Abstract:
A multi-bladed tail rotor assembly is disclosed that provides higher aerodynamic performance, damage tolerant design with 10,000-hour life expectancy, and which requires low maintenance through the use of composites and elastomerics. The tail rotor hub assembly includes two stacked yoke assemblies having multi-bladed teetering rotors, each mounted on a single drive mast. Each yoke assembly includes a yoke hub having a transverse bore therethrough, a bearing assembly disposed within the bore, and retention means for aligning and securing the bearing assembly within the bore. Each bearing assembly includes a trunnion portion having trunnion arms that extend outwardly from a trunnion body portion, and an elastomeric bearing disposed about each trunnion arm. The tail rotor assembly utilizes a composite twist strap flexure to accommodate collective pitch control integral with each rotor blade.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/273,534, filed Mar. 6, 2001, titled “Four-Bladed Tail Rotor Hub Design for Coriolis Relief,” and U.S. Provisional Application No. 60/289,265, filed May 7, 2001, titled “Elastomeric Bearing and Trunnion Rotor Hub Assembly.” 
    
    
     GOVERNMENT LICENSE RIGHTS 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00019-96-C-0128 awarded by NAVAIR. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to tail rotors for helicopters and other rotary wing aircraft. In particular, the present invention relates to a multi-bladed tail rotors and their ability to accommodate potentially powerful Coriolis torque. 
     2. Description of Related Art 
     One of the significant challenges involved with the design of multi-bladed tail rotors is their ability to accommodate potentially powerful Coriolis torque. When the rotor plane of a helicopter rotor is tilted relative to the shaft, 1/rev and 2/rev Coriolis torque is generated. Because the 1/rev Coriolis torque is proportional to the coning angle, it is usually negligible for most tail rotors. For two-bladed tail rotors, the 2/rev Coriolis is also not a problem because both blades speed up and slow down at the same time, and the drive system is usually sufficiently flexible to provide the necessary torsional freedom. However, the 2/rev Coriolis torque becomes a problem with multi-bladed tail rotors when no lead-lag articulation is provided. 
     Various methods are used on existing helicopters with multi-bladed tail rotors to provide the necessary relief for 2/rev Coriolis torque. For example: the Sikorsky S-56 uses a fully articulated rotor having lead-lag hinges and dampers; the Sikorsky S-61 has a flexible spindle at the blade root combined with restricted flapping motion to limit stresses due to Coriolis; the Kaman UH-2 allows a small amount of lead-lag motion by using a rocking pin arrangement in its flapping hinge; and the Lockheed AH-56 uses a gimbaled tail rotor hub that relieves the 2/rev Coriolis torque in the same manner as a two-bladed teetering rotor. Unfortunately, all of these approaches tend to be heavy and complex. They each require highly loaded bearings oscillating at tail rotor frequency. This results in a design that requires a lot of maintenance and a significant amount of downtime. 
     One of the ways to approach this problem is to mount two, two-bladed rotors on the same shaft. This arrangement provides a four-bladed tail rotor with the mechanical and structural simplicity of a two-bladed teetering rotor. By using this concept, no bearings are required to oscillate while carrying the full centrifugal force of the blade. 
     The AH-1Z/UH-1Y tail rotor also utilizes this approach, where two 2-bladed rotors are mounted on the same drive shaft. Each assembly is a two-bladed teetering rotor; they are independently mounted on a single output shaft. The span wise axes of the blade-pairs are perpendicular to each other, and are separated axially to provide adequate space for accommodating hub attachment hardware and operational clearance between them. However, this configuration does not inherently provide relief for the 2/rev Coriolis torque. Whenever the tail rotor experiences first harmonic flapping, one pair of blades is trying to speed up at the same instant in time that the other pair of blades is trying to slow down. Thus, the two rotors are trying to move like a pair of scissors. 
     This approach has been used on several research and production models throughout the rotorcraft industry. Bell Helicopter Textron Inc. has successfully flown a double-teetering tail rotor with coaxial shafts on one of its research aircraft. The AH-64D Apache uses a double-teetering tail rotor with flexible forks. While both these approaches provide the desired relief for 2/rev Coriolis torque, there are several disadvantages associated with each one: the mechanical complexity, heavier design, problems associated with tailoring stiffness of critical metal parts—possibly resulting in a degraded structural design and potentially catastrophic failure modes—just to name a few. 
     Although the foregoing approaches represent significant strides in the area of tail rotor design, significant challenges remain with regard to the ability of multi-bladed tail rotors to accommodate this potentially powerful Coriolis torque. 
     SUMMARY OF THE INVENTION 
     While various multi-bladed tail rotor designs presently in use compensate for Coriolis torque differently, the tail rotor system of the present invention offers a simpler and more cost-effective solution by making use of existing parts that are required to perform other functions. 
     There is a need for a multi-bladed tail rotor system that can accommodate potentially powerful Coriolis torque without the need for heavy, complex components, such as highly loaded bearings oscillating at tail rotor frequencies. 
     Therefore, it is an object of the present invention to provide a multi-bladed tail rotor system that can accommodate 2/rev Coriolis torque without the need for heavy, complex components that require significant maintenance and downtime. 
     This object is achieved by providing a four-bladed tail rotor system in which 2/rev Coriolis relief is provided by optimizing the dynamic characteristics of an existing component in the system, i.e., an elastomeric bearing that accommodates rotor flapping. The tail rotor system of the present invention utilizes two stacked two-bladed teetering rotors, each rotor pair being mounted onto the same single drive shaft through a unique rotor yoke assembly. The span wise axes of the two pairs of blades are perpendicular to each other and are separated axially to provide adequate space for accommodating hub attachment hardware and operational clearance. Each rotor yoke assembly is mounted to the drive shaft with a bearing and trunnion assembly in which a pair of trunnion arms having a generally conical shape extend radially outward from a cylindrical body portion. 
     The trunnion arms are preferably shaped to fit securely within an elastomeric bearing. The elastomeric bearings may be either molded to the trunnion arms or pre-molded and secured to the trunnion arms after molding. A rigid sleeve is disposed around each elastomeric bearing. These sleeves are configured to fit securely within a transverse bore that passes through each rotor yoke. The elastomeric bearings and sleeves are held in place within the yoke by retention fittings that are coupled to the rotor yokes at each end of the transverse bore. The sleeves may include stop members that are received by the retention fittings to limit the movement of the yoke relative to the drive shaft. 
     In the preferred embodiment of the present invention, an inboard bearing and trunnion assembly, a hub adapter, and an outboard bearing and trunnion assembly are coupled together on the drive shaft by an inboard cone, an outboard cone, and a mast nut. Drive torque is transferred from the drive shaft to the inboard bearing and trunnion assembly through splines on the exterior of the drive shaft which mate with splines on the interior of the body portion of the inboard bearing and trunnion assembly. The drive torque is transferred from the inboard bearing and trunnion assembly to the hub adapter through a toothed coupling on one end of the hub adapter, and from the hub adapter to the outboard bearing and trunnion assembly through another toothed coupling on the other end of the hub adapter. 
     The multi-bladed tail rotor system according to the present invention provides the significant advantages. Conventional teetering rotors that use elastomeric bearings to provide flapping degrees of freedom, require that the radial stiffness of the bearings to be very high to minimize radial deflection under rotor torque. However, in the multi-bladed tail rotor system according to the present invention, the radial stiffness of a uniquely designed elastomeric flapping bearing is tailored to provide adequate stiffness to react to rotor torque and to provide adequate softness to relieve the 2/rev Coriolis torque, without adding additional hardware. Because this Coriolis relief is provided by tailoring the spring rate of an existing component, the resulting hub assembly provides a much simpler configuration with reduced weight and cost, and higher reliability due to reduction in the number of parts in the system. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. However, the invention itself, as well as, a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a helicopter having a multi-bladed tail rotor assembly according to the present invention. 
     FIG. 2 is a perspective view of the multi-bladed tail rotor assembly according to the present invention. 
     FIG. 3 is an enlarged perspective view of the multi-bladed tail rotor assembly of FIG.  2 . 
     FIG. 4 is an exploded view of the multi-bladed tail rotor hub assembly of FIG.  3 . 
     FIG. 5A is a cut-away view of the mast and trunnion assemblies of the multi-bladed tail rotor hub assembly according to the present invention. 
     FIG. 5B is a perspective view of one of the bearing and trunnion assemblies of the multi-bladed tail rotor hub assembly according to the present invention. 
     FIG. 5C is an exploded view of the bearing and trunnion assembly of FIG.  5 B. 
     FIGS. 6 through 9 are principal axis views of one of the tail rotor yoke assemblies of the multi-bladed tail rotor hub assembly according to the present invention. 
     FIG. 10 is a perspective view of a rotor blade of the multi-bladed tail rotor assembly according to the present invention. 
     FIG. 11 is an enlarged perspective view of the rotor blade cuff of the rotor blade of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 in the drawings, an aircraft  10  having a multi-bladed tail rotor hub assembly for Coriolis relief according to the present invention is illustrated. Aircraft  10  comprises a fuselage  12  and a main rotor  14 . Torque imparted to fuselage  12  by main rotor  14  is counter-acted by a multi-bladed tail rotor assembly  16  mounted on a tail portion  22  of fuselage  12 . Main rotor  14  and multi-bladed tail rotor assembly  16  are powered by a drive means  18  under the control of a pilot in a cockpit  20 . 
     It will be noted that tail rotor assembly  16  of aircraft  10  is a “pusher” type design, wherein tail rotor assembly  16  is located on the left side of aircraft  10  looking forward. This design is desirable because in conventional “tractor” designs in which the tail rotor assembly is located on the right side of the aircraft, side loading of tail portion  22  caused by the tail rotor wake has been shown to subtract significantly from available tail rotor thrust. In certain instances, a net thrust loss due to the interference of tail portion  22  may be as high as twenty percent. For this reason, in the preferred embodiment of the present invention, tail rotor assembly  16  is located on the “pusher” side of tail portion  22 . The fin-to-tail rotor separation distance has been optimized for weight, flapping clearance, and aerodynamic efficiency. 
     In the preferred embodiment of the present invention, multi-bladed tail rotor assembly  16  utilizes four rotor blades. The use of four blades provides lower blade loading, i.e., thrust per blade, as compared to two-blade designs. The use of four rotor blades results in improved aerodynamic performance due to lower tip losses associated with high aspect ratio blades. The use of four tail rotor blades also results in reduced control loads. 
     Referring now to FIGS. 2 through 4 in the drawings, tail rotor assembly  16  of aircraft  10  is illustrated in perspective views. FIG. 2 is an assembled view of tail rotor assembly  16 , FIG. 3 is an enlarged assembled view of tail rotor assembly  16 , and FIG. 4 is an exploded view of a tail rotor hub assembly  16   a . Tail rotor hub assembly  16   a  includes a mast  30  having a mast axis  31  coupled to two virtually identical hub assemblies: an outboard hub assembly and an inboard hub assembly. The outboard hub assembly includes an outboard yoke  32 , an outboard trunnion and elastomeric bearing assembly  56  carried within outboard yoke  32 , and outboard retention fittings  64  coupled to the ends of outboard yoke  32 . Two outboard blades  34  are coupled to outboard yoke  32 , as will be described in detail below. Although not completely visible in the figures, the inboard hub assembly includes an inboard yoke  33 , an inboard trunnion and elastomeric bearing assembly  57  carried within inboard yoke  33 , and inboard retention fittings  59  coupled to the ends of inboard yoke  33 . The outboard hub assembly is spatially separated from the inboard hub assembly by a hub adapter  54 , as will be described in further detail below. As will be explained in more detail below, inboard hub assembly, outboard hub assembly, and hub adapter  54  are sandwiched together and held in place over mast  30  by an inboard cone  94  (see FIG.  5 A), an outboard cone  58 , a spacer  61 , and a mast nut  60 . 
     Outboard trunnion and elastomeric bearing assembly  56  is held in place within outboard yoke by retention fittings  64 . Likewise, inboard trunnion and elastomeric bearing assembly  57  is held in place within inboard yoke  33  by retention fittings  59 . Retention fittings  64  are coupled to outboard yoke  32  by bolts  66  that pass through bores  78  (see FIG.  6 ). Retention fittings  59  are coupled to inboard yoke  33  in a similar fashion. In the preferred embodiment, an additional lug  80  integral to retention fitting  64  provides one of the redundant load paths for the rotor blade to yoke attachment. 
     A rotating control system  41  is oriented generally coaxial with and on the outside of mast  30 . Rotating control system  41  includes a rotating crosshead  44 , a thrust bearing housing  42 , a thrust bearing  43 , an input lever  40 , a plurality of pitch links  46  and  48 , and a plurality of U-shaped pitch horns  50  and  52 . Thrust bearing  43 , along with a system of links and levers, provides an interface between a non-rotating control system and rotating control system  41 . Rotating crosshead  44  controls blade pitch by transmitting control inputs from the non-rotating system through pitch links  46  and  48  to the cuff-mounted U-shaped pitch horns  50  and  52 . 
     Tail rotor hub assembly  16   a  includes a plurality of shear spindles  74 . Each shear spindle  74  is coupled to an inboard end of rotor blades  32  and  34  to provide a blade shear load path to a bearing  76  housed in a corresponding retention fitting  64 . Each shear spindle  74  includes at least one coning stop  79  to limit blade coning. The coning stops  79  prevent damage from strong side gust winds and ground handling. 
     In the preferred embodiment, outboard yoke  32  and inboard yoke  33  each include a set of multiple redundant load paths. For clarity, these multiple redundant load paths will be described with respect to outboard yoke  32  only. It will be understood that multiple redundant load paths associated with outboard yoke  32  are also associated with inboard yoke  33 . Yoke  32  includes multiple lugs  68  having redundant load paths at each end. Each pair of lugs  68  is configured to receive blade lugs  70  of rotor blade  34 . Each rotor blade  34  is attached to lugs  68  of outboard yoke  32  with bolts  72  in a multiple shear connection. Rotor blades  34  are preferably separated by 180 degrees. In the preferred embodiment, outboard yoke  32  is configured inboard of lugs  68  such that multiple load paths for structural redundancy in reacting to blade-to-blade centrifugal forces is provided. It should be understood that yoke  32  and rotor blades  34  may be assembled in alternate geometries. 
     Referring now to FIG. 5A in the drawings, the trunnion-to-mast attachment structure of tail rotor hub assembly  16   a  is illustrated. As is shown, an inboard cone  94 , an inboard trunnion  92 , hub adapter  54 , an outboard trunnion  90 , outboard cone  58 , and spacer  61  are sandwiched together over mast  30  between an inboard shoulder  101  of mast  30  and mast nut  60 . As mast nut  60  is tightened down onto mast  30 , outboard trunnion  90 , hub adapter  54 , and inboard trunnion  92  are compressed together and positively centered. Inboard cone  94  blocks out radial looseness in the spline section  96 , and outboard cone  58  provides positive centering of outboard trunnion  90 . Mast  30  transmits drive torque to inboard trunnion  92  by means of a spline section  96  disposed on mast  30 . Inboard trunnion  92  has mating splines on its inside surface that mate with spline section  96  of mast  30 . Inboard trunnion  92  forms a toothed coupling  98  with hub adapter  54 . The drive torque is transmitted from inboard trunnion  92  to hub adapter  54  through toothed coupling  98 . Hub adapter  54  forms a toothed coupling  100  with outboard trunnion  90 . The drive torque is transmitted from hub adapter  54  to outboard trunnion  90  through toothed coupling  100 . 
     As is shown, a portion of mast  30  outboard of inboard trunnion  92  has a reduced outside diameter. This reduced outside diameter produces a torsional stiffness significantly lower than the tortional stiffness of hub adapter  54 . Thus, for any rotational deflection of outboard trunnion  90 , mast  30  will rotate an equivalent amount, but with the rotation occurring in the reduced-diameter section of mast  30 , and not at the interface of outboard cone  58  and mast  30 . Mast nut  60  produces an axial preload across inboard cone  94 , inboard trunnion  92 , hub adapter  54 , outboard trunnion  90 , outboard cone  58 , and spacer  61 . This axial preload generates a desirable frictional clamp up at outboard cone  58  and counteracts separation force from toothed couplings  98  and  100 . 
     The primary purpose of hub adapter  54  is to transfer drive torque from inboard trunnion  92  to outboard trunnion  90 . Because inboard trunnion  92  is splined to mast  30 , all of the steady drive torque from mast  30  is transferred to inboard trunnion  92 . However, only about one-half of that drive torque is transferred to rotor blades  36  through inboard yoke  33 . The remaining drive torque is transferred from inboard trunnion  92 , through hub adapter  54 , through outboard trunnion  90 , to rotor blades  34  through outboard yoke  32 . It should be noted that because outboard trunnion  90  is not splined to mast  30 , hub adapter  54  experiences about one-half of the mast torque as a steady load. 
     By configuring tail rotor hub assembly  16   a  in this manner, several benefits are provided, including: (1) reduced failure due to fretting and wear; (2) the absence of relative motion at the attachment joints; and (3) commonality between the inboard and outboard rotor assemblies. Because the 2/rev Coriolis torque loads between inboard trunnion  92  and outboard trunnion  90  are counteracted by toothed couplings  98  and  100 , and not splined section  96  of mast  30 , the potential failure due to fretting is reduced. Because the two stacked trunnions  90  and  92  are clamped together through toothed couplings  98  and  100 , they are securely fixed to one another via a tight joint, which is desirable for minimizing the fretting and wear common to joints that see high oscillatory loads. The torsionally soft outboard section of mast  30  accommodates the angular deflection between inboard trunnion  92  and outboard trunnion  90  with minimal relative motion occurring at the toothed joints of toothed couplings  98  and  100 . In addition, this unique configuration allows for common inboard and outboard rotor assemblies that can be assembled, replaced, and shipped as individual two-bladed assemblies. The configuration of toothed couplings  98  and  100  of hub adapter  54  include important design considerations. First, each toothed coupling  98  and  100  must be capable of counteracting the steady, oscillatory, and limit torque loads imposed by tail rotor hub assembly  16   a . Second, it is desirable that the axial preload across toothed couplings  98  and  100  be sufficient to prevent joint separation during operation. Toothed couplings  98  and  100 , along with the surrounding hardware, must also be capable of carrying the preload requirement. Therefore, it should be understood that the size and pitch of toothed couplings  98  and  100  may vary from one application to another. 
     Referring now to FIGS. 5B and 5C in the drawings, elastomeric bearing and trunnion assembly  56  is illustrated. In FIG. 5B, bearing and trunnion assembly  56  is shown in an assembled view; while in FIG. 5C, bearing and trunnion assembly  56  is shown in an exploded view. Bearing and trunnion assembly  56  includes a trunnion  90  having a pair of trunnion arms  110  extending radially outward therefrom. In the preferred embodiment, trunnion arms  110  have a generally conical shape in which they taper inwardly as they extend outwardly from a body portion  113 . Each trunnion arm  110  is configured to fit securely within an interior portion  116  of an elastomeric bearing  112 . Elastomeric bearings  112  accommodate rotor flapping motions and forces, and each elastomeric bearing and trunnion assembly  56  and  57  provides load paths for rotor torque and thrust. A rigid annular sleeve  114  is disposed around each elastomeric bearing  112 . Sleeves  114  are configured to fit securely within a transverse bore  104  (see FIGS. 6 through 9) through inboard yoke  33  and outboard yoke  32 . In the preferred embodiment, sleeves  114  include stops  118  that register against retention fittings  64  to limit the radial movement of sleeves  114  within bore  104 . In this manner, the movements of inboard yoke  33  and outboard yoke  32  relative to mast  30  are limited. 
     As set forth above, the 2/rev Coriolis relief provided by the present invention is achieved by optimizing the spring rate characteristics of elastomeric bearings  112 , rather than by adding additional hardware. Conventional teetering rotors that use elastomeric bearings to provide a flapping degree of freedom allow the radial stiffness of the bearings to be very high in order to minimize weight and size. However, according to the present invention, the radial stiffness of elastomeric bearings  112  is selectively tailored to provide adequate stiffness to react to rotor torque, while at the same time providing adequate softness to relieve the 2/rev Coriolis loads. Because the Coriolis relief is provided by tailoring the spring rate of an existing component, the resulting hub assembly provides a much simpler configuration with reduced weight and cost, and higher reliability due to reduction in the number of parts required. 
     In the preferred embodiment, trunnion  90  is made of stainless steel; however, it will be understood that other suitable materials may be used. Likewise, it should be understood that the construction materials and dynamic characteristics of elastomeric bearings  112  may vary from one application to another. In the preferred embodiment, elastomeric bearings  112  are molded and placed, or vulcanized, directly onto trunnion arms  110 . It should be understood, that elastomeric bearings  112  may also be pre-molded and then later bonded to, adhered to, or otherwise secured to, trunnion arms  110  after molding. In the preferred embodiment, elastomeric bearings  112  are selectively tailored to provide an axial spring rate in a direction parallel to the axis of the trunnion arms; a flapping spring rate; and a radial, or torque, spring rate radially about the axis of the mast to relieve the 2/rev Coriolis torque. It should be noted that any one of these characteristics may vary depending upon the requirements of a particular application. 
     Tail rotor hub assembly  16   a  provides far superior performance as compared to conventional tail rotor hub assemblies, particularly in regard to the handling of 2/rev Coriolis torque. In general, the known solutions for dealing with 2/rev Coriolis torque involve heavy and complex mechanisms. Some require the use of highly loaded bearings oscillating at tail rotor frequencies, resulting in designs that require high levels of maintenance and excessive down times. In certain conventional designs, problems associated with tailoring the stiffness of critical metal parts exist, which can result in degraded structural designs and potentially catastrophic failure modes. However, tail rotor hub assembly  16   a  according to the present invention overcomes these problems. 
     The 2/rev Coriolis torque relief of tail rotor hub assembly  16   a  is provided by optimizing the spring rate characteristics of elastomeric bearings  112 , which are existing components in multi-bladed tail rotor system. In other words, tail rotor hub assembly  16   a  uses an existing elastomeric bearing used to accommodate rotor flapping, rather than introducing a separate mechanism. Conventional teetering rotors that use elastomeric bearings to provide flapping degrees of freedom allow the radial stiffness of the bearings to be very high in order to minimize weight and size. In the present invention, however, the bearing radial stiffness is tailored to provide adequate stiffness to react rotor torque and to provide adequate softness to relieve 2/rev Coriolis torque loads. 
     Because the Coriolis torque relief is provided by tailoring the spring rate of an existing component necessary to accommodate the flapping degrees of freedom, the resulting hub assembly provides a much simpler configuration, having reduced weight and costs, and providing higher reliability due to a reduction in the number of parts required to achieve that result. Certain parts of multi-bladed tail rotor hub assembly  16   a  according to the present invention may be designed to function as independent fail safe load paths to protect against catastrophic failure of the tail rotor hub assembly  16   a . For example, certain embodiments of tail rotor hub assembly  16   a  incorporate redundant load paths in the lug areas of outboard yoke  32  and inboard yoke  33 . Specifically, outboard yoke  32  and inboard yoke  33  are designed to allow each lug  68  to function independently of the other lugs  68  as a fail safe load path. With this configuration, even in the event of a complete mechanical failure of one of the lugs  68 , the other lug  68  can continue to carry loads. 
     Referring now to FIGS. 6 through 9 in the drawings, outboard yoke  32  is illustrated in four principal-axis views. It will be appreciated that outboard yoke  32  is identical in form and function as inboard yoke  33 . Outboard yoke  32  transfers drive torque to rotor blades  34 , reacts to rotor loads, and transfers blade thrust to mast  30  through elastomeric bearing and trunnion assembly  56 . In the preferred embodiment, outboard yoke  32  and inboard yoke  33  are forged from titanium. 
     Mast  30  passes through a mast bore  102 . A bearing bore  104 , which intercepts mast bore  102  and is indexed to a pitch axis, receives elastomeric bearing and trunnion assembly  56 . In the preferred embodiment, bearing bore  104  is indexed at forty degrees to the pitch change axis. When fully assembled, retention fittings  64  are coupled to yoke  32  at each end of bearing bore  104 , such that a compressive axial preload is created across elastomeric bearing and trunnion assembly  56 . As is best seen in FIG. 4, retention fitting  64  is held in place by two bolts  66  that pass through bores  78 . 
     In the preferred embodiment, there are six independent primary load paths. In the event of failure of any of these load paths, outboard yoke  32  will continue to provide a high level of structural integrity to tail rotor hub assembly  16   a . For example, if tail rotor hub assembly  16   a  suffers complete failure of any single load path, tail rotor hub assembly  16   a  can maintain structural integrity for at least six flight hours of an unrestricted flight spectrum, including all limit and ultimate load conditions. 
     Referring to FIG. 10 in the drawings, a rotor blade  34  according to the present invention is illustrated. In the preferred embodiment, rotor blade  34  includes three distinct portions: an integral cuff  120 , an outboard blade section  124 , and an integral twist strap (not shown). The integral twist strap within cuff  120  functions as the main centrifugal force load path for rotor blade  34 , and accommodates both pitch change and coning motions. The integral twist strap is rigidly bolted to yoke lugs  68  through blade lugs  70 . Integral cuffs  120 , which are coupled to the upper and lower surfaces of rotor blade  34  at interfaces  122 , interface with outboard yoke  32  through shear spindles  74 . Integral cuffs  120  deliver control system pitch inputs to rotor blades  34  via U-shaped pitch horns  50 . U-shaped pitch horns  50  are also coupled to cuffs  120 . Outboard blade section  124  generates an aerodynamic thrust for rotor blade  34 . Although tail rotor hub assembly  16   a  has been described herein with respect to four rotor blades, it should be understood that tail rotor hub assembly  16   a  may utilize more or fewer than four rotor blades. 
     In the preferred embodiment, cuff section  120  of rotor blade  34  has a hollow airfoil shape. It is preferred that cuff  120  be manufactured primarily from off-axis fiberglass/epoxy tape in combination with several unidirectional layers of carbon fiber. Of course, it should be understood that other forms of construction and choices of materials may be utilized for rotor blade  34 . 
     An inside opening of cuff  120  is large enough to accommodate the pitch change motion of the twist strap. Cuff  120  interfaces with rotor yoke  32  through shear spindle  74 , which is bolted to the upper and lower surfaces of rotor blade  34 , and delivers control system pitch inputs to rotor blade  34  through U-shaped pitch horn  50 . As set forth above, the outboard end of cuff  120  is integral with rotor blade  34 . 
     In addition to the above-described distinctions, the multi-bladed tail rotor system according to the present invention may utilize a bearingless pitch mechanism to accommodate rotor pitch. Conventional rotor assemblies differ from that of the subject invention in that conventional rotor assemblies rely on spherical bearings between the yokes and the blades to accommodate the pitching motion of the blades relative to the yoke. As best seen in FIGS. 2-4, rotor blades  34  and  36  are coupled to yokes  32  and  33 , respectively, and in turn couple to mast  30 , with no provision within tail rotor hub assembly  16   a  for accommodation of rotor blade pitch. Each rotor blade  34  and  36  incorporates an integral flexing strap that replaces the functionality of the bearings found in conventional designs by flexing about the lengthwise axis of each rotor blade  34  and  36 , so as to allow for adjustment of the pitch of each rotor blade  34  and  36  without spherical bearings. 
     Inboard cuff  120  is configured to provide protection over the integral twist strap, thereby maintaining the aerodynamic contour of rotor blade  34  and preventing contact between the integral strap and the hollow structure of rotor blade  34  during flight. In the preferred embodiment, cuff  120  is configured to be sufficiently torsionally stiff to function as the pitching mechanism transmitting pitching torque from pitch horn  50  into rotor blade  34 . 
     It is apparent that an invention with significant advantages has been described and illustrated. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.