Abstract:
A hydraulic system for raising and lowering aircraft landing gear ( 11, 12 ) includes an actuator ( 18 ) which is extendible and retractable to operate the landing gear, the actuator ( 18 ) including a movable member ( 19 ) in a casing ( 20 ), the movable member ( 19 ) being moved relative to the casing ( 20 ) in a first direction to extend the actuator ( 18 ) when fluid under pressure is supplied to a first side ( 22 ) of the movable member ( 19 ) while fluid is exhausted from a second side ( 23 ) of the movable member ( 19 ), and the movable member ( 19 ) being moved in a second direction to retract the actuator ( 18 ) when fluid under pressure is supplied to the second side ( 23 ) of the movable member ( 19 ) while fluid is exhausted from the first side ( 22 ) of the movable member ( 19 ), and there being selector valve ( 25 ) selectively to supply pressurized fluid to the first ( 22 ) or second ( 23 ) side of the movable member ( 19 ), and a check valve ( 40 ) to permit exhausted fluid from at least one of the first ( 22 ) and second ( 23 ) sides of the movable member ( 19 ) to augment the supplied fluid from the selector valve ( 25 ) and thus be directed with the supplied fluid, to the second ( 23 ) or first side ( 22 ) respectively of the movable member ( 19 ).

Description:
FIELD OF THE INVENTION 
     This invention relates to a hydraulic system for raising and lowering an aircraft landing gear, and more particularly but not exclusively useful for a kind of aircraft landing gear known as a side brace, where the landing gear is movable between a generally vertical condition for landing, and a generally horizontal condition for stowage e.g. within a wing of the aircraft. 
     BACKGROUND OF THE INVENTION 
     A particular feature of such side brace landing gears is that landing loads act through an actuator which is provided to raise and lower the landing gear. Accordingly such actuators have to be more substantial than would be required simply for raising and lowering the landing gear. Typically such actuators are piston and cylinder arrangements, and the piston diameter is made larger than is necessary just for raising and lowering the landing gear. 
     SUMMARY OF THE INVENTION 
     As a result, th actuator demands a large volume of hydraulic fluid for operation. Particularly during landing, when it is desired to lower the landing gear, other aircraft services will be demanding hydraulic fluid, for example flap lowering actuators may require fluid, which can place high demands on the hydraulic pump(s) supplying pressurised hydraulic fluid to the various services. 
     According to one aspect of the invention we provide a hydraulic system for raising and lowering aircraft landing gear, the system including an actuator which is extendible and retractable to operate the landing gear, the actuator including a movable member in a casing, the movable member being moved relative to the casing in a first direction to extend the actuator when fluid under pressure is supplied to a first side of the movable member while fluid is exhausted from a second side of the movable member, and the movable member being moved in a second direction to retract the actuator when fluid under pressure is supplied to the second side of the movable member while fluid is exhausted from the first side of the movable member, and there being selector valve means selectively to supply pressurized fluid to the first or second side of the movable member, and means provided to permit exhausted fluid from at least one of the first and second sides of the movable member to augment the supplied fluid from the selector valve means and thus be directed with the supplied fluid, to the second or first side respectively of the movable member. 
     By virtue of the system of the present invention, the demand on the hydraulic pump(s) is thus reduced during landing gear operation as exhausted fluid is recirculated to augment the fluid supply, thus reducing the volume of fluid required from the hydraulic pump(s) to operate the landing gear. 
     In one arrangement the means which permit exhausted fluid to augment the supplied fluid includes a check valve which is opened as tile movable member of the actuator moves relatively in the casing in the first direction to extend the actuator and lower the landing gear. 
     The check valve mast be adapted to open to permit exhausted fluid from the second side of the movable member to augment the supplied fluid in response to the pressure of the fluid supplied to the first side of the member or alternatively in response to a pressure build up in a line carrying exhausted fluid from the second side of the movable member. In each case preferably closure means are provided positively to close the check valve when pressurised fluid is supplied by the selector salve to the second side of the movable member. 
     Such closure means may be of a hydraulic and/or mechanical nature. 
     Preferably means are provided to relieve exhausted fluid which is not recirculated from the at least one of the first and second sides of the movable member as the movable member reaches tile end of travel in the casing. Thus there is no risk of trapped fluid interfering Faith the propel operation of the landing gear. 
     In one arrangement the hydraulic system includes a first fluid supply line to the first side of the movable member for supplied fluid from the selector valve means when the selector valve means is in a first position, and a second supply line to the second side of the movable member for supplied fluid from the selector valve means when the selector valve means is in a second position, and the means which permit exhausted fluid from at least one of the first and second sides of the movable member to augment the supplied fluid from the selector valve means and thus be directed with the supplied fluid, to the second or first side respectively of the movable member, permitting the exhausted fluid to flow from the second supply line to the first supply line. 
     To ensure that exhausted fluid is available to augment the supplied fluid to extend the actuator, the second supply line may include non-return means at least to restrict the flow of exhausted fluid from the hydraulic system. However desirably a restrictor means is provided to enable a restricted flow of exhausted fluid to by-pass the non-return means so that fluid which is not recirculated, is not trapped in the second supply line which could interfere with the proper operation of the landing gear. 
     The selector valve means may be movable to a first position to permit the flow of fluid therethrough from a source of pressurized fluid to the first side of the movable member, and to a second position to permit the flow of fluid therethrough from the source to the second side of the movable member, and to a rest position in which the source is isolated and fluid may pass from the system tank. 
     According to a second aspect of the invention we provide an aircraft having landing gear which is raised and lowered by a hydraulic system according to the first aspect of the invention. 
     According to a third aspect of the invention we provide a valve including, a valve member and a piston each received in a passage in a valve body, the valve member and piston being biased apart by resilient means such that the valve member is urged towards a valve seat towards one end of the passage, and the piston is urged towards a stop towards an opposite end of the passage, a fluid inlet and a fluid outlet, the pressure of fluid at the inlet when sufficient, acting to move tile valve member against the force of the biasing means off the valve seat to permit fluid flow from the inlet, past the valve seat, to the outlet, and the piston being movable in the passage away from the stop in response to a pilot pressure delivered to a pilot pressure port of the body against the force of the biasing means to a position in die passage in which the piston engages the valve member and restrains the valve member against movement off the valve seat in response to the inlet pressure. 
     Means may be provided to permit fluid pressure at the outlet to be communicated to an intermediate region of the passage between the valve member and the piston at least when the piston is engaged with the stop. For example the outlet may open into the valve passage and there may be a flow path for the fluid at the outlet past and/or through the valve member to the intermediate region when the valve member is in engagement with the valve seat. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
     Such a valve in accordance with the third aspect of the invention may be a check valve to permit the flow of exhausted fluid from at least one of the first and second sides of the movable member of the actuator of the hydraulic system according to the first aspect of the invention to augment the supplied fluid from the selector valve means and thus be directed with the supplied fluid, to the second or first side respectively of the movable members. 
     The invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 is a front illustrative view of an aircraft showing the landing gear thereof on one side in a lowered lauding condition and the landing gear at the other side partly raised towards a stowed condition, tile landing gear being operated by a hydraulic system in accordance with the invention. 
     FIG. 2 is an illustrative view of a hydraulic system in accordance with the invention; 
     FIG. 3 is a more detailed but illustrative view of a check valve for use in the system of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1 of the drawings an aircraft  10  has landing gear  11 ,  12  one at each side thereof, the landing gears  11 ,  12  each including a strut assembly  13  which carries a landing wheel  14 , which strut assembly  13  is movable between a generally vertical lowered condition for landing, and a raised stowed condition in which the strut assembly  13  is generally horizontal and located within a wing : 15  where the landing gear  11 ,  12  is stowed. 
     The landing gears  11 ,  12  are operable by means actuators  18  which are extendible and retractable by means of pressurized hydraulic fluid. 
     Referring now to FIG. 2, a hydraulic system is shown. It can be seen that in this example each actuator  18  has a movable member or piston  19  in this example, which moves inside a casing or cylinder  20 , as is well known in the art of hydraulic systems. 
     When pressurized hydraulic fluid is supplied to a head end  11  of the actuator  18  at a first side of the piston  19 , the piston  19  moves in the cylinder  20  so as to extend the actuator  18 , at the same time forcing fluid at a second opposite side of the piston  19 , to be exhausted from an actuator rod end  23  of the actuator  18 . Conversely, when pressurized fluid is supplied to the actuator rod end  23  of the actuator  18 , the piston  19  moves in the cylinder  20  so as to retract the actuator  18 , at the same time forcing fluid to be exhausted from the head end  22 . 
     Hydraulic fluid is fed to the actuator  18  via a selector valve means  25  to which a source of fluid under pressure i.e. a hydraulic pump  26  is connected. The pump  26  draws hydraulic fluid for pumping, from a tank  27 . 
     The selector valve means  25  is movable between three positions in this example. When in a first raised position, i.e. when a spool  29  thereof is in the position indicated in the drawings at I, pressurized fluid is supplied to a first supply line  30  which extends to the head end  22  of the actuator  18 . In the first supply line  30  there is a flow regulating means  31  which controls the pressure of fluid which is supplied to the head end  22  of the actuator  18 . Also there is a by-pass one way valve  32  which enables fluid from the first supply line  30  to flow freely to tank  27  as hereinafter described. 
     When the spool  29  of the selector valve means  25  is in an intermediate or rest position as shown and indicated at R, the pressurized fluid source i.e. pump  26  is isolated from the actuator  18  and moreover fluid may flow from the first supply line  30  as well as from a second supply line  34  to be described, back to tank. 
     When the spool  29  of the selector valve means  25  is in a second lowered position as indicated at II in FIG. 2, pressurized fluid is fed from the pump  26  to a second supply  34  which extends to the rod end  23  of the actuator  18 . The second supply line  34  includes a one way valve  35  through which pressurized fluid may freely flow to the rod end  23  of the actuator  18 , and a by-pass restrictor  36  which allows fluid to by-pass the one way vale  35  as hereinafter described. 
     Between the first  30  aid sec id  34  supply lines, there is a check valve  40 . In FIG. 2 the operation of the check valve  40  is shown illustratively, and the operation of a practical such check valve  40  will be described below with reference to FIG.  3 . 
     The operation of the hydraulic system described will now be outlined. 
     Starting with the actuator  18  in a retracted position in which the landing gear  11  or  12  is stowed, when it is desired to lower the landing gear for landing, the selector valve means  25  is moved to the first position  1 . 
     Fluid thus flows along the first supply line  30 , through the flow regulating means  31  to the head end  22  of the actuator  18  and the piston  19  is caused to commence movement to extend the actuator  18  to lower the landing gear  11  or  12 . Fluid flow from the rod end  23  of the actuator  18  is however restricted to a small flow through the by-pass restrictor  36  from where the exhausted fluid passes to tang  27  via the selector valve means  25 . 
     Because exhausted fluid flow is thus restricted, pressure will build up in both the first and second  30 , 34  lines. This pressure build up results in the check valve  40  being opened, as more particularly described below with reference to FIG. 3, thus to permit exhausted fluid from the rod end  23  of the actuator  18  to flow into the first supply line  30  to augment the supply of fluid from the selector valve means  25 , resulting in less fluid being demanded from the pump  26 . 
     Thus piston  19  movement in the cylinder  20  may continue fully to lower the landing gear  11  or  12 . 
     The presence of the by-pass restrictor  36  in the line  34  between the rod end  23  and the selector valve means  25  allows fluid flow from the line  34  when the check valve  40  is closed so that there is no risk of residual uncirculated fluid preventing the piston  19  from moving through its full range. 
     Typically, a mechanical or other sensing arrangement is provided which may interface with an interlock which operates mechanically to retain the landing gear in its fully lowered condition and may cause the selector valve means  25  to assume the rest position R once the landing gear  11  or  12  is fully lowered. In the rest condition R, fluid may flow from the first and second supply lines  30 ,  34  to tank  27  so that the system is unpressurized when the landing gear  11  or  12  is in a fully lowered condition and the pump  26  is indicated. 
     When the selector valve means  25  is moved to the rest position R, the check valve  40  will be closed by the action of a rerun spring  42  or the like. 
     When it is desired to raise the landing gear  11  or  12  the selector valve means  25  may be moved to the second position II in which pressurized fluid is fed to the second supply line  34  through the one way valve  35  to the rod end  23  of the actuator  28 , and the piston  19  will be moved to begin retraction of the landing gear  11  or  12 . When there is an interlock mechanically to retain the landing gear in a lowered condition, this needs to be released before the piston  19  can move. Such release may be arranged to occur simultaneously with selector valve means  25  movement. 
     Fluid which is exhausted from the head end  22  of the actuator  18  may flow freely past the one way valve  32  in the first line  30 , to tank  27  via the selector valve means  25 . The check valve  40  will remain closed when the selector valve means  25  is in the second condition and there is no exchange of fluid between the two lines  30  and  34 . However, when fluid is supplied to the second supply line  34 , a portion of (lie fluid is bled to a pilot line  39  which flows to the check valve  40  and operates to maintain the check valve  40  in the closed condition. 
     As the landing gear  11  or  12  fully retracts, an uplock may be operated mechanically to hold the landing gear in its raised condition. At the same time, the selector valve means  25  may be moved to the rest position R so that again, the hydraulic system is unpressurized when the landing gear  11  or  12  is in its stowed condition. 
     Referring now to FIG. 3 particular example of a check valve  40  is illustrated. 
     The check valve  40  has a valve body  50  in which there is provided a valve passage  51 . The passage  51  contains a valve member  52  which is movable in the passage  51  into and away from engagement with a valve seat  54 , towards one end  60  of the passage, and a valve piston  55 . The valve piston  55  has a larger cross sectional area than the valve member  52 , and accordingly the passage  51  is stepped. The valve piston  55  is in sealing engagement with the passage  51 . 
     Between the valve member  52  and the valve piston  55  there is provided a resilient biasing means which in this example is a coil spring  56  which urges the valve member  52  into engagement with its seat  54 , and the valve piston  55  into engagement with a stop  58  at one closed end  59  of the passage  51 . 
     At the opposite end  60  of the passage  51  to the closed end  59 , there is a fluid inlet  61 , arranged such that fluid pressure at the inlet  61  acts to lift the valve member  52  from the seat  54  when sufficient to overcome the force of the coil spring  56 . A fluid outlet  59  intersects with the passage  51  and is positioned such that as the valve member  52  is lifted off its seat  54 , fluid flow from the inlet  61  to the outlet  59  is permitted. Moreover, the valve member  52  has a circumferential recess  64  which is aligned with the outlet when the valve member  52  is on its seat  54 . The recess  64  opens to an axially extending channel  65  along the valve member  52  so that fluid pressure at the outlet  59  is communicated to an intermediate region  66  between the valve member  52  and the valve piston  55 . The valve member  52  and the valve piston  55  are each cut away to provide a location for the coil spring  56  and to enhance the volume of the intermediate region  66 . Thus when the valve member  52  is on its seat  54 , outlet fluid pressure assists the spring  56  in urging tile valve member  52  and pistol  55  apart. 
     In FIG. 3, the connections to the hydraulic system of FIG. 2 are intimated. In FIG. 2, there is shown line  70  from the first supply line  30  to the check valve  40  to intimate that the check valve  40  responds to sufficient fluid pressure in the first supply line  30 . In FIG. 3 though there is intimated a line  71  from the second supply line  34  to the check valve  40 . In both cases because there is a pressure rise as the landing gear  11  or  12  is lowered because fluid cannot freely flow from the rod end  23  of the actuator  18 , the check valve  40  will be opened. 
     Referring again to FIG. 3, when such pressure rise is experienced in the second supply line  34  and line  71 , this will be communicated to the face  74  of the valve member  52  and will lift the valve member  52  off the seat  54 . Thus during lowering of die landing gear  11  or  12 , fluid may flow past the valve member  52  from the second line  34  into the first supply line  30  to augment the supplied fluid. 
     When fluid is supplied under pressure to the second supply line  34  to raise the landing gear  11  or  12 , a pilot pressure is delivered to the check valve along line  39  and acts to urge the valve piston  55  away from its stop  58  and against the force of the coil spring  56  until the valve piston  55  engages the valve member  52  to urge the valve member  52  firmly onto the seat  54 , notwithstanding high supply fluid pressures exerted on the face  74  of the valve member  52 , because the valve piston  55  are of on which the pilot pressure acts, is greater than that of the valve member  52 . 
     Various modifications may be made without departing from the scope of the invention. 
     Particularly, the check valve  40  described with reference to FIG. 3 is only an example of a check valve suitable for the purpose of allowing fluid expelled from the actuator  18  to augment the flow of fluid to the actuator. 
     The various components of the hydraulic system have been described as separate components although it will be appreciated that multiple components may be provided e.g. in a common valve block. Thus the one way valves  32 ,  35  and/or the restrictor means  36  and/or the floss control means  31  may be provided in a common valve block together or not with the check valve  40  and possibly the selector valve means  25  too. 
     The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination be utilized for realizing the invention in diverse forms thereof.