Abstract:
An annulus filler for mounting to a rotor disc of a gas turbine engine is provided to bridge the gap between adjacent blades. A first part is connectable to the rotor disc between adjacent blades. There is a separate second part that engages with the first part after connecting the rotor blades to the rotor disc. When installed, the filler is spaced from each blade by a respective clearance gap (G), and an operational configuration in which it contacts each of said blades. Engagement of the second part with the first part is effective to urge the first part from said installation configuration to said operational configuration and thus into blade contact. The first part may have a mounting region for connection to the rotor disc and allow, in said first step of said procedure, the mounting region to remain visible from a radially outer viewpoint.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is entitled to the benefit of British Patent Application No. GB 0910752.5, filed on Jun. 23, 2009. 
     FIELD OF THE INVENTION 
     The present invention relates to annulus fillers for bridging gaps between adjacent blades of a gas turbine engine stage. 
     BACKGROUND OF THE INVENTION 
     Conventionally, each compressor rotor stage of a gas turbine engine comprises a plurality of radially extending blades mounted on a rotor disc. The blades are mounted on the disc by inserting a root portion of the blade in a complementary retention groove in the outer face of the disc periphery. To ensure a smooth radially inner surface for air to flow over as it passes through the stage, annulus fillers are used to bridge the spaces between adjacent blades. Typically, seals between the annulus fillers and the adjacent fan blades are also provided by resilient strips bonded to the annulus fillers adjacent the fan blades. 
     Annulus fillers of this type are commonly used in the fan stage of gas turbine engines. The fillers may be manufactured from relatively lightweight materials and, in the event of damage, may be replaced independently of the blades. 
     It is known to provide annulus fillers with features for removably attaching them to the rotor disc. For example, it has been proposed to provide annulus fillers with axially spaced hook members, the hook members sliding into engagement with respective parts of the rotor disc.  FIG. 1  shows an example of such an annulus filler viewed from the side, and  FIG. 2  shows the annulus filler fitted to the rotor disc as viewed in transverse cross-section. 
     In use, the upper surface or lid  2  of the annulus filler  1  bridges the gap between two adjacent fan blades  3  (one of which is shown in outline in  FIG. 2 ) and defines the inner wall of the flow annulus of a fan stage. The annulus filler  1  is mounted on a fan disc  4  by two hook members  5 ,  6  respectively towards the forward and rearward ends of the annulus filler  1 . The hook members are configured to engage with outwardly directed hooks provided on the fan disc  4 . The annulus filler is also attached to a support ring  7  by a retention flange  8  provided at the forward end of the annulus filler. Along its rear edge, the annulus filler is provided with a rear lip  9  which is configured to fit under a rear fan seal  10  located axially behind the rotor disc  4  to limit deflection under running conditions. Similarly, the front edge of the annulus filler defines a front lip  11 , which is configured to fit under a spinner fairing  12  located axially ahead of the annulus filler. The two opposed side faces  13 ,  14  of the annulus filler are provided with respective seal strips (not shown) and confront the aerofoil surfaces of the adjacent fan blades  3  in a sealing manner. 
     As illustrated in more detail in  FIG. 3 , the retention flange  8  carries a forwardly extending spigot or pin  15 . The spigot or pin  15  is arranged for engagement within a corresponding aperture or recess provided in the support ring  7 . At a position circumferentially adjacent the spigot or pin  15 , the retention flange is also provided with a mounting aperture  16  which is arranged for co-alignment with a corresponding mounting aperture (not shown) provided through the support ring  7 . The co-aligned mounting apertures are sized to receive a mounting bolt. Thus, it will be appreciated that the retention flange  8  is pinned and bolted to the front support ring  7 . 
       FIG. 4  illustrates the typical form of the rear hook member  6 , as viewed from behind. As can be seen, the hook member defines an arcuate channel  17 . The channel  17  is curved in such a manner as to be centred on the rotational axis of the engine (not shown), and cooperates with a correspondingly arcuate hook on the rotor disc  4 . The front hook member  5  has a similar arcuate configuration. 
     A problem which has been experienced with prior art annulus fillers of the general type described above is that of reliable installation during engine assembly. As will be appreciated by those of skill in the art, the annulus filler must be fitted after the radially extending fan blades have been attached to the rotor disc. This means when the fitter then comes to install the annulus fillers between adjacent blades, his or her line of sight is obstructed by the presence of the fan blades. Also, the unitary construction of the annulus filler exacerbates this problem, because the filler lid  2  also obstructs the fitter&#39;s view when attempting to engage the hook members  5 ,  6  with the rotor disc  4 . Misassembly of the rear hook member  6  has been found to be a particular problem in this regard and has been attributed to the release of annulus fillers in operation. 
     Annulus fillers of the prior-art type described above are self-loading in the sense that, as a rotating component, the majority of forces on the filler are generated by its own mass. This can be modelled as an approximately radial force acting through the centre of gravity of the annulus filler. However, in the event of a bird-strike, or a fan blade otherwise becoming detached from the rotor (i.e. a so-called “fan-blade-off” event), the blades can apply tangential pushing forces to the adjacent annulus fillers thereby tending to pinch the annulus fillers between the blades as the blades pivot tangentially in their retention grooves. This can cause the annulus fillers to become detached from the rotor. In this regard, it is to be noted that a bird-strike or fan-blade-off event creates substantial imbalance in the rotor, and so even the remaining fan blades can deflect considerably due to their tips impinging on the outer casing surrounding the rotor. Thus it is not unknown to lose annulus fillers from circumferential positions well away from the primary release blade. 
     It has been found that the above-described configuration of annulus filler can increase the likelihood of the filler failing under the action of the tangential forces applied to it by the adjacent fan blades. Due to the curved nature of the interface between the hook members  5 ,  6  on the annulus filler and the cooperating hooks formed on the rotor disc  4 , the natural tendency of an annulus filler pushed from the side by an adjacent fan blade is to move rotationally relative to the disc, about the engine axis. However, because the front end of the filler is securely fixed by being pinned and bolted to the support ring, the front region of the filler is not permitted to deflect in this manner. The result is that the annulus filler becomes twisted along its length, which can lead to the filler fracturing between the retention flange  8  and the front hook member  5 . As will be appreciated, failure of annulus fillers in this manner is problematic as it increases the amount of shrapnel moving around inside engine during a bird-strike or fan-blade-off event, which can have serious consequences for the integrity of the engine. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved annulus filler. 
     According to a first aspect of the invention there is provided a method of mounting an annulus filler to a rotor disc of a gas turbine engine, the annulus filler bridging the gap between two adjacent blades attached to the rotor disc, the annulus filler having: 
     a first part which is connectable to the rotor disc between the positions of said adjacent blades, and a separate second part configured for engagement with the first part, characterised in that the method comprises the steps of installing the first part on the rotor disc in an installation configuration in which it is spaced from each said blade by a respective clearance gap, and subsequently engaging the second part with the first part to urge the first part from the installation configuration to an operational configuration in which it substantially contacts each of said blades. 
     The first part may be installed on the rotor disc in the installation configuration prior to connection of said blades to said rotor disc. 
     The step of installing the first part to the disc may include securing the first part on the rotor disc using a mechanical fastener. The mechanical fastener may be releasable and include a threaded shank and corresponding receptacle, rivet or other appropriate device. 
     The step of installing the first part to the disc may include the step of inspecting the mechanical fastener after securing the first part on the rotor disc and prior to the engagement of the second part with the first part. 
     The first part may have, in transverse cross-section, a pair of spaced-apart and generally radially oriented arms, wherein on engagement of said second part with said first part the radially outer regions of said arms are urged further apart from one another. 
     The second part may be slid into engagement with said first part in a direction perpendicular to the transverse cross-section. 
     The second part may be removably engaged with axial grooves provided in each arm with each groove receiving a respective edge of said second part. 
     The first part may be provided with a pair of seals that contact and substantially seal against respective blades when in said operational configuration. 
     According to a second aspect of the present invention, there is provided an annulus filler for mounting to a rotor disc of a gas turbine engine and for bridging the gap between two adjacent blades attached to the rotor disc, the annulus filler having: 
     a first part which is connectable to the rotor disc between the positions of said adjacent blades, and a separate second part configured for engagement with the first part, characterised in that said first part has, in transverse cross-section, a pair of spaced-apart and generally radially orientated arms resiliently biased towards an installation configuration in which the first part is spaced from each said blade by a respective clearance gap (G), and an operational configuration in which it substantially contacts each of said blades, wherein engagement of the second part with the first part is effective to urge the first part from said installation configuration to said operational configuration and thus towards contact with said blades. 
     The first and second parts may be configured to allow a procedure for mounting the annulus filler to the rotor disc, the procedure having a first step in which the first part is connected to the rotor disc without the second part and whilst in said installation configuration, and a subsequent second step in which the second part is engaged with the first part to urge the first part from said installation configuration to said operational configuration and thus towards contact with said blades. 
     Said first step may occur prior to connection of said blades to said rotor disc, and said second step may occur after connection of said blades to said rotor disc. 
     The first part may have at least one mounting region for connection to the rotor disc and may be configured to allow the or each mounting region to remain substantially visible from a radially outer viewpoint after the first part is mounted to the rotor disc. 
     Conveniently, said first and second parts may be configured to allow the engaging regions of said first and second parts to remain substantially visible from a radially outer viewpoint ( 37 ) during said second step. 
     The second part may be configured for engagement with said first part in a sliding manner, in a substantially axial direction. 
     The first part may be configured such that when in said installation configuration, the arms lie substantially parallel to one another in transverse cross-section. 
     Each arm may be provided with an axial groove configured to slideably receive a respective edge of said second part. 
     Said first part may be provided with a pair of seals to contact and substantially seal against respective blades when in said operational configuration. Each said seal may be provided in the radially outer region of a respective said arm. 
     The first part may be formed from a first material and the second part formed from a different second material. More particularly, the first part may be formed from a metal material. The second part may be formed from plastics material. 
     At least one of said first and second parts may define part of an airflow surface for air drawn through the engine. 
     Said first and second parts may define respective regions of an airflow surface for air drawn through the engine, the first and second parts having respective outer surfaces which lie substantially flush when the parts are engaged with one another. 
     A stage for a gas turbine engine may have: a rotor disc; a plurality of circumferentially spaced apart blades attached to the rotor disc; and a plurality of annulus fillers in accordance with a second aspect of the invention. Optional features of the first or second aspect may apply, as appropriate. 
     A stage for a gas turbine engine may have: a rotor disc; a plurality of circumferentially spaced apart blades attached to the rotor disc; and a plurality of annulus fillers mounted to the rotor disc in accordance with the first aspect of the invention. Optional features of the first or second aspect may apply, as appropriate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior-art annulus filler, viewed from the side; 
         FIG. 2  shows the annulus filler of  FIG. 1 , installed in a gas turbine engine; 
         FIG. 3  is an enlarged view of part of the annulus filler shown in  FIGS. 1 and 2 , as viewed from the front; 
         FIG. 4  is an enlarged view of another part of the annulus filler shown in  FIGS. 1 and 2 , as viewed from the rear; 
         FIG. 5  is a transverse cross-sectional view showing a first part of an annulus filler in accordance with the present invention connected to a rotor disc between the positions of a pair of adjacent blades, and in a first configuration; 
         FIG. 6  is a cross-sectional view similar to that of  FIG. 5 , showing the first part in combination with a second part of the annulus filler, and with the first part in a second configuration in which it contacts the adjacent blades; and 
         FIG. 7  is a transverse cross-sectional view taken through a region of an annulus filler in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now in more detail to  FIG. 5 , there is shown a first part  20  of a two-part annulus filler  21 . A portion of the radially outer region of a compressor fan rotor disc  22  is also shown. In a generally conventional manner, the radially outer surface of the rotor disc  22  is provided with a plurality of circumferentially spaced-apart retention grooves  23  (parts of two such grooves being illustrated in  FIG. 5 ) for receiving and retaining the root portions  24  of respective fan blades  25 . The retention grooves  23  may be straight or curved and extend generally in the axial direction of the engine. In the particular arrangement illustrated in  FIG. 5 , the retention grooves  23  have a generally “fir-tree”-” shaped cross-sectional profile and the root portions  24  of the blades have a complementary fir-tree profile in order to provide an accurate and strong connection between each blade and the rotor disc  22 . However, it is to be appreciated that in alternative embodiments, particularly those intended for use in the fan of a gas turbine engine, the retention grooves  23  and the root portions  24  of the blades could have complementary dovetail profiles instead. 
     The first part  20  of the annulus filler takes the form of a generally elongate body extending in the axial direction of the engine.  FIG. 5  illustrates the body part  20  in transverse cross-section and shows it in an initial installation configuration, which will be described in more detail below. The body part is resiliently deformable and is configured such that in its natural relaxed condition, it adopts the installation configuration illustrated in  FIG. 5 . The body part is preferably formed from metal such as aluminium, titanium or magnesium alloys and may be extruded or metal injection moulded. 
     In transverse cross-section (as shown in  FIG. 5 ), the body part  20  has a pair of spaced-apart arms  26  which are arranged so as to extend generally radially outwardly from a mounting region  27 . The mounting region  27  forms an integral part of the body  20  and serves to interconnect the two arms  26  at their radially innermost ends. The mounting region  27  has a curved profile and is thus configured for intimate engagement against the outer surface of the rotor disc  22 . 
       FIG. 5  shows the body part  20  connected to the rotor disc  22 . This connection can be effected in a number of alternative ways. In the particular arrangement illustrated, the mounting region  27  of the first part  20  is provided with a number of mounting apertures  28  at spaced-apart positions along its axial length. Each mounting aperture  28  is configured to receive therethrough the threaded shank  29  of a mounting bolt  30  for threaded engagement within an aligned mounting recess  31  provided in the outer region of the rotor disc  22 . Thus, it will be appreciated that the particular mounting arrangement illustrated in  FIG. 5  uses generally radially oriented mounting bolts  30 . However, as indicated above, alternative mounting arrangements could also be used which could, for example, use axially orientated mounting bolts or the like. Other mounting arrangements are also possible. 
     Each arm  26  supports an enlarged formation  32  at its radially outermost end, each formation extending both inwardly into the space defined between the two arms  26  and outwardly so as to extend generally towards the respective adjacent rotor blade  25 . More particularly, each formation  32  presents a generally radially-outwardly directed surface  33  and defines an axially extending side edge  34 . In the arrangement illustrated in  FIG. 5 , the body part  20  is provided with a pair of sealing members  35  each of which is mounted along a respective side edge  34 . 
     The region of each formation  32  extending generally inwardly into the space defined between the two supporting arms  26  is configured so as to define a generally axially extending groove  36 . The two grooves  36  are arranged so as to oppose one another and are each open in a direction facing the opposite groove. 
     As indicated above,  FIG. 5  shows the resilient body part  20  in a relaxed condition in which it adopts an initial installation configuration. In this configuration, it is to be noted that each outwardly extending sealing member  35  is spaced from the adjacent rotor blade  25  by a clearance gap G, whilst the inwardly directed regions of the formations  32  defining the opposed grooves  36  are spaced from one another by a clearance gap g which is of a size sufficient to permit the passage therethrough of a tool for use in installing and tightening the mounting bolts  30 . This configuration of the body part  20  thus permits the rotor blades  25  to be easily mounted to the rotor disc  22  after the body part  20  has been mounted to the rotor disc  22 . The clearance gaps G between each side of the body part  20  and the adjacent rotor blades  25  allows the rotor blades  25  to be properly located and offered up to the rotor disc  22  without hindrance by body parts  20 , the gaps allowing movement of the blades from side to side as might be necessary as they are manipulated into engagement with their respective retention grooves  23 . However, it is to be noted that whilst it is envisaged that the body parts  20  of respective annulus fillers will usually be mounted to the rotor disc prior to the rotor blades  25 , the configuration of the body part would also permit an alternative assembly order in which the rotor blades  25  are mounted to the rotor disc first, followed by the body parts. 
     Additionally, the clearance gap g between the inwardly directed regions of the formations  32  allows a person fitting the annulus filler to the rotor disc  22  to view the mounting region  27  in a generally radial direction denoted by arrow  37 , through the gap, thereby allowing accurate alignment of the mounting apertures  28  with respective mounting recesses  31  formed in the outer periphery of the rotor disc  22 . The clearance gap g also permits the passage therethrough of a tool for installation and tightening of the mounting bolts  30 , whilst simultaneously allowing clear sight of the bolts. As will be appreciated, it will be generally easier to mount the body part  20  to the rotor disc in this manner in the absence of the rotor blades  25  as the fitter will be afforded a clearer view and easier tool access. 
     Turning now to consider  FIG. 6 , the above-described body part  20  of the annulus filler  21  is shown in combination with a separate second part  38 . The second part  38  takes the form of an elongate slider which is configured for engagement with the body part  20  in a manner effective to urge the body part  20  against the bias of its inherent resiliency, so as to move from the initial installation configuration illustrated in  FIG. 5  towards an alternate, operational configuration as illustrated in  FIG. 6 . 
     The second part, or slider  38 , has a radial cross-sectional profile, which presents a generally smooth radially outer surface  39 . The slider  38  is provided with a pair of oppositely directed flanges  40  running along respective side edges. As thus illustrated in  FIG. 6 , the oppositely directed side flanges  40  of the slider  38  are thus configured for sliding engagement within respective grooves  36  formed in the body part  20 . After the rotor blades  25  have been connected to the rotor disc, the slider  38  may thus be slidingly engaged with the body part  20  in a substantially axial direction relative to the axis of the engine (i.e. into the page as viewing  FIG. 6 ). In this regard, it is to be noted that a person fitting the annulus filler to the rotor disc  22  is afforded a clear view of the slider  38  in the radial viewing direction  37  as it is engaged with the body part  20 , thereby ensuring reliable connection of the two components. 
     Sliding engagement of the slider  38  with the body part  20  is effective to drive the support arms  26  outwardly, as indicated by arrows  41  in  FIG. 5 , such that they move from being substantially parallel to one another as illustrated in  FIG. 5  to being divergent as illustrated in  FIG. 6 . It will thus be appreciated that in the configuration illustrated in  FIG. 6 , the transverse cross-sectional profile of the body part  20  is generally V-shaped, and in this configuration the clearance gaps G between the side edges of the two sealing members  35  and the adjacent rotor blades  25  have been closed such that the sealing members  35  are brought into close and intimate sealing contact with the surfaces of the rotor blades  25 . 
     When the slider  38  is fully engaged with the body  20  such that the body  20  adopts the operational configuration illustrated in  FIG. 6 , the radially outer surfaces  33  of the body part  20  lie substantially flush with the radially outer surface  39  of the slider  38 . The flush-lying surfaces  33 ,  39  thus cooperate to define respective regions of an airflow surface for air drawn through the engine, the airflow surface extending generally between the adjacent rotor blades  25 . 
     It is envisaged that the slider  38  could either be made from suitable metal material such as aluminium, titanium or magnesium alloys. Alternatively, however, the slider  38  could be formed from plastic material. For example, material for the slider may be a carbon- or glass-fibre reinforced thermoplastic, such as Torlon™ 5030/7030 (polyamide-imide) from Solvay Advanced Polymers. Such a slider could be formed by injection or compression moulding. Alternatively, the slider could be formed from fibre reinforced epoxy, for example by compression moulding. Injection moulding generally requires short reinforcing fibres. Compression moulding could use longer fibres. 
     As will thus be appreciated, the two-part annulus filler  21  of the present invention offers significant advantages over prior art annulus filler designs in that it permits an installation process in which the fitter has substantially unobstructed sight of the mounting region  27  of the annulus filler as it is offered up to and connected to the rotor disc, and substantially unobstructed sight of the flanges  40  of the slider  38  and the cooperating grooves  36  formed in the body part as the slider is offered up to and engaged with the body part, even in the event that the adjacent rotor blades have already been assembled. This significantly reduces the potential for mal-assembly of the annulus filler, which in turn reduces the likelihood of the annulus filler becoming detached from the rotor in service. 
     Additionally, the annulus filler design of the present invention also provides distinct advantages in the event of a fan-blade-off event. The generally V-shaped transverse cross-sectional profile of the body part  20  when in its operational configuration, and its deformable nature, provides a degree of flexibility that allows the annulus filler to rotate relative to the axis of the engine when pushed from the side by a deflecting rotor blade. Should the filler nevertheless fail due to the forces exerted on it by an adjacent deflecting blade, it is likely that only the slider  38  (and perhaps also the radially outer region of the arms  26  supporting the formations  32 ) will fail, leaving intact the radially inner region of the arms, which will thus remain securely connected to the rotor disc. As only the slider  38  (and perhaps also a portion of the body part  20 ) is thus likely to be released under such circumstances, the mass and therefore energy of the resulting debris will thus be reduced in comparison to the sort of failure experienced with prior art annulus fillers. This reduces the amount of shrapnel moving around in the fan-case of the engine, thereby reducing the risk of high-energy debris causing further damage to the engine. Also, by making the slider  38  from plastic or composite materials proposed above rather than metal, the weight of any such shrapnel will be significantly reduced, thereby reducing the likelihood of the shrapnel causing serious damage to the engine. 
     Turning now to consider  FIG. 7 , there is illustrated an alternative embodiment of the present invention in which the side flanges  40  of the slider  38 , and the cooperating axial grooves  36  of the body part  20  have a modified cross-sectional profile. In this arrangement, it will be seen that the flanges  40  of the slider  38  are each provided with a small radially outwardly directed lip  42 . The cooperating grooves  36  in the body part are configured so as to have a corresponding re-entrant region  43  sized and shaped to receive a respective side lip  42  of the slider  38 . This modified form of engagement between the slider  38  and the body part  20  serves to further resist possible release of the slider  38  due to circumferential deflection of the arms  26  of the body part  20  during operation of the engine. Engagement of the side lips  42  within the re-entrant regions  43  of the grooves  36  is thus effective to prevent disengagement of the side flanges  40  of the slider  38  from the grooves  36  during significant circumferential deflection of the arms  26 . 
     When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. 
     The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. 
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.