Abstract:
A method of manufacturing a hollow aerofoil component ( 100 ) for a gas turbine engine ( 10 ) comprises using a capping panel ( 200 ) to cover a pocket ( 310 ) in a pocketed aerofoil body ( 300 ). During manufacture, a mandrel ( 400 ) is provided to support the capping panel ( 200 ) in the correct position. This ensures that the outer surface of the capping panel ( 200 ) is located as accurately as possible. This means that the capping panel ( 200 ) can be made to be as thin as possible, which in turn reduces weight and material wastage. Remotely detectable elements ( 700 ) may be provided to the mandrel ( 400 ) to enable the location of the pocket ( 310 ) to be accurately determined from outside the aerofoil ( 100 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from British Patent Application Number 1316731.7 filed 20 Sep. 2013, the entire contents of which are incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Disclosure 
         [0003]    The present invention relates to the manufacture of a hollow aerofoil, in particular the manufacture of a hollow aerofoil component for use in a gas turbine engine. 
         [0004]    2. Description of the Related Art 
         [0005]    Aerofoil shaped components are used throughout gas turbine engines. For example, aerofoil shaped stator vanes and rotor blades are used to guide gas through the engine, for example both in the turbine and the compressor, including the fan and associated guide vanes. 
         [0006]    Weight reduction is an important consideration in gas turbine engines, particularly, although not exclusively, for gas turbine engines used to power aircraft. Generally, the lower the weight of the component the better the performance of the aircraft to which it is fitted, for example in terms of fuel consumption. To this end, it is known to use hollow aerofoils, e.g. rotor blades and/or stator vanes, in some stages of gas turbine engines. 
         [0007]    One method of producing a hollow aerofoil involves forming the structure using a skin. This involves creating an internal cavity (which may be filled with another, lighter weight, material) using hot creep or super plastic forming processes. Such processes may generate aerofoils with some advantageous properties, such as thin skin thickness and tight dimensional tolerance, but they involve significant material wastage. This material wastage makes these processes expensive, due at least to high material cost for a given size of hollow aerofoil component. 
         [0008]    An alternative method for producing hollow aerofoil components involves attaching a plate to an aerofoil structure out of which a pocket has been machined. The plate is placed into the pocket and attached (for example welded or bonded) therein to produce a hollow aerofoil component. 
         [0009]    An advantage of producing the hollow aerofoil by using a plate to cover a pocket in an aerofoil structure is that there is less material wastage than using a skin to produce the hollow aerofoil. However, the dimensional tolerances are not so accurate. This may be because distortion is introduced in the process of attaching the plate to the pocketed aerofoil, which typically involves local heating at the interface between the plate and the pocketed aerofoil. Additionally, tolerance errors may “stack-up” in the process used to produce the pocketed aerofoil, the process used to produce the plate, and the process/feature used to locate the plate into position in the pocket, which typically involve placing the plate onto a supporting ledge inside the pocket. 
         [0010]    The lack of dimensional accuracy means that the plate generally has to be manufactured to be thicker than would otherwise be required. For example, the extra thickness may be required in order to ensure that there is enough material to be machined back to produce the desired aerofoil shape after it has been fixed into the pocket. Without the extra thickness, the dimensional variation resulting from tolerance “stack-up” and/or distortion may mean that there is not sufficient material to produce the desired aerofoil shape in some of the aerofoils produced by the method. 
         [0011]    However, this extra thickness means both that the component is heavier than it would otherwise need to be, and also that there is more material wastage. 
       OBJECTS AND SUMMARY 
       [0012]    It is therefore desirable to manufacture hollow aerofoil components by using a plate to cover a pocket (so as to take advantage of the generally lower material wastage), but with improved dimensional tolerance. 
         [0013]    According to an aspect, there is provided a method of manufacturing a hollow aerofoil comprising: 
         [0014]    providing a pocketed aerofoil body having an open pocket formed in a surrounding hollowed surface; 
         [0015]    placing a temporary mandrel into the pocket of the pocketed aerofoil body; 
         [0016]    locating a capping panel over the pocketed aerofoil body and temporary mandrel, the capping panel having an inner surface and an opposing outer surface; 
         [0017]    joining a first region of the inner surface of the of the capping panel to the surrounding hollowed surface of the pocketed aerofoil body so as to form the hollow aerofoil; and 
         [0018]    removing the temporary mandrel from the hollow aerofoil after the step of joining. The entire temporary mandrel may be removed from the hollow aerofoil. 
         [0019]    The temporary mandrel is shaped so as to support the capping panel over a second region of its inner surface during the joining, such that the outer surface of the capping panel forms a desired aerodynamic surface of the hollow aerofoil. 
         [0020]    According to such a method, the capping panel is supported during joining by the temporary mandrel, meaning that the shape of the capping panel is accurately controlled during the manufacture. This means that a hollow aerofoil can be manufactured with good dimensional accuracy (i.e. to tight tolerance), with minimal residual stress. Because of the good dimensional accuracy, the aerofoil (including the capping panel) can have thinner walls than would be possible with conventional methods, thereby reducing material wastage, manufacture time, and the overall cost and/or weight of the hollow aerofoil. 
         [0021]    Furthermore, using a thinner capping panel may reduce the amount of input power required in the joining step. In turn, this means that the amount of distortion produced in the joining process may be reduced, resulting in a more accurate profile that may require less (or no) machining to produce the finished aerofoil surface. 
         [0022]    The capping panel may be supported by the temporary mandrel during the joining step such that it is held in a position in which it forms a continuous aerodynamic surface with the pocketed aerofoil body. 
         [0023]    The capping panel may form part of the gas-washed surface of the manufactured hollow aerofoil, for example either a pressure surface or a suction surface of an aerofoil. When the capping panel is located in position, it may cover at least a part of (for example all of) the surrounding hollowed surface. 
         [0024]    The temporary mandrel may comprise at least one detectable element whose location is remotely detectable. Such a remotely detectable element may allow the position of the temporary mandrel to be detected relative to the hollow aerofoil, for example even when it is covered by the capping panel. Such a detectable element may be detectable (for example using suitable detection apparatus) when not visible. Accordingly, the detectable element may allow the position of the void in the hollow aerofoil to be determined from outside the hollow aerofoil, for example in relation to external surfaces of the hollow aerofoil. Equally, the detectable element mat allow the external surfaces to be referenced (for example defined) relative to the temporary mandrel. 
         [0025]    The temporary mandrel may comprise at least three such detectable elements. Three or more detectable elements may, in some cases, allow the position of the temporary mandrel to be determined particularly accurately, for example particularly accurately in three dimensions. 
         [0026]    Any suitable detectable element may be used. For example, the or each detectable element may be magnetic. Such magnetic detectable elements may be detected using Hall effect sensors, for example. Such a detectable element may be detected in any suitable manner, for example using magnetic, radio frequency (RF) and/or ultrasonic detectors. By way of non-limitative example, other types of detectable elements that could be used include sensor coils. 
         [0027]    Detectable elements may be provided in any suitable manner. For example, they may be provided to the temporary mandrel as separate elements and/or they may be formed integrally with the temporary mandrel. 
         [0028]    The step of locating the capping panel may comprise detecting the or each detectable element so as to accurately position capping panel relative to the temporary mandrel. In this way, the capping panel may be positioned accurately relative to the void in the finished hollow aerofoil. This accurate positioning may allow the capping panel to have thinner wall thickness, because less machining may be required after joining the capping panel to the pocketed aerofoil body in order to achieve the desired shape. 
         [0029]    The method may comprise machining an outer surface of the hollow aerofoil after the step of joining but before the step of removing the temporary mandrel. As such, the mandrel may continue to provide support to the capping panel during any such machining step, thereby ensuring that it is retained in the desired position. 
         [0030]    Where a machining step is used, it may be based at least in part on the position of the or each detectable element. Thus, the machining may be based at least in part on the position of the temporary mandrel (the position of which may be known accurately from the detectable element), and thus also based at least in part on the position of the resulting void left in the hollow aerofoil once the temporary mandrel has been removed. The detectable element(s) may be said to act as a datum for a machining step. 
         [0031]    The joining step may comprise diffusion bonding. The joining step may involve diffusion bonding the first region of the inner surface of the capping panel to the hollowed surface of the pocketed aerofoil body. Using diffusion bonding has the advantage of producing a strong joint that is free from residual stress. 
         [0032]    The method may comprise locating one or both of the capping panel and the pocketed aerofoil body in a respective fixture prior to the joining step, for example prior to a diffusion bonding step. This may be a particularly convenient way to locate (and optionally subsequently hold) the capping panel relative to the hollowed surface of the pocketed aerofoil body into the position for joining the two together. 
         [0033]    Where one or more fixtures is used, pressure may be applied to and/or through the respective fixture or fixtures so as to perform diffusion bonding. 
         [0034]    Where one or more fixtures is used, the method may comprise heating the or each respective fixture so as to perform diffusion bonding. The heating of the fixtures may be before and/or during any pressure is applied during the diffusion bonding. 
         [0035]    The joining step may comprise liquid interface diffusion (LID) bonding (which may be referred to as liquid activated diffusion bonding). The method may comprise providing an interface foil layer between the surfaces being bonded, for example between part or all of the first region of the inner surface of the capping panel and the surrounding hollowed surface of the pocketed aerofoil body in or to facilitate the liquid interface diffusion bonding. 
         [0036]    The temporary mandrel may be substantially incompressible throughout the joining step. Such an incompressible core would ensure that the temporary mandrel retains the desired shape throughout the joining process, and thus that the cover plate is supported in and/or forms the desired shape during the joining step. 
         [0037]    The temporary mandrel may be constructed and/or arranged such that it can be activated so as to urge the capping panel away from the pocket. For example, where the capping panel is located in a fixture, the capping panel may be urged towards the fixture by such a temporary mandrel when it is activated. By way of example, the temporary mandrel may comprise a cavity that may be sealed using a flexible material, for example it may not allow gas to pass therethrough, i.e. it may be impermeable. Such a cavity may be filled with a gas, for example a non-reactive or inert gas. Activation of such a temporary mandrel may comprise generating a pressure differential across the flexible material containing the gas so that it is urged to expand, and thus act on the capping panel to urge it away from the pocket. Such a pressure differential may be generated by increasing the pressure inside the cavity, for example by heating. 
         [0038]    The temporary mandrel may be coated with a material comprising a rare earth element oxide. This may prevent interaction between the temporary mandrel material and the pocketed aerofoil body or capping panel, including preventing contamination of the pocketed aerofoil body or capping panel and the material of the temporary mandrel. For example, the temporary mandrel may be coated with an oxide of yttrium. 
         [0039]    Purely by way of example, a suitable material for the temporary mandrel may comprise ceramic and/or rammed graphite. 
         [0040]    The temporary mandrel may be formed in any suitable manner, for example using injection moulding and/or compression moulding. 
         [0041]    Any suitable technique may be used to remove the temporary mandrel from the hollow aerofoil, for example ultrasonic shattering or leeching. 
         [0042]    The method of manufacturing a hollow aerofoil may further comprise filling the hollow aerofoil with a core material after the step of removing the temporary mandrel. The core material may be a permanent core material. The core material may have different properties from the temporary mandrel, for example it may be less dense and/or have better vibration absorption characteristics. The core material may be a lightweight filling medium. Thus, the method and apparatus described herein may be used to manufacture a filled aerofoil, and so references herein to the manufacture of a hollow aerofoil should be taken to include the manufacture of an aerofoil that is filled with a core material, i.e. a filled aerofoil. 
         [0043]    The method may comprise machining a pocket into an aerofoil blank in order to produce the pocketed aerofoil body. The method may comprise forming the aerofoil blank, for example by forging. Alternatively, the pocketed aerofoil body may be formed in any other suitable way, for example comprising a casting process, and the method may comprise such a process for forming the pocketed aerofoil body. 
         [0044]    According to an aspect of the invention, there is also provided an aerofoil manufactured according to the method described above and elsewhere herein. Such an aerofoil may be an aerofoil for a gas turbine engine. Such an aerofoil may be (or may form a part of), for example, a rotating blade or a stationary vane. Such an aerofoil may be a part of a compressor or a turbine. Purely by way of example, such an aerofoil may be an outlet guide vane of a turbofan gas turbine engine. 
         [0045]    According to an aspect of the invention, there is provided a gas turbine engine comprising an aerofoil (or aerofoil component) manufactured using a method as described above and elsewhere herein in relation to the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]    Embodiments of the invention will now be described by way of example only, with reference to the accompanying diagrammatic drawings, in which: 
           [0047]      FIG. 1  is a schematic sectional side view of a gas turbine engine; 
           [0048]      FIG. 2  is a schematic perspective view showing a hollow aerofoil in accordance with the invention; 
           [0049]      FIG. 3  is a schematic cross-section through a capping panel, pocketed aerofoil body and temporary mandrel during manufacture of a hollow aerofoil; and 
           [0050]      FIG. 4  is a schematic perspective view showing a pocketed aerofoil body and a temporary mandrel having detectable elements. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0051]    With reference to  FIG. 1 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis X-X. The direction X-X may be referred to as the axial direction of the engine. The engine  10  comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , an intermediate pressure turbine  17 , a low-pressure turbine  18  and a core engine exhaust nozzle  19 . A nacelle generally surrounds the engine  10  and defines the intake  11 , a bypass duct  22  and a bypass exhaust nozzle  23 . 
         [0052]    The gas turbine engine  10  works in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow A into the intermediate pressure compressor  13  and a second air flow B (which may be referred to as a bypass flow B) which passes through the bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  13  compresses the air flow A directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
         [0053]    The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  16 ,  17 ,  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  16 ,  17 ,  18  respectively drive the high and intermediate pressure compressors  14 ,  13  and the fan  12  by suitable interconnecting shafts. 
         [0054]    As the air passes through the gas turbine engine  10  it is heated to high temperatures. In particular, the first airflow A reaches high temperatures as it passes through the core of the engine. Typically, particularly high temperatures may be reached at the exit of the combustion equipment  15 , and as the air subsequently passes through the high, intermediate and low-pressure turbines  16 ,  17 ,  18 . 
         [0055]    The gas turbine engine  10  comprises outlet guide vanes (OGVs)  100  extending across the bypass duct  22 , which therefore sit in the bypass flow B. Each OGV  100  takes the form of a large stator vane, and thus may be referred to as an aerofoil or aerofoil component  100 . A plurality of OGVs  100  is typically provided as an annular array in the bypass duct  22 . 
         [0056]    Because each OGV  100  is an especially large aerofoil component, it is particularly advantageous to reduce its weight. Thus, the OGV  100  in the gas turbine engine  10  is hollow. 
         [0057]      FIG. 2  is a schematic of a hollow aerofoil in accordance with the invention. In the  FIG. 2  example, the hollow aerofoil is a hollow outlet guide vane (OGV)  100 . The hollow OGV  100  is manufactured by joining a capping panel  200  to a pocketed aerofoil body  300 . The capping panel  200  and the pocketed aerofoil body  300  may be any suitable material, for example metallic and/or composite, and may be the same or different materials. 
         [0058]    In  FIG. 2 , the interface between the pocketed aerofoil body  300  and the capping panel  200  is indicated by the line  110 . However, it will be appreciated that after manufacture of the hollow aerofoil  100 , the join between the original capping panel  200  and the pocketed aerofoil body  300  will be seamless, and may not be visible. As such, the line  110  in  FIG. 1  is shown merely for the purpose of indicating an example of the position of the original interface between the capping panel  200  and the pocketed aerofoil body  300  prior to the joining and optional finishing of the hollow aerofoil  100 . Furthermore, it will be appreciated that the extent of the capping panel  200  is not limited to that illustrated by the line  110  in  FIG. 2 , which is merely illustrative. 
         [0059]    In the example shown in  FIG. 2 , the hollow OGV comprises an inner attachment  330  (which may be a platform) and an outer attachment  340 , which may be used to attach the finished OGV  100  to the rest of the gas turbine engine. Such inner and outer attachments  330 ,  340  may be a part of the pocketed aerofoil body  300 . However, it will be appreciated that some pocketed aerofoil bodies  300  may not include one or both of the inner attachment  330  and outer attachment  340 . For example, the inner and/or outer attachments  330 / 340  may be separate features that may be attached to the hollow aerofoil  100  (for example by welding) after the capping panel  200  and the pocketed aerofoil body  300  have been joined together. 
         [0060]      FIG. 3  shows a close-up cross sectional view of the capping panel  200  being joined to the pocketed aerofoil body  300  during manufacture of the hollow OGV  100 .  FIG. 4  also shows a temporary mandrel  400 , which is explained in greater detail elsewhere herein. It will be appreciated that  FIG. 3  shows only a part of the capping panel  200 , pocketed aerofoil body  300 , and temporary mandrel  400 , and that the full parts extend in the chordwise direction, to the left in  FIG. 3 . 
         [0061]    As shown in  FIG. 3 , the pocketed aerofoil body  300  comprises a pocket  310  which, prior to being covered by the capping panel  200 , is an open pocket  310 . The pocket  310  is formed in, and surrounded by, a surrounding hollowed surface  320 . 
         [0062]    During manufacture, a mandrel  400  is positioned in the pocket  310 . The mandrel  400  may also be referred to as a core  400 , or a temporary core  400 . The mandrel  400  is arranged, for example sized and/or shaped, so as to support the capping panel  200  during manufacture. The mandrel  400 , for example an upper (or outer) surface  410  of the mandrel  400 , may support the capping panel  200  over all, or substantially all, of the pocket  310 , as shown in the  FIG. 3  example. In this regard, the capping panel  200  has an inner surface  210  that has a first portion  214  that is supported by the mandrel  400 . 
         [0063]    In the  FIG. 3  example, the capping panel  200  is held in a capping panel fixture  510 , and the pocketed aerofoil body  300  is held in an aerofoil body fixture  520 . During manufacture, a diffusion bonding process is used to join the capping panel  200  to the pocketed aerofoil body  300 . Using a diffusion bonding process may help to ensure that the resulting joint is free from residual stress. However, it will be appreciated that other joining process could be employed. 
         [0064]    In the  FIG. 3  example, the surrounding hollowed surface  320  of the pocketed aerofoil body  300  is joined to an opposing, or adjacent, portion  212  of the inner surface  210  of the capping panel  200  by diffusion bonding. The diffusion bonding may follow any suitable process. For example, the capping panel  200  and the pocketed aerofoil body  300  may be pressed or forced together, for example by applying pressure P through one or more of their respective fixtures  510 ,  520 . The diffusion bonding typically comprises raising the temperature of the pocketed aerofoil body  300  and the capping panel  200 , at least in the regions being joined. The heating energy used to raise the temperature may, for example, be applied through their respective fixtures  510 ,  520 . 
         [0065]    A liquid interface diffusion (LID) bonding process may be used to join the pocketed aerofoil body  300  and the capping panel  200  together. Indeed, this is the joining process that is illustrated in  FIG. 3 . As such, the  FIG. 3  arrangement includes an interface foil layer  600  between the surrounding hollowed surface  320  of the pocketed aerofoil body  300  and the opposing portion  212  of the inner surface  210  of the capping panel  200 . The interface foil layer  600  may comprise copper and/or silver and/or nickel, or indeed any other material that may be used in a LID bonding process. 
         [0066]    As mentioned elsewhere herein, during the joining process, the mandrel  400  supports the capping panel  200 . In particular, the mandrel  400  supports the first portion  214  of the inner surface  210  of the capping panel during the joining process in a position such that the outer surface  220  of the capping panel  200  maintains (or is held in) the desired position. In this regard, the desired position may be that position in which the outer surface  220  takes the correct shape to form an aerodynamic surface of the finished hollow OGV  100 , such as at least a part of the pressure surface or suction surface, optionally allowing for a slight change in shape resulting from an optional finishing step. Also as shown in the  FIG. 3  example, once the capping panel  200  has been positioned onto the surrounding hollowed surface  320 , the surrounding hollowed surface  320  may no longer form an external surface of the aerofoil  100 . 
         [0067]    To this end, the mandrel  400  may be substantially incompressible throughout the joining process. For example, the mandrel  400  may be substantially incompressible even when subjected to elevated pressure and/or temperature resulting from a diffusion bonding process. The mandrel may, for example, comprise ceramic or graphite. As an alternative to being substantially incompressible, the mandrel  400  may experience a degree of compression or deformation during the manufacture of the hollow OGV  100 , but in that case the compression/deformation would be specifically designed to ensure that the capping panel  200  is retained in the desired position during welding. 
         [0068]    During manufacture (for example during a diffusion bonding process), the capping panel  200  (and possibly the pocketed aerofoil body  300 ) may soften. As such, the shape of the capping panel  200  may be defined at least in part by the mandrel  400  on which it is supported. As such, the mandrel  400  may at least in part define the shape of the capping panel  200 , for example the shape of the inner surface  210  and/or the outer surface  220  of the capping panel  200 . Even where the capping panel  200  does not soften appreciably during manufacture, its shape may be at least in part defined by the mandrel  400 . For example, the support provided by the mandrel  400  may prevent the capping panel  400  from sagging during manufacture. The tooling  510 ,  520  may also at least in part define the external shape of the finished hollow OGV  100 . 
         [0069]    The mandrel  400  may be precision formed so as to ensure that the capping panel  200  takes the desired shape. Any suitable process may be used to form the mandrel  400 , such as injection moulding and/or compression moulding. 
         [0070]    In order to avoid unwanted reaction between the mandrel  400  and the capping panel  200  and/or the pocketed aerofoil, the mandrel may be coated with an unreactive material, such as a rare earth oxide, such as yttria. 
         [0071]    The mandrel  400  of  FIG. 3  comprises a detectable element  700  that is detectable even when not visible. In the example of  FIG. 3 , the detectable element  700  is a magnetic element (for example a metallic sphere). The position of such a magnetic element  700  may be detected by, for example, a Hall effect sensor or a Reed sensor. However, other remotely detectable sensors may be used, such as sensor coils or trace paints. 
         [0072]    The position of the detectable element  700  in relation to the mandrel  400  is known accurately. As such, detection of the position of the detectable element  700  allows accurate determination of the position of the mandrel  400  in the pocket  310 , even when it is covered by the capping panel  200 , and thus not visible. In turn, this allows accurate determination of the position of the pocket  310  within the hollow OGV once the capping panel  200  and the pocketed aerofoil body  300  have been joined. 
         [0073]      FIG. 4  is a schematic showing an example of a mandrel  400  in a pocketed aerofoil body  300  prior to the capping panel  200  being introduced. In the  FIG. 4  example, the mandrel  400  is provided with four detectable elements  700 . Any number of detectable elements  700  may be provided to the mandrel  400 , but as the number of detectable elements  700  increases, so the accuracy with which the location of the mandrel  400  may increase. Thus, for example, whilst a mandrel  400  may be provided with one or two detectable elements  700 , providing three, four, five or more than five detectable elements  700  may be advantageous in this regard. 
         [0074]    Precise knowledge of the position of the mandrel  400 , and thus of the internal definition of the pocket  310 , allows the capping panel  200  to be accurately located. This means that the wall thickness of the capping panel can be reduced, because the positional variability and thus the required tolerance can be reduced. This results in a lighter component with less material wastage. 
         [0075]    Precise knowledge of the position of the mandrel  400 , and thus of the internal definition of the pocket  310 , may allow any finishing or post-joining machining to be datumed to the internal pocket  310  geometry/position. Again, this may reduce the required tolerance in wall thickness, and thus allow a reduced wall thickness to be used. 
         [0076]    The mandrel  400  may remain in the aerofoil  100  during any post-joining process, such as machining, finishing, and/or heat treatment. This may ensure that the capping panel  200  retains the correct position and/or shape throughout such processes. 
         [0077]    After manufacture of the hollow OGV  100  (for example after joining the capping panel  200  and the pocketed aerofoil body together, or after any optional post-joining processes have been finished), the mandrel  400  is removed from inside the OGV, leaving a hollow pocket  310 . As such, the mandrel  400  may be referred to as a temporary mandrel  400 . The temporary mandrel  400  may be removed by any suitable techniques, such as ultrasonic destruction or chemical etching. The temporary mandrel  400  may be removed via an opening (which may commonly be referred to as a “letter-box”) in the OGV  100 . 
         [0078]    Optionally, the void, or pocket,  310  remaining after the mandrel  400  has been removed may be at least partially (for example completely) filled using a core material. Such a core material may have properties that provide advantages during use of the aerofoil  100 , such as high vibration damping and light weight. Thus, such a core would typically have different properties to the temporary mandrel  400 . 
         [0079]    Although the invention has largely been described herein in relation to an OGV  100 , it will be appreciated that it could be applied to any aerofoil or aerofoil component, such as any rotor blade or stator vane, for example for use in a turbine, compressor, or other aerofoil-shaped component of a gas turbine engine 
         [0080]    It will be appreciated that many designs and/or arrangements of features, such as capping panel  200 , pocketed aerofoil body  300  or mandrel  400 , other than those shown in and described in relation to  FIGS. 1 to 4  and not explicitly described herein fall within the scope of the invention. Furthermore, any feature described and/or claimed herein may be combined with any other compatible feature described in relation to the same or another embodiment.