Abstract:
A bifurcation aerofoil having a leading edge portion and flanks where the leading edge portion is mounted to an inner wall of the bypass duct using a floating seal which permits circumferential movement of the leading edge. A cowl has a wall that provides the flanks of the aerofoil. On closing of the cowl into its flight position any contact between the flanks and the leading edge portion enables realignment of the leading edge portion to the inner wall of the bypass duct.

Description:
TECHNICAL FIELD OF INVENTION 
       [0001]    The invention relates to a floating seal and a sealing method for aerofoil-shaped structures to a fixed structure within a gas turbine engine. The invention finds particular application for aerofoil bifurcations within a bypass duct of a gas turbine engine 
       BACKGROUND OF INVENTION 
       [0002]    Civil aircraft gas turbine engines are mounted to aircraft sub-structures through a pylon. The engines typically comprise an engine core surrounded by an aerodynamic nacelle. A fan at the engine inlet pushes a large volume of air through a bypass duct defined between the engine core and the nacelle. The pylon extends through the bypass duct and is located within an aerodynamically formed fairing that is known in the industry as a bifurcation that smooths the air flow through the bypass duct around the mounting structure. 
         [0003]    The nacelles typically have cowls that move relative to the engine core to allow access to the engine core. The cowls are arranged to abut fixed sections within the engine and a seal is required between the fixed section in the bypass duct and the cowl. 
         [0004]    Cowling doors can be large components and forming a repeatable seal that can be made, broken and remade has proved difficult. In particular, rigidly fixing the bifurcation aerofoil to the engine structure can, when the cowl doors are closed, lead to gaps giving poor sealing between the bifurcation aerofoil and the cowl door, or excessive pressure on the bifurcation leading edge that can damage the component. The radial join and existing manufacturing and assembly tolerances along with aero and inertia loads creates steps and gaps that are significant drag generators to the flow in the bypass duct. 
         [0005]    It is an object of the present invention to seek to provide an improved sealing arrangement. 
       STATEMENTS OF INVENTION 
       [0006]    According to a first aspect of the invention there is provided a sealing joint between an aerofoil and a wall of a bypass duct on a gas turbine engine, wherein the aerofoil extends radially across the bypass duct and can move circumferentially with respect to the wall; wherein the aerofoil has a circumferentially extending sealing element that cooperates with a seal element on the wall to provide the sealing joint; wherein the sealing joint is maintained on circumferential movement between the duct wall and the aerofoil. 
         [0007]    Axial alignment of the cowling and the aerofoil may be provided by a radially extending groove which engages a flange protruding from the cowling. Advantageously this axial alignment connection is easily assembled and disassembled. The cowl may move radially with respect to the aerofoil. 
         [0008]    Preferably the sealing element is a lip integral with a fixed part of the aerofoil which can also have a moveable part. The fixed part is preferably the leading and/or trailing edge. The moveable part may be the aerofoil flanks that may be moveable with the cowling. The duct wall seal element may comprises a channel defined between a wall of the duct wall and a seal retainer. 
         [0009]    Preferably the sealing element is locatable between the wall and the seal retainer. The channel may have a first flexible seal member on the wall and a second flexible seal member on the seal retainer. 
         [0010]    Preferably the first flexible seal member seals against a first surface of the sealing element and the second flexible seal member seals against a second surface of the sealing element. 
         [0011]    The duct wall is preferably part of the radially inner wall of the bypass duct or part of the radially outer wall of the bypass duct. 
         [0012]    The aerofoil may comprise a leading edge portion and a first and second flank extending from the leading edge portion. The leading edge portion may be detachable from and attachable to one or more of the first and second flanks. 
         [0013]    The leading edge portion preferably has a radially extending groove towards its axially rearward edge. The groove may have a “V” cross section and each flank may have a corresponding radially extending flange that is located within the groove when the flank is attached to the leading edge portion. 
         [0014]    The flanks are preferably provided by walls extending between a surface defining part of the outer periphery of the bypass duct and a surface defining part of the inner periphery of the bypass duct. 
         [0015]    The mounting structure preferably carries a cowling comprising portions of the radially inner wall and the radially outer wall of the bypass duct and the first and second flanks. The cowling may be in two sections each separately mounted to the mounting structure, wherein each structure comprises a portion of the radially inner wall and a portion of the radially outer wall and either the first or second flanks. 
         [0016]    Each section may be mounted to the mounting structure through a hinge that permits rotation of the cowling section away from the engine. 
         [0017]    Alternatively, the aerofoil may be located around a drive shaft that extends across the bypass duct of a gas turbine between an engine core and an auxiliary gearbox. This aerofoil is known as the lower bifurcation. 
         [0018]    The aerofoil may have a sealing element towards both its radially inner and radially outer extents. 
         [0019]    Advantageously, the arrangement permits the aerofoil to move relative to the duct wall in a transverse direction and/or a radial direction. The movement permits a loose installation of the aerofoil to the duct wall that increases the tolerance of the components in manufacture and aids installation by providing a self aligning effect. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0020]    The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0021]      FIG. 1  depicts an exemplary ducted gas turbine; 
           [0022]      FIG. 2  shows a view of the bypass duct with a bifurcation aerofoil with closed cowling; 
           [0023]      FIG. 3  depicts the bypass duct of  FIG. 2  with open cowling 
           [0024]      FIG. 4  depicts a rear view of the leading edge of the bifurcation aerofoil; 
           [0025]      FIG. 5  is a view of the sealing joint between the aerofoil and the radially outer cowl wall; 
           [0026]      FIG. 6  is a cross-section of the sealing joint of  FIG. 5   
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0027]    Referring to  FIG. 1 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis  11 . The engine  10  comprises a propulsive fan  13  and a core engine  9  having, in axial flow series, an air intake  12 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine, an intermediate-pressure turbine  18 , a low-pressure turbine  19  and terminating with a core exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines the intake  12 , a bypass duct  22  and an exhaust nozzle  23 . 
         [0028]    The gas turbine engine  10  works in the conventional manner so that air entering the intake  12  is accelerated by the fan  13  to produce two air flows: a first airflow A into the intermediate pressure compressor  14  and a second airflow B which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the airflow A directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
         [0029]    The compressed air exhausted from the high pressure compressor  15  is directed into the combustor  16  where it is mixed with fuel and combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzles  20  to provide additional propulsive thrust. The high, intermediate and low pressure turbines  17 ,  18 ,  19  respectively drive the high, intermediate pressure compressors  15 ,  14  and the fan  13  by suitable interconnecting shafts. 
         [0030]    A centre plug  29  is positioned within the core exhaust nozzle  20  to provide a form for the core gas flow A to expand against and to smooth its flow from the core engine. The centre plug  29  extends rearward of the cone nozzle&#39;s exit plane  27 . 
         [0031]    The fan is circumferentially surrounded by a structural member in the form of a fan casing  24  which is supported by an annular array of outlet guide vanes  28 . The fan casing  24  comprises a rigid containment casing  25  and attached rearwardly thereto is a rear fan casing  26 . 
         [0032]    The gas turbine engine  10  is installed under an aircraft wing  7  via a pylon  8 . The nacelle  21  comprises an axially forward cover  35  and a translatable cowl  37 . Both the cover and the cowl are provided by C-shaped openable doors with each door being separately hinged to the aircraft pylon  8 . The nacelle has a thrust reverser unit  31  which is formed from a number of cascade panels arranged sequentially around the circumference of the engine  10 . The hinged doors permit access to the engine core for maintenance or inspection purposes. 
         [0033]    A bifurcation aerofoil  60  surrounds the pylon mounting structure as it extends across the bypass duct  22  as shown in  FIG. 2 . The aerofoil has side panels  62 ,  63  and a floating leading edge portion  64  which together enclose a volume that contains mechanical struts, engine mounts, cabling, and pipework through to the engine core. The aerofoil presents a smooth surface to the flow through the bypass duct to minimise flow disruptions and pressure loss. 
         [0034]    The cowling  26 ,  40  is connected by a hinge to the pylon that allows the cowling to be opened by rotating it away from the engine about the hinge. The rotation allows access to the engine core for maintenance or inspection. In the embodiment shown the radially inner wall of the bypass duct  40  and the radially outer wall  26  of the bypass duct are connected by walls  62  and  63  that forms the side panels of the bifurcation aerofoil. 
         [0035]    Efficiency of the engine is kept high by minimising air loss within the engine to ensure that the maximum amount of air possible can be used to generate thrust. Minimising drag and aerodynamic losses is also important. A seal is therefore provided between the cowling and the aerofoil that inhibits air loss from the bypass duct. 
         [0036]    The leading edge portion is  64  is loosely connected to the outlet guide vane outer casing and inner casing  92  to permit limited axial and circumferential or transversal movement. By allowing the leading edge to float the manufacturing tolerance can be relaxed. Cowling doors can be large components—up to four metres in diameter—and forming a repeatable seal that can be opened and closed has proved difficult. 
         [0037]    Shown in  FIG. 4 , the bifurcation leading edge  64  is a lightweight, thin walled component of metal or composite that is reinforced with a plurality of stiffening ribs  66 . The ribs have holes  68  through which cabling and pipework can pass, or just for the weight reduction purpose. 
         [0038]    Towards the radially inner and radially outer extent of the bifurcation leading edge  64  a lip  72  is provided. The lip extends both axially and circumferentially and advantageously acts both to stiffen the edge of the aerofoil and also to provide a sealing feature which cooperates with a further sealing feature on the outlet guide vane inner casing  90 . 
         [0039]      FIG. 5  shows an exemplary seal between the radially outer end of the bifurcation leading edge  64  and the outlet guide vane casing  92 . It is to be understood that a corresponding seal may be provided between the radially inner end of the bifurcation leading edge  64  and the radially inner outlet guide vane casing  90 . Floating point fasteners (not shown) may be used to limit the movement of the leading edge with respect to the casings of the outlet guide vane. 
         [0040]    The casing  92  which forms part of the outer wall of the bypass duct has a seal retainer  80  on the side of the wall that does not form the inner surface of the bypass duct. The seal retainer is cantilevered from the casing to define a channel which holds a flexible seal member. The flexible seal member is formed in two halves and mounted between the seal retainer and the casing  92 , one half mounted  82   a  to the casing and one half to the seal retainer  82   b . The two halves of the flexible member abut each other but flex to allow the bifurcation lip to separate them.. 
         [0041]    Advantageously, this creates a convoluted seal where any air that escapes through the seal has to pass across two flexible members and around the bifurcation lip  70  before exiting the bypass duct  22 . 
         [0042]    A portion  84  of the lip  70  protrudes through the flexible members  82   a ,  82   b . The lip can therefore be imprecisely located axially and circumferentially whilst still providing the required seal. Advantageously, the bifurcation aerofoil leading edge can therefore be loosely located onto the engine with a permitted degree of circumferential or axial movement that reduces the risk of damage to the part.. 
         [0043]    The rear of the leading edge  64  of the bifurcation aerofoil is provided with grooves  86  as receptacles for male elements that are provided on the cowling side walls  62 ,  63 . One groove is provided for each flank but it will be appreciated that multiple flanges may be provided on each flank which will require multiple grooves on each side of the leading edge portion. When the cowl is closed the male elements, which are preferably flanges extending from the cowling, locate within the grooves on the leading edge portion. This both seals the cowling in place and locates the leading edge axially against the cowl. Where the cowl induces movement of the leading edge the movement is enabled by the floating nature of the seal joint and realignment of the leading edge with the outlet guide vane casing structures is enabled in a simple and elegant manner whilst maintaining the sealing joint. 
         [0044]    Use of a “V” shaped groove advantageously helps locate the leading edge portion  64  axially to the outlet guide vane casing. As the flange contacts the groove surface the force of the closing cowl moves the leading edge portion axially forward or rearward depending on whether the flange contacts the axially forward or axially rearward surface of the groove. The floating join between the aerofoil leading edge and the outlet guide vane casing maintaining the seal despite the axial and/or circumferential movement. 
         [0045]      FIG. 6  depicts a schematic of a seal for the radially inner wall of the bypass duct that is of a similar arrangement to that of  FIG. 5 . The bifurcation leading edge  64  has a lip  72  that is located between a seal retainer  80  and the inner wall  40 . The flexible seal members are not shown for clarity. 
         [0046]    With this self-alignment effect, steps and gaps normal to the air flow direction are minimised with a similar reduction in drag. The steps and gaps around the groove  86  and flange joint can change but with a minor effect on the overall drag partially because over those corners integration angle wake is already generated. It will be appreciated that with this invention drag issue of the joint line is reduced and its variation is moved towards the area at which there is already a wake generated. Integration of the drag sources gives less total drag than the separate drag sources. 
         [0047]    Further, if elements of the engine have to be routed through the bifurcation&#39;s leading edge  64 , reinforcing ribs  66  could be split into multiple parts, to provide sufficient access. 
         [0048]    It will be apparent that the invention can also apply to trailing edges of the bifurcation aerofoils which can also be loosely located onto the engine. The trailing and leading edges of the bifurcation duct may therefore move independently of each other as the cowl is opened and closed. 
         [0049]    As discussed earlier the bifurcation aerofoil may also enclose other structures than the pylon mounting structure. For example, it may surround drive shafts from the accessory gearbox. The invention may be used on multiple bifurcation aerofoils that extend across the bypass duct. 
         [0050]    Although described with respect to aerofoils in bifurcation duct of a gas turbine it will be appreciated that the invention could also be applied to other aerofoil structures that require such a floating sealing joint.