A liquid spring assembly

Abstract

Liquid spring 50 having chamber 56 containing liquid 70 and no gas, and piston rod 58 reciprocally slidable within chamber 56. Piston rod retracts into the chamber to pressurise and compress the liqui

Description

BACKGROUND OF THE INVENTION

[0001] The present disclosure relates to the field of liquid springs which have particular, but not exclusive, application to aircraft landing gear. The disclosure relates particularly, but not exclusively, to a liquid spring assembly, an aircraft landing gear, and aircraft and a method of using a liquid spring assembly.

[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] The reason liquid springs have seen successful use only in military aircraft is due to their short stroke length and considerably greater stiffness. In essence, the “ride quality” they offer is generally excessively harsh. Some military aircraft are built sufficiently robustly and / or are sufficiently light to be able to tolerate the shocks involved, particularly for nose landing gear which generally support less of an aircraft’s weight. Also, military aircraft often have a very short service interval so fatigue from excessive shock loading has a shorter time-frame in which to propagate. Commercial airliners, however, weigh considerably more, are designed with the focus on efficiency rather than absolute strength, and work with long service intervals. Thus, Oleo struts have remained the sole choice for commercial airliners.

[0006] Instead or as well, in some applications it would be desirable for liquid spring assemblies to exhibit different behaviour at different displacements (i.e. a non-linear spring curve). 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 assembly, aircraft landing gear, aircraft or method of using a liquid spring assembly. SUMMARY OF THE INVENTION

[0008] According to a first aspect of the present invention there is provided a liquid spring assembly comprising: a first end connected to a sealed chamber which contains a liquid and substantially no gas; and 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, wherein: the first and second ends are resiliently movable towards one another, movement of the first and second ends towards one another inserting the piston rod further into the sealed chamber and thereby pressurising and compressing the liquid, compression of the liquid creating a restorative force which urges the first and second ends apart from one another; the sealed chamber is configured to deform, and thereby lengthen in a direction parallel to the longitudinal axis, in response to said pressurisation of the liquid.

[0009] Lengthening of the sealed chamber may advantageously increase the stroke length of the liquid spring assembly, both in terms of there being a greater length of chamber into which the piston rod can be inserted, and in terms of there being more volume within the chamber in which the liquid (and part of the piston rod) can be accommodated. In contrast, if hypothetically a conventional liquid spring were loaded sufficiently to affect the sealed chamber, the sealed chamber would fail catastrophically rather than deforming, or would “balloon” outwards without changing m length.

[0010] 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.

[0011] The sealed chamber may be generally cylindrical, for instance substantially concentric around the longitudinal axis of the piston rod.

[0012] The sealed chamber may be configured to deform such that a percentage change in length of the sealed chamber is greater than any percentage change in diameter of the sealed chamber in a direction normal to the longitudinal axis.

[0013] Put simply, the sealed chamber may lengthen more than it changes diameter. Less of a change in diameter for a given change in length may reduce the effect of the deformation on components inside the chamber (such as a piston head), or on the interactions between those components and the chamber (such as the ability of the piston head to move within the chamber in close engagement therewith.

[0014] The sealed chamber may be configured to deform such that said percentage change in length of the sealed chamber is at least 1.5 times, for instance at least twice or at least three times, any percentage change in diameter of the sealed chamber in a direction normal to the longitudinal axis.

[0015] This may further improve the extent to which the sealed chamber can lengthen without affecting components inside the chamber or affecting its interaction with such components.

[0016] The sealed chamber may be configured to deform with substantially no change in diameter in any direction normal to the longitudinal axis.

[0017] This may allow the sealed chamber can lengthen without affecting components inside the chamber, or affecting its interaction with such components, at all.

[0018] Said deformation of the sealed chamber may be elastic deformation.

[0019] The liquid spring assembly may therefore return to its original shape after deformation. Deformation of the sealed chamber may provide a restorative force, contributing to the resilient behaviour of the liquid spring assembly. The liquid spring assembly may in essence function like two springs in series.

[0020] Optionally: for a given movement of the piston rod into the sealed chamber, compression of the liquid produces a first restorative force which acts to urge the first and second ends of the liquid spring assembly away from one another; for an equal magnitude of movement of the piston rod into the sealed chamber, elastic deformation of the sealed chamber produces a second restorative force which acts, through the fluid, to urge the first and second ends of the liquid spring assembly away from one another; and the second restorative force is larger in magnitude than the first restorative force.

[0021] The liquid spring assembly may therefore provide different levels of response depending on stroke length, with (a) lower loads mainly compressing the liquid and the liquid spring assembly providing a less stiff response, and (b) higher loads compressing the liquid and also deforming the sealed chamber, the liquid spring then providing a stiffer response. This may be of particular benefit where the liquid spring assembly is used in aircraft landing gear, as it may provide the aircraft with suspension which rides softly over smaller shocks but provides a firmer response to larger shocks which could otherwise damage the landing gear by moving it beyond its intended range of motion.

[0022] The magnitude of the second restorative force may be at least double, for instance at least triple or at least quadruple, that of the first restorative force.

[0023] As an alternative to elastic deformation, said deformation of the sealed chamber may be plastic deformation.

[0024] The sealed chamber may therefore function in a manner akin to a “mechanical fuse”, reacting in safe but noticeable and irreversible manner to an overload of the system.

[0025] The sealed chamber may be configured to undergo at least some of said deformation after the liquid has been compressed to a volume at which it is substantially incompressible.

[0026] This may allow the response of the liquid spring assembly to be split into two phases - one where the liquid is compressed and one where the sealed chamber is deformed. The response may therefore be more predictable than if the sealed chamber deformed during compression of the liquid.

[0027] The sealed chamber may be configured to undergo substantially all said deformation after the liquid has been compressed to said volume at which it is substantially incompressible.

[0028] The liquid spring assembly may be configured as a liquid spring shock absorber which damps movement of the first and second ends relative to one another.

[0029] The liquid spring assembly may therefore provide dual functionality - resilient deformation and damping action. This may allow apparatus using the liquid spring assembly, such as aircraft landing gear, to be more compact, simpler, lighter and / or quicker to assemble than if it contained a liquid spring assembly and a separate shock absorber.

[0030] The liquid spring assembly may have a piston head provided on the piston rod and located within the sealed chamber, the piston head being configured to impart viscous friction to the liquid during movement of the piston rod within the sealed chamber so as to damp movement of the first and second ends relative to one another.

[0031] The piston head may have a conduit through which liquid in the chamber can flow during movement of the piston head within the chamber.

[0032] The use of a conduit may allow motion of liquid in the sealed chamber to be more accurately or reliably controlled, for instance in comparison to an arrangement which relied on leakage of liquid between the piston head and an inner wall of the chamber.

[0033] 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.

[0034] 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.

[0035] Where the liquid spring assembly has a piston head with a conduit, the piston head being a close fit to the chamber, it may be particularly beneficial for the diameter of the chamber to remain relatively unchanged as discussed above. In such an arrangement a change in diameter could seize the piston head in the chamber, or allow too much leakage of fluid around the piston head.

[0036] The sealed chamber may be formed partially or substantially entirely from an anisotropic material, for instance an orthotropic material.

[0037] This may allow the sealed chamber to preferentially expand in a particular direction (for instance lengthways) while placing fewer design constraints on the shape of the sealed chamber.

[0038] Said anisotropic material may be a composite material made up of elongate fibres in a matrix, said elongate fibres running non-parallel to the longitudinal axis.

[0039] The elongate fibres may be carbon fibres, glass fibres, or fibres of a polymer such as Kevlar, for example.

[0040] The matrix may be a polymeric resin, for instance a UV-curing or oven-curing resin such as epoxy.

[0041] The sealed chamber may be a filament wound cylinder, the wound filament providing the elongate fibres of the composite material.

[0042] The wound filament may be helical wound or hoop wound, for example.

[0043] It is to be understood that referenced to the sealed chamber being a filament wound chamber is not intended to preclude the possibility of the sealed chamber including additional components such as a lining, a cover or one or more attachment members.

[0044] Said elongate fibres may be positioned at an angle of at least 30 degrees, for instance at least 45 degrees or at least 60 degrees, to the longitudinal axis.

[0045] This may increase the ability of the sealed chamber to lengthen, with the fibres tending to elastically “uncoil” rather than having to stretch.

[0046] At least part of the sealed chamber may have a concertinaed shape which is configured to straighten in response to said pressurisation of the liquid so as to provide at least some of said lengthening.

[0047] This may allow the sealed chamber to provide the necessary lengthening with fewer design constraints being placed on the material from which the sealed chamber is made. In some embodiments where at least part of the sealed chamber has a concertinaed shape, said part (for instance substantially all of the sealed chamber) may be made from an isotropic material.

[0048] According to a second aspect of the present invention there is provided an aircraft landing gear comprising a liquid spring assembly according to any preceding claim.

[0049] Such a landing gear may provide one or more of the advantages discussed above.

[0050] According to a third aspect of the present invention there is provided an aircraft comprising a landing gear according to the second 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] According to a fourth aspect of the present invention there is provided a method of using a liquid spring assembly, the liquid spring assembly comprising: a first end connected to a sealed chamber which contains a liquid; and a second end connected to a piston rod which extends along a longitudinal axis into the sealed chamber through an aperture therein, the method comprising: moving the first and second ends towards one another, thereby inserting the piston rod further into the sealed chamber and consequently pressurising and compressing the liquid, compression of the liquid creating a restorative force which urges the first and second ends apart from one another; and during and / or after compression of the liquid, causing the pressurised liquid to deform the sealed chamber and thereby lengthen it in a direction parallel to the longitudinal axis.

[0054] Lengthening the sealed chamber may advantageously increase the stroke length of the liquid spring assembly, both in terms of there being a greater length of chamber into which the piston rod can be inserted, and in terms of there being more volume within the chamber in which the liquid (and part of the piston rod) can be accommodated.

[0055] The method may further comprise: moving the first and second ends towards one another using a first force, thereby compressing the liquid to a volume at which it is substantially incompressible, then subsequently moving the first and second ends further towards one another using a second force in addition to the first force, thereby pressurising the liquid to a pressure at which said deformation of the sealed chamber takes place.

[0056] It will of course be appreciated that features described m 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

[0057] 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 a first 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 schematic side view of a step in the manufacture of a chamber of the liquid spring assembly of Figure 3; Figure 5 shows a plot of stroke length against load for the liquid spring shock absorber of Figure 3. Figure 6 shows a kinematic diagram of the landing gear of Figure 2; Figure 7 shows a flow-chart of a method according to the first embodiment of the invention; and Figure 8 is a schematic cross-sectional side view of a liquid spring assembly, more particularly a liquid spring shock absorber, according to a second embodiment of the invention. DETAILED DESCRIPTION

[0058] Figure 1 shows an aircraft, more particularly a commercial airliner 2, according to a first 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.

[0059] The airliner 2 also has a nose landing gear 12 and two mam 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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 pin 31, which forms a pivot joint. The shock absorber 50 can therefore pivot relative to the support arm 20 about the pivot pin 31.

[0064] 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 pin 46.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] The shock absorber 50 provides additional functionality to that described above in that the sealed chamber 56 is configured to deform in response to pressurisation of the liquid 70, thereby lengthening in a direction parallel to (and in this case collinear with) the longitudinal axis 60. In other words, the pressure of the liquid 70 can force the sealed chamber 56 to lengthen. This deformation is provided by the material from which the sealed chamber 56 is made. The chamber 56 is a filament wound cylinder produced from carbon fibre composite.

[0075] Figure 4 shows a step in the production of the sealed chamber 56. A continuous filament 84 made up of carbon fibres 86 is wound around a mandrel 88. In this case the filament 84 is helically wound, following a helical path wrapping back and forth over the mandrel 88 and overlapping with previous layers. Coatings of a liquid resin matrix 90 are applied intermittently. Once the filament 84 has fully enveloped the mandrel 88 to the required wall thickness, the structure is removed from the mandrel 88 and cured to solidify the resin matrix 90. The result is a carbon fibre composite material made up of carbon fibres 86 (provided by the carbon fibre filament 84) in the solid matrix 90. This material is anisotropic, more particularly orthotropic.

[0076] It is noteworthy that the fibres 86 are positioned at an angle of around 50 degrees to the longitudinal axis 60 of the piston rod when the shock absorber 50 is assembled. This makes the chamber 56 considerably less stiff in the direction parallel to the longitudinal axis 60 than it is in a direction radial to that axis 60 or in a direction circumferential about that axis 60. The chamber 56 therefore deforms longitudinally considerably more (in terms of percentage change) than it does radially or circumferentially. Indeed, when it deforms the chamber 56 lengthens while undergoing substantially no change in diameter.

[0077] Although the sealed chamber 56 can deform, that is not to say that it does so easily. In this embodiment the composite material of the chamber 56 is stiff enough that it only begins to lengthen appreciably when the liquid is at a pressure at which it is substantially incompressible. Accordingly, when the ends 52, 54 of the shock absorber 50 are moved towards one another, at first the liquid 70 compresses and the chamber 56 does not appreciably deform, and only once the liquid 70 can be compressed substantially no further does its pressure reach a level at which the chamber 56 extends to any noticeable extent.

[0078] In other embodiments the deformation of the chamber 56 may be plastic deformation. In the present embodiment, however, the deformation is elastic deformation. In essence, therefore, the shock absorber 50 acts as a two-stage spring with a first, lower stiffness while the liquid is compressing and a second, higher stiffness while the chamber 56 is deforming. Figure 5 shows a plot of stroke length against load for the liquid spring shock absorber 50. Point X is the point at which the liquid 70 substantially ceases to be compressible and substantive deformation of the chamber 56 begins. From comparing the gradients of the line before and after this point, it can be seen that for a given stroke length (i.e. a given movement of the piston rod 58), the restorative force provided by deformation of the chamber 56 is around four times as large as that provided by compression of the liquid 70. In other words, deformation of the chamber 56 has an effective spring constant around four times as high as compression of the liquid 70.

[0079] A method of using the liquid spring assembly 50, more particularly a method of absorbing shocks in the landing gear 14, according to the present embodiment will now be described with reference to Figures 6 and 7 (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 5.

[0080] 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.

[0081] In step 220 a first 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 6) which exceeds the moment (clockwise from the perspective of Figures 2 and 6) 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 6) 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 6) relative to the support arm 20 about pivot pin 31, and clockwise (from the perspective of Figures 2 and 6) relative to the trailing arm 32 about pivot pin 46.

[0082] 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).

[0083] 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.

[0084] 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 6) 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 6) relative to the support arm 20 about pivot pin 31, and clockwise (from the perspective of Figures 2 and 6) relative to the trailing arm 32 about pivot pin 46.

[0085] 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.

[0086] 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.

[0087] In step 260 a second, larger, shock loading is applied to the landing gear 14, for instance from the aircraft 2 hitting a harsh bump or a piece of debris when taxiing, taking off or landing. Again, this temporarily increases the ground reaction force R, applying a moment to the trailing arm 32 which exceeds the moment which is exerted by the shock absorber 50 and pivoting the trailing arm 32 upwards. In this case, the ground reaction force R is large enough that even when the liquid 70 is substantially fully compressed, the restorative force it generates is still insufficient. As the trailing arm 32 continues to be forced upwards, the now-substantially-incompressible liquid reaches a pressure at which the sealed chamber 56 begins to deform (i.e. lengthen). The chamber 56 continues to lengthen until the restorative force produced thereby, in combination with the restorative force from the compressed liquid 70, is sufficient to counteract the ground reaction force R. The landing gear 14 can then reach an equilibrium again. Throughout the motion of the piston rod 58, movement of the piston head 74 causes liquid 70 to move through the conduit, dissipating energy through viscous friction and thereby damping the motion.

[0088] In step 270 the shock loading is removed (for instance due to the airliner 2 passing over the bump or debris in the runway. The ground reaction force R therefore drops to a lower level and the moment exerted on the trailing arm 32 by the restorative forces from the compressed liquid 70 and deformed chamber 56 more than counteract the moment exerted by the ground reaction force R. The trailing arm 32 therefore pivots downward again, the chamber 56 returns to its original length and then the liquid 70 begins to expand, until a new equilibrium is reached.

[0089] Figure 8 shows a liquid spring assembly, in the form of a liquid spring shock absorber 50, according to a second embodiment of the invention. The shock absorber 50 of this embodiment is similar to that of the first embodiment, therefore only the differences will be described here.

[0090] In this embodiment the sealed chamber 56 is made from an isotropic material, more particularly stainless steel. The sealed chamber 56 is nonetheless configured to deform m such a manner that the chamber 56 lengthens while undergoing substantially no change in diameter. This is achieved due to the sealed chamber 56 having a part 92 which has a concertinaed shape. When the liquid 70 is pressurised sufficiently, the concertinaed part 92 straightens out. This lengthens the sealed chamber 56 without significantly changing its diameter. The deformation of the sealed chamber 56, namely the straightening of the concertinaed part 92, provides a restorative force in corresponding manner to the first embodiment. When the pressure of the liquid 70 drops again, the concertinaed part 92 returns to its original shape.

[0091] 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.

[0092] The term ‘or’ shall be interpreted as ‘and / or’ unless the context requires otherwise.

Claims

1. A liquid spring assembly comprising:a first end connected to a sealed chamber which contains a liquid and substantially no gas; anda 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, wherein:the first and second ends are resiliently movable towards one another, movement of the first and second ends towards one another inserting the piston rod further into the sealed chamber and thereby pressurising and compressing the liquid, compression of the liquid creating a restorative force which urges the first and second ends apart from one another;the sealed chamber is configured to deform, and thereby lengthen in a direction parallel to the longitudinal axis, in response to said pressurisation of the liquid.

2. A liquid spring assembly according to claim 1 wherein the sealed chamber is configured to deform such that a percentage change in length of the sealed chamber is greater than any percentage change in diameter of the sealed chamber in a direction normal to the longitudinal axis.

3. A liquid spring assembly according to claim 2 wherein the sealed chamber is configured to deform such that said percentage change in length of the sealed chamber is at least twice any percentage change in diameter of the sealed chamber in a direction normal to the longitudinal axis.

4. A liquid spring assembly according to claim 3 wherein the sealed chamber is configured to deform with substantially no change in diameter in any direction normal to the longitudinal axis.

5. A liquid spring assembly according to any preceding claim wherein said deformation of the sealed chamber is elastic deformation.

6. A liquid spring assembly according to claim 5 wherein:for a given movement of the piston rod into the sealed chamber, compression of the liquid produces a first restorative force which acts to urge the first and second ends of the liquid spring assembly away from one another;for an equal magnitude of movement of the piston rod into the sealed chamber, elastic deformation of the sealed chamber produces a second restorative force which acts, through the fluid, to urge the first and second ends of the liquid spring assembly away from one another; andthe second restorative force is larger in magnitude than the first restorative force.

7. A liquid spring assembly according to claim 6 wherein the magnitude of the second restorative force is at least triple that of the first restorative force.

8. A liquid spring assembly according to any one of claims 1 to 4 wherein said deformation of the sealed chamber is plastic deformation.

9. A liquid spring assembly according to any preceding claim wherein the sealed chamber is configured to undergo at least some of said deformation after the liquid has been compressed to a volume at which it is substantially incompressible.

10. A liquid spring assembly according to claim 9 wherein the sealed chamber is configured to undergo substantially all said deformation after the liquid has been compressed to said volume at which it is substantially incompressible.

11. A liquid spring assembly according to any preceding claim wherein the liquid spring assembly is configured as a liquid spring shock absorber which damps movement of the first and second ends relative to one another.

12. A liquid spring assembly according to claim 11 wherein the liquid spring assembly has a piston head provided on the piston rod and located within the sealed chamber, the piston head being configured to impart viscous friction to the liquid during movement of the piston rod within the sealed chamber so as to damp movement of the first and second ends relative to one another.

13. A liquid spring assembly according to claim 12 wherein the piston head has a conduit through which liquid in the chamber can flow during movement of the piston head within the chamber.

14. A liquid spring assembly according to any preceding claim wherein the sealed chamber is formed partially or substantially entirely from an anisotropic material.

15. A liquid spring assembly according to claim 14 wherein said anisotropic material is an orthotropic material.

16. A liquid spring assembly according to claim 14 or 15 wherein said anisotropic material is a composite material made up of elongate fibres in a matrix, said elongate fibres running non-parallel to the longitudinal axis.

17. A liquid spring assembly according to claim 16 wherein the sealed chamber is a filament wound cylinder, the wound filament providing the elongate fibres of the composite material.

18. A liquid spring assembly according to claim 16 or 17 wherein said elongate fibres are positioned at an angle of at least 45 degrees to the longitudinal axis.

19. A liquid spring assembly according to any preceding claim wherein at least part of the sealed chamber has a concertinaed shape which is configured to straighten in response to said pressurisation of the liquid so as to provide at least some of said lengthening.

20. An aircraft landing gear comprising a liquid spring assembly according to any preceding claim.

21. An aircraft comprising a landing gear according to claim 20.

22. An aircraft according to claim 21 wherein said landing gear is a main landing gear.

23. A method of using a liquid spring assembly, the liquid spring assembly comprising:a first end connected to a sealed chamber which contains a liquid; anda second end connected to a piston rod which extends along a longitudinal axis into the sealed chamber through an aperture therein, the method comprising:moving the first and second ends towards one another, thereby inserting the piston rod further into the sealed chamber and consequently pressurising and compressing the liquid, compression of the liquid creating a restorative force which urges the first and second ends apart from one another; andduring and / or after compression of the liquid, causing the pressurised liquid to deform the sealed chamber and thereby lengthen it in a direction parallel to the longitudinal axis.

24. A method according to claim 23 further comprising:moving the first and second ends towards one another using a first force, thereby compressing the liquid to a volume at which it is substantially incompressible, then subsequentlymoving the first and second ends further towards one another using a second force in addition to the first force, thereby pressurising the liquid to a pressure at which said deformation of the sealed chamber takes place.

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