Patent Publication Number: US-10330166-B2

Title: Rotorcraft vibration suppression system in a four corner pylon mount configuration

Description:
BACKGROUND 
     Technical Field 
     The present application relates in general to vibration control. More specifically, the present application relates to systems for isolating mechanical vibrations in structures or bodies that are subject to harmonic or oscillating displacements or forces. The systems of the present application are well suited for use in the field of aircraft, in particular, helicopters and other rotary wing aircraft. 
     Description of Related Art 
     For many years, effort has been directed toward the design of an apparatus for isolating a vibrating body from transmitting its vibrations to another body. Such apparatuses are useful in a variety of technical fields in which it is desirable to isolate the vibration of an oscillating or vibrating device, such as an engine, from the remainder of the structure. Typical vibration isolation and attenuation devices (“isolators”) employ various combinations of the mechanical system elements (springs and mass) to adjust the frequency response characteristics of the overall system to achieve acceptable levels of vibration in the structures of interest in the system. One field in which these isolators find a great deal of use is in aircraft, wherein vibration-isolation systems are utilized to isolate the fuselage or other portions of an aircraft from mechanical vibrations, such as harmonic vibrations, which are associated with the propulsion system, and which arise from the engine, transmission, and propellers or rotors of the aircraft. 
     Vibration isolators are distinguishable from damping devices in the prior art that are erroneously referred to as “isolators.” A simple force equation for vibration is set forth as follows:
 
 F=m{umlaut over (x)}+c{dot over (x)}+kx  
 
     A vibration isolator utilizes inertial forces (m{umlaut over (x)}) to cancel elastic forces (kx). On the other hand, a damping device is concerned with utilizing dissipative effects (c{dot over (x)}) to remove energy from a vibrating system. 
     One important engineering objective during the design of an aircraft vibration-isolation system is to minimize the length, weight, and overall size including cross-section of the isolation device. This is a primary objective of all engineering efforts relating to aircraft. It is especially important in the design and manufacture of helicopters and other rotary wing aircraft, such as tilt rotor aircraft, which are required to hover against the dead weight of the aircraft, and which are, thus, somewhat constrained in their payload in comparison with fixed-wing aircraft. 
     Another important engineering objective during the design of vibration-isolation systems is the conservation of the engineering resources that have been expended in the design of other aspects of the aircraft or in the vibration-isolation system. In other words, it is an important industry objective to make incremental improvements in the performance of vibration isolation systems which do not require radical re-engineering or complete redesign of all of the components which are present in the existing vibration-isolation systems. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the embodiments of the present application are set forth in the appended claims. However, the embodiments themselves, 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 side view of a rotorcraft having a vibration suppression system, according to an illustrative embodiment of the present application; 
         FIG. 2A  is a plan view of a tilt rotor aircraft, according to the present application in an airplane mode; 
         FIG. 2B  is a perspective view of a tilt rotor aircraft, according to the present application in a helicopter mode; 
         FIG. 3  is a side view of the vibration suppression system, according to an illustrative embodiment of the present application; 
         FIG. 4  is a top view of the vibration suppression system, according to an illustrative embodiment of the present application; 
         FIG. 5  is a partial side view of the vibration suppression system, according to an illustrative embodiment of the present application; 
         FIG. 6  is a side view of an exemplary embodiment of a vibration isolator, according to an illustrative embodiment of the present application; 
         FIG. 7  is a section view of the vibration isolator, taken at section lines VII-VII, according to an illustrative embodiment of the present application; 
         FIG. 8  is a mechanical equivalent force diagram of the vibration isolator of  FIGS. 6 and 7 ; and 
         FIG. 9  is a schematic view of an active vibration control system, according to an illustrative embodiment of the present application. 
     
    
    
     While the system and method of the present application is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the application to the particular embodiment disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the process of the present application as defined by the appended claims. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrative embodiments of the system and method of the present application are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. 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&#39;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 nevertheless 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 application, the devices, members, apparatuses, etc. 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. 
     Referring to  FIG. 1  in the drawings, a rotorcraft  11  is illustrated. Rotorcraft  11  has a rotor system  13  with a plurality of rotor blades  21 . Rotorcraft  11  further includes a fuselage  15 , landing gear  17 , and an empennage  19 . A main rotor control system can be used to selectively control the pitch of each rotor blade  21  in order to selectively control direction, thrust, and lift of rotorcraft  11 . It should be appreciated that even though rotorcraft  11  is depicted as having certain illustrated features, it should be appreciated that rotorcraft  11  can take on a variety of implementation specific configurations, as one of ordinary skill in the art would fully appreciate having the benefit of this disclosure. Further, it should be appreciated that rotorcraft  11  can have variety of rotor blade quantities. It should be understood that the systems of the present application may be used with any aircraft on which it would be desirable to have vibration isolation, including unmanned aerial vehicles that are remotely piloted. 
     The systems of the present application may also be utilized on other types of rotary wing aircraft. Referring now to  FIGS. 2A and 2B  in the drawings, a tilt rotor aircraft  111  according to the present application is illustrated. As is conventional with tilt rotor aircraft, rotor assemblies  113   a  and  113   b  are carried by wings  115   a  and  115   b , and are disposed at end portions  116   a  and  116   b  of wings  115   a  and  115   b , respectively. Tilt rotor assemblies  113   a  and  113   b  include nacelles  120   a  and  120   b , which carry the engines and transmissions of tilt rotor aircraft  111 , as well as, rotor hubs  119   a  and  119   b  on forward ends  121   a  and  121   b  of tilt rotor assemblies  113   a  and  113   b , respectively. 
     Tilt rotor assemblies  113   a  and  113   b  move or rotate relative to wing members  115   a  and  115   b  between a helicopter mode in which tilt rotor assemblies  113   a  and  113   b  are tilted upward, such that tilt rotor aircraft  111  flies like a conventional helicopter; and an airplane mode in which tilt rotor assemblies  113   a  and  113   b  are tilted forward, such that tilt rotor aircraft  111  flies like a conventional propeller driven aircraft. In  FIG. 2A , tilt rotor aircraft  111  is shown in the airplane mode; and in  FIG. 2B , tilt rotor aircraft  111  is shown in the helicopter mode. As shown in  FIGS. 2A and 2B , wings  115   a  and  115   b  are coupled to a fuselage  114 . Tilt rotor aircraft  111  also includes a vibration isolation system according to the present application for isolating fuselage  114  or other portions of tilt rotor aircraft  111  from mechanical vibrations, such as harmonic vibrations, which are associated with the propulsion system and which arise from the engines, transmissions, and rotors of tilt rotor aircraft  111 . 
     Referring to  FIGS. 3-5 , a vibration suppression system  601  is illustrated. System  601 , also termed a vibration isolator system, includes a vibration isolator  401  located in each corner in a four corner pylon mount structural assembly. The combination of four vibration isolators  401 , two being forward of transmission  607 , and two being aft of transmission  607 , collectively are effective at isolating main rotor vertical shear, pitch moment, as well as roll moment induced vibrations. For example, rotor hub induced pitch moment vibrations, which can become relatively large in high-speed forward flight, can be effectively isolated with the four vibration isolators, corner located as shown in  FIGS. 3 and 4 . Locating isolators  401  away from the transmission is an improvement over legacy configurations which typically couple the transmission directly to the isolator. However, this is not the case in the present application. 
     The four corner pylon mount structural assembly includes a first pylon structure  615   a , second pylon structure  615   b , a first roof beam  603   a , a second roof beam  603   b , a forward cross member  201   a , and an aft cross member  201   b . Structural adapters can be used to structurally couple roof beams  603   a  and  603   b  with cross members  201   a  and  201   b . In the illustrated embodiment, roof beams  603   a  and  603   b  are coupled to an airframe  605 , while pylon structures  615   a  and  615   b  are coupled to isolators  401 . First pylon structure  615   a  is mounted with a first vibration isolator  401   a  and a second vibration isolator  401   b , while a second pylon structure  615   b  is mounted with a third vibration isolator  401   c  and a fourth vibration isolator  401   d . Each vibration isolator  401   a - d  is mounted substantially vertical, as illustrated in  FIG. 5 . Transmission  607  is coupled to pylon structures  615   a  and  615   b  as opposed to direct coupling to the isolators. A driveshaft  609  carries mechanical power from an engine  611  to transmission  607 . It should be appreciated that embodiments of pylon system  601  may employ any practical number of engines and transmissions. Furthermore, it is contemplated that any plurality of pylon structures and vibration isolators may be used in a variety of orientations spaced fore, aft, and even outboard of transmission  607 . 
     As seen in  FIGS. 4 and 5 , isolators  401   a - d  are mounted away from transmission  607 . For example, isolators  401   a - d  are mounted forward and aft of transmission  607 . Additionally, isolators  401   a - d  are mounted outboard from transmission  607 . As depicted in  FIG. 4 , isolators  401   a - d  are mounted sufficiently outboard so as to be located further outboard than the point of coupling  606  between transmission  607  and pylon structures  615   a  and  615   b . The point of coupling  606  is inboard between roof beams  603   a ,  603   b . In so doing, two isolators  401   a ,  401   c  are positioned above roof beams  603   a ,  603   b  forward of transmission  607 . Likewise two isolators  401   b ,  401   d  are positioned above roof beams  603   a ,  603   b  aft of transmission  607 . Isolators  401   a - d  are spaced away from the point of coupling between pylon structures  615   a  and  615   b  and the transmission in fore, aft, and outboard directions in the preferred embodiment. However, it is understood that other embodiments may adjust the spacing do affect dynamics from different aircraft or transmissions. 
     Pylon structures  615   a ,  615   b  are configured to correlate motion of the transmission between a plurality of isolators  401  simultaneously by suspending a portion of transmission  607  between a plurality of isolators located on opposing ends of the pylon structure. The use of pylon structures  615   a ,  615   b  permits an aircraft to space the location of isolators  401   a - d  to an infinite number of locations independent of transmission  607 . Locating isolators forward and aft of transmission permits the pylon mount structural assembly minimizes the size of each isolator  401   a - d  and avoids the use of additional elements to control the dynamics of transmission  607 . For example, the pylon mount structural assembly is springless in that the assembly does not use a spring mounted externally beneath the transmission to control dynamics of the transmission. The pylon mount structural assembly is configured to control pitch and roll dynamics by spacing of isolators  401   a - d  and the use of pylon structures  615   a  and  615   b.    
     Further, implementing active vibration isolators, such as piezoelectric vibration isolators, can be effective for vibration isolation for a multiple RPM rotorcraft. It should be appreciated that other active actuation methods can be used as well, such as hydraulic, electromagnetic, electromechanical, to name a few. Active vibration isolators can also achieve better vibration isolation by overcoming damping losses, and adjusting the frequency response characteristics. Further, each opposing pair of vibration isolators  401  can efficiently react against the moment oscillations because the moment can be decomposed into two antagonistic vertical oscillations at each vibration isolator  401 . 
     Referring now also to  FIGS. 6 and 7  in the drawings, isolator  401  comprises an upper housing  403  and a lower housing  405 . An upper reservoir housing  427  and a lower reservoir housing  429  are coupled to end portions of upper housing  403  and a lower housing  405 , respectively. Each upper reservoir housing  427  and a lower reservoir housing  429  define an upper fluid chamber  407  and a lower fluid chamber  409 , respectively. A piston spindle  411  includes a cylindrical portion that is at least partially disposed within the interior of upper housing  403  and lower housing  405 . A plurality of studs  417  rigidly couple together upper housing  403  and a lower housing  405  via an upper ring  439  and a lower ring  441 , respectively, so that upper housing  403  and lower housing  405  function as a single rigid body. Studs  417  extend through piston spindle  411  within apertures sized to prevent any contact between studs  417  and piston spindle  411  during operation. Further, piston spindle  411  is resiliently coupled to upper housing  403  and lower housing  405  via an upper elastomer member  413  and a lower elastomer member  415 , respectively. Upper elastomer member  413  and lower elastomer member  415  each function similar to a journal bearing, as further discussed herein. 
     Piston spindle  411  is coupled to a vibrating body, such as a transmission of an aircraft via a pylon assembly, such as a pylon assembly  601 . A spherical bearing assembly  425  is coupled to lower housing  405 . Spherical bearing assembly  425  includes an attachment member  431  configured for coupling the spherical bearing assembly  425  to a body to be isolated from vibration, such as a roof beam of an airframe in an aircraft, such as roof beam  603 . In such an arrangement, the airframe serves as the body to be isolated from vibration, and the transmission of the aircraft serves as the vibrating body. Spherical bearing assembly  425  includes a spherical elastomeric member  433  having an elastomeric material bonded between a non-resilient concave member and a non-resilient convex member. Spherical elastomeric member  433  is configured to compensate for misalignment in loading between the pylon assembly  601  and roof beam  603  through shearing deformation of the elastomeric material. Spherical elastomeric member  433  is partially spherical shaped with a rotational center point  445  that lies on a centerline plane  443  of attachment member  431 . Furthermore, spherical bearing assembly  425  is positioned and located to reduce an overall installation height of vibration isolator  401 , as well is provide optimized performance of pylon assembly  601  and related propulsion components. 
     Upper elastomer member  413  and lower elastomer member  415  seal and resiliently locate piston spindle  411  within the interior upper housing  403  and lower housing  405 . Upper housing  403  and lower housing  405  can each be coupled to piston spindle  411  with an upper adapter  435  and lower adapter  437 , respectively. Upper elastomer member  413  and lower elastomer member  415  function at least as a spring to permit piston spindle  411  to move or oscillate relative to upper housing  403  and lower housing  405 . Upper elastomer member  413  and lower elastomer member  415  can be a solid elastomer member, or alternatively can be alternating layers of non-resilient shim members and elastomer layers. 
     Isolator  401  further includes an elongated portion  419  integral with piston spindle  411 , the elongated portion  419  being configured to define a tuning passage  421 . Tuning passage  421  axially extends through elongated portion  419  to provide for fluid communication between upper fluid chamber  407  and lower fluid chamber  409 . The approximate length of tuning passage  421  preferably coincides with the length of elongated portion  419 , and is further defined by L 1 . Tuning passage  421  is generally circular in cross-section and can be partially tapered longitudinally in order to provide efficient fluid flow. 
     A tuning fluid  423  is disposed in upper fluid chamber  407 , lower fluid chamber  409 , and tuning passage  421 . Tuning fluid  423  preferably has low viscosity, relatively high density, and non-corrosive properties. For example, tuning fluid  423  may be a proprietary fluid, such as SPF I manufactured by LORD CORPORATION. Other embodiments may incorporate hydraulic fluid having suspended dense particulate matter, for example. 
     The introduction of a force into piston spindle  411  translates piston spindle  411  and elongated portion  419  relative to upper housing  403  and lower housing  405 . Such a displacement of piston spindle  411  and elongated portion  419  forces tuning fluid  423  to move through tuning passage  421  in the opposite direction of the displacement of piston spindle  411  and elongated portion  419 . Such a movement of tuning fluid  423  produces an inertial force that cancels, or isolates, the force from piston spindle  411 . During typical operation, the force imparted on piston spindle  411  is oscillatory; therefore, the inertial force of tuning fluid  423  is also oscillatory, the oscillation being at a discrete frequency, i.e., isolation frequency. 
     The isolation frequency (f i ) of vibration isolator  401  can be represented by the following equation: 
     
       
         
           
             
               f 
               i 
             
             = 
             
               
                 1 
                 
                   2 
                   ⁢ 
                   π 
                 
               
               ⁢ 
               
                 
                   K 
                   
                     
                       R 
                       ⁡ 
                       
                         ( 
                         
                           R 
                           - 
                           1 
                         
                         ) 
                       
                     
                     ⁢ 
                     
                       m 
                       t 
                     
                   
                 
               
             
           
         
       
     
     In the above equation, R represents the ratio of the functional area A p  of piston spindle  411  to the total area A T  inside the tuning passage  421 . As such, R=A p /A T  Mass of tuning fluid  423  is represented by m t . The combined spring rate of elastomer members  413  and  415  is represented by K. 
     It should be appreciated that isolator  401  is merely exemplary of a wide variety of vibration isolators that may be used. For example, vibration isolator  401  is illustrated as a passive vibration isolator; however, it should be fully appreciated that vibration isolator  401  can also be of an active isolator. An active isolator is configured so that the isolation frequency can be selective changed during operation. For example, an active vibration isolator is illustrated in U.S. Patent Application Publication No. US 2006/0151272 A1, titled “Piezoelectric Liquid Inertia Vibration Eliminator”, published 13 Jul. 2006, to Michael R. Smith et al., which is hereby incorporated by reference. 
     Vibration suppression system  601  is configured such that transmission  607  is “soft mounted” with a vibration isolator  401   a - d  located at each end of a pylon structure  615 . During operation, each vibration isolator  401   a - d  allows each pylon structure  615   a ,  615   b  to float relative to roof beams  603   a ,  603   b  through the deformation of upper elastomer member  413 , lower elastomer member  415 , and spherical elastomeric member  433 . If coupling  613  is required to compensate for a large amount of axial and angular misalignment, then the size and complexity of coupling  613  is undesirably large. Further, it is desirable to minimize the size and complexity of aircraft components in order to minimize weight and expense of the aircraft, thereby maximizing performance and reducing manufacturing associated expenditure. As such, vibration isolators  401   a - d  are uniquely configured to reduce the size and complexity of drive system components, such as coupling  613 . More specifically, spherical bearing assembly  425  is configured so that centerline plane  443  of attachment member  431  lies on or near a waterline plane of driveshaft axis  617  so as to reduce a moment arm that could otherwise contribute to axial (chucking) misalignment. An undesirable moment arm could be produced if centerline plane  443  of attachment member  431  were to lie a significant moment arm distance, as measured in the waterline direction, from driveshaft axis  617 . Chucking occurs essentially when engine  611  and transmission translate towards or away from each other. Further, the location of spherical bearing assembly  425  circumferentially around lower housing  405  reduces the overall height of vibration isolators  401   a - d . A compact pylon system  601  improves performance by reducing moment arms that can react between components. 
     Referring briefly to  FIG. 8  in the drawings, a mechanical equivalent model  701  for vibration isolator  401  of  FIGS. 4 and 5  is illustrated. In mechanical equivalent model  701 , a box  703  represents the mass of the fuselage M fuselage ; a box  705  represents the mass of the pylon assembly M pylon ; and a box  707  represents the mass of the tuning mass M t , in this case, the mass of tuning fluid  423 . A vibratory force F·sin(ωt) is generated by the transmission and propulsion system. Force F·sin(ωt) is a function of the frequency of vibration of the transmission and propulsion system. 
     Force F·sin(ωt) causes an oscillatory displacement up of the pylon assembly; an oscillatory displacement of the fuselage u f ; and an oscillatory displacement of the tuning mass u t . Elastomer members  413  and  415  are represented by a spring  709  disposed between the fuselage M fuselage  and the pylon assembly M pylon . Spring  709  has a spring constant K. 
     In mechanical equivalent model  701 , tuning mass M t  functions as if cantilevered from a first fulcrum  711  attached to the pylon assembly M pylon , and a second fulcrum  713  attached to the fuselage M fuselage . The distance a from first fulcrum  711  to second fulcrum  713  represents the cross-sectional area of tuning passage  421 , and the distance b from first fulcrum  711  to the tuning mass M t  represents the effective cross-sectional area of piston spindle  411 , such that an area ratio, or hydraulic ratio, R is equal to the ratio of b to a. Mechanical equivalent model  701  leads to the following equation of motion for the system: 
     
       
         
           
             
               
                 
                   [ 
                   
                     
                       
                         
                           
                             M 
                             pylon 
                           
                           + 
                           
                             
                               
                                 ( 
                                 
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                                   1 
                                 
                                 ) 
                               
                               2 
                             
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                               ⁡ 
                               
                                 ( 
                                 
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                             M 
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                               R 
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                                 ( 
                                 
                                   R 
                                   - 
                                   1 
                                 
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                           ⁢ 
                           
                             M 
                             t 
                           
                         
                       
                       
                         
                           
                             M 
                             fuselage 
                           
                           + 
                           
                             
                               R 
                               2 
                             
                             ⁢ 
                             
                               M 
                               t 
                             
                           
                         
                       
                     
                   
                   ] 
                 
                 ⁢ 
                 
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                   } 
                 
               
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                         K 
                       
                       
                         
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                         K 
                       
                     
                   
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                 ⁢ 
                 
                   { 
                   
                     
                       
                         
                           u 
                           p 
                         
                       
                     
                     
                       
                         
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                           f 
                         
                       
                     
                   
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             = 
             
               { 
               
                 
                   
                     
                       F 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             ω 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             t 
                           
                           ) 
                         
                       
                     
                   
                 
                 
                   
                     0 
                   
                 
               
               } 
             
           
         
       
     
     As is evident, no means for actively tuning vibration isolator  401  is available. Once the cross-sectional areas of tuning passage  421  and piston spindle  411  are determined, and the tuning fluid is chosen, the operation of vibration isolator  401  is set. However, an embodiment of vibration isolator  401  can be configured such that the isolation frequency can be selectively altered and optimized by the removing and replacing elongated portion  419  from piston spindle  411  with another elongated portion  419  having a different diameter tuning passage  421 . As such, vibration isolator  401  can be adaptable to treat a variety of isolation frequencies, as well as being adaptable for variances in stiffness K of upper and lower elastomer members  413  and  415 . 
     Referring now also to  FIG. 9 , an active vibration control system  801  is illustrated. System  801  can includes a plurality of vibration feedback sensors  803   a - 803   d  in communication with a vibration control computer (VCC)  805 . VCC  805  is in communication with each active vibration isolator in system  601  so that the isolation frequency of each active vibration isolator can be actively modified during operation. The vibration control system is configured to detect and convey vibration data through a plurality of feedback sensors  803   a - 803   d  to regulate the isolation frequency of at least one vibration isolator  401   a - d.    
     The vibration suppression system of the present application provides significant advantages, including: 1) efficient and effective vibration suppression rotor induced vertical hub shear forces, hub pitch moments, and hub roll moments; 2) improved occupant ride quality; 3) improved life of life critical rotorcraft components; 4) decreased size of isolators; and 5) ability to control the roll, pitch, and shear without the assistance of externally mounted systems to the transmission. 
     It is apparent that embodiments with significant advantages have been described and illustrated. Although the embodiments in the present application are 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.