Patent Publication Number: US-2009218444-A1

Title: undercarriage shock absorber with positive retention in a retracted position and with crash overtravel

Description:
The invention relates to an undercarriage shock absorber with positive retention in a retracted position and with crash overtravel. 
     BACKGROUND OF THE INVENTION 
     Telescopic shock absorbers are known for the undercarriage of an aircraft, in particular a helicopter, that are constituted by a strut in which a rod is mounted to slide, the shock absorber including controlled retraction means for retracting the rod into the strut to a retracted position. 
     By way of example, document FR 2 608 242 describes one such shock absorber in which the retracted position is defined by an abutment of the strut. The rod is moved to the retracted position by injecting hydraulic fluid into the annular chamber that extends between the rod and the strut. The rod is then held in the retracted position by keeping the hydraulic fluid captive in the annular chamber. 
     The abutment is mounted in the strut at the end of a support tube that is suitable for buckling under a force exceeding a predetermined threshold so that in the event of the aircraft crash landing, a compression force is generated on the shock absorber that is greater than said threshold, so the rod deforms the support tube and can thus be pushed into the strut beyond the retracted position. 
     In the event of hydraulic fluid leaking while the rod is held in the retracted position, the retracted position is no longer guaranteed and the shock absorber lengthens progressively under thrust from the compressed gas contained in the shock absorber and as a function of the volume of fluid that escapes from the annular chamber. 
     In order to maintain the retracted position even in the event of hydraulic fluid leaking, the precaution has been taken to provide the shock absorber with mechanical means for positively retaining the rod in the retracted position inside the strut. Such positive retention means are well known in the field of actuators comprising a cylinder in which a rod is mounted to slide, with some such actuators being fitted with means for positively retaining the rod in its fully retracted position inside the cylinder. By way of example, such retention means can be constituted by a catch or hook mechanism. In this respect, document GB 561 275 describes means for retaining a rod in a cylinder in a fully pushed-home position, such that those means are arranged in a manner that is not compatible with overtravel beyond the retracted position. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to the invention, there is provided a telescopic shock absorber for an aircraft undercarriage, the shock absorber comprising a strut in which a rod is mounted to slide, the shock absorber including controlled retraction means for retracting the rod into the strut to a retracted position. According to the invention, the shock absorber is fitted with positive retention means for retaining the rod in the strut and adapted to retain the rod in the retracted position when the rod is moved thereto by the retraction means, the positive retention means being arranged, at least during a severe landing of the aircraft, to enable the rod to be pushed into the strut beyond the retracted position, either by leaving the rod free to slide beyond the retracted position, or by yielding under thrust from the rod. 
     Thus, the positive retention means as arranged in this way ensure that the rod is held in the retracted position even in the event of hydraulic fluid leaking, but they do not prevent the rod from being pushed into the strut beyond the retracted position. 
     This thus provides push-in overtravel extending beyond the retracted position that can be used to advantage for absorbing additional energy and thus to reduce the risks associated with a severe landing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood in the light of the following description with reference to the figures of the accompanying drawings, in which: 
         FIGS. 1 to 4  are section views of an aircraft shock absorber in a first particular embodiment of the invention, shown in various length positions; 
         FIG. 5  is a section view of an aircraft shock absorber in a variant embodiment of the invention; 
         FIG. 6  is a view showing a detail of  FIG. 5  concerning means for positive retention of the rod; and 
         FIG. 7  is a section view of an aircraft shock absorber in a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , the shock absorber of the invention comprises in conventional manner a strut  1  having a rod  2  mounted to slide in sealed manner therein. 
     For this purpose, the strut  1  and the rod  2  carry sealing gaskets  3  that define an annular chamber  4  between the rod  2  and the strut  1 . 
     The strut is subdivided into a bottom chamber  5  and a top chamber  6  by a diaphragm  7  having throttling orifices  8 . The bottom chamber  5  extends from the diaphragm  7  to the gasket  3  carried by the rod  2 . 
     A calibration member  9  serves to modify the effective throttling diameter depending on the flow direction of the hydraulic fluid through the throttling orifices  8 . 
     In the position shown in  FIG. 1 , which is the rest position of the shock absorber, hydraulic fluid fills the bottom chamber  5  and a portion of the top chamber  6 . The other portion of the top chamber  6  is subdivided into two gas compartments (here containing nitrogen): a low-pressure compartment  10  in contact with the hydraulic fluid, and a high-pressure compartment  11  separated from the low-pressure compartment  10  by a separator piston  12  that is held against an abutment  13  by the effect of the pressure difference acting thereon. 
     The rod  2  is hollow and has a top end  14  in the form of a chimney open to the bottom chamber  5  of the strut  1 . A plug  15  is mounted to slide in sealed manner in the rod  2  between a bottom abutment and a top abutment to define within the rod a retraction chamber  61  and to isolate the fluid contained in the strut  1  from the fluid contained in the retraction chamber  61 . 
     The bottom end of the rod  2  has a controlled valve  20  that, on being activated, puts the retraction chamber  61  into communication with an external hydraulic circuit. In the rest position shown here, the retraction chamber  61  of the rod  2  is filled with fluid such that the plug  15  is against its top abutment, the valve  20  being kept closed by a spring  21 . The plug  15  is then prevented from moving down into the rod  2 . 
     According to the invention, the strut includes a conically-shaped support  25  extending under the diaphragm  7  and carrying a plate  26  that forms an abutment for the rod  2  in the retracted position, as explained below. The plate  26  carries a catch  27  adapted to co-operate with a step  28  carried by the end  14  of the rod  2  in order to hold the rod  2  in its retracted position in the strut  1 . 
     A piston  29  for locking the catch  27  is mounted in the strut to slide axially between the rest position shown in this figure in which it leaves the catch  27  free to expand radially when the step  28  goes past it, and a locking position shown in  FIG. 2  in which it prevents the catch from expanding radially so that the catch  27  holds the rod  2  positively. 
     The locking piston  29  is held in the rest position by a spring  30  extending between the locking piston  29  and the strut  1 . When the rod moves to the retracted position, the locking piston  29  is moved into the locking position by a collar  31  slidably mounted on the top end of the rod  2  and held in abutment against the step  28  by a spring  32  that is stiffer than the spring  30 . 
     The locking piston  29  co-operates with the strut  1  to define an annular unlocking chamber  33  that, on being pressurized, forces the locking piston  29  into the rest position. 
     The operation of the shock absorber shown in  FIG. 1  requires four connections with an external circuit, via four selectors, comprising:
         a first selector  41  adapted by default to put the annular chamber  4  into communication with the hydraulic return (at a pressure of a few bars) of the aircraft, and when operated, to put the annular chamber  4  into communication with a pressure source of the aircraft (typically at several hundreds of bars);   a second selector  42  adapted by default to put the annular unlocking chamber  33  into communication with the hydraulic return of the aircraft, and when operated, to put the annular unlocking chamber  33  into communication with the pressure source of the aircraft;   a third selector  43  adapted by default to put the inside of the rod  2  into communication with the hydraulic return via the valve  20 , and when operated, to put the inside of the rod  2  into communication with the pressure source of the aircraft; and finally   a fourth selector  44  adapted to control the valve  20  via a control piston  22  that co-operates with the rod to define a control chamber  23  and that by default is placed by a spring  24  in a rest position (as shown) in which it does not act on the valve  20 . The fourth selector  44  is adapted by default to put the control chamber  23  into communication with the hydraulic return of the aircraft, and when operated, to put the control chamber  23  into communication with the pressure source of the aircraft so that the control piston  22  is moved against the spring  24  to place the valve  20  in the open position.       

     The shock absorber of the invention is implemented as follows. 
     Starting from the position shown in  FIG. 1 , which is the position of the shock absorber once the aircraft has left the ground, it is desired to cause the shock absorber to retract, e.g. for the purpose of minimizing the aerodynamic drag of the associated undercarriage. 
     For this purpose, and as shown in  FIG. 2 , the first selector  41  is operated to put the annular chamber  4  into communication with the pressure source of the aircraft. Simultaneously, the fourth selector  44  is operated to put the control chamber  23  into communication with the pressure source of the aircraft, thereby having the effect of opening the valve  20 . 
     The fluid under pressure penetrates into the annular chamber  4 , having the effect of moving the rod  2  into the strut  1 . This shortening causes the volume of the bottom chamber  5  to be reduced so that some of the hydraulic fluid contained in the bottom chamber  5  is forced to migrate into the rod  2 , pushing back the plug  15 . The hydraulic fluid contained in the retraction chamber  61  of the rod  2  under the plug  15  is then forced towards the hydraulic return of the aircraft through the open valve  20  and the third selector. 
     The rod  2  then moves progressively into the strut  1  without the gas compartments  10  and  11  being compressed. Thus, the pressure that exists in the shock absorber never exceeds the inflation pressure of the low-pressure compartment  10 . 
     At the end of the stroke, the step  28  carried by the top end of the rod  2  penetrates into the catch  27  until the top end  14  comes into abutment against the plate  26 . The collar  31  moves the locking piston  29  into the locking position, such that the catch can no longer expand and thereby prevents the step  28  from moving back out. The rod  2  is held positively in the retracted position. 
     The first selector  41  is then released so that the annular chamber is again in communication with the hydraulic return of the aircraft. The fourth selector  44  is also released so as to close the valve  20 . 
     To cause the shock absorber to return to the rest position shown in  FIG. 1 , the second selector  42  is operated to place the locking piston  29  in its rest position in which it does not oppose radial expansion of the catch  27 , and the third selector  43  is operated to put the inside of the rod  2  into communication with the pressure source of the aircraft. Fluid reaches the valve against the spring  21 , such that the valve  20  opens automatically without it being necessary to operate the fourth selector  44 . The fluid penetrates into the retraction chamber  61  of the rod  2 , pushing back the plug  15 , which in turn pushes back the hydraulic fluid that has penetrated into the rod  2  from the bottom chamber  5 . The fluid pushed back in this way migrates into the bottom chamber  5 , thereby having the effect of forcing the rod  2  to be extended from the strut  1 . In so doing, the hydraulic fluid contained in the hydraulic chamber  4  is delivered towards the hydraulic return via the first selector  41 . 
     The rod then extends progressively until it reaches its extended abutment. The second selector  42  and the third selector  43  are then returned to the rest position. 
     In the landing configuration, the shock absorber is in the relaxed position shown in  FIG. 1 , with all of its selectors being at rest. The plug  15  is then in top abutment within the rod  2  and the retraction chamber  61  is closed by the valve  20 . The shock absorber acts as though the plug  15  forms a barrier for the fluid contained in the bottom chamber  5  so that fluid cannot penetrate into the rod  2 . 
     As can be seen in  FIG. 3 , pushing the rod  2  into the strut  1  under the effect of landing forces then causes fluid to be transferred from the bottom chamber  5  towards the top chamber  6  through the throttling orifices  8 , which orifices give rise to high levels of head loss, thereby slowing down the movement of the rod  2  into the strut  1 . 
     The fluid as transferred in this way reduces the volume available for the gas in the top chamber  6 , such that at least the low-pressure gas compartment  10  becomes compressed. As soon as the pressure in the low-pressure gas compartment  10  reaches or exceeds the inflation pressure of the high-pressure gas compartment  11 , the high-pressure compartment is in turn compressed. 
     The annular chamber  4  remains connected to the hydraulic return of the aircraft, such that it remains continuously under pressure and no cavitation occurs therein. 
     During ordinary landings, the mechanical impact energy to be absorbed in order to stop the downward movement of the aircraft can be absorbed (by heat being generated during throttling of the fluid or by compression of the gas compartments) over a compression stroke of the rod into the strut that is such that the rod remains below the retracted position shown in  FIG. 2 . 
     Nevertheless, in the event of a severe landing, e.g. a crash landing at a very high vertical speed, such a strut is not sufficient for absorbing the mechanical energy of the impact. 
     Under such situations, and as shown in  FIG. 4 , the rod  2  reaches the retracted position, exerting a force on the abutment  26  that is greater than the deformation threshold of the conical support  25  such that it deforms and the rod  2  continues moving into the strut  1 , taking the plate  26  and the catch  27  with it. Simultaneously, the collar  31  comes to bear against the locking piston and then yields, not opposing movement of the rod  2  beyond the retraced position. 
     This thus releases a certain amount of crash overtravel beyond the retracted position whereby additional mechanical energy can be absorbed by throttling hydraulic fluid and compressing the gas compartments (not counting the mechanical energy that is absorbed by deforming the conical support  25 ). 
     Thus, in accordance with the invention, the means for positively retaining the rod in the retracted position are adapted to allow the rod  2  to be pushed into the strut  1  beyond the retracted position. 
     This possibility of going beyond the retracted position relies in particular in the elements of the positive retention means that are carried away by the rod (catch, step, abutment, collar) being capable of sliding without jamming (or while yielding) relative to the elements of the positive retention means that remain stationary relative to the strut (locking piston). 
     With reference to  FIG. 5  and in a shock absorber similar to that shown in  FIGS. 1 to 4 , one possible variant embodiment of the invention consists in replacing the overridable catch mechanism with a retractable finger mechanism. In  FIG. 5 , elements that are common with elements of  FIGS. 1 to 4  are given the same numerical references plus one hundred. 
     The strut  101  carries two fingers  151  that are slidably mounted in housings  152  to move in a direction perpendicular to the direction in which the rods  201  slide in the strut  101 . The fingers  151  are urged by springs  153  into a latching position as shown in which the fingers project from the housings  152 . 
     The fingers  151  are associated with pistons mounted to slide in sealed manner in the housings  152  and defining therein respective annular chambers that can be put into communication with the pressure source of the aircraft via a selector  154  so as to cause the fingers  151  to be retracted against the springs  153 . 
     The fingers  151  are designed to co-operate with the top end  114  or the rod  102  in order to hold it positively in the retracted position, as shown here. 
     As can be seen more clearly in  FIG. 6 , the top end  114  has a conical first portion  115  that, on going past the fingers  151 , forces the fingers to retract into the housings  152  against the springs  153 . 
     The conical portion  155  is followed by a groove  156  into which the fingers are pushed by the springs  153 , as shown in  FIG. 6 . The fingers  151  then hold the rod  102  positively in the retracted position. To release the rod  102 , the selector  154  is operated to retract the fingers  151  and allow the rod  102  to move downwards. 
     Unlike the shock absorber shown in  FIGS. 1 to 4 , it should be observed that here there is no abutment for defining the retracted position of the rod. This position is defined by the fingers  151  engaging in the groove  156 . This engagement can be identified by a finger position sensor (not shown) that changes the value of an output signal when engagement occurs, which change is advantageously used to stop feeding fluid to the annular chamber  104  and to close the valve  120 . 
     The groove  156  is followed by a conical portion  157  extending the surface defining the wall of the groove  156 . In the event of a severe landing, the rod tends to be pushed further into the strut and the conical portion  156  forces the fingers  151  to retract into the housings  152  against the springs  153 . The rod  102  can then move beyond the retracted position without the positive retention means opposing this movement. 
     The shock absorber thus presents a retracted position that is guaranteed by the positive retention means constituted by the fingers  151 , while also presenting overtravel beyond the retracted position in the event of a severe landing. 
     Unlike the above-described embodiment (and also the embodiment described below), pushing the rod beyond the retracted position does not require an internal part of the shock absorber to be broken or deformed. 
     This possibility of moving beyond the retracted position relies in particular on the elements of the positive retention means that are entrained by the rod (groove  156 ) being capable of sliding without jamming relative to the elements of the positive retention means (fingers) that do not move along the strut. 
     In a second embodiment shown in  FIG. 7 , modifications are made to a shock absorber of the inverted type, e.g. as described in document EP 0 533 530, so as to fit it with overridable positive retention means of the invention. 
     The shock absorber comprises a strut  201  in which a rod  202  is mounted to slide in sealed manner. As above, the strut  201  and the rod  202  define an annular chamber  204 . 
     The shock absorber includes a dip tube  251  that extends inside the rod and that carries at its bottom end a diaphragm  207  separating a bottom chamber  205  filled with hydraulic fluid and extending in the rod  202  under the diaphragm  207  from a top chamber  206  extending above the diaphragm  207  in the strut  201 . 
     The top chamber  206  is partially filled with hydraulic fluid, with the remaining space forming a low-pressure gas compartment  210 . A high-pressure gas compartment  211  extends in the bottom end of the rod  2 , being separated from the hydraulic fluid by a separator piston  212 . 
     The annular chamber  204  is split into two by a floating piston  252  that subdivides the annular chamber into a thrust chamber  253  and a relaxation chamber  254 . 
     The diaphragm  207  has throttling orifices  208  for allowing hydraulic fluid to be transferred between the bottom chamber  205  and the top chamber  206 . Similarly, the rod  202  has throttling orifices  209  for allowing hydraulic fluid to be transferred between the top chamber  206  and the relaxation chamber  254 . 
     During shortening while landing, fluid is transferred from the bottom chamber  205  towards the top chamber  206  via the orifices  208  that give rise to high levels of head loss, slowing down the movement of the rod  202  into the strut  201 . In parallel, fluid is transferred from the top chamber  206  towards the relaxation chamber  254 . 
     The fluid as transferred in this way reduces the volume available for gas in the top chamber  206  such that at least the low-pressure gas compartment  210  is compressed. Once the pressure in the bottom chamber  205  reaches or exceeds the inflation pressure of the high-pressure gas compartment  211 , the high-pressure compartment is compressed in turn. 
     The dip tube  251  defines a retraction chamber  261  that is closed by a plug  215  slidable in leaktight manner in the dip tube  251 . The retraction chamber is closed by a controlled valve  220 . 
     In the same manner as for the shock absorber shown in  FIGS. 1 to 4 , in order to move the rod  202  into the retracted position, hydraulic fluid is admitted into the thrust chamber  253  and the controlled valve  220  is opened to allow the fluid contained in the retraction chamber  251  to escape. 
     The strut  201  has an abutment  226  carried by a conical structure  225 . The abutment  226  co-operates with the top end  214  of the rod  202  in order to define the retracted position. 
     According to the invention, the strut is fitted with a catch  227  that extends to co-operate with a step  228  carried by the top end  214  of the rod  202 . A locking piston  229  is mounted to slide in the strut between a rest position (as shown) in which it leaves the catch  227  free to expand radially when the step  228  goes past it, and a locking position in which it prevents such expansion so that the catch  227  positively retains the rod  202 . 
     The locking piston  229  is held in the rest position by a spring  230  that extends between the locking piston  229  and the strut  201 . The locking piston  229  is moved into the locking position by a collar  231  that is slidably mounted on the top end  214  of the rod  202  and that is held in abutment against the step  228  by a spring  232  that is stiffer than the spring  230 . 
     The locking piston  229  co-operates with the strut  1  to define an annular unlocking chamber  223  that, on being pressurized, forces the unlocking piston  229  into the rest position. 
     This shock absorber operates in entirely similar manner to the shock absorber shown in  FIGS. 1 and 2 , so its operation is not described again in detail below. 
     In the event of a severe landing, e.g. in the event of a crash, the top end  214  of the rod  202  bears against the abutment  226  and deforms the conical structure  225  so that the rod can continue to be pushed in beyond the retracted position. It should be observed that unlike the shock absorber shown in  FIGS. 1 to 4 , the catch  227  is not secured to the abutment  226 , such that the catch  227  is not entrained by the rod  202  when it moves beyond the retracted position. 
     Once more, the possibility of being pushed in beyond the retracted position relies in particular on the elements of the positive retention means that are carried away by the rod (step, abutment, collar) being capable of sliding without jamming (or while yielding) relative to the elements of the positive retention means that remain stationary relative to the strut (catch, locking piston). 
     The invention is not restricted to the above description, but on the contrary covers any variant coming within the ambit defined by the claims. 
     In particular, although the invention is shown in an application to shock absorbers having two gas compartments, this example is not limiting and the invention can be applied in the same manner to shock absorbers having only one, or on the contrary having more than two gas compartments. 
     Although in the examples shown, the controllable portion of the positive retention means (catch/piston or finger/spring) is placed inside the strut while the passive portion of the positive retention means (step or groove) is placed on the rod, it would naturally be possible to invert that arrangement and place the controllable portion of the positive retention means on the rod and the passive portion of the positive retention means in the strut. 
     Furthermore, although in the examples shown, the rod is locked in the retracted position in purely passive manner, with only unlocking being a controlled action, it would be possible to use positive retention means that need to be activated in order to retain the rod.