Patent Publication Number: US-2022228609-A1

Title: Power transfer unit with breakout friction reduction and leakage reduction

Description:
FIELD OF INVENTION 
     The present invention relates to power transfer units that are used to transfer hydraulic power from one hydraulic system to another without mixing fluid, and more particularly to power transfer units for the hydraulic systems in an aircraft. 
     BACKGROUND 
     Power transfer units (PTUs) are used in various applications for transferring hydraulic fluid from one system to another without sharing fluid between the two systems. For example, PTUs may be used in aircraft hydraulic systems that operate landing gears, thrust reversers, flight control surfaces, brakes, cargo doors, and many other components. The PTU is generally used to ensure control of the aircraft when one of the hydraulic systems has lost or reduced hydraulic power from another pump within the hydraulic system by supplementing power to the inoperable hydraulic system. Independent hydraulic systems may be used for different components, or two independent hydraulic systems may be used for a single component to ensure control of the single component during failure. 
     PTUs generally include two axial-piston rotating groups that are housed separately and connected by a common driveshaft and mounting adapter. The PTU mechanically couples two hydraulic systems such that, when activated, hydraulic pressure from the first system drives the hydraulic motor which transmits rotational power through the common shaft to the hydraulic pump. The rotational energy is then converted back into hydraulic pressure for the second system. The PTU is restricted from starting in response to high static friction within the unit, or a “breakout” state of the PTU. However, the friction is inherently difficult to control due to performance constraints and certain temperature ranges in aircraft applications. Friction may also vary for different fluid properties that change over the life of the aircraft. Thus, conventional PTUs may be disadvantageous during the breakout state of the aircraft. Still another disadvantage of conventional PTUs is that leakage may occur when the PTU is in a stalled state, which may consequently cause overheating of the system and excessive fuel burning for the aircraft. 
     SUMMARY OF INVENTION 
     The present application is directed towards a power transfer unit (PTU) that is connected to two hydraulic systems and is configured to supplement power to one of the hydraulic systems during failure of the hydraulic system. The PTU includes a first hydraulic circuit that corresponds to the first hydraulic system, a second hydraulic circuit that corresponds to the second hydraulic system to be supplemented, and a pump and motor assembly that is connected between the hydraulic circuits. The first hydraulic circuit includes a high-flow isolation valve that is piloted by an arming valve and controls flow to the inlet of the pump and motor assembly. The second hydraulic circuit includes a normally open unloader valve and an orifice that are used to reduce pump pressure from the outlet of the pump and motor assembly during the breakout stage of the PTU when rotation of the PTU starts. 
     The PTU is particularly advantageous for use in the hydraulic systems for components in an aircraft. The hydraulic circuits are configured to prevent leakage when pressurizing both hydraulic circuits such that the potential for system overheating is reduced and less aircraft fuel is burned. Another advantage of the PTU is reducing or eliminating breakout pressure constraints during the start-up of the PTU for supplementing power to the second hydraulic system, such that the PTU and the hydraulic systems will have smoother operation. Still another advantage of the PTU is that using the arming solenoid valve arms the PTU for operation and operates the isolation valve. By providing the arming valve, conventionally used additional PTU valves may no longer be required such that additional valves, fittings, tubing, installation time, and leakage points may be eliminated. 
     According to an aspect of the invention, a power transfer unit includes a first hydraulic circuit, a second hydraulic circuit fluidly connected to the first hydraulic circuit, a pump and motor assembly fluidly connected between the first hydraulic circuit and the second hydraulic circuit, and an isolation valve arranged along the first hydraulic circuit and fluidly connected to an inlet of the pump and motor assembly. The isolation valve is movable between a closed position and an open position to prevent and enable high-pressure fluid flow to the inlet, respectively. The power transfer unit includes an unloader valve arranged along the second hydraulic circuit and fluidly connected to an outlet of the pump and motor assembly, and an orifice arranged along the second hydraulic circuit and fluidly connected to the unloader valve to reduce back pressure in the second hydraulic circuit. 
     According to another aspect of the invention, a method of power transfer from a first hydraulic system to a second hydraulic system includes fluidly connecting a first hydraulic circuit to the first hydraulic system, fluidly connecting a second hydraulic circuit to the second hydraulic system through a pump and motor assembly, arranging an isolation valve along the first hydraulic circuit between the first hydraulic system and an inlet of the pump and motor assembly to isolate high-pressure fluid between the first hydraulic system and the motor inlet, and arranging an unloader valve and an orifice along the second hydraulic circuit between an outlet of the pump and motor assembly and a discharge line of the second hydraulic circuit to reduce back pressure in the second hydraulic circuit. 
     Other systems, devices, methods, features, and advantages of the present invention will be or become apparent to one having ordinary skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing showing an exemplary power transfer unit (PTU) and a hydraulic control circuit in an isolated state in accordance with an aspect of the present invention. 
         FIG. 2  is a schematic drawing showing the PTU and the hydraulic control circuit of  FIG. 1  in a breakout state. 
         FIG. 3  is a schematic drawing showing the PTU and the hydraulic control circuit of  FIG. 1  in a low-power operation state. 
         FIG. 4  is a schematic drawing showing the PTU and the hydraulic control circuit of  FIG. 1  in a high-power operation state. 
         FIG. 5  is a schematic drawing showing the PTU and the hydraulic control circuit of  FIG. 1  in a stopping state. 
         FIG. 6  is a sectional view of the PTU of  FIG. 1 . 
         FIG. 7  is another sectional view of the PTU of  FIG. 6 . 
         FIG. 8  is a sectional view of a front of the PTU of  FIG. 6 . 
         FIG. 9  is a cross-sectional view of the PTU of  FIG. 8  taken along a cut line A-A and showing the power transfer unit having two axial piston assemblies. 
         FIG. 10  is a cross-sectional view of the PTU of  FIG. 9  taken along a cut line B-B and showing a hydraulic circuit of the power transfer unit that corresponds to one of the two axial piston assemblies and a hydraulic system to be supplemented by the PTU. 
         FIG. 11  is a cross-sectional view of the PTU of  FIG. 9  taken along a cut line C-C and showing another portion of the hydraulic circuit of the PTU. 
         FIG. 12  is a cross-sectional view of the PTU of  FIG. 11  taken along a cut line D-D and showing an isolation valve of the hydraulic circuit. 
         FIG. 13  is a cross-sectional view of the PTU of  FIG. 11  taken along a cut line E-E and showing a connection between the isolation valve and a hydraulic system to be supplemented by the PTU. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present invention relate to a power transfer unit (PTU) for transferring hydraulic fluid from one hydraulic system to another hydraulic system without sharing fluid between the two systems. The PTU and power transfer method described herein may be suitable for many different applications that use two or more hydraulic systems to ensure operation when one of the hydraulic systems fails. An example of a suitable application is in an aircraft that uses independent hydraulic systems for operating landing gear, thrust reversers, flight control surfaces, brakes, cargo doors, and other aircraft components. For example, a hydraulic actuation system may be used for extending and retracting a landing gear. The PTU may be arranged for transferring power between different hydraulic systems for different components, or both of the hydraulic systems may be used in operating a single component to ensure control of the single component during a failure state. 
     Referring first to  FIG. 1 , a hydraulic control system  20  in a PTU  22  connected between a first hydraulic system  24  and a second hydraulic system  26  is shown. The first hydraulic system  24  and the second hydraulic system  26  may be configured for operation of different components, such as in an aircraft, or for a single component to enable operation of the component during a failure state. The hydraulic control system  20  includes a first hydraulic circuit  28  that is fluidly connected to the first hydraulic system  24  and a second hydraulic circuit  30  that is fluidly connected to the second hydraulic system  26 . In the exemplary embodiment described herein, the first hydraulic circuit  28  is arranged to unidirectionally supplement the second hydraulic circuit  30 . 
     A pump and motor assembly  31  is fluidly connected between the first hydraulic circuit  28  and the second hydraulic circuit  30 , and includes a motor  32  and a pump  34  that is rotatably coupled to the motor  32  via a common coupling shaft  36 . In an exemplary embodiment, the motor  32  may be arranged along the first hydraulic circuit  28  and the pump  34  may be arranged along the second hydraulic circuit  30 . In another exemplary embodiment, the motor  32  may be arranged along the second hydraulic circuit  30  and the pump  34  may be arranged along the first hydraulic circuit  28 . The PTU  22  may be suitable for use with in-line pumps, bent axis pumps, or a combination thereof. 
     The first hydraulic circuit  28  includes an isolation valve  38  that is fluidly connected between the first hydraulic system  24 , or a high-pressure source of the first hydraulic system  24 , and the motor  32 . The isolation valve  38  may be spring-biased in a normally closed position, as shown in  FIG. 1 , to prevent flow from the hydraulic system  24  from reaching an inlet  40  of the pump and motor assembly  31 , such as an inlet of the motor  32 , such as during a non-operational state of the PTU  22 . The isolation valve  38  is piloted by an arming valve  42  that is arranged along the first hydraulic circuit  28  and fluidly connected between the first hydraulic system  24  and a return line  44 . The arming valve  42  may be a three-way normally open solenoid valve that is configured to remove and apply pilot pressure from and to the isolation valve  38  for opening and closing the isolation valve  38 . Using the isolation valve  38  is advantageous in enabling an isolated state of the hydraulic control system  20  in which the high-pressure fluid of the first hydraulic system  24  is blocked from reaching the inlet  40  of the pump and motor assembly  31 . Thus, the PTU  22  will not rotate or produce any pressure when in the isolated state. For example, the PTU  22  may be non-rotatable during an overheat of the hydraulic system or when a low level of fluid is present in the reservoir of the hydraulic system. 
     The second hydraulic circuit  30  includes an unloader valve  46  that is fluidly connected to an outlet  48  of the pump and motor assembly  31 , such as an outlet of the pump  34 . The unloader valve  46  may be a spring-biased normally open valve that is used to reduce the pump pressure during operation of the PTU  22 . When the PTU  22  starts to rotate, discharge fluid may flow from the pump  34  to either a discharge line  50  of the second hydraulic circuit  30  or to the unloader valve  46  of the second hydraulic circuit  30 . The second hydraulic circuit  30  also includes an orifice  46   a  that is fluidly connected to the unloader valve  46  such that flow traveling to the unloader valve  46  from the pump  34  will then pass through the orifice  46 a. Providing the unloader valve  46  and the orifice  46   a  is advantageous in reducing back pressure and offloading the pump discharge line  50  to reduce an amount of breakout friction in the PTU  22 . The system may also include any suitable check valves that enable unidirectional flow from the pump  34  to the second hydraulic system  26  for supplementing the second hydraulic system  26  while also preventing high-pressure flow from the second hydraulic system  26  from entering the fluid system. 
     The second hydraulic circuit  30  may further include an orifice  52  and at least one pressure relief valve  54 . The orifice  52  is fluidly connected to the outlet of a low-pressure pilot pump of the pump. Providing the pressure relief valve  54  is advantageous in limiting pressure within the pilot system during high-speed rotation of the PTU  22 . The second hydraulic system  26  may be communicatively coupled to the isolation valve  38  via a mechanical connection  58  for sending a pilot signal to the isolation valve  38  when pressure in the hydraulic systems  24 ,  26  is equalized such that the PTU  22  may move to a stopping state. 
     A low-pressure positive displacement or pilot pump  60  is rotatably coupled with the motor  32  and the pump  34  via a shaft  62  connected to the pump  34 . The low-pressure pilot pump  60  may be a small sized fixed displacement low-pressure pilot pump that is used to activate the isolation valve  38  and the unloader valve  46 . The low-pressure pilot pump  60  is communicatively coupled to the isolation valve  38  via a connection  66  and to the unloader valve  46  via a connection  68 . When the system pressure in the hydraulic systems  24 ,  26  is equalized, the pressure from the low-pressure pilot pump  60  generates a countering signal. The countering signal counters against the pilot signal that is communicated by the second hydraulic system  26  to the isolation valve  38  via a mechanical connection  58 . The compensator valve  70  is fluidly connected to the motor  32 , the isolation valve  38  and a return line  72  for automatically regulating or stopping pump flow if the pressure in the pump and motor assembly  31  exceeds a predetermined maximum differential pressure between the hydraulic systems  24 ,  26 . 
     With further reference to  FIG. 2-5 , operation of the PTU  22  and the hydraulic control system  20  for transferring power from the first hydraulic system  24  to the second hydraulic system  26 , such as during a failure or an emergency operation, occurs as follows.  FIG. 1  shows the PTU  22  in an isolated state  74  in which the arming valve  42  is energized to remove pilot pressure from the isolation valve  38 . When the pilot pressure is removed from the isolation valve  38 , the isolation valve  38  is normally spring-biased in the closed position  38   a.  When the isolation valve  38  is in the closed position  38   a,  high-pressure from the first hydraulic system  24  is prevented from reaching the motor  32  such that the PTU  22  does not rotate or produce pressure. Accordingly, the PTU  22  may have a speed that is zero or nearly zero when in the isolated state  74 . 
       FIG. 2  shows the PTU  22  in a breakout state  76  in which pressure in the second hydraulic system  26  may drop below a predetermined amount, such as during an emergency situation. For example, the second hydraulic system  26  may have a pressure that is between 340 to 685 psi lower as compared with the first hydraulic system  24 , such that the PTU  22  is automatically actuated to supplement the second hydraulic system  26  and maintain a predetermined pressure. In an exemplary application, the PTU  22  may be operable to maintain a pressure of  3000  psi during the emergency situation. When the arming valve  42  is de-energized, the pilot pressure is applied to the isolation valve  38  via a communication line  78  to transition the isolation valve  38  to an open position  38   b.  When the isolation valve  38  is open, high-pressure fluid from the first hydraulic system  24  will flow to the inlet  40  of the motor  32 . The high-pressure in the system will start rotation of the motor  32 , the pump  34 , and the low-pressure pilot pump  60 . 
     As the PTU  22  starts to rotate, fluid will be discharged from the pump  34 . Fluid may flow from the outlet  48  of the pump  34  to either the discharge line  50  or to the unloader valve  46  which is in a normally open position  46 a. The fluid flow traveling to the unloader valve  46  may pass through the unloader valve  46  and then through an orifice  52  arranged in the second hydraulic circuit  30 . The fluid flow may further pass through the pressure relief valve  54 . Using the unloader valve  46  and the orifice  46   a  will reduce back pressure in the system as compared with the pressure in the discharge line  50 , such that the breakout friction of the PTU  22  will be reduced for a more stable control during the operation of the PTU  22 . 
     When the PTU  22  is in the breakout state  76 , the compensator valve  70  may be configured to receive a signal  80  from the pump  34  pertaining to an amount of pressure in the second hydraulic circuit  30  and a signal  81  pertaining to the amount of pressure in the first hydraulic circuit  28 . The compensator valve  70  may compare the pressures and is configured to move between an open position  70 a and a closed position  70 b in which fluid flows from the motor  32  to the return line  72 , such that the compensator valve  70  is used to enable stability and further control of flow through the pump and motor assembly  31 . The check valve  56  is also used to control flow through the system in that if the pressure in the second hydraulic system  26  is high, the check valve  56  is unidirectional and operable to prevent high-pressure from entering the system. 
       FIG. 3  shows a low-power state  82  of the PTU  22 , which occurs after the breakout state  76  and during which the PTU  22  is operable at a low speed for supplying power to the second hydraulic system  26 . During the low-power state  82 , for example, the demand of the second hydraulic system  26  may be between  15  and  30  liters per minute. As the PTU  22  rotates, the fluid that is discharged from the pump  34  will increase pressure in the second hydraulic circuit  30  to supply power to the second hydraulic system  26 , such as during an emergency operation. The fluid flows from the pump  34  and through the unidirectional check valve  56  to the second hydraulic system  26 . Fluid flow will continue to travel through the unloader valve  46 . The flow through the unloader valve  46  maintains the rotational speed of the PTU  22  as the second hydraulic system  26  is supplemented by the PTU  22 . Using the unloader valve circuit, including the unloader valve  46 , the orifice  52  and the pressure relief valve  54 , is particularly advantageous in preventing stops in rotation or “chugging” to maintain a smooth operation of the PTU  22 . 
       FIG. 4  shows a high-power state  84  in which the PTU  22  continues to increase rotational speed from a low speed to a medium or high speed. During the high-power state  84 , for example, the demand of the second hydraulic system  26  may be between  30  and  102  liters per minute. The pump and motor assembly  31  continues to receive fluid flow from the open isolation valve  38 . As the PTU  22  increases in speed, the low-pressure pilot pump  60  increases flow and pressure output from the pump and motor assembly  31  which overcomes the spring force and provides pilot pressure to the unloader valve  46 , via a connection  68 , such that the unloader valve  46  moves from an open position  46   a  to a closed position  46   b.  When in the closed position  46   b,  fluid flow is blocked from passing through the normally open unloader valve  46 . When in the high-power state  84 , the PTU  22  has reached an optimal speed such that all of the fluid that is flowing out of the pump  34  may flow out of the PTU  22  directly through the discharge line  50 . 
       FIG. 5  shows a stopping state  86  in which pressure has been equalized between the first hydraulic system  24  and the second hydraulic system  26 . During the stopping state  86 , for example, the demand of the second hydraulic system  26  may be less than 15 liters per minute and the difference in pressure between the first hydraulic system  24  and the second hydraulic system  26  may fall below 350 psi. Thus, the PTU  22  may be operated to move to the stopping state  86 . When the pressure is equalized, rotation of the PTU  22  becomes slower. The second hydraulic system  26  will send a pilot signal  88  to the isolation valve  38  and the pressure from the low-pressure pilot pump  60  will provide a countering signal  90  to the isolation valve  38  to continue low speed stable operation of the unit to prevent the unit from prematurely shutting off if additional flow is needed. When the high-pressure pilot signal  88  overcomes the countering signal  90 , the high-pressure pilot signal  88 , via the mechanical connection  58 , will transition the isolation valve  38  to the closed position  38 a in which fluid flow is prevented from reaching the motor  32  and rotation of the PTU  22  is stopped. The isolation valve  38  may be damped when moving from the open position  38   b,  i.e. the run state, to the closed position  38   a,  i.e. the isolated state, such that the transition is smooth. In the event that the pressure in the second hydraulic system  26  again falls to less than 350 psi below the pressure of the first hydraulic system  24 , the PTU  22  will return to the breakout state  76  of  FIG. 2 . 
     Referring now to  FIGS. 6-13 , various views of the PTU  22  including the hydraulic control system  20  of  FIGS. 1-5  are shown.  FIGS. 6-8  show various views of the outside of a PTU case  92  in which the first hydraulic circuit, the second hydraulic circuit, and the pump and motor assembly are housed. The PTU case  92  may define high-pressure ports  94 ,  96 , low-pressure ports  98 ,  100 , and case drain ports  102 ,  104  that each corresponds to one of the first hydraulic circuit  28  and the second hydraulic circuit  30 , as shown in  FIGS. 1-5 , and are configured to receive fluid flow therethrough. The ports  94 ,  98  and  102  may correspond to the second hydraulic circuit  30  and the second hydraulic system  26 , as shown in  FIG. 1-5 , and the ports  96 ,  100  and  104  may correspond to the first hydraulic system  24 , as shown in  FIGS. 1-5 . The PTU case  92  may be formed of separate housings and adapters that are bolted or attached together to enclose the PTU  22 . 
       FIGS. 9-13  show cross-sectional views of the PTU  22 .  FIG. 9  is a cross-sectional view as taken along cut line A-A of  FIG. 8 . As shown in  FIG. 9 , the PTU  22  includes a first axial piston assembly  106  having a first cylinder block  108  and a second axial piston assembly  110  having a second cylinder block  112 . The first axial piston assembly  106  and the second axial piston assembly  110  correspond to the pump and motor assembly  31  shown in  FIGS. 1-5 . For example, the first axial piston assembly  106  may correspond to the motor  32  and the second axial piston assembly  110  may correspond to the pump  34  that is rotatably coupled to the motor  32  along the shaft  36 . In another exemplary embodiment, at least one of the axial piston assemblies  106 ,  110  may be arranged along a bent axis. The PTU  22  further includes the low-pressure pilot pump  60  rotatably coupled to the pump  34  along the shaft  62 , and an impeller  113 . The fluid flowing from the low-pressure pilot pump  60  may also flow through the orifice  52  arranged along the second hydraulic circuit  30  prior to discharge. The fluid may flow from the low-pressure pilot pump  60  through the pressure relief valve  54  to the case drain port as the speed of the unit increases due to increased flow from the low-pressure pilot pump  60 . 
       FIG. 10  shows a cross-sectional view of the PTU  22  taken along a cut line B-B of  FIG. 9 .  FIG. 10  shows the high-pressure port  94  and the second hydraulic circuit  30 . The second hydraulic circuit  30  includes the unloader valve  46 , the pressure relief valve  54 , and the check valve  56 , as schematically shown in  FIGS. 1-5 . The second hydraulic circuit  30  further includes a fluid passage  113  that is configured to receive fluid from the second axial piston assembly  110 , or the pump  34 , and through which the fluid flows to the high-pressure port  94  for the second hydraulic system and the unloader valve  46 . 
     Referring now to  FIGS. 11 and 12 ,  FIG. 11  shows a cross-sectional view of the PTU  22  taken along a cut line C-C of  FIG. 9  and  FIG. 12  shows a cross-sectional view of the PTU  22  taken along a cut line D-D of  FIG. 11 .  FIGS. 11 and 12  show the case drain ports  102 ,  104 , the low-pressure ports  98 ,  100 , the isolation valve  38 , and the check valve  56 . As shown in  FIG. 11 , the first hydraulic circuit  28  includes the isolation valve  38  that is connected with the motor  32  or the first axial piston assembly  106  which is coupled with the second axial piston assembly  110 , or the pump  34 . As shown in  FIG. 12 , the first hydraulic circuit  28  also includes the arming valve  42  that pilots the isolation valve  38  as described above.  FIG. 12  also shows the high-pressure port  96  being fluidly connected to the isolation valve  38  and the arming valve  42  via fluid paths  114 ,  116 . The communication line  78  between the arming valve  42  and the isolation valve  38  for piloting the isolation valve  38  is also shown. 
     With further reference to  FIG. 13 ,  FIG. 13  shows a cross-sectional view of the PTU  22  taken along cut line E-E of  FIG. 11 .  FIGS. 11 and 13  show the compensator valve  70  that is arranged along the first hydraulic circuit  28  and fluidly coupled to the isolation valve  38 .  FIG. 13  also shows the mechanical connection  58  for sending a pilot signal from the second hydraulic system  26  to the isolation valve  38 . 
     Using the PTU with the hydraulic control system described herein is advantageous in preventing leakage when pressurizing both hydraulic circuits such that the potential for system overheating is reduced and less aircraft fuel is burned. In an exemplary application, the PTU may have a leakage that is less than 50 cubic centimeters per minute at 3000 psi which will further result in cost savings during operation. Another advantage of the PTU and the hydraulic control system is using the unloader valve and isolation valve to reduce or eliminate breakout pressure constraints during the start-up of the PTU. Still another advantage of the PTU and the hydraulic control system is that using the arming solenoid valve arms the PTU for operation and operates the isolation valve. By providing the arming solenoid valve in the PTU, conventional PTU valves may no longer be required such that additional valves, fittings, tubing, installation time, and leakage points may all be eliminated. 
     A power transfer unit includes a first hydraulic circuit, a second hydraulic circuit fluidly connected to the first hydraulic circuit, a pump and motor assembly fluidly connected between the first hydraulic circuit and the second hydraulic circuit, an isolation valve arranged along the first hydraulic circuit and fluidly connected to an inlet of the pump and motor assembly, with the isolation valve being movable between a closed position and an open position to prevent and enable high-pressure fluid flow to the inlet, respectively, an unloader valve arranged along the second hydraulic circuit and fluidly connected to an outlet of the pump and motor assembly, and an orifice arranged along the second hydraulic circuit and fluidly connected to the unloader valve to reduce back pressure in the second hydraulic circuit. 
     The power transfer unit may include an arming valve arranged along the first hydraulic circuit for piloting the isolation valve. 
     The arming valve may be a three-way normally open solenoid valve. 
     The power transfer unit may include a fluid pressure source that is fluidly connected to the arming valve and the isolation valve. 
     The power transfer unit may include a low-pressure pilot pump arranged along the second hydraulic circuit and coupled for rotation with the pump and motor assembly. 
     The low-pressure pilot pump may be communicatively coupled to the unloader valve and the isolation valve. 
     The low-pressure pilot pump may be a gerotor. 
     The power transfer unit may include a check valve arranged along the second hydraulic circuit. 
     The power transfer unit may include a fluid pressure source fluidly connected to the second hydraulic circuit, with the check valve being fluidly connected between the outlet of the pump and motor assembly and the fluid pressure source. 
     The fluid pressure source may be communicatively coupled to the isolation valve via a mechanical connection. 
     The power transfer unit may include a compensator valve that is arranged along the first hydraulic circuit and fluidly connected to the isolation valve. 
     The outlet of the pump and motor assembly may be communicatively coupled with the compensator valve. 
     The first hydraulic circuit and the second hydraulic circuit may be arranged for unidirectional flow from the first hydraulic circuit to the second hydraulic circuit. 
     The unloader valve may be spring-biased in a normally open position. 
     The isolation valve may be spring-biased in a normally closed position. 
     The power transfer unit may include a relief valve arranged along the second hydraulic circuit. 
     The pump and motor assembly may include a motor arranged along the first hydraulic circuit and a pump arranged along the second hydraulic circuit. 
     An aircraft control system includes a first hydraulic system and a second hydraulic system independent from the first hydraulic system, with the power transfer unit as described herein being connected between the first hydraulic system and the second hydraulic system for transferring power between the first hydraulic system and the second hydraulic system. 
     A method of power transfer from a first hydraulic system to a second hydraulic system includes fluidly connecting a first hydraulic circuit to the first hydraulic system, fluidly connecting a second hydraulic circuit to the second hydraulic system through a pump and motor assembly, arranging an isolation valve along the first hydraulic circuit between the first hydraulic system and an inlet of the pump and motor assembly to isolate a high-pressure fluid between the first hydraulic system and the inlet, and arranging an unloader valve and an orifice along the second hydraulic circuit between an outlet of the pump and motor assembly and a discharge line of the second hydraulic circuit to reduce back pressure in the second hydraulic circuit. 
     The method may include fluidly connecting an arming solenoid valve to the isolation valve, and rotatably coupling a low-pressure pilot pump to the pump and motor assembly along the second hydraulic circuit. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.