Abstract:
In an engine disk and blade combination, the metallic disk has a plurality of first blade attachment slots and a plurality of second blade attachment slots circumferentially interspersed with each other. There is a circumferential array of a plurality of first blades. Each first blade has an airfoil and an attachment root. The attachment roots are respectively received in associated said first attachment slots. There is a circumferential array of second blades. Each second blade has an airfoil and an attachment root. The attachment roots are respectively received in associated said second slots. The first blades and second blades are non-metallic. The first blades are radially longer than the second blades. The first slots are radially deeper than the second slots.

Description:
BACKGROUND 
       [0001]    The disclosure relates to turbine blades. More particularly, the disclosure relates to attachment of non-metallic blades to turbine disks in gas turbine engines. 
         [0002]    Gas turbine engines contain rotating blade stages in fan, compressor, and/or turbine sections of the engine. 
         [0003]    In the turbine sections, high temperatures have imposed substantial constraints on materials. An exemplary turbine section blade is formed of a cast nickel-based superalloy having an internal air cooling passageway system and a thermal barrier coating (TBC). The exemplary blade has an airfoil extending radially outward from a platform. A so-called fir tree/dovetail attachment root depends from the platform and is accommodated in a complementary slot in a disk. The exemplary disk materials are powder metallurgical (PM) nickel-based superalloys. 
         [0004]    The weight of nickel-based superalloys and the dilution associated with cooling air are both regarded as detrimental in turbine engine design. 
       SUMMARY 
       [0005]    One aspect of the disclosure involves an engine disk and blade combination. A metallic disk has a plurality of first blade attachment slots and a plurality of second blade attachment slots circumferentially interspersed with each other. There is a circumferential array of a plurality of first blades. Each first blade has an airfoil and an attachment root. The attachment roots are respectively received in associated said first attachment slots. There is a circumferential array of second blades. Each second blade has an airfoil and an attachment root. The attachment roots are respectively received in associated said second slots. The first blades and second blades are non-metallic. The first blades are radially longer than the second blades. The first slots are radially deeper than the second slots. 
         [0006]    In various implementations, the combination may be a turbine stage. The disk may comprise a nickel-based superalloy. The first blades and second blades may comprise a structural ceramic or ceramic matrix composite (CMC). The second blades may have a characteristic chord, less than a characteristic chord of the first blades. The second blades may have a characteristic leading edge axial position axially recessed relative to a characteristic leading edge axial position of the first blades. 
         [0007]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a partially schematic axial/radial sectional view of a gas turbine engine. 
           [0009]      FIG. 2  is a partial axial schematic view of turbine disk and associated blade stage. 
           [0010]      FIG. 3  is a partial radially inward view of blades of the stage of  FIG. 2 . 
           [0011]      FIG. 4  is a circumferential projection of first and second blades of the stage of  FIG. 2 . 
       
    
    
       [0012]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0013]      FIG. 1  schematically illustrates an exemplary gas turbine engine  10  including (in serial flow communication from upstream to downstream and fore to aft) a fan section  14 , a low-pressure compressor (LPC) section  18 , a high-pressure compressor (HPC) section  22 , a combustor  26 , a high-pressure turbine (HPT) section  30 , and a low-pressure turbine (LPT) section  34 . The gas turbine engine  10  is circumferentially disposed about an engine central longitudinal axis or centerline  500 . During operation, air is: drawn into the gas turbine engine  10  by the fan section  14 ; pressurized by the compressors  18  and  22 ; and mixed with fuel and burned in the combustor  26 . The turbines  30  and  34  then extract energy from the hot combustion gases flowing from the combustor  26 . 
         [0014]    In a two-spool (two-rotor) design, the blades of the HPC and HPT and their associated disks, shaft, and the like form at least part of the high speed spool/rotor and those of the LPC and LPT form at least part of the low speed spool/rotor. The fan blades may be formed on the low speed spool/rotor or may be connected thereto via a transmission. The high-pressure turbine  30  utilizes the extracted energy from the hot combustion gases to power the high-pressure compressor  22  through a high speed shaft  38 . The low-pressure turbine  34  utilizes the extracted energy from the hot combustion gases to power the low-pressure compressor  18  and the fan section  14  through a low speed shaft  42 . The teachings of this disclosure are not limited to the two-spool architecture. Each of the LPC, HPC, HPT, and HPC comprises interspersed stages of blades and vanes. The blades rotate about the centerline with the associated shaft while the vanes remain stationary about the centerline. 
         [0015]      FIG. 2  shows one of the stages  50  of blades. As is discussed further below, the stage comprises alternatingly interspersed pluralities of first blades  52 A and second blades  52 B. Each blade comprises an attachment root  54 A,  54 B and an airfoil  56 A,  56 B. The roots are received in respective slots  58 A,  58 B extending radially inward from the periphery  60  of a disk  62 . The exemplary disk is metallic (e.g., a nickel-based superalloy which may be of conventional disk alloy type). The exemplary blades, however, are non-metallic. The exemplary non-metallic blades are ceramic based (e.g., wherein at least 50% of a strength of the blade is a ceramic material). Exemplary non-metallic materials are monolithic ceramics, ceramic matrix composites (CMCs) and combinations thereof. 
         [0016]    Attachment of such non-metallic blades poses problems. Relative to metallic blades, the non-metallic blades may have low modulus and low volumetric strength. Additionally, various ceramic-based materials may have particular strength deficiencies. For example, CMC materials have relatively high tensile strength yet relatively low interlaminar tensile strength. An exemplary ceramic matrix composite comprises a stack of plies extending generally radially through the root and the blade. Attachment stresses may cause interlaminar stresses to the plies within the root. Retaining the blades may require a relatively large attachment root compared with a metal blade of similar size. The increased root size may be needed to provide sufficient strength at the root and/or provide its efficiently distributed engagement of contact forces between the slot and the root. Providing such an attachment root might otherwise necessitate either too tight a root-to-root spacing (thereby weakening the disk) or too long (axially) of a root (thereby increasing stage-to-stage axial spacing and correspondingly reducing efficiency). 
         [0017]      FIG. 2  further shows each airfoil as extending from an inbourd end at a platform  78 A,  78 B to a tip  80 A,  80 B. Each airfoil has ( FIG. 3 ) a leading edge  82 A,  82 B; a trailing edge  84 A,  84 B, a pressure side  86 A,  86 B, and a suction side  88 A,  88 B. The exemplary tips  80 A and  80 B are in close facing proximity to inboard faces  90  of an array of blade outer air seal (BOAS) segments  92 . The blade platforms have respective arc widths or circumferential extents W A  and W B . Exemplary W A  is larger than W B . Exemplary W B  is 33-100% of W A , more narrowly, 50-90% or 75-85%. An inter-platform gap  94  has a circumferential extent W G  which is relatively small. Alternatively defined, W A , W B , W G  may be measured as linear lengths measured circumferentially in a platform radius R P  (e.g., measured at the outboard boundary of the platform). The exemplary first platforms occupy approximately 50-75% of the total circumference, more narrowly, 60-70%. The exemplary second platforms may represent 25-50%, more narrowly, 30-40%. An exemplary width of the gap is 0.000-0.005 inch (0.0-0.13 mm) accounting for a very small percentage of total circumference. 
         [0018]    To provide sufficient attachment strength, the exemplary slots  58 A and  58 B and their associated blade roots are radially staggered. The first slots  58 A have a characteristic radius Z A . The exemplary second slots have a characteristic radius Z B . Radius Z is defined as the radial distance from the disk center of rotation to a line connecting the mid-points of the blade to disk contact surface from the pressure side to the suction side of the attachment. This radial dimension is typically measured on a plane, normal to the axis of rotation, described by line going from the center of disk rotation through the centerline of the defined attachment configuration, and roughly half the axial distance, of the blade attachment, from the front of the blade attachment. 
         [0019]    Robust blade-to-disk attachment may be provided in one or more of several ways. First, the radial stagger alone may provide more of an interfitting of the two groups of roots. Additionally, one of the groups (e.g., the outboard shifted second group) may have smaller airfoils (weighing less and, thereby, necessitating a correspondingly smaller attachment root and slot). 
         [0020]    In a first example,  FIGS. 3 and 4  show the exemplary second blade airfoils  56 B as having a similar radial span to the first blade airfoils  56 A (i.e., so that the respective tips  80 B and  80 A are at the same radial position relative to the engine centerline  500 ). An exemplary reduced size of the second airfoils results from reduced chord length.  FIG. 3  shows the airfoils  56 B of the second blades as having a relatively greater spanwise taper than the airfoils  56 A of the first blades (so that the tip chord of the airfoils of the second blades is smaller than the tip chord of the airfoils of the first blades whereas, near the root, the chords are closer to equal).  FIG. 3  shows the forward extremes of the tips of the second airfoils recessed axially aftward by a separation S 1  relative to those of the first airfoils.  FIG. 3  further shows a forward recessing of the trailing extremes by a distance S 2 . In the exemplary embodiment, at a given axial position, the tips of the first and second blades are at like radial positions (e.g., so that they may have similar interactions with outer air seals or other adjacent structures). 
         [0021]    Exemplary Z B  is 105-125% of Z A , more narrowly, 110-115%. An exemplary mass of the second blades is 50-100% of a mass of the first blades, more narrowly, 60-95% or 75-85%. An exemplary longitudinal span S B  of the second blade airfoils is 50-100% of a longitudinal span S A  of the first blade airfoils at the tips, more narrowly, 70-95% or 85-95%.  FIG. 2  further shows exemplary blade centers of gravity C GA  and C GB . Broadly, exemplary C GB  and C GA  are radially within a few percent of each other (90-110% of each other). Although either can be radially outboard, exemplary C GB  is slightly radially outboard of C GA  (e.g., at a radius of 100-110% of C GA , more narrowly, 101-105%). Exemplary C GA  and C GB  may be at the same axial position (e.g., along the transverse centerplane of the disk for balance). Alternative implementations may axially stagger C GA  and C GB  while maintaining balance. 
         [0022]    One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when implemented in the remanufacture of the baseline engine or the reengineering of a baseline engine configuration, details of the baseline configuration may influence details of any particular implementation. Although an ABAB . . . pattern is shown, alternative patterns may have unequal numbers of the respective blades (e.g., an AABAAB . . . pattern or an ABBABB . . . pattern). Accordingly, other embodiments are within the scope of the following claims.