Patent Publication Number: US-2023145902-A1

Title: Rotor Assemblies for Vehicle Propulsion

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of provisional application number 63/278,472, filed Nov. 11, 2021. 
     TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure relates, in general, to rotor assemblies operable to generate thrust for the propulsion of a vehicle and, in particular, to rotor assemblies utilizing metallic coupling assemblies that provide centrifugal force load paths between composite rotor blades and the rotor hub. 
     BACKGROUND 
     Many vehicles use rotor assemblies to generate thrust that propels the vehicle on or through the water or in the air. A typical rotor assembly has a rotating hub with a plurality of rotor blades radiating therefrom that exert a linear force upon a working fluid, such as water or air, when the rotor assembly is rotated. Specifically, due to the shape of the rotor blades, the rotational motion of the rotor assembly in the fluid causes a pressure difference between the forward and aft surfaces of the rotor blades according to Bernoulli’s principle. Many marine vehicles use screw propellers with helical blades that rotate on a horizontal shaft. Aircraft utilize a variety of rotor assemblies for propulsion including twisted airfoil shaped propellers on fixed wing aircraft, rotary wings on helicopters and proprotors on tiltrotor aircraft. 
     Amphibious air-cushion vehicles that can travel over water and land supported by a downwardly ejected cushion of air are another type of vehicle that uses rotor assemblies to generate thrust. Certain air-cushion vehicles utilize dual ducted rotor assemblies having variable pitch rotor blades to control the speed and direction of the vehicle. The versatile amphibious capability of these vehicles not only enables them to transverse deep water, shallows and reefs but also enables them to drive onto land such as beaches. For example, large air-cushion vehicles such as the Landing Craft Air Cushion (LCAC) and the Ship-to-Shore Connector (SSC) are capable of accessing more than seventy percent of the world’s coastline for rapid deployment of large payloads such as vehicles, heavy equipment, supplies and troops. In addition, these large air-cushion vehicles are valuable in supporting humanitarian relief efforts throughout the world including delivering life-saving equipment, food, water and medical supplies. 
     SUMMARY 
     In a first aspect, the present disclosure is directed to a rotor assembly for generating thrust for a vehicle. The rotor assembly includes a rotor hub and a plurality of rotor blade assemblies coupled to the rotor hub. Each rotor blade assembly includes a metallic bearing race, a composite rotor blade and a metallic coupling assembly. The composite rotor blade has a root section with a radially outwardly tapered outer surface. The metallic coupling assembly has a radially inwardly tapered inner surface that receives the radially outwardly tapered outer surface of the root section of the rotor blade therein to provide a centrifugal force seat for the rotor blade. The coupling assembly includes at least two circumferentially distributed coupling members. The coupling assembly is configured to couple the rotor blade to the bearing race and to provide a centrifugal force load path therebetween. 
     In some embodiments, the rotor assembly may include a duct and a plurality of stators that couple the duct to the rotor hub such that the rotor blade assemblies are disposed within the duct. In certain embodiments, each bearing race may include a ball bearing race and a roller bearing race. In some embodiments, each bearing race may be a steel bearing race, each rotor blade may be a carbon fiber rotor blade and/or each coupling assembly may be a titanium coupling assembly. In certain embodiments, for each rotor blade assembly, the bearing race may include a flange end and the coupling assembly may include a flange end. In such embodiments, the flange end of the bearing race may be coupled to the flange end of the coupling assembly with a plurality of bolts that provide a centrifugal force load path between the coupling assembly and the bearing race. In some embodiments, for each rotor blade assembly, the bearing race may include a radially outwardly extending conical end and the coupling assembly may include a radially inwardly extending conical end. In such embodiments, the conical end of the bearing race may be received within the conical end of the coupling assembly to provide a centrifugal force load path between the coupling assembly and the bearing race. Also, in such embodiments, a conical wear ring, such as a segmented conical wear ring, may be disposed between the conical end of the bearing race and the conical end of the coupling assembly to provide a sacrificial element therebetween. 
     In certain embodiments, for each rotor blade assembly, the coupling assembly may include first and second coupling members each having a flange end. In such embodiments, the flange end of the first coupling member may be coupled to the flange end of the second coupling member with a plurality of bolts to circumferentially secure the coupling assembly about the root section of the rotor blade. In some embodiments, each rotor blade assembly may include at least one circumferential band positioned around the coupling assembly that is configured to circumferentially secure the coupling assembly about the root section of the rotor blade. In certain embodiments, each rotor blade assembly may include a metallic ring disposed within the root section of the rotor blade radially opposite of the radially outwardly tapered outer surface. In some embodiments, each rotor blade assembly may include an anti-rotation element configured to prevent relative rotation between the coupling assembly and the rotor blade. For example, the anti-rotation element may be at least one anti-rotation key disposed between the coupling assembly and the rotor blade or a polygonal interface, such as an octagonal interface, between the coupling assembly and the rotor blade. 
     In a second aspect, the present disclosure is directed to a vehicle operable for forward motion responsive to thrust. The vehicle includes a rotor assembly having a rotor hub and a plurality of rotor blade assemblies coupled to the rotor hub. Each rotor blade assembly includes a metallic bearing race, a composite rotor blade and a metallic coupling assembly. The composite rotor blade has a root section with a radially outwardly tapered outer surface. The metallic coupling assembly has a radially inwardly tapered inner surface that receives the radially outwardly tapered outer surface of the root section of the rotor blade therein to provide a centrifugal force seat for the rotor blade. The coupling assembly includes at least two circumferentially distributed coupling members. The coupling assembly is configured to couple the rotor blade to the bearing race and to provide a centrifugal force load path therebetween. 
     In some embodiments, the rotor assembly may be configured to produce thrust when rotating in a working fluid of air. In certain embodiments, the vehicle may be is an amphibious air-cushion vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the features and advantages of the present disclosure, reference is now made to the detailed description along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
         FIGS.  1 A- 1 C  are schematic illustrations of an amphibious air-cushion vehicle having dual ducted rotor assemblies for generating thrust in accordance with embodiments of the present disclosure; 
         FIGS.  2 A- 2 B  are schematic illustrations of a ducted rotor assembly for generating vehicle thrust in accordance with embodiments of the present disclosure; 
         FIGS.  3 A- 3 B  are front and cross sectional views of a metallic coupling assembly coupling a composite the rotor blade to a metallic bearing race to provide a centrifugal force load path therebetween in accordance with embodiments of the present disclosure; 
         FIGS.  4 A- 4 B  are front and cross sectional views of a metallic coupling assembly coupling a composite the rotor blade to a metallic bearing race to provide a centrifugal force load path therebetween in accordance with embodiments of the present disclosure; and 
         FIGS.  5 A- 5 B  are front and cross sectional views of a metallic coupling assembly coupling a composite the rotor blade to a metallic bearing race to provide a centrifugal force load path therebetween in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     While the making and using of various embodiments of the present disclosure are discussed in detail below, it should be appreciated that the present disclosure provides many applicable inventive concepts, which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative and do not delimit the scope of the present disclosure. In the interest of clarity, not all features of an actual implementation may be described in the present disclosure. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer’s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     In the specification, reference may be made to the spatial relationships between various components and to the spatial orientation of various aspects of components as the devices are depicted in the attached drawings. However, as will be recognized by those skilled in the art after a complete reading of the present disclosure, the devices, members, apparatuses, and the like described herein may be positioned in any desired orientation. Thus, the use of terms such as “above,” “below,” “upper,” “lower” or other like terms to describe a spatial relationship between various components or to describe the spatial orientation of aspects of such components should be understood to describe a relative relationship between the components or a spatial orientation of aspects of such components, respectively, as the device described herein may be oriented in any desired direction. As used herein, the term “coupled” may include direct or indirect coupling by any means, including moving and/or non-moving mechanical connections. 
     Referring to  FIGS.  1 A- 1 C  in the drawings, an amphibious air-cushion vehicle is schematically illustrated and generally designated  10 . Vehicle  10  includes hull  12  having a bow portion  14 , a stern portion  16 , a port side  18  and a starboard side  20 . In the illustrated embodiment, vehicle  10  includes an open deck space  22 , a bow ramp  24  and a stern ramp  26  enabling roll-on/roll-off loading and unloading of cargo and equipment including, for example, high-speed offloading during a cargo drop scenario into a hostile landing area. Vehicle  10  is equipped with dual ducted air propulsors depicted as port rotor assembly  28  and starboard rotor assembly  30 . Rotor assembly  28  is driven by a gas turbine engine  32  via a drive shaft  34 . Rotor assembly  30  is driven by a gas turbine engine  36  via a drive shaft  38 . Gas turbine engine  32  also drives a lift fan  40  and a bow thruster  42 . Likewise, gas turbine engine  36  also drives a lift fan  44  and a bow thruster  46 . Lift fans  40 ,  44  are used to produce large volumes air that form an air cushion below hull  12  and within a skirt  48  that lifts hull  12  such that vehicle  10  floats above a running surface such as water or land. Bow thrusters  42 ,  46  are used to enhance the maneuverability of vehicle  10  particularly at lower speeds. Vehicle  10  includes a pilot and copilot module  50  that supports the command and control station of vehicle  10 . Vehicle  10  also includes a personnel and equipment module  52 . 
     Referring additionally to  FIGS.  2 A- 2 B  in the drawings, rotor assembly  28  is disclosed in further detail. Rotor assembly  30  is substantially similar to rotor assembly  28  therefore, for sake of efficiency, certain features will be disclosed only with regard to rotor assembly  28 . One having ordinary skill in the art, however, will fully appreciate an understanding of rotor assembly  30  based upon the disclosure herein of rotor assembly  28 .  FIG.  2 A  depicts rotor assembly  28  from the inlet side or a forward perspective and  FIG.  2 B  depicts rotor assembly  28  from the outlet side or an aft perspective. A forward screen, as best seen in  FIG.  1 A , has been removed from rotor assembly  28  for clarity. Rotor assembly  28  is coupled to hull  12  of vehicle  10  by a suitable support structure depicted as a pair of pedestals  54 ,  56 . Rotor assembly  28  includes a generally cylindrical outer duct  58  that is preferrable formed from a strong and lightweight material such as a carbon fiber composite. In the illustrated embodiment, rotor assembly  28  includes a stator system  60  having seven twisted stators that are formed from a strong and lightweight material such as a carbon fiber composite. In other embodiments, stator system  60  may include other numbers of stators both greater than or less than seven, the stators could have other designs such as straight stators and/or the stators could be formed from other materials such as metals including aluminum. 
     Stator system  60  supports a rotor hub  62  within duct  58  of rotor assembly  28 . Rotor hub  62  includes a fixed portion  62   a  that is coupled to stator system  60  and a rotating portion  62   b  that is coupled to a plurality of rotor blade assemblies  64 . Rotor hub  62  houses structural and control components that support the centrifugal force generated by rotor blade assemblies  64  and enable collective pitch operations of rotor blade assemblies  64  to provide variable thrust to vehicle  10 . Specifically, each rotor blade assembly  64  is coupled to a bearing assembly  64   a  disposed within rotor hub  62 , such as a bearing assembly that includes a ball bearing set and a roller bearing set, that react centrifugal and bending loads from the rotor blade assembly  64  during rotary operations and collective pitch change operations. In the illustrated embodiment, rotor assembly  28  includes six rotor blade assemblies  64  that are formed from a strong and lightweight material such as a carbon fiber composite. In other embodiments, rotor assembly  28  may include other numbers of rotor blade assemblies both greater than or less than six. In the illustrated embodiment, rotor assembly  28  includes a pair of vertically extending rudders  66   a ,  66   b  that are coupled to the aft side of duct  58 . Rudders  66   a ,  66   b  are rotatable relative to duct  58  to control the direction of thrust from rotor assembly  28  and thus the direction of travel of vehicle  10 . 
     Referring next to  FIGS.  3 A- 3 B  in the drawings, component parts of a rotor blade assembly  100  that is representative of rotor blade assemblies  64  will now be discussed. Rotor blade assembly  100  includes a metallic bearing race  102 , depicted as including a ball bearing race  104  and a roller bearing race  106 , that is received within and supported by one of the bearing assemblies  64   a  of rotor hub  62 . Bearing race  102  includes a flange end  108  having a plurality of bolt openings. In the illustrated embodiment, bearing race  102  is a steel component machined to the desired specifications and tolerances. Rotor blade assembly  100  includes a composite rotor blade  110  with only the root section  112  being visible in the drawings. Rotor blade  110  is preferably a monolithic structure formed using a broad goods and/or layered tape construction process having a manual or automated layup of a plurality of composite broad goods material layers such as carbon fabric, carbon tape and combinations thereof. After curing, the material layers form a high strength, lightweight solid composite member. 
     Root section  112  of rotor blade  110  has a generally cylindrical shape with a constant inner diameter along its length but a changing outer diameter along its length. More specifically, root section  112  has an outboard section  114  having a first wall thickness and an inboard section  116  having a second wall thickness that is greater than the first wall thickness with a tapered section  118  therebetween having a radially outwardly tapered outer surface  120  extending from outboard section  114  to inboard section  116 . In the illustrated embodiment, the ratio of the wall thicknesses of inboard section  116  to outboard section  114  is about 2 to 1. In other embodiments, the wall thicknesses of inboard section  116  and outboard section  114  may have other ratios both greater than or less than 2 to 1 including 4 to 1, 3 to 1, 5 to 2, 3 to 2 or other desired wall thickness ratio. Radially outwardly tapered outer surface  120  may progress in a linear or nonlinear manner and may have different contours or slopes in different portions thereof. As such, it should be understood by those skilled in the art that the profile of radially outwardly tapered outer surface  120  will be determined based upon structural and dynamic analysis for the specific implementation including, for example, the centrifugal load requirement of tapered section  118 . In the illustrated embodiment, inboard section  116  of root section  112  includes a pair of oppositely disposed pockets  116   a ,  116   b . 
     In the illustrated embodiment, rotor blade assembly  100  includes a metallic coupling assembly  122  depicted as two semi-cylindrical coupling members  122   a ,  122   b  that together form a substantially cylindrical coupling assembly. In other embodiments, a multi-piece coupling assembly may include more than two circumferentially distributed coupling members that together form a substantially cylindrical coupling assembly. Coupling assembly  122  has a generally constant outer diameter along its length but a changing inner diameter along its length. More specifically, an inboard section  124  of coupling assembly  122  has a first wall thickness and an outboard section  126  of coupling assembly  122  has a second wall thickness that is greater than the first wall thickness with a tapered section  128  therebetween having a radially inwardly tapered inner surface  130  extending from inboard section  124  to outboard section  126 . In the illustrated embodiment, the ratio of the wall thicknesses of outboard section  126  and inboard section  124  is about 2 to 1. In other embodiments, the wall thicknesses of outboard section  126  and inboard section  124  may have other ratios both greater than or less than 2 to 1 including 4 to 1, 3 to 1, 5 to 2, 3 to 2 or other desired wall thickness ratio. Radially inwardly tapered inner surface  130  has a matching profile with radially outwardly tapered outer surface  120  such that radially inwardly tapered inner surface  130  provides a centrifugal force seat for radially outwardly tapered outer surface  120  and thus for rotor blade  110 . A compliant layer, such as an adhesive layer or polymer layer, may provide an interface between radially inwardly tapered inner surface  130  and radially outwardly tapered outer surface  120  that may allow for certain strain resolution between rotor blade  110  and coupling assembly  122 . In the illustrated embodiment, coupling assembly  122  includes a pair of oppositely disposed pockets  132   a ,  132   b . In addition, coupling assembly  122  includes a flange end  134  having a plurality of bolt openings. In the illustrated embodiment, coupling assembly  122  is a titanium component machined to the desired specifications and tolerances. In other embodiments, coupling assembly  122  could be formed from other metals such as aluminum. 
     An assembly process for rotor blade assembly  100  will now be described. As best seen in  FIG.  3 B , a metallic support ring  136  is disposed within root section  112  of rotor blade  110  generally radially aligned with tapered section  118 . Metallic support ring  136  provides added hoop strength to root section  112  at tapered section  118  to prevent any deformation of root section  112  responsive to centrifugal and/or bending loads. Coupling member  122   b  is then positioned relative to root section  112  such that radially inwardly tapered inner surface  130  mates with radially outwardly tapered outer surface  120  and such that pockets  116   a ,  116   b  of inboard section  116  are aligned with pockets  132   a ,  132   b  of coupling member  122   b . Anti-rotation keys  138   a ,  138   b  are placed within the cavities created respectively by pockets  116   a ,  132   a  and pockets  116   b ,  132   b . Coupling member  122   a  may now be positioned relative to root section  112  such that radially inwardly tapered inner surface  130  mates with radially outwardly tapered outer surface  120  and such that pockets  116   a ,  116   b  of inboard section  116  are aligned with pockets  132   a ,  132   b  of coupling member  122   a . In this manner, anti-rotation keys  138   a ,  138   b  are captured between rotor blade  110  and coupling assembly  122  and serve as an anti-rotation element to prevent relative rotation therebetween when rotor blade assembly  100  is fully assembled. Even though two anti-rotation keys have been depicted and described, it should be understood by those having ordinary skill in the art that any number of anti-rotation keys could be circumferentially distributed between rotor blade  110  and coupling assembly  122 . 
     The two parts of coupling assembly  122  may now be circumferentially secured together about root section  112  of rotor blade  110 . In the illustrated embodiment, this is achieved using three circumferential bands  140   a ,  140   b ,  140   c  that are received within radially inwardly projecting channels in coupling members  122   a ,  122   b . Circumferential bands  140   a ,  140   b ,  140   c  each include two semi-circumferential members that are coupled together with bolts, as best seen in  FIG.  3 A . In this manner, coupling members  122   a ,  122   b  are secured together and secured to root section  112  of rotor blade  110 . Even though circumferential bands  140   a ,  140   b ,  140   c  have been depicted and described as being secured about coupling assembly  122  with tension bolts, it should be understood by those having ordinary skill in the art that circumferential bands of the present disclosure could alternatively be secured about coupling assembly  122  with one or more shear bolts. Flange end  134  of coupling assembly  122  is now positioned relative to flange end  108  of bearing race  102  such that the respective bolt openings are aligned. A plurality of bolts  142  may now be used to secured coupling assembly  122  to bearing race  102  such that bolts  142  provide a centrifugal force load path therebetween and prevent relative rotation therebetween. In this manner, a centrifugal force load path is provided between composite rotor blade  110  and metallic bearing race  102  by metallic coupling assembly  122 . 
     Referring next to  FIGS.  4 A- 4 B  in the drawings, component parts of a rotor blade assembly  200  that is representative of rotor blade assemblies  64  will now be discussed. Rotor blade assembly  200  includes a metallic bearing race  202 , depicted as including a ball bearing race  204  and a roller bearing race  206 , that is received within and supported by one of the bearing assemblies  64   a  of rotor hub  62 . Bearing race  202  includes a radially outwardly extending conical end  208  having an outer conical surface  208   a . Rotor blade assembly  200  includes a composite rotor blade  210  with only the root section  212  being visible in the drawings. Root section  212  of rotor blade  210  has a generally cylindrical shape with a constant inner diameter along its length but a changing outer diameter along its length. More specifically, root section  212  has an outboard section  214  having a first wall thickness and an inboard section  216  having a second wall thickness that is greater than the first wall thickness with a tapered section  218  therebetween having a radially outwardly tapered outer surface  220  extending from outboard section  214  to inboard section  216 . Radially outwardly tapered outer surface  220  may progress in a linear or nonlinear manner and may have different contours or slopes in different portions thereof. In the illustrated embodiment, inboard section  216  of root section  212  includes a pair of oppositely disposed pockets  216   a ,  216   b . 
     Rotor blade assembly  200  includes a metallic coupling assembly  222  depicted as two semi-cylindrical coupling members  222   a ,  222   b  that together form a substantially cylindrical coupling assembly with a generally constant outer diameter along its length but a changing inner diameter along its length. More specifically, an inboard section  224  of coupling assembly  222  has a first wall thickness and an outboard section  226  of coupling assembly  222  has a second wall thickness that is greater than the first wall thickness with a tapered section  228  therebetween having a radially inwardly tapered inner surface  230  extending from inboard section  224  to outboard section  226 . Radially inwardly tapered inner surface  230  has a matching profile with radially outwardly tapered outer surface  220  such that radially inwardly tapered inner surface  230  provides a centrifugal force seat for radially outwardly tapered outer surface  220  and thus for rotor blade  210 . A compliant layer, such as an adhesive layer or polymer layer, may provide an interface between radially inwardly tapered inner surface  230  and radially outwardly tapered outer surface  220  that may allow for certain strain resolution between rotor blade  210  and coupling assembly  222 . In the illustrated embodiment, coupling assembly  222  includes a pair of oppositely disposed pockets  232   a ,  232   b . In addition, coupling assembly  222  includes a radially inwardly extending conical end  234  having an inner conical surface  234   a . 
     An assembly process for rotor blade assembly  200  will now be described. As best seen in  FIG.  4 B , a metallic support ring  236  is disposed within root section  212  of rotor blade  210  generally radially aligned with tapered section  218 . An upper surface of bearing race  202  is positioned relative to a lower surface of rotor blade  210 . A conical wear ring  244 , such as a segmented conical wear ring, is positioned proximate outer conical surface  208   a  of bearing race  202 . Coupling member  222   b  is positioned relative to root section  212  such that radially inwardly tapered inner surface  230  mates with radially outwardly tapered outer surface  220  and such that pockets  216   a ,  216   b  of inboard section  216  are aligned with pockets  232   a ,  232   b  of coupling member  222   b . At the same time, coupling member  222   b  is positioned relative to bearing race  202  such that radially outwardly extending conical end  208  of bearing race  202  is positioned within radially inwardly extending conical end  234  with conical wear ring  244  positioned between inner conical surface  234   a  and outer conical surface  208   a . Conical wear ring  244  is a metal wear ring that is softer than the metal of bearing race  202  and coupling assembly  222  such that conical wear ring  244  acts as a sacrificial element. In certain embodiments, inner conical surface  234   a  and/or outer conical surface  208   a  may be surface treated such that they are much harder than conical wear ring  244 . Anti-rotation keys  238   a ,  238   b  are placed within the cavities created respectively by pockets  216   a ,  232   a  and pockets  216   b ,  232   b . 
     Coupling member  222   a  may now be positioned relative to root section  212  such that radially inwardly tapered inner surface  230  mates with radially outwardly tapered outer surface  220  and such that pockets  216   a ,  216   b  of inboard section  216  are aligned with pockets  232   a ,  232   b   of coupling member  222   a . At the same time, coupling member  222   a  is positioned relative to bearing race  202  such that radially outwardly extending conical end  208  of bearing race  202  is positioned within radially inwardly extending conical end  234  with conical wear ring  244  positioned between inner conical surface  234   a  and outer conical surface  208   a . In this manner, anti-rotation keys  238   a ,  238   b  are captured between rotor blade  210  and coupling assembly  222  and serve as an anti-rotation element to prevent relative rotation therebetween when rotor blade assembly  200  is fully assembled. The two parts of coupling assembly  222  may now be circumferentially secured together about root section  212  of rotor blade  210  and radially outwardly extending conical end  208  of bearing race  202 . In the illustrated embodiment, this is achieved using three circumferential bands  240   a ,  240   b ,  240   c  that are received within radially inwardly projecting channels in coupling members  222   a ,  222   b . Circumferential bands  240   a ,  240   b ,  240   c  each include two semi-circumferential members that are coupled together with bolts, as best seen in  FIG.  4 A . In this manner, coupling members  222   a ,  222   b  are secured together and secured to root section  212  of rotor blade  210  and radially outwardly extending conical end  208  of bearing race  202 . In this manner, a centrifugal force load path is provided between composite rotor blade  210  and metallic bearing race  202  by metallic coupling assembly  222 . In the illustrated embodiment, the friction between inner conical surface  234   a , conical wear ring  244  and outer conical surface  208  generated by the centrifugal force supported between metallic bearing race  202  and metallic coupling assembly  222  during rotary operations, provides an anti-rotation element therebetween. 
     Referring next to  FIGS.  5 A- 5 B  in the drawings, component parts of a rotor blade assembly  300  that is representative of rotor blade assemblies  64  will now be discussed. Rotor blade assembly  300  includes a metallic bearing race  302 , depicted as including a ball bearing race  304  and a roller bearing race  306 , that is received within and supported by one of the bearing assemblies  64   a  of rotor hub  62 . Bearing race  302  includes a radially outwardly extending conical end  308  having an outer conical surface  308   a . Bearing race  302  also includes radially extending lugs  302   a ,  302   b . Rotor blade assembly  300  includes a composite rotor blade  310  with only the root section  312  being visible in the drawings. Root section  312  of rotor blade  310  has a generally cylindrical shape with a constant inner diameter along its length but a changing outer diameter along its length. More specifically, root section  312  has an outboard section  314  having a first wall thickness and an inboard section  316  having a second wall thickness that is greater than the first wall thickness with a tapered section  318  therebetween having a radially outwardly tapered outer surface  320  extending from outboard section  314  to inboard section  316 . Radially outwardly tapered outer surface  320  may progress in a linear or nonlinear manner and may have different contours or slopes in different portions thereof. In the illustrated embodiment, inboard section  316  of root section  312  has a polygonal shaped outer perimeter depicted as an octagonal shaped outer perimeter  316   a . 
     Rotor blade assembly  300  includes a metallic coupling assembly  322  depicted as two semi-cylindrical coupling members  322   a ,  322   b  each having a flange end that together form a substantially cylindrical coupling assembly with a generally constant outer diameter along its length but a changing inner diameter along its length. More specifically, an inboard section  324  of coupling assembly  322  has a first wall thickness and an outboard section  326  of coupling assembly  322  has a second wall thickness that is greater than the first wall thickness with a tapered section  328  therebetween having a radially inwardly tapered inner surface  330  extending from inboard section  324  to outboard section  326 . Radially inwardly tapered inner surface  330  has a matching profile with radially outwardly tapered outer surface  320  such that radially inwardly tapered inner surface  330  provides a centrifugal force seat for radially outwardly tapered outer surface  320  and thus for rotor blade  310 . A compliant layer, such as an adhesive layer or polymer layer, may provide an interface between radially inwardly tapered inner surface  330  and radially outwardly tapered outer surface  320  that may allow for certain strain resolution between rotor blade  310  and coupling assembly  322 . In the illustrated embodiment, coupling assembly  322  has a polygonal shaped inner perimeter depicted as an octagonal shaped inner perimeter  332 . In addition, coupling assembly  322  includes a radially inwardly extending conical end  334  having an inner conical surface  334   a . 
     An assembly process for rotor blade assembly  300  will now be described. As best seen in  FIG.  5 B , a metallic support ring  336  is disposed within root section  312  of rotor blade  310  generally radially aligned with tapered section  318 . An upper surface of bearing race  302  is positioned relative to a lower surface of rotor blade  310 . A conical wear ring  344 , such as a segmented conical wear ring, is positioned proximate outer conical surface  308   a  of bearing race  302 . Coupling member  322   b  is positioned relative to root section  312  such that radially inwardly tapered inner surface  330  mates with radially outwardly tapered outer surface  320  and such that octagonal shaped outer perimeter  316   a  mates with octagonal shaped inner perimeter  332 . At the same time, coupling member  322   b  is positioned relative to bearing race  302  such that radially outwardly extending conical end  308  of bearing race  302  is positioned within radially inwardly extending conical end  334  with conical wear ring  344  positioned between inner conical surface  334   a  and outer conical surface  308   a . Lugs  302   a ,  302   b  are received within notches in the flange end of coupling member  322   b . 
     Coupling member  322   a  may now be positioned relative to root section  312  such that radially inwardly tapered inner surface  330  mates with radially outwardly tapered outer surface  320  and such that octagonal shaped outer perimeter  316   a  mates with octagonal shaped inner perimeter  332 . At the same time, coupling member  322   a  is positioned relative to bearing race  302  such that radially outwardly extending conical end  308  of bearing race  302  is positioned within radially inwardly extending conical end  334  with conical wear ring  344  positioned between inner conical surface  334   a  and outer conical surface  308   a . Lugs  302   a ,  302   b  are also received within notches in the flange end of coupling member  322   a . In this manner, octagonal shaped outer perimeter  316   a  and octagonal shaped inner perimeter  332  create a polygonal interface and in this case an octagonal interface that serves as an anti-rotation element to prevent relative rotation between coupling assembly  322  and rotor blade  310  when rotor blade assembly  300  is fully assembled. The two parts of coupling assembly  322  may now be circumferentially secured together about root section  312  of rotor blade  310  and radially outwardly extending conical end  308  of bearing race  302 . In the illustrated embodiment, this is achieved using a plurality of bolts  340  to couple flange ends of coupling members  322   a ,  322   b  together, as best seen in  FIG.  5 A . In this manner, coupling members  322   a ,  322   b  are secured together and secured to root section  312  of rotor blade  310  and radially outwardly extending conical end  308  of bearing race  302 . A centrifugal force load path is provided between composite rotor blade  310  and metallic bearing race  302  by metallic coupling assembly  322 . In addition, as lugs  302   a ,  302   b  of bearing race  302  are clamped and bolted between the flange ends of coupling members  322   a ,  322   b , this provides an anti-rotation element between metallic coupling assembly  322  and bearing race  302 . 
     Compared to prior mechanisms for coupling composite rotor blades to metallic bearing races, the present embodiments provide for the use of a larger diameter root section without requiring a larger diameter hub, thereby increasing the root to chord ratio of the rotor blade as well as the strength and durability of the root section thereof. In addition, the use of the coupling assemblies of the present embodiments, allows for inspection of the rotor blades and in particular inspection of the root sections of the rotor blades as each rotor blade is removable from the rotor hub by disconnection of the coupling assembly. This removability feature also allows for the replacement of wearable components or any individual components of the rotor assembly as requiring according to a maintenance schedule or as discovered during an inspection. 
     The foregoing description of embodiments of the disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principals of the disclosure and its practical application to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the embodiments without departing from the scope of the present disclosure. Such modifications and combinations of the illustrative embodiments as well as other embodiments will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.