Abstract:
A tiltrotor rotor hub comprises first and second configurations of yokes arranged in two parallel and axially offset planes, each having an equal number of two or more yokes substantially equally spaced about a mast, wherein a portion of each yoke overlaps with a portion of each azimuthally adjacent yoke. Another tiltrotor rotor hub is selectively positionable for operation in helicopter/airplane/transition modes and comprises substantially parallel and axially offset first and second planes, each containing a plurality of blade yokes arranged about a central axis and a portion of each yoke overlaps with a portion of each azimuthally adjacent yoke. Another tiltrotor rotor hub is coupled to a tiltrotor mast and comprises a stacked arrangement of blade yokes wherein a portion of each yoke overlaps with a portion of each azimuthally adjacent yoke, and a plurality of mounting components coupling each yoke to the overlapping azimuthally adjacent yoke.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure generally relates to a rotor hub, and more particularly, to a tiltrotor aircraft rotor hub having yokes in two or more axially offset planes. 
       BACKGROUND 
       [0002]    Rotor hubs are used to mount the rotor blades of tiltrotor aircraft in predetermined geometric configurations, and also serve to oppose centrifugal forces acting to pull the spinning blades away from a centerline of rotation. Generally, hubs having a radially compact arrangement about the centerline of rotation experience lower relative loads, as the moment arm between system forces and the center of rotation is reduced. Lower loads allow components and other structure to be leaner, resulting in reduced system weight. Additionally, compact hub designs may have lower drag coefficients due to having smaller profiles. However, the extent of compact radial packaging may be limited by physical interferences between components, especially in systems having multiple spinning elements. In such systems, hub components may be arranged at a radius from the centerline of rotation sufficient to provide circumferential space to physically accommodate each spinning element and its given range of motion in a given plane, thereby limiting the compactness of the design. 
         [0003]    In addition, rotor blade diameter on tiltrotor aircraft often results from a design compromise between desired vehicle performance in “helicopter mode” (primarily vertical takeoff/landing, hover, and low speed flight) and “airplane mode” (primarily high speed forward flight). Generally speaking, larger diameter rotors offer favorable performance in helicopter mode, but may degrade performance in airplane mode, and vice versa. Improvements in rotor efficiency in helicopter mode may provide for a smaller diameter rotor to be used, thereby potentially improving performance in airplane mode, resulting in overall improved vehicle performance. 
       SUMMARY 
       [0004]    Embodiments of the present disclosure generally provide rotor hubs for tiltrotor aircraft. 
         [0005]    The present disclosure is directed to a rotor hub of a tiltrotor aircraft comprising a first configuration of two or more yokes arranged in a first plane about a mast of the tiltrotor aircraft and having substantially equal angular spacing therebetween, and a second configuration of an equal number of yokes as the first configuration, the equal number of yokes being arranged in a second plane about the mast of the tiltrotor aircraft and having substantially equal angular spacing therebetween, wherein the second plane is substantially parallel to and axially offset from the first plane, and wherein a portion of each yoke in the first configuration overlaps with a portion of each azimuthally adjacent yoke in the second configuration. 
         [0006]    In various embodiments, the second configuration yokes are angularly offset from the first configuration yokes. In an embodiment, the second configuration yokes substantially bisect the angular spaces separating the first configuration yokes. 
         [0007]    In an embodiment, at least some of the yokes of the first configuration and second configuration may undergo flapping motion. In another embodiment, the second plane is axially offset from the first plane by a predetermined distance sufficient to accommodate flapping without interference. In yet another embodiment, the second plane is axially offset from the first plane by a predetermined distance substantially equal to about one or two rotor chord lengths. 
         [0008]    In an embodiment, the hub further comprises a common mounting component coupling each first configuration yoke to at least one azimuthally adjacent second configuration yoke. In an embodiment, at least some of the yokes of the first configuration and the second configuration are substantially stiff in-plane. In yet another embodiment, the first configuration yokes and the second configuration yokes are arranged at a common radius from the mast. 
         [0009]    In an embodiment, the first configuration has two yokes and the second configuration has two yokes. In another embodiment, the first configuration has three yokes and the second configuration has three yokes. In yet another embodiment, the first configuration has four yokes and the second configuration has four yokes. 
         [0010]    In another aspect, the present disclosure is directed to a rotor hub of a tiltrotor aircraft comprising a plurality of blade yokes arranged in a first plane about a central axis, and a plurality of blade yokes arranged in a second plane about the central axis, wherein the second plane is substantially parallel to and axially offset from the first plane, a portion of each yoke in the first plane overlaps with a portion of each azimuthally adjacent yoke in the second plane, and the rotor hub is selectively positionable for operation of the tiltrotor aircraft in helicopter mode, airplane mode, and transition modes there between. In various embodiments, the plurality of blade yokes in each of the first plane and second plane are positioned with substantially equal angular spacing therebetween. In various embodiments, the plurality of blade yokes in the first plane are angularly offset from the plurality of blade yokes in the second plane. In an embodiment, the plurality of blade yokes in the second plane substantially bisect the angular spaces separating the plurality of blade yokes in the first plane. 
         [0011]    In an embodiment, the hub further comprises a common mounting component coupling each yoke in the first plane to the overlapping azimuthally adjacent yoke in the second plane. 
         [0012]    In another aspect, the present disclosure is direct to a rotor hub of a tiltrotor aircraft comprising a plurality of blade yokes configured in a stacked arrangement such that a portion of each yoke in a first axial plane overlaps with a portion of each azimuthally adjacent yoke in a second axial plane, and a plurality of mounting components, each mounting component coupling each yoke in the first axial plane to the overlapping azimuthally adjacent yoke in the second axial plane, wherein the rotor hub is coupled to a mast of the tiltrotor aircraft. In various embodiments, the plurality of blade yokes in each of the first plane and the second plane are positioned with substantially equal angular spacing therebetween. In an embodiment, the plurality of blade yokes in the second plane substantially bisect the angular spaces separating the plurality of blade yokes in the first plane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a more complete understanding of this disclosure, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
           [0014]      FIG. 1  depicts a perspective view of an offset stacked yoke hub coupled to a mast according to an embodiment of the present disclosure; 
           [0015]      FIG. 2  depicts a perspective view of the offset stacked yoke hub of  FIG. 1 , schematically representing four yokes undergoing flapping motion according to an embodiment of the present disclosure; 
           [0016]      FIG. 3A  depicts a top view of a yoke of an offset stacked yoke hub according to an embodiment of the present disclosure; 
           [0017]      FIG. 3B  depicts a side cutaway view of a yoke according to an embodiment of the present disclosure; 
           [0018]      FIG. 3C  depicts another side cutaway view of the yoke of  FIG. 3B  with a blade cuff coupled thereto according to the embodiment of the present disclosure; 
           [0019]      FIG. 4A  depicts a top view of an arrangement of yokes in an offset stacked yoke hub according to an embodiment of the present disclosure; 
           [0020]      FIG. 4B  depicts a side cutaway view of the arrangement of yokes in an offset stacked yoke hub of  FIG. 4A  according to an embodiment of the present disclosure; 
           [0021]      FIG. 4C  depicts a side cutaway view of the arrangement of yokes in an offset stacked yoke hub of  FIG. 4A , shifted 90° about a vertical axis with respect to the side cutaway view of  FIG. 4B , according to an embodiment of the present disclosure; 
           [0022]      FIG. 5A  depicts a top view of mounting hardware used to secure yokes to a mast according to an embodiment of the present disclosure; 
           [0023]      FIG. 5B  depicts a side cutaway view of the mounting hardware used to secure yokes to a mast of  FIG. 5A  according to an embodiment of the present disclosure; 
           [0024]      FIG. 6A  depicts an assembled perspective view of an offset stacked yoke hub having four yokes according to an embodiment of the present disclosure; 
           [0025]      FIG. 6B  depicts an exploded perspective view of the offset stacked yoke hub having four yokes of  FIG. 6A  according to an embodiment of the present disclosure; 
           [0026]      FIG. 7A  depicts an assembled perspective view of an offset stacked yoke hub having six yokes according to an embodiment of the present disclosure; 
           [0027]      FIG. 7B  depicts an exploded perspective view of the offset stacked yoke hub having six yokes of  FIG. 7A  according to an embodiment of the present disclosure; 
           [0028]      FIG. 8A  depicts an assembled perspective view of an offset stacked yoke hub having eight yokes according to an embodiment of the present disclosure; and 
           [0029]      FIG. 8B  depicts an exploded perspective view of the offset stacked yoke hub having eight yokes of  FIG. 8A  according to an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Embodiments of the present disclosure generally provide an offset stacked yoke rotor hub for use on a tiltrotor aircraft. As described herein, the rotor hub may package tightly to a rotorcraft mast, thereby reducing weight, drag profile, and loads on the hub and accompanying structure. The rotor hub may also provide for improved aerodynamic rotor performance in axial flow compared to single-plane rotor hubs, resulting in possible production of a comparable thrust/power ratio using a smaller diameter rotor disk. In various embodiments, blade yokes may be arranged in two or more axially offset planes, may partially overlap, and may share common axial retainer bolts. 
         [0031]      FIGS. 1-8B  illustrate representative embodiments of offset stacked yoke hubs  100 ,  500 ,  600  and parts thereof. It should be understood that the components of offset stacked yoke hubs  100 ,  500 ,  600  and parts thereof shown in  FIGS. 1-8B  are for illustrative purposes only, and that any other suitable components or subcomponents may be used in conjunction with or in lieu of the components comprising offset stacked yoke hubs  100 ,  500 ,  600  and the parts of offset stacked yoke hubs  100 ,  500 ,  600  described herein. 
         [0032]    Offset stacked yoke hubs  100 ,  500 ,  600  according to the present disclosure may be used to secure rotor blades on tiltrotor aircraft. Hubs  100 ,  500 ,  600  may support blades or other items attached thereto in a predetermined geometric arrangement and may oppose centrifugal forces acting on those items during rotation of the system. Hubs  100 ,  500 ,  600  may be configured to provide for yoke flapping and pitching motion. The present disclosure is directed to various embodiments of offset stacked yoke hubs  100 ,  500 ,  600  that closely package multiple yokes  200  about a central axis  130 , and may thereby reduce weight, drag profile, and loads on the hubs  100 ,  500 ,  600  and accompanying structure. 
         [0033]    Referring now to  FIG. 1 , an offset stacked yoke hub  100  is depicted coupled to a tiltrotor mast  132 . The offset stacked yoke hub  100  may generally comprise a plurality of blade yokes  200 , such as an even number of blade yokes  200 , arranged about a central axis  130  in a predetermined geometric configuration. As described in more detail herein, yokes  200  may be axially offset from one another along the central axis  130 , and yokes  200  may substantially overlap one another (thereby being “stacked”) so as to package tightly with respect to central axis  130 . Offset stacked yoke hub  100  may be coupled to a tiltrotor mast  132  as further described herein and may rotate about central axis  130 . Rotation of hub  100  may be driven by the tiltrotor mast  132  or by other external forces acting on hub  100  or the blades attached thereto. 
         [0034]    In various embodiments, construction of hub  100  may provide for motion of blade yokes  200 , as the flapping motion schematically depicted  FIG. 2 . 
       Yokes 
       [0035]    As depicted in  FIG. 1 , offset stacked yoke hub  100  may comprise four or more substantially identical yokes  200 .  FIGS. 3A and 3B  depict a top view and a side cutaway view, respectively, of a representative yoke  200  having a longitudinal axis  205  and a lateral axis  207 , and  FIG. 3C  depicts a side cutaway view of the yoke  200  with a blade cuff  246  coupled thereto. The yoke  200  comprises a yoke body  210  extending longitudinally from an inner end  231  to an outer end  235  and having a substantially elongated planform (for example, rectangular, ovular, triangular, or a variant thereof). The yoke body  210  may be constructed of any suitable material able to withstand the forces and moments of the dynamic system including, but not limited to, laminated fiberglass composites, carbon composites, or any combination thereof. Yokes  200  of hubs  100 ,  500 , and  600  may be limited to stiff in-plane constructions in order to handle loads associated with strong axial flow in tiltrotor airplane mode. One or more retainer holes  230  may be laterally disposed through the inner end  231 . 
         [0036]    Each yoke  200  may further comprise an inboard beam assembly  240 . In an embodiment, the inboard beam assembly  240  comprises a substantially “C” shaped member constructed of forged metallic material. The inboard beam assembly  240  may be disposed within a cutout  242  in the yoke  200  and may be oriented such that the open part of the “C” faces radially outward with respect to central axis  130 . Inboard beam assembly  240  may be rotationally coupled to yoke  200  about longitudinal axis  205 , providing for possible pitching motion of components coupled thereto. In an embodiment, a blade (or intermediate structure, such as a composite blade grip  246 ) may be coupled to inboard beam assembly  240  via inboard beam attachment holes  244  using any suitable mechanism, such as one or more bolts. 
         [0037]    Each yoke  200  may further comprise coupling means by which a blade (or intermediate structure) may couple thereto and be restrained against centrifugal forces acting to pull the blade away from a centerline of rotation during rotation. In one embodiment, a blade (or intermediate structure, such as a spindle assembly, not shown) may be coupled to yoke  200  via yoke attachment holes  232  using any suitable mechanism, such as one or more bolts. 
       Arrangement of Yokes in Yoke Planes 
       [0038]      FIG. 4A  depicts a top view, and  FIGS. 4B and 4C  depict side cutaway views offset from one another by 90°, of an arrangement of four yokes  200  of offset stacked yoke hub  100 . The yokes  200  may be arranged in two or more planes  215 , each containing the same number of yokes  200 . In one embodiment, yokes  200  and planes  215  may comprise two groups—those yokes  212  situated in a first plane  210 , and those yokes  222  situated in a second plane  220 . As best shown in  FIG. 4A , yokes  212  are arranged about a central axis  130  (possibly defined by a mast  132 ) at a radius  312 , and are equally spaced at planar spacing angles  310  within plane  210 . An equal number of yokes  222  are similarly arranged about the same mast  132  at the same radius  312 , and are equally spaced at planar spacing angles  310  in plane  220 . Such an arrangement provides for balanced mass distribution within the plane  215  to manage dynamic loads and stability in rotating systems, such as rotors. 
         [0039]    Mechanical packaging and load considerations may influence radii  312 . Generally speaking, flapping forces are concentrated at a flapping hinge  314 , and the radius  312  (also known as hinge offset) between the central axis  130  and the flapping hinge  314  defines a moment arm. Therefore, the closer a flapping hinge  314  is situated to a mast  132 , the lower the flapping moment imparted to the mast  132 . This may result in reduced weight by allowing the mast  132  and other components to be leaner, and may result in reduced drag by narrowing the profile of the hub  100 . In embodiments using substantially rigid, stiff in-plane yokes  200 , a flapping hinge  314  may comprise an actual hinge within the yoke body  210  of yoke  200 , or the flapping hinge  314  may instead coincide with the mechanical juncture of the yoke  200  to offset stacked yoke hub  100 . Radius  312  may, however, be conversely influenced by mechanical packaging considerations, such as establishing a minimum radius  312  to physically assemble hub  100  about a mast  132  without interferences. One having ordinary skill in the art will recognize a desirable radius  312  for a given application. 
       Axial Arrangement of Yoke Planes 
       [0040]    Still referring to  FIGS. 4A ,  4 B, and  4 C, each yoke plane  215  may be substantially parallel, and may share a common central axis  130 . Each plane  215  may be axially offset from the others at a predetermined axial offset distance  320 , as best shown in  FIGS. 4B and 4C . Predetermined axial offset distance  320  may be influenced by design considerations including, but not limited to, flapping angle, weight, and performance characteristics. Predetermined axial offset distance  320  may be of sufficient length to avoid physical interference of yokes  200  in the various planes  210 ,  220  and the blades attached thereto when undergoing flapping motion. While larger offset distances  320  may generally provide ample flapping clearance, they may extend the height of hub  100 , which may result in increased weight due to the additional material. Additionally, radial forces acting on a taller hub  100  may result in greater moments thereby driving increased structural weight to handle the loads. Similarly, a taller hub  100  may result in increased drag. However, other aerodynamic performance benefits may drive predetermined offset distance  320  to be greater than that necessary to provide flapping clearance. Offset rotor disks often demonstrate improved aerodynamic efficiency in axial flow conditions. At certain offset distances, the rotor may perform as though it has a larger effective diameter, resulting in increased thrust/power ratio than a similar single-plane rotor. As such, the proper axial offset distance  320  may provide for desired aerodynamic performance to be achieved using a smaller rotor diameter. Accordingly, the rotors may be disposed further inward toward the tiltrotor fuselage, possibly resulting in reduced aircraft weight. In one embodiment, aerodynamic performance improvements are maximized using a predetermined axial offset distance  320  corresponding with one to two rotor blade chord lengths. One having ordinary skill in the art will recognize a desirable predetermined axial offset distance  320  that may balance the considerations described herein for a given application. 
         [0041]    In an embodiment, yokes  212  of plane  210  and yokes  222  of plane  220  are situated about a common central axis  130  (possibly defined by a mast  132 ), and plane  210  is parallel to and axially offset a predetermined distance  320  from plane  220 . In another embodiment, a 3-inch thick yoke  212  in plane  210  may flap at ±12 degrees of flapping angle  316  in each direction. A minimum predetermined axial offset distance  320  between plane  210  and  220  may be set at 6.25 inches to provide clearance between a yoke  212  and an azimuthally adjacent yoke  222  in plane  220  having similar dimensions and flapping characteristics. However, the predetermined axial offset distance  320  of this embodiment may be larger to take advantage of the aerodynamic performance benefits described in the previous paragraph. 
       Angular Arrangement of Yoke Planes 
       [0042]    Still referring to  FIGS. 4A ,  4 B, and  4 C, yokes  200  in a given plane  215  may be angularly offset an angular offset angle  330  from yokes  200  in other plane(s)  215 . This may provide for blade flapping clearance, and improved airflow through blades  246  that may be attached to yokes  200  in planes  215 . 
         [0043]    In one embodiment, as best shown in  FIG. 4A , yokes  212  of plane  210  may be angularly offset by an angular offset angle  330  from yokes  222  of plane  220 . In another embodiment, yokes  212  of plane  210  substantially bisect the equal angular spacing angles  310  separating yokes  222  in plane  220 , and vice versa. 
         [0044]    One having ordinary skill in the art will recognize that the number of blade yokes  200 , as well as the axial offset distance  320  and the angular offset distance  330  between them, may be determined by a variety of design factors including, but not limited to, performance characteristics; blade flapping and pitching angles; and weight, drag, and load characteristics. 
       Mechanical Embodiment 
       [0045]    Referring now to  FIGS. 5A and 5B , offset stacked yoke hub  100  may include mounting hardware  400 , which may comprise mounting plates  410  and axial mounting bolts  420 . Hub  100  may comprise any number of mounting plates  410 , and mounting plates  410  may be of any suitable material, shape, size, and construction to couple yokes  200  to a mast  132  or similar structure. Mounting plate  410  may comprise a mast cutout  412  forming an aperture of sufficient diameter to allow a mast  132  to pass there through. Each mounting plate  410  may further comprise mounting holes  414  that are sized and arranged to be substantially concentric with the retainer holes  230  disposed through the inner ends  231  of yokes  200  when arranged in a desired configuration about the central axis  130 . In such a configuration, mounting bolts  420  may pass through each set of axially-aligned mounting holes  414  and retainer holes  230  to secure yokes  200  to mounting plates  410 . 
         [0046]    In an embodiment, as best shown in  FIG. 5B , hub  100  comprises three mounting plates  411 ,  413 ,  415 . First mounting plate  411  and second mounting plate  413  may be positioned about the mast  132  on the axially outer sides of planes  210  and  220 , respectively. A third mounting plate  415  may be positioned about the mast  132  axially between yokes  212  and yokes  222 . In this particular embodiment, each of yokes  212  and  222  comprises two retainer holes  230 , and the retainer holes  230  of each yoke  212  in plane  210  substantially axially align with the retainer holes  230  of each azimuthally adjacent yoke  222  in plane  220 . Mounting plates  410  are oriented such that mounting holes  414  substantially align with the aforementioned axially aligned retainer holes  230  of yokes  212  and  222 . A mounting bolt  420  is disposed through each set of axially aligned mounting holes  414  and retainer holes  230  to secure yokes  212  and  222  to the mounting plates  410 , and thereby to mast  132 . 
         [0047]    Referring now to  FIGS. 6A and 6B , an offset stacked yoke hub  100  having four yokes  200  is depicted in assembled and exploded views, respectively. Two mounting bolts  420  secure each yoke  200 , and each mounting bolt  420  is shared by two azimuthally adjacent yokes  200 . As such, the number of mounting holes  414  in a mounting plate  410  is equal to the number of yokes  200  in hub  100 . In this embodiment, the four total yokes  200  may be arranged in two axially offset planes  215 , namely two yokes  212  in an upper plane  210 , and two yokes  222  in a lower plane  220 . Each pair of yokes  212  and  222  has equal angular spacing  310  within their respective planes  210  and  220 —that is, yokes  200  in each given plane  215  are spaced 180° apart. The planes  210  and  220  are substantially parallel, share a common central axis  130 , and are angularly offset such that yokes  212  in plane  210  bisect the planar spacing angle  310  between yokes  222  in plane  220 —that is, each yoke  212  is positioned about 90° from each azimuthally adjacent yoke  222 . Yokes  212  and  222  are also “stacked” at an axial offset distance  320 . 
         [0048]    Each yoke  212  and  222  comprises two retainer holes  230  in its base, and yokes  212  and  222  are set at a radius  312  from the central axis  130  such that a retainer hole  230  on any given yoke  212  aligns axially with a retainer hole  230  on an azimuthally adjacent yoke  222 . Mounting holes  414  are arranged to coincide with the axially-aligned retainer holes  230  of the stacked azimuthally adjacent yokes  212  and  222 , and mounting bolts  420  may be disposed therein. By stacking yokes  212  and  222  such that their inner ends  231  partially overlap, offset stacked yoke hub  100  may be packaged tighter to a central axis  130  than if all yokes  212  and  222  were arranged in a single plane  215 , thereby reducing hub loads, weight, and drag. Axial offset distance  320  between yokes  212  and  222  may accommodate flapping motion of the yoke  200 . 
         [0049]    Referring now to  FIGS. 7A and 7B , an offset stacked yoke hub  500  having six yokes  200  is depicted in assembled and exploded views, respectively. This embodiment comprises mounting plates  410  having six mounting holes  414  arranged in a hexagonal pattern to coincide with axially-aligned retainer holes  230  of yokes  212  and  222  stacked in a manner similar to that described with respect to offset stacked yoke hub  100  having four yokes  200 . The six total yokes  200  may be arranged in two axially offset planes  215 , namely three yokes  212  in an upper plane  210 , and three yokes  222  in a lower plane  220 . Each set of yokes  212  and  222  has equal angular spacing  310  within their respective planes  210  and  220 —that is, yokes  200  in a given plane  215  are spaced 120° apart. The planes  210  and  220  are substantially parallel, share a common central axis  130 , and are angularly offset such that yokes  212  in plane  210  bisect the planar spacing angle  310  between yokes  222  in plane  220 —that is, each yoke  212  is positioned about 60° from each azimuthally adjacent yoke  222 . Yokes  212  and  222  are also “stacked” at an axial offset distance  320 . 
         [0050]    Each yoke  212  and  222  comprises two retainer holes  230  in its base, and yokes  212  and  222  are set at a radius  312  from the central axis  130  such that a retainer hole  230  on any given yoke  212  aligns axially with a retainer hole  230  on an azimuthally adjacent yoke  222 . Mounting holes  414  are arranged to coincide with the axially-aligned retainer holes  230  of the stacked azimuthally adjacent yokes  212  and  222 , and mounting bolts  420  may be disposed therein. Flapping motion may be similarly accommodated by this embodiment. 
         [0051]    Referring now to  FIGS. 8A and 8B , an offset stacked yoke hub  600  having eight yokes  200  is depicted in assembled and exploded views, respectively. This embodiment comprises mounting plates  410  having eight mounting holes  414  arranged in an octagonal pattern to coincide with axially-aligned retainer holes  230  of yokes  212  and  222  stacked in a manner similar to that described with respect to offset stacked yoke hub  100  having four yokes  200  and offset stacked yoke hub  500  having six yokes  200 . The eight total yokes  200  may be arranged in two axially offset planes  215 , namely four yokes  212  in an upper plane  210 , and four yokes  222  in a lower plane  220 . Each set of yokes  212  and  222  has equal angular spacing  310  within their respective planes  210  and  220 —that is, yokes  200  in a given plane  215  are spaced 90° apart. The planes  210  and  220  are substantially parallel, share a common central axis  130 , and are angularly offset such that yokes  212  in plane  210  bisect the planar spacing angle  310  between yokes  222  in plane  220 —that is, each yoke  212  is positioned about 45° from each azimuthally adjacent yoke  222 . Yokes  212  and  222  are also “stacked” at an axial offset distance  320 . 
         [0052]    Each yoke  212  and  222  comprises two retainer holes  230  in its base, and yokes  212  and  222  are set at radius  312  from the central axis  130  such that a retainer hole  230  on any given yoke  212  aligns axially with a retainer hole  230  on an azimuthally adjacent yoke  222 . Mounting holes  414  are arranged to coincide with the axially-aligned retainer holes  230  of the stacked azimuthally adjacent yokes  212  and  222 , and mounting bolts  420  may be disposed therein. Flapping motion may be similarly accommodated by this embodiment. 
         [0053]    It may be advantageous to set forth definitions of certain words and phrases used in this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more elements, whether or not those elements are in physical contact with one another. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. 
         [0054]    Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.