A liquid spring shock absorber

Abstract

A liquid spring shock absorber 50 has one end 52 with a sealed chamber 56 containing a liquid 70, the other end 54 has a piston rod 58 which is reciprocally received in the chamber 56. The piston rod

Description

BACKGROUND OF THE INVENTION

[0001] The present disclosure relates to the field of liquid spring shock absorbers. It relates particularly, but not exclusively, to liquid spring shock absorbers of the kind which may be used in an aircraft landing gear such as a trailing-link type main landing gear of a commercial airliner.

[0002] In the field of commercial airliners, rather than other technical fields such as military aircraft, UAVs and aircraft for general aviation, most main landing gear use an oleo-pneumatic shock absorber or “Oleo strut”. These are well known in the art. In brief, these shock absorbers have a piston which is reciprocally movable within a chamber or (often referred to as a cylinder). The cylinder is filled with both a liquid (such as hydraulic fluid) and a gas (such as nitrogen or air). When the shock absorber is loaded, its two ends are urged towards one another, which pushes the piston further into the cylinder. This movement of the piston compresses the gas within the cylinder, and the compressed gas in turn acts on the piston and urges it back out of the cylinder, providing a restorative force. The Oleo strut therefore acts as a gas spring. Equally, movement of the piston within the cylinder also displaces the liquid in the cylinder, for instance forcing it through ducts in the cylinder and / or the piston, which dissipates energy through viscous friction. The Oleo strut therefore acts as a damper or dashpot, as well as a gas spring.

[0003] Generally speaking, in commercial airliners the Oleo struts extends downward from the aircraft and directly support an axle or bogey on which wheels are provided. The Oleo struts are sometimes positioned at a slight angle to the vertical, but beyond a limit a few degrees from vertical the internal friction within the struts obstructs their function. When the aircraft rests on its landing gear, its weight and the resulting ground reaction force load the Oleo strut as described above. Any shocks the aircraft experiences, such as when touching down or when hitting a bump while taxiing, are absorbed by the Oleo strut - the strut compresses and the wheels move upward towards the body of the aircraft, while the Oleo strut dissipates energy, then the restorative force from the compressed gas returns the wheels to their default position.

[0004] In a few military aircraft, the nose landing gear does not use an Oleo strut but rather uses a liquid spring assembly. Liquid springs share some similarity with Oleo struts in that they utilise a piston reciprocally movable within a chamber. However, whereas Oleo struts utilise compression of a gas to provide the restorative force, in liquid springs the chamber does not contain any gas and the restorative force is provided by compression of liquid within the chamber. In many cases liquid spring assemblies are also configured to act as shock absorbers, with movement of the piston moving some of the liquid in the chamber and dissipating energy in similar fashion to Oleo struts.

[0005] One problem with liquid spring shock absorbers is that their behaviour changes with temperature. In particular, an increase in the temperature of the liquid reduces its viscosity or a reduction in temperature increases its viscosity, which in either case changes the behaviour of the liquid within the chamber (for instance how much energy is dissipated). In contrast, it is generally desirable for liquid spring shock absorbers to exhibit the same behaviour regardless of operating temperature. For sake of example, warming the liquid reduces its viscosity and thus reduces the amount of energy which is dissipated by the shock absorber by viscous friction.

[0006] While it is normally beneficial for shock absorber response to be independent of temperature, it is desirable in some contexts for the behaviour of a liquid spring shock absorber to vary according to the “sharpness” of the shock loading it experiences (i.e. the rate of change of force applied to the shock absorber). This is not possible with conventional designs.

[0007] The present invention seeks to mitigate one or more of the above-mentioned limitations. Alternatively or additionally, the present invention seeks to provide an improved or alternative liquid spring shock absorber, aircraft landing gear or aircraft. SUMMARY OF THE INVENTION

[0008] According to a first aspect of the present invention there is provided a liquid spring shock absorber comprising: a first end connected to a sealed chamber which contains a liquid and substantially no gas; a second end connected to a piston rod which extends along a longitudinal axis mto the sealed chamber through an aperture therein, the piston rod being movable along the longitudinal axis within the aperture while in sealing engagement therewith; and a piston head provided on the piston rod and located within the liquid in the sealed chamber, the piston head having a conduit through which liquid in the chamber can flow during movement of the piston head within the chamber, wherein: the shock absorber is configured whereby movement of the first and second ends towards one another inserts the piston rod further into the sealed chamber, thereby reducing the volume of the sealed chamber and thus pressurising and compressing the liquid, compression of the liquid in the chamber creating a restorative force which urges the first and second ends apart from one another; the piston head is configured to displace some of the liquid in the sealed chamber through the conduit during said further insertion of the piston rod, movement of said liquid dissipating energy through viscous friction; the piston head further comprises an obstructing member movable along the longitudinal axis of the piston rod, between a set of positions in which the obstructing member obstructs flow of liquid through the conduit to differing extents; and the obstructing member is movable between said positions under the influence of a passively-responsive control mechanism.

[0009] The obstructing member being movable can allow the flow through the conduit, and thus the response of the shock absorber, to differ according to particular circumstances. Use of a passively-responsive control mechanism can allow the response of the shock absorber to be adjusted for different circumstances without the need for active control. This, in turn, may make the shock absorber cheaper, more reliable, smaller and / or lighter, simpler, easier to service or the like. Instead or as well, it may reduce the severity of shocks transmitted by the shock absorber to other components attached thereto. In the case of the shock absorber being used in an aircraft landing gear, for example, the shock absorber may reduce the maximum severity of shock which may be transmitted to the airframe, thereby allowing the airframe (and thus the aircraft as a whole) to be lighter and / or able to work under longer service intervals.

[0010] The obstructing member being movable along the longitudinal axis can make the piston head, and thus the shock absorber as a whole, narrower. Such narrowness may be beneficial e.g. in aircraft landing gear, both in terms of aerodynamic drag and in terms of the space required for stowing of the landing gear.

[0011] For the avoidance of doubt, reference to the sealed chamber containing a liquid and substantially no gas is not intended to mean that the liquid can have a gas dissolved therein. Rather, it is intended to mean that substantially no material in the gaseous phase (with the exception, perhaps, of one or two small bubbles due to slight leakage of a seal or the like) is present in the sealed chamber.

[0012] A passively-responsive control mechanism may be considered to be a control mechanism which responds autonomously to changes in circumstances without requiring input from e.g. a user, sensors, a computer or the like. By way of example, a general example of a passive control mechanism is a centrifugal governor whereas the active equivalent would be an electronic speed controller or a user operated accelerator pedal.

[0013] The conduit may define a single flow path, or two or more flow paths running partially or entirely in parallel through different branches of the conduit.

[0014] The conduit may be permanently open, or may be selectively closable (in one or both directions of flow) for instance by a bistable valve which opens at a predetermined pressure.

[0015] The obstructing member may be movable substantially parallel to or collinear with the longitudinal axis of the piston rod.

[0016] This may increase the extent to which the beneficial narrowness discussed above can be provided.

[0017] As an alternative, the obstructing member may be movable in a direction which is within 40 degrees, for instance within 30 degrees or within 20 degrees or within 10 degrees, of the longitudinal axis of the piston rod.

[0018] The passively-responsive control mechanism may comprise an actuator configured to move the obstructing member according to the temperature of the shock absorber.

[0019] This may allow the passively-responsive control mechanism to compensate for changes in response of the shock absorber which may otherwise occur due to changes in temperature, thereby allowing the response of the shock absorber to be more predictable or reliable.

[0020] The actuator may comprise a predetermined volume of a material which has a positive coefficient of thermal expansion.

[0021] Such a material expands when heated, which may allow a beneficially straightforward and / or reliable mechanism to be used for moving the obstructing member.

[0022] Said material may have a constant coefficient of thermal expansion.

[0023] In other words, the material may exhibit a linear relationship between temperature and expansion. This may allow the response of the actuator to be more predictable over a range of temperatures, in comparison to an arrangement where the coefficient of thermal expansion of the material changed with temperature.

[0024] Said material is a liquid at room temperature.

[0025] An expanding liquid may be more easily used to move the obstructing member due to its ability to be redirected, whereas an expanding solid may require use of a complex, bulky and / or fragile mechanism to be used.

[0026] Said material may be a eutectic or near-eutectic alloy of gallium, indium and tin.

[0027] Such a material is generally known by the name Gallistan, which is a brand name but is generally used to describe equivalent materials manufactured by other brands.

[0028] At least some of said material may be contained within a linearly expandable / contractable bellows.

[0029] Such a bellows may translate expansion of the material into linear motion, rather than three-dimensional expansion for example, which may amplify the movement created by the expansion of the material.

[0030] The bellows may be metal, which may enable good heat transfer into or out of said material and thereby make the actuator more responsive.

[0031] The passively-responsive control mechanism may comprise a resistor unit which resists movement of the obstructing member between said positions to different extents according to a rate of change of force exerted on the obstructing member by liquid flowing through the conduit.

[0032] The resistor unit may allow the shock absorber to exhibit different levels of response depending on how sharply a load is applied thereto.

[0033] The resistor unit may comprise a volume of non-Newtonian fluid, for instance a shear-thickening fluid.

[0034] This may allow for a simpler, cheaper, lighter and / or more compact mechanism to be used.

[0035] At least part of the volume of non-Newtonian fluid may be housed in a linearly expandable / contractable bellows.

[0036] The obstructing member may be biased to a predetermined one of said positions, for instance by a biasing member such as a spring.

[0037] This can improve the predictability of response of the shock absorber by returning the obstructing member to the same position after each shock loading.

[0038] The obstructing member may be biased to the position in which the conduit is blocked by the obstructing member to the greatest extent.

[0039] As an alternative to being biased, the obstructing member may be free-floating or urged to a particular position merely by gravity.

[0040] The liquid spring shock absorber may be configured to prevent the obstructing member from being able to completely block the conduit.

[0041] The shock absorber may thereby be configured to avoid a situation in which no liquid can flow through the piston head, which would effectively reduce the total volume of liquid which could be compressed and provide restorative force, which in turn could have the effect of giving the shock absorber an unduly stiff response.

[0042] The conduit may extend generally along the longitudinal axis of the piston rod.

[0043] For instance, the conduit may follow a path which overall runs within 40 degrees, for instance within 30 degrees or within 20 degrees, of the longitudinal axis.

[0044] According to a second aspect of the present invention there is provided a liquid spring shock absorber comprising: a first end having a sealed cylinder filled with liquid-phase material and substantially no gaseous-phase material; a second end having a plunger which extends in a length direction thereof into the cylinder, the plunger being movable in the length direction relative to the cylinder, wherein: the plunger has an end located within the liquid-phase material in the cylinder, the end having a passage through which liquid-phase material in the cylinder can pass during movement of the plunger; the shock absorber is configured whereby movement of the first and second ends towards one another pushes the plunger further into the cylinder, thereby reducing the volume which can be occupied by the liquid-phase material and thus compressing the liquid-phase material, compression of the liquid-phase material producing a restorative force which opposes said insertion of the plunger; the end of the plunger is configured to move some of the liquid-phase material in the cylinder through the passage as the plunger is pushed further into the cylinder, movement of said liquid-phase material dissipating energy through viscous friction; the end of the plunger also has a constriction member which is movable along the length direction of the plunger so as to change an extent to which the constriction member can constrict flow of liquid-phase material through the passage; and the constriction member is movable between said positions by a mechanism which is not actively controlled.

[0045] The constriction member being movable can allow the flow through the passage, and thus the response of the shock absorber, to differ according to particular circumstances. Use of a mechanism which is not actively controlled can allow the response of the shock absorber to be adjusted for different circumstances without the need for active control. This, in turn, may make the shock absorber cheaper, more reliable, smaller and / or lighter, simpler, easier to service or the like. Instead or as well, it may reduce the severity of shocks transmitted by the shock absorber to other components attached thereto. In the case of the shock absorber being used in an aircraft landing gear, for example, the shock absorber may reduce the maximum severity of shock which may be transmitted to the airframe, thereby allowing the airframe (and thus the aircraft as a whole) to be lighter and / or able to work under longer service intervals.

[0046] The constriction member being movable along the length direction of the plunger can make the end of the plunger, and thus the shock absorber as a whole, narrower. Such narrowness may be beneficial e.g. in aircraft landing gear, both in terms of aerodynamic drag and in terms of the space required for stowing of the landing gear.

[0047] According to a third aspect of the present invention there is provided an aircraft landing gear comprising a liquid spring shock absorber according to the first or second aspect of the invention.

[0048] Such an aircraft landing gear may provide one or more of the advantages discussed above.

[0049] The landing gear may be a trailing-link type landing gear.

[0050] According to a fourth aspect of the present invention there is provided an aircraft comprising a landing gear according to the third aspect of the invention.

[0051] Such an aircraft may provide one or more of the advantages discussed above.

[0052] Said landing gear may be a main landing gear.

[0053] It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. Further, it is to be understood that different steps of a method described herein need not necessarily be performed in the order in which they are recited. DESCRIPTION OF THE DRAWINGS

[0054] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a front view of a commercial airliner according to an embodiment of the invention; Figure 2 shows a schematic side view of a main landing gear of the commercial airliner of Figure 1; Figure 3 shows a schematic cross-sectional side view of a liquid spring assembly, more particularly a liquid spring shock absorber, of the landing gear of Figure 2; Figure 4 shows a kinematic diagram of the landing gear of Figure 2; and Figure 5 shows a flow-chart of a method according to the embodiment of the invention. DETAILED DESCRIPTION

[0055] Figure 1 shows an aircraft, more particularly a commercial airliner 2, according to an embodiment of the invention. It has a body 4 made up of a fuselage 6, a tail 8 and two wings 10. Within the fuselage 6 is a cockpit and room for around 200 passengers seated in conventional fashion. Each wing 10 has a skin which envelops a substructure (not visible) made up of a front spar, a rear spar and a set of ribs, as is known in the art. Though only the length and thickness of the wings 10 is shown in Figure 1, it is noteworthy that each wing 10 has an aspect ratio of around 14.

[0056] The airliner 2 also has a nose landing gear 12 and two main landing gear 14, each of which is shown in a deployed configuration in Figure 1. The nose landing gear 12 depends from the fuselage 6 beneath the nose of the aircraft. Each main landing gear 14 is attached to a respective wing 10, more particularly the rear spar (not visible) of that wing 10. In known fashion, the landing gear 12, 14 are movable to a stowed configuration in which the nose landing gear is received within the fuselage 6 and each main landing gear 14 is received in its respective wing 10. In the present embodiment the airliner 2 has a maximum take off weight of around 250 tonnes.

[0057] Figure 2 shows one of the main landing gear 14, schematically, from the side. Figure 2 will now be referred to in combination with Figure 1. The landing gear 14 has a rigid support arm 20 which extends from a proximal end 22 to a distal end 24. With the landing gear 14 in the deployed configuration with the airliner 2 resting on the ground, the support arm 20 is positioned substantially vertically. The proximal end 22 of the support arm has a pair of flanges 26 which receive an axle 27 therethrough so as to attach the support arm 20, and thus the landing gear 14 as a whole, to the rear spar (not visible) of one of the wings 10. The distal end 24 of the support arm 20 also has a pair of flanges 28 (one of which is visible from the perspective of Figure 2). Flanges 28, which are oriented generally perpendicularly to flanges 26, support a pivot pin 30. The support arm 20 also has a pair of fins 29 (one of which is visible from the perspective of Figure 2) projecting generally rearwardly therefrom. The fins 29 support a pivot pin 31.

[0058] The landing gear 14 also has a trailing arm 32 which extends along a length 34 thereof from a proximal end 36 to a distal end 38. The trailing arm 32 is rotatably coupled to the support arm 20. More particularly, the trailing arm 32 and the support arm 20 are pivotally connected to one another by the pivot pin 30, which forms a pivot joint. The trailing arm 32 can therefore pivot about the pivot pin 30 relative to the support arm 20.

[0059] The distal end 38 of the trailing arm 32 has a wheel support in the form of an axle 40 which supports a pair of wheels 42 either side of the trailing arm 32, only one of which is shown in Figure 2. The trailing arm 32 also has a pair of fins 44 (one of which is visible from the perspective of Figure 2) which support a pivot pin 46 at substantially the same position along the length 34 of the trailing arm as the axle 40.

[0060] The landing gear 14 also comprises a liquid spring assembly 50 which is configured as a liquid spring shock absorber. The liquid spring shock absorber 50 has a first end in the form of a proximal end 52 and a second end in the form of a distal end 54. The proximal end 52 is connected to the support arm 20. More particularly, the proximal end 52 of the liquid spring shock absorber 50 is pivotally connected to the support arm by the pivot pm 31, which forms a pivot joint. The shock absorber 50 can therefore pivot relative to the support arm 20 about the pivot pin 31.

[0061] The distal end 54 of the shock absorber 50 is connected to the trailing arm 32. More particularly, the distal end 54 is pivotally attached to the trailing arm 32 by the pivot pin 46, which forms a pivot joint. The shock absorber 50 and trailing arm 32 can thus pivot relative to one another about the pivot pm 46.

[0062] The proximal end 52 of the shock absorber 50 is connected to, and in this embodiment integrally formed with, a sealed chamber 56. The distal end 54 of the shock absorber is connected to, and in this case integrally former with, a piston rod 58 which extends along a longitudinal axis 60 into the sealed chamber 56.

[0063] It is noteworthy that in this embodiment the support arm 20, the trailing arm 32 and the shock absorber 50 are all positioned in the same plane, namely a plane which is normal to the pitch axis of the aircraft (parallel to the page from the perspective of Figure 2). The axle 27 which passes through the flanges 26 of the support arm 20 also lies within this plane. The axle 27 provides an axis about which the support arm 20, trailing arm 32 and shock absorber 50 pivot together when the landing gear 14 moves between the stowed and deployed configurations.

[0064] The landing gear 14 defines a length direction 64 which runs between the proximal end 22 of the support arm 20 and the wheels 42. The landing gear 14 also defines a thickness direction which is perpendicular to the length direction 64 and also to the roll axis of the airliner 2. The thickness direction runs through the page from the perspective of Figure 2. It is noteworthy that the thickness of the shock absorber 50, in the thickness direction, is no larger than that of the support arm 20 or that of the trailing arm 32. Indeed, in this case the shock absorber 50 has a thickness which is slightly smaller than that of the support arm 20 and that of the trailing arm 32.

[0065] For the sake of completeness, the landing gear 14 also has a foldable inboard sidestay which braces the landing gear against lateral loads. However, the foldable inboard sidestay is not material to the present invention. It has therefore been omitted from the drawings for the sake of simplicity, and will not be described here.

[0066] The shock absorber 50 will now be described in more detail with reference to Figure 3, which shows the shock absorber 50 in schematic cross section, in combination with Figures 1 and 2. As noted above, the proximal end 52 of the shock absorber 50 is connected to a sealed chamber 56 and the distal end 54 is connected to a piston rod 58. The sealed chamber 56 is generally cylindrical in shape and contains a liquid 70 in the form of hydraulic oil. Unlike the cylinder / chamber of an Oleo strut, the sealed chamber 56 of the shock absorber contains substantially no gas. The chamber 56 has an aperture 72 through which the piston rod 58 is slidingly received.

[0067] The piston rod 58 extends into the chamber 56 through the aperture 72, and is movable along the longitudinal axis 60 within the aperture 72 while remaining in sealing engagement with the aperture 72 (thereby maintaining the seal of the sealed chamber 56). The piston rod 58 terminates in a piston head 74 which is located within the liquid 70 inside the chamber 56. The piston head 74 is generally hollow. It has a solid front wall 76, a solid peripheral wall 77 and a porous rear wall 78 by which the piston head 74 is attached to the piston rod 58. The peripheral wall 77 is a close fit to the interior of the sealed chamber, minimising the potential for leakage of liquid therebetween while allowing the piston head 74 to move within the chamber 56.

[0068] The piston head 74 has a conduit 80 which extends generally along the longitudinal axis 60. The conduit 80 runs through a hole 82 in the front wall 76 and into the piston head 74, then out to behind the piston head 74 through the porous rear wall 78. Some of the liquid 70 in the sealed chamber 56 can flow through the conduit 80 during movement of the piston head 74 within the chamber 56, as described in more detail later.

[0069] The shock absorber 50 is configured whereby movement of its ends 52, 54 towards one another inserts the piston rod 58 further into the sealed chamber 56. With more of the piston rod 58 received in the chamber 56, the volume of the chamber is reduced. Being forced to occupy a smaller volume, the liquid 70 within the chamber 56 is pressurised and compressed. Compression of the liquid 70 creates a restorative force which urges the piston rod out of the chamber 56. Accordingly, when the ends 52, 54 of the shock absorber 50 are moved towards one another the shock absorber 50 resiliently biases them apart again.

[0070] When the piston rod 58 is inserted further into the sealed chamber 56, the piston head 74 moves along the chamber 56. This has the effect of pushing some of the liquid 70 through the conduit 80 in the piston head 74, from in front of the piston head to behind it. The liquid being forced through the conduit 80, in particular the hole 82 in the front wall 76, acts to dissipate energy through viscous friction in the liquid 70, thereby exerting a damping action on relative movement of the ends 52, 54 of the shock absorber 50. It is noteworthy that this damping action is also provided when the piston rod moves further out of the sealed chamber 56 under action of the restorative force from the compressed liquid 70 (i.e. when the ends 52, 54 are moved apart), with some of the liquid passing through the conduit 80 from behind the piston to in front of it.

[0071] A method of using the liquid spring assembly 50, more particularly a method of absorbing shocks in the landing gear 14, will now be described with reference to Figures 4 and 5 (which show a kinematic diagram of the landing gear 14 and a flow-chart of the method respectively) along with continued reference to Figures 1 to 3.

[0072] In a first step 210 the airliner 2 is rested on the ground with the landing gear 12, 14 in the deployed configuration. When the airliner 2 rests on the ground, a portion of its weight W acts downwards through the support arm 20. The ground reaction force R acts upwards through the wheels 42. With the support arm 20 being urged downwards and the distal end 38 of the trailing arm 32 being urged upwards by the wheels, the trailing arm 32 is urged anticlockwise from the perspective of Figure 3, about the pivot pin 30, relative to the support arm 20. This acts to push the ends 52, 54 of the shock absorber 50 together, pressurising and compressing the liquid 70 in the chamber 56. The landing gear 14 reaches an equilibrium point when the ends 52, 54 of the shock absorber 50 have moved together sufficiently (i.e. the liquid 70 in the chamber 56 has been compressed sufficiently) for the restorative force exerted by the liquid 70 to exert a moment on the trailing arm 32 which counteract the moment exerted by the ground reaction force R.

[0073] In step 220 a shock loading is applied to the landing gear 14, for instance from the aircraft 2 hitting a bump in the runway when taxiing. The bump temporarily increases the ground reaction force R, applying a moment (anticlockwise from the perspective of Figures 2 and 4) which exceeds the moment (clockwise from the perspective of Figures 2 and 4) which is exerted by the shock absorber 50 (due to the restorative force from the compressed liquid 70). As a result of these imbalanced moments, the trailing arm 32 pivots upwards (anticlockwise from the perspective of Figures 2 and 4) relative to the support arm 20 about pivot pin 30. This pivoting motion is accompanied by slight pivoting of the shock absorber 50, both anticlockwise (from the perspective of Figures 2 and 4) relative to the support arm 20 about pivot pin 31, and clockwise (from the perspective of Figures 2 and 4) relative to the trailing arm 32 about pivot pin 46.

[0074] Upward pivoting of the trailing arm 32 moves the ends 52, 54 of the shock absorber 50 further together, which inserts the piston rod 58 further into the sealed chamber 56. This further reduces the volume within the chamber 56, further compressing the liquid 70 and increasing the restorative force generated thereby. Also as the piston rod 58 is further inserted into the chamber 56, the piston head 74 moves forward within the chamber 56. This forces some of the liquid 70 in front of the piston head 74 through the conduit 80 and to the rear of the piston head 74, dissipating energy through viscous friction and thereby damping the movement of the ends 52, 54 of the shock absorber 50 (and thus the movement of the trailing arm 32).

[0075] In some cases, in step 230 the trailing arm 32 pivots and the restorative force from the shock absorber 50 increases until the trailing arm reaches a position in which the moment exerted by the shock absorber 50 again counteracts the moment exerted by the (increased) ground reaction force R. The landing gear 14 then reaches equilibrium again. In other cases the shock loading may be removed before the landing gear reaches the new equilibrium state.

[0076] In either case, in step 240 the shock loading is removed (for instance due to the airliner 2 passing over the bump in the runway. The ground reaction force R therefore drops to a lower level. At this point the moment exerted on the trailing arm 32 by the restorative force applied by the shock absorber 50 more than counteracts the moment exerted by the ground reaction force R, and the trailing arm 32 pivots downward (clockwise from the perspective of Figures 2 and 4) relative to the main arm 20 about the pivot pin 30. This pivoting motion is accompanied by slight pivoting of the shock absorber 50, both anticlockwise (from the perspective of Figures 2 and 4) relative to the support arm 20 about pivot pin 31, and clockwise (from the perspective of Figures 2 and 4) relative to the trailing arm 32 about pivot pin 46.

[0077] As the trailing arm 32 pivots downward, the ends 52, 54 of the shock absorber are permitted to move apart from one another, withdrawing the piston rod 58 from the sealed chamber 56 to some extent. This has the effect of increasing the volume of the chamber 56, allowing the liquid to expand and become a little less pressurised. Also, this movement of the piston rod 58 moves the piston head 74 rearwards, forcing some of the liquid 70 back through the conduit 80 and dissipating energy as discussed above.

[0078] As the trailing arm 32 pivots, the shock absorber 50 lengthens and the liquid 70 in the sealed chamber 56 becomes less compressed. The restorative force provided by the liquid 70 therefore decreases. In step 250 the trailing arm 32 reaches a position where moment applied to it by the restorative force of the shock absorber 50 is counteracted by the moment applied by the (reduced) ground reaction force R, and the landing gear 14 again reaches an equilibrium position.

[0079] Returning primarily to Figure 3, it is noteworthy that the piston head 74 of the shock absorber 50 has a generally conical obstructing member 84. The obstructing member 84 is movable relative to the conduit 80 in a direction parallel to (and in this case collinear with) the longitudinal axis 60 of the piston rod 58 between different positions. In the different positions of the obstructing member 84, it projects into the hole 82 in the front wall to differing extents, thereby obstructing the flow of the liquid 70 through the conduit 80 to differing extents. When the obstructing member 84 is in its furthest forwards position (towards the top from the perspective of Figure 3) it blocks around 90% of the cross sectional area of the hole 82 and therefore greatly obstructs flow of liquid 70 through the conduit 80. With the obstructing member 84 in its furthest rearwards position (towards the bottom from the perspective of Figure 3) it barely blocks the hole 82 at all and therefore barely obstructs flow of liquid 70 through the conduit 80. Figure 3 shows the obstructing member 84 in an intermediate position, between these two extremes.

[0080] The obstructing member 84 is movable between its positions under the influence of a control mechanism 86. The control mechanism 86 is passively responsive, rather than being actively operated e.g. by a user or a computer. In this embodiment the control mechanism has an actuator 88 and a resistor unit 90. The obstructing member 84 is mounted to the actuator 88, which in turn is mounted to the resistor unit 90. The resistor unit is attached to the rear wall 78 of the piston head 74.

[0081] The actuator 88 is configured to move the obstructing member 84 between its positions based on the temperature of the shock absorber 50 (more precisely based on the temperature of the actuator 88). The actuator 88 has a rigid main tank 92 connected by a rigid pipe 94 to a metal expansion bellows 96. The main tank 92, pipe 94 and bellows 96 are filled with an eutectic alloy of gallium, indium and tin known as Galinstan, which is liquid at room temperature. The Galinstan has a constant positive coefficient of thermal expansion. In other words, its volume is linearly related to temperature. When temperature increases, the Galinstan in the bellows 96 expands and causes the bellows to expand (in this case extending along the longitudinal axis 60 of the piston rod). Further, expansion of the Galinstan in the main tank 92 and the pipe 94 pushes more Galinstan into the bellows 96, expanding it further. Accordingly, an increase in temperature results in a considerable extension of the bellows.

[0082] Equally, when the temperature decreases the Galinstan in the bellows 96 contracts, and contraction of the Galinstan in the main tank 92 and the pipe 94 pulls some of the Galinstan out of the bellows 96. Accordingly, a reduction in temperature results in a considerable retraction of the bellows 96.

[0083] Expansion / extension of the bellows 96 pushes the obstructing member 84 forwards to a position in which it occludes the hole 82 (and thus the conduit 80) to a greater extent, whereas contraction / retraction of the bellows 96 pulls the obstructing member backwards to a position in which it occludes the hole 82 (and thus the conduit 80) to a lesser extent. It therefore follows that the hotter the shock absorber 50 is, the more blocked the conduit is. The shock absorber 50 is configured whereby for a given temperature change, the change in extent to which the conduit is blocked counteracts the change in the viscosity of the liquid. For example, heating of the shock absorber 50 makes the liquid 70 less viscous and thus able to flow more easily, but that heating also makes the conduit 80 more constricted. In other words, the warmer and thinner liquid 70 flowing though the more constricted conduit 80 dissipates substantially the same amount of energy as the colder and thicker liquid flowing through the less constricted conduit 80. Thus, the behaviour of the shock absorber 50 is substantially constant across a range of operating temperatures.

[0084] The resistor unit 90 does not move based on temperature. When liquid 70 flows through the conduit 80 from in front of the piston head 74 to behind it, the liquid exerts a force on the obstructing member 84. The resistor unit 90 resists the movement of the obstructing member 84 between its positions, to differing extents according to the rate of change of the force exerted on the obstructing member 84 by the liquid 70 flowing through the conduit 80.

[0085] The resistor unit 90 is a further metal bellows 98 that is filled with a non-Newtonian fluid, more particularly a dilatant or shear-thickening fluid. This fluid increases in viscosity according to the rate of shear strain, put simply it gets thicker when it is forced to move more quickly. A weak compression spring (not visible) is also positioned inside the bellows 98, which urges them to extend with a weak force. However, the range of motion of the bellows 98 and bellows 96 is such that the obstructing member 84 is prevented from being able to completely block the conduit 80.

[0086] When the shock absorber 50 is subjected to a more gentle load, liquid 70 begins moving through the conduit slowly and the force it exerts on the obstructing member 84 is applied gently. In other words, the rate of change of force exerted by the liquid 70 on the obstructing member 84 is relatively low. This gently pushes the obstructing member (and the actuator 88) backwards, against the bias from the spring (not visible), urging the bellows 98 of non-Newtonian fluid to contract. Subject to this gentle motion, the nonNewtonian fluid exhibits a relatively low viscosity and the bellows 98 is therefore able to contract relatively easily. Contraction of the bellows 98 allows the obstructing member 84 to move backwards and make the conduit 80 less constricted. When the flow of liquid 70 subsides, the obstructing member 84 slowly moves forward again under action of the spring (not visible) inside the bellows 98.

[0087] In contrast, when the shock absorber 50 is subjected to a rapid loading, i.e. a harsh shock load, the liquid 70 begins moving through the conduit 80 rapidly and the force it exerts on the obstructing member 84 is applied rapidly. In other words, the rate of change of force exerted by the liquid 70 on the obstructing member 84 is relatively high. This sudden force pushes the obstructing member (and the actuator 88) backwards, urging the bellows 98 of non-Newtonian fluid to contract rapidly. Subject to this rapid motion, the non-Newtonian fluid exhibits a relatively high viscosity and the bellows 98 which strongly resists contraction of the bellows 98. The bellows 98 therefore remains largely unchanged in shape, keeping the obstructing member 84 close to its original, relatively obstructive, position.

[0088] The resistor unit 90 therefore allows the response of the shock absorber 50 to vary depending on whether the shocks it experiences are more gentle or more sharp. The piston rod 58 can move relatively easily under gentle loads (due to the obstructing member 84 moving to obstruct the conduit less), but under harsher loads the piston rod 58 offers more resistance (due to the obstructing member 84 remaining in an obstructive position). This behaviour can be beneficial for instance in avoiding unusually sharp loading forcing the shock absorber beyond its intended range of motion, damaging it.

[0089] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

[0090] The term ‘or’ shall be interpreted as ‘and / or’ unless the context requires otherwise.

Claims

1. A liquid spring shock absorber comprising:a first end connected to a sealed chamber which contains a liquid and substantially no gas;a second end connected to a piston rod which extends along a longitudinal axis into the sealed chamber through an aperture therein, the piston rod being movable along the longitudinal axis within the aperture while in sealing engagement therewith; anda piston head provided on the piston rod and located within the liquid in the sealed chamber, the piston head having a conduit through which liquid in the chamber can flow during movement of the piston head within the chamber, wherein:the shock absorber is configured whereby movement of the first and second ends towards one another inserts the piston rod further into the sealed chamber, thereby reducing the volume of the sealed chamber and thus pressurising and compressing the liquid, compression of the liquid in the chamber creating a restorative force which urges the first and second ends apart from one another;the piston head is configured to displace some of the liquid in the sealed chamber through the conduit during said further insertion of the piston rod, movement of said liquid dissipating energy through viscous friction;the piston head further comprises an obstructing member movable along the longitudinal axis of the piston rod, between a set of positions in which the obstructing member obstructs flow of liquid through the conduit to differing extents; andthe obstructing member is movable between said positions under the influence of a passively-responsive control mechanism.

2. A liquid spring shock absorber according to claim 1 wherein the obstructing member is movable substantially parallel to or collinear with the longitudinal axis of the piston rod.

3. A liquid spring shock absorber according to claim 1 or 2 wherein the passively -responsive control mechanism comprises an actuator configured to move the obstructing member according to the temperature of the shock absorber.

4. A liquid spring shock absorber according to claim 3 wherein said actuator comprises a predetermined volume of a material which has a positive coefficient of thermal expansion.

5. A liquid spring shock absorber according to claim 4 wherein said material has a constant coefficient of thermal expansion.

6. A liquid spring shock absorber according to claim 4 or 5 wherein said material is a liquid at room temperature.

7. A liquid spring shock absorber according to claim 6 wherein said material is a eutectic or near-eutectic alloy of gallium, indium and tin.

8. A liquid spring shock absorber according to claim 6 or 7 wherein at least some of said material is contained within a linearly expandable contractable bellows.

9. A liquid spring shock absorber according to any preceding claim wherein the passively-responsive control mechanism comprises a resistor unit which resists movement of the obstructing member between said positions to different extents according to a rate of change of force exerted on the obstructing member by liquid flowing through the conduit.

10. A liquid spring shock absorber according to claim 9 wherein the resistor unit comprises a volume of non-Newtonian fluid.

11. A liquid spring shock absorber according to claim 10 wherein at least part of the volume of non-Newtonian fluid is housed in a linearly expandable / contractable bellows.

12. A liquid spring shock absorber according to any preceding claim wherein the obstructing member is biased to a predetermined one of said positions.

13. A liquid spring shock absorber according to any preceding claim wherein the liquid spring shock absorber is configured to prevent the obstructing member from being able to completely block the conduit.

14. A liquid spring shock absorber according to any preceding claim wherein the conduit extends generally along the longitudinal axis of the piston rod.

15. A liquid spring shock absorber comprising:a first end having a sealed cylinder filled with liquid-phase material and substantially no gaseous-phase material;a second end having a plunger which extends in a length direction thereof into the cylinder, the plunger being movable in the length direction relative to the cylinder, wherein:the plunger has an end located within the liquid-phase material in the cylinder, the end having a passage through which liquid-phase material in the cylinder can pass during movement of the plunger;the shock absorber is configured whereby movement of the first and second ends towards one another pushes the plunger further into the cylinder, thereby reducing the volume which can be occupied by the liquid-phase material and thus compressing the liquid-phase material, compression of the liquid-phase material producing a restorative force which opposes said insertion of the plunger;the end of the plunger is configured to move some of the liquid-phase material in the cylinder through the passage as the plunger is pushed further into the cylinder, movement of said liquid-phase material dissipating energy through viscous friction;the end of the plunger also has a constriction member which is movable along the length direction of the plunger so as to change an extent to which the constriction member can constrict flow of liquid-phase material through the passage; andthe constriction member is movable between said positions by a mechanism which is not actively controlled.

16. An aircraft landing gear comprising a liquid spring shock absorber according to any preceding claim.

17. An aircraft landing gear according to claim 16 wherein the landing gear is a trailing-link type landing gear.

18. An aircraft comprising a landing gear according to claim 16 or claim 17.

19. An aircraft according to claim 18 wherein said landing gear is a main landing gear.

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