Abstract:
An undercut is provided in a gas turbine engine disk to smooth out an uneven axial distribution of radial stress in the disk. The undercut is defined radially inwardly of the blade attachment slots provided at the periphery of the disk.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to gas turbine engines and, more particularly, to rotor disks of such engines.  
         [0003]     2. Background Art  
         [0004]     Fan rotors can be manufactured integrally or as an assembly of blades around a disk. In the case where the rotor is assembled, the fixation between each blade and the disk has to provide retention against extremely high radial loads. This in turn causes high radial stress in the disk retaining the blades.  
         [0005]     In the case of “swept” fans, the blades are asymmetric with respect to their redial axis. A significant portion of the weight of these blades is cantilevered over the front portion of the fixation, which causes an uneven axial distribution of the radial load on the fixation and disk. This load distribution causes high local radial stress in the front of the disk and high contact forces between the blade and the front of the disk.  
         [0006]     Although a number of solutions have been provided to even axial distribution of stress in blades, such as grooves in blade platforms to alleviate thermal and/or mechanical stresses, these solutions do not address the problem of high local radial stress in the disk supporting the blades.  
         [0007]     Accordingly, there is a need for a disk for a gas turbine engine fan having a smoother axial distribution of radial stress.  
       SUMMARY OF INVENTION  
       [0008]     It is therefore an aim of the present invention to provide an improved rotor disk for a gas turbine engine.  
         [0009]     It is also an aim of the present invention to provide a method for smoothing an axial distribution of radial stress in a rotor disk.  
         [0010]     Therefore, in accordance with a general aspect of the present invention, there is provided a gas turbine engine rotor disk comprising a disk body having a plurality of blade attachment slots circumferentially distributed about a periphery thereof, and wherein an undercut is provided radially inwardly of said blade attachment slots.  
         [0011]     In accordance with a further general aspect of the present invention, there is provided a gas turbine engine rotor comprising a plurality of blades, each of said blades having a root received in a corresponding blade attachment slot defined in a disk adapted to be mounted for rotation about an axis, and wherein an axial distribution of radial stress in the disk is smoothed by providing an undercut in the disk radially inwardly of the blade attachment slots.  
         [0012]     In accordance with a still further general aspect of the present invention, there is provided a method to smooth out an uneven axial distribution of radial stress in a gas turbine engine rotor disk having a plurality of blade attachment slots in which are retained a corresponding number of blades, the method comprising the step of: providing an undercut radially inwardly of said plurality of blade attachment slots. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment of the present invention and in which:  
         [0014]      FIG. 1  is a side view of a gas turbine engine, in partial cross-section; and  
         [0015]      FIG. 2  is a partial side view of a fan, in cross-section, showing a disk according to a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]      FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases.  
         [0017]     Referring to  FIG. 2 , part of the fan  12 , which is a “swept” fan, is illustrated. Although the present invention applies advantageously to such fans, it is to be understood is can also be used with other types of radial fans, as well as other types of rotating equipment having a disk requiring a smoother axial distribution of radial stress including, but not limited to, compressor and turbine rotors.  
         [0018]     The fan  12  includes a disk  30  mounted on a rotating shaft  31  and supporting a plurality of blades  32  which are asymmetric with respect to their radial axis. Each blade  32  comprises an airfoil portion  34  including a leading edge  36  in the front and a trailing edge  38  in the back. The airfoil portion  34  extends radially outwardly from a platform  40 . A blade root  42  extends from the platform  40 , opposite the airfoil portion  34 , such as to connect the blade  32  to the disk  10 . The blade root  42  includes an axially extending dovetail  44 , which is designed to engage a corresponding dovetail groove  46  in the disk  30 . Other types of attachments can replace the dovetail  44  and dovetail groove  46 , such as a bottom root profile commonly known as “fir tree” engaging a similarly shaped blade attachment slot in the disk  10 . The airfoil section  34 , platform  40  and root  42  are preferably integral with one another.  
         [0019]     As stated above, the asymmetry of the blade  32  cause a significant portion of the blade weight to be cantilevered over the front portion of the dovetail  44 . This creates an uneven axial distribution of the radial load on the dovetail  44  and disk  30 . Such a load distribution produces unacceptably high local radial stress in the front of the disk  30  and contact forces between the dovetail  44  and the front of the dovetail groove  46 .  
         [0020]     According to an embodiment of the present invention, the axial distribution of the radial stresses in the disk  30  is smoothed by way of a continuous annular undercut  50  provided in the front of the disk  30 , radially inwardly of the dovetail groove  46 . The undercut  50  is preferably rounded and generally slightly curved toward the rotating shaft  31 .  
         [0021]     Although a number of different geometries are possible for the undercut  50 , the geometry must be carefully selected in order to produce a favorable change in the load path of the disk  30 . For example, in the case of a “swept” fan, a simple straight undercut will lower the stress at the leading edge of the disk but cause a sharp peak further back, which is undesirable. By contrast, the undercut  50  having the geometry shown in  FIG. 2  will produce a radial stress having a maximum generally constant value along a significant middle portion of the disk  30 , with a generally progressively lower value toward both the leading and trailing edge of the disk. A preferred way of determining the appropriate undercut geometry is through 3D finite element analysis according to methods well known in the art.  
         [0022]     The undercut  50  thus eliminates the unacceptably high local radial stress in the front of the disk  30  and contact forces between the dovetail  44  and the front of the dovetail groove  46  by evening the axial distribution of the radial stresses in the disk  30 .  
         [0023]     The undercut  50 , among other things, allows for a simple way to balance the axial distribution of radial stress in a disk of a “swept” fan, as well as in other types of disks requiring similar balancing of the axial distribution of radial stress.  
         [0024]     The embodiments of the invention described above are intended to be exemplary. Those skilled in the art will therefore appreciate that the foregoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. Accordingly, the present is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.