Abstract:
A removable fan blade of a turbofan engine includes a blade root at one end thereof. A rotor disk has a retention slot having dimensions corresponding for receiving the root of the fan blade in the slot. An elongated resilient member extends within the retention slot in a lengthwise direction between the root of the fan blade and the bottom of the retention slot and an elongated member extends lengthwise between the root of the fan blade and the elastomeric member to compress the resilient member and provide a radial preload to the fan blade.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/358,729 filed on Jan. 23, 2009, the content of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The technical field generally relates to fan blades for use in turbofan gas turbine engines. 
     BACKGROUND 
     Rotor disks used in some turbofan engines can have blades removably mounted in circumferentially-disposed blade retention slots provided at their periphery. These blades have blade roots that are somewhat loose in their corresponding blade retention slots when the engine is shut down. However, when the engine is running rotor disk rotates at high speeds and the centrifugal force pushes the blades radially outwardly for a firm connection between the blade roots and the respective blade retention slots. 
     Windmilling is the passive rotation of an engine spool at very low speeds when the gas turbine engine is not operating (i.e. on the ground) in response to environmental wind blowing on the engine. The clearance between a blade root and its blade retention slot can cause the blade root to flop around in the blade retention slot. After many hours of windmilling, the mating surfaces on the blade root and the blade retention slot will be subject to wear. This wear can have a detrimental impact on the low cycle fatigue life of the rotor disk and of the blades. 
     It is known to provide devices to bias the blades outwardly so as to reduce blade friction wear while windmilling, but known devices are often relatively complex to assemble. Room for improvements thus exists. 
     SUMMARY 
     In one aspect, there is provided a fan blade assembly for a turbofan engine, comprising a plurality of fan blades each having a root at one end thereof, the root having a lengthwise direction and a widthwise direction, a rotor disk having a plurality of retention slots each with a lengthwise and widthwise direction corresponding to and for retaining the root of the fan blade; an elongated resilient first member extending within the retention slot in the lengthwise direction between the root of the fan blade and the bottom of the retention slot, and a second member extending lengthwise between the root of the fan blade and the first member while compressing the first member to provide a radial preload to the root of the fan blade. 
     In a second aspect, there is provided a fan blade assembly of a turbofan engine, comprising a fan blade having a root having a longitudinal axis and a lateral axis, a rotor disk which has a retention slot corresponding to the root of the fan blade, an elongated resilient first member insertable in the retention slot between the root of the fan blade and the bottom of the retention slot and a second member insertable between the root and the first member in a manner that the second member compresses the first member for radially preloading the fan blade in the rotor disk. 
     In a third aspect, there is provided a method of assembling an assembly of fan blades and a rotor disk of a turbofan engine, wherein the fan blades each include a root having a longitudinal axis and a lateral axis and the rotor disk has retention slots corresponding to the root of the fan blades, the method comprising the steps of: inserting the fan blades into the slots, inserting a first elongated resilient member longitudinally into each retaining slot between the root of the fan blade and the bottom of the retention slot; and then separately inserting a second elongated member between the root and the first elongated resilient member to compress the first member in a direction of the fan blade to thereby preload the fan blade in the rotor disk in a radial outward direction. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures, in which: 
         FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine; 
         FIG. 2  is a fragmentary axial cross section showing a detail of an embodiment of the preload device; 
         FIG. 3  is a fragmentary radial cross section showing the detail of  FIG. 2 ; 
         FIG. 4  is a fragmentary perspective view of the fan blade and root showing the embodiment of  FIG. 2 ; 
         FIG. 5  is a fragmentary perspective view of the detail shown in  FIG. 4  but taken from below; 
         FIG. 6  is a fragmentary rear elevation of the detail shown in  FIG. 4 ; 
         FIG. 7   a  is a perspective view of a further detail of  FIG. 4 ; 
         FIG. 7   b  is a perspective view of a still further detail of  FIG. 4 ; and 
         FIG. 7   c  is a view similar to  FIG. 7   a  showing a set of strips of different lengths and weights that can be used to balance the fan rotor in addition of providing assistance in pre-loading the fan blades around the fan hub. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a turbofan 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. Referring concurrently to  FIGS. 3 and 4 , it can be appreciated that fan assembly  12  includes a plurality of blades  36 , each with a root  42  inserted into a corresponding slot (not indicated) on a hub  20 . A blade pre-loading apparatus  54 / 64  is also provided, as described further below. 
     Referring now to  FIGS. 2 to 7  there is shown a portion of the rotor disk or hub  20  in which retention slots  22  are disposed somewhat axially and spaced apart circumferentially.  FIGS. 2 and 3  show the retention slot  22  as having a bottom groove  24  and side walls  26  and  28 . Referring to  FIG. 2  there is an axial rim  30  that is concentric with the rotor disk  20 . The rim  30  includes a radial flange  32  and a rim extension  34 . The purpose of the rim  30  and the radial flange  32  is for anchoring weights in order to balance the rotor disk  20  with the assembled fan blades  36 . The rim extension  34  acts as a support rim for the rotor disk when it is being serviced and laid on a flat surface. 
     The fan blade  36  is shown in  FIG. 4 . The fan blade  36  includes an airfoil  38  with a leading edge  39  extending above a land or platform  40 . Below the land  40  is a root  42  adapted to be inserted in the retention slot  22  of the rotor disk  20 . The root  42  includes a groove  44  extending longitudinally thereof. The root  42  also includes side walls  46  and  48  as well as stoppers  50  and  52  at the front end. These stoppers  50  and  52  prevent the root from sliding beyond the rear end of the retention slot  22 . 
     Since the root  42  fits somewhat loosely in the retention slot  22  there is a need to preload the fan blade  36  so that it does not slop around in the rotor when the engine is stopped with the aircraft on the ground. The preloading device in one embodiment includes a resilient strip which can, for instance, be made of an elastomeric material such as rubber. The strip  54  as shown in  FIG. 7B  includes a downward hook portion  55  at the front end thereof and a groove  60  on the top surface  56 . The bottom surface  58  is at least contoured to fit in the groove  24  of the retention slot  22 . The top surface of the strip  54  includes parallel lobes  62  on either side of the groove  60 . The purpose of the hook portion  55 , at the front end of the strip  54 , is to retain the strip  54  within the groove  24  and to prevent it from sliding beyond the rear face of the rotor disk  20 . The strip  54  is prevented from moving forwardly in the groove  24  by a retaining ring (not shown) which will eventually be bolted to the front of the rotor disk  20  when all the blades  36  have been loaded on the rotor  20 . The ring will encompass the root  42  as well as the strip  54 . 
     A further separate strip  64  is provided to function with the strip  54 . The strip  64 , shown in  FIG. 7   a , is metallic and can be produced from titanium in order to minimize corrosion. The strip  64  also has a front bend in the shape of a hook  66  for the purposes of preventing the strip  64  from moving rearwardly and also to allow a tool to grab onto the strip so that it can be removed. The retaining ring, as previously discussed, will prevent the strip  64  from moving forwardly. 
     Once the fan blade  36  has been mounted on the rotor disk  20  with the root  42  inserted into the retention slot  22 , the strip  54  will be inserted in the clearance between the groove  44  shaped in the root  42  and the groove  24  formed in the bottom of the retention slot  22 . The metal strip  64  is then inserted between the rubber strip  54  and the groove  44  of the root  42 . By inserting the metal strip  64 , the rubber strip  54  is compressed thereby providing radial pressure on the strip  64  and the root  42 . This provides the necessary preloading of the fan blade  36  on the rotor. 
     The metal strip  64  (or the resilient strips  54 ) can serve the further purpose of balancing the rotor disk  20  when the fan blades  36  are mounted thereon. For instance, as shown in  FIG. 7   c , a set of different metal strips  64 ′ can be provided to enable the operator to place strips  64 ′ of different weights from one fan blade to the next and thus ensure a uniform distribution of the weight around the fan rotor. These strips  64 ′ could be of different lengths (i.e. from one blade to the next) so that the different strips have different weights and can therefore be used to balance the fan rotor assembly, thereby providing for selective balancing within the retention slot  22 . 
     It is assumed that the rear extension  34  may be all but eliminated since the rear rim  30  and the radial flange  32  are for the purposes balancing rotor disk  20  and blades  36 . All that would be retained would be a short rim extension for the purpose of laying the rotor disk on a flat surface for servicing. This would eliminate weight which compensates to a certain extent to the added weight of the strips  54  and  64 . 
     Little or no modification may need to be done to the root  42  or the retention slot  22  to implement the present approach, relative to a traditional root/slot design. The rubber strip  54  may be made of any elastomeric or other suitably resilient material, and the density, composition, shape, etc. thereof can be selected to obtain the proper preload on the fan blade  36 . The strip  64  may be made of any suitable material. The strip  64  may be flexible but its main purpose is to apply pressure on the rubber strip  54  and therefore should have enough rigidity to perform this function and allow it to be forced in after the resilient piece  54  is in place. The strip  64  could for instance be of the same material as the strip  54 , but with a metal rod down its core or simply of a greater density to make it more rigid. The rigidity of the strip  64  in the longitudinal direction is selected so that it can be forced in the slot after the resilient piece is in place. 
     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 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.