Abstract:
A fan blade has a main body extending between a leading edge and a trailing edge. Channels are formed into the main body from an open side extending toward an opposed closed side. A plurality of ribs extending across the main body intermediate the channels, the fan blade has a dovetail, and an airfoil extending radially outwardly from said dovetail. A bottom surface of the channels is defined at the closed side of the channels. Sides of the channel merge into sides of the ribs, with a compound fillet at the bottom surface. A first radius of curvature is used along the bottom, and merging into at least a second radius of curvature at the sides. The first radius of curvature is larger than the second radius of curvature.

Description:
BACKGROUND 
     This application relates to a hollow fan blade for a gas turbine engine, wherein a unique rib geometry is utilized. 
     Gas turbine engines may be provided with a fan for delivering air to a compressor section. From the compressor section, the air is compressed and delivered into a combustion section. The combustion section mixes fuel with the air and combusts the combination. Products of the combustion pass downstream over turbine rotors, which in turn are driven to rotate and rotate the compressor and fan. 
     The fan may include a rotor having a plurality of blades. 
     One type of fan blade is a hollow fan blade having a plurality of channels defined by intermediate ribs in a main fan blade body. An outer skin is attached over the main fan blade body to close off the cavities. The blades are subject to a number of challenges, including internal stresses that vary along a length of the fan blade. 
     SUMMARY 
     A fan blade has a main body extending between a leading edge and a trailing edge. Channels are formed into the main body from an open side extending toward an opposed closed side. A plurality of ribs extending across the main body intermediate the channels, the fan blade has a dovetail, and an airfoil extending radially outwardly from said dovetail. A bottom surface of the channels is defined at the closed side of the channels. Sides of the channel merge into sides of the ribs, with a compound fillet at the bottom surface. A first radius of curvature is used along the bottom, and merging into at least a second radius of curvature at the sides. The first radius of curvature is larger than the second radius of curvature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with regard to the specific and drawings, the following of which is a brief description. 
         FIG. 1A  shows a fan blade. 
         FIG. 1B  shows another feature of the  FIG. 1A  fan blade. 
         FIG. 2  is a cross-sectional view along line  2 - 2  as shown in  FIG. 1A . 
         FIG. 3  shows a main body of the  FIG. 1A  fan blade. 
         FIG. 4  is a simplified view of one rib. 
         FIG. 5A  is a first embodiment taken along line  5 - 5  of  FIG. 4 . 
         FIG. 5B  is a second embodiment taken along line  5 - 5  of  FIG. 4 . 
         FIG. 5C  is a third embodiment taken along line  5 - 5  of  FIG. 4 . 
         FIG. 6A  is a first embodiment rib break-edge. 
         FIG. 6B  is another embodiment rib break-edge. 
         FIG. 7  shows another area within the fan blade. 
         FIG. 8  shows a radially inner end of the channels. 
     
    
    
     DETAILED DESCRIPTION 
     A fan blade  20  is illustrated in  FIG. 1A  having an airfoil  18  extending radially outwardly from a dovetail  24 . A leading edge  21  and a trailing edge  22  define the forward and rear limits of the airfoil  18 . 
     As shown in  FIG. 1B , a fan rotor  16  receives the dovetail  24  to mount the fan blade  20  with the airfoil  18  extending radially outwardly. As the rotor  16  is driven to rotate it carries the fan blades  20  with it. There are higher stresses adjacent to the rotor  16 , than occur radially outwardly of the rotor. 
       FIG. 2  shows a cross-section of the fan blade  20 , at the airfoil  18 . As shown, the leading edge  21  carries a cap  37  secured to a main body  28 . A cover skin  32  closes off cavities or channels  30  in the main body  28 . The main body  28 , the cap  37  and the skin  32  may all be formed of various aluminum alloys. While aluminum alloys or aluminum may be utilized, other materials, such as titanium, titanium alloys, or other appropriate metals may be utilized. 
     As shown, a plurality of ribs  26  separate channels  30  in the cross-section illustrated in  FIG. 2 . These channels  30  are closed off by the skin  32 . As shown, the channels  30  extend from an open end inwardly to a closed side. The open end is closed off by skin  32 . It is within the scope of this invention, however, that the channel extends across the width of the main body  28 , and there are two skins on opposed sides of the main body  28 . 
     In addition, the channels may be filled with lighter weight filler material to provide stiffness, as known. 
     A contact area  132  at the forward face of the ribs  26  serves as a mount point for the skin  32 , and receives an adhesive. Chamfers  38  are formed at the break-edges, or the edges of the ribs  26 , and will be described in more detail below. As shown, the channels  30  have a side extent formed by a compound radius  34  and  36 , again to be described in greater detail below. 
       FIG. 3  shows the main body  28 . There are a plurality of channels  30  from the front or leading edge  21 , to the back or trailing edge  22 , and varying from the radially inner end toward the radially outer tip. As shown, some of the channels  30  extend generally radially upwardly. Other channels, such as channel  40 , bend toward the leading edge  21 . Other channels  41  simply extend generally from the middle of the main body  28  toward the leading edge  21 . 
     To reduce the weight, it is desirable to maximize the amount of channels and minimize the amount of rib. However, there is also a need for additional stiffness adjacent the radially inner edge  42 , to provide greater durability, and minimize blade pull. Thus, the ribs  26  may be formed such that they tend to be thicker adjacent a radially inner edge  42 , and become thinner when moving toward the radially outer portions  44 . 
     It is also desirable to form a blade which avoids certain operational modes across the engine operational range. Additional mass toward the tip or outer end of the blade raises challenges against tuning away from fundamental modes. 
     As shown schematically in  FIG. 4 , ribs  26  are thinner at radially outer end  44  than at the inner end  42 . A thickness t 1  at the radially inner end  42  is greater than then the thickness t 2  at the tip or radially outer end  44 . In embodiments, a ratio of t 1  to t 2  may be between 1.1 and 8. As can be appreciated from  FIG. 3 , the variation need not be linear as shown in  FIG. 4 , and may be different across the several ribs. 
     As shown in  FIG. 5A , a cross-section through the rib could be a trapezoid as shown in  FIG. 5A , wherein the bottom  50 , which extends into the main body  28 , is larger than the outer end  48  which attaches to the skin  32 . Sides  46  are angled between the two ends  48  and  50 . 
       FIG. 5B  shows a rectangular cross-section for the rib  26  wherein the ends  52  and  54  are generally of the same thickness, and the sides  56  are generally perpendicular to those ends. 
       FIG. 5C  shows yet another embodiment, wherein the ends  58  and  60  are of different thicknesses, and the sides  62  curve relative to each other along a particular radius. 
     By modifying these several variables, a designer is able to tune or optimize the operation of the fan blade for its use in a gas turbine engine. 
     The features of the thinner ribs are disclosed in co-pending U.S. patent application Ser. No. 13/241,756, filed on even date herewith, and entitled “HOLLOW FAN BLADE RIB GEOMETRY.” 
     Notably, as will be explained below, it is desirable that the upper end  48 / 52 / 58  actually has a more complex surface at its break-edges. 
       FIG. 6A  shows the actual break-edge  38  on a rib  26 . The contact area  132  which will actually contact the skin, and provide a surface for receiving adhesive and securing the skin should be maximized On the other hand, there are stresses which are induced at the break-edges, and thus a chamfer  38  is formed in this embodiment. 
     As shown in  FIG. 6A , the rib  26  has a nominal thickness t 3  at the upper end, if not for the chamfers  38 . Stated another way, t 3  is the distance between sides  200  at the end of chamfers  38 . The chamfers  38  extend for a thickness c measured in a plane perpendicular to the top edge  132 . 
     A ratio of c to t 3  may be between 0.02-0.15. The use of the chamfer at the break-edge location reduces the stress. There would otherwise be stress concentrations at that area. On the other hand, by utilizing a chamfer within the disclosed range, the amount of surface area available to provide a good adhesion to the cover is still adequate. 
       FIG. 6B  shows an embodiment of a rib  64 , wherein the break-edges are provided along a radius r 1 . In embodiments, the ratio of r 1  to t 3  is between 0.02-0.15. 
     The features of the break-edges are disclosed in co-pending U.S. patent application Ser. No. 13/241,868, filed on even date herewith and entitled “FAN BLADE HAVING INTERNAL RIB BREAK-EDGE.” 
       FIG. 7  shows the surfaces  34  and  36  as illustrated in  FIG. 2 . The areas at that side of the channels  30  are prone to stress concentrations. A typical fillet, or single curve, may be considered for formation at that area to reduce stress. However, in the disclosed embodiment, a compound fillet having two curves  34  and  36  is utilized. Curve  34  is formed along a radius r 2  while curve  36  is formed along a radius r 3 . A ratio of r 3  to r 2  is between 0.03 and 0.25. As is clear, r 2  is greater than r 3 . More narrowly, it may be between 0.06 and 0.13. The use of the compound fillet provides a great reduction in stress concentration, which would otherwise be maximized at the general location of the curve  36 . 
     Finally  FIG. 8  shows a radially inner end, bottom or termination  100  of a channel  30 . As shown, there is a compound curve or fillet including a bottom portion  104  formed at a radius r 4  and a side portion  102  formed at a radius r 5 , which merges into the side of the ribs. As is clear, r 5  is greater than r 4 . Again, this arrangement reduces a stress concentration at the corners which would otherwise be induced into the cavity terminations. In embodiments, a ratio of r 4  to r 5  is between 0.03 and 0.25. 
     An Application directed to the features of  FIG. 8  has been filed as U.S. patent application Ser. No. 13/241,821, filed on even date herewith and entitled “FAN BLADE CHANNEL TERMINATION.” 
     The compound fillets as disclosed in  FIGS. 7 and 8  reduce stress concentrations with minimum weight increase. Further, the compound fillets may be provided with minimal additional cost, because multi-pass machining is not required. Instead, a cutter with a compound radius shape may be utilized. 
     The fan blade as described above reduces stresses that are raised during operations when mounted in a gas turbine engine. 
     Although embodiments have been disclosed, a worker of ordinary skill in the art would recognize the modifications which come within the scope of this Application. Thus, the following claims should be studied to determine the true scope and content.