Abstract:
Apparatuses and systems that use a variable displacement motor-pump (VDMP) to control the position of a hose of an aerial refueling system are disclosed. At VDMP displacements greater than required to hold a position of the hose, the VDMP operates in a motor mode to retract the hose. For lesser VDMP displacements, the VDMP operates in a pump mode to control extension of the hose. In accordance with some embodiments, a dual orifice conveys hydraulic fluid at varying rates depending upon the VDMP operational mode.

Description:
BACKGROUND 
       [0001]    Aerial refueling of a receiver aircraft from a tanker aircraft is commonly performed. Nevertheless, aerial refueling is a difficult and dangerous maneuver that is typically attempted only by military personnel throughout the world. Today, usually two types of aerial refueling systems are used: extendable boom systems and hose-and-drogue systems. 
         [0002]    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. 
         [0003]    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 tanker and receiver 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. 
         [0004]    After the drogue is engaged, fuel can be pumped from the tanker aircraft to the receiver aircraft. When refueling is complete, the pilot of the receiver aircraft disengages the refueling probe from the drogue. When the receiver aircraft tries to disengage within the refueling range, it is referred to as “flowing disconnect”, and pulling forces on the hose can increase significantly. The hose can then be retracted onto the hose reel for storage by rotating the hose reel in a retract direction. 
         [0005]    Thus, the hydraulic motor-pump operates in a pump mode to rotate the hose reel in a trail direction and to extend the hose. Conversely, the hydraulic motor-pump operates in a motor mode to rotate the hose reel in the retract direction and to retract the hose 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. 
         [0006]    Aerial refueling systems have utilized hydraulic motor-pumps incorporating 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 
       [0007]    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 convey hydraulic fluid in parallel with the VDMP at varying rates depending upon the VDMP operational mode. Embodiments are described in relation to their use in an aerial refueling system, but it will be appreciated that the embodiments may also be used in other hydraulic systems and applications. For example, the embodiments can relate to applications such as ship-to-ship refueling and offshore oil operations. 
         [0008]    In an embodiment, a hydraulic motor assembly (HMA) includes a VDMP coupled with a supply conduit and a return conduit. The VDMP has a spline shaft coupled with a hose reel of an aerial refueling system and when the VDMP operates in a pump mode at a flow rate, the VDMP rotates the hose reel in a trail direction. The HMA also includes a dual orifice (DO) coupled with the supply conduit and the return conduit in parallel with the VDMP, which can function as a bypass valve. The DO includes a fixed orifice, which continuously conveys hydraulic fluid from the supply conduit to the return conduit, and a variable orifice, which only conveys hydraulic fluid from the supply conduit to the return conduit when the VDMP operates in the pump mode and the flow rate reaches a predetermined rate. The HMA can also include a check valve located between the VDMP and an aircraft hydraulic system to prevent hydraulic fluid from being conveyed into the aircraft hydraulic system. 
         [0009]    In an embodiment, the fixed orifice of the DO continuously conveys hydraulic fluid at a rate in a range of about 1 to 5 gallons per minute (GPM) when the hydraulic fluid in the supply conduit is at about 3000 psig pressure. In an embodiment, the variable orifice conveys hydraulic fluid at a rate in a range of about to 15 GPM when it is open and the hydraulic fluid in the supply conduit is at about 3000 psig pressure. In one embodiment, the variable orifice comprises a solenoid valve. 
         [0010]    In another embodiment, a method includes operating a VDMP in a first pump mode at a first flow rate to rotate a hose reel of an aircraft refueling system in a trail direction. Hydraulic fluid is conveyed from a supply conduit to a return conduit through a fixed orifice of a DO while the VDMP is in the first pump mode. In the method, the VDMP is transitioned to operate in a second pump mode at a second flow rate. In response to the second pump mode, hydraulic fluid is conveyed from the supply conduit to the return conduit through both the fixed orifice and a variable orifice of the DO. In an embodiment, the second flow rate is more than 1 GPM. For example, the second flow rate can be in a range of about 1 to 5 GPM. In an embodiment, the variable orifice includes a solenoid valve. 
         [0011]    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 
         [0012]    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. 
           [0013]      FIG. 1  is a schematic view illustration of an aerial refueling system in accordance with an embodiment. 
           [0014]      FIG. 2  is a schematic view illustration of a fixed orifice component of a dual orifice in accordance with an embodiment. 
           [0015]      FIG. 3  is a schematic view illustration of a variable orifice component of a dual orifice in accordance with an embodiment. 
           [0016]      FIG. 4A  is a schematic view illustration of a hydraulic motor assembly operating with a variable displacement motor-pump in a motor mode in accordance with an embodiment. 
           [0017]      FIG. 4B  is a schematic view illustration of a hydraulic motor assembly operating with a variable displacement motor-pump in a pump mode at low flow in accordance with an embodiment. 
           [0018]      FIG. 4C  is a schematic view illustration of a hydraulic motor assembly operating with a variable displacement motor-pump in a pump mode at high flow in accordance with an embodiment. 
           [0019]      FIG. 5  is a method of using a hydraulic motor assembly in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    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. 
         [0021]    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. 
         [0022]    Referring to  FIG. 1 , a schematic view illustration of an aerial refueling system is shown in accordance with an embodiment. The aerial refueling system includes hydraulic motor assembly (HMA)  101  that has a conduit system to interconnect various components of HMA  101 . The conduit system includes various conduits, such as supply conduit  102  and return conduit  104 . Furthermore, HMA  101  includes variable displacement motor-pump (VDMP)  110  that connects with supply conduit  102  and return conduit  104 . HMA  101  also includes dual orifice (DO)  112  that connects with supply conduit  102  and return conduit  104 , in parallel with VDMP  110 . 
         [0023]    Supply conduit  102  can extend from HMA  101  to aircraft hydraulic system  113  through a series of hydraulic pumps, valves, fittings, conduits, etc. This series of fluid pathways can connect with a reservoir (not shown) of aircraft hydraulic system  113  and can be duplicated in whole or in part to create redundant aircraft hydraulic systems that ensure supply of hydraulic fluid to HMA  101  in the event of a failed sub-system, e.g., a failed pump or valve. Return conduit  104  can return hydraulic fluid from HMA  101  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 . 
         [0024]    Still referring to  FIG. 1 , hydraulic fluid is conveyed 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  101  is operating, the direction and or path that hydraulic fluid flows may vary. For example, when HMA  101  is operating with VDMP  110  in a motor mode, hydraulic fluid is conveyed directly from supply conduit  102  to return conduit  104  through both VDMP  110  and DO  112 . In contrast, when HMA  101  is operating with VDMP  110  in pump mode, flow in VDMP  110  is reversed such that hydraulic fluid is conveyed from return conduit  104  to supply conduit  102  through VDMP  110  while hydraulic fluid is conveyed from supply conduit  102  to return conduit  104  through DO  112 . Thus, in at least one embodiment, hydraulic fluid recirculates through VDMP  110  via DO  112  when VDMP  110  operates in a pump mode. Additional details regarding flow patterns in HMA  101  are provided below. 
         [0025]    It will be appreciated that a conduit as used herein refers generally to a fluid pathway, such as a fluid pathway that exists between two components of HMA  101 . 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, conduit system can include 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. 
         [0026]    HMA  101  includes VDMP  110  that has a first port connected with supply conduit  102  and a second port connected with return conduit  104 . As described above, when VDMP  110  operates in a motor mode, hydraulic fluid is conveyed from supply conduit  102  to return conduit  104  through VDMP  110  ports. In contrast, when VDMP  110  operates in a pump mode, hydraulic fluid can be sucked from return conduit  104  to supply conduit  102  through VDMP  110  ports. 
         [0027]    VDMP  110  also includes 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  converts mechanical energy in spline shaft  118  into fluid energy in the hydraulic fluid of HMA  101 . Conversely, in the motor mode, VDMP  110  converts fluid energy in the hydraulic fluid of HMA  101  into mechanical energy in spline shaft  118 . 
         [0028]    Spline shaft  118  connects with hose reel  119  of the aerial refueling system  100  through various gear boxes, shafts, and couplings, as is known in the art. Additionally, hose reel  119  can connect with a hose and drogue of the aerial refueling system  100 . 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 and retraction of the hose. 
         [0029]    Thus, whether VDMP  110  operates in a pump or motor mode can be characterized by observing the net torque applied to spline shaft  118  by fluid energy in HMA  101  and by hose reel  119 . More particularly, the net torque applied to 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 spline shaft  118 , i.e., the torque applied to spline shaft  118  by the fluid energy in HMA  101 , can be controlled by electro-hydraulic control valve  121 . By way of summary description, electro-hydraulic control valve  121  increases or decreases the pressure of hydraulic fluid within 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 fluid energy in HMA  101  for a given hydraulic pressure supplied to VDMP  110 . 
         [0030]    By way of example, 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 HMA  101 . Therefore, if the hose is deployed from the tanker aircraft fuselage, there would be negligible resistance torque to counteract the torque applied to spline shaft  118  by aerodynamic drag on the hose through hose reel  119 . 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”. Conversely, when electro-hydraulic valve  121  manages control piston  125  so that VDMP  110  displacement is maximized, there is maximum torque transmitted to spline shaft  118  by fluid energy in HMA  101 . Thus, under most conditions, spline shaft  118  is able to overcome torque applied by aerodynamic drag and hose reel  119  is rotated in a retract direction to retract the hose onto hose reel  119 . In this mode of operation, VDMP  110  can be described as operating in the “motor mode”. 
         [0031]    It will be appreciated that 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 tanker aircraft pilot, avionics equipment, or various sensors of HMA  101 . Thus, HMA  101  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  101  is a feedback positioning system whose primary function is to maintain tension on the refueling hose and thus can control torque applied to spline shaft  118  even when the hose is extending. Additionally, VDMP  110  is a constant pressure system in which torque is controlled by varying displacement to balance spline shaft  118  torque with 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  101  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 to rotate hose reel  119  in a retract direction and to retract the hose. It will be appreciated that not all sensors and feedback loops required for operation of VDMP  110  are shown in the accompanying figures, and these sensors and feedback loops can also be controlled by a microprocessor represented by computer  123 . 
         [0032]    In an embodiment, HMA  101  includes DO  112  connected with supply conduit  102  and return conduit  104  in parallel with VDMP  110 . The term “connected in parallel” does not imply that flow through VDMP  110  and DO  112  must be in the same direction. Indeed, flow through VDMP  110  and DO  112  can be in opposite directions, e.g., in the case where VDMP  110  operates in a pump mode, as described above. DO  112  includes fixed orifice  130  and variable orifice  132 , which allow for flow through the DO  112  between the supply conduit  102  and the return conduit  104 . 
         [0033]    Fixed orifice  130  is sized and configured to allow the flow of hydraulic fluid from the supply conduit  102  to return conduit  104  at a given rate. More specifically, fixed orifice  130  can be a flow restriction, such as a precision machined hole formed in a cylinder with a diameter chosen to restrict flow to a predetermined rate when VDMP  110  operates in a motor mode. As an example, the predetermined rate can be about 1 to 5 GPM when the pressure of hydraulic fluid in supply conduit  102  is about 3000 psig as VDMP  110  operates in a motor mode. One skilled in the art will appreciate that various fixed orifice diameters and cross-sections can be used to achieve these flow characteristics. More specifically, the choice of orifice configurations can depend on the system characteristics, such as the properties of the hydraulic fluid used and the pressure of hydraulic fluid supplied by the aircraft hydraulic system in normal operation. In an embodiment, flow through fixed orifice  130  is continuous, allowing for a constant bleed of hydraulic fluid from supply conduit  102  to return conduit  104 . However, it is contemplated that fixed orifice  130  can be sized to limit bleed flow in order to minimize wasted flow of hydraulic fluid and associated energy. 
         [0034]    Variable orifice  132  is also sized and configured to allow hydraulic fluid to flow from supply conduit  102  to return conduit  104 . Variable orifice  132  can provide a flow restriction that varies depending on one or more operating conditions of HMA  101 . For example, variable orifice  132  can be configured to disallow flow when VDMP  110  is operating in a motor mode or when VDMP  110  is operating in a pump mode at a low flow rate. More specifically, variable orifice  132  can be configured to remain closed unless VDMP  110  is operating, or is expected to operate, in a pump mode with a pump flow of at least 1 GPM. Even more particularly, the variable orifice  132  can be configured to remain closed unless VDMP  110  is operating, or is expected to operate, in a pump mode with a pump flow in a range of 1 to 5 GPM. Computer  123  can actuate variable orifice  132  based on signals provided by sensors throughout HMA  101 . For example, components of HMA  101  can be coupled with various sensors to provide an output signal to computer  123  proportional to displacement of VDMP  110 , rotational speed and direction of spline shaft  118 , or other system conditions. Based on these signals, computer  123  can calculate an expected flow of VDMP  110 , and can open or close variable orifice  132  to increase or decrease hydraulic fluid flow through DO  112  in response to the expected flow. Thus, in an embodiment, variable orifice  132  can be opened prior to changing a displacement of VDMP  110 . 
         [0035]    In an embodiment, HMA  101  can include valve  134  to prevent backflow of hydraulic fluid from VDMP  110  and DO  112  into aircraft hydraulic system  113 . For example, valve  134  can be a check valve that limits flow in a single direction. Thus, valve  134  can be oriented to allow hydraulic fluid to flow from aircraft hydraulic system  113  to VDMP  110  and DO  112  when the pressure of hydraulic fluid at an inlet of valve  134  exceeds the pressure of hydraulic fluid at an outlet of valve  134 . Conversely, valve  134  can disallow backflow of hydraulic fluid from VDMP  110  and DO  112  into aircraft hydraulic system  113  if the pressure of hydraulic fluid at the outlet exceeds the pressure of hydraulic fluid at the inlet. It will be appreciated that valve  134  need not be a check valve. For example, valve  134  can be an electromechanically controlled valve, such as a two-port valve that is controlled by computer  123  in response to various feedback data from, e.g., pressure and flow sensors. Thus, valve  134  can emulate the operation of a check valve by being controlled according to various system conditions. 
         [0036]    Having generally described an embodiment of HMA  101 , several components of DO  112  will now be given further attention. Referring now to FIG.  2 , a schematic view illustration of a fixed orifice component of a dual orifice is shown in accordance with an embodiment. Fixed orifice  130  includes supply port  202  and return port  204 . Supply port  202  can be connected with supply conduit  102  and return port  204  can be connected with return conduit  104 . For example, supply port  202  and return port  204  can include any of various fittings, e.g., threaded or clamp, that are known in the art for interconnecting hydraulic conduits. Fixed orifice  130  also includes channel  206  between supply port  202  and return port  204 . Channel  206  conveys fluid from supply conduit  102  to return conduit  104 . In addition, fixed orifice  130  includes restriction  308  located within channel  206 . Restriction  308  limits flow through fixed orifice  130 . It will be appreciated that restriction  308  can be configured with numerous shapes to produce the desired flow effect. For example, restriction  308  can be shaped in a manner to maintain laminar flow of hydraulic fluid or to induce turbulent flow, if desired. Without limitation, restriction  308  can be a smooth orifice, a sharp-edged orifice, or a roughened orifice of various diameters and cross-sections. 
         [0037]    Thus, it will be appreciated that in at least one embodiment, fixed orifice  130  can be configured as a simple flow restriction formed in a channel. However, it will be appreciated that fixed orifice  130  can be embodied by numerous other valves and flow restrictors, as well. For example, in at least one alternative embodiment, fixed orifice  130  can include a solenoid valve that is maintained in an open state to allow for the continuous flow of hydraulic fluid through the solenoid valve. This and other embodiments will be contemplated by one skilled in the art within the scope of this description. 
         [0038]    Referring now to  FIG. 3 , a schematic view illustration of a variable orifice component of a dual orifice is shown in accordance with an embodiment. Variable orifice  132  can include a solenoid valve, as represented by  FIG. 3 . The various components of variable orifice  132  having a solenoid valve can be packaged within one or more subassemblies, such as housing  314 . More specifically, variable orifice  132  can include supply port  302  connected with supply conduit  102  and return port  304  connected with return conduit  104 . Hydraulic fluid flow from supply port  302  to return port  304  can be affected by the actuation of plunger  306 . More specifically, plunger  306  can be moved between a first position, in which it restricts or prevents flow through return conduit  104 , to a second position, in which it permits such flow. Even more particularly, when the solenoid valve is deenergized, plunger  306  can be biased toward the return port  304  by spring  308  in order to obstruct hydraulic fluid flow through the return port  304 . However, when the solenoid valve is energized, plunger  306  can be moved away from return port  304  by a magnetic force applied to the plunger  306 , or a related component. The magnetic force can result from current flow through coil windings  310 , as is well known in the art. Coil windings  310  can be energized by electricity provided by wiring  312 . 
         [0039]    Thus, it will be appreciated that in at least one embodiment, variable orifice  132  can include, or be represented by, a solenoid valve. However, it will further be appreciated that variable orifice can be embodied by any number of variable output valves and fittings, such as those that are driven by motors or hydraulics, as is well known in the art. Furthermore, it will be appreciated that although variable orifice  132  has mainly been described as having discrete open and closed states, variable orifice  132  could alternatively include a hydraulic fluid pathway with a varying diameter, such that flow can be increased and decreased gradually from a closed configuration to a maximum flow configuration, rather than being fully open or fully closed. 
         [0040]    In yet another embodiment, fixed orifice  130  and variable orifice  132  can be replaced with a single orifice component that simulates the function of the two orifices. For example, a single variable orifice like variable orifice  132  shown in  FIG. 4  can be used that has a low flow and a high flow configuration. In the low flow configuration, a plunger of the single variable orifice can be positioned to allow a first flow. For example, in the low flow configuration, single variable orifice can allow flow in a range of 1 to 5 GPM when hydraulic fluid pressure in supply conduit  102  is at 3000 psig pressure. In the high flow configuration, the plunger can be moved to allow a second flow, such as a flow in a range of 6 to 20 GPM. Thus, the alternative variable orifice can simulate fixed orifice  130  in the low flow configuration and it can simulate the combination of fixed orifice  130  and variable orifice  132  in the high flow configuration. This varying flow can be achieved by designing the single orifice component flow to depend on the stroke as well as the diametric gap between plunger  306  and return port  304 , as will be apparent to one skilled in the art. 
         [0041]    In still another embodiment, DO  112  can be replaced with a multi-orifice bypass valve with more than two orifices. For example, DO  112  can have three or more orifices of various flow rate capacities. These orifices can be fixed or variable. Thus, the term “dual orifice” as used in this description is not to be interpreted as being limited to two orifices. 
         [0042]    The various components of HMA  101  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. 
         [0043]    Having outlined several features of HMA  101  and its components, additional detail will now be provided related to patterns of hydraulic fluid flow through HMA  101  under different operating conditions. Referring now to  FIG. 4A , a schematic view illustration of a hydraulic motor assembly operating with a VDMP  110  in a motor mode is shown in accordance with an embodiment. With VDMP  110  operating in this mode, spline shaft  118  and hose reel  119  are rotated in a retract direction to cause the hose to be retracted onto hose reel  119 . Again, this is a process that involves operation of the feedback controlled HMA  101  system using active system commands and various feedback loops. 
         [0044]    As previously discussed, while VDMP  110  operates in a motor mode, hydraulic fluid is circulated from supply conduit  102  through both VDMP  110  and DO  112 . More specifically, hydraulic fluid flows through VDMP  110  and bleeds through fixed orifice  130  of DO  112 . 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 variable orifice  132  of DO  112  in motor mode. There is typically no need for such flow since the hydraulic fluid is optimally directed through VDMP  110  in order to generate sufficient torque to retract the hose onto hose reel  119 . Thus, bleeding of hydraulic fluid through DO  112  is limited to flow through fixed orifice  130  by closing variable orifice  132  while VDMP  110  operates in motor mode. Limitation of bleeding is desirable because the hydraulic fluid bleed is essentially wasted flow and lost energy, given that the bleed flow is not used to generate torque through VDMP  110 . In an embodiment, fixed orifice  130  can continuously bleed hydraulic fluid at a rate in a range of 1 to 5 GPM. 
         [0045]    Referring now to  FIG. 4B , a schematic view illustration of a hydraulic motor assembly operating with a VDMP  110  in a pump mode at low flow is shown in accordance with an embodiment. Building on the previous description, hydraulic fluid is still conveyed through DO  112  from supply conduit  102  to return conduit  104  when VDMP  110  operates in a low flow pump mode. However, in this mode of operation, hydraulic fluid flows through VDMP  110  from return conduit  104  to supply conduit  102 . Flow through VDMP  110  can be relatively low when the hose is trailing at a low speed or low tension. More specifically, flow generated by VDMP  110  in a low flow pump mode can generally be in a range of about 1 to 5 GPM to produce sufficient output torque at spline shaft  118  in order to maintain hose reel  119  rotation at the appropriate rate. It will be appreciated that this rate could actually be zero, as in the case when hose reel  119  is kept stationary to maintain the hose at a constant position. 
         [0046]    As discussed above, given that flow through VDMP  110  remains below about 1 to 5 GPM in the low flow pump mode, fixed orifice  130  typically provides a sufficient passage for hydraulic fluid to circulate through DO  112 . Thus, variable orifice  132  can remain closed to limit bleeding from the aircraft hydraulic system  113 . As will be described below, hydraulic fluid can be supplied by aircraft hydraulic system  113  to DO  112  to maintain fixed orifice  130  at a rated flow rate when flow through VDMP  110  operating in the low flow pump mode is below the rated flow rate. 
         [0047]    Referring to  FIG. 4C , a schematic view illustration of a hydraulic motor assembly operating with a VDMP  110  in a pump mode at high flow is shown in accordance with an embodiment. In the high flow pump mode, hydraulic fluid flows through VDMP  110  from return conduit  104  to supply conduit  102 . As discussed above, flow through VDMP  110  can be increased to accommodate increased output torque requirements. Output torque requirements can increase due to, e.g., high aerodynamic loads from adverse weather conditions or increased loads applied by the receiver aircraft during the flowing disconnect stage. In these scenarios, increased torque must be transmitted to hose reel  119  through spline shaft  118 . Thus, displacement of VDMP  110  is controlled by computer  123  to operate VDMP  110  in a high flow pump mode. In this mode, flows can exceed the low flow upper limit of about 1 to 5 GPM. As an example, hydraulic fluid flow through VDMP  110  can be increased to about 12.5 GPM in the high flow pump mode. 
         [0048]    To accommodate the increased flow through VDMP  110 , hydraulic fluid can be directed through both fixed orifice  130  and variable orifice  132  of DO  112 . More specifically, variable orifice  132  can be actuated to an open position by computer  123  in response to determining that pump flow from VDMP  110  will exceed predetermined levels. This determining can include calculating expected flow that will result from changes to VDMP  110  operation, e.g., displacement or speed, required to achieve the necessary torque output. Even more particularly, variable orifice  132  can be opened to permit a flow rate of about 5 to 15 GPM of hydraulic fluid to flow from supply conduit  102  to return conduit  104 . Thus, the combined flow through fixed orifice  130  and variable orifice  132  of DO  112  can result in total flow in a range of about 6 to 20 GPM through DO  112 . As will be described below, hydraulic fluid can be supplied by aircraft hydraulic system  113  to DO  112  to maintain fixed orifice  130  and variable orifice  132  at a combined rated flow rate when flow through VDMP  110  operating in the high flow pump mode is below the combined rated flow rate. 
         [0049]    Referring to  FIG. 5 , a method of using a hydraulic motor assembly is shown in accordance with an embodiment. At operation  501 , VDMP  110  is operated in a first pump mode. For example, VDMP  110  can be operated in a low flow pump mode. It will now be appreciated that this mode may be used when the hose is trailing at low speed and/or low tension, i.e., when low output torque is required. As discussed above, the displacement and speed of VDMP  110  can be controlled by computer  123  through actuation of control piston  125  in response to various feedback signals from sensors, e.g., linear variable differential transformer displacement transducers, dual speed sensors, and load cell sensors connected with a planetary gear box (not shown) that is placed between spline shaft  118  and hose reel  119  indicating that low output torque is required. More particularly, in this mode, VDMP  110  may be operated to generate only about 1 to 5 GPM of hydraulic fluid flow from return conduit  104  to supply conduit  102 . 
         [0050]    At operation  505 , hydraulic fluid is conveyed through fixed orifice  130  of DO  112  while VDMP  110  operates in the low flow pump mode. In an embodiment, this fluid can be a combination of hydraulic fluid flowing from aircraft hydraulic system  113  and hydraulic fluid flowing in reverse from the first port of VDMP  110 . For example, in an embodiment, fixed orifice  130  allows 5 GPM of hydraulic fluid flow when hydraulic fluid in supply conduit  102  is at about 3000 psig. However, if VDMP  110  is operated in a low flow pump mode with a flow of only 3 GPM to achieve the necessary torque output, then additional flow is required to maintain fixed orifice  130  at optimal flow conditions. Thus, aircraft hydraulic system  113  can supply 2 GPM of hydraulic fluid flow to fixed orifice  130  to maintain fixed orifice  130  at the maximum flow capacity of 5 GPM. If flow through VDMP  110  decreases to generate less output torque, then hydraulic fluid supply from aircraft hydraulic system  113  will increase, maintaining fixed orifice  130  at maximum flow capacity. Likewise, if flow through VDMP  110  increases to generate more output torque, then hydraulic fluid supply from aircraft hydraulic system  113  will decrease, maintaining fixed orifice  130  at maximum flow capacity. Thus, the hydraulic fluid recirculated to VDMP  110  can be a mixture of hydraulic fluid coming from aircraft hydraulic system  113  and hydraulic fluid coming from VDMP  110 . 
         [0051]    In an embodiment, the mixing of hydraulic fluid from aircraft hydraulic system  113  and VDMP  110  as it flows through fixed orifice  130  has an overall cooling effect on the hydraulic fluid that is recirculated to VDMP  110 . 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 fixed orifice  130 . 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, hydraulic fluid flowing from aircraft hydraulic system  113  can be cooler than the recirculated hydraulic fluid. Therefore, mixing hydraulic fluid from aircraft hydraulic system  113  and VDMP  110  in DO  112  will supply cooler hydraulic fluid at an outlet of DO  112  than would otherwise be the case if the same hydraulic fluid was recirculated continuously during the pump mode. A portion of the mixed fluid will be returned to aircraft hydraulic system  113  through return conduit  104  where the imparted heat can be dissipated. Additionally, a portion of the mixed fluid will be recirculated to VDMP  110  in the pump mode at the cooled temperature. Thus, the mixing function of DO  112  can prevent damage to the hydraulic fluid inside HMA  101  as well as to VDMP  110 . 
         [0052]    At operation  510 , feedback signals are received by computer  123  indicating that increased torque output is required by VDMP  110 . For example, feedback signals indicating that high aerodynamic loads require high braking torque or that the receiver aircraft is trying to disengage within the refueling range, i.e., during a flowing disconnect stage of the refueling cycle, can be received by computer  123 . These signals can be generated by any of the previously described sensors. In response to these signals, computer  123  calculates an expected pump flow resulting from adjustment of VDMP  110  to achieve the required output torque. For example, computer  123  can determine that the expected flow from VDMP  110  will enter a high flow pump mode and exceed a predetermined rate as a result of changes to VDMP  110  displacement to achieve a necessary high braking torque. As described earlier, this predetermined rate could be greater than 1 GPM, such as in a range of about 1 to 5 GPM. 
         [0053]    At operation  515 , in response to computer  123  calculating the expected flow based on feedback signals received at operation  510  and determining that the expected flow is above the predetermined level, computer  123  actuates variable orifice  132  to increase the allowable flow rate of DO  112 . Actuation can be caused by, for example, energization of a solenoid valve of variable orifice  132 . 
         [0054]    At operation  520 , VDMP  110  can be operated in a second pump mode. For example, VDMP  110  can be operated in the high flow pump mode. As described above, computer  123  can control the displacement and speed of VDMP  110  through actuation of control piston  125 . As a result, VDMP  110  pump flow will increase until VDMP  110  enters the high flow pump mode. In an embodiment, VDMP  110  will enter the high flow pump mode when the flow generated by VDMP  110  exceeds the predetermined rate. This predetermined rate can be chosen to coincide with the flow capacity of fixed orifice  130 . For example, as described above, in at least one embodiment this predetermined rate can be in a range of about 1 to 5 GPM. 
         [0055]    At operation  525 , hydraulic fluid is conveyed through both fixed orifice  130  and variable orifice  132  of DO  112 . DO  112  can allow about 6 to 20 GPM hydraulic fluid flow when both fixed orifice  130  and variable orifice  132  are open and the pressure of hydraulic fluid in supply conduit  102  is about 3000 psig. For example, under such conditions, fixed orifice  130  can accommodate hydraulic fluid flow in a range of about 1 to 5 GPM and variable orifice  132  can accommodate hydraulic fluid flow in a range of about 5 to 15 GPM. However, as noted above, DO  112  can accommodate other maximum flow conditions through fixed orifice  130  and variable orifice  132 . Furthermore, it will be appreciated that hydraulic fluid flow through DO  112  in this mode can be a mixture of hydraulic fluid recirculated from VDMP  110  and supplied by aircraft hydraulic system  113 , in a manner similar to that described above with respect to operation  505 . More specifically, aircraft hydraulic system  113  can supply additional hydraulic fluid to DO  112  to maintain DO  112  at a maximum flow capacity when flow generated by VDMP  110  is less than the maximum flow capacity. Thus, the output of DO  112  can be a cooled mixture of hydraulic fluid that prevents damage to VDMP  110  and returns heat generated by DO  112  to the aircraft hydraulic system  113 , where it can be dissipated. 
         [0056]    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.