Abstract:
A rotary wing aircraft yoke is provided including an arcuate medial portion. A pair of arms extends from the medial portion and each terminates in a distal end. The yoke is made from multiple plies of glass and graphite fibers of varying orientations. The number of graphite plies increases along each arm from adjacent the medial portion to the distal end.

Description:
BACKGROUND OF THE INVENTION 
     This invention was made with Government support under Agreement N00014-06-D-0045 D.O. 0013 for Composite Main Rotor Blade Attachment Yoke Manufacturing Technology Phase 1. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to rotorcraft rotors and, more particularly, to a rotor blade mounting assembly. 
     A helicopter rotor hub is the primary structural assembly for driving torque to each rotor blade, balancing centrifugal loads between opposing rotor blades and transferring lift loads to the aircraft fuselage. Within the class of articulated rotors are those which include a rotor yoke, one per rotor blade, which is driven by a central hub plate via a multi-laminate elastomeric bearing. The dimensions of the rotor yoke are determined by the operational motion envelope of the rotor system and blade loads. 
     Historically, helicopter rotor hubs and their associated parts have been made of high strength, lightweight, critical metals or alloys. Although these metal components perform adequately, it is always desirable to reduce the empty weight of the helicopter. Presently, titanium and aluminum are used extensively because of their generally light weight and strength. There is a constant search for lighter and stronger materials. One consideration for decreasing the empty weight of the helicopter is to use fiber reinforced composites to replace metal components. However, the cost of a composite part, especially a part requiring a significant number of plies, is often not competitive with the comparable metal part that it is designed to replace. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment of the invention, a rotary wing aircraft yoke is provided including an arcuate medial portion. The yoke also includes a pair of arms extending from the medial portion and terminating at a distal end. The yoke is made from multiple plies of glass and graphite fibers having varying orientations. The number of graphite plies increases along each arm from adjacent the medial portion to the distal end. 
     According to another embodiment of the invention, a rotor mount assembly for a rotary wing aircraft is provided including a hub plate having a plurality of spokes and arcuate segments. The rotor mount assembly also includes at least one rotor blade. At least one yoke connects the at least one rotor blade to the hub plate. The yoke includes an arcuate medial portion. A pair of arms extends from the medial portion and terminates in a distal end. The yoke is made from multiple plies of glass and graphite fibers of varying orientations. The number of graphite plies increases along each arm from adjacent the medial portion to the distal end. An elastomeric bearing is disposed between the medial portion of the yoke and an arcuate segment of the hub plate to accommodate the loading and displacement of the rotor blade. A cap fastened to the elastomeric bearing around the medial portion of the yoke holds the elastomeric bearing to the yoke. 
     According to yet another embodiment of the invention, a method of manufacturing a yoke for a rotary wing aircraft is provided including attaching a preform to a tool head. The preform is contoured to form at least two yokes facing each other. A plurality of glass fibers of varying orientations are then wound around the preform. A plurality of plies of graphite fibers of varying orientations are placed around at least a portion of the preform. The preform is then cured and multiple yokes are machined from the fibers around the preform. 
    
    
     
       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 a rotary wing aircraft for use with the present invention; 
         FIG. 2  is a detailed perspective view of a rotor mount assembly; 
         FIG. 3  is a perspective view of a yoke assembly; 
         FIG. 4  is a perspective view of a yoke assembly including an elastomeric bearing; and 
         FIG. 5  is a perspective view of a manufacturing tool for making a plurality of yokes. 
     
    
    
     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 
       FIG. 1  schematically illustrates a rotary-wing aircraft  10  having a main rotor system  12 . The aircraft  10  includes an airframe  14  having an extending tail  16  which mounts a tail rotor system  18  as an anti-torque system. The main rotor assembly  12  is driven about an axis of rotation A through a main gearbox (illustrated schematically at T) by one or more engines E. The main rotor system  12  includes multiple rotor mount assemblies  30  each for connecting a blade  28  to a rotor hub  20 . Although a particular helicopter configuration is illustrated and described in the disclosed non-limiting embodiment, other configurations and/or machines, such as high speed compound rotary wing aircraft with supplemental translational thrust systems, dual contra-rotating coaxial rotor system aircraft, turboprops, tilt-rotors, and tilt-wing aircraft, will also benefit from the present invention. 
       FIG. 2  depicts a perspective view of a rotor mount assembly  30  for driving a plurality of rotor blades  28  about an axis of rotation A. The rotor mount assembly  30  includes a hub plate  20  which is adapted for mounting to a rotor shaft (not shown) and which includes a plurality of radial spokes  22  and arcuate segments  24  for structurally interconnecting adjacent spokes  22 . The radial spokes  22  and arcuate segments  24  define hub plate apertures  26  for accepting rotor yoke assemblies  40 . Each yoke assembly is structurally interconnected to a rotor blade  28  by a mounting sleeve  34  joined to a blade attachment  32 . 
     Referring now to  FIGS. 3 &amp; 4 , the rotor yoke assembly  40  has a generally U-shaped yoke  41  and includes a curved medial portion  42  which extends through the respective hub plate aperture  26 . In a preferred embodiment, the curve of the medial portion  42  has a radius R 1 . Upper arm  44   a  and lower arm  44   b  project radially outboard from the ends of medial portion  42 , such that the yoke  41  therefore loops about the respective arcuate segment  24  so that the upper and lower arms  44   a ,  44   b  are disposed above and below the arcuate segment  24  and extend radially outboard thereof. A bend that is more acute than the curvature of the medial portion, joins each arm  44   a ,  44   b  to the medial portion  42 . In a preferred embodiment, the bend is a radius R 2  that is smaller than radius R 1  of the medial portion  42 . Each arm  44   a ,  44   b  of yoke  41  includes a distal end  46   a  and  46   b  respectively. At least one mounting aperture  48  is formed through the distal ends  46   a ,  46   b  of the upper and lower arms  44   a ,  44   b  for connecting the yoke  41  with the mounting sleeve  34  to transfer the operational loads of the rotor system. A bushing  50  is located within each mounting aperture  48 , and a pad  52  is disposed adjacent the top and bottom surfaces of the distal ends  46   a ,  46   b  between the yoke  41  and mounting sleeve  34  to prevent damage at the connection with mounting sleeve  34 . 
     An elastomeric bearing  60  is interposed between the arcuate segment  24  of the rotor hub  20  and the medial portion  42  of the rotor yoke assembly  40  to accommodate the load and multidirectional displacement of the rotor blade. Outer member  64  of the elastomeric bearing  60  contacts the arcuate segment  24  of hub  20  and inner member  62  of the elastomeric bearing  60  contacts the medial portion  42  of the yoke  41 . A cap  70  holds the inner member  62  in place against the medial portion  42  of the yoke  41 . The cap  70  includes a channel portion that contacts the medial portion  42  of the yoke  41 . This channel portion has a complementary curvature to en enhance the surface contact between the cap  70  and the yoke  41  and to limit relative movement. The cap  70  is wider than the medial portion  42  of the yoke  41  such that the holes  72  extending through the cap  70  are located near the edges of the medial portion  42 . Bolts  74  extend from the cap  70 , around medial portion  42 , to engage the inner member  62  thereby compressing the medial portion  42  of yoke  41  between the cap  70  and the inner member  62  of the elastomeric bearing  60 . 
     In one embodiment, the yoke  41  is made from a plurality of materials. The materials are selected so that the yoke  41  has generally similar strength, flexibility and dimensions as a metal yoke intended for use in the same application. The medial portion  42  of yoke  41  must be able to bend and may be made from multiple layers or plies of glass fiber. Any suitable glass fiber, such as S-2 Glass® for example, may be used. The fibers within any one glass ply are continuous and unidirectionally oriented, but the plies are stacked one atop the other at various angles, such as in a ±4° orientation for example. Because the yoke  41  connects to the mounting sleeve  34  at distal ends  46   a  and  46   b , these ends  46   a ,  46   b  require additional tensile and shear strength and may consist of a combination of materials. In addition to the glass plies, the distal ends  46   a ,  46   b  may include multiaxially oriented multiple plies of unidirectionally oriented graphite fibers. For example, the distal ends  46   a ,  46   b  may include a plurality of graphite plies stacked at a ±45° orientation and a 90° orientation. In one embodiment, the distal ends  46   a ,  46   b  are made up of generally 49% glass fibers stacked at a ±4° orientation, 41% graphite fibers stacked at a ±45° orientation, and 10% graphite fibers stacked at a 90° orientation. However, the yoke  41  may include any of a number of variations of the percentages of glass and graphite fibers, as well as orientation angles of the fibers in each ply. The plies of the distal ends  46   a ,  46   b  are layered such that no two consecutive plies are identical in material and orientation. Additionally, the material and the orientation of the fibers of the plies may generally alternate. 
     The arms  44   a ,  44   b  connecting the medial portion  42  and the distal ends  46   a ,  46   b  has a combination of the materials included in the distal ends  46   a ,  46   b . Arms  44   a ,  44   b  increase in thickness as they extend from the medial portion  42  toward the distal ends  46   a ,  46   b . The graphite composition in each of the arms increases as the thickness increases. This is accomplished by, for example, increasing the number of graphite plies progressively along the arms  44   a ,  44   b  from a section adjacent medial portion  42  to a section of the arms  44   a ,  44   b  adjacent the distal ends  46   a ,  46   b . In a preferred embodiment, the sections of the arms  44   a ,  44   b  immediately adjacent the medial portion  42  and distal ends  46   a ,  46   b , each includes a number of graphite plies substantially equal to the number of graphite plies in the medial portion  42  and distal ends  46   a ,  46   b , respectively. 
     Referring now to  FIG. 5 , one method of manufacturing a yoke  41  includes placing multiple layers of fibers onto a tool head  80  to form a preform  82 . The side surfaces of the tool head  80  have a radius R 1  and the side surfaces converge with the top and bottom surfaces of the tool head  80  at a bend area having a radius R 2 . The tool head  80  is shaped to match the contour of the yoke such that two yokes may be formed having their distal ends  46   a ,  46   b  adjacent one another in the center of the top and bottom surface. The tool head  80  may have an extended length for manufacturing a plurality of yokes  41  during a single manufacturing operation, thereby reducing manufacturing costs. 
     Plies of fibers are placed individually around at least a portion of the surface of the tool head  80  to form the preform  82 . Numerous sequences for the order in which the plies may be placed exist. In one embodiment, no two consecutive layers are identical in material and fiber orientation. Additionally, the material of each layer can, but need not alternate, such as positioning a graphite layer between glass layers for example. For example, a first glass ply may be placed around the entire tool head  80 , having a 0° or a ±4° orientation. The next ply may be a first graphite ply having fibers at either a ±45° or a 90° orientation. This first graphite layer will be laid on only a portion of the two longer surfaces of the tool head  80  which form the arms  44   a ,  44   b  and distal ends  46   a ,  46   b  of a plurality of yokes  41 . A second glass layer, having a different fiber orientation than the prior glass ply, may then be wound around the entire tool head  80 . A second graphite ply may then be laid on top of the second glass ply. The second graphite ply may have a different fiber orientation than the first graphite layer. Consecutive graphite plies become increasingly shorter to create the tapered thickness of arms  44   a ,  44   b . Once all of the fiber layers have been placed, the tool head  80  is then cured at an optimal temperature and time based on the composition of the materials to form a preform  82 . Individual yokes  41  and their corresponding mounting apertures  48  are then machined from the cured composite preform  82 . 
     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.