Abstract:
A method for preventing cracking of a turbine engine case includes the steps of disposing at least two rails upon an exterior surface of a turbine engine case; and securing a first rail to a first means for attaching at least one fan exit guide vane to the turbine engine case and securing a second rail to a second means for attaching the at least one fan exit guide vane to the turbine engine case.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a Continuation-In-Part of Ser. No. 12/333,613, filed Dec. 12, 2008, and entitled APPARATUS AND METHOD FOR PREVENTING CRACKING OF TURBINE ENGINE CASES, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure relates to turbine engines cases and, more particularly, relates to an apparatus and method for preventing cracking of turbine engine cases. 
     BACKGROUND OF THE DISCLOSURE 
     Stationary airfoils disposed aft of a rotor section within a gas turbine engine help direct the gas displaced by the rotor section in a direction chosen to optimize the work done by the rotor section. These airfoils, commonly referred to as “guide vanes”, are radially disposed between an inner casing and an outer casing, spaced around the circumference of the rotor section. Typically, guide vanes are fabricated from conventional aluminum as solid airfoils. The solid cross-section provides the guide vane with the stiffness required to accommodate the loading caused by the impinging gas and the ability to withstand an impact from a foreign object. 
     “Gas path loading” is a term of art used to describe the forces applied to the airfoils by the gas flow impinging on the guide vanes. The magnitudes and the frequencies of the loading forces vary depending upon the application and the thrust produced by the engine. If the frequencies of the forces coincide with one or more natural frequencies of the guide vane (i.e., a frequency of a bending mode of deformation and/or a frequency of a torsional mode of deformation), the forces could excite the guide vane into an undesirable vibratory response. The guide vanes are secured between the inner and outer cases of a turbine engine case by a series of bolts. 
     Historically, the undesirable vibratory response at times excites the guide vane so much that the guide vane pulls the bolts through the outer case and cracks the case. As a result, the aircraft must be taken out of service in order to repair and/or replace the case and other necessary components. 
     Therefore, there exists a need to secure the guide vane to the outer case in order to prevent cracking or mitigate existing cracking or cracks. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with one aspect of the present disclosure, a method for preventing cracking of a turbine engine case broadly comprises disposing at least two rails upon an exterior surface of a turbine engine case; and securing a first rail to a first means for attaching at least one fan exit guide vane to the turbine engine case and securing a second rail to a second means for attaching the at least one fan exit guide vane to the turbine engine case. 
     In accordance with another aspect of the present disclosure, a method for remanufacturing a turbine engine broadly comprises replacing at least one means for attaching at least one fan exit guide vane to a turbine engine case with at least one rail; and securing a first rail to a first means for attaching the at least one fan exit guide vane to the turbine engine case and securing a second rail to a second means for attaching the at least one fan exit guide vane to the turbine engine case. 
     In accordance with yet another aspect of the present disclosure, a turbine engine broadly comprises a fan section; a low pressure compressor; an engine case disposed about the fan section and the low pressure compressor; and, wherein the engine case comprises at least one rail disposed upon an exterior surface and in connection with a first means for attaching a fan exit guide vane to the engine case for reinforcing the engine case. 
     The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified representation of a cross-sectional view of a turbine engine; and 
         FIG. 2  is a partial representation of a fan exit guide vane and attachment of the present disclosure. 
         FIG. 3  is a simplified outer diameter (OD) view of a rail of the attachment of  FIG. 2 . 
         FIG. 4  is a simplified outer diameter (OD) view of the attachment of  FIG. 2 . 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a gas turbine engine  10  includes a fan section  12 , a low pressure compressor  14 , a high pressure compressor  16 , a combustor  18 , a low pressure turbine  20 , and a high pressure turbine  22 . The fan section  12  and the low pressure compressor  14  are directly connected to one another and are driven by the low pressure turbine  20 . In some configurations, the fan section  12  is driven separately through a gearbox at a lower speed than the low pressure turbine  20 . The high pressure compressor  16  is directly driven by the high pressure turbine  22 . Air compressed by the fan section  12  will either enter the low pressure compressor  14  as “core gas flow” or will enter a bypass passage  23  outside the engine core as “bypass air”. Bypass air exiting the fan section  12  travels toward and impinges against a plurality of fan exit guide vanes  24 , or “FEGV&#39;s”, disposed about the circumference of the engine  10 . The FEGV&#39;s  24  straighten and guide the bypass air into ducting (not shown) disposed outside the engine  10 . 
     Now referring to  FIGS. 1 and 2 , the FEGV&#39;s  24  extend between fan inner case  26  and outer case  28 . The inner case  26  is disposed radially between the low pressure compressor  14  and the FEGV&#39;s  24  and the outer case  28  is disposed radially outside of the FEGV&#39;s  24 . Each FEGV  24  includes an airfoil  30  and means for attaching the airfoil  30  between the inner and outer cases  26 ,  28 . 
     Referring specifically now to  FIG. 2 , each FEGV  24  may be attached to the outer case  26  by at least one rail, for example, a first rail  32  and a second rail  34 , disposed about an exterior surface  36  of the outer case  28 . The rail is elongated in a circumferential direction. The first rail  32  and second rail  34  may be aligned approximately parallel to one another and secured to the outer case  28  by a first means for attaching  38  and a second means for attaching  40 , respectively. Each means for attaching  38 ,  40  secure each FEGV  24  to the outer case  28  and also secure each rail  32 ,  34  to the outer case  28 . The means for attaching  38 ,  40  may include at least one of the following: bolts, rivets, screws, and the like, as known to one of ordinary skill in the art. There may be at least two circumferentially spaced apart means (e.g., nut, washer, and screw/bolt combinations) for attaching  38 , 40  for each rail  32 , 34  (e.g., a front pair and a rear pair). 
     The rails  32 ,  34  may be installed where each FEGV  24  is mounted. Each rail may be circumferentially-shaped, or at least substantially circumferentially-shaped, to complement the shape of the exterior surface of the outer case  28 . As can be seen, each rail  32 , 34  has an L-shaped cross section with a first portion  50  which extends along the case  28  (and has holes  54  for accommodating the associated means for attaching) and a second portion  52  protruding radially outward. This second portion forms a stiffening flange for the rail  32 , 34 . 
     The rails  32 ,  34  may distribute the load experienced by the FEGV during operation and help support the outer case  28 . As the FEGV vibrates, the rails  32 ,  34  may prevent the FEGV  24  from pulling the means for attaching through the outer case  28  as well as also prevent the case from cracking. A typical gas turbine engine contains approximately eighty (80) FEGV&#39;s, and thus approximately one hundred sixty (160) rails may be installed to stiffen the outer case and either mitigate existing cracking or cracks and/or prevent cracking from occurring. By stiffening the outer case, the entire turbine engine casing may be reinforced to withstand torsional modes of vibration experienced during operation of the turbine engine. 
     A pair of rails each having the following dimensions axial length L of 0.5 inches (12.7 millimeters)×radial height of 0.5 inches (12.7 millimeters)×width or length along the case circumference W of 3.0 inches (76.2 millimeters) and composed of 0.0625 inches (1.5875 millimeters) thick sheet metal (e.g., stainless steel) were bolted to a piece of an outer case and an FEGV. The structure was mounted to a hydraulic cylinder and a simulated air load was applied. One cycle constituted one stroke actuated by the hydraulic cylinder upon the structure. After subjecting the structure to ten-thousand (10,000) cycles, no crack growth was observed in the outer case and the outer case maintained an overall stiffness of between approximately eighty percent (80%) to approximately one hundred percent (100%) of the original stiffness. Exemplary ranges for axial height and length are each 10-30 mm, more narrowly 12-20 mm and for end-to-end width W 2.5-10 cm, more narrowly 6-9 cm. 
     One or more embodiments of the present disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.