Abstract:
A gas turbine engine and a method of tuning a rotor in the gas turbine engine wherein the rotor includes an array of blades extending from a rotor hub each having an airfoil mounted to a blade platform. The method includes adding or removing material from bladed rotor projections to alter the mass of the rotor and change the frequency of the respective airfoil.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority on U.S. Provisional Application No. 61/420,927 filed on Dec. 8, 2010, the content of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application relates to gas turbine engines and more particularly to improvements in a method and an arrangement for tuning/detuning a rotor blade array. 
       BACKGROUND ART 
       [0003]    Gas turbine rotor assemblies rotate at extreme speeds. Inadvertent excitation of resonant frequencies by the spinning rotor can cause an unwanted dynamic response in the engine, and hence it is desirable to be able to tune, or mistune, the rotor in order to avoid specific frequencies or to lessen their effect. 
       SUMMARY 
       [0004]    In accordance with an general aspect, there is provided a method of tuning a bladed rotor in a gas turbine engine, wherein the bladed rotor includes a circumferential array of blades extending from a rotor hub, each blade having an airfoil extending from a blade platform; the method comprising: providing a platform projection depending from every second blade, the platform projections together forming a circumferentially interrupted rib on the hub, and tuning the bladed rotor by adding or removing mass from at least one platform projection to alter the natural frequency of the rotor. 
         [0005]    In accordance with another aspect, there is provided a bladed rotor for a gas turbine engine, the bladed rotor comprising a hub and a circumferential array of blades extending from the hub; each blade having an airfoil extending from a gaspath side of a platform provided at a periphery of the hub; and an annular array of projections depending from an interior side of the blade platform at circumferential locations generally corresponding to every second blade, the projections cooperating to form a circumferentially interrupted rib, the interrupted rib configured to provide a desired frequency response to the bladed rotor. 
         [0006]    In accordance with a further general aspect, there is provided a method of tuning a bladed rotor for a gas turbine engine, the bladed rotor including a rotor hub having a circumferential array of airfoil blades extending therefrom, the hub having a gas path side defining a portion of the gas path in which the bladed assembly is to be mounted and an interior side opposite the gas path side; the method comprising: providing at least one projection extending from the rotor hub interior side, determining a frequency response of the bladed assembly in an as-manufactured condition, determining a desired frequency response, and then modifying the at least one projection to provide the bladed assembly with the desired frequency response. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Reference is now made to the accompanying figures in which: 
           [0008]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine illustrating a turbofan configuration; 
           [0009]      FIG. 2  is an isometric view partly fragmented showing a rib feature of a rotor blade that may be used for blade tuning; and 
           [0010]      FIG. 3  is an isometric view of a portion of a bladed rotor illustrating an alternate rib-no- rib configuration for mistuning blade frequencies. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]      FIG. 1  schematically depicts a turbofan engine A which, as an example, illustrates the application of the described subject matter. The turbofan engine A includes a nacelle  10 , a low pressure spool assembly which includes at least a fan  12  and a low pressure turbine  14  connected by a low pressure shaft  16 , and a high pressure spool which includes a high pressure compressor  18  and a high pressure turbine  20  connected by a high pressure shaft  22 . The engine A further comprises a combustor  26 . 
         [0012]    The fan  12 , the high pressure compressor  18 , the high pressure turbine  20  and the low pressure turbine  14 , for the purposes of the present description include rotors represented by the blades  30  in  FIG. 1 . 
         [0013]    The rotors, especially the fan  12 , may be provided in the form of blisks, that is, in the form of integrally bladed disks (IBR). As shown in  FIG. 2 , the blades  30  are integrally formed with a rotor hub  34  in a unitary construction. Each blade  30  comprises an airfoil  32  extending from a gas path side of an annular platform  34   a  formed at the periphery of the rotor hub  34 . In use, the airfoils  32  may vibrate at different frequencies and in order to tune the rotor, the individual airfoils  32  must be tuned or mistuned. For instance, where adjacent airfoils have the same natural frequencies, the airfoils can excite each other. Thus, the airfoils may be mistuned to avoid the excitation. 
         [0014]    As shown in  FIGS. 2 and 3 , a series of projections  36  may be provided below the platform  34   a  or on the interior side of the platform  34   a  opposite to the gas path side thereof. The projections  36  may be integrally formed with the platform  34   a . The projections  36   a  may be provided in the form of rib features depending radially inwardly from the platform  34   a . The projections  36  may be identical in term of shapes and sizes. The projections  36  may also be circumferentially spaced-apart in annular alignment forming a regular rib but which is interrupted by voids or spaces  38 . In the embodiment shown in  FIG. 3 , a projection  36  is provided at alternate or on every second blade  30  and, therefore, at every second airfoil for the purpose of tuning or mistuning the airfoil. However, it is understood that various number of projections may be provided. As shown in  FIGS. 2 and 3 , the projections  36  may be provided at the leading edge of the platform  34   a  forwardly of the center of gravity of the blades  30 , but other suitable locations for the projection may be used (e.g. platform trailing edge). 
         [0015]    If the airfoils  32  of two adjacent blades  30  have the same natural frequency, one may mistune the blade  30  to which a projection  36  is dependent so that the frequency of the respective airfoil  32  will be mismatched to the frequency of the airfoil  32  on the adjacent blade  30 . 
         [0016]    The projections  36  may be tuned or mistuned by removing material therefrom thereby altering the mass thereof, causing the respective airfoil  32  to be modified in terms of its frequency. Alternately, material can be added to the projection  36  by a bonding process like welding. A projection  36  or similar rib features depending from the blade platform may be in this manner used to control blade frequencies. 
         [0017]    The array of projections  36  are shown as being located at the leading edge of the platform  34   a  but it is understood that the array of projections  36  may be located at the trailing edge or other suitable location on the platform  34   a . The shape of the projections  36  making up the array may be identical forming a regular shaped rib albeit interrupted. 
         [0018]    It can be appreciated that a gas turbine engine rotor may be tuned by providing at least one projection extending from a platform interior side, determining a frequency response of the bladed rotor in an as-manufactured condition, determining a desired frequency response, and then modifying the at least one projection to provide the bladed rotor with the desired frequency response. Modifying the at least one projection may be done by removing material from the projection or by adding material thereto. 
         [0019]    The material addition (i.e. the projections  36 ) on the disk provides a convenient way of changing the blade frequencies. The projections  36  may be used to tune or mistune the blades (where frequencies of adjacent blades are different) to provide the bladed rotor with the desired frequency response. 
         [0020]    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 instance, it will be understood that he present teaching may be applied to any bladed rotor assembly, including but not limited to fan and compressor rotors, and may likewise be applied to any suitable rotor configuration, such as integrally bladed rotors, conventional bladed rotors etc. Any 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 scope of the appended claims.