Tail rotor isolation systems for rotorcraft

A tail rotor isolation system for rotorcraft includes a secondary engine, first and second freewheeling units, an isolation assembly and a tail rotor system. The secondary engine is coupled to the input race of the first freewheeling unit. A main rotor system is coupled to the output race of the second freewheeling unit. The isolation assembly is coupled to the output race of the first freewheeling unit and has a fully engaged position coupling the input and output races of the second freewheeling unit and a partially engaged position coupled to the input race but decoupled from the output race of the second freewheeling unit. The tail rotor system is coupled to the input race of the second freewheeling unit such that in the partially engaged position of the isolation assembly, the overrunning mode of the second freewheeling unit isolates the tail rotor system from the main rotor system.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to improving rotorcraft safety during ground operations and, in particular, to a tail rotor isolation system configured to selectively couple and decouple the tail rotor from torque generated by the main rotor system, selectively transmit torque to the tail rotor from a secondary engine and stop rotation of the tail rotor during ground operations.

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. During ground operations, the tail rotor of a running rotorcraft can be dangerous to crew, passengers or other ground personnel as an encounter with a rotating tail rotor can be fatal. Exacerbating matters, a rotating tail rotor can be difficult to see. Accordingly, a need has arisen for improved rotorcraft systems that enable power for preflight operations, supplemental power during high-power demand operations and/or non-rotation of the tail rotor during ground operations.

SUMMARY

In a first aspect, the present disclosure is directed to a tail rotor isolation system for rotorcraft having a main rotor system including a main engine, a main rotor gearbox and a main rotor. The tail rotor isolation system includes a secondary engine, first and second freewheeling units, an isolation assembly and a tail rotor system. The first and second freewheeling units each have an input race and an output race such that torque applied to the input race is transferred to the output race in a driving mode and torque applied to the output race is not transferred to the input race in an overrunning mode. The input race of the first freewheeling unit is coupled to the secondary engine. The output race of the second freewheeling unit is coupled to the main rotor system. The isolation assembly is disposed between the first and second freewheeling units. The isolation assembly is coupled to the output race of the first freewheeling unit. The isolation assembly has a fully engaged position in which the isolation assembly couples the input and output races of the second freewheeling unit and a partially engaged position in which the isolation assembly is coupled to the input race of the second freewheeling unit and decoupled from the output race of the second freewheeling unit. The tail rotor system is coupled to the input race of the second freewheeling unit. In the partially engaged position of the isolation assembly, the overrunning mode of the second freewheeling unit isolates the tail rotor system from torque generated by the main rotor system. Also, in the partially engaged position of the isolation assembly, the tail rotor system is coupled to torque generated by the secondary engine. In the fully engaged position of the isolation assembly, the tail rotor system is coupled to torque generated by the main rotor system and the secondary engine.

In some embodiments, the secondary engine may be configured to generate between 5 percent and 20 percent of the power of the main engine. In other embodiments, the secondary engine may be configured to generate approximately the same amount of power as the main engine. In certain embodiments, the secondary engine may be a gas turbine engine. In other embodiments, the secondary engine may be an electric motor. In some embodiments, the isolation assembly may include an outer housing and a splined adaptor that is disposed within the outer housing, rotatable with the outer housing and translatable relative to the outer housing between the fully engaged position and the partially engaged position with the second freewheeling unit. In such embodiments, the splined adaptor may have a splined coupling with the input race of the second freewheeling unit in both the fully engaged position and the partially engaged position. Also, in such embodiments, the splined adaptor may have a splined coupling with the output race of the second freewheeling unit in the fully engaged position and may be decoupled from the output race of the second freewheeling unit in the partially engaged position. In certain embodiments, the splined adaptor may have outer splines and inner splines such that the outer splines of the splined adaptor may have a splined coupling with inner splines of the input race of the second freewheeling unit in both the fully engaged position and the partially engaged position and such that the inner splines of the splined adaptor may have a splined coupling with outer splines of the output race of the second freewheeling unit in the fully engaged position and may be decoupled from the outer splines of the output race of the second freewheeling unit in the partially engaged position.

In some embodiments, an actuator may be coupled to the splined adaptor and configured to shift the splined adaptor between the fully engaged position and the partially engaged position with the second freewheeling unit. In such embodiments, the actuator may be a linear actuator such as a hydraulic actuator, an electromechanical actuator or a pneumatic actuator. In certain embodiments, the tail rotor system may include a tail rotor and a tail rotor brake. In such embodiments, in the partially engaged position of the isolation assembly and with the main rotor system operating, the tail rotor brake may be configured to stop rotation of the tail rotor when the secondary engine is shut down to establish a ground safety configuration of the rotorcraft. In some embodiments, in the partially engaged position of the isolation assembly, the overrunning mode of the second freewheeling unit may be enabled such that the second freewheeling unit is configured for unidirectional torque transfer from the input race to the output race. In certain embodiments, in the fully engaged position of the isolation assembly, the overrunning mode of the second freewheeling unit may be disabled such that the second freewheeling unit is configured for bidirectional torque transfer between the input race and the output race.

In a second aspect, the present disclosure is directed to a rotorcraft. The rotorcraft includes a main rotor system including a main engine, a main rotor gearbox coupled to the main engine and a main rotor coupled to the main rotor gearbox. The rotorcraft also includes a secondary engine, first and second freewheeling units, an isolation assembly and a tail rotor system. The first and second freewheeling units each have an input race and an output race such that torque applied to the input race is transferred to the output race in a driving mode and torque applied to the output race is not transferred to the input race in an overrunning mode. The input race of the first freewheeling unit is coupled to the secondary engine. The output race of the second freewheeling unit is coupled to the main rotor system. The isolation assembly is disposed between the first and second freewheeling units. The isolation assembly is coupled to the output race of the first freewheeling unit. The isolation assembly has a fully engaged position in which the isolation assembly couples the input and output races of the second freewheeling unit and a partially engaged position in which the isolation assembly is coupled to the input race of the second freewheeling unit and decoupled from the output race of the second freewheeling unit. The tail rotor system is coupled to the input race of the second freewheeling unit. In the partially engaged position of the isolation assembly, the overrunning mode of the second freewheeling unit isolates the tail rotor system from torque generated by the main rotor system. Also, in the partially engaged position of the isolation assembly, the tail rotor system is coupled to torque generated by the secondary engine. In the fully engaged position of the isolation assembly, the tail rotor system is coupled to torque generated by the main rotor system and the secondary engine.

In certain embodiments, the rotorcraft may be a helicopter. In some embodiments, in a ground safety configuration, the isolation assembly is in the partially engaged position, the main rotor system is operating, the secondary engine is shut down and the tail rotor brake is engaged to stop rotation of the tail rotor. In certain embodiments, in an enhanced power configuration, the isolation assembly is in the fully engaged position, the main engine provides power to the main rotor gearbox and the secondary engine provides power to the tail rotor system and the main rotor system through the first and second freewheeling units and the isolation assembly. In some embodiments, in a high efficiency configuration, the isolation assembly is in the fully engaged position, the secondary engine is in standby mode and the main engine provides power to the main rotor gearbox and the tail rotor system through the second freewheeling unit. In certain embodiments, in an enhanced autorotation configuration, the isolation assembly is in the fully engaged position, the main engine is not operating and the secondary engine provides power to the main rotor system through the first and second freewheeling units and the isolation assembly.

DETAILED DESCRIPTION

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 toFIGS.1A-1Cin the drawings, a rotorcraft in the form of a helicopter is schematically illustrated and generally designated10. The primary propulsion assembly of helicopter10is a main rotor12. Main rotor12includes a plurality of rotor blades14extending radially outward from a main rotor hub16. Main rotor12is coupled to a fuselage18and is rotatable relative thereto. The pitch of rotor blades14can be collectively and/or cyclically manipulated to selectively control direction, thrust and lift of helicopter10. A tailboom20is coupled to fuselage18and extends from fuselage18in the aft direction. Anti-torque is provided to helicopter10by a tail rotor system22that includes a tail rotor24, a tail rotor gearbox26and a tail rotor brake28. Tail rotor system22controls the yaw of helicopter10by counteracting the torque exerted on fuselage18by main rotor12. In the illustrated embodiment, helicopter10includes a vertical tail fin30that provide stabilization to helicopter10during high-speed forward flight. In addition, helicopter10includes wing members32that extend laterally from fuselage18and wing members34that extend laterally from tailboom20. Wing members32,34provide lift compounding to helicopter10responsive to the forward airspeed of helicopter10, thereby reducing the lift requirement on main rotor12and increasing the top speed of helicopter10

Main rotor12receive torque and rotational energy from a main engine36. Main engine36is coupled to a main rotor gearbox38by suitable clutching and shafting. Main rotor gearbox38is coupled to main rotor12by a mast40. As discussed herein, main rotor gearbox38is also selectively coupled to tail rotor system22through a secondary gearbox42and a tail rotor drive shaft44. In the illustrated embodiment, a secondary engine46is also coupled to secondary gearbox42. Collectively, main engine36, main rotor gearbox38and main rotor12may be referred to herein as the main rotor system48.

During ground operations, a rotating tail rotor can be dangerous to crew, passengers or other ground personnel as an encounter with a rotating tail rotor can be fatal. Exacerbating matters, a rotating tail rotor can be difficult to see. In the present embodiments, tail rotor system22can be selectively isolated from torque generated by main rotor system48based upon the configuration of secondary gearbox42. When tail rotor system22is isolated from torque generated by main rotor system48, rotation of tail rotor24is controlled by operation of secondary engine46and tail rotor brake28. In some embodiments, secondary engine46is operable as an auxiliary power unit to provide preflight power to the accessories of helicopter10such as electric generators, air pumps, oil pumps, hydraulic systems and the like as well as to provide the power required to start main engine36. In addition, secondary engine46is operable to provide supplemental power to main rotor12that is additive with the power provided by main engine36during, for example, high-power demand conditions including takeoff, hover, heavy lifts and high-speed flight operations. Secondary engine46is also operable to provide emergency power to main rotor12. For example, in the event of a failure of main engine36, secondary engine46is operable to provide emergency power to enhance the autorotation and flare recovery maneuver of helicopter10. Use of secondary engine46not only enhances the safety of helicopter10but also increases the efficiency of helicopter10. For example, having the extra power provided by secondary engine46during high-power demand operations allows main engine32to be downsized for more efficient single engine operations such as during high efficiency cruise operations.

It should be appreciated that helicopter10is merely illustrative of a variety of aircraft that can implement the embodiments disclosed herein. Indeed, the tail rotor isolation system of the present disclosure may be implemented on any rotorcraft. Other aircraft implementations can include hybrid aircraft, unmanned aircraft, gyrocopters, compound helicopters, drones and the like. As such, those skilled in the art will recognize that the tail rotor isolation system 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 now toFIG.2Ain the drawings, various mechanical systems of a rotorcraft which is representative of helicopter10are illustrated in a block diagram format and are generally designated100. Systems100include a main engine102such as a gas turbine engine capable of producing 2000 to 4000 horsepower or more, depending upon the particular implementation. Main engine102is coupled to a freewheeling unit depicted as sprag clutch104that acts as a one-way clutch enabling a driving mode in which torque from main engine102is coupled to main rotor gearbox106when the rotating speed of the input race, on the main engine side of sprag clutch104, is matched with the rotating speed of the output race, on the main rotor gearbox side of sprag clutch104. Importantly, sprag clutch104has an overrunning mode in which main engine102is decoupled from main rotor gearbox106when the rotating speed of the input race is less than the rotating speed of the output race of sprag clutch104. Operating sprag clutch104in the overrunning mode allows, for example, main rotor108to engage in autorotation in the event of a failure of main engine102.

In the illustrated embodiment, main rotor gearbox106is coupled to sprag clutch104via a suitable drive shaft. In addition, main rotor gearbox106is coupled to main rotor108by a suitable mast. Main rotor gearbox106includes 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 engine102and main rotor108may each rotate at optimum speed during flight operations. Collectively, main engine102, sprag clutch104, main rotor gearbox106and main rotor108may be referred to herein as the main rotor system. In the illustrated embodiment, systems100also include a secondary engine110such as a gas turbine engine or an electric motor capable of producing 200 to 400 horsepower or more, depending upon the particular implementation. For example, secondary engine110may generate between 5 percent and 20 percent of the power of main engine102. In other embodiments, secondary engine110may generate approximately the same amount of power as main engine102, in which case, secondary engine110may be referred to as a second main engine suitable for use in a twin engine rotorcraft. In the illustrated embodiment, secondary engine110is coupled to a secondary gearbox112by a suitable driveshaft. Main rotor gearbox106is also coupled to secondary gearbox112by a suitable driveshaft. A tail rotor system including tail rotor114, tail rotor gearbox116and tail rotor brake118is coupled to secondary gearbox112by a suitable tail rotor driveshaft. Optionally, accessories120may be coupled to secondary gearbox112. In other embodiments, the accessories may be powered directly from the main rotor system.

InFIG.2B, systems100are depicted in an auxiliary power configuration that is beneficial for embodiments in which accessories120are coupled to secondary gearbox112. In the illustrated embodiment, secondary engine110is providing torque to accessories120through secondary gearbox112as indicated by the arrows between secondary engine110, secondary gearbox112and accessories120. As discussed herein, certain embodiments of secondary gearbox112have a disengaged position which isolates the tail rotor system from torque from secondary gearbox112as well as torque from the main rotor system as indicated by the dashed lines between secondary gearbox112and the components of the tail rotor system. Optionally, tail rotor brake118may be engaged as a further safety measure against rotation of tail rotor114. As illustrated, the main rotor system is not operating as indicated by the dashed lines between the components of the main rotor system and the dashed line between main rotor gearbox106and secondary gearbox112.

InFIG.2C, systems100are depicted in a main engine ramp up configuration as indicated by the arrows between the components of the main rotor system and the arrow from main rotor gearbox106to secondary gearbox112. In the illustrated embodiment, secondary engine110is providing torque to accessories120through secondary gearbox112as indicated by the arrows between secondary engine110, secondary gearbox112and accessories120. In this configuration, secondary gearbox112is isolating the tail rotor system from torque from secondary gearbox112as well as torque from the main rotor system as indicated by the dashed lines between secondary gearbox112and the components of the tail rotor system. Optionally, tail rotor brake118may be engaged as a further safety measure against rotation of tail rotor114.

InFIG.2D, systems100are depicted in an auxiliary power off configuration as indicated by the dashed line between secondary engine110and secondary gearbox112. With the main rotor system operating, as indicated by the arrows between the components of the main rotor system and the arrow from main rotor gearbox106to secondary gearbox112, accessories120are now powered by the main rotor system as indicated by the arrow from secondary gearbox112to accessories120. In this configuration, secondary gearbox112may be shifted from the disengaged position to a partially engaged position which couples secondary engine110and the tail rotor system while neither is rotating, as indicated by the dashed lines between secondary engine110and secondary gearbox112as well as the dashed lines between secondary gearbox112and the components of the tail rotor system. In this configuration of secondary gearbox112, the tail rotor system remains isolated from torque from the main rotor system, also as indicated by the dashed lines between secondary gearbox112and the components of the tail rotor system. Optionally, tail rotor brake118may be engaged as a further safety measure against rotation of tail rotor114and if engaged, should be disengaged prior to the entering the secondary engine ramp up configuration described next.

InFIG.2E, systems100are depicted in the secondary engine ramp up configuration as indicated by the arrow from secondary engine110to secondary gearbox112as well as the arrows between the components of the main rotor system and the arrow from main rotor gearbox106to secondary gearbox112. As secondary gearbox112was shifted to the partially engaged position, tail rotor gearbox116and tail rotor114ramp up with secondary engine110as indicated by the arrows between secondary gearbox112and the components of the tail rotor system. When secondary engine110is fully ramped up to match the rotating speed of the main rotor system, one or both of secondary engine110and the main rotor system provide torque to accessories120and the tail rotor system via secondary gearbox112, as indicated by the arrow from secondary gearbox112to accessories120and by the arrows between secondary gearbox112and the components of the tail rotor system. In this configuration, secondary gearbox112may be shifted from the partially engaged position to a fully engaged position that couples the main rotor system and the tail rotor system as indicated by the arrow from main rotor gearbox106to secondary gearbox112and by the arrows between secondary gearbox112and the components of the tail rotor system. This configuration of systems100may represent a normal flight configuration for a twin engine rotorcraft with both main engine102and secondary engine110operating together during flight. Alternatively, this configuration of systems100may represent an enhanced power configuration for a rotorcraft having a primary engine that provide cruise power and a secondary engine the provides supplemental power.

It should be noted that the fully engaged position of secondary gearbox112enables secondary engine110to provide torque to the main rotor system through secondary gearbox112including in the event of a failure in main engine102. In this case, an autorotation maneuver may be performed in which the descent of helicopter10creates an aerodynamic force on main rotor108as air moves up through main rotor108generating rotational inertia. Upon final approach during the autorotation landing, helicopter10performs a flare recovery maneuver in which the kinetic energy of main rotor108is converted into lift using aft cyclic control. Both the autorotation maneuver and the flare recovery maneuver are enhanced by operating secondary engine110and sending power through secondary gearbox112to the main rotor system. It is noted that rotational energy is also sent to sprag clutch104, which is operating in its overrunning mode while main engine102is not operating. This configuration may be referred to as the enhanced autorotation configuration in which main engine102is not operating but secondary engine110is providing power to main rotor108.

InFIG.2F, systems100are depicted in a high efficiency configuration in which secondary engine110has been shut down or placed in standby mode, as indicated by the dashed lines between secondary engine110and secondary gearbox112, while main engine102provide power for all components as indicated by the arrows between the components of the main rotor system, the arrow from main rotor gearbox106to secondary gearbox112, the arrow from secondary gearbox112to accessories120and by the arrows between secondary gearbox112and the components of the tail rotor system.

Referring next toFIGS.3A-3Bof the drawings, an embodiment of a secondary gearbox200will be discussed in greater detail. Secondary gearbox200may be representative of secondary gearbox42and/or secondary gearbox112. Secondary gearbox200includes a freewheeling unit depicted as sprag clutch202that acts as a one-way clutch enabling a driving mode in which torque from the secondary engine110(seeFIGS.4A-4D) is coupled through sprag clutch202from an input race204to an output race206. In the illustrated embodiment, output race206is coupled to an output gear208that provides torque to an isolation assembly210. Sprag clutch202has an overrunning mode in which secondary engine110is decoupled from torque transfer through sprag clutch202when the rotating speed of input race204is less than the rotating speed of output race206. Operating sprag clutch202in the overrunning mode allows, for example, the main rotor system to drive torque to the tail rotor system when secondary engine110is in standby mode or not operating, as discussed herein.

Secondary gearbox200includes a multimode clutch assembly212that is coupled to output gear208of sprag clutch202. Multimode clutch assembly212includes a freewheeling unit depicted as sprag clutch214, isolation assembly210coupled to sprag clutch214and an actuator coupled to isolation assembly210(seeFIGS.4A-4D). Sprag clutch214may function as a one-way clutch enabling a driving mode in which torque from secondary engine110is coupled through sprag clutch214from an input race216to an output race218. In the illustrated embodiment, input race216is coupled to an input gear220that provides torque to the tail rotor system. When sprag clutch214is operating as a one-way clutch, sprag clutch214has an overrunning mode in which torque is not transferred through sprag clutch214when the rotating speed of input race216is less than the rotating speed of output race218. In the illustrated embodiment, output race218is coupled to a driveshaft222from the main rotor system. Operating sprag clutch214in the overrunning mode allows, for example, the isolation of the tail rotor system from torque generated by the main rotor system.

Multimode clutch assembly212has a unidirectional torque transfer mode and a bidirectional torque transfer mode. In the illustrated embodiment, isolation assembly210includes an outer housing224integral with an input gear226and a splined adaptor228that is disposed within outer housing224. A set of outer splines (not visible) of splined adaptor228form a splined coupling with a set of inner splines230of outer housing224such that splined adaptor228is rotatable with outer housing224and translatable relative to outer housing224. In addition, splined adaptor228has a set of inner splines232and a set of outer splines (not visible) on a flanged end234thereof. The outer splines mesh with a set of inner splines236to form a splined coupling with input race216of sprag clutch214. Inner splines232selectively mesh with a set of outer splines238to form a splined coupling with output race218of sprag clutch214.

As best seen inFIG.3A, isolation assembly210is in a partially engaged position wherein splined adaptor228has a splined coupling with input race216of sprag clutch214and is decoupled from output race218of sprag clutch214. In this configuration, sprag clutch214acts as a one-way clutch enabling the driving mode in which torque transfers from input race216to output race218and the overrunning mode in which torque does not transfer from output race218to input race216, thereby isolating torque generated by the main rotor system from the tail rotor system. When sprag clutch214is acting as a one-way clutch, multimode clutch assembly212is in its unidirectional torque transfer mode.

As best seen inFIG.3B, isolation assembly210has been shifted from the partially engaged position to a fully engaged position. In the fully engaged position, splined adaptor228has a splined coupling with input race216of sprag clutch214and a splined coupling with output race218of sprag clutch214which couples input race216to output race218and functionally forms a connected shaft. In this configuration, isolation assembly210prevent sprag clutch214from operating in the overrunning mode which causes multimode clutch assembly212to operate in its bidirectional torque transfer mode. In the bidirectional torque transfer mode of multimode clutch assembly212, torque can be driven from input race216to output race218or from output race218to input race216enabling, for example, torque generated by the main rotor system to drive the tail rotor system.

The operation of secondary gearbox200will now be described with reference toFIGS.4A-4D. InFIG.4A, systems100are in the normal flight configuration and/or the high efficiency configuration in which secondary engine110has been shut down or placed in standby mode, as indicated by the dashed lines between secondary engine110and sprag clutch202, while main rotor system240provides power for all components as indicated by the arrows between main rotor system240, sprag clutch214, isolation assembly210, sprag clutch202and the components of the tail rotor system; namely, tail rotor114, tail rotor gearbox116and tail rotor brake118. During all flight configurations including the normal flight configuration, isolation assembly210is in the fully engaged position with sprag clutch214as indicated by the double arrows between isolation assembly210and sprag clutch214as well as the arrow from actuator242to isolation assembly210. With isolation assembly210in the fully engaged position, multimode clutch assembly212is operating in its bidirectional torque transfer mode which enables torque generated by the main rotor system240to drive the tail rotor system through sprag clutch214.

InFIG.4B, systems100are depicted in the secondary engine ramp up configuration as indicated by the arrow from secondary engine110to sprag clutch202as well as the arrows between sprag clutch202, isolation assembly210, sprag clutch214, main rotor system240and the components of the tail rotor system which are now driven by torque from secondary engine110when to rotating speed of secondary engine110matches that of main rotor system240. This configuration may be considered as the enhanced power configuration of the rotorcraft. In the illustrated embodiment, this configuration is also required when it is desired to isolate the tail rotor system from main rotor system torque. Specifically, once the rotating speeds of secondary engine110and main rotor system240are matched with input race216rotating at the same speed as output race218of sprag clutch214, isolation assembly210can now be actuated from the fully engaged position to the partially engaged position.

InFIG.4C, systems100are depicted in a tail rotor isolation configuration which preferably occurs after the rotorcraft has landed. This configuration is achieved by linear operation of actuator242to shift splined adaptor228from the fully engaged position (seeFIG.3B) to the partially engaged position (seeFIG.3A). The operation of actuator242may be pilot controlled and/or may be automated by the flight control computer of the rotorcraft. In the illustrated embodiment, splined adaptor228is shifted between the fully engaged and partially engaged positions responsive to linear forces supplied by actuator242, which may be generated mechanically, electrically, hydraulically, pneumatically and/or combinations thereof or by other suitable actuation signaling means. The partially engaged position of isolation assembly210is indicated by the single arrow and dashed line between isolation assembly210and sprag clutch214as well as the arrow from isolation assembly210to actuator242. In this configuration, torque from secondary engine110is driving the tail rotor system as indicated by the arrows between secondary engine110, sprag clutch202, isolation assembly210, sprag clutch214and the components of the tail rotor system. With isolation assembly210in the partially engaged position, multimode clutch assembly212is operating in its unidirectional torque transfer mode which enables the overrunning mode of sprag clutch214, thereby isolating the tail rotor system from torque generated by the main rotor system240.

InFIG.4D, systems100are depicted in a ground safety configuration in which the main rotor system240remains in operation as in indicated by the arrow from main rotor system240to sprag clutch214and in which secondary engine110and tail rotor114are not operating as indicated by the dashed lines between secondary engine110, sprag clutch202, isolation assembly210, sprag clutch214and the components of the tail rotor system. Isolation assembly210remains in the partially engaged position such that multimode clutch assembly212is operating in its unidirectional torque transfer mode which enables the overrunning mode of sprag clutch214and the isolation of the tail rotor system from torque generated by main rotor system240. Once secondary engine110is powered down, tail rotor brake118may be used to rapidly stop the rotation of tail rotor114and lock tail rotor114in a non-rotating state, thereby enhancing the safety of a rotorcraft utilizing the tail rotor isolation system of the present disclosure.

When the rotorcraft is ready to return to flight modes, secondary engine110is ramped up together with the tail rotor system which can be represented byFIG.4C. When secondary engine110is rotating at the same speed as main rotor system240, actuator242may be used to shift splined adaptor228from the partially engaged position (seeFIG.3A) to the fully engaged position (seeFIG.3B) which can be represented byFIG.4B. With isolation assembly210in the fully engaged position, multimode clutch assembly212is operating in its bidirectional torque transfer mode which enables torque generated by the main rotor system240to drive the tail rotor system. Thereafter, the rotorcraft may operate in the enhanced power configuration which can be represented byFIG.4Band/or the high efficiency configuration which can be represented byFIG.4A.

Referring next toFIGS.5A-5Cof the drawings, another embodiment of a secondary gearbox300will be discussed in greater detail. Secondary gearbox300may be representative of secondary gearbox42and/or secondary gearbox112. Secondary gearbox300includes a freewheeling unit depicted as sprag clutch302that acts as a one-way clutch enabling a driving mode in which torque from the secondary engine110(seeFIGS.6A-6F) is coupled through sprag clutch302from an input race304to an output race306. In the illustrated embodiment, output race306is coupled to an output gear308that provides torque to an isolation assembly310. Sprag clutch302has an overrunning mode in which secondary engine110is decoupled from torque transfer through sprag clutch302when the rotating speed of input race304is less than the rotating speed of output race306. Operating sprag clutch302in the overrunning mode allows, for example, the main rotor system to drive torque to accessories and the tail rotor system when secondary engine110is in standby mode or not operating, as discussed herein.

Secondary gearbox300includes a multimode clutch assembly312that is coupled to output gear308of sprag clutch302. Multimode clutch assembly312includes a freewheeling unit depicted as sprag clutch314, isolation assembly310coupled to sprag clutch314and an actuator coupled to isolation assembly310(seeFIGS.6A-6F). Sprag clutch314may function as a one-way clutch enabling a driving mode in which torque from secondary engine110is coupled through sprag clutch314from an input race316to an output race318. In the illustrated embodiment, input race316is coupled to an input gear320that provides torque to the tail rotor system. When sprag clutch314is operating as a one-way clutch, sprag clutch314has an overrunning mode in which torque is not transferred through sprag clutch314when the rotating speed of input race316is less than the rotating speed of output race318. In the illustrated embodiment, output race318is coupled to a driveshaft322from the main rotor system. Operating sprag clutch314in the overrunning mode allows, for example, the isolation of the tail rotor system from torque generated by the main rotor system.

Multimode clutch assembly312has a unidirectional torque transfer mode and a bidirectional torque transfer mode. In the illustrated embodiment, isolation assembly310includes an outer housing324integral with an input gear326and a splined adaptor328that is disposed within outer housing324. A set of outer splines (not visible) of splined adaptor328form a splined coupling with a set of inner splines330of outer housing324such that splined adaptor328is rotatable with outer housing324and translatable relative to outer housing324. In addition, splined adaptor328has a set of inner splines332and a set of outer splines (not visible) on a flanged end334thereof. The outer splines selectively mesh with a set of inner splines336to form a splined coupling with input race316of sprag clutch314. Inner splines332selectively mesh with a set of outer splines338to form a splined coupling with output race318of sprag clutch314.

As best seen inFIG.5A, isolation assembly310is in a disengaged position wherein splined adaptor328is decoupled from input race316of sprag clutch314and decoupled from output race318of sprag clutch314. In this configuration, sprag clutch314acts as a one-way clutch enabling the driving mode in which torque transfers from input race316to output race318and the overrunning mode in which torque does not transfer from output race318to input race316isolating torque generated by the main rotor system from the tail rotor system.

As best seen inFIG.5B, isolation assembly310has been shifted from the disengaged position to a partially engaged position. In the partially engaged position, splined adaptor328has a splined coupling with input race316of sprag clutch314and is decoupled from output race318of sprag clutch314. In this configuration, sprag clutch314acts as a one-way clutch enabling the driving mode in which torque transfers from input race316to output race318and the overrunning mode in which torque does not transfer from output race318to input race316isolating torque generated by the main rotor system from the tail rotor system. When sprag clutch314is acting as a one-way clutch, multimode clutch assembly312is in its unidirectional torque transfer mode.

As best seen inFIG.5C, isolation assembly310has been shifted from the partially engaged position to a fully engaged position. In the fully engaged position, splined adaptor328has a splined coupling with input race316of sprag clutch314and a splined coupling with output race318of sprag clutch314which couples input race316to output race318and functionally forms a connected shaft. In this configuration, isolation assembly310prevent sprag clutch314from operating in the overrunning mode which causes multimode clutch assembly312to operate in its bidirectional torque transfer mode. In the bidirectional torque transfer mode of multimode clutch assembly312, torque can be driven from input race316to output race318or from output race318to input race316enabling, for example, torque generated by the main rotor system to drive the tail rotor system.

Secondary gearbox300also provides multiple torque paths to power accessories120(seeFIGS.6A-6F). Secondary gearbox300includes a freewheeling unit depicted as sprag clutch342that acts as a one-way clutch enabling a driving mode in which torque from output race306of sprag clutch302is coupled through sprag clutch342from an input race344to an output race346. In the illustrated embodiment, output race346is coupled to a driveshaft348that provides torque to an accessories gear train350that powers accessories120. Sprag clutch342has an overrunning mode in which secondary engine110is decoupled from torque transfer through sprag clutch342when the rotating speed of input race344is less than the rotating speed of output race346. Operating sprag clutch342in the overrunning mode allows, for example, the main rotor system to drive torque to accessories120when secondary engine110is in standby mode or not operating.

Secondary gearbox300includes a freewheeling unit depicted as sprag clutch352that acts as a one-way clutch enabling a driving mode in which torque from the main rotor system through input gear358is coupled through sprag clutch352from an input race354to an output race356. In the illustrated embodiment, output race356is coupled to a driveshaft360that provides torque to accessories gear train350that powers accessories120. Sprag clutch352has an overrunning mode in which the main rotor system is decoupled from torque transfer through sprag clutch352when the rotating speed of input race354is less than the rotating speed of output race356. Operating sprag clutch352in the overrunning mode allows, for example, secondary engine110to drive torque to accessories120when the main rotor system is not operating.

The operation of secondary gearbox300will now be described with reference toFIGS.6A-6F. InFIG.6A, systems100are in the preflight configuration in which secondary engine110operating as an auxiliary power unit to provide torque that powers accessories120as indicated by the arrows between secondary engine110, sprag clutch302, sprag clutch342and accessories120. In addition, sprag clutch352is operating in the overrunning mode. Isolation assembly310is in the disengaged position (FIG.5A) so no torque is being transferred to sprag clutch314as indicated by the double dashed lines between isolation assembly310and sprag clutch314as well as the double dashed lines between isolation assembly310and actuator362. In this configuration, the tail rotor system is isolated from torque generated by secondary engine110. Main rotor system340has not yet been started up as indicated by the dashed lines between main rotor system340and sprag clutches314,352.

InFIG.6B, systems100are in the main rotor system ramp up configuration in which secondary engine110continues to operate as an auxiliary power unit to provide torque that powers accessories120as indicated by the arrows between secondary engine110, sprag clutch302, sprag clutch342and accessories120. Main rotor system340is operating as indicated by the arrows from main rotor system340to sprag clutches314,352. When the rotation speed of main rotor system340matches or exceeds that of secondary engine110, sprag clutch352provides power to accessories120as indicted by the arrow from sprag clutch352to accessories120. Sprag clutch314is operating in its overrunning mode such that torque from main rotor system340is not transferred to the tail rotor system as indicated by the dashed lines between sprag clutch314and the components of the tail rotor system. In addition, isolation assembly310remains in the disengaged position (FIG.5A) so no torque is being transferred to sprag clutch314as indicated by the double dashed lines between isolation assembly310and sprag clutch314as well as the double dashed lines between isolation assembly310and actuator362. In this configuration, the tail rotor system is isolated from torque generated by secondary engine110and from main rotor system340. This configuration is considered to be a ground safety configuration in which tail rotor114is not rotating. Tail rotor brake118may be engaged to lock tail rotor114in the non-rotating state.

InFIG.6C, systems100are depicted in an auxiliary power off configuration as indicated by the dashed line between secondary engine110, sprag clutch302, sprag clutch342and isolation assembly310. Main rotor system340continues to operate as indicated by the arrows from main rotor system340to sprag clutches314,352and accessories120. Sprag clutch342is operating in its overrunning mode. In addition, the tail rotor system remains isolated from torque from main rotor system340with sprag clutch314operating in the overrunning mode as indicated by the dashed lines between sprag clutch314and the components of the tail rotor system. As both isolation assembly310and input race316are not rotating, splined adaptor328may be shifted from the disengaged position (FIG.5A) to the partially engaged position (FIG.5B) which couples the outer splines of splined adaptor328with inner splines336of input race316but does not couple inner splines332with outer splines338of output race318, as indicated by the solid line and dashed line between isolation assembly310and sprag clutch314as well as the arrow and dashed line between actuator362and isolation assembly310. Optionally, tail rotor brake118may be engaged as a further safety measure against rotation of tail rotor114and if engaged, should be disengaged prior to the entering the secondary engine ramp up configuration described next.

InFIG.6D, systems100are depicted in the secondary engine ramp up configuration as indicated by the arrows between secondary engine110, sprag clutch302, sprag clutch342and isolation assembly310. As isolation assembly310is in the partially engaged position, tail rotor114ramps up with secondary engine110as indicated by the arrows between sprag clutch314and the components of the tail rotor system. When secondary engine110is fully ramped up to match the rotating speed of main rotor system340, secondary engine110may provide torque to main rotor system340as indicated by the arrow between sprag clutch314and main rotor system340.

InFIG.6E, systems100are depicted in the tail rotor coupling configuration in which secondary engine110continues to drive sprag clutches302,342,352, insolation assembly310, sprag clutch314and the tail rotor system as indicated by the arrows therebetween. With input race316and output race318of sprag clutch314rotating at the same speed, splined adaptor328may be shifted from the partially engaged position to the fully engaged position responsive to operation of actuator362as indicated by the dual arrows between isolation assembly310and sprag clutch314as well as the dual arrows from actuator362to isolation assembly310. Splined adaptor328couples input race316to output race318which also couples main rotor system340to the tail rotor system as indicated by the arrows between sprag clutch314and the components of the tail rotor system. This configuration of systems100may represent the enhanced power configuration.

InFIG.2F, systems100are depicted in a high efficiency configuration in which secondary engine110has been shut down or placed in standby mode, as indicated by the dashed lines between secondary engine110and sprag clutch302, while main rotor system340provides power for all components as indicated by the arrows between main rotor system340, sprag clutches314,352,342,302, accessories120, isolation assembly310and the components of the tail rotor system. Sprag clutch302is in its overrunning mode. Isolation assembly310remains in the fully engaged position as indicated by the dual arrows between isolation assembly310and sprag clutch314as well as the dual arrows from actuator362to isolation assembly310. This configuration of systems100may represent the high efficiency or normal cruise configuration.

When the rotorcraft is ready to return to the ground safety configuration, secondary engine110is ramped up to match the rotating speed of main rotor system340, which can be represented byFIG.6E. Actuator362may then be used to shift splined adaptor328from the fully engaged position (FIG.5C) to the partially engaged position (FIG.5B) which isolates the tail rotor system from torque generated by main rotor system340and can be represented byFIG.6D. Secondary engine110may now be powered down which also powers down tail rotor114. Tail rotor brake118may be used to rapidly stop the rotation of tail rotor114and lock tail rotor114in a non-rotating state, placing the rotorcraft in the ground safety configuration, which can be represented byFIG.6C.

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.