Abstract:
A hub assembly for a tilt rotor includes a yoke having a plurality of yoke arms located substantially orthogonal to a central axis of the hub assembly. An inboard pitch change bearing assembly substantially surrounds a yoke arm of the plurality of yoke arms. An outboard pitch change bearing assembly is located at the yoke arm. The inboard pitch change bearing assembly and the outboard pitch change bearing assembly are operably connectable with a rotor blade to allow pitch change of the rotor blade relative to the yoke about a pitch change axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a nonprovisonal application of U.S. Provisional Application No. 61/307,511, filed on Feb. 24, 2010, the disclosure of which is also incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein generally relates to rotors for aircraft use. More specifically, the subject disclosure relates to hub configurations for tilt rotors. 
     Rotor blades utilized in tilt-rotor aircraft typically must be able to pitch significantly relative to a blade pitch change axis, be capable of allowing flapping motion of the rotor blades, have proper frequency placement, proper kinematic couplings, among other requirements. To meet these requirements in the past, a gimbal-style rotor hub has been utilized as shown in  FIG. 7 . A gimbal style hub offers the benefit of a low head moment and, consequently, less structure required to support the rotor shaft within the gearbox and less structure required to support the gearbox on the airframe. The rotor hub  100  includes a trunnion  114 , a series of drive links  106 , and a thrust bearing  116  arranged at an interface of the rotor hub  100  and a rotor shaft  104 . The trunnion  114  is connected to the rotor shaft  104  via splines. The links  106  are connected between the trunnion  114  and to the rotor hub  100  via a number of pillow blocks  102  thereby allowing relative motion between the hub  100  and the rotor shaft  104  while transferring torque between the rotor shaft  104  and the hub  100 . The trunnion  114  is located between the upper and lower thrust bearings  116 . The upper and lower thrust bearings  116  are also connected to the shaft via splines and allow relative motion between the rotor hub  100  and the rotor shaft  104  while absorbing the thrust loads of the rotor. The elastomerics within the links  106  and upper and lower thrust bearings  116  are thermally limited, which limits operations of the tilt rotor and maneuverability of the aircraft in which the tilt rotor is utilized. Further, the gimbal-style hub has many parts and is of considerable weight. 
     Pitch change of the blades of such a rotor is typically accommodated by inboard pitch change bearing assemblies (IPCBAs)  108  and outboard pitch change bearing assemblies (OPCBAs)  110 . The IPCBAs  108  are typically located in holes through the yoke  112  of the rotor hub  100 , which presents structural issues for the yoke  112 , as it must react IPCBA  108  shear loads. The art would well-receive a less complex, lighter weight tilt rotor hub configuration, while maintaining lower head moments than other lightweight configurations. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a hub assembly for a tilt rotor includes a yoke having a plurality of yoke arms located substantially orthogonal to a central axis of the hub assembly. An inboard pitch change bearing assembly substantially surrounds a yoke arm of the plurality of yoke arms. An outboard pitch change bearing assembly is located at the yoke arm. The inboard pitch change bearing assembly and the outboard pitch change bearing assembly are operably connectable with a rotor blade to allow pitch change of the rotor blade relative to the yoke about a pitch change axis. 
     According to another aspect of the invention, a rotor assembly includes a hub assembly having a yoke having a plurality of yoke arms located substantially orthogonal to a central axis of the hub assembly. An inboard pitch change bearing assembly substantially surrounds a yoke arm of the plurality of yoke arms, and an outboard pitch change bearing assembly is located at the yoke arm. A rotor blade is operably connected to the inboard pitch change bearing assembly and the outboard pitch change bearing assembly such that the inboard pitch change bearing assembly and the outboard pitch change bearing assembly absorb aerodynamic loads of the rotor blade about a pitch change axis. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view of an embodiment of a tilt rotor hub; 
         FIG. 2  is a cross-sectional view of an embodiment of a tilt rotor hub; 
         FIG. 3  is a partial cross-sectional view of an embodiment of a tilt rotor hub; 
         FIG. 4  is a cross-sectional view of another embodiment of a tilt rotor hub; 
         FIG. 5  is a partial cross-sectional view of another embodiment of a tilt rotor hub; 
         FIG. 6  is a perspective view of another embodiment of a tilt rotor hub; and 
         FIG. 7  is a perspective view of a typical tilt rotor hub. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , shown is an embodiment of an improved rotor assembly  10 . The rotor assembly  10  includes a rotor hub  12 , best shown in  FIG. 2 . The rotor hub  12  includes a yoke  14  surrounded by a spindle  16 . In some embodiments, the yoke  14  is constructed of at least one flex beam  18 . Each flex beam  18  extends along a diameter of the yoke  14  toward a rotor axis  20 . In some embodiments, each flex beam  18  extends across the rotor axis  20  and includes a hub opening  22 . Each flex beam  18  is configured to be supportive of an outboard spindle  70 , as shown in  FIG. 1 . Referring again to  FIG. 2 , to support additional rotor blades  24 , additional flex beams  18  can be stacked at the rotor axis  20 . For example, as shown, to construct a rotor hub  12  supportive of four rotor blades  24 , two flex beams  18  may be stacked and offset at an angle of approximately ninety degrees about the rotor axis  20 , though the flex beams  18  may be offset at other suitable angles for dynamic and/or acoustic reasons. While a rotor hub  12  supportive of four rotor blades  24  is shown in  FIG. 2 , it is to be appreciated that embodiments having, for example, two, three or six rotor blades  24 , are contemplated within the scope of the present disclosure. Each flex beam  18  includes at least one virtual hinge line  28 , which in some embodiments is located at about 5% of a rotor blade  24  length from the rotor axis  20 . The virtual hinge line  28 , which may comprise a thinning of the cross-section of the flex beam  18 , acts as a flapping and coning hinge allowing motion of an attached rotor blade  24  in a flapping direction  30 . In the prior art gimbal type hub, the flapping hinge is located within the gimbal itself which adds substantial structure and large elastomeric bearings to the gimbal and the hub. 
     The spindle  16  surrounds the at least one flex beam  18  and extends at least partially along a length of each flex beam  18 . In some embodiments, the spindle  16  is a hollow structure and, as shown in  FIG. 2 , is secured to a shaft flange  32  on a rotor shaft  34 , and at least one flex beam  18  at the hub opening  22 . The need for a spindle  16  depends at least in part on the edgewise and beamwise shear loads the spindle  16  must have the capability to withstand. The hub opening  22  is configured to be receptive of the rotor shaft  34 . A hub plate  36  is disposed at the hub opening  22  and includes an attachment flange  38  and internal spline  42 . Internal spline  42  meshes with a corresponding external spline (not shown) on the rotor shaft  34 . Torque is transferred from the rotor shaft  34  into the rotor hub  12  via the internal spline  42  and a plurality of hub bolts  44 . While an internal spline  42  and shaft flange  32  are shown in  FIG. 2 , other means of transferring rotor moments and torque between the rotor shaft  34  and the rotor hub  12  are contemplated within the present scope. The yoke  14 , the spindle  16 , the shaft flange  32 , and the hub plate  36  are joined together via the plurality of hub bolts  44  extending through the attachment flange  38 , the spindle  16 , the shaft flange  32  and the yoke  14 . 
     The spindle  16  may be formed from any suitable material, for example, a composite material, titanium, steel, or aluminum or an alloy thereof. The spindle  16  is configured to abut the yoke  14  at the hub opening  22 , and includes a plurality of spindle arms  46  which extend along a plurality of yoke arms  48  of the yoke  14 . As shown in  FIG. 3 , the spindle arms  46  are configured to provide a spindle gap  50  between the spindle arms  46  and the yoke arms  48  to allow movement of the yoke arms  48  relative to the spindle arms  46  during operation of the rotor assembly  10 . 
     The spindle  16  may be formed as a unitary piece, or in some embodiments, formed of multiple pieces and assembled to facilitate a desired yoke  14  configuration. For example, if the spindle  16  is a two-piece assembly, then the multiple flex beams  18  can be consolidated into a single unitary yoke  14 , such as is shown in  FIG. 4 . 
     Referring again to  FIG. 1 , the rotor assembly  10  includes a plurality of inboard pitch change bearings (IPCBAs)  52 . Each IPCBA  52  is secured to one of the spindle arms  46  of the plurality of spindle arms  46  by, for example, a plurality of bolts (not shown), and in some embodiments is located such that an IPCBA centroid is coincident with the virtual hinge line  28 . It is to be appreciated, however, that other attachment means are contemplated within the present scope. Further, in some embodiments, as shown in  FIG. 5 , each IPCBA  52  may be secured to a yoke arm  48  rather than, or in addition to, being secured to the spindle arm  46 . The ability to secure the IPCBA  52  to the yoke arm  48  depends on the ability of the yoke arm  48  to withstand the blade loads with adequate structural margin. 
     As shown in  FIG. 3 , each IPCBA  52  surrounds one of the yoke arms  48  which extends through an opening  54  in the IPCBA  52  and consequently has an inner member  56  with a width or diameter greater than a width of the yoke arm  48 . To support the inner member  56 , a plurality of race supports  58  extend from the opening  54  to the inner member  56 . Each IPCBA  52  includes an outer race  60  located circumferentially around the inner member  56 . 
     For such a large spherical bearing a plain, non-lubricated bearing may be used, but may be limited depending on the liner system used between the ball and race. Another approach is illustrated in  FIG. 6 . The embodiment shown utilizes a roller bearing  62  which surrounds the spindle arms  46  in conjunction with elastomeric edgewise bearings  64  and elastomeric beamwise bearings  66 , which connect the roller bearing  62  to the spindle arm  46 . In some embodiments, the edgewise bearing  64  is a flat ring laminate, while the beam wise bearing  66  is a cylindrical or spherical laminate. 
     Referring again to  FIG. 1 , the rotor blade  24  fits around the IPCBA  52  and interfaces therewith to allow for rotation of the rotor blade  24  about a pitch change axis  68 , in some embodiments up to +/−35 degrees, while minimizing torsion loads transferred to the yoke arm  48 . Further, in some embodiments, the IPCBA  52  is located at the virtual hinge line  28  which allows for flap of the rotor blade  24  of about +/−10 degrees. 
     The outboard spindle  70  is secured to an outboard end  72  of each yoke arm  48 . In some embodiments, the outboard spindles  70  are bolted through a plurality of through holes  74  in the yoke arm  48  (shown in  FIG. 2 ). Mounted on each outboard spindle  70  is an outboard pitch change bearing assembly (OPCBA)  76  and a centrifugal force (CF) bearing  78 . The OPCBAs  76 , which in some embodiments are elastomeric bearings or non-lubricated bearings, include a plurality of blade attachment holes  80  for securing the rotor blade  24  to the OPCBA  76 . The OPCBAs  76  are configured to react beamwise and edgewise loads generated by the rotor blade  24 . The CF bearing  78  reacts centrifugal force loads of the rotor blade  24 , and in some embodiments may be an elastomeric bearing or a thrust bearing. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.