Abstract:
A method for repairing a blade in a gas turbine engine comprises the steps of: isolating the damage on the airfoil of the blade; forming a cut back in the shape of elongated “D” shaped recess with a pair of fillets, a depth and a longitudinal axis of the “D” shaped recess having a length along the leading or trailing edge of the airfoil; and the fillets having a respective radius.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority on U.S. Provisional Application Ser. No. 61/838,022, filed on Jun. 21, 2013. 
     
    
     TECHNICAL FIELD 
       [0002]    The described subject matter relates generally to gas turbine engines, and more particularly to a method for repairing a damaged blade. 
       BACKGROUND ART 
       [0003]    Compressor blades of gas turbine engines are subject to foreign object damage (FOD). The nature of the damage could vary depending on the type of the foreign object: nicks, tears, dings and blade bending are common types of damages seen in the field. In order to make the damaged blades flight worthy again, the damaged areas of the airfoil are repaired in a well-defined fashion as outlined in repair and overhaul manuals. A typical blade repair scheme involves a cut out in the area of interest that is in the shape of an arc or “C” shape. 
         [0004]    The typical blade repair scheme is not always successful because peak steady stress and peak vibratory stress locations may both coincide at the cutback radius. The peak vibratory stress may correspond to a resonance condition. This coincidence of vibratory and steady stress peaks is a concern from a durability stand point. 
         [0005]    There is a need to improve such repair methods. 
       SUMMARY 
       [0006]    In accordance with the present disclosure, there is provided a method for repairing a blade in a gas turbine engine comprising: identifying a damage on an edge of an airfoil of the blade; forming a cutback around the damage in the edge, the cutback shaped to comprise at least a pair of fillets r 1 , r 2  in the edge on opposite ends of the cutback, a depth d from the edge, and a length l along the edge. 
         [0007]    Further in accordance with the present disclosure, there is provided a blade in a gas turbine engine comprising: an airfoil having a leading edge and a trailing edge; and a cutback machined in at least one edge among the leading and trailing edges at a location of damage, the cutback comprising a shape defined by at least a pair of fillets r 1 , r 2  on opposite ends of the cutback, a depth d from the edge, and a length l along the edge. 
         [0008]    Still further in accordance with the present disclosure, there is provided a gas turbine engine comprising: at least one blade having a leading edge and a trailing edge; and a cutback machined in at least one edge among the leading and trailing edges at a location of damage, the cutback comprising a shape defined by at least a pair of fillets r 1 , r 2  on opposite ends of the cutback, a depth d from the edge, and a length l along the edge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Reference is now made to the accompanying figures in which: 
           [0010]      FIG. 1  is a schematic view of a longitudinal section of an embodiment of a turbofan gas turbine engine; 
           [0011]      FIG. 2  is a fragmentary perspective view of a blade repaired with a conventional “C” shaped cutback; 
           [0012]      FIG. 3   a  is a graphical representation of  FIG. 2  showing the peak vibratory stress; 
           [0013]      FIG. 3   b  is a graphical representation showing the peak steady stress; 
           [0014]      FIG. 4  is a fragmentary perspective view of a blade repaired in accordance with an embodiment of the present disclosure; 
           [0015]      FIG. 5   a  is a graphical exemplary representation of  FIG. 4  showing the peak vibratory stress on the blade of  FIG. 4 ; 
           [0016]      FIG. 5   b  is a graphical exemplary representation showing the peak steady stress on the blade of  FIG. 4 ; 
           [0017]      FIG. 6   a  is a schematic view of another shape of the cutback of  FIG. 4 ; and 
           [0018]      FIG. 6   b  is a schematic view of another shape of the cutback of  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]      FIG. 1  schematically depicts a turbofan engine  10  which, as an example, illustrates the application of the described subject matter. The turbofan engine  10  includes a nacelle  11 , a fan  12 , a compressor module  14 , a combustor module  16  and a high pressure turbine module  18 . 
         [0020]      FIG. 2  of the prior art shows a typical compressor disc  20  with an airfoil  22 , a leading edge  24  a trailing edge  26 , and hub  27 . As shown in  FIG. 2 , a repair in the form of a conventional “C” shaped cutback  28  is applied to a mid-span area of the leading edge  24 . As shown in the graphs represented in  FIGS. 3   a  and  3   b , the peak vibratory stress and the steady stress peaks may coincide at the mid-span area where the repair  28  is made. This may be a cause for concern of reduced durability. 
         [0021]      FIG. 4  shows a similar compressor disc  30  having an airfoil  32 , a leading edge  34  and a trailing edge  36 . A repair has the form of a “D” shaped cutback  38  (hereinafter referred to as “D” shaped for simplicity. The “D” shaped cutback  38  may be compared to an elongated recess resembling a geometric form between a rectangle and an ellipse. It is characterized by fillets r 1  and r 2 . The radii of the fillets r 1  and r 2  may or may not be equal in value. It may be possible to use the same tooling if the radii of the fillets r 1  and r 2  is equal. In an embodiment, the fillets r 1  and r 2  may be spaced apart by a generally straight cutback edge f. By generally straight, it is understood that the cutback edge f may be substantially straight, or may have a radius that is substantially greater than the fillet r 1  and r 2 , i.e., be quasi-straight. It is also considered not to have any edge spacing apart the fillet r 1  and r 2 , whereby 1=r 1 +r 2 , in a limit case for the cutback  38  which would have more of a “C” shape in this limit case. 
         [0022]    Still referring to  FIG. 4 , the length l and depth d will vary depending on the damage to be repaired. Fillets r 1  and r 2  may vary as a function of the depth d. For instance, an appropriate ratio range for l/d is 1 to 20, while r 1 /d=0.2 to 20 and r 2 /d= 0 . 2  to  20 . The depth d is within the maximum blend limit. 
         [0023]    For example, in proposed applications the length l may be between 0.060″ and 3.0″, for d between 0.030″ and 1.5″, and for r 1 , r 2  between 0.030″ and 1.5″. 
         [0024]    Referring now to  FIGS. 5   a  and  5   b , it will be seen how the peak vibratory stress is concentrated more in the area of r 1  ( FIG. 5   a ) with a critical stress location shown as A, while the peak steady stress is located closer to the r 2  zone with a critical stress location shown as B. Hence, the “D” shaped cutback of repair  38  helps in decoupling peak steady stress and peak vibratory stress locations. With a “D” shaped cutback in place and appropriately located, two critical locations can be well separated, thus making the blade repair scheme acceptable. 
         [0025]    Referring to  FIGS. 6   a  and  6   b , other alternative shapes of the cutback  38  are shown, in which the fillets r 1 , r 2  are offset from the leading edge  34  (although a similar configuration could be used on the trailing edge  36  as well). The fillets r 1 , r 2  are offset from the edge by straight portions as in  FIG. 6   a , or by arcuate portions, as in  FIG. 6   b , or by a combination of both, etc. The straight portions of  FIG. 6   a  may be angled or perpendicular to the edge  34 ,  36 , and may be quasi-straight, etc. In the instances of  FIGS. 6   a  and  6   b , the depth d includes the offset (if any). The offset of  FIGS. 6   a  and  6   b  may be used in larger blades, for instance. 
         [0026]    The method to repair a damage blade in accordance with the present disclosure comprises identifying a damage on a leading and/or trailing edge of an airfoil of the blade. A cutback  38  is formed about the damage in the leading and/or trailing edge, the cutback shaped to comprise at least a pair of fillets r 1 , r 2  in the edge on opposite ends of the cutback, a depth d from the leading edge, and a length l in the leading or trailing edge. As the skilled reader will appreciate, a d′ is selected to be suitable for the airfoil in question. For example, on larger airfoils like turbofan fan blades, a d′=10d may be appropriate, while on smaller airfoils like high pressure compressor airfoils, it may not be appropriate as d′ would be too large. 
         [0027]    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. For example, blades in any other suitable type of engines may be repaired with the cutback  38 . 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.