Abstract:
Apparatuses and systems that use a variable displacement motor-pump (VDMP) to control the position and speed of a hose for an aerial refueling system are disclosed. At displacements greater than required to hold a position of the hose, the VDMP operates in a motor mode to retract the hose. For lesser displacements, the VDMP operates in a pump mode to control extension of the hose. In accordance with some embodiments, a pump-motor relief valve operates to throttle hydraulic fluid flow from the VDMP and to control mixing of hydraulic fluid flowing from the VDMP with system hydraulic fluid.

Description:
BACKGROUND 
     Aerial refueling of a receiver aircraft from a tanker aircraft is commonly performed. Nevertheless, aerial refueling is still a difficult and dangerous maneuver that is typically attempted only by military personnel throughout the world. Today, usually only two types of aerial refueling systems are used: extendable boom systems and a hose-and-drogue systems. 
     In a hose-and-drogue system, the drogue is attached to the outlet end of a hose. The inlet end of the hose is attached to a hose reel onto which the hose is wound. The hose reel is typically mounted either within a tanker aircraft fuselage or on a refueling pod or module which is attached to the bottom of the tanker aircraft. The hose reel is commonly connected to a motor and/or pump that is hydraulically driven. The hydraulic motor-pump can be connected through a coupling system, which may include, e.g., various gear boxes, shafts, and couplings. When the hose is deployed from the tanker aircraft, the drogue encounters drag and the hose reel rotates in a trail direction in which the hose extends behind the tanker aircraft. 
     When the hose and the drogue are fully extended, a pilot of a receiver aircraft maneuvers the receiver aircraft to engage a refueling probe of the receiver aircraft with the drogue. Danger arises because the high speeds of the aircrafts relative to the ground and to each other can result in the drogue being hit with considerable force during engagement. Such engagements may create slack in the hose that must be quickly eliminated. Otherwise, the risk of aircraft accidents increases substantially. Retracting the hose onto the hose reel eliminates the slack. 
     After the drogue is engaged, fuel can be pumped from the tanker aircraft to the receiver aircraft. When refueling is completed, the pilot of the receiver aircraft disengages the refueling probe from the drogue. The hose can then be retracted onto the hose reel for storage by rotating the hose reel in a retract direction. 
     Thus, when the hose extends, it drives the hose reel in a trail direction while the hydraulic motor-pump operates in a pump mode. Conversely, operating the hydraulic motor-pump in a motor mode rotates the hose reel in the retract direction, causing the hose to be retracted onto the hose reel. In the trail mode, hose position can be controlled independently from variations in hose tension. In the retract mode, hose tension can be controlled independently from variations in hose position. 
     Aerial refueling systems have utilized hydraulic motor-pumps that incorporate fixed displacement hydraulic motors that control the extension of the hose in a pump mode and control the retraction of the hose in a motor mode. However, such systems suffer from low hose retraction rates and accessory components that increase overall weight and response time of the system. Information relevant to attempts to address these problems can be found in U.S. Pat. Nos. 6,454,212 and 6,866,228, which disclose variable displacement hydraulic motor-controlled hose reel drive systems. 
     SUMMARY OF THE DESCRIPTION 
     Embodiments generally provide apparatuses and systems that regulate the output of a variable displacement motor-pump (VDMP) by varying the VDMP displacement, with relatively constant system pressure. Embodiments also generally provide apparatuses and systems that control the mixing of hydraulic fluid recirculating to a VDMP in, for example, an aerial refueling system while the VDMP operates in a pump mode. 
     One embodiment relates to a hydraulic motor assembly (HMA). The embodiment is described in relation to its use in an aerial refueling system, but it will be appreciated that it may also be used in other hydraulic systems and applications. The HMA can include a supply conduit for conveying hydraulic fluid from an aircraft hydraulic system, a return conduit for conveying hydraulic fluid back to the aircraft hydraulic system, and a pump conduit for conveying hydraulic fluid between the supply conduit and the return conduit. The HMA can also include a valve, such as a check valve, that includes an inlet connected to the supply conduit and an outlet connected to the pump conduit. The valve can isolate the pump conduit and allow independent metering of the supply conduit to the return conduit. The HMA can also include a VDMP having a first port connected to the return conduit, a second port connected to the pump conduit, and a spline shaft connected to a hose reel of the aerial refueling system. The VDMP can be capable of operating in a pump mode in which hydraulic fluid is conveyed through the VDMP from the first port to second port, i.e., in which hydraulic fluid flows from the return conduit to the pump conduit, when the hose reel rotates in a trail direction. The VDMP can also be operated in a motor mode in which hydraulic fluid is conveyed through the VDMP from the second port to the first port, i.e., in which hydraulic fluid flows from the pump conduit to the return conduit, to rotate the hose reel in a retract direction. 
     The HMA can also include a pump-motor relief valve (PMRV) which has a dual function. The primary function is to limit the VDMP output pressure by opening a throttling orifice which connects the VDMP output, e.g., the pump conduit, to the return conduit. In this way, energy generated by an extending hose and hose reel rotating in the extend direction is dissipated by the pressure drop across the orifice. The second function is to mix enough aircraft hydraulic system fluid with the hydraulic fluid from the VDMP to limit the combined fluid temperature to a safe level for recirculation to the system. 
     The PMRV can include a control chamber and a mixing chamber. The control chamber can be divided into an actuator chamber and a regulation chamber by a spool. The actuator chamber can be placed in fluid communication with an inlet conduit and can also house a bias spring, or a portion thereof, configured to exert a load on the spool. The regulation chamber can be placed in fluid communication with the pump conduit. The mixing chamber can be placed in fluid communication with the supply conduit, the pump conduit, and the return conduit and can also house a portion of the spool. The mixing chamber can be separated from the control chamber by the spool, or by another structure, and can be configured to control mixing of hydraulic fluid flowing from the supply conduit and the pump conduit through the mixing chamber into the return conduit when the VDMP operates in the pump mode. Furthermore, the spool can be configured to prevent hydraulic fluid from flowing through the mixing chamber into the return conduit when the VDMP operates in the motor mode. 
     In an aspect of an embodiment, the bias spring regulates a pressure of hydraulic fluid in the pump conduit to a load pressure when the VDMP operates in the pump mode. Such regulation can regulate the pressure of the hydraulic fluid in the pump conduit to an absolute value or to a predetermined offset pressure above or below a pressure of hydraulic fluid in the supply conduit, return conduit, or another conduit of the conduit system. Thus, the VDMP of the aerial refueling system remains consistently loaded while the VDMP operates in the pump mode. 
     In an aspect of an embodiment, the spool is further configured to control mixing of hydraulic fluid flowing from the supply conduit and the pump conduit at a predetermined ratio when the VDMP operates in the pump mode. Such controlled mixing can mix hydraulic fluid flowing across a throttling orifice from the pump conduit with cooler hydraulic fluid flowing from the supply conduit to maintain the hydraulic fluid recirculating to the VDMP of the aerial refueling system at a constant temperature while the VDMP operates in the pump mode. 
     The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, and also those disclosed in the detailed description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar, but not necessarily identical, elements. 
         FIG. 1  is a schematic view illustration of a hydraulic motor assembly in accordance with an embodiment. 
         FIG. 2A  is a schematic view illustration of a hydraulic motor assembly operating in a pump mode in accordance with an embodiment. 
         FIG. 2B  is a schematic view illustration of a hydraulic motor assembly operating in a motor mode in accordance with an embodiment. 
         FIG. 3  is a schematic view illustration of a pump-motor relief valve in accordance with an embodiment. 
         FIG. 4  is a schematic view illustration of a pump-motor relief valve in accordance with an embodiment. 
         FIG. 5  is a schematic view illustration of a pump-motor relief valve in accordance with an embodiment. 
         FIG. 6  is a schematic view illustration of a pump-motor relief valve in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments and aspects will be described with reference to details discussed below, and the accompanying drawings will illustrate the various embodiments. The following description and drawings are illustrative and are not to be construed as limiting the invention. Numerous specific details are described to provide a thorough understanding of various embodiments. However, in certain instances, well-known or conventional details are not described in order to provide a concise discussion of embodiments. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in conjunction with an embodiment can be included in at least one embodiment. In addition, the appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. 
     Referring to  FIG. 1 , a schematic view illustration of hydraulic motor assembly (HMA)  100  in accordance with an embodiment is shown. HMA  100  includes a conduit system to interconnect various components of HMA  100  through various conduits, such as supply conduit  102 , return conduit  104 , and pump conduit  106 . For example, HMA  100  includes valve  108  disposed between supply conduit  102  and pump conduit  106  of the conduit system. Furthermore, HMA  100  includes variable displacement motor-pump (VDMP)  110  that connects with one or more other conduits of the conduit system. HMA  100  also includes pump-motor relief valve (PMRV)  112  that connects with several of the conduits of the conduit system. 
     Supply conduit  102  can convey hydraulic fluid from aircraft hydraulic system  113  to HMA  100  through a series of hydraulic pumps, valves, fittings, conduits, etc. This series of fluid pathways can originate in aircraft hydraulic system  113  reservoir, and can be duplicated in whole or in part to create redundant aircraft hydraulic systems that ensure supply of hydraulic fluid to supply conduit  102  in the event of a failed sub-system, e.g., a failed pump or valve. 
     Return conduit  104  can return hydraulic fluid from HMA  100  to aircraft hydraulic system  113 . Hydraulic fluid can be conveyed from return conduit  104  to the reservoir of aircraft hydraulic system  113  through a series of hydraulic pumps, valves, fittings, conduits, etc. similar or different from the series described above with respect to supply conduit  102 . 
     Pump conduit  106  can convey hydraulic fluid between supply conduit  102  and return conduit  104 . The conveyance of hydraulic fluid between supply conduit  102  and return conduit  104  need not be between the same two points. Depending on the mode in which HMA  100  is operating, the direction and or path that hydraulic fluid flows through pump conduit  106  may vary. For example, when HMA  100  is operating with VDMP  110  in a motor mode, pump conduit  106  may convey hydraulic fluid directly from supply conduit  102  to return conduit  104  through valve  108  and VDMP  110 . In contrast, when HMA  100  is operating with VDMP  110  in pump mode, flow in pump conduit  106  is reversed and must be routed through PMRV  112  to return conduit  104 . Simultaneously, flow from the aircraft hydraulic system  113  in supply conduit  102  proportional to flow from pump conduit  106  is routed to return conduit  104 . The VDMP  110  flow and aircraft hydraulic system  113  flow are thoroughly mixed in PMRV  112 , cooling the hydraulic fluid which is recirculated to first port  114  of VDMP  110 . Mixed hydraulic fluid from PMRV  112  also returns to aircraft hydraulic system  113 , and excess energy can be returned to aircraft hydraulic system  113  by this warmer fluid. 
     The conduit system described in reference to HMA  100  in  FIG. 1  can include other conduits as well. For example, in at least one embodiment, an inlet conduit is connected with PMRV  112 . The inlet conduit may be the same or different than any of the other conduits. In other words, the inlet conduit can be connected to the same source or fluid pathway of another conduit, e.g., supply conduit  102 , with minimal resistance to fluid flow therebetween. Thus, a conduit as used herein refers generally to a fluid pathway, such as a fluid pathway that exists between two components of HMA  100 . Therefore, any conduit may be composed of one or more tubes, hoses, fittings, etc., that can create a continuous fluid pathway between the components that the conduit is described as being connected with. 
     In various embodiments, the conduits of the conduit system can be rigid fluid lines, flexible hose, drilled passages in manifolds, or any communicating volumes in which the fluid is in a functionally equivalent state. More particularly, the various conduits may be rigid tubing fabricated from copper, aluminum alloy, steel, or titanium 3Al-2.5V alloy, as is commonly used in aircraft hydraulic systems. However, conduit selection may be based on considerations such as operating pressures, space limitations, and routing requirements through the aircraft body. Thus, one skilled in the art will appreciate that the conduits may be formed from various other known materials and forms that meet the design requirements of a particular case. 
     Referring again to  FIG. 1 , HMA  100  can include valve  108  connected to supply conduit  102  and pump conduit  106 . Thus, hydraulic fluid can flow from supply conduit  102  through an inlet of valve  108  to pump conduit  106  through an outlet of valve  108 . Valve  108  can operate to control the flow of hydraulic fluid through pump conduit  106 . For example, opening valve  108  can allow hydraulic fluid to flow through pump conduit  106  to VDMP  110  when HMA  100  is operating with VDMP  110  in a motor mode. Alternatively, valve  108  can be closed when HMA  100  is operating with VDMP  110  in a pump mode in order to isolate pump conduit  106  from supply conduit  102  and thus allow independent metering of flow from supply conduit  102  and pump conduit  106  through PMRV  112 . 
     In an embodiment, valve  108  is a check valve that allows hydraulic fluid to flow in only one direction, e.g., from the inlet to the outlet of valve  108 . Thus, hydraulic fluid is allowed to flow into pump conduit  106  from supply conduit  102  through valve  108 , but hydraulic fluid is not allowed to flow into supply conduit  102  from pump conduit  106  through valve  108 . The flow of hydraulic fluid through valve  108  can therefore depend on the relative pressures of hydraulic fluid in supply conduit  102  and pump conduit  106 . When a pressure of hydraulic fluid in supply conduit  102  exceeds a pressure of hydraulic fluid in pump conduit  106 , hydraulic fluid will flow from supply conduit  102  to pump conduit  106  through valve  108 . Conversely, when the pressure of hydraulic fluid in pump conduit  106  exceeds the pressure of hydraulic fluid in supply conduit  102 , hydraulic fluid will be checked by valve  108  and will not flow from pump conduit  106  through valve  108 . 
     In an alternative embodiment, valve  108  can be a valve type other than a check valve. For example, valve  108  can be a shuttle valve to allow for multiple inlets and directions of flow through valve  108 . Alternatively, valve  108  can be a two-port valve that is electromechanically controlled and, in one embodiment, placed in communication with a separate flow sensor. In this way, valve  108  can rely on flow sensor information to control valve  108  and thereby emulate the operation of a check valve. It will be apparent to one skilled in the art that many other valve configurations may be used to achieve the functionality that is within the scope of this description. For example, in an embodiment described further below, PMRV  112  can include features that provide a check valve equivalent function. 
     Still referring to  FIG. 1 , in this embodiment, HMA  100  can include VDMP  110  that includes first port  114 , second port  116 , and spline shaft  118 . First port  114  can be connected to return conduit  104  and second port  116  can be connected to pump conduit  106 . Thus, when VDMP  110  operates in a pump mode, hydraulic fluid can be sucked from return conduit  104  through first port  114  and conveyed through VDMP  110  to second port  116  and into pump conduit  106 . In contrast, when VDMP  110  operates in a motor mode, hydraulic fluid is conveyed from pump conduit  106  to return conduit  104  in an opposite manner. 
     VDMP  110  can include spline shaft  118  that is driven by, or drives, the internal mechanisms of VDMP  110 . Without going into detail, as the internal mechanisms and functions of VDMP  110  will be known to one skilled in the art, VDMP  110  can comprise inner structures, e.g., pistons, piston mounting plates, etc., that facilitate the conversion of fluid energy into mechanical energy, and vice versa. More particularly, in the pump mode VDMP  110  will convert mechanical energy in spline shaft  118  into fluid energy in the hydraulic fluid of HMA  100 . Conversely, in the motor mode, VDMP  110  will convert fluid energy of HMA  100  into mechanical energy in spline shaft  118 . 
     Spline shaft  118  can be connected to hose reel  119  of the aerial refueling system through various gear boxes, shafts, and couplings, as is known in the art. Additionally, hose reel  119  can couple with a hose and drogue of the aerial refueling system. Thus, spline shaft  118  will rotate in opposite directions corresponding to the extension and retraction of the hose. More particularly, in the pump mode, spline shaft  118  will rotate in a direction corresponding to rotation of hose reel  119  in a trail direction and extension of the hose. Conversely, in the motor mode, spline shaft  118  will rotate in a direction corresponding to rotation of hose reel  119  in a retract direction causing retraction of the hose. 
     Thus, whether VDMP  110  operates in a pump or motor mode can be characterized by observing the net torque applied to spline shaft  118 . The net torque is roughly equivalent to the torque applied to spline shaft  118  by the fluid energy in HMA  100  minus the torque applied to spline shaft  118  by hose reel  119 . Thus, the net torque of spline shaft  118  can be considered positive when operating in a motor mode and negative when operating in a pump mode. One skilled in the art will appreciate that the torque at which VDMP  110  drives its spline shaft  118 , i.e., the torque applied to spline shaft  118  by the fluid energy in HMA  100 , can be controlled by an electro-hydraulic control valve  121 . By way of summary description, the electro-hydraulic control valve  121  increases or decreases the pressure of hydraulic fluid within a spring-biased displacement control piston  125 . The hydraulic pressure in control piston  125  causes that piston to move into a position corresponding to such pressure. The position of control piston  125  determines the displacement of VDMP  110 , which in turn determines the torque applied to spline shaft  118  by the fluid energy at a given hydraulic pressure supplied to VDMP  110 . Furthermore, it will be appreciated that the electro-hydraulic control valve  121  can be controlled by a microprocessor (e.g., as represented by computer  123 ) based, for example, on flight data and commands provided by, e.g., the pilot or the avionic equipment of the tanker aircraft. 
     When electro-hydraulic control valve  121  manages control piston  125  so that VDMP  110  displacement is zero, there is minimal torque transmitted to spline shaft  118  by fluid energy in VDMP  110 . Essentially, spline shaft  118  is able to rotate freely under such conditions. Therefore, if the hose is deployed from the tanker aircraft fuselage, there would be negligible resistance torque to counteract the torque applied from hose reel  119  to spline shaft  118 , and thus, spline shaft  118  would rotate freely and the hose would extend at a maximum rate. In this mode of operation, VDMP  110  can be described as operating in the “pump mode”. 
     It will be appreciated that the HMA  100  is not a passive system, but is rather an actively controlled system, even when the net torque on the spline shaft is negative, i.e., when VDMP  110  is operating in the pump mode. More specifically, HMA  100  is a feedback positioning system whose primary function is to maintain tension on the refueling hose and thus can control torque applied to the spline shaft even when the hose is extending. The VDMP  110  is a constant pressure system in which the torque of the motor-pump is controlled by varying the displacement to maintain motor-pump shaft torque equal to the load torque. As stated above, the displacement is controlled by electro-hydraulic control valve  121 , which operates control piston  125  in a feedback control loop. When HMA  100  controls the displacement (output torque) of VDMP  110  at a point that is greater than required to maintain the load, VDMP  110  acts as a motor and retracts the hose. It will be appreciated that satisfactory operation of the system requires various sensors and feedback loops not shown in the accompanying figures, and these sensors and feedback loops can be controlled by a microprocessor represented by computer  123 . 
     Referring to  FIG. 2A , a schematic view illustration of HMA  100  is shown in accordance with an embodiment. This schematic illustrates the hydraulic fluid flow through HMA  100  while VDMP  110  operates in a pump mode. With VDMP  110  operating in this mode, the net torque on spline shaft  118  is negative, meaning that spline shaft  118  rotates in a direction consistent with the torque applied to hose reel  119  by the hose. 
     While VDMP  110  is operating in a pump mode, hydraulic fluid is recirculated through VDMP  110  from return conduit  104  to pump conduit  106 . In this mode, the energy from VDMP  110  is dissipated across PMRV  112 , where it is mixed with a proportional flow from aircraft hydraulic system  113  via supply conduit  102  to prevent overheating the fluid. Flow is discharged into return conduit  104  and recirculated to VDMP  110 . In steady state, the portion that returns to VDMP  110  passes into pump conduit  106  at essentially the same temperature at which it entered VDMP  110  via return conduit  104 . 
     In one aspect, the mixture of flow from VDMP  110  and aircraft hydraulic system  113  is mixed in PMRV  112 . In PMRV  112 , the flow from VDMP  110  in pump mode is mixed with aircraft hydraulic system fluid to cool it below the temperature of hydraulic fluid in return conduit  104 . For example, as VDMP  110  fluid crosses throttling orifices in PMRV  112 , it can be heated by about 21 degrees Fahrenheit, for phosphate ester fluid and 2900 psi differential across the throttling orifice. Therefore, the VDMP fluid is mixed with enough aircraft hydraulic system fluid to cool it by about 21 degrees Fahrenheit, leaving the temperature of the hydraulic fluid mixture entering the return conduit  104  approximately the same as VDMP fluid before it was pumped to PMRV  112 . 
     Thus, the hydraulic fluid from VDMP  110  and aircraft supply system  113  are mixed within PMRV  112  before discharging into return conduit  104 . It will also be appreciated that as the hydraulic fluid flows through return conduit  104 , in order to maintain continuity of mass in the conduit system, a portion of the mixed hydraulic fluid can be diverted to aircraft hydraulic system  113  and a portion of the mixed hydraulic fluid can be returned to VDMP  110 . 
     Referring to  FIG. 2B , a schematic view illustration of HMA  100  is shown in accordance with an embodiment. This schematic illustrates a scenario of hydraulic fluid flow through HMA  100  while VDMP  110  operates in a “motor mode”, as described above. More specifically, the feedback controlled HMA  100  can vary electro-hydraulic control valve  121  to control control piston  125  so that VDMP  110  displacement continues to increase. Assuming that VDMP  110  is properly sized for the application, the torque transmitted to spline shaft  118  by the fluid energy in VDMP  110  will increase beyond the torque applied to hose reel  119  by the extending hose. Thus, the net torque on spline shaft  118  will be positive, meaning that spline shaft  118  rotates in a direction that opposes the torque applied by the hose on hose reel  119 . As a result, hose reel  119  will reverse its direction of rotation and the hose will be retracted onto hose reel  119 . Again, this is a process that involves operation of the feedback controlled HMA  100  system using active system commands and various feedback loops. 
     While VDMP  110  is operating in a motor mode, hydraulic fluid is circulated from aircraft supply system  113  through valve  108  to pump conduit  106 . Hydraulic fluid then flows through pump conduit  106  through VDMP  110 . The hydraulic fluid drives VDMP  110  in motor mode to retract the hose, and returns to aircraft hydraulic system  113  via return conduit  104 . In at least one embodiment, there is no flow through PMRV  112  in motor mode. Thus, there can be no hydraulic fluid flow in the lines leading to and from PMRV  112  while operating in the motor mode. 
     Building on the previous description, in pump mode the hydraulic fluid that flows from second port  116  can pass through some form of throttling orifice in PMRV  112  before it enters return conduit  104 . From there it reenters VDMP  112  at first port  114  to complete the cycle. Since the total volume of fluid in the recirculating loop is small, the fluid temperature will quickly reach an intolerable level if the fluid is repeatedly passed through the throttling orifice. Excess heating may damage the fluid to the point of having to be replaced because it becomes corrosive, elastomer seals may be damaged or destroyed, and overheated fluid may present a fire danger. However, this can be avoided by a mechanism incorporated in HMA  100  for cooling the hydraulic fluid. 
     To regulate load on VDMP  110  and cool hydraulic fluid in the pump mode, HMA  100  can include PMRV  112 . Referring now to  FIG. 3 , a schematic view illustration of PMRV  112  is shown in accordance with an embodiment. In this embodiment, PMRV  112  can include control chamber  302  for regulating pressure in pump conduit  106  to load VDMP  110  operating in the pump mode. Control chamber  302  is fluidly connected with pump conduit  106  and inlet conduit  318 . PMRV  112  also includes mixing chamber  304  that controls the mixing of hydraulic fluid flowing from supply conduit  102  and pump conduit  106  into return conduit  104  as the hydraulic fluid circulates through VDMP  110  operating in a pump mode. This controlled mixing can serve a cooling function to cool circulating hydraulic fluid in HMA  100 . 
     Still referring to  FIG. 3 , mixing chamber  304  is connected with supply conduit  102 , pump conduit  106 , and return conduit  104 . More particularly, mixing chamber  304  is represented schematically as a pair of connected flow control valves, such as relief valves, where each flow control valve is connected with one of either the supply conduit  102  or pump conduit  106 . Moreover, flow of hydraulic fluid through the flow control valves is variable. This variation can be provided by the mechanical actuation of the valve orifices, e.g., by actuating a spool that is apposed with the orifices. Thus, flow from supply conduit  102  and the flow from pump conduit  106  are metered through the throttling orifices of mixing chamber  304 . After passing through the orifices, the hydraulic fluid converges and mixes before entering return conduit  104 . Although mixing chamber  304  has been shown schematically in  FIG. 3 , one skilled in the art will appreciate that numerous physical embodiments may be used to achieve the represented hydraulic flow pattern. Several such embodiments are described in detail below. 
     Referring now to  FIG. 4 , a schematic view illustration of a pump-motor relief valve is shown in accordance with an embodiment. The PMRV  112  includes various spaces within a cylinder, such as within sleeve  400 . Sleeve  400  can be physically combined or separated from the cylinders or other structures used to form PMRV  112  portions, such as control chamber  302  and mixing chamber  304 . More specifically, sleeve  400  can be encased by a separate housing (not shown) and be sealed against the separate housing by various o-rings or other seals that allow the sleeve to move rotationally and axially within the housing. 
     Control chamber  302  includes actuator chamber  402  and regulation chamber  404 . These chambers can be separated by spool  406 . Control chamber  302  and its portions may be disposed within a cylinder, e.g., sleeve  400 , that defines a chamber space within an inner wall of the cylinder. Alternatively, actuator chamber  402  and regulation chamber  404  can be disposed within entirely separate structures. For example, actuator chamber  402  may be disposed within a first cylinder and regulation chamber  404  may be disposed within a second cylinder. In such a case, the combination of the first cylinder and second cylinder could be considered to define control chamber  302 , in accordance with at least one embodiment. 
     Spool  406  that separates actuator chamber  402  and regulation chamber  404  can be slidably disposed within sleeve  400 . For example, spool  406  may include one or more landings  408  that slide along a surface of the inner wall of sleeve  400 . In an alternative embodiment, spool landings  408  may further include grooves that constrain o-rings, and these o-rings can form a sliding seal with the inner wall of sleeve  400  in order to separate actuator chamber  402  and regulation chamber  404 . In an embodiment, spool  406  includes first face  410  that is either directly or indirectly exposed to actuator chamber  402 . Spool  406  can also include second face  412  that is either directly or indirectly exposed to regulation chamber  404 . Thus, forces acting on first face  410  sum with forces acting on second face  412  to cause spool  406 , or a portion thereof, to move within sleeve  400 . 
     As shown in  FIG. 4 , in an embodiment, actuator chamber  402  further includes bias spring  414 . For example, actuator chamber  402  may house a portion of a compression spring that has a first end and a second end. Whereas the first end of the compression spring can act on a surface of actuator chamber  402 , a second end of the compression spring can act on first face  410  of spool  406 . Thus, in the absence of any pressurized fluids applied to actuator chamber  402  or regulation chamber  404 , spool  406  can be biased by the spring toward a mechanical stop  416 . The mechanical stop  416  may simply be an end of control chamber  302  that spool  406  can slide against. However, in operation, with hydraulic fluid flowing through HMA  100  as VDMP  110  operates in the pump mode, spool  406  may operate in a floating condition with its position determined by the pressure difference across its faces. 
     In an embodiment, bias spring  414 , e.g., as embodied by a compression spring, can have an inherent spring rate. For example, the compression spring could have a spring rate in the range of about 100 pounds per inch to about 500 pounds per inch. By way of example, the compression spring could have a spring rate of about 350 pounds per inch. One skilled in the art would appreciate that various other spring rates within or even outside of this range could be used within the scope of this description. 
     In an alternative embodiment, mechanical stop  416  can be positioned within actuator chamber  402 , or it can exist in another structure of PMRV  112 . Furthermore, the mechanical stop  416  can be adjustable, such that movement of mechanical stop  416  alters the range of motion of spool  406  within control chamber  302 , or another chamber of PMRV  112 . More particularly, mechanical stop  416  could be a steel shim that can be moved, or replaced by other steel shims having varying dimensions. Thus, the position of the mechanical stop  416  can be altered through adjustment. 
     In the embodiment shown in  FIG. 4 , actuator chamber  402  is connected with inlet conduit  318 , which is in turn connected with supply conduit  102 . Furthermore, regulation chamber  404  is connected with pump conduit  106 . Thus, when hydraulic fluid is conveyed from supply conduit  102  to actuator chamber  402  and from pump conduit  106  to regulation chamber  404 , a pressure differential may exist across first face  410  and second face  412  of spool  406 . Accordingly, the hydraulic fluid pressure in actuator chamber  402  exerts a load on spool  406  in one direction and the hydraulic fluid pressure in regulation chamber  404  exerts a load on spool  406  in a direction. In at least one embodiment, these directions can be partially or directly opposite to one another. 
     Thus, in an embodiment, the position of spool  406  within PMRV  112  can depend upon the sum of the load exerted on first face  410  of spool  406  by bias spring  414 , the load exerted on first face  410  of spool  406  by hydraulic fluid in actuator chamber  402 , and the load exerted on second face  412  of spool  406  by hydraulic fluid in regulation chamber  404 . Furthermore, unless spool  406  is biased against a mechanical stop  416 , the pressure within regulation chamber  404  may be roughly equivalent to the pressure exerted on first face  410  by bias spring  414  and the hydraulic fluid in actuator chamber  402 . In other words, the pressure of hydraulic fluid in regulation chamber  404  will be higher than the pressure of hydraulic fluid in supply conduit  102  by an offset pressure that is proportional to the load exerted by bias spring  414  on first face  410 . This load will of course vary in some embodiments with the position of spool  406 , since the movement of spool  406  may compress bias spring  414 , which can exert a load proportional to its compression distance, as in the case of a compression spring. 
     Spool  406  can have various dimensions and features, such as landings, grooves, ports, protrusions, or any other features that enable the functionality described throughout this description. Dimensions can depend upon the overall system specifications and requirements. For example, it is contemplated that spool  406  diameter could be about 0.5 inches in an embodiment. However, spool  406  diameter could be in the range of about 0.1 inches to 2 inches in another embodiment. Even this range should not be considered restrictive, since spool  406  diameter could be any diameter that is necessary to regulate flow of hydraulic fluids in the system, consistent with the principles described herein. 
     Similarly, the motion of spool  406  or bias spring  414  can be selected and modified based on the principles of operation described throughout this description. For example, while in one embodiment, the stroke of spool  406  can be about 0.1 inches, it is also possible for the stroke of spool  406  to be several inches or more. Bias spring strokes can be similarly selected to meet the various operational and dimensional constraints of the system design. 
     Having discussed the basic interactions in control chamber  302 , it will now be apparent that the pressure of hydraulic fluid within regulation chamber  404  can be regulated by adjusting the loads applied to first face  410 . Thus, either the force applied through bias spring  414  or the pressure of hydraulic fluid from inlet conduit  318  can be adjusted in order to produce a corresponding change to the pressure of hydraulic fluid in regulation chamber  404 . Since regulation chamber  404  and pump conduit  106  are connected together, the hydraulic fluid pressure in pump conduit  106  can be regulated in a similar manner. 
     Given that the pressure of hydraulic fluid in pump conduit  106  can be controlled using hydraulic fluid in inlet conduit  318  as an input, it will be appreciated that the hydraulic pressure in pump conduit  106  can be regulated to any desired pressure. For example, in one embodiment, the pressure in pump conduit  106  can be regulated to a predetermined absolute value. For example, if a pump conduit pressure of 3000 psi is desired and bias spring  414  exerts 100 psi across first face  410 , inlet conduit  318  can be connected to a reservoir or other fluid source that supplies hydraulic fluid at 2900 psi in order to generate the desired pressure. It will be appreciated that the fluid source that is connected with inlet conduit  318  can be separate from any other fluid conduit or source in HMA  100 , including supply conduit  102  that delivers hydraulic fluid from aircraft hydraulic system  113 . 
     In an embodiment, pump conduit  106  pressure can be regulated to a predetermined offset pressure above the pressure of hydraulic fluid in supply conduit  102 . By way of example, if bias spring  414  applies a load to first face  410  resulting in a 100 psi pressure distributed across the face surface area and the input conduit  318  is connected with supply conduit  102  to flow hydraulic fluid at a pressure of 2900 psi into actuator chamber  402 , then when spool  406  is operating in a floating condition, the pressure of hydraulic fluid in regulation chamber  404  and pump conduit  106  would be regulated to 3000 psi, as exerted through second face  412  of spool  406 . Notably, if the pressure of hydraulic fluid in supply conduit  102  drifts, for example, to 3100 psi, the pressure of hydraulic fluid in regulation chamber  404  and pump conduit  106  would be regulated to 4000 psi. Thus, the pressure in pump conduit  106  will always differ from supply conduit  102  by an offset pressure that is proportional to the load exerted by bias spring  414  on first face  410  while VDMP  110  operates in a pump mode. 
     In yet another embodiment, pump conduit  106  pressure can be regulated to a predetermined offset pressure above the pressure of hydraulic fluid in return conduit  104 . By way of example, if bias spring  414  applies a load to first face  410  resulting in a 100 psi pressure distributed across the face surface area and the input conduit is connected with return conduit  104  to flow hydraulic fluid at a pressure of 2900 psi into actuator chamber  402 , then when spool  406  is operating in a floating condition, the pressure of hydraulic fluid in regulation chamber  404  and pump conduit  106  would be regulated to 3000 psi, as exerted through second face  412  of spool  406 . Notably, if the pressure of hydraulic fluid in return conduit  104  drifts, for example, to 3100 psi, the pressure of hydraulic fluid in regulation chamber  404  and pump conduit  106  would be regulated to 4000 psi. Thus, the pressure in pump conduit  106  will always differ from return conduit  104  by an offset pressure that is proportional to the load exerted by bias spring  414  on first face  410  while VDMP  110  operates in a pump mode. 
     In an alternative embodiment, the offset pressure can be below the pressure of hydraulic fluid in another conduit in the system. For example, bias spring  414  may be disposed within regulation chamber  404  instead of the actuation chamber  402 . Thus, bias spring  414  can exert a load on second face  412  of spool  406 . In this case, the pressure of hydraulic fluid within regulation chamber  404  will be offset below the pressure of hydraulic fluid within actuator chamber  402  by a pressure proportional to the load exerted by bias spring  414  on second face  412 . For example, in the case where inlet conduit  318  is connected with an external reservoir, regulation chamber  404 , and thus pump conduit  106 , would be regulated to a pressure below the pressure of the external reservoir. 
     In yet another embodiment, inlet conduit  318  can be connected to a valve, such as a three-way valve, that would allow actuator chamber  402  to be connected to supply conduit  102 , return conduit  104 , or another system conduit or reservoir, depending on the preference of the pilot and/or avionics equipment of the tanker aircraft. 
     It will be appreciated that bias spring  414  may be embodied by various other actuator types and configurations. In an alternative embodiment, rather than being a compression spring, bias spring  414  could be a tension spring disposed within regulation chamber  404 . The tension spring can pull second face  412  toward a wall of regulation chamber  404 . Additionally, bias spring  414  may not be a spring at all. For example, bias spring  414  can be an electric motor, a pneumatic actuator, a hydraulic actuator, or any other mechanism or object that stores energy or exerts a load. Further still, bias spring  414  could be used in combination with various sensors, microprocessors, and controllers in order to exert a variable load on spool  406  based on pressures, flow rates, temperatures, and other characteristics that are monitored throughout the system. Control of bias spring  414  could be based on calculations involving such sensor data. Thus, numerous potential actuator configurations exist within the scope of this description. 
     In at least one embodiment, bias spring  414  may be adjustable in that adjustments may be made to bias spring  414  or PMRV  112  that result in a change to the load exerted on spool  406  by bias spring  414 . For example, the load exerted by a compression spring actuator can be adjusted with a pressure adjusting screw (not shown) that changes the location of one end of the compression spring, and thus, changes the displacement of the compression spring when spool  406  is biased against a mechanical stop  416 . That is, the preload of the spring may be adjusted. In one embodiment, the preload of a compression spring bias spring could be in the range of about 1 to 10 pound force. In another embodiment, the preload of a compression spring bias spring could be in the range of about 3 to 7 pound force. In yet another embodiment, the preload of a compression spring bias spring could be about 5 pound force. However, it will be appreciated that these ranges for spring preloads are not restrictive and that suitable spring preloads exist beyond these ranges. 
     Other means of adjusting bias spring  414  can be contemplated by one skilled in the art. For example, in the case where the bias spring is a hydraulic actuator, bias spring  414  could be controlled by varying the pressure applied to a hydraulic piston in bias spring  414 . Alternatively, in the case of an electromechanical actuator, bias spring  414  could be controlled by varying the current supplied to a motor in bias spring  414 . One skilled in the art can contemplate various other means of adjusting bias spring  414  within the scope of this description. 
     Still referring to  FIG. 4 , mixing chamber  304  can be in fluid communication with supply conduit  102 , pump conduit  106 , and return conduit  104 . Mixing chamber  304  can include premix chambers  420 ,  420 ′ and admix chamber  422 . More particularly, supply conduit  102  can flow through a first orifice  424  formed in spool  400  into a premix chamber  420  and pump conduit  106  can flow through a second orifice  426  formed in spool  400  into a premix chamber  420 ′. These flows can subsequently enter into admix chamber  422  through a third orifice  424 ′, also formed in spool  400 , and a fourth orifice  426 ′, also formed in spool  400 . In an embodiment, premix chambers  420 ,  420 ′ are defined by a space between the sleeve  400  inner wall and an outer surface of spool  406 . Furthermore, the premix chambers  420 ,  420 ′ can be separated from each other by one or more landings  408  of spool  406 . The admix chamber  422  can be defined by an annular space formed within sleeve  400  or a space between sleeve  400  and an outer housing or encasement that sleeve  400  is disposed within. Thus, hydraulic fluid from supply conduit  102  can flow through first orifice  424 , over a surface of spool  406  housed within premix chamber  420 , and through third orifice  424 ′ into admix chamber  422 . Similarly, hydraulic fluid from pump conduit  106  can flow through second orifice  426 , over a surface of spool  406  housed within premix chamber  420 ′, and through third orifice  426 ′ into admix chamber  422 . Hydraulic fluid can exit mixing chamber  304  into return conduit  104  from admix chamber  422 . 
     When hydraulic fluid from supply conduit  102  and pump conduit  106  enters admix chamber  422 , it can be thoroughly mixed and cooled. For example, hydraulic fluid flowing from pump conduit  106  can be heated as it passes through second orifice  426  and or fourth orifice  426 ′. Thus, heating can result from the work done to force the hydraulic fluid through the orifice. However, upon entering admix chamber  422 , it can be mixed with hydraulic fluid flowing from supply conduit  102 , which is cooler. Hydraulic fluid from supply conduit  102  can be maintained at a cooler temperature, for example, by forcing it through larger orifices at lower rates or pressures. Thus, the temperature of the mixed hydraulic fluid will be less than one of the constituent hydraulic fluid parts. 
     The configuration of spool landings  408  and the orifices that connect the conduits with the various portions of mixing chamber  304  can control the flow of hydraulic fluid from the conduits through mixing chamber  304 . More particularly, the flow control can depend on a position and size of the orifices formed in sleeve  400  and the relative locations of the spool landing surfaces. Thus, movement of spool  406  as managed by the control chamber in the manner described above will produce a corresponding movement of spool  406  within sleeve  400 . More particularly, movement of spool  406  within sleeve  400  can cause spool landings  402  to interact with the sleeve orifices in such a way that the flow through supply conduit  102  and pump conduit  106  into mixing chamber  304  is varied. Even more particularly, the flow through the orifices depends on the dimensions of spool landings  402  and the position of spool  406  relative to first orifice  424 , second orifice  426 , third orifice  424 ′, and fourth orifice  426 ′ formed in spool  400 . 
     By way of example, when VDMP  110  operates in a pump mode, spool  400  will be in a floating configuration as described above, with the pressure of hydraulic fluid in regulation chamber  404  being regulated to a pressure above or below the hydraulic fluid in actuator chamber  402 . In this floating configuration, the landings  402  of spool  406  in sleeve  400  can either not obstruct, or only partially obstruct, the orifices that connect with supply conduit  102  and pump conduit  106 . Thus, when VDMP  110  operates in a pump mode, hydraulic fluid will flow through mixing chamber  304  into return conduit  104 . 
     In an alternative scenario of the same embodiment, when VDMP  110  operates in a motor mode, valve  108  of HMA  100  will open as the pressure in supply conduit  102  exceeds the pressure that is generated at the outlet, i.e., second port  116 , of VDMP  110 . Supply conduit  102  pressure exceeds the pressure in pump conduit  106  at this stage. As a result, valve  108  opens and hydraulic fluid from supply conduit  102  will flow into pump conduit  106 . Thus, in the case where inlet conduit  318  of actuator chamber  402  is connected with supply conduit  102 , the pressure of the hydraulic fluid in both actuator chamber  402  and regulation chamber  404 , which is connected with pump conduit  106 , will be equal. However, since bias spring  414  will exert an additional load on first face  410 , spool  406  will be biased toward a mechanical stop  416 . 
     Referring to  FIG. 4 , in the current example, spool  406  would be biased fully to the right. In this position, spool  406 , or more particularly spool landings  408 , can be configured to completely obstruct the flow of hydraulic fluid from the supply and pump conduits into PMRV  112 . Thus, when VDMP  110  operates in a motor mode, PMRV  112  configuration can prevent hydraulic fluid from flowing through mixing chamber  304  into return conduit  104 . It will be appreciated that this functionality improves the efficiency of the system because the hydraulic fluid conveyed through supply conduit  102  will be directed through VDMP  110  for generating torque in spline shaft  118 , rather than leaking to return conduit  104  through PMRV  112  without doing beneficial work in HMA  100 . 
     Having discussed the basic interactions inherent in mixing chamber  304 , it will be appreciated that configuration of spool  406  and the placement and shape of the orifices within sleeve  400  can be modified in many ways to create the flow characteristics and mixing profiles that are desired in either the pump or motor mode. For example, spool  406  can be configured to control the flow of hydraulic fluid from supply conduit  102  and pump conduit  106  to facilitate the mixing of the hydraulic fluid from those sources at a predetermined ratio. By way of example, spool  406  can be configured to create a mixing ratio in the range of about one to three parts hydraulic fluid flowing from supply conduit  102  to every two to ten parts hydraulic fluid flowing from pump conduit  106 . In an alternative example, spool  406  can be configured to create a mixing ratio of about one part hydraulic fluid flowing from supply conduit  102  to two parts hydraulic fluid flowing from pump conduit  106 . It will be appreciated that this ratio can be varied such that more hydraulic fluid flows from supply conduit  102  than pump conduit  106 , or such that the ratio of fluid flowing from those sources is much higher or lower. 
     In an embodiment, the predetermined ratio is achieved by maintaining a ratio of orifice sizes regardless of spool position. More particularly, the spool and orifices can be sized and positioned such that the area of the opening of first orifice  424  is a predetermined ratio of the area of the opening of second orifice  426 . Any other orifices can be selected to create this predetermined ratio. For example, the ratio of area of openings in third orifice  424 ′ and second orifice  426  could be used. As discussed above, the ratio of areas will correspond to the flow rates through the orifices, and thus maintain the area of the opening of third orifice  424 ′ to be one half of the area of opening of fourth orifice  426 ′ will result in a mixture of approximately one part supply hydraulic fluid to two parts pump hydraulic fluid. This is only an example and it will be appreciated that other ratios can be maintained within the scope of this description. 
     One skilled in the art will appreciate that a predetermined ratio can be closely maintained through precise fabrication of sleeve  400  and orifices  424 ,  424 ′,  426 , and  426 ′. For example, landing lengths, orifice sizes, and orifice shapes can be precisely machined as required. Furthermore, additional components can be used to sense system fluctuations and provide appropriate system inputs. By way of example, flow sensors could be used to monitor flow through pump conduit  106  and supply conduit  102  to ensure that the desired mixing ratio is being achieved. If the mixing ratio drifts from the predetermined ratio, inputs may be supplied to move spool  406 , such as by regulating pressure that is supplied to inlet conduit  318 , by adjusting a valve placed within supply conduit  102  or pump conduit  106 , or any number of other system modifications that can be contemplated by one skilled in the art to achieve this goal. 
     Despite the preceding description, it should still be recognized that a precise predetermined ratio is difficult to achieve, and thus, this description is not intended to be so limited that the predetermined ratio is considered to be constant throughout the entire operation of HMA  100 . In other words, although a predetermined ratio is contemplated in one embodiment, e.g., about one part to two parts flow, as the pressure in pump conduit  106  varies according to the variable displacement of VDMP  110 , then the flow from pump conduit  106  will vary too. Therefore, the ratio of flow may fluctuate according to transient system responses such as these pressure and flow fluctuations. Therefore, any reference to a predetermined ratio should be considered to be approximate, in at least one embodiment. 
     Referring to  FIG. 5 , a schematic view illustration of a PMRV  112  is shown in accordance with yet another embodiment. A housing  500  can include features that emulate the control chamber and mixing chamber of the preceding embodiments. Housing  500  can include first orifice  502  connected with supply conduit  102  and second orifice  504  connected with pump conduit  106 . Thus, when unobstructed, hydraulic fluid from each conduit can flow into mixing chamber  512  at a rate determined by the orifice dimensions, hydraulic fluid viscosities, pressures, etc. 
     In an embodiment, mixing of the hydraulic fluid is controlled by a stopper plate  506 , which in one embodiment, is embodied by a bar having a width sufficient to occlude the orifices  502 ,  504 . The stopper plate  506  can be hinged at pivot  508 , using a clevis fastener, for example. Furthermore, stopper plate  506  can be biased in a closed direction by bias spring  510 , which can be a compression spring, for example. The bias spring  510  can bias stopper plate  506  against the orifices, and thus, flow through the orifices will be stopped until the flow pressure through either orifice exceeds the bias force exerted on the stopper plate  506  by the bias spring  510 . 
     When stopper plate  506  is open, i.e. when the pressure in the conduits exceeds the bias force, the flow of fluid through the orifices connecting with the supply and pump conduits may be unrestricted. Thus, the ratio of flow through these orifices will be based on the orifice flow characteristics and the pressures within the connected conduits, rather than upon any configuration of a spool landing. In essence, mixing chamber  512  operates as a simple orifice with mixing of the hydraulic fluid corresponding directly to the flow characteristics of the input orifices. 
     However, in an alternative scenario of the same embodiment, the stopper plate  506  can be configured to close off a portion or all of one orifice, while not restricting flow through another orifice. For example, the stopper plate  506  can be configured to slide from side to side, rather than being configured to rotate up and down relative to the orifices. In such an embodiment, as stopper plate  506  moves from being fully biased in a direction that blocks both orifices to being fully biased in another direction, e.g., as it moves from left to right in housing  500 , it may first fully block both orifices, then not block the supply conduit orifice while partially blocking the pump conduit orifice, then not block either orifice. Thus, flow through mixing chamber  512  can be varied according to the mode of operation, i.e., motor mode or pump mode. The transition of stopper plate  506  through this range can be precisely controlled by adjusting orifice spacing, bias spring design, and so forth. This operation is consistent with the principles of operation described above. 
     Referring now to  FIG. 6 , a schematic view illustration of a pump-motor relief valve is shown in accordance with an embodiment. The structure of this embodiment is similar in some respects to that shown in PMRV  112  previously illustrated in  FIG. 4 . However, PMRV  112  embodiment shown here does not include external valve  108 . Instead, external valve  108  is replaced in HMA  100  system by a functional equivalent integrated within PMRV  112 . More specifically, PMRV  112  includes check orifice  602  formed within sleeve  400 . As in the case of an external valve  108 , check orifice  602  functions to permit flow from supply conduit  102  into pump conduit  106  when VDMP  110  operates in motor mode, but facilitates flow of hydraulic fluid from supply conduit  102  into mixing chamber  304  of PMRV  112  when VDMP  110  operates in a pump mode. 
     It will be appreciated that as in the embodiments described above, flow through check orifice  602  can depend on the interaction between check orifice  602  and a landing  408  of spool  406 . More specifically, landing  408  and check orifice  602  can be precisely fabricated to ensure that hydraulic fluid from the supply conduit  102  connects to return conduit  104  when hydraulic fluid from the pump conduit  106  is at a higher pressure than hydraulic fluid from the supply conduit  102 . Consistent with this objective, PMRV  112  also includes second bias spring  604  acting on second face  412 . 
     Second bias spring  604  exerts a force on second face  412  that counteracts the force exerted on first face  410  by bias spring  414 . Thus the position of spool  406  within sleeve  400  depends on the net load applied to it in actuator chamber  402  and the regulation chamber  404 . The load applied in actuator chamber  402  is the sum of the pressure of hydraulic fluid from aircraft hydraulic system  113  across first face  410  and the load applied by bias spring  414  to first face  410 . The load applied in regulation chamber  402  is the sum of the pressure of hydraulic fluid from pump conduit  106  across second face  412  and the load applied by second bias spring  604  to second face  412 . 
     In an embodiment, the bias springs maintain spool  406  in a floating condition within sleeve  400 . More particularly, as pressure of hydraulic fluid within pump conduit  106  increases, spool  406  will bias toward the left. However, as the pressure in pump conduit  106  decreases, spool  406  will bias toward the right. 
     In an embodiment, premix chamber  420 ′ can be formed between landing  606  and  608  of spool  406 . Furthermore, the landing size and spacing can be such that when spool  406  is biased leftward, a fluid pathway is created between first orifice  424  and third orifice  424 ′, and between second orifice  426  and fourth orifice  426 ′. In contrast, when spool  406  is biased rightward, a fluid pathway is created between check orifice  602  and second orifice  426 , while fluid flow through first orifice  424  is blocked. Thus, when spool  406  is biased leftward, hydraulic fluid flows from supply conduit  102  and pump conduit  106  to return conduit  104  through PMRV  112 . In contrast, when spool  406  is biased rightward, hydraulic fluid flows from supply conduit  102  into pump conduit  106  through PMRV  112 , but no fluid is returned to return conduit  104 . 
     Thus, by sizing the bias springs, spool, and orifices appropriately, PMRV  112  provides a check valve equivalent function, in which hydraulic fluid is mixed within PMRV  112  when supply conduit  102  pressure is less than pump conduit  106  pressure, i.e., when VDMP  110  operates in a pump mode. Furthermore, PMRV  112  flows hydraulic fluid directly from supply conduit  102  to pump conduit  106  without mixing the fluid when supply conduit  102  pressure exceeds pump conduit  106  pressure, i.e., when VDMP  110  operates in a motor mode. 
     The various components of HMA  100  described above, as well as the subcomponents of those components, can be fabricated from materials that are commonly used in aircraft hydraulic systems. For example, in at least one embodiment, one or more components may be wholly or partially formed from material groups including copper, aluminum alloy, steel, or titanium 3Al-2.5V alloy. Furthermore, it will be appreciated by one skilled in the art that the various components can be designed with various shapes, profiles, and cross-sections to achieve the functionality described above. These various features and modifications have been omitted in some cases for the sake of brevity, but they are considered to be within the scope of the description. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.