Abstract:
A prop rotor hub includes a constant velocity joint in the same plane as a prop blade yoke. Torque is transmitted from a shaft to the blades through the CV joint, hub plates attached to the CV joint, and the yoke, which is attached to the hub plates. Providing all elements in a substantially planar arrangement results in a hub assembly which has significantly less height than a traditional design. The hub itself is in-plane with the yoke, resulting in a more efficient torque transmission from the mast to the rotor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention The present invention relates generally to aircraft rotors, and more particularly to a hub suitable for use with a tilt rotor aircraft. 
     2. Description of the Prior Art 
     Design of rotors and propellers for aircraft is often extremely complex. A large number of factors must be taken into account, including flexure of the rotor under heavy loads and the required motions of the rotor blades with respect to the drive mechanism. The considerations for prop rotors, used as both propellers and rotors in aircraft such as a tiltrotor aircraft, can be more complex than usual. 
     Constant velocity joints must be provided between the rotor shaft and the blades, giving rise to a relatively complex assembly at the hub of the rotor shaft. An example of such an assembly, useful for certain helicopter designs, is described in U.S. Pat. No. 4,729,753. As illustrated therein, numerous approaches to making helicopter and prop rotor assemblies have been tried. Many of these are suitable for a given application, but not for others. As aircraft designs progress, the hub assemblies used on their rotors must meet new specifications which render older designs unsuitable. 
     The advent of the tilt rotor aircraft has added performance requirements to the hub assembly, resulting from the more complex operation of the craft. The prop systems on a tilt rotor are very large by comparison with standard aircraft, and size becomes an issue. In some designs of a tilt rotor aircraft, particularly suitable for use in light and medium duty models, certain design choices must be made in order that there is simply room for all of the required parts. 
     Standard rotor hub designs are relatively large, influencing the design of mechanical systems associated with the rotor. For example, a large rotor hub requires a relatively long mast. The hub itself is heavy, and associated systems, such as the control rods, are relatively long and heavy. Systems must be designed so that the control system is not interfered with. 
     Therefore, it would be desirable to provide a rotor hub design which is suitable for use with the design constraints of a prop rotor aircraft. Such hub must provide proper support for the blades while remaining small compared to prior art designs. A thinner hub design would shorten the mast, lower the height of the associated systems, and save weight. 
     SUMMARY OF THE INVENTION 
     Therefore, in accordance with the present invention, a prop rotor hub includes a constant velocity joint in the same plane as a prop blade yoke. Torque is transmitted from a shaft to the blades through the CV joint, hub plates attached to the CV joint, and the yoke, which is attached to the hub plates. Providing all elements in a substantially planar arrangement results in a hub assembly which has significantly less height than a traditional design. The hub itself is in-plane with the yoke, resulting in a more efficient torque transmission from the mast to the rotor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a preferred rotor hub assembly according to the present invention; 
     FIG. 2 is a view of a yoke used in the preferred rotor hub assembly; 
     FIG. 3 is a perspective view oi a lower hub spring assembly for the preferred rotor hub assembly; 
     FIG. 4 shows the upper and lower hub spring assemblies connected together into a hub spring set; 
     FIG. 5 illustrates a preferred constant velocity joint assembly used with the prop rotor hub assembly; 
     FIG. 6 is a view of the preferred rotor hub assembly with the upper hub spring assembly removed; 
     FIG. 7 is a graph illustrating loads on the rotor hub assembly; and 
     FIG. 8 is a top view of a yoke illustrating the effects of bolt hole spacing on yoke stiffness. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The description which follows is directed to a prop rotor hub assembly suitable for use in a tilt rotor aircraft. The preferred assembly carries three blades. It will be appreciated by those skilled in the art that the described design could be used with a helicopter if desired, or that a different number of blades could be used if the design was otherwise suitable. 
     Referring to FIG. 1, a hub assembly  10  is shown which provides a constant velocity (CV) joint suitable for use with a tilt rotor aircraft. As will be appreciated by the following description, the described assembly provides a stiff in-plane rotor. In the preferred embodiment, the assembly is constant velocity up to approximately 12 degrees of flap, including approximately ½ degree of yoke flexure. The stiff in-plane design provides for no lead/lag flexure. 
     The assembly includes a yoke  12 , and upper  14  and lower  16  hub plates. Details of these sub-assemblies are described in connection with FIGS. 2-6. Attached to the ends of the yoke arms are centrifugal force bearings  18 . Bearings  18  include a pitch bearing internally, and absorb chord and beam loads generated by the blades (not shown) as well as centrifugal force loads. These CF bearings are well known in the art, and any suitable structures may be used. 
     FIG. 2 illustrates a yoke for the hub assembly  10 . Yoke  12  has three arms  20 . At the tip of each arm  20  are a pair of through holes  22  to which the CF bearings  18  are bolted. At an inboard end of each arm  20  are a pair of through holes  24  for mounting a carrier for an inboard spindle. More details of this structure are described in connection with FIG.  6 . 
     As can be seen from FIG. 2, the center region  26  of yoke  12  is cut away. As will be described, a constant velocity joint assembly is located in center region  26  when the hub  10  is assembled. Alongside center region  26 , between arms  20 , are pairs of through holes  28 . Holes  28  receive bolts which connect the upper and lower hub plates  14 ,  16  to yoke  12 . 
     Chord stiffness for the assembly is very important. As shown in FIG. 2, chord stiffness is the resistance to bending in the plane of the yoke. The depicted design enhances such stiffness in several ways. First, the fiberglass material itself is quite stiff, particularly when laid up in the preferred manner described below. Second, the shape of the yoke is selected to maximize stiffness. Finally, the positioning of bolt holes enhances yoke stiffness. 
     Each arm  20  has a flexure region  21  which is somewhat flat and wide. This allows a small amount of needed flexure in a vertical direction caused by normal forces on the rotor blades. However, flexure regions  21  have less flex in the plane of the yoke  12 . Transition regions  23  on each arm  20  connect the relatively thinner flexure regions  21  with thicker central support regions  25 . As the arms  20  tend to flex relative to each other in 12  plane, large bending forces are generated in the central support regions. These therefore need to be thicker than the flexure regions  21  to withstand these higher forces. 
     In addition, spacing of bolt holes  28  significantly impacts the overall in-plane stiffness of the assembly. When upper and lower plates  14 ,  16  are bolted together, bolts go through bolt holes  28 . This provides locations of relative immobility within central support regions  25 . The spacing of these holes contributes greatly to the overall stiffness of the yoke. The fact that such spacing can be varied to change the overall stiffness of the hub assembly enhances the utility of the design. 
     Fiberglass yoke  12  is made in expensive, permanent tooling. However, bolt holes  28  are drilled into the yoke  12  after fabrication, so their spacing can be adjusted without changing the yoke tooling. If design considerations change, adjustments to yoke stiffness can be made by adjusting spacing rather than re-working the yoke fabrication tooling. Relatively large changes in assembly stiffness can be made with relatively small spacing changes in bolt hole  28  location. 
     The geometry of yoke  12  allows the assembly stiffness to be varied by moving bolt holes  28 . The opening  26  and the overall layout of yoke  12  provides a generally triangular structure, with central support regions  25  being the sides of the triangle. The triangular shape itself provides considerable in-plane stiffness, and the in-plane deflection of the side beams of the triangular center section of the yoke can be used as a means of varying the beam stiffness of the overall hub assembly. As shown in FIG. 8, locating the bolts in approximately the middle of the sides of the triangle greatly enhances overall assembly stiffness because the greatest amount of flexing naturally occurs there. Thus, the geometrical design of the yoke, in combination with the design of the overall hub assembly, hub plates, and arms, makes the assembly stiff and tunable. For example, with the illustrated design, assuming each arm  20  to be approximately 20 inches long and 2-4 inches thick, chord stiffness (Elc) of the overall assembly as described herein might be 450 lb-in 2  for a bolt hole  28  spacing of 3 inches. Changing the hole  28  spacing to 6 inches can increase assembly chord stiffness to over 650 lb-in 2  without making any other changes to the yoke  12 . This large variation allows fine tuning of hole location spacing to give a desired stiffness consistent with other design considerations. 
     A graph showing variations of chord stiffness as a function of bolt spacing is shown in FIG.  7 . Curve Elc shows that spacing of the bolt holes  28 , without changing other design factors other than those needed to accommodate the spacing change, greatly influences the overall assembly stiffness. The Bolt Load line indicates that load on the bolts increases with stiffness in a manner tracking stiffness. Curves such as those in FIG. 7 can be generated using finite element modeling of the assembly. 
     As will be appreciated from the description of the hub plate spring assemblies and hub plates which is detailed in connection with FIGS. 3 and 4, changing the location of bolt holes  28  affects the size and construction of the hub plates  14 ,  16 . As the holes  28  are moved further apart, hub plates  14 ,  16  must be made larger. Moving holes  28  closer together allows smaller hub plates  14 ,  16  to be used. Because changing the size of hub plates  14 ,  16  changes their weight, trade offs must be made regarding bolt hole location. The holes  28  should be far enough apart to provide enough stiffness for the overall hub assembly  10 , but preferably no farther than required. Although increasing the hole spacing beyond that required increases assembly stiffness, it also increases the weight and size of the assembly. Additionally, increasing spacing increases the loads seen by the bolts, which must be large enough to withstand increased loads with increased spacing. 
     As is known in the art, the entire rotational assembly has a lowest resonant frequency which is a function of size, weight, stiffness and similar material properties of the rotational assembly elements. One acceptable design criterion is to provide an assembly having a lowest resonant frequency which is at least 1.25 time greater than the rotational frequency of the rotating assembly. Increasing stiffness of the hub assembly by positioning the holes  28  can be used to achieve this design criterion. Because increasing stiffness also increases weight, because of larger hub plates  14 ,  16 , computerized finite element analysis is preferably used to optimize stiffness of hub assembly  10  to reach the target assembly stiffness. As known in the art, several iterations are generally needed in the analysis process to provide a minimal size and weight hub assembly which is stiff enough to meet design criteria. FIG. 3 shows a lower hub spring assembly  17 . Assembly  17  includes the lower hub plate  16 , hub spring  36 , and spherical center element  39 . Through holes  30  are spaced to align with holes  28  in the yoke  12 . Bushings (not shown) are inserted in through holes  30  when the lower and upper hub assemblies are assembled. Through holes  32 ,  34  are used to receive pillow block studs described in connection with FIG.  5 . Only one set of holes  32  or  34  is used during hub assembly, depending on whether the hub is used with a clockwise or counterclockwise rotating shaft. 
     Hub spring  36  is securely mounted on hub plate  16 , preferably being glued or vulcanized to both hub plate  16  and center element  39 . Hub spring  36  is preferably constructed from layers of rubber and shims as well known in the art. Construction of hub spring  36 , and its mating with the prop shaft (not shown) is conventional. Center element  39  contains a plurality of splines in a central shaft opening  40  for mating with the prop shaft. Referring to FIG. 4, upper and lower hub plates  14 ,  16  are shown mated without yoke  12 . Bolts  42  attach the hub plates  14 ,  16  together through holes  30  in lower hub plate  16 , and matching holes in upper hub plate  14 . As previously described, bushings are used to mate the hub plates  14 ,  16  to the bolts. 
     Upper hub plate  14  has through holes  44 ,  46  which align and correspond to through holes  32  and  34 , respectively. Upper hub plate  14  includes a hub spring (not shown) corresponding to hub spring  36 , and a spherical center element corresponding to center element  39  of the lower hub spring assembly  17 . Together, the upper and lower hub spring assemblies allow limited movement of the hub assembly  10  with respect to the shaft as known in the art. 
     Referring to FIG. 5, a constant velocity (CV) joint assembly is shown. CV joint assembly  50  includes trunnion  52 , which is internally splined to mate with splines on the rotor shaft (not shown). Three drive links  54  are connected at equally spaced intervals around trunnion  52 , and provide a constant velocity joint for the hub assembly  10  in combination with trunnion  50 . Drive links  54  provide the required degrees of freedom for the yoke  12  and attached blades to flap relative to the rotor shaft. Use of these links is described in detail in U.S. Pat. No. 5,186,686, assigned to Lord Corporation, which is incorporated by reference herein as if set forth in full. 
     Each link  54  has a first end  56  rotatably coupled to trunnion  52 , and a second, free, end  58  rotatably coupled to a pillow block  60 . Each pillow block  60  carries two studs  62 , which will extend through the appropriate holes in the top and bottom hub plates. Studs  62  will each extend through one set of holes  32  and  44 , or the other set of holes,  34  and  46 , depending on the orientation of the CV joint  50 . 
     As can be seen in FIG. 5, CV joint  50  is symmetrical about a plane passing through trunnion  52  at an orientation perpendicular to the rotor shaft. By simply flipping CV joint assembly  50  about this plane, pillow block studs  62  will be aligned to match up with one set or the other os the holes in hub plates  14  and  16 . CV joint  50  should be oriented so that, as the shaft rotates, first end  56  leads its corresponding free end  58 . 
     FIG. 6 illustrates final assembly of the unit  10 , with upper hub plate  14 , and its associated spring assembly, removed for clarity. As shown, CV joint assembly  50  rests within opening  26  in the middle of yoke  12 . Preferably, CV joint assembly  50  is substantially coplanar with yoke  12 . In the preferred embodiment, CV joint  50  is approximately 0.36 inches above the plane of yoke  12 , but it will be recognized that the relative locations of CV joint  50  and yoke  12  may change while still using the teachings herein. Shown in FIG. 6 are the previously described elements, assembled except for upper hub plate  14 , and additionally shown are inboard anchoring elements  64 . Elements  64  are conventionally constructed, and carry chord and beam loads from the blade. Elements  64  are connected only to the yoke  12 , and do not make any contact with pillow block assembly  50  or either hub plate  14 ,  16 . The torque path through assembly  10  is as follows: torque is transferred from the shaft through trunnion  52  to the three links  54 . It is then transferred from the drive links to the pillow blocks  60 , and then to the upper and lower hub plates  14 ,  16  through pillow block studs  62 . Finally, torque is transferred through hub plates  14 ,  16  to yoke  12  through bolts  42  and their associated bushings. Thus, hub plates  14 ,  16  are an integral part of the torque transfer path from the shaft to the blades. The hub plates and yoke act as a unit, improving efficiency of torque transfer in the system. 
     In the preferred embodiment, trunnion  50  is fabricated from high strength stainless steel to provide the required strength for this highly stressed part. The drive links and pillow blocks are formed from titanium, which combines suitable weight with adequate strength and fatigue properties. Upper and lower hub plates can be fabricated from aluminum. Due to their size, hub plates are preferably low weight combined with adequate strength, and aluminum fits this requirement for a light to medium duty tilt rotor aircraft. 
     The pitch bearings are preferably aluminum with uniball bearings and teflon liners as known in the art. The bolts and bushings used to attach the hub plates to the yoke are preferably high strength steel, while the yoke itself is a fiber composite material. The yoke can be formed using any of several specialized techniques known in the industry. An example of a suitable technology which can be used to form the yoke is described in U.S. Pat. No. 4,293,276, which is hereby incorporated by reference. 
     The preferred yoke uses three separate sets of fiber belts, wound between two arms for each set. For example, referring back to FIG. 2, one belt set can be wound between the left arm and that toward the lower right as shown in the figure. The belt is wound around hollow posts used to define bolt holes  22 . Multiple belts are stacked in sets to obtain the required thickness of the yoke. The preferred embodiment includes  8  belts in each set to result in the desired yoke thickness. 
     In a preferred embodiment, the rotor shaft is  4  inches in diameter, and each yoke arm  20  is 18 to 24 inches long. Of course, these sizes can be varied as needed to work with any design appropriate for the aircraft being built. 
     In summary, in improved prop rotor hub assembly has been described which utilizes an in-plane design. The design is substantially symmetrical around a plane passing through the center of the yoke. All of the torque transfer elements, including the described trunnion, drive links, pillow blocks, hub plates, and yoke, are all symmetrical about this plane. As will be appreciated by those skilled in the art, the assembly need not be precisely symmetrical to reap the benefits of the invention. 
     One feature of the design is the ability to easily adjust the in-plane stiffness of the overall hub assembly by properly locating the bolts which couple the hub plates to the yoke. This allows relatively simple design changes to be implemented as design criteria are changed, and these changes do not require expensive retooling for the yoke. This technique can be used with rotor designs other than that described herein in order to achieve required stiffness within the assembly. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.