Abstract:
A turbine blade growth pocket provides a feature to measure blade growth due to engine operation while maintaining dynamic characteristics of the turbine blade within acceptable limits by providing a variable radius fillet between the pocket and a side rail of the pocket.

Description:
TECHNICAL FIELD 
       [0001]    This application relates to gas turbine engines and, in particular, to the design of rotary airfoil blades therefore. 
       BACKGROUND 
       [0002]    The design of gas turbine blades is an area of continuous improvement as blade geometry and material directly impact engine performance. Blade creep growth is a perennial issue with high pressure turbine blades (i.e. blades mounted on the high pressure turbine, or compressor turbine) due to the hot environment in which they operate. A feature, sometimes known as a “growth pocket”, may be incorporated at or near the tip of a high pressure turbine blade to assist with monitoring blade creep growth over the life of the part and establish at which point the blade needs replacement. However, introducing a growth pocket can introduce stress concentrations, structural or dynamic weakness, and/or vibration issues, depending on the blade design and the specific environment to which the blade is subjected in use. Accordingly, there is room for improvement. 
       SUMMARY 
       [0003]    In a first aspect, provided is a turbine blade comprising an airfoil extending from a root to a tip, the airfoil defined by a suction side and a pressure side each extending from a leading edge to a trailing edge, the tip having a suction side rail of constant thickness defining a suction side outer periphery of the tip, the tip further having a substantially constant depth pocket defined in the tip, the pocket extending from the suction side rail chordwise to a rail-less pressure side, the pocket having a planar floor extending from the suction side rail to the pressure side and meeting the pressure side substantially perpendicularly along an entire length of the pocket, the length of the pocket extending substantially from the blade leading edge to the blade trailing edge, the pocket further comprising a fillet radius between the planar floor and the suction side rail along an entire length of the suction side rail, the fillet radius having at least a first radius in a region of the pocket corresponding to a maximum width of the pocket and a second radius in a region of the pocket adjacent the blade trailing edge, wherein the second radius is larger than the first radius. 
         [0004]    In a second aspect, provided is a turbine blade comprising an airfoil extending from a root to a tip, the airfoil defined by a suction side and a pressure side each extending from a leading edge to a trailing edge, the tip being a substantially planar surface extending perpendicularly from the pressure side, the tip having a suction side rail extending radially outwardly from the planar surface along the suction side of the airfoil, the suction side rail having substantially constant thickness, the rail and surface being substantially perpendicular to one another and having a fillet radius extending between the rail and surface along an entire length of a rail-surface interface, the fillet radius having a changing radius along its length, wherein the fillet radius has a smaller radius adjacent the blade leading edge and a larger radius adjacent the blade trailing edge. 
         [0005]    In a third aspect, provided is a method of making a turbine blade comprising a method of manufacturing a turbine blade, the method comprising the steps of: casting a turbine blade having at least an airfoil having a leading edge a trailing edge, pressure and suctions sides and a tip; electron discharge machining (EDM) a tip using an electrode to provide an airfoil-shaped tip pocket, the tip pocket having a floor extending substantially perpendicularly relative to the pressure side, the pocket being defined on at least one side by a rail extending above the pocket. 
     
    
     
       DRAWINGS 
         [0006]    The figures depict aspects of the disclosed apparatus and method, in which: 
           [0007]      FIG. 1  shows an isometric version of a turbine blade according to the present description; 
           [0008]      FIG. 2  shows an enlarged portion of the cross-section taken along line  2 - 2  in  FIG. 1 ; 
           [0009]      FIG. 3  shows an enlarged portion of the cross-section taken along line  3 - 3  in  FIG. 1 ; 
           [0010]      FIG. 4  shows a plan view of the blade tip of  FIG. 1 ; and 
           [0011]      FIG. 5  schematically shows how a tip portion of the blade of  FIG. 1  may be made. 
       
    
    
     DESCRIPTION 
       [0012]      FIG. 1  depicts a turbine blade  10  having an internally uncooled airfoil body  12  (i.e. the body is solid, free from internal cooling cavities, orifices, etc.), defined by a pressure side  14  and a suction side  16  each extending from a blade leading edge  18  to a blade trailing edge  20 . The airfoil body extends radially outwardly (relative to an engine axis, not shown, when the blade is installed in the engine) from a root platform  22  to an unsupported or free end terminating in tip  24 . The tip  24  is shroud-less, meaning that there is no shroud mounted to the tip, and as such the outer periphery of the tip is substantially the same shape as the outer periphery of the airfoil body  12  (i.e. airfoil shaped). A “fir tree” blade fixing  26  extends radially inwardly from an underside of the root platform and provides a means for fixing the blade to an engine rotor disc (not shown). 
         [0013]    A growth pocket  30  is provided in the blade tip  24 , the pocket having a tip platform  32  bounded on the suction side  16  of the blade by a suction-side rail  34 . The tip platform  32  has a planar portion  36  which extends axially (i.e. substantially perpendicular to a radius extending from the engine axis) across the blade. The tip platform extends chord-wise across the blade from the suction-side rail to a rail-less pressure side  14  of the blade, the absence of a rail on the pressure side resulting in a generally perpendicular intersection of the tip platform  32  and the blade pressure side  14 . A tip platform of this configuration is provided to minimize tip weight, to reduce centrifugally-induced steady stresses and for vibration interference tuning. Though not depicted in the Figures, a slight radius or break-edge may be provided between platform  32  and pressure side  14 . The tip platform is a flat, featureless surface, uninterrupted by cooling holes. 
         [0014]    The growth pocket  30  further includes a fillet radius  38  between tip platform  32  and suction-side rail  34 , extending generally from a leading edge end  40  to a trailing edge end  42  of the fillet radius  38  along the entire length of the platform-rail interface. However, the size of the fillet radius  38  is not constant along its length, as it extends from end  40  to end  42 , as will now be described. 
         [0015]    As shown in  FIG. 2 , fillet radius  38  has a smaller radius  38   a  near the leading edge end  40  to provide sufficient room on tip platform  32  for a blade growth measurement probe to adequately measure blade growth (e.g. due to creep) in a zone  50  of the pocket (see  FIG. 4 ). Since measurement is best taken from a flat (planar) surface perpendicular to the engine radius, it is desirable to provide a measurement surface (surface  36 ) free from the effects of a fillet radius, and thus a small radius achieves the largest possible measurement surface. However, as shown in  FIG. 3 , fillet radius  38  has a larger radius  38   b  near the trailing edge end  42  to better distribute vibratory stresses in the thin region of the airfoil body  12 , indicated as zone  52  in the Figures. The radius of fillet radius  38  may transition smoothly between radiuses  38   a  and  38   b , and may consistently increase from a region where radius  38   a  is present to a region where  38   b  is present. The radius may increase at a constant rate between radius  38   a  and  38   b,  or may increase in increments, or in any suitable manner. 
         [0016]    The growth pocket  30  is radially positioned on the blade to have a specific desired radial depth from the blade tip (i.e. from the height of the suction-side rail), which may be 0.105″, for example. The pocket depth may be selected to optimize negative effects due to airfoil torsional vibration mode (if the pocket was too deep) and negative effects due to airfoil bending vibration mode (if the pocket is too shallow). 
         [0017]    The growth pocket axial position is specifically set to leave a sufficient wall thickness at the airfoil tip (i.e. the thickness of suction-side rail  34 ), which may be a minimum of 0.017″, such that the blade can withstand a blade tip rub without excessive damage to the blade. The suction-side rail may be constant thickness along its length from leading edge to trailing edge. With a rail of constant thickness, the planar portion  36  of the tip platform  32  itself has an airfoil shape (i.e. the airfoil  12  cross-sectional shape less the cross-sectional shape of the rail and fillet radius). 
         [0018]    The blade may be cast from a single crystal alloy. The pocket, however, cannot easily be provided in a casting, and it is difficult to conventionally machine reliably, due to the variable fillet radius and thin wall thickness for suction-side rail  34 . Therefore, referring to  FIG. 5 , once the blade is cast, the growth pocket may be provided using a Electron Discharge Machining (EDM) process with a formed electrode  60 , the formed electrode having a shape suitable to provide the desired pocket shape described above, including a radius portion  38 ′ to provide radius  38 , is used to form the pocket  30  in an end of the as-cast blade  62 . Once formed, the pocket may be bordered on at least one side by the rail  34 . Subsequent to EDM machining of growth pocket, the growth pocket can be finished by honing or polishing to improve surface finish and have remove/negate any debit in material properties. 
         [0019]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.