Abstract:
Fan for gas turbine engine includes hub and annular array of fan blades mounted to hub and extending radially outwardly from hub, each fan blade includes blade root. The hub includes plurality of hub grooves having shape complimentary to shape of blade root and each hub groove receives and radially retains one blade root. A slot is provided on the suction side and on the pressure side of each blade root. Plurality of keys is provided, each key engages with slots in one blade root and with corresponding hub groove so as to axially retain fan blade. The key includes two arms and joining member, two arms are received in slots provided on blade root and joining member extends between the arms. Interface is defined between key arms and slots in blade root, and greater or equal percentage of interface is curved than is planar.

Description:
FIELD OF INVENTION  
       [0001]    The present invention relates to a fan for a gas turbine engine, a fan blade, and/or a gas turbine engine. 
       BACKGROUND  
       [0002]    Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Typically a turbofan gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core. 
         [0003]    The fan comprises an array of radially extending fan blades mounted on a rotor and will usually provide, in current high bypass gas turbine engines, around seventy-five percent of the overall thrust generated by the gas turbine engine. The remaining portion of air from the fan is ingested by the engine core and is further compressed, combusted, accelerated and exhausted through a nozzle. The engine core exhaust mixes with the remaining portion of relatively high-volume, low-velocity air bypassing the engine core through a bypass duct. 
         [0004]    The rotor of the fan can be considered to be a hub. The fan blades are connected to the hub via a blade root. The hub generally includes a plurality of axially extending grooves in the periphery of the hub to receive the roots of the fan blades. To axially retain the fan blades in the hub a shear key and shear key slot arrangement is used. The design of the shear key and shear key slot arrangement can improve accuracy of positioning of the fan blades into the hub and contribute to preventing a fan blade being released from the hub in the event of the blade being impacted, e.g. by a foreign object such as a bird. 
         [0005]    An exemplary shear key  172  and shear key slot  160  is shown in  FIGS. 1A and 1B . The key locates in the shear key slot provided in the blade root  152 . One slot is located each side of the root so that each slot portion receives one arm of the key. An undercut  162  is provided in the base of the root portion to receive the bridging portion (or joining member)  176  of the key which interconnects its arms  174 . The groove in the hub which receives the fan blade root is also provided with two generally radially extending slots. The axial extent of each of the slots in the groove of the hub is approximately equal to the thickness of the arms of the key. When the fan blade root portion is correctly positioned within the fan hub, the slots in the blade root and hub groove respectively are radially aligned. This permits the arm of the key to simultaneously locate in the slots in the blade root and the slots in the groove of the hub. As a consequence of this, the fan blade root portion is prevented by the key from translating axially relative to the hub. 
         [0006]    As can be seen from  FIG. 1A , the shear key slot  160  has two flat faces extending in a thickness direction of the blade, the two flat faces are linked by a further flat face extending substantially in the chordwise direction of the blade (if the blade root is curved the flat face intersects two points on a chord line of the root). The corners between the planar faces are curved to reduce the stress concentration. The circumferentially extending face is provided so that the shear key slot arrangement is capable of resisting bird strike impact forces. 
         [0007]    Typically the surface of the shear key slot  160  and key  172  is treated to mitigate fatigue failure. An exemplary surface treatment method is laser shock peening. Laser shock peening is an expensive treatment method. Furthermore, care needs to be taken that the corners between the planar faces of the key slot are correctly treated, because the corners are geometrically complicated to treat but are also generally the region of highest stress concentration. 
       SUMMARY OF INVENTION  
       [0008]    The present invention seeks to mitigate one or more of the problems associated with the shear key slot arrangements of the prior art. 
         [0009]    A first aspect of the disclosure provides a fan for a gas turbine engine, the fan comprises a hub and an annular array of fan blades mounted to the hub and extending radially outwardly from the hub. Each of the fan blades comprises a blade root. The hub comprises a plurality of hub grooves having a shape complimentary to the shape of the blade root and each hub groove receives and radially retains one blade root. A slot is provided on the suction side and on the pressure side of each blade root. A plurality of keys is provided, each key engages with the slots in one blade root and with the corresponding hub groove so as to axially retain the fan blade. The key comprises two arms and a joining member, the two arms are received in the slots provided on the blade root and the joining member extends between the two arms. An interface is defined between the key arms and the slots in the blade root, and a greater or equal percentage of the interface is curved than is planar. 
         [0010]    It is understood in the art that the surface of the slot in the blade root should include a planar section in order for the key and slot arrangement to have the required strength to resist a bird strike impact. In the prior art, this planar section is maximised so as to improve resistance to bird strike by the sides of the slot being planar, and the only curved surface being a smaller radius region connecting between the planar sections. 
         [0011]    The present inventors have gone against this prejudice in the art and have surprisingly found that the planar section can be substantially reduced in size or eliminated and still meet the requirements for resisting bird strike impact. 
         [0012]    The slot of the first aspect has the advantage that the highest stress concentration is not in a tight corner of the slot (as in the prior art). This means that the geometry is such that the position of the stress concentration is away from the areas that are most difficult to surface treat. In some cases it may not be necessary to surface treat the areas of most challenging geometry. 
         [0013]    A further advantage of the slot and key arrangement of the first aspect is that the surface treatment time (e.g. laser shot peening time) can be reduced. 
         [0014]    A yet further advantage of the slot and key arrangement of the first aspect is that a wider range of surface treatments are available, for example low plasticity burnishing, which would not be possible with the conventional slot and key design. When using treatment methods employing a tool applying a compressive stress, it may be possible to use a single tool to treat the entire geometry and/or apply a high compressive stress. 
         [0015]    In the present application, when directions are defined with respect to the blade, a chordwise direction refers to a direction extending from the leading edge to the trailing edge of the blade. A spanwise direction refers to a direction extending from the blade root to a tip of the blade. A thickness direction refers to a direction extending from a pressure side of the blade to the suction side of the blade. 
         [0016]    The surface of the slot and the surface of the key defining the interface between the key arms and the slots may extend substantially in the spanwise direction. 
         [0017]    The curvature of the interface may lead to a planar section. 
         [0018]    The radius of curvature may be approximately equal to or between 3 and 15 mm. In exemplary embodiments the radius of curvature may be equal to or between 3 and 7 mm. 
         [0019]    The interface may have one or more curved sections having a constant and/or a single radius. Alternatively the interface may have one or more curved sections having a varying radius, for example a larger radius leading to a smaller radius. 
         [0020]    The transition between a spanwise extending surface of the blade slot and the remainder of the root may be curved. The curvature of the transition region may be greater in a section of the slot that extends generally in the thickness direction than in a section of the slot that extends generally in the chordwise direction of the slot. 
         [0021]    An undercut may be provided in each blade root. The undercut may extend between and connect opposing slots in the blade root. The undercut may receive the joining member of the key. 
         [0022]    A slot may be provided in the hub groove. The slot may be aligned with the slots in the blade root. The slot in the hub groove may receive a portion of the key. 
         [0023]    The slot in the blade root may be surface treated using low plasticity burnishing. 
         [0024]    A second aspect of the disclosure provides a fan blade comprising a fan blade root. A slot is provided in the suction side and the pressure side of the fan blade root. The slot may extend through the root in a spanwise direction to define a slot surface. A larger or equal percentage of the slot surface may be curved than planar. 
         [0025]    The fan blade of the second aspect may include one or more of the optional features of the fan blade of the fan of the first aspect. 
         [0026]    A third aspect of the disclosure provides a fan blade comprising a fan blade root. A slot is provided in the suction side and the pressure side of the fan blade root. The slots may extend through the root in a spanwise direction to define a slot surface, and wherein the slot surface includes a curved portion having a radius of approximately 3 to 7 mm. 
         [0027]    The fan blade of the third aspect may include one or more of the optional features of the fan blade of the fan of the first aspect. 
         [0028]    A fourth aspect of the disclosure provides a fan blade comprising a fan blade root. A slot is provided in the suction side and the pressure side of the fan blade root. The slot extends through the root in a spanwise direction to define a slot surface, and wherein a larger percentage of a portion of the slot extending in the chordwise direction is curved than planar. 
         [0029]    The fan blade of the fifth aspect may include one or more of the optional features of the fan blade of the fan of the first aspect. 
         [0030]    A fifth aspect of the disclosure provides a method of manufacturing a fan blade having a blade root. The method comprises providing a slot in the blade root; and surface treating the slot using low plasticity burnishing. 
         [0031]    The fan blade may be a fan blade of the fan of the first aspect, and/or a fan blade of the second, third and/or fourth aspects. 
     
    
     
       DESCRIPTION OF DRAWINGS  
         [0032]    The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0033]      FIG. 1A  illustrates a perspective view of a shear key of the prior art; 
           [0034]      FIG. 1B  illustrates a perspective view of a shear key slot in a blade root of the prior art; 
           [0035]      FIG. 2  illustrates a cross section of a gas turbine engine; 
           [0036]      FIG. 3  illustrates a perspective view of a fan blade of the gas turbine engine of  FIG. 2 ; 
           [0037]      FIG. 4  illustrates a partial perspective view of a hub of the fan of the gas turbine engine of  FIG. 2 ; 
           [0038]      FIG. 5  illustrates a perspective view of a slot formed in a root of the blade of  FIG. 3 ; 
           [0039]      FIG. 6  illustrates a perspective view of a key for use with the slot of  FIG. 5 ; and 
           [0040]      FIG. 7  illustrates a schematic view of an arrangement used for low plasticity burnishing. 
       
    
    
     DETAILED DESCRIPTION  
       [0041]    With reference to  FIG. 2  a bypass gas turbine engine is indicated at  10 . The engine  10  comprises, in axial flow series, an air intake duct  11 , fan  12 , a bypass duct  13 , an intermediate pressure compressor  14 , a high pressure compressor  16 , a combustor  18 , a high pressure turbine  20 , an intermediate pressure turbine  22 , a low pressure turbine  24  and an exhaust nozzle  25 . The fan  12 , compressors  14 ,  16  and turbines  20 ,  22 ,  24  all rotate about the major axis of the gas turbine engine  10  and so define the axial direction of the gas turbine engine. 
         [0042]    Air is drawn through the air intake duct  11  by the fan  12  where it is accelerated. A significant portion of the airflow is discharged through the bypass duct  13  generating a corresponding portion of the engine thrust. The remainder is drawn through the intermediate pressure compressor  14  into what is termed the core of the engine  10  where the air is compressed. A further stage of compression takes place in the high pressure compressor  16  before the air is mixed with fuel and burned in the combustor  18 . The resulting hot working fluid is discharged through the high pressure turbine  20 , the intermediate pressure turbine  22  and the low pressure turbine  24  in series where work is extracted from the working fluid. The work extracted drives the intake fan  12 , the intermediate pressure compressor  14  and the high pressure compressor  16  via shafts  26 ,  28 ,  30 . The working fluid, which has reduced in pressure and temperature, is then expelled through the exhaust nozzle  25  generating the remainder of the engine thrust. 
         [0043]    The intake fan  12  comprises an array of radially extending fan blades  40  that are mounted to the shaft  26 . The shaft  26  may be considered a hub at the position where the fan blades  40  are mounted. 
         [0044]    Referring to  FIG. 3 , the fan blades  40  each comprise an aerofoil portion having a leading edge  44 , a trailing edge  46 , a concave pressure surface  48  extending from the leading edge to the trailing edge and a convex suction surface  49  extending from the leading edge to the trailing edge. The fan blade has a root  50  via which the blade can be connected to the hub. The root  50  has a dovetail shaped cross section. The fan blade has a tip  52  at an opposing end to the root. The fan blade may also have an integral platform (not shown in  FIG. 2 ) which may be hollow or ribbed for out of plane bending stiffness. In the present embodiment the fan blade is a metallic fan blade. 
         [0045]    In the present application, a chordwise direction C is a direction extending between the leading edge and the trailing edge; a spanwise direction S is a direction extending between the tip of the blade and the root  50  of the blade  40 ; and the thickness direction T is a direction extending between the pressure surface  48  and the suction surface  50  of the blade  40 . 
         [0046]    Referring now to  FIG. 4 , a portion of the hub  54  is shown. The hub includes a plurality of grooves  56 . In the present embodiment the grooves extend in an axial direction, but in alternative embodiments the grooves may be angled to the axial direction and/or may be curved. The grooves have a dove tail shaped cross section, which is complimentary to the shape of the blade root. 
         [0047]    Referring now to  FIG. 5 , a slot  60  is provided in the blade root  50  at the pressure side of the blade and at the suction side of the blade. An undercut  62  is provided in the blade root and extends between and connects the slots  60 . 
         [0048]    As can be seen in  FIG. 5 , the slot  60  cuts into the root in the thickness direction T of the root and extends entirely through the root (in the region of the slot) in the spanwise direction. The slot on the pressure side of the blade root extends from the pressure side towards the suction side. The slot includes a surface wall  64  extending substantially in the spanwise direction S. The surface  64  is substantially an inverted U shape in plan view, that is the surface  64  extends from the pressure side towards the suction side, the surface then curves in a generally chordwise direction C before curving back towards the pressure side of the blade root. 
         [0049]    The majority of the surface  64  is curved in a plane defined by the thickness direction T and the chordwise direction C (e.g. the majority of the surface  64  is curved when viewed in plan view). The portions of the surface  64  extending from the pressure side end of the slot towards the suction side end of the slot have a curvature of approximately 3 to 7 mm, but this curvature can vary depending on given loading requirements and blade geometry. 
         [0050]    The surface  64  includes a planar section  66 . The planar section  66  extends substantially in the chordwise direction of the blade root  50 . The planar section  66  is much smaller in extent than the comparable planar section of the prior art, due to the curvature of the remainder of the surface  64 . Indeed, the majority of the surface  64  of the slot  60  is curved, as opposed to slots of the prior art where the majority of the slot is planar. 
         [0051]    The upper edge  68  of the slot  60  are rounded. That is, the transition between the surface  64  of the slot and the outer surface of the root  50  is curved. The curvature of the edge  68  is larger in the region where the surface extends from the pressure side to the suction side than in the region where the surface extends in the chordwise direction. 
         [0052]    The transition between the slots  60  and the undercut  62  is curved (the curved region is labelled  70 ). In the present embodiment, the transition has a curvature of approximately 3 to 8 mm, but it will be appreciated that any suitable curvature may be selected. 
         [0053]    The slot on the suction side of the fan blade has a similar profile as the slot on the pressure side, but is a mirror image. 
         [0054]    The shape of the slot  60  means that the slot does not have tight corners, as compared to the slots of the prior art, which aids with surface treatment of the slot. Surprisingly, when tested the blade having the described root configuration (and using the key as will be described later) was able to resist bird strike impact. It is commonly understood in the art that the planar portion of the slot needs to be much larger than that of the described embodiment (feature  66 ) to resist bird strike impact, but the present inventors have unexpectedly found this not to be the case. 
         [0055]    Referring now to  FIG. 6 , the key  72  is shown in more detail. The key includes two arms  74  and a joining member  76  extending between the two arms. The arms and the joining member form a substantially U-shaped key. The shape of the key is complimentary to the slots  60  and undercut  62  on the blade root, so that the arms and joining member of the key can be at least partially received in the slots and undercut. 
         [0056]    The arms  74  of the key  72  each include a surface (referred to from hereon in as the inner surface) that interfaces with the slot. The inner surface is shaped so that a larger percentage of said surface is curved than planar. As can be seen the inner surface of the arms curve in a direction towards the opposing arm (i.e. in the thickness direction when connected to the blade root). At a position proximal to the opposing arm the inner surface includes a planar portion. The planar portion is much smaller than the comparable planar portion of the prior art. As is apparent from  FIGS. 5 and 6 , the inner surface of the arms  74  of the key  72  follows the profile defined by the surface  64  of the slot  60 . 
         [0057]    The joining member  76  has substantially planar surfaces. A curved region  78  provides a transition from the inner surface of the arms  74  to the joining member. The joining member is thinner in width than the arms (similar to the undercut  62  which is thinner in width than the slots  60 ). A portion of the side walls of the key  72  that transition from the joining member to the arms is curved. 
         [0058]    To manufacture the fan blade, the blade is formed in the usual way using techniques known in the art. The blade may be manufactured with the slot  60  in the blade root or the slot  60  may be machined into the blade. The slot  60  is then surface treated to improve fatigue performance. 
         [0059]    In the present embodiment the slot  60  is surface treated using low plasticity burnishing. The process of low plasticity burnishing is illustrated in  FIG. 7 . As can be seen in  FIG. 7 , to low pressure burnish a surface  80  (such as the surface  64  of the slot  60 ) a ball element  82  (or other type of rolling element) is rolled along the surface  80 . During the rolling of the element  82  a compressive pressure is applied to the surface  80  using hydraulic pressure. A hydraulic pump  84  supplies a fluid (usually oil based) through a central channel  86  of a burnishing tool  88 . The ball element  82  is provided at an axial end of the burnishing tool and the fluid flows from the central channel  86  around the ball element, such that the ball element does not directly contact the burnishing tool during operation. 
         [0060]    Low plasticity burnishing is generally a faster and cheaper surface treatment process than the conventional method of laser peening. To the inventors knowledge, it is not currently possible to effectively use low plasticity burnishing to treat slots of the prior art blade roots because it is not possible to get the ball element  82  into the tight corners of the slot. This is of particular importance because in blade slots of the prior art the tight corners are the regions of the highest stress concentration. 
         [0061]    In alternative embodiments, laser peening could be used to treat the surface of the slot  60  of the described embodiment. If laser peening is used, in many cases, the peening time can be reduced compared to the slots of the prior art. 
         [0062]    When treating the surface of the slot  60 , it may be possible, depending on loadings for a specific application, to not treat the regions of the smallest radius (i.e. the transition between the planar region and the curved region), which would further reduce treatment time and complexity. It is possible to not treat said regions because the critical regions in terms of stress have been moved away from the regions of most complex geometry. 
         [0063]    It will be appreciated by one skilled in the art that, where technical features have been described in association with one or more embodiments, this does not preclude the combination or replacement with features from other embodiments where this is appropriate. Furthermore, equivalent modifications and variations will be apparent to those skilled in the art from this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. 
         [0064]    The described fan blade is a metallic fan blade, but in alternative embodiments the fan blade may be a composite fan blade. 
         [0065]    In the present embodiment the planar surface is flat, e.g. if the root of the blade is curved the planar surface intersects the chord of the blade at two points and deviate from the chord for the remainder of the surface. Alternatively, the planar surface may follow the general chordal direction of the root. In further alternative embodiments the slot may not include a planar section and instead the entirety of the surface of the slot that interfaces with the key arm may be curved.