Abstract:
A rotor assembly includes a rotor hub rotatable about a central axis including a plurality of rotor hub arms and blade yokes, each blade yoke including yoke arms located at opposing lateral sides of a corresponding one of the rotor hub arms to which the rotor hub arm is operably connected. The rotor assembly further includes blade retention bearings, each blade retention bearing being a single-element bearing disposed at a corresponding one of the rotor hub arms and which is supportive of a corresponding one of the blade yokes. Rotor blades are secured to corresponding ones of the blade yokes.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to the art of rotary wing aircraft and, more specifically, to rotor systems for rotary wing aircraft. 
         [0002]    The rotor systems of rotary wing aircraft, for example, helicopters, are a major source of aerodynamic drag during operation of the helicopter. The rotor system typically includes a rotor hub with a number of rotor blades secured to and extending from the rotor hub. In many rotor systems, the rotor blades are connected to the rotor hub via an intermediate component or components, such as a yoke. The yoke is fixed to the rotor blade and rotatably secured to the rotor hub via a bearing set to allow for rotation of the rotor blade about a rotor blade axis, otherwise known as pitch change. Further, to enable and/or control pitch change of the rotor blades, pitch change arms connect to the yoke, such that movement of the pitch change arms adjust pitch of the rotor blades. Moreover, helicopter rotor systems often include large, bulky vibration absorbers at the rotor hub. 
         [0003]    Rotor systems have previously included fairings in an attempt to reduce aerodynamic drag, but the fairings were unable to enclose the entire rotor system due to the design or requirements of one or more of the components described above. Often, one or more of the components is left protruding from the fairing or the resultant fairing-enclosed rotor system has much less than ideal impact on aerodynamics, because the attempt to make the rotor system more aerodynamic occurs after the rotor system has already been designed and configured, causing the fairing to be designed around existing features of the rotor system. 
       BRIEF DESCRIPTION 
       [0004]    In one embodiment, a rotor assembly includes a rotor hub rotatable about a central axis including a plurality of rotor hub arms and blade yokes, each blade yoke including yoke arms located at opposing lateral sides of a corresponding one of the rotor hub arms to which the rotor hub arm is operably connected. The rotor assembly further includes blade retention bearings, each blade retention bearing being a single-element bearing disposed at a corresponding one of the rotor hub arms and which is supportive of a corresponding one of the blade yokes. Rotor blades are secured to corresponding ones of the blade yokes. 
         [0005]    In another embodiment, a rotor assembly includes a rotor hub rotatable about a central axis including a plurality of rotor hub arms and blade yokes, each blade yoke including yoke arms located at opposing lateral sides of a corresponding one of the rotor hub arms to which the rotor hub arm is operably connected. The rotor assembly further includes blade pitch rods, each blade pitch rod being operably connected to a trailing edge side of one of the blade yokes, relative to a direction of rotor assembly rotation about the central axis. Rotor blades are secured to a corresponding one of the blade yokes. 
         [0006]    In yet another embodiment, a helicopter includes an airframe, a drive system, and a rotor assembly operably connected to the drive system. The rotor assembly includes a rotor hub rotatable about a central axis including a plurality of rotor hub arms and blade yokes, each blade yoke including yoke arms located at opposing lateral sides of a corresponding one of the rotor hub arms to which the rotor hub arm is operably connected. The rotor assembly further includes blade retention bearings, each blade retention bearing being a single-element bearing disposed at a corresponding one of the rotor hub arms and which is supportive of a corresponding one of the blade yokes. Rotor blades are secured to corresponding ones of the blade yokes. An aerodynamic fairing is installed over the rotor hub and at least partially over the blade yokes. 
         [0007]    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 
         [0008]    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: 
           [0009]      FIG. 1  is a schematic view of an embodiment of a helicopter; 
           [0010]      FIG. 2  is a plan view of an embodiment of a main rotor assembly; 
           [0011]      FIG. 3  is a cross-sectional view of a hub arm of a main rotor assembly; 
           [0012]      FIG. 4  is a plan view of another embodiment of a main rotor assembly; 
           [0013]      FIG. 5  is a plan view of yet another embodiment of a main rotor assembly; and 
           [0014]      FIG. 6  is a cross-sectional view of another embodiment of a rotor assembly. 
       
    
    
       [0015]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION 
       [0016]    Shown in  FIG. 1  is schematic view of an embodiment of an aircraft, in this embodiment a helicopter  10 . The helicopter  10  includes an airframe  12  with an extending tail  14  and a tail rotor  16  located thereat. While the embodiment of a helicopter  10  described herein includes an extending tail  14  and tail rotor  16 , it is to be appreciated that the disclosure herein may be applied to other types of rotor craft as well as helicopters  10  of other configurations. A main rotor assembly  18  is located at the airframe  12  and rotates about a main rotor axis  20 . The main rotor assembly  18  is driven by a drive shaft  22  connected to a power source, for example, an engine  24  by a gearbox  26 . 
         [0017]    The main rotor assembly  18  includes a rotor hub  28  located at the main rotor axis  20  and operably connected to the drive shaft  22 . A plurality of blade assemblies  30  are connected to the rotor hub  28 . As will be described in greater detail below, the main rotor assembly  18  includes a number of features to reduce aerodynamic drag of the main rotor assembly  18 , and to more easily accommodate an aerodynamic fairing (not shown) installed to the main rotor assembly  18 . While shown is a single axis rotor system, aspects of the invention can be used in multi-axis systems, such as the X2® helicopter produced by Sikorsky Aircraft Corporation. 
         [0018]    Shown in  FIG. 2  is a plan view of an embodiment of a main rotor assembly  18 . The rotor hub  28  includes a plurality of hub arms  32  extending substantially radially outwardly. A blade assembly  30  is secured at each hub arm via a blade yoke  34 . The blade yoke  34  includes a yoke base  36  to which the blade assembly  30  is secured via, for example, a plurality of screws or bolts  38 . The blade yoke  34  has two yoke arms  40  extending from the yoke base  36 , with the yoke arms  40  are located at opposing lateral sides  42  of the hub arm  32 . With the yoke arms  40  located at opposing lateral sides  42  of the hub arm  32 , the yoke arms  40  are arranged substantially horizontally. Thus, the main rotor assembly  18  is referred to as having a “horizontal yoke”, as opposed to a convention “vertical yoke” main rotor assembly where yoke arms are arranged substantially vertically. The horizontal yoke arrangement allows for more compact arrangement of components, such as blade damper  48  and blade pitch rod  50 , thus reducing an effective diameter  44  of the rotor hub  28 , which reduces aerodynamic drag, once a fairing (not shown) having the effective diameter  44  is attached. 
         [0019]    Referring to  FIG. 3 , the blade yoke  34  is connected to the hub arm  32  via a spherical elastomeric bearing  54 , which has an outer race  46  extending between the yoke arms  40  and through, for example, an arm opening  52  in the hub arm  32 . The outer race  46  of the elastomeric bearing  56  is bonded to an outermost layer of an elastomer package  56 , which is made up of many layers. An inner race  58  of the bearing  54  is attached to the hub arms  32 , positioning the outer race  46  in the arm opening  52 . In the embodiment of  FIG. 3 , the elastomeric package  56  is located such that the entirety of the bearing  54  is radially inboard of an outer diameter  60  of the blade arms  32 , thus further reducing the effective diameter  44  of the rotor hub  28  compared to a convention main rotor with back-to-back bearing elements. 
         [0020]    Referring now to  FIG. 4 , a blade pitch rod  50  is connected to each blade yoke  34 . The blade pitch rod  50  receives inputs to urge rotation of the blade yoke  34  (and thus the blade assembly  30 ) about a blade axis  62 , thus changing a pitch of the blade assembly  30 . The blade pitch rod  50  is connected to the blade yoke  34  at a yoke arm  40  closest to a trailing edge  64  of the blade assembly  30 , relative to a direction of rotation  66  of the main rotor assembly  18  about the rotor axis  20 . In some embodiments, as shown in  FIG. 4 , the blade pitch rod  50  may be located at substantially the same radial position as a flapping axis  68 . The flapping axis  68  is an axis about which the blade assembly  30  moves up and down relative to the flight path of the helicopter  10 . In other embodiments, as shown in  FIG. 5 , the blade pitch rod  50  is connected to the yoke am  40  at a radial position inboard of the flapping axis  68 . For example, the pitch rod  50  and the flapping axis  68  may be offset by an angle of about  3  degrees. Locating the blade pitch rod  50  inboard of the flapping axis  68  further reduces the effective diameter  44  of the main rotor assembly  18 . 
         [0021]    Referring to  FIG. 6 , an aerodynamic fairing  70  is installed over the rotor hub  28  and blade yokes  34 , with the blade yokes  34  extending through fairing openings  72 . While not required in all aspects, the fairing  70  can be a fiberglass and/or graphite fairing, although any material which maintains a shape can be used. Due to a combination of the horizontal yoke configuration, single element blade retention bearing  54 , and the trailing edge  64  location of the blade pitch rod  50 , a fairing diameter  74  can be greatly reduced when compared to a traditional fairing. Further, the configuration allows for the components such as blade dampers  48  to be enveloped by the fairing  70 , not protrude through the fairing  70 , which would reduce the effectiveness of the fairing  70  in reducing aerodynamic drag. Is some embodiments, the main rotor assembly  18  includes a vibration absorber  76  located, for example, vertically above the rotor hub  28 . An upper surface  78  of the vibration absorber  76  forms a portion of the fairing  70 , thus further reducing an aerodynamic profile of the main rotor assembly  18 . However, the vibration absorber  76  need not form a portion of the fairing  70  in all aspects, such as when no absorber  76  is used or where the fairing  70  also covers the absorber  76 . 
         [0022]    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. For instance, aspects can be used with propeller assemblies and/or fans where blade pitch control and compactness of design may be useful. 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.