Abstract:
A composite aerofoil vane for a gas turbine engine extends in use in a generally radial direction across an axially- and circumferentially-extending annular duct. The vane comprises a plurality of first reinforcing fibers extending in a generally radial direction and a plurality of second reinforcing fibers extending in generally axial and circumferential directions. The second fibers are interlaced with the first fibers so as to form a more unified and integrated structure, and to prevent the first reinforcing fibers from moving relative to one another.

Description:
BACKGROUND 
     This invention relates to non-rotating aerofoil vanes for gas turbine engines, and more particularly to fibre-reinforced composite vanes. 
     Gas turbine engines comprise stages of rotating aerofoil blades, which turn the gas flow and either do work on, or extract work from, the gas flow (depending on whether they are compressor or turbine blades). Interposed between the stages of rotating blades are stages of non-rotating aerofoil vanes, whose primary purpose is to straighten the gas flow to deliver it at the correct angle of incidence to the next stage of rotating blades. 
     Vanes are also provided elsewhere in gas turbine engines where straightening of the gas flow is required.  FIG. 1  shows a sectional view of part of a gas turbine engine, which has a principal rotational axis X-X. In use, air enters the engine through an annular intake  10 , and passes along a duct defined by an inner annulus wall  12  and an outer annulus wall  14 . The air passes through an annular array of rotating fan blades  16 , which impart energy to the air flow, following which an annular splitter  18  divides the air flow into two streams. A core air flow  20  passes through an annular array of vanes  21 , commonly known as engine section stators, and thereafter through the engine core. A bypass flow  22  passes through the bypass duct  24 . The details of operation of these two streams are well known and will not be described here. The bypass flow passes through an annular array of fan outlet guide vanes  26 . The vanes  26  straighten the air flow leaving the fan blades  16 , and thereby reduce the aerodynamic losses in the bypass duct  24 . 
     SUMMARY 
     In addition to their aerodynamic function, fan OGVs must also resist aerodynamic loads and loads arising from impact of foreign objects. Depending on the engine design, they may also have to carry structural loads. Current trends in engine architecture are for structural OGVs, and with the deletion of features such as A-frames and rear fan cases the structural requirements on the OGVs are becoming even more challenging. 
     In use, OGVs have to resist buckling loads, tensile loads and torsional assembly loads exerted by a number of external forces, including gust loading on the nacelle and fan blade off. The OGVs must also maintain their integrity under bow and torsional vibration. 
     Fan OGVs are commonly made from metal, and both hollow and solid metal vanes are known. It is also known to make them from fibre-reinforced composite material. 
     Organic matrix composites are commonly considered where a weight reduction is desired. However, in the case of OGVs the conflicting loading requirements, and in particular the torsional vibration requirement, mean that a composite vane must be some 35% thicker than a corresponding metal one, which is detrimental to the weight and aerodynamic performance. 
     It is therefore an aim of this invention to provide a composite aerofoil vane with superior mechanical properties to known vanes, so that the thickness penalty compared with a metal vane is reduced to an acceptable level. 
     The invention provides a composite aerofoil vane for a gas turbine engine as set out in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described in more detail, with reference to the attached drawings, in which 
         FIG. 1  shows a sectional view of part of a gas turbine engine; 
         FIG. 2  shows a cross-section of a known composite aerofoil vane; 
         FIG. 3  shows a view on arrow B of  FIG. 2 ; 
         FIG. 4  shows a cross-section of a first embodiment of a composite aerofoil vane according to the invention; 
         FIG. 5  shows a partial cross-section of the trailing edge region of a second embodiment of a composite aerofoil vane according to the invention; and 
         FIG. 6  shows a schematic side view of part of the vane of  FIG. 4 , in the direction of the arrow VI. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 2  shows a cross-section of a known composite aerofoil vane  30 , approximately at the position shown by the line A-A in  FIG. 1 . The vane  30  extends in a generally axial direction between a leading edge  32  and a trailing edge  34 . The aerodynamic profile of the vane  30  is formed by a shell  36  formed of one or more layers of woven ±45° fibres (that is, fibres whose directions lie at 45° either side of the axial direction of the vane).  FIG. 3  shows a view on arrow B of  FIG. 2 , and illustrates the weave of the shell fibres. In this embodiment, the shell comprises two layers of ±45° fibres, but it will be appreciated that in different embodiments fewer or more layers may be used. Furthermore, it will be appreciated that other weave patterns and fibre orientations may be employed. Other common fibre orientations are 0°, ±60° and 90°. The ±45° woven fabric of the shell typically provides substantially all the torsional stiffness of the vane  30 . The outer surface  38  of the shell  36  also provides a smooth gas-washed surface for the vane  30 , which is important for its aerodynamic performance. 
     Within the shell  36  are a plurality of bundles or tows  40  of unidirectional fibres, oriented in a generally radial direction (substantially at 90° to the axial direction). There will also be a relatively small number of fibres at 0° to the axial direction, loosely connecting the dry fibres that form the tows. Only a few tows  40  are shown, but in practice they would fill the space within the shell  36 . Each tow  40  comprises typically 12000-24000 fibres, though it may have as few as 1000. Sets of tows can be bundled together to speed production. The unidirectional tows provide substantially all the radial strength and bow stiffness of the vane  30 . The remaining space within the shell is filled with resin, and the whole structure is cured together. Typically, the dry fibres are placed in a mould tool and the resin is introduced in a resin transfer moulding (RTM) process. 
     Usually, the shell layers and the unidirectional bundles will be made from the same fibre-resin system, for example AS7/IM7 intermediate modulus fibres in an RTM6/PR520 epoxy resin. Of course, alternative fibre/resin systems may be used to suit particular applications—for example, an HTS fibre (higher strength) with BMI resin (higher temperature). 
     This two-part structure, in which substantially all the torsional stiffness of the vane is provided by the outer shell  36  and substantially all the bow stiffness and radial strength is provided by the unidirectional tows  40 , is relatively inefficient and means that a vane of this construction must typically be some 35% thicker than an equivalent metal vane. It will be appreciated that such an increase in thickness is very damaging to the aerodynamic performance of the vane. A further disadvantage is that the increased bulk of the vane inevitably leads to an increase in weight and manufacturing cost. 
     The inventors have realised that a more integrated structure can be more structurally efficient, and can therefore deliver the required stiffness and strength with a smaller aerodynamic penalty. Vanes according to the invention are, furthermore, lighter and offer a reduced performance penalty compared with known composite vanes. Manufacturing costs may also be reduced, although there is a trade-off between increased complexity of the vane architecture and a reduction in material input. 
       FIG. 4  shows a cross-section of a first embodiment of a composite aerofoil vane  70  according to the invention. (As for  FIG. 2 , the cross-section is approximately at the position shown by the line A-A in  FIG. 1 .) The vane  70  has a pressure surface  72  and a suction surface  74 , which extend in a generally axial direction between a leading edge  76  and a trailing edge  78 . 
     The vane comprises a plurality of bundles, tows or structures  80  of first fibres. These fibres are unidirectional and are aligned in a generally radial direction. By “tows” is meant sheaves of aligned fibres, which may be dry (i.e. with no resin between them). By “structures” is meant solid or hollow pre-cured rods or bundles of fibres. If the tows are formed of dry fibres they will generally be loosely tied together. A fibre bundle may be made from several tows and these may be of different materials; for example, a bundle may comprise tows of carbon, glass and aramid fibres. Because different fibres have different properties, the mix of fibres may be varied to provide optimum properties for different regions of the vane. This is illustrated in  FIG. 4  by the two different types of bundles  80  and  80 ′. Interlaced with the tows  80  are a plurality of second fibres  82 ,  84 . These fibres run in generally axial and circumferential directions. These fibres may be arranged in rods, bundles or tows. As with the first fibres, the second fibres  82 ,  84  may be of different materials, separately or mixed within a tow or bundle; for example, carbon, aramid or metal fibres. 
     The second fibres  82 ,  84  are provided over at least part of the span of the vane. They can be over any part of the span, but will generally be provided in the middle two-thirds where enhanced stiffness will improve the resistance to bow and torsional vibration. The spacing, in the radial direction, of the second fibres can be adjusted to suit the particular stiffness requirements, and ultimately will be limited by the fibre gauge (finer fibres permitting a higher volume fraction of fibres). 
     The leading edge and trailing edge portions  76 ,  78  are constructed in much the same way as the first fibre bundles  80 ,  80 ′, but are appropriately shaped to define the leading and trailing edges of the vane. Typically, the vanes will be provided with metal leading edge protection made from stainless steel or from nickel alloy. 
     The second fibres are arranged in two distinct patterns, which alternate along the radial direction of the vane. The spacing of the second fibres in the radial direction will vary as required. In the first pattern, second fibres  82 ′ and  82 ″ are interlaced between the tows  80  so as to extend from the pressure surface  72  to the suction surface  74  of the vane. In the second pattern, second fibres  84 ′ and  84 ″ are interlaced between the tows  80 ′ nearest to the pressure surface  72  of the vane, but do not pass through the central portion of the vane. Similarly, second fibres  84 ″′ and  84 ″″ are interlaced between the tows  80 ′″ nearest to the suction surface  74  of the vane. Both second fibres  82  and second fibres  84  can extend over the full chord, from leading edge to trailing edge. In  FIG. 5 , the axial extent of these fibres is limited to different axial zones only to improve the clarity of the drawing. 
       FIG. 6  shows a side view of part of the vane of  FIG. 4 , in the direction of the arrow VI. Second fibres  82  are interlaced between the first fibres  80 . It can be seen that the weaving pattern alternates by half a pitch between successive second fibres  82   a ,  82   b ,  82   c . Although it is not shown in  FIG. 6 , the weaving pattern will also alternate in exactly the same way between successive second fibres  84 , and (in places where second fibres  82  and  84  are adjacent to each other) between successive second fibres  82 ,  84 . 
     These arrangements of first and second fibres can be produced on a loom, using a 3D weaving machine or using robotic placement. It would also be possible to manufacture them by hand, although of course this would be slower. In one embodiment of the invention, the fibres are woven into a dry fibre pre-form, which is then fitted into a mould tool. Resin is injected into the mould tool, in a known resin transfer moulding process, and the component is then cured in the mould. 
     In this way, the tows  80  of first fibres are bound together by the second fibres  82  and  84 , so as to form a more unified and integrated structure. In particular, the tows  80  are prevented from moving relative to one another as shown in  FIG. 3 . The result is a vane with much better mechanical properties; in particular, the torsional stiffness is greatly improved by the interlacing such that an adequately stiff vane can be made with only a small thickness penalty compared with a metal vane. 
       FIG. 5  shows the trailing edge region of a second embodiment of a vane according to the invention, in which a different combination of first fibre bundles  80 ,  80 ′ is employed to form the desired trailing edge shape. 
     The vane  70  may optionally be provided with a thin surface layer  86 . This helps improve the integrity of the thin leading and trailing edges of the vane, and also helps to improve the profile tolerance and surface finish of the aerodynamic shape of the gas-washed surface. In contrast to the shell of the prior art vane of  FIG. 2 , the surface layer is thin and does not contribute to the mechanical properties of the vane. The surface layer may comprise a fine woven layer wrapped or braided around the vane. The surface layer may comprise a woven mat of reed-like flat elements of polymer or metal. A woven structure will generally be easier to handle, and will be easier to attach to the structure with infused resin than a continuous sheet would be. It will also be more resistant to delamination. As discussed previously, the vane will typically have metal leading edge protection. In line with normal practice, erosion protection (such as a layer of polyurethane) may be applied to the leading edge and/or pressure surface of the vane. If required, colourants or barrier layers (against UV, moisture etc.) may be applied or may be mixed with the surface layer.