Patent Publication Number: US-2022212783-A1

Title: Multimode Clutch Assemblies having Engagement Status Sensors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of co-pending application Ser. No. 17/063,712, filed Oct. 5, 2020, which is a continuation-in-part of application Ser. No. 16/567,086, filed Sep. 11, 2019, which is a continuation-in-part of application Ser. No. 16/274,520, filed Feb. 13, 2019, which claims the benefit of provisional application Ser. No. 62/801,621, filed Feb. 5, 2019, the entire contents of each are hereby incorporated by reference. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with Government support under Agreement No. W911W6-19-9-0002, awarded by the Army Contracting Command-Redstone Arsenal. The Government has certain rights in the invention. 
    
    
     TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure relates, in general, to clutch assemblies operable for use on rotorcraft and, in particular, to multimode clutch assemblies having engagement status sensors that are operable to enable the selective use of secondary engine power independent of or together with main engine power to drive the main rotor, the tail rotor and/or the accessories of a rotorcraft. 
     BACKGROUND 
     Many rotorcraft are capable of taking off, hovering and landing vertically. One such rotorcraft is a helicopter, which has a main rotor that provides lift and thrust to the aircraft. The main rotor not only enables hovering and vertical takeoff and landing, but also enables forward, backward and lateral flight. These attributes make helicopters highly versatile for use in congested, isolated or remote areas. It has been found that the power demand of a rotorcraft can vary significantly based upon the operation being performed. For example, low power demand exists during preflight operations, when power is only needed to operate accessories such as generators, air pumps, oil pumps, hydraulic systems and the like as well as to start the main engine. Certain rotorcraft utilize a dedicated auxiliary power unit to generate preflight accessory power. During takeoff, hover, heavy lifts and/or high speed operations, rotorcraft experience high power demand. Certain rotorcraft utilize multiple main engines or one main engine and a supplemental power unit to generate the required power for the main rotor during such high power demand flight operations. In conventional rotorcraft, the dedicated auxiliary power unit is not operable to provide supplemental power to the main rotor during high power demand flight operations. Accordingly, a need has arisen for improved rotorcraft systems that enable an auxiliary power unit to not only provide accessory power during preflight operations but also to operate as a supplemental power unit to provide power to the main rotor during high power demand flight operations. 
     SUMMARY 
     In a first aspect, the present disclosure is directed to a multimode clutch assembly for a rotorcraft. The clutch assembly includes a freewheeling unit having an input race and an output race. The freewheeling unit has a driving mode in which torque applied to the input race is transferred to the output race and an overrunning mode in which torque applied to the output race is not transferred to the input race. A bypass assembly has an engaged position in which the bypass assembly couples the input and output races of the freewheeling unit and a disengaged position in which the bypass assembly does not couple the input and output races of the freewheeling unit. An actuator assembly has an engagement configuration supplying an engagement force to shift the bypass assembly from the disengaged position to the engaged position and a disengagement configuration supplying a disengagement force to shift the bypass assembly from the engaged position to the disengaged position. An engagement status sensor is operably associated with at least one of the bypass assembly and the actuator assembly. The engagement status sensor is configured to determine an engagement status of the bypass assembly. In the disengaged position of the bypass assembly, the overrunning mode of the freewheeling unit is enabled such that the clutch assembly is configured for unidirectional torque transfer from the input race to the output race. In the engaged position of the bypass assembly, the overrunning mode of the freewheeling unit is disabled such that the clutch assembly is configured for bidirectional torque transfer between the input and output races. 
     In some embodiments, the engagement status sensor may be a proximity sensor. In such embodiments, the actuator assembly may include a liner, a piston and a bearing sled wherein the piston is slidably disposed relative to the liner and the bearing sled is coupled between the piston and the bypass assembly and wherein the proximity sensor may be an inductive proximity sensor configured to monitor the position of the bearing sled relative thereto to determine the engagement status of the bypass assembly. Alternatively, the proximity sensor may be a load cell, such as a strain sensor, that is configured to monitor the position of the bypass assembly relative thereto to determine the engagement status of the bypass assembly. In certain embodiments, the engagement status sensor may be an oil pressure sensor. In some embodiments, the engagement status sensor may be a tooth passage frequency sensor such as a variable reluctance sensor or a hall-effect sensor. In other embodiments, the engagement status sensor may be a variable differential transformer such as a linear variable differential transformer or a rotary variable differential transformer. 
     In a second aspect, the present disclosure is directed to a powertrain for a rotorcraft. The powertrain has a main drive system including a main engine. The powertrain also has a secondary engine and a multimode clutch assembly that is positioned between the main drive system and the secondary engine. The clutch assembly includes a freewheeling unit having an input race coupled to the main drive system and an output race coupled to the secondary engine. The freewheeling unit has a driving mode in which torque applied to the input race is transferred to the output race and an overrunning mode in which torque applied to the output race is not transferred to the input race. A bypass assembly has an engaged position in which the bypass assembly couples the input and output races of the freewheeling unit and a disengaged position in which the bypass assembly does not couple the input and output races of the freewheeling unit. An actuator assembly has an engagement configuration supplying an engagement force to shift the bypass assembly from the disengaged position to the engaged position and a disengagement configuration supplying a disengagement force to shift the bypass assembly from the engaged position to the disengaged position. An engagement status sensor is operably associated with at least one of the bypass assembly and the actuator assembly. The engagement status sensor is configured to determine an engagement status of the bypass assembly. In the disengaged position of the bypass assembly, the overrunning mode of the freewheeling unit is enabled such that the clutch assembly is configured for unidirectional torque transfer from the input race to the output race. In the engaged position of the bypass assembly, the overrunning mode of the freewheeling unit is disabled such that the clutch assembly is configured for bidirectional torque transfer between the input and output races. 
     In some embodiments, the main engine may be a gas turbine engine and the secondary engine may be a gas turbine engine. In other embodiments, the main engine may be a gas turbine engine and the secondary engine may be an electric motor. In certain embodiments, the secondary engine may be configured to generate between about 5 percent and about 20 percent of the power of the main engine or between about 10 percent and about 15 percent of the power of the main engine. 
     In a third aspect, the present disclosure is directed to a rotorcraft. The rotorcraft includes a main rotor coupled to a main drive system including a main engine. The rotorcraft also includes a secondary engine and a multimode clutch assembly that is positioned between the main drive system and the secondary engine. The clutch assembly includes a freewheeling unit having an input race coupled to the main drive system and an output race coupled to the secondary engine. The freewheeling unit has a driving mode in which torque applied to the input race is transferred to the output race and an overrunning mode in which torque applied to the output race is not transferred to the input race. A bypass assembly has an engaged position in which the bypass assembly couples the input and output races of the freewheeling unit and a disengaged position in which the bypass assembly does not couple the input and output races of the freewheeling unit. An actuator assembly has an engagement configuration supplying an engagement force to shift the bypass assembly from the disengaged position to the engaged position and a disengagement configuration supplying a disengagement force to shift the bypass assembly from the engaged position to the disengaged position. An engagement status sensor is operably associated with at least one of the bypass assembly and the actuator assembly. The engagement status sensor is configured to determine an engagement status of the bypass assembly. In the disengaged position of the bypass assembly, the overrunning mode of the freewheeling unit is enabled such that the clutch assembly is configured for unidirectional torque transfer from the input race to the output race. In the engaged position of the bypass assembly, the overrunning mode of the freewheeling unit is disabled such that the clutch assembly is configured for bidirectional torque transfer between the input and output races. 
     In a preflight configuration of the rotorcraft, the bypass assembly is in the disengaged position, the main engine is not operating and the secondary engine provides power to at least one rotorcraft accessory. In an enhanced power configuration of the rotorcraft, the bypass assembly is in the engaged position, the main engine provides power to the main drive system and the secondary engine provides power to at least one rotorcraft accessory and to the main drive system through the clutch assembly. In a high efficiency configuration of the rotorcraft, the bypass assembly is in the engaged position, the secondary engine is in standby mode and the main engine provides power to the main drive system and to at least one rotorcraft accessory through the clutch assembly. In an enhanced autorotation configuration of the rotorcraft, the bypass assembly is in the engaged position, the main engine is not operating and the secondary engine provides power to the main drive system through the clutch assembly. 
    
    
     
       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. 1A-1C  are schematic illustrations of a rotorcraft having a multimode clutch assembly in accordance with embodiments of the present disclosure; 
         FIGS. 2A-2E  are block diagrams of a powertrain including a multimode clutch assembly for a rotorcraft in various operating configurations in accordance with embodiments of the present disclosure; 
         FIGS. 3A-3C  are cross sectional views of a rotorcraft gearbox assembly including a multimode clutch assembly in various operating configurations in accordance with embodiments of the present disclosure; 
         FIGS. 4A-4B  are cross sectional views of a rotorcraft gearbox assembly including a multimode clutch assembly and depicting a lubrication circuit in accordance with embodiments of the present disclosure; 
         FIGS. 5A-5B  are cross sectional views of a rotorcraft gearbox assembly including a multimode clutch assembly in various operating configurations in accordance with embodiments of the present disclosure; 
         FIGS. 6A-6B  are cross sectional views of a rotorcraft gearbox assembly including a multimode clutch assembly in various operating configurations in accordance with embodiments of the present disclosure; and 
         FIGS. 7A-7E  are cross sectional views of a rotorcraft gearbox assembly including a multimode clutch assembly and depicting various engagement status sensors 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, all features of an actual implementation may not be 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 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 devices 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 by mere contact or by moving and/or non-moving mechanical connections. 
     Referring to  FIGS. 1A-1C  in the drawings, a rotorcraft in the form of a helicopter is schematically illustrated and generally designated  10 . The primary propulsion assembly of helicopter  10  is a main rotor assembly  12 . Main rotor assembly  12  includes a plurality of rotor blades  14  extending radially outward from a main rotor hub  16 . Main rotor assembly  12  is coupled to a fuselage  18  and is rotatable relative thereto. The pitch of rotor blades  14  can be collectively and/or cyclically manipulated to selectively control direction, thrust and lift of helicopter  10 . A tailboom  20  is coupled to fuselage  18  and extends from fuselage  18  in the aft direction. An anti-torque system  22  includes a tail rotor assembly  24  coupled to an aft end of tailboom  20 . Anti-torque system  22  controls the yaw of helicopter  10  by counteracting the torque exerted on fuselage  18  by main rotor assembly  12 . In the illustrated embodiment, helicopter  10  includes a vertical tail fin  26  that provide stabilization to helicopter  10  during high speed forward flight. In addition, helicopter  10  includes wing members  28  that extend laterally from fuselage  18  and wing members  30  that extend laterally from tailboom  20 . Wing members  28 ,  30  provide lift to helicopter  10  responsive to the forward airspeed of helicopter  10 , thereby reducing the lift requirement on main rotor assembly  12  and increasing the top speed of helicopter  10   
     Main rotor assembly  12  and tail rotor assembly  24  receive torque and rotational energy from a main engine  32 . Main engine  32  is coupled to a main rotor gearbox  34  by suitable clutching and shafting. Main rotor gearbox  34  is coupled to main rotor assembly  12  by a mast  36  and is coupled to tail rotor assembly  24  by tail rotor drive shaft  38 . In the illustrated embodiment, a secondary engine  40  is coupled to tail rotor drive shaft  38  by a secondary gearbox  42 . Together, main engine  32 , main rotor gearbox  34 , tail rotor drive shaft  38 , secondary engine  40  and secondary gearbox  42  as well as various other shafts and gearboxes coupled therein may be considered as the powertrain of helicopter  10 . 
     Secondary engine  40  is operable as an auxiliary power unit to provide preflight power to the accessories of helicopter  10  such as electric generators, air pumps, oil pumps, hydraulic systems and the like as well as to provide the power required to start main engine  32 . In addition, secondary engine  40  is operable to provide supplemental power to main rotor assembly  12  that is additive with the power provided by main engine  32  during, for example, high power demand conditions including takeoff, hover, heavy lifts and high speed flight operations. Secondary engine  40  is also operable to provide emergency power to main rotor assembly  12 . For example, in the event of a failure of main engine  32 , secondary engine  40  is operable to provide emergency power to enhance the autorotation and flare recovery maneuver of helicopter  10 . Use of secondary engine  40  not only enhances the safety of helicopter  10  but also increases the efficiency of helicopter  10 . For example, having the extra power provided by secondary engine  40  during high power demand operations allows main engine  32  to be downsized for more efficient single engine operations such as during cruise operations. 
     It should be appreciated that helicopter  10  is merely illustrative of a variety of aircraft that can implement the embodiments disclosed herein. Indeed, the multimode clutch assembly of the present disclosure may be implemented on any rotorcraft. Other aircraft implementations can include hybrid aircraft, tiltwing aircraft, tiltrotor aircraft, quad tiltrotor aircraft, unmanned aircraft, gyrocopters, propeller-driven airplanes, compound helicopters, drones and the like. As such, those skilled in the art will recognize that the multimode clutch assembly of the present disclosure can be integrated into a variety of aircraft configurations. It should be appreciated that even though aircraft are particularly well-suited to implement the embodiments of the present disclosure, non-aircraft vehicles and devices can also implement the embodiments. 
     Referring to  FIG. 2A  in the drawings, a powertrain  100  of a rotorcraft is illustrated in a block diagram format. Powertrain  100  includes a main engine  102  such as a turbo shaft engine capable of producing 2000 to 4000 horsepower or more, depending upon the particular implementation. Main engine  102  is coupled to a freewheeling unit depicted as sprag clutch  104  that acts as a one-way clutch enabling a driving mode in which torque from main engine  102  is coupled to main rotor gearbox  106  when the rotating speed of the input race, on the main engine side of sprag clutch  104 , is matched with the rotating speed of the output race, on the main rotor gearbox side of sprag clutch  104 . Importantly, sprag clutch  104  has an overrunning mode in which main engine  102  is decoupled from main rotor gearbox  106  when the rotating speed of the input race is less than the rotating speed of the output race of sprag clutch  104 . Operating sprag clutch  104  in the overrunning mode allows, for example, main rotor  108  of helicopter  10  to engage in autorotation in the event of a failure of main engine  102 . 
     In the illustrated embodiment, main rotor gearbox  106  is coupled to sprag clutch  104  via a suitable drive shaft. In addition, main rotor gearbox  106  is coupled to main rotor  108  by a suitable mast. Main rotor gearbox  106  includes a gearbox housing and a plurality of gears, such as planetary gears, used to adjust the engine output speed to a suitable rotor speed so that main engine  102  and main rotor  108  may each rotate at optimum speed during flight operations of helicopter  10 . Main rotor gearbox  106  is coupled to a tail rotor gearbox  110  via a suitable tail rotor drive shaft. Tail rotor gearbox  110  includes a gearbox housing and a plurality of gears that may adjust the main rotor gearbox output speed to a suitable rotational speed for operation of tail rotor  112 . Main engine  102 , sprag clutch  104 , main rotor gearbox  106  and tail rotor gearbox  110  as well as various shafts and gearing systems coupled therewith may be considered the main drive system of powertrain  100 . 
     Powertrain  100  includes a secondary engine  114  such as a turbo shaft engine or an electric motor capable of producing 200 to 400 horsepower or more, depending upon the particular implementation. In the illustrated embodiment, secondary engine  114  may generate between about 5 percent and about 20 percent or more of the horsepower of main engine  102 . In other embodiments, secondary engine  114  may generate between about 10 percent and about 15 percent of the horsepower of main engine  102 . Secondary engine  114  is coupled to a secondary gearbox  116 . Secondary engine  114  and secondary gearbox  116  as well as various shafts and gearing systems coupled therewith may be considered the secondary drive system of powertrain  100 . 
     Referring additionally to  FIG. 3A , secondary gearbox  116  includes a freewheeling unit depicted as sprag clutch  118  that acts as a one-way clutch enabling a driving mode in which torque from secondary engine  114  is coupled through sprag clutch  118  from an input race  120  to an output race  122 . In the illustrated embodiment, output race  122  is coupled to an output gear  126  that provides power to accessories  124  such as one or more generators, air pumps, oil pumps, hydraulic systems and the like. Sprag clutch  118  has an overrunning mode in which secondary engine  114  is decoupled from torque transfer through sprag clutch  118  when the rotating speed of input race  120  is less than the rotating speed of output race  122 . Operating sprag clutch  118  in the overrunning mode allows, for example, main engine  102  to drive accessories  124  when secondary engine  114  is in standby mode or not operating, as discussed herein. 
     Secondary gearbox  116  includes a multimode clutch assembly  128  that is coaxially aligned with sprag clutch  118  and secondary engine  114 , in the illustrated embodiment. In other embodiments, multimode clutch assembly  128  may operate on a separate axis than sprag clutch  118  and/or secondary engine  114 . Multimode clutch assembly  128  has a unidirectional torque transfer mode and a bidirectional torque transfer mode. In the illustrated embodiment, multimode clutch assembly  128  includes a freewheeling unit depicted as sprag clutch  130 , a bypass assembly  132  and an actuator assembly  134 . Sprag clutch  130  has an input race  136  that is coupled to main rotor gearbox  106  via the tail rotor drive shaft and one or more gears including input gear  138 . Sprag clutch  130  has an output race  140  that is coupled to output race  122  of sprag clutch  118  via shaft  122   a.  Shaft  122   a  has outer splines (not visible) that are coupled to inner splines  140   a  of output race  140 . Likewise, shaft  122   a  has outer splines (not visible) that are coupled to inner splines (not visible) of output race  122 . Sprag clutch  130  may act as a one-way clutch enabling a driving mode in which torque from the main drive system is coupled through sprag clutch  130  from input race  136  to output race  140 . Sprag clutch  130  also has an overrunning mode in which the main drive system is decoupled from torque transfer with sprag clutch  130  when the rotating speed of input race  136  is less than the rotating speed of output race  140  of sprag clutch  130 . When sprag clutch  130  is acting as a one-way clutch, multimode clutch assembly  128  is in its unidirectional torque transfer mode. In the unidirectional torque transfer mode of multimode clutch assembly  128 , torque can be driven from the main drive system through secondary gearbox  116  but torque cannot be driven from secondary gearbox  116  to the main drive system of powertrain  100 . 
     Referring additionally to  FIG. 3C , the overrunning mode of multimode clutch assembly  128  can be disabled by engaging bypass assembly  132  to couple input race  136  and output race  140  of sprag clutch  130  to functionally form a connected shaft. In this configuration with bypass assembly  132  preventing sprag clutch  130  from operating in the overrunning mode, multimode clutch assembly  128  is in its bidirectional torque transfer mode. In the bidirectional torque transfer mode of multimode clutch assembly  128 , torque can be driven from the main drive system through secondary gearbox  116  and torque can be driven from secondary gearbox  116  to the main drive system of powertrain  100 . 
     Multimode clutch assembly  128  is operated between the unidirectional and bidirectional torque transfer modes by shifting bypass assembly  132  between its disengaged position ( FIG. 3A ) and its engaged position ( FIG. 3C ). The operations of engaging and disengaging bypass assembly  132  may be pilot controlled and/or may be automated by the flight control computer of helicopter  10  and may be determined according to the operating conditions of helicopter  10 . In the illustrated embodiment, bypass assembly  132  is shifted between the engaged and disengaged positions responsive to engagement and disengagement forces supplied by actuator assembly  134 , which may be generated mechanically, electrically, hydraulically, pneumatically and/or combinations thereof or by other suitable actuation signaling means. 
     In the illustrated embodiment, actuator assembly  134  includes an actuator liner  142  that is fixed relative to the housing of secondary gearbox  116 . A piston  144  is slidably and sealingly received within actuator liner  142 . In the illustrated embodiment, piston  144  is coupled to a piston extension depicted as an oil jet  146 . In other embodiments, piston  144  and oil jet  146  may be integral or oil jet  146  may be omitted. Actuator assembly  134  also includes a bearing sled  148  that is slidably received about actuator liner  142  and that slidably receives piston  144  therein. Bearing sled  148  and actuator liner  142  preferably including an anti-rotation feature that prevents relative rotation therebetween such as a tab and slot assembly wherein, for example, one or more tabs of actuator liner  142  extend radially outwardly into slots of bearing sled  148  or wherein one or more tabs of bearing sled  148  extend radially inwardly into slots of actuator liner  142  (not pictured). In the illustrated embodiment, a mechanical biasing element depicted as wave spring  150  is positioned between a shoulder of piston  144  and a shoulder bearing sled  148 . A bearing assembly depicted as a ball bearing set  152  couples bearing sled  148  with bypass assembly  132  such that bypass assembly  132  translates with bearing sled  148  and is rotatable relative to bearing sled  148  as well as the other components of actuator assembly  134 . In the illustrated embodiment, the inner race of ball bearing set  152  has an anti-rotation coupling with bearing sled  148 . In addition, actuator assembly  134  includes an actuator  154  having a cylinder  156  that is shiftable responsive to an electric signal, a hydraulic signal, a pneumatic signal or the like. When actuator  154  is electrically signaled, actuator  154  may be referred to herein as an electric switch. When actuator  154  is hydraulically or pneumatically signaled, actuator  154  may be referred to herein as a pressure switch and more precisely a hydraulic switch or a compressed air switch, respectively. Operation of cylinder  156  by actuator  154  causes piston  144  to shift relative to actuator liner  142  between first and second positions. Shifting of piston  144  causes bypass assembly  132  to shift between engaged and disengaged positions with sprag clutch  130 . More specifically, bypass assembly  132  includes a shaft  132   a  having outer splines (not visible) and a ring gear  132   b  having outer splines (not visible). The outer splines of shaft  132   a  are in mesh with inner splines  140   a  of output race  140  of sprag clutch  130  such that when output race  140  is rotating, bypass coupling  132  also rotates. The outer splines of ring gear  132   b  are selectively engaged with and disengaged from inner splines  136   a  of input race  136  to operate multimode clutch assembly  128  between the unidirectional and bidirectional torque transfer modes. 
     Returning to  FIGS. 2A-2E , operating scenarios for helicopter  10  will now be described. In  FIG. 2A , powertrain  100  is in a preflight configuration in which main engine  102  is not yet operating as indicated by the dashed lines between the components of the main drive system. As the main drive system is not turning, no torque is being applied to secondary gearbox  116  from the main drive system as indicated by the dashed line therebetween. Prior to starting secondary engine  114 , an engagement status of multimode clutch assembly  128  should be checked. In the illustrated embodiment, an engagement status sensor includes three circumferentially distributed inductive proximity sensors  158  (only one being visible in  FIGS. 3A-3C ) that are used to determine the engagement status of bypass assembly  132  by measuring the position of bearing sled  148  relative to proximity sensors  158  by detecting the presence or absence of the metal of bearing sled  148  adjacent to the faces of proximity sensors  158 . For example, as best seen in  FIG. 3C , proximity sensors  158  detect the absence of bearing sled  148  relative thereto indicating bypass assembly  132  is in the engaged position. In addition, as best seen in  FIGS. 3A and 3B , proximity sensors  158  detect the presence of bearing sled  148  relative thereto indicating bypass assembly  132  is not in the engaged position. In other embodiments, other numbers of proximity sensors  158  in other orientations may be used. In still other embodiments, other types of engagement status sensors may be used to determine the engagement status of bypass assembly  132 , as will be discussed herein. In addition to determining the engagement status of bypass assembly  132  in pre-flight, the use of an engagement status sensor is also beneficial in determining, for example, a malfunction of actuator assembly  134 , breakage of wave spring  150 , partial engagement or disengagement of bypass assembly  132 , disengagement of bypass assembly  132  during flight, disengagement of bypass assembly  132  under torque, engagement of bypass assembly  132  at a differential speed relative to outer race  136  as well as other undesirable conditions. 
     Following the status check, if multimode clutch assembly  128  is not in the unidirectional torque transfer mode with bypass assembly  132  in the disengaged position, actuator  154  provides a suitable disengagement signal (hydraulic, pneumatic, electric) to operate cylinder  156  and shift piston  144  to the position shown in  FIG. 3A , thereby shifting bypass assembly  132  to the disengaged position. It is noted that in the disengaged position, contact between bypass assembly  132  and the housing of secondary gearbox  116  is prevented by bearing sled  148 . Another status check may now be performed. Following the status check, if multimode clutch assembly  128  is in the unidirectional torque transfer mode with bypass assembly  132  is in the disengaged position, secondary engine  114  may be started such that secondary engine  114  provides torque and rotational energy within the secondary drive system, as indicated by the arrows between secondary engine  114 , secondary gearbox  116  and accessories  124 , in  FIG. 2A . More specifically, secondary engine  114  is driving input race  120  of sprag clutch  118 , which causes output race  122  of sprag clutch  118  to drive output gear  126  which in turn provides power to accessories  124 . It is noted that rotation of output race  122  causes rotation of shaft  122   a  which in turn causes rotation of output race  140  of sprag clutch  130 , which is operation in its overrunning mode. In addition, rotation of shaft  122   a  causes rotation bypass assembly  132  via inner splines  140   a.  While operating in the preflight configuration, the pilot of helicopter  10  can proceed through the startup procedure. Prior to starting main engine  102 , the status of multimode clutch assembly  128  may be checked again using proximity sensors  158 . This process step provides further assurance that bypass assembly  132  is secured in the disengaged position prior to starting main engine  102 . 
     Once main engine  102  is started, torque is delivered through the main drive system as indicated by the arrows between the components within the main drive system, as best seen in  FIG. 2B . In addition, the main drive system may supply torque to secondary gearbox  116 , as indicated by the arrow therebetween. When power is applied to input race  136  of sprag clutch  130  via input gear  138  from the main drive system such that input race  136  and output race  140  of sprag clutch  130  are turning together at the same speed, multimode clutch assembly  128  may be operated from the unidirectional torque transfer mode to the bidirectional torque transfer mode. Specifically, bypass assembly  132  can now be shifted from the disengaged position to the engaged position responsive to pilot input and/or operation of the flight control computer of helicopter  10 . In the illustrated embodiment, actuator  154  provides a suitable engagement signal (hydraulic, pneumatic, electric) to operate cylinder  156  and shift piston  144  to the position shown in  FIG. 3B . In the illustrated configuration, the movement of piston  144  relative to actuator liner  142  and bearing sled  148  has compressed wave spring  150  between piston  144  and bearing sled  148  due to contact between the faces of the outer splines of ring gear  132   b  and inner splines  136   a  of input race  136 . Wave spring  150  assists in overcoming such misalignment in the clocking of the outer splines of ring gear  132   b  and inner splines  136   a  of input race  136  by allowing full actuation of piston  144  while maintaining pressure between ring gear  132   b  and input race  136  so that when bypass assembly  132  and input race  136  start to rotate relative to each other, the outer splines of ring gear  132   b  will mesh with inner splines  136   a  of input race  136 , thereby shifting bypass assembly  132  to the engaged position and multimode clutch assembly  128  to the bidirectional torque transfer mode, as best seen in  FIG. 3C . 
     If the outer splines of ring gear  132   b  and inner splines  136   a  of input race  136  are aligned prior to operating cylinder  156 , bypass assembly  132  may be shifted directly from the disengaged position ( FIG. 3A ) to the engaged position ( FIG. 3C ) without compressing spring  150  or being in the intermediate position depicted in  FIG. 3B . In the bidirectional torque transfer mode of multimode clutch assembly  128 , when input race  136  of sprag clutch  130  is driven by the main drive system, bypass assembly  132  and output race  140  rotate therewith. In addition, when output race  140  of sprag clutch  130  is driven by secondary engine  114 , bypass assembly  132  and input race  136  rotate therewith to supply power to main drive system, thereby bypassing the overrunning mode of sprag clutch  130  such that multimode clutch assembly  128  operates with the functionality of a connected shaft. Actuator assembly  134  preferably has a suitable locking mechanism to maintain bypass assembly  132  in the engaged position until it is desired to shift bypass assembly  132  to the disengaged position. 
     In the engaged position, bypass assembly  132  couples input race  136  with output race  140  such that multimode clutch assembly  128  is in the bidirectional torque transfer mode. In this configuration, secondary engine  114  may be operated in standby mode or powered down as indicated by the dashed line between secondary engine  114  and secondary gearbox  116  in  FIG. 2C , such that main engine  102  is driving not only the main drive system but also accessories  124 , as indicated by the arrows to secondary gearbox  116  and accessories  124 . This configuration of powertrain  100  may be referred to as a high efficiency configuration. In addition, secondary engine  114  may be operated to provide supplemental power to the main drive system as indicated by the arrow between secondary gearbox  116  and the tail rotor drive shaft in  FIG. 2D . This configuration of powertrain  100  may be referred to as an enhanced power configuration. 
     Continuing with the operating scenarios of helicopter  10 , once multimode clutch assembly  128  is in the bidirectional torque transfer mode, helicopter  10  is ready for takeoff. Assuming a high power demand takeoff and/or hover, powertrain  100  is preferably in the enhanced power configuration of  FIG. 2D  for takeoff. Once helicopter  10  has completed the takeoff and is flying at a standard speed cruise, it may be desirable to place secondary engine  114  in standby mode such as idle operations or even shut secondary engine  114  down to operate helicopter  10  in the high efficiency configuration depicted in  FIG. 2C . In this configuration, secondary engine  114  provide no power as indicated by the dashed line between secondary engine  114  and secondary gearbox  116  with torque and rotational energy being provided by main engine  102  through the main drive system to secondary gearbox  116  and accessories  124 . More specifically, power from the main drive system is transferred through multimode clutch assembly  128  to output gear  126  by input race  136  and output race  140  that are coupled together by bypass assembly  132  then by shaft  122   a  and output race  122  of sprag clutch  118 . Rotational energy is not sent to input race  120 , as sprag clutch  118  is operating in its overrunning mode. Thus, in addition to powering main rotor  108  and tail rotor  112 , in the high efficiency configuration of powertrain  100 , main engine  102  also powers accessories  124 . 
     It should be noted that multimode clutch assembly  128  is preferably maintained in its bidirectional torque transfer mode during all flight operations. For example, having multimode clutch assembly  128  in its bidirectional torque transfer mode is a safety feature of helicopter  10  in the event of a failure in main engine  102  during flight, as indicated by the dashed lines between main engine  102  and sprag clutch  104  in  FIG. 2E . In this case, an autorotation maneuver may be performed in which the descent of helicopter  10  creates an aerodynamic force on main rotor  108  as air moves up through main rotor  108  generating rotational inertia. Upon final approach during the autorotation landing, helicopter  10  performs a flare recovery maneuver in which the kinetic energy of main rotor  108  is converted into lift using aft cyclic control. Both the autorotation maneuver and the flare recovery maneuver are enhanced by operating secondary engine  114  and sending power through secondary gearbox  116  to the main drive system, as indicated by the arrow therebetween, and more particularly by sending power to main rotor  108  as indicated by the arrows leading thereto. It is noted that rotational energy is also sent to sprag clutch  104 , which is operating in its overrunning mode while main engine  102  is not operating. This configuration may be referred to as the enhanced autorotation configuration of powertrain  100  in which main engine  102  is not operating but secondary engine  114  is providing power to main rotor  108  through multimode clutch assembly  128 , which is in the bidirectional torque transfer mode. 
     Continuing with the operating scenarios of helicopter  10 , after a conventional landing, when it is desired to operate multimode clutch assembly  128  from the bidirectional to the unidirectional torque transfer mode, main engine  102  continues to provide torque and rotational energy to input race  136 , which in turn drives output race  140  of sprag clutch  130 . Actuator  154  then provides a suitable disengagement signal (hydraulic, pneumatic, electric) to operate cylinder  156  and shift piston  144  to the position shown in  FIG. 3A  such that the outer splines of ring gear  132   b  shift out of mesh with inner splines  136   a  of input race  136 , thereby shifting bypass assembly  132  to the disengaged position. Actuator assembly  134  preferably has a suitable locking mechanism to maintain bypass assembly  132  in the disengaged position until it is desired to shift bypass assembly  132  to the engaged position. 
     Referring next to  FIGS. 4A-4B , the lubrication strategy for secondary gearbox  116  will now be described. Secondary gearbox  116  includes a lubrication circuit in which pressurized lubricating oil is depicted as heavy dashed lines  200 . The lubrication circuit includes an oil pump (not pictured) that pressurizes and routes lubricating oil to secondary gearbox  116  and in particular to supply port  202 . Pressurized lubricating oil  200  is then routed to an annular passageway  204  defined between the housing of secondary gearbox  116  and actuator liner  142  by a pair of seals depicted as O-rings. Actuator liner  142  includes one or more passageways  206  that route pressurized lubricating oil  200  to an annular oil chamber  208  defined between actuator liner  142  and piston  144  by a pair of seals depicted as O-rings  144   a,    144   b.  Pressurized lubricating oil  200  then enters the interior of piston  144  via one or more ports  210  that are in fluid communication with annular oil chamber  208 . From piston  144 , pressurized lubricating oil  200  flows into oil jet  146  that includes a plurality of nozzles  146   a,    146   b,    146   c,    146   d,    146   e,    146   f.  A filter or debris screen (not pictured) may be positioned within piston  144  to prevent any solids within pressurized lubricating oil  200  from entering oil jet  146  and the plugging nozzles. 
     Each of the nozzles directs pressurized lubricating oil  200  into a specific region within shaft  122   a  defined between adjacent oil dams. More specifically, one or more nozzles  146   a  direct pressurized lubricating oil  200  into region  212 , one or more nozzles  146   b  direct pressurized lubricating oil  200  into region  214 , one or more nozzles  146   c  direct pressurized lubricating oil  200  into region  216 , one or more nozzles  146   d  direct pressurized lubricating oil  200  into region  218 , one or more nozzles  146   e  direct pressurized lubricating oil  200  into region  220  and one or more nozzles  146   f  direct pressurized lubricating oil  200  into region  222 . The centrifugal force generated by rotation of shaft  122   a  during operation of helicopter  10  aids in oil flow from the interior of shaft  122   a  to the desired locations within secondary gearbox  116 . For example, pressurized lubricating oil  200  from region  212  flows to ball bearing set  152  for lubrication thereof. Similarly, pressurized lubricating oil  200  from region  216  flows to sprag clutch  130  to provide lubrication for the sprag elements  130   a  between input race  136  and output race  140  as well as for clutch bearing sets  130   b,    130   c.  Oil dams within sprag clutch  130  keep sprag elements  130   a  submerged in pressurized lubricating oil  200 . The oil dams may also include metering orifices that route pressurized lubricating oil  200  to clutch bearing sets  130   b,    130   c.  Likewise, pressurized lubricating oil  200  from region  222  flows to sprag clutch  118  to provide lubrication for the sprag elements  118   a  between input race  120  and output race  122  as well as for clutch bearing sets  118   b,    118   c.  Oil dams within sprag clutch  118  keep sprag elements  118   a  submerged in pressurized lubricating oil  200 . The oil dams may also include metering orifices that route pressurized lubricating oil  200  to clutch bearing sets  118   b,    118   c.  Importantly, lubrication circuit integrity is maintained when bypass assembly  132  is shifted between the engaged and disengaged positions as the oil inlet to annular oil chamber  208  remains between O-ring  144   a,    144   b  as piston  144  shifts within actuator liner  142  between the disengaged position of bypass assembly  132  ( FIG. 4A ) and the engaged position of bypass assembly  132  ( FIG. 4B ). 
     As discussed herein, multimode clutch assembly  128  is preferably maintained in its bidirectional torque transfer mode during all flight operations. This is achieved in the embodiment depicted in  FIGS. 5A-5B  using a mechanical biasing element that maintains bypass assembly  132  in the engaged position unless a disengagement force sufficient to overcome the engagement force of the mechanical biasing element is applied. Specifically, a secondary gearbox  300  includes sprag clutch  118  having input race  120  and output race  122  which is coupled to output gear  126  that provides power to accessories  124 . Secondary gearbox  300  also includes a multimode clutch assembly  128  that is coaxially aligned with sprag clutch  118 . Multimode clutch assembly  128  has a unidirectional torque transfer mode and a bidirectional torque transfer mode. Multimode clutch assembly  128  includes sprag clutch  130 , bypass assembly  132  and an actuator assembly  302 . Sprag clutch  130  includes input race  136  that is coupled to main rotor gearbox  106  via the tail rotor drive shaft and one or more gears including input gear  138 . Sprag clutch  130  includes output race  140  that is coupled to output race  122  of sprag clutch  118  via shaft  122   a.  Sprag clutch  130  may act as a one-way clutch enabling a driving mode in which torque from the main drive system is coupled through sprag clutch  130  from input race  136  to output race  140 . Sprag clutch  130  also has an overrunning mode in which the main drive system is decoupled from torque transfer with sprag clutch  130  when the rotating speed of input race  136  is less than the rotating speed of output race  140  of sprag clutch  130 . When sprag clutch  130  is acting as a one-way clutch, multimode clutch assembly  128  is in its unidirectional torque transfer mode. In the unidirectional torque transfer mode of multimode clutch assembly  128 , torque can be driven from the main drive system through secondary gearbox  300  but torque cannot be driven from secondary gearbox  300  to the main drive system of powertrain  100 . 
     The overrunning mode of multimode clutch assembly  128  can be disabled by engaging bypass assembly  132  to couple input race  136  and output race  140  of sprag clutch  130  to functionally form a connected shaft. In this configuration with bypass assembly  132  preventing sprag clutch  130  from operating in the overrunning mode, multimode clutch assembly  128  is in its bidirectional torque transfer mode. In the bidirectional torque transfer mode of multimode clutch assembly  128 , torque can be driven from the main drive system through secondary gearbox  300  and torque can be driven from secondary gearbox  300  to the main drive system of powertrain  100 . 
     Multimode clutch assembly  128  is operated between the unidirectional and bidirectional torque transfer modes by shifting bypass assembly  132  between its disengaged position ( FIG. 5A ) and its engaged position ( FIG. 5B ). The operations of engaging and disengaging bypass assembly  132  may be pilot controlled and/or may be automated by the flight control computer of helicopter  10  and may be determined according to the operating conditions of helicopter  10 . In the illustrated embodiment, bypass assembly  132  is shifted between the engaged and disengaged positions responsive to engagement and disengagement forces supplied by actuator assembly  302 . 
     Actuator assembly  302  includes an actuator liner  304  that is fixed relative to the housing of secondary gearbox  300 . A piston  306  is slidably and sealingly received within actuator liner  304 . In the illustrated embodiment, piston  306  is coupled to a piston extension depicted as oil jet  146 . Actuator assembly  302  also includes a bearing sled  308  that is slidably received about actuator liner  304 . Bearing sled  308  is coupled to piston  306  to prevent relative translation therebetween and thus, may be considered part of piston  306 . In the illustrated embodiment, a mechanical biasing element depicted as wave spring  310  is positioned between a shoulder of actuator liner  304  and an end of bearing sled  308 . A bearing assembly depicted as ball bearing set  152  couples bearing sled  308  with bypass assembly  132  such that bypass assembly  132  is rotatable relative to bearing sled  308  as well as the other components of actuator assembly  302 . In addition, actuator assembly  302  includes an actuator  312  having a cylinder  314  that is shiftable responsive to an electric signal, a hydraulic signal, a pneumatic signal or the like. In the illustrated embodiment, actuator assembly  302  has an energized configuration in which cylinder  314  is retracted, as depicted in  FIG. 5A , and an unenergized or default configuration in which cylinder  314  is released, as depicted in  FIG. 5B . 
     When actuator  312  is not activated, the biasing force generated by wave spring  310  acts on bearing sled  308  and serves as an engagement force to shift bypass assembly  132  from the disengaged position ( FIG. 5A ) to the engaged position ( FIG. 5B ). In addition, once bypass assembly  132  is in the engaged position, the biasing force generated by wave spring  310  continues to act on bearing sled  308  to maintain the engagement force on bypass assembly  132 , thereby preventing bypass assembly  132  from shifting out of the engaged position. The use of actuator assembly  302  with wave spring  310  makes multimode clutch assembly  128  a mechanically failsafe multimode clutch assembly that remains in the bidirectional torque transfer mode even if a failure occurs in a related electric, hydraulic and/or pneumatic system. When helicopter  10  has landed and it is desired to shift bypass assembly  132  from the engaged position ( FIG. 5B ) to the disengaged position ( FIG. 5A ), actuator  312  is energized with the appropriate electric signal, hydraulic signal, pneumatic signal or the like to generate a disengagement force that overcomes the engagement force of wave spring  310  causing cylinder  314  to shift piston  306  relative to actuator liner  304  which compresses wave spring  310  between actuator liner  304  and bearing sled  308  and shifts bypass assembly  132  to the disengaged position. In the illustrated embodiment, actuator  312  must remain energized to overcome the engagement force of wave spring  310 . Actuator assembly  302  may have a suitable locking mechanism to secure bypass assembly  132  in the disengaged position until it is desired to shift bypass assembly  132  to the engaged position, in which case, actuator assembly  302  may be deenergized after the locking mechanism has been deployed. 
     Alternatively or additionally, actuator  312  may be used to provide at least a portion of the engagement force to shift bypass assembly  132  from the disengaged position ( FIG. 5A ) to the engaged position ( FIG. 5B ). For example, actuator  312  may be energized with the appropriate electric signal, hydraulic signal, pneumatic signal or the like to generate at least a portion of the engagement force that together with the biasing force generated by wave spring  310  shifts bypass assembly  132  from the disengaged position to the engaged position. In this embodiment, once bypass assembly  132  is in the engaged position, actuator  312  may be unenergized as the biasing force generated by wave spring  310  continues to act on bearing sled  308  to maintain the engagement force on bypass assembly  132 , thereby preventing bypass assembly  132  from shifting out of the engaged position. 
       FIGS. 6A-6B  depict another embodiment of a secondary gearbox that includes a failsafe multimode clutch assembly. In this embodiment, a pressurized fluid maintains bypass assembly  132  in the engaged position unless a disengagement force sufficient to overcome the engagement force of the pressurized fluid is applied. Specifically, secondary gearbox  400  includes sprag clutch  118  having input race  120  and output race  122  which is coupled to output gear  126  that provides power to accessories  124 . Secondary gearbox  400  also includes a multimode clutch assembly  128  that is coaxially aligned with sprag clutch  118 . Multimode clutch assembly  128  has a unidirectional torque transfer mode and a bidirectional torque transfer mode. Multimode clutch assembly  128  includes sprag clutch  130 , bypass assembly  132  and an actuator assembly  402 . Sprag clutch  130  includes input race  136  that is coupled to main rotor gearbox  106  via the tail rotor drive shaft and one or more gears including input gear  138 . Sprag clutch  130  includes output race  140  that is coupled to output race  122  of sprag clutch  118  via shaft  122   a.  Sprag clutch  130  may act as a one-way clutch enabling a driving mode in which torque from the main drive system is coupled through sprag clutch  130  from input race  136  to output race  140 . Sprag clutch  130  also has an overrunning mode in which the main drive system is decoupled from torque transfer with sprag clutch  130  when the rotating speed of input race  136  is less than the rotating speed of output race  140  of sprag clutch  130 . When sprag clutch  130  is acting as a one-way clutch, multimode clutch assembly  128  is in its unidirectional torque transfer mode. In the unidirectional torque transfer mode of multimode clutch assembly  128 , torque can be driven from the main drive system through secondary gearbox  400  but torque cannot be driven from secondary gearbox  400  to the main drive system of powertrain  100 . 
     The overrunning mode of multimode clutch assembly  128  can be disabled by engaging bypass assembly  132  to couple input race  136  and output race  140  of sprag clutch  130  to functionally form a connected shaft. In this configuration with bypass assembly  132  preventing sprag clutch  130  from operating in the overrunning mode, multimode clutch assembly  128  is in its bidirectional torque transfer mode. In the bidirectional torque transfer mode of multimode clutch assembly  128 , torque can be driven from the main drive system through secondary gearbox  400  and torque can be driven from secondary gearbox  400  to the main drive system of powertrain  100 . 
     Multimode clutch assembly  128  is operated between the unidirectional and bidirectional torque transfer modes by shifting bypass assembly  132  between its disengaged position ( FIG. 6A ) and its engaged position ( FIG. 6B ). The operations of engaging and disengaging bypass assembly  132  may be pilot controlled and/or may be automated by the flight control computer of helicopter  10  and may be determined according to the operating conditions of helicopter  10 . In the illustrated embodiment, bypass assembly  132  is shifted between the engaged and disengaged positions responsive to engagement and disengagement forces supplied by actuator assembly  402 . 
     Actuator assembly  402  includes an actuator liner  404  that is fixed relative to the housing of secondary gearbox  400 . A piston  406  is slidably and sealingly received within actuator liner  404 . In the illustrated embodiment, piston  406  is coupled to a piston extension depicted as oil jet  146 . Actuator assembly  402  also includes a bearing sled  408  that is slidably received about actuator liner  404  and that slidably receives piston  406  therein. In the illustrated embodiment, a mechanical biasing element depicted as wave spring  410  is positioned between a shoulder of piston  406  and a shoulder of bearing sled  408 . Wave spring  410  operates in a manner similar to wave spring  150  discussed herein to assist in overcoming any misalignment in the clocking between splines of bypass assembly  132  and input race  136  during engagement operations. A bearing assembly depicted as ball bearing set  152  couples bearing sled  408  with bypass assembly  132  such that bypass assembly  132  is rotatable relative to bearing sled  408  as well as the other components of actuator assembly  402 . In addition, actuator assembly  402  includes an actuator  412  having a cylinder  414  that is shiftable responsive to an electric signal, a hydraulic signal, a pneumatic signal or the like. In the illustrated embodiment, actuator assembly  402  has an energized configuration in which cylinder  414  is retracted, as depicted in  FIG. 6A , and an unenergized or default configuration in which cylinder  414  is released, as depicted in  FIG. 6B . 
     Similar to the lubrication circuit described herein with reference to  FIGS. 4A-4B , secondary gearbox  400  includes a lubrication circuit that not only provides pressurized lubricating oil to various components within secondary gearbox  400  but also provides a pressure source for failsafe operations of bypass assembly  132 . In particular, the lubrication circuit of secondary gearbox  400  includes an oil pump (not pictured) that pressurizes and routes lubricating oil  416  to supply port  418 . Pressurized lubricating oil  416  is then routed to an annular passageway  420  defined between the housing of secondary gearbox  400  and actuator liner  404  by a pair of seals depicted as O-rings. Actuator liner  404  includes one or more passageways  422  that route pressurized lubricating oil  416  to an annular oil chamber  424  defined between actuator liner  404  and piston  406  by a pair of seals depicted as O-rings  406   a,    406   b.  While not illustrated, pressurized lubricating oil  416  then enters the interior of piston  406  for distribution to various components via nozzles of oil jet  146 , as discussed herein. In the illustrated embodiment, annular oil chamber  424  and O-rings  406   a,    406   b  defined a differential pressure chamber as the annular area defined by O-ring  406   b  is larger than the annular area defined by O-ring  406   a  such that when pressurized lubricating oil  416  flows through annular oil chamber  424 , a biasing force is generated that acts on piston  406  and serves as an engagement force to shift bypass assembly  132  from the disengaged position ( FIG. 6A ) to the engaged position ( FIG. 6B ). In addition, once bypass assembly  132  is in the engaged position, the biasing force generated by pressurized lubricating oil  416  in annular oil chamber  424  continues to act on piston  406  to maintain the engagement force on bypass assembly  132 , thereby preventing bypass assembly  132  from shifting out of the engaged position. 
     The use of actuator assembly  402  with pressurized lubricating oil  416  in annular oil chamber  424  makes multimode clutch assembly  128  a hydraulically failsafe multimode clutch assembly that remains in the bidirectional torque transfer mode even if a failure occurs in a related electric, hydraulic and/or pneumatic system. When helicopter  10  has landed and it is desired to shift bypass assembly  132  from the engaged position ( FIG. 6B ) to the disengaged position ( FIG. 6A ), actuator  412  is energized with the appropriate electric signal, hydraulic signal, pneumatic signal or the like to generate a disengagement force that overcomes the engagement force of pressurized lubricating oil  416  in annular oil chamber  424  causing cylinder  414  to shift piston  406  relative to actuator liner  404  which in turn shifts bypass assembly  132  to the disengaged position. In the illustrated embodiment, actuator  412  must remain energized to overcome the engagement force of pressurized lubricating oil  416  in annular oil chamber  424 . Actuator assembly  402  may have a suitable locking mechanism to secure bypass assembly  132  in the disengaged position until it is desired to shift bypass assembly  132  to the engaged position, in which case, actuator assembly  402  may be deenergized after the locking mechanism has been deployed. 
     Alternatively or additionally, actuator  412  may be used to provide at least a portion of the engagement force to shift bypass assembly  132  from the disengaged position ( FIG. 6A ) to the engaged position ( FIG. 6B ). For example, actuator  412  may be energized with the appropriate electric signal, hydraulic signal, pneumatic signal or the like to generate at least a portion of the engagement force that together with the biasing force generated by pressurized lubricating oil  416  in annular oil chamber  424  shifts bypass assembly  132  from the disengaged position to the engaged position. In this embodiment, once bypass assembly  132  is in the engaged position, actuator  412  may be unenergized as the biasing force generated by pressurized lubricating oil  416  in annular oil chamber  424  continues to act on piston  406  to maintain the engagement force on bypass assembly  132 , thereby preventing bypass assembly  132  from shifting out of the engaged position. 
     As discussed herein, maintaining bypass assembly  132  in the engaged position during all flight operations is an important safety feature of the present helicopter to ensure, for example, that the secondary engine can provide power to the main rotor in the event of a main engine failure. Depending upon the specific configuration of the multimode clutch assembly, a variety of engagement status sensors may be used to monitor the engagement status of the multimode clutch assembly. In one example,  FIG. 7A  depicts secondary gearbox  116  with bypass assembly  132  in the engaged position. In the illustrated embodiment, multimode clutch assembly  128  includes one or more proximity sensors depicted as one or more load cells  500 . Load cells  500  may be coupled to an end of shaft  122   a  such that translation of bypass assembly  132  brings an end of shaft  132   a  into contact with load cells  500  when bypass assembly  132  is in the engaged position. In one example, load cells  500  may be compression load cells having strain gauges that provide an electrical signal to indicate the presence or absence of a load and/or an absolute load between a no-load condition and a full-capacity load. Such compression load cells may also be referred to herein as strain sensors. In operation, a no-load reading by load cells  500  indicates bypass assembly  132  is not in the engaged position while a load reading indicates bypass assembly  132  is in the engaged position, thereby providing the engagement status of bypass assembly  132 . Alternatively, a load reading below a predetermined threshold by load cells  500  indicates bypass assembly  132  is not in the engaged position while a load reading above a predetermined threshold indicates bypass assembly  132  is in the engaged position, thereby providing the engagement status of bypass assembly  132 . 
     In another example,  FIG. 7B  depicts secondary gearbox  116  with bypass assembly  132  in the engaged position. In the illustrated embodiment, multimode clutch assembly  128  includes one or more engagement status sensors depicted as one or more tooth passage frequency sensors  502 . For example, tooth passage frequency sensors  502  could be variable reluctance sensors, monopole sensors, hall-effect sensors, optical sensors or the like. In the illustrated embodiment, bypass assembly  132  includes two ring gears  132   b,    132   c.  The number of splines on ring gear  132   b  is different from the number of teeth on ring gear  132   c  such as in a ratio of 2 or 3 to 1 or in a ratio of 1 to 2 or 3. When bypass assembly  132  is in the engaged position, tooth passage frequency sensors  502  are aligned with rotating ring gear  132   c  such that the alternating presence and absence of the passing gear teeth has a first frequency. When bypass assembly  132  is in the disengaged position, tooth passage frequency sensors  502  are aligned with rotating ring gear  132   b  which has a different number of splines than the number of teeth of ring gear  132   c  such that the alternating presence and absence of the passing splines has a second frequency. The frequency detected by tooth passage frequency sensors  502  is different for ring gear  132   b  versus ring gear  132   c  such that the change in frequency and/or the absolute frequency provides the engagement status of bypass assembly  132 . 
     When tooth passage frequency sensors  502  are variable reluctance sensors, for example, the alternating presence and absence of the passing gear teeth vary the reluctance of a magnetic field, which dynamically changes the magnetic field strength. This changing magnetic field strength induces a current into a coil winding which is attached to the output terminals such that the variable reluctance sensors provide a frequency output. Alternatively or additionally, tooth passage frequency sensors  502  may be used to detect a change in the annular speed of bypass assembly  132  in the engaged position versus the disengage position, even in embodiments having the same number of teeth on both ring gears  132   b,    132   c.  In this implementation, tooth passage frequency sensors  502  provide a first frequency reading when bypass assembly  132  is in the engaged position and a second frequency reading, based upon a lower or a higher annular speed of bypass assembly  132  depending upon the status of secondary engine  114 , when bypass assembly  132  is in the disengaged position, thereby providing the engagement status of bypass assembly  132 . 
     In a further example,  FIG. 7C  depicts secondary gearbox  300  with bypass assembly  132  in the engaged position. In the illustrated embodiment, multimode clutch assembly  128  includes one or more oil pressure sensors  504  which may be positioned within actuator liner  304  or may be otherwise located within the secondary gearbox downstream of an oil pressure passageway. Oil pressure sensors  504  are selectively aligned with one or more ports  506  of piston  306  that are in communication with the lubrication circuit of secondary gearbox  300  to detect the presence or absence of oil pressure and/or a high pressure or low pressure condition. Specifically, when bypass assembly  132  is in the disengaged position, ports  506  are not aligned with oil pressure sensors  504  whereas, when bypass assembly  132  is in the engaged position, ports  506  are aligned with oil pressure sensors  504 . A pressure reading below a predetermined threshold by oil pressure sensors  504  indicates bypass assembly  132  is not in the engaged position while a pressure reading above a predetermined threshold indicates bypass assembly  132  is in the engaged position, thereby providing the engagement status of bypass assembly  132 . 
       FIG. 7D  depicts secondary gearbox  300  with bypass assembly  132  in the engaged position. In the illustrated embodiment, multimode clutch assembly  128  includes an engagement status sensor depicted as a variable differential transformer in the form of a linear variable differential transformer  508 . Linear variable differential transformer  508  includes a core  508   a  that is coupled to the end of oil jet  146  that translates within a coil assembly  508   b  such that an input voltage within coil assembly  508   b  induces two output voltages as piston  306  is shifted between first and second positions. The electrical signals generated in responsive to the rectilinear motion of core  508   a  relative to coil assembly  508   b  is used to determine the engagement status of bypass assembly  132 . 
       FIG. 7E  depicts secondary gearbox  300  with bypass assembly  132  in the engaged position. In the illustrated embodiment, multimode clutch assembly  128  includes an engagement status sensor depicted as a variable differential transformer in the form of a rotary variable differential transformer  510 . An input shaft of rotary variable differential transformer  510  is rotated by gear teeth  512  on piston  306  as piston  306  is shifted between first and second positions. Rotary variable differential transformer  510  provides a variable alternating current output voltage that is linearly proportional to the angular displacement of the input shaft. These electrical signals are used to determine the engagement status of bypass assembly  132 . 
     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.