule of this casing. 
     Finally, another object of the invention is a method for the assembly of such a structural connecting arm on an intermediate casing of a turbomachine with ducted fan, where the method includes the following steps:
         positioning of the arm facing the annular space demarcated between the outer ferrule and the hub of the intermediate casing;   positioning of the arm between the outer ferrule and the hub of the intermediate casing, by moving the arm in the axial direction of the intermediate casing; and   attachment of the arm on the outer ferrule and on the hub of the intermediate casing.       

     This method is extremely easy to implement, since the arm is positioned simply by moving it in the axial direction of the intermediate casing, between the outer ferrule and the hub, which do not require to be moved. Moreover, the arm can equally easily be removed from the intermediate casing, during handling operations the purpose of which is, for example, to repair or exchange it, giving it the character of an item of equipment which can be replaced during a stopover, also called an LRU (Line Replaceable Unit). 
     Other advantages and characteristics of the invention will appear in the non-restrictive detailed disclosure below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This description will be made with reference to the attached illustrations, among which: 
         FIG. 1  represents a longitudinal half-section view of a forward part of an aircraft turbojet according to a preferred embodiment of the present invention; 
         FIG. 2  represents a perspective view of a part of one of the structural connecting arms fitted to the intermediate casing of the turbojet shown in  FIG. 1 ; 
         FIG. 3  represents a section view taken in plane P of  FIG. 2 ; 
         FIGS. 4   a  and  4   b  shows diagrams of a method of manufacture of the arm shown in  FIGS. 2 and 3 ; 
         FIG. 5  shows a perspective view of the arm represented in  FIG. 2 , fitted with its means for attachment to the elements of the intermediate casing; 
         FIG. 6  represents a section view of a radially inner part of the structural arm shown in the previous figures; 
         FIG. 7  represents a perspective view of a radially outer part of the structural arm shown in the previous figures; 
         FIG. 8  represents a perspective view of the structural arm shown in the previous figures, assembled on the hub and the outer ferrule of the intermediate casing; 
         FIG. 9  shows a diagram of a method of assembly of the structural connecting arm on the intermediate casing, according to a preferred embodiment of the invention; and 
         FIG. 10  shows a perspective view of the intermediate casing fitted to the turbojet shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , a front part  1  of a turbofan for aircraft, according to a preferred embodiment of the present invention, can be seen. 
     In  FIG. 1 , only the low-pressure compressor  3  of the gas generator has been represented, which is, for example, a two-compressor generator. 
     The turbomachine has, in a general direction of outflow of the fluid through this turbomachine, moving from the front to the rear, as is represented diagrammatically by the arrow  9 , an air inlet  4 , a fan  6 , and a flow separation nozzle  14 , from which emerge an annular primary channel  16  and an annular secondary channel  18 , the latter being positioned radially towards the outside relative to the primary channel  16 . These traditional elements known to the skilled man in the art each naturally are annular in shape, and are centred on a longitudinal axis  22  of the turbomachine. 
     Thus, the air flow F traversing fan  6  is divided into two separate flows after it comes into contact with the upstream end of separation nozzle  14 , namely into a primary flow F 1  entering channel  16 , and a secondary flow F 2  entering channel  18 . 
     In addition, fan  6  is surrounded by a fan casing  24  extended downstream by an outer ferrule  28  of an intermediate casing  26 , attached to casing  24  by means of bolts. Intermediate casing  26  also has, positioned concentrically and radially towards the interior relative to ferrule  28 , a hub  30  centred on axis  22  and located downstream from flow separation nozzle  14 . 
     Structural connecting arms  32  provide the mechanical connection between ferrule  28  and hub  30 , these arms being spaced circumferentially relative to one another, in regular fashion, and each extending roughly in the radial direction of the turbojet. Structural arms  32  therefore have a high mechanical resistance, allowing firstly the efforts between ferrule  28  and hub  30  to be transmitted, and secondly allowing the projectiles likely to impact it to be able to be resisted. 
     In addition, each arm  32  traversing secondary channel  18  has an aerodynamic outer surface  36  shaped such that the arm also performs the function of an outlet guide vane, or OGV, the aim of which is to straighten the secondary air flow F 2  escaping from fan  6 , in order to limit its whirling. 
     Consequently, there is no requirement for additional outlet guide vanes to be interposed between fan  6  and structural arms  32 , and the latter then constitute the first elements which the air of secondary flow F 2  traverses after going beyond separation nozzle  14 . 
     With reference at present  FIG. 2 , one of the structural connecting arms  32  fitted to the intermediate casing is shown. One of the features of this arm lies in the fact that the elements used to perform the mechanical function are disassociated from those used to perform the aerodynamic function of straightening of the flow. Indeed, to perform the mechanical resistance function, arm  32  includes multiple metal ties which extend in the longitudinal direction of the arm, schematised by double arrow  38 . There are, for example, three such ties  40 , spaced relative to one another according to the structure of the arm forming the outlet guide vane. 
     As mentioned above, the longitudinal direction  38  in this case is the radial direction of the turbojet on which arm  32  is intended to be installed. Moreover, it is noted that each arm of the intermediate casing has a design comparable to the one described here. 
     To perform the aerodynamic function of straightening of the secondary flow, the arm has a shell  42  made from composite material, preferably of the type of glass and/or carbon fibres with a resin, for example an epoxy resin. This shell therefore takes the shape of a continuous structure, produced using several folds, and forming the leading edge  44  of the arm, the concave side  45 , the trailing edge  46 , and the convex side  47 . Thus, shell  42  defines the entire outer aerodynamic surface  36  of the arm forming the outlet guide vane. 
     As can best be seen in  FIG. 3 , the support on which the entire inner surface of shell  42  preferably rests is a filling material  50  filling an inner space  52  demarcated by this same shell, and also traversed by ties  40 . In this case, the segments of the ties which traverse shell  42  are completely sunken in this filling material  50 , so as to separate these ties from the shell, and thus to prevent a direct contact between these elements, which might cause a floating of the shell during operation of the turbojet. In addition, as can be seen in  FIG. 3 , this enables a uniform and continuous surface to be presented for the support of the shell, a surface which thus proves to be perfectly suitable for the manufacture by moulding of this same shell made of composite material. In this preferred embodiment, the inner space  52  demarcated by the inner surface of shell  42  is completely filled by filling material  50  and ties  40 . 
     Lastly in the area of the leading edge of shell  42 , a foil of material  54  is positioned externally, used to strengthen the mechanical rigidity of the arm, and thus suitable to resist any impacts which the latter might incur. 
     With reference at present  FIGS. 4   a  and  4   b , a method of manufacture of the structural arm  32  described above is schematised. Firstly with reference to  FIG. 4   a , the first operation consists in putting in position ties  40 , for example in an appropriate mould (not represented), by positioning them relative to one another in positions such as those adopted in the finalised arm. The filling material is then injected into the abovementioned mould, so as to sink metal ties  40  in a manner set out above, the aim being to cover the entire lateral surface of the tie segments intended to be surrounded by the shell produced subsequently. This involves, for example, injection moulding of expanded foam, or again of any other elastomer judged appropriate by the skilled man in the art. Be that as it may, this filling material  50  is chosen such that it has a low density, such that structural arm  32  has a lower mass. 
     When the assembly has been obtained, including filling material  50  and ties  40  which are sunk in it, as shown in  FIG. 4   b , this assembly placed is once again in another mould in which the composite material folds cover filling material  50 , before the firing operation the purpose of which is to obtain shell  42 . The foil  54  is positioned in this same mould, in order that it adheres to the shell during the said firing. In this case, the composite material folds intended to form shell  42  rest entirely on the outer surface of filling material  50 , which therefore forms a uniform and continuous surface along a closed line. 
     For this moulding, any technique known to the skilled man in the art may be used, such as that known as vacuum injection, also called RTM (Resin Transfer Moulding). At the end of this firing operation arm  32  as shown in  FIG. 3  is obtained. 
     In the preferred embodiment, each tie  40  extends beyond shell  42  in direction  38 , as can be seen in  FIG. 2 . Thus, ties  40  have radially outer ends protruding from filling material  50  and from shell  42 ; there are also radially inner ends, which also protrude from both these elements. In  FIG. 5 , it is shown that the radially outer ends are designed to support the means  60  intended to attach the arm on the outer ferrule of the intermediate casing, whereas the radially inner ends are designed to support the means  62 , roughly similar to means  60 , and intended to attach the arm on the hub of the intermediate casing. 
       FIG. 6  shows a turbojet transverse plane section of the radially inner part of arm  32 . Thus, it can be seen that means  62  have an omega-shaped section, with the hollow  66  of this omega defined jointly by a central face  68  and two lateral faces  70 , from which protrude two bases  72  forming the base of the omega. Holes  74  traversed by the radially inner end  40   a  of ties  40 , respectively, are present in the central face  68 . Moreover, each end  40   a  has a shoulder  76  resting on central face  68 , the mechanical attachment being provided by a nut  78  housed in the inner space  66  of the omega, and screwed on to end  40   a  such that it is pressed against the inner surface of central face  68 . To facilitate the assembly of arm  72  on the hub, as will be explained below, end  40   a  and nut  78  remain housed in inner space  66 , such that they do not protrude beyond bases  72 , each of which also have several attachment holes  84  for assembling the arm on the hub. 
     There is a comparable configuration for means  60 , the bracket of which also has the shape of an inverted omega, with an inner space  82  demarcated jointly by a central face  84  and two lateral surfaces  86 , from which two bases  88  emerge forming the base of this inverted omega. In this case too, the radially outer ends  40   b  of the ties are assembled on central face  84  using nuts  90 , which are pressed against central face  84 , which has holes for attaching the ties. Each of both bases  88  also has attachment holes  92  for assembling the arm on the outer ferrule of the intermediate casing. 
     This is notably represented in  FIG. 8 , showing one of structural arms  32 , the means  60  of which have both its bases  88  resting on the inner radial surface  94  of the outer ferrule  28  of the intermediate casing  26 , and the bases  72  of the means  62  of which are resting on the outer radial surface  95  of hub  30 . In both cases, these bases  72 ,  88  are joined to the surfaces with which they are in contact by means of screwed elements traversing orifices  80 ,  92 , and co-operating with means of the nut type, which have, for example, previously been secured to ferrule  28  and to hub  30 . 
     By virtue of the original design of the fastenings  60 ,  62 , the method of assembly of structural arm  32  on the intermediate casing proves extremely simple to accomplish. The preferred manner of such a method is shown in  FIG. 9 , schematising by dotted lines the positioning of arm  32  opposite annular space  18 , demarcated between outer ferrule  28  and hub  30 , where both these elements occupy their definitive positions within intermediate casing  26 . After this, as is schematised by arrow  96 , arm  32  is put in position between ferrule  28  and hub  30 , by moving it in the axial direction of arrow  96 , parallel to axis  22  of the turbojet and of the intermediate casing. During this movement, bases  88  slide over inner surface  94  of ferrule  28 , while bases  72  simultaneously slide over outer surface  95  of hub  30 , until the final position of this arm within casing  26  is reached. 
     After this, bases  88 ,  72  are attached to ferrule  28  and hub  32  using screwed elements as described above, and represented here schematically with the elements referenced  98 . With this configuration, the assembly of an arm  32 , and also its disassembly, are extremely easy, making it possible for it to be an item of equipment which can easily be replaced during a stopover. In addition, the easy character of the assembly and disassembly is accentuated by the fact that screwed elements  98  can be assembled and disassembled by an operator from annular space  18 , without requiring additional access in the area of ferrule  28  or hub  30 . 
     Naturally, arms  32  can be assembled one after another in the manner which has just been described above, in order to accomplish intermediate casing  26  shown in  FIG. 10 . 
     Naturally, various modifications can be made by the skilled man in the art to the invention which has just been described, solely as non-restrictive examples.