REPLENISHMENT DEVICE

Background Art

Aircraft landing gear are commonly provided with a damping system for reducing the forces applied to the aircraft body upon the aircraft landing. Such a damping system may incorporate a telescopic shock absorber. A shock absorber defines a compression chamber, the volume of the compression chamber increasing and decreasing as the shock absorber extends and compresses, respectively.

Oleo -pneumatic shock absorbers are now commonly used for aircraft landing gear, this type of shock absorber having a compression chamber often divided by a floating piston into a first space containing gas (usually nitrogen) and a second space containing oil. When the aircraft lands, the shock absorber is compressed, reducing the volume of the compression chamber and, in doing so, compressing the gas. This provides the shock absorbing "spring" function.

Previously liquid-spring shock absorbers were often used, where the spring function was performed by the compression of a liquid.

A problem with shock absorbers is that the volume of oil within the shock absorber can change, which can adversely affect the working of the system. In particular, oil can leak from the shock absorber, which can affect the energy absorbing capability of the shock absorber as well as the ride height of the aircraft when taxiing.

The operating pressure of a liquid- spring shock absorber typically cycles between some 13,790,031 Pa (20,000 psi) when landing or taxiing, to a much lower pressure when in flight, often near zero Pa.

It is known to replenish oil lost from a liquid-spring shock absorber by connecting it via a pressure-operated one-way valve to the aircraft hydraulic system (which is typically at some 20,685,005 Pa (3,000 psi)). The valve remains closed during the landing and taxiing phases, but will open during the flight phase if the shock absorber pressure drops below the aircraft hydraulic system pressure. It will remain open until enough fluid has been added to cause the shock absorber pressure to rise to meet the system pressure. Because the liquid-spring has a very high stiffness, the volume of fluid admitted to the shock absorber is small, and any effect on the subsequent performance of the shock absorber is acceptably small.

The same method, with a pressure-operated one-way valve, will not work with a conventional oleo -pneumatic shock absorber since the operating pressure of an oleo -pneumatic shock absorber is usually below that of the aircraft hydraulic system pressure (except during some landing conditions) and the "stiffness" of an oleo -pneumatic spring is relatively low. This would result in a large volume of fluid entering the shock absorber, raising it to an unacceptably high pressure, and having a significantly detrimental effect on its performance and characteristics.

Disclosure of the Invention

According to a first aspect of the present invention, there is provided an aircraft landing gear including:

a shock absorber having a compression chamber containing hydraulic fluid;

a fluid replenishment device including a valve having an open configuration in which auxiliary hydraulic fluid is admitted to the compression chamber and a closed configuration in which auxiliary hydraulic fluid is inhibited from entering the compression chamber; and a movable element, wherein the aircraft landing gear is arranged such that the movable element moves in response to the volume of hydraulic fluid within the compression chamber and can move to a position where the movable element mechanically couples with the valve to change the valve from the closed configuration to the open configuration.

The fluid replenishment device may include a recuperator chamber for providing the auxiliary hydraulic fluid to the compression chamber of the shock absorber, the recuperator chamber including:

a closed container defining an internal space and having:

a first port coupled in a fluid tight manner to the compression chamber of the shock absorber; and

a second port arranged to be coupled in a fluid tight manner to a pressurised source of auxiliary hydraulic fluid; the movable element, the movable element being movably mounted within the container and arranged to divide the internal space into first and second subspaces, the first and second ports being in fluid communication with the first subspace;

the valve, wherein in the open configuration the auxiliary hydraulic fluid may pass into the first subspace and in the closed configuration auxiliary hydraulic fluid is inhibited from passing into the first subspace; and

a biasing device arranged to bias the movable element towards the valve,

wherein the fluid replenishment device is arranged such that, when the force acting on the movable element due to the biasing device is greater than the force acting on the movable element due to the pressure of fluid within the first subspace, the movable element moves towards the valve such that it can mechanically couple with the valve so as to open the valve

The biasing device may be implemented as the closed container defining a third port arranged to be coupled in a fluid tight manner to a second source of pressurised f uid, the third port being in fluid communication with the second subspace, when the pressure of fluid within the first subspace is greater than the pressure of fluid within the first subspace, the movable element mechanically couples with the valve so as to change it to the open configuration.

The fluid replenishment device may be arranged such that, when the pressure of fluid within the second subspace is less than the pressure of fluid within the first subspace, the movable element moves away from the valve such that it can move out of mechanical coupling with the valve, such that the valve may change from the open configuration to the closed configuration. The first and second subspaces may be sealed in a fluid tight manner from one another by the movable element.

The aircraft landing gear may be provided in combination with an auxiliary fluid reservoir arranged to provide the source of pressurised auxiliary f uid.

The auxiliary f uid reservoir may include:

a second closed container defining a second internal space and having a fifth port arranged to be coupled in a fluid tight manner to the second port; a second movable element arranged to divide the second internal space into third and fourth subspaces, the fourth port being in fluid communication with the third subspace and the fifth port being in fluid communication with the fourth subspace; and

a third biasing device arranged to bias the second movable element in a direction resulting in contraction of the fourth subspace.

The third biasing device may be provided by a the second closed container having a fourth port arranged to be coupled to a third source of pressurised fluid. The second movable element may include an elongate filling port being in fluid communication with the fourth subspace and the second closed container further includes a sixth port arranged to receive the elongate filling port such that a seal exists between the two.

A stopper may be disposed within the second closed container and arranged to inhibit the entirety of the elongate filling port from being received by the sixth port.

The auxiliary fluid reservoir may be arranged such that the fluid pressure within the fourth subspace is greater than the fluid pressure within the second subspace. The shock absorber may be an oleo -pneumatic shock absorber, the compression chamber having a shock absorber movable element movably mounted within an internal space of the compression chamber, the movable element being arranged to divide the internal space of the compression chamber into first and second shock absorber subspaces, the first shock absorber subspace containing hydraulic fluid and the second shock absorber subspace being arranged to contain a second fluid that more compressible than the hydraulic fluid.

The landing gear may be arranged such that the pressure of the auxiliary fluid is less than the pressure within the second shock absorber subspace when the shock absorber is extended. The biasing device may be a spring or the like.

The landing gear may be arranged such that the shock absorber is an oleo -pneumatic shock absorber including: a compression chamber defining an internal space and including the movable element movably mounted within an internal space of the compression chamber, the movable element being arranged to divide the internal space into first and second subspaces, the first subspace containing hydraulic fluid and the second subspace containing a second fluid that is more compressible than the hydraulic fluid,

and the fluid replenishment device includes:

a first port coupled in a fluid tight manner to the first subspace of the shock absorber;

a second port arranged to be coupled in a fluid tight manner to a pressurised source of auxiliary fluid; and

the valve, wherein in the open configuration the auxiliary fluid may pass into to the first subspace and in the closed configuration wherein the auxiliary fluid is inhibited from passing into to the first subspace,

wherein the landing gear is arranged such, when the pressure of fluid within the second subspace is greater than the pressure of fluid within the first subspace, the movable element mechanical couples with the valve so as to change it to the open configuration.

The landing gear may be arranged such that, when the pressure of fluid within the second subspace is less than the pressure of fluid within the first subspace, the movable element moves away from the valve such that it can move out of mechanical coupling with the valve, such that the valve may move from the open configuration to the closed configuration.

The movable element may include a longitudinal push rod having a free end arranged be brought into mechanical coupling with the valve by way of movement of the movable element.

The movable element may be a piston.

The valve may include a valve member and a valve seat, the valve being arranged such that net pressure from pressurised fluid within the first subspace loads the valve member towards the valve seat.

The valve member may include an internal bore providing a passageway for fluid communication between opposite faces of the valve member. The landing gear may include a second biasing device arranged to apply a biasing force to the valve to bias the valve towards the closed configuration. The second biasing device may be arranged to apply a closing force to the valve that is greater than a predetermined opening force applied to the valve by the pressurised auxiliary fluid.

According to a second aspect of the present invention, there is provided an aircraft including a landing gear according to the first aspect.

According to a third aspect of the present invention, there is provided fluid replenishment device specially adapted for use with a landing gear according to the first aspect. According to a fourth aspect of the present invention, there is provided an aircraft landing gear including:

shock absorber means having a compression chamber containing hydraulic fluid;

fluid replenishment means including valve mean having an open configuration in which auxiliary hydraulic fluid is admitted to the compression chamber and a closed configuration in which auxiliary hydraulic fluid is inhibited from entering the compression chamber; and movable means, wherein the aircraft landing gear is arranged such that the movable means moves in response to the volume of hydraulic fluid within the compression chamber and can move to a position where the movable means mechanically couples with the valve means to change the valve means from the closed configuration to the open configuration.

In some embodiments, the aircraft landing gear may include a shock absorber and a fluid replenishment device, the shock absorber having a compression chamber containing hydraulic fluid and the fluid replenishment device including a recuperator chamber for providing hydraulic fluid to the compression chamber of the shock absorber, the recuperator chamber including:

a closed container defining an internal space and having:

a first port coupled in a fluid tight manner to the compression chamber of the shock absorber; and a second port arranged to be coupled in a fluid tight manner to a pressurised source of auxiliary hydraulic fluid;

a movable element movably mounted within the container and arranged to divide the internal space into first and second subspaces, the first and second ports being in fluid communication with the first subspace;

a valve having a open configuration in which the auxiliary hydraulic fluid may pass into the first subspace and a closed configuration wherein the second port is sealed in a fluid tight manner; and

a biasing device arranged to bias the movable element towards the valve,

wherein the fluid replenishment device is arranged such that, when the force acting on the movable element due to the biasing device is greater than the force acting on the movable element due to the pressure of fluid within the first subspace, the movable element moves towards the valve such that it can mechanically couple with the valve so as to open the valve.

In some embodiments, the aircraft landing gear may include an oleo -pneumatic shock absorber and a fluid replenishment device, the oleo-pneumatic shock absorber including:

a compression chamber defining an internal space and including a movable element movably mounted within the internal space of the compression chamber, the movable element being arranged to divide the internal space into first and second subspaces, the first subspace being arranged to contain a first fluid and the second subspace being arranged to contain a second fluid,

the fluid replenishment device including:

a first port coupled in a fluid tight manner to the first subspace of the shock absorber;

a second port arranged to be coupled in a fluid tight manner to a pressurised source of auxiliary fluid; and

a valve having a open configuration in which fluid may pass from the pressurised source of auxiliary fluid to the first subspace and a closed configuration wherein the second port is sealed in a fluid tight manner,

wherein the landing gear is arranged such that, when the fluid pressure within the second subspace is greater than the fluid pressure within the first subspace, the movable element moves towards and into mechanical coupling with the valve so as to open the valve. These and other aspects of the present invention will be apparent from, and clarified with reference to, the embodiments described herein.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Description of the Drawings

Figure la shows a schematic drawing, in cross section, of an oleo -pneumatic shock absorber including a recuperator cylinder, where the shock absorber is compressed;

Figure lb shows a schematic drawing, in cross section, the oleo -pneumatic shock absorber of Figure la, where the shock absorber is almost fully extended;

Figure lc shows a schematic drawing, in cross section, the oleo -pneumatic shock absorber of Figure la, where the shock absorber is fully extended;

Figure 2a is a partial schematic drawing, in cross section, of a landing gear according to a first embodiment of the present invention, including a fluid replenishment device, where the shock absorber of the landing gear is extended;

Figure 2b is a schematic drawing of a valve of the fluid replenishment device of is a partial schematic drawing, in cross section, of the landing gear of Figure 2a;

Figure 2c is a schematic drawing, in cross section, of the fluid replenishment device of the landing gear of Figure 2a, where the shock absorber of the landing gear is compressed;

Figure 3 is a partial schematic drawing, in cross section, of a landing gear according to a second embodiment of the present invention, including a fluid replenishment device, where the shock absorber of the landing gear is extended;

Figure 4 is a schematic drawing, in cross section, of a conventional oleo -pneumatic shock absorber; and Figure 5 is a partial schematic drawing, in cross section, of a landing gear according to a third embodiment of the present invention, including a fluid replenishment device, where the shock absorber of the landing gear is extended. Embodiments of the Invention

Referring to Figures la to lc, schematic drawings, in cross section, of an oleo -pneumatic shock absorber 110 including a recuperator cylinder 120 are shown. The oleo -pneumatic shock absorber 110 has a casing 112 within which a slider 114 is telescopically housed. The travel of the slider 1 14 is limited by a slider end stop 114' contacting a casing end stop 112'. The slider end stop 114' defines a shoulder portion arranged to contact an outer shoulder portion of the casing end stop 112'. The slider end stop 114' also has an inner shoulder portion. A shock absorber floating piston 116, which functions as a gas spring piston, is slidably constrained within the slider 114, so as to partition the shock absorber 110 into a shock absorber oil chamber 113 and a shock absorber gas chamber 115. The inner shoulder of the slider end stop 114' limits upward travel of the shock absorber floating piston 116. With the shock absorber floating piston 116 in contact with the inner shoulder of the slider end stop 114', the pressure within the shock absorber gas chamber 115 is approximately 1,379,000 Pa (200 psi) and the pressure within the shock absorber oil chamber 113 is less than or equal to the shock absorber gas chamber 115.

The recuperator 120 is a closed cylinder containing a recuperator sliding piston 122 that divides the internal space of the cylinder into a recuperator oil chamber 124 and a recuperator gas chamber 126. The recuperator oil chamber 124 is in fluid communication with the shock absorber oil chamber 113 via pipe 118. Pipe 118 may also include some restriction to fluid flow, in particular a one-way restriction limiting flow from the shock absorber oil chamber 113 to the recuperator oil chamber 124. Pressurised gas, such as compressed air, circa 137,900 Pa (20 psi), bled from the aircraft engine is in fluid communication with the recuperator gas chamber 126 via pipe 126. The pressure within the recuperator gas chamber 126 is thus less than the pressure within the shock absorber gas chamber 115, but can be greater than the pressure within the shock absorber oil chamber 113.

A purpose of the recuperator 120 is to change the spring characteristics of the oleo -pneumatic shock absorber 110. The recuperator 120 allows a reduction in the spring force at high levels of extension of the shock absorber 110. However, the total shock absorber travel is retained for damped energy absorption, due to the fact that the shock absorber floating piston 116 contacts the inner shoulder of the slider end stop 114' before the shock absorber 110 fully extends, as shown in Figure lb, and thereafter the shock absorber 110 is fully extended, as shown in Figure lc, using the compressed gas in the recuperator gas chamber 126. The shock absorber 110 makes it final extension under low pressure, such as aircraft bleed air pressure, which is lower that the nitrogen gas spring pressure. This is possible because the shock absorber floating piston 116 is in contact with the inner shoulder of the slider end stop 114' and thus the nitrogen gas can expand no further.

Figure 2a shows a partial schematic drawing, in cross section, of a landing gear according to a first embodiment of the present invention. The landing gear includes an oleo-pneumatic shock absorber, such as the shock absorber 110 shown in Figures la to lc, that is connected to the airframe at its upper end and connected to a wheeled assembly at its lower end and may be pivotally deployed, for landing, and retracted, for stowing (however, for clarity, these parts are not shown in the accompanying figures).

The landing gear includes a fluid replenishment device 10. The fluid replenishment device

10 includes a recuperator chamber 11 and an auxiliary oil reservoir 40. The recuperator chamber 11 is coupled with the oleo-pneumatic shock absorber and is arranged to exchange

011 with the shock absorber oil chamber. The fluid replenishment device 10 is arranged such that the auxiliary oil reservoir 40 can supply oil to the recuperator chamber 11. The recuperator 11 also provides the known functionality of reducing the spring force at high levels of extension of the shock absorber.

The recuperator chamber 11 includes a closed container 12 defining an inner space and having a first port 14, a second port 16 and a third port 18 arranged to enable fluid communication between the inner space and the exterior of the recuperator chamber 11. The first port 14 is arranged to be coupled in a fluid-tight manner to the shock absorber oil chamber, via a first pipe 20 defining a first fluid passageway for oil.

The second port 16 is arranged to be coupled in a fluid tight manner to the auxiliary oil reservoir 40, via a second pipe 22 defining a second fluid passageway. The auxiliary reservoir 40, which is described in more detail below, is arranged to provide a pressurised supply of auxiliary oil to the recuperator chamber 11.

The third port 18 is arranged to be coupled in a fluid tight manner to a supply of pressurised air, via a third pipe 24 defining a third fluid passageway. The purpose of the pressurised air is to provider a counterbalance of pressure within the internal space of the recuperator chamber 11 , as described in more detail below.

A movable element 26 in the form of recuperator piston 26 is disposed within the internal space of the recuperator chamber 11 and divides the internal space into a first subspace 12' and a second subspace 12". The first subspace 12' and second subspace 12" are sealed in a substantially air-tight manner from one another by the piston 26. As will be appreciated, the first subspace 12' thus forms part of the oil side of the shock absorber compression chamber. The piston 26 is movable by virtue of the fluid conditions (pressure and volume) within the first 12' and second subspace 12". The piston 26 thus moves in response to the volume of oil within the shock absorber compression chamber.

The first port 14 and second port 16 are in fluid communication with the first subspace 12'. The third port 18 is in fluid communication with the second subspace 12". Consequently, positive pressure at the first or second ports 14, 16 results in a pressure increase within the first subspace 12' and positive pressure at the third port 18 results in a pressure increase within the second subspace 12". Such pressure increases bring about movement of the piston 26 to equalise the pressure in the subspaces 12', 12". It should be noted that the recuperator chamber 11 can take any suitable configuration having two ports in exclusive fluid communication with a first subspace and a third port in exclusive fluid communication with a second subspace, the volume of the subspaces being dependent upon the pressure of fluid therein. A movably mounted movable element is intended to encompass a structure arranged to move relative to the camber, or a structure arranged to deform by virtue of applied pressure so as to change the respective volumes of the subspaces.

Referring to Figure 2b, a valve 30 isolates the second port 16 from the second pipe 22. The valve 30 includes a valve body 32 defining a valve cavity 34 that provides a passageway for enabling fluid communication between the second port 16 and the second pipe 22. The valve cavity 34 narrows towards one end defining a generally frusto-conical valve seat 32'. The valve seat terminates in a generally cylindrical well 34' into which the second pipe 22 opens.

A valve member 36 is disposed within the valve cavity 34 and includes a hollow tubular portion disposed within the second port 16 such that the hollow tubular part can slide axially within the second port 16. A seal is formed between the outer surface of tubular portion of the valve member 36 and the second port 16. The lower end of the valve member 36 projects from the second port 16 into the first subspace 12' and includes a plurality of lateral apertures 36" arranged to permit oil exchange between the interior of the hollow tubular portion of the valve member 36 and the first subspace 12' when the piston 26 opens the valve 30. The upper end of the valve member 36 radially enlarges to form a generally frusto-conical shoulder portion 36' arranged to correspond in shape to the generally frusto-conical valve seat 32'. A compression spring 38, or other suitable biasing device, is mounted within the valve cavity 34 so as to bias the shoulder portion 36' of the valve member 36 against the valve seat 32'. To enable pressurised filling of the first subspace 12', which may ease accessibility and may in some cases prevent air ingestion, the biasing force provided by the spring 38 is greater than the load from the pressurised auxiliary oil acting over the shoulder portion 36' of the valve member 36. Thus, the biasing force applied by the spring 38 is great enough to prevent the pressurised auxiliary oil from opening the valve 30.

With the shoulder portion 36' of the valve member 36 abutting the valve seat 32', the valve 30 is closed and isolates the second port 16 from the second pipe 22. The valve 30 can be moved to an open configuration by an upward force applied to the hollow lower end of the valve member 36 by the piston 26. The piston 26 thus mechanically couples with the valve member to lift the shoulder portion 36' of the valve member 36 away from the valve seat 32', so as to define a fluid channel between them via which oil can be exchanged between the auxiliary oil reservoir 40 and the first subspace 12'. It should be noted that the valve is arranged to be biased towards the closed position by fluid pressure within the valve 30 when the piston 26 has moved out of contact with the valve member 36. This is achieved in the illustrated embodiment by the configuration of the hollow valve member 36 and valve cavity 34 such that net pressure from the oil loads downwards onto the seat 32'. It should be noted that the valve 30 may take any suitable configuration that at rest isolates the auxiliary fluid supply from the internal volume of the replenishment chamber and can be mechanically changed to an open configuration by causing the movable element to directly or indirectly contact a part of the valve to mechanically open it.

Referring back to Figure 2a, the auxiliary oil reservoir 40 includes a closed container 42 defining an inner space and having a fourth port 44, a fifth port 46 and a sixth port 45 arranged to enable fluid communication between the inner space and the exterior of the closed container 42.

The fourth port 44 is arranged to be coupled in a fluid tight matter to the second supply of pressurised fluid, via a fourth pipe 52 defining a fourth fluid passageway. As for the recuperator chamber 11 , the purpose of the second supply of pressurised fluid is to provide a counterbalance of pressure within the internal space of the oil reservoir 40, as described in more detail below.

The fifth port 46 is arranged to be coupled in a fluid tight manner to the recuperator chamber 11 , via the second pipe 22 The sixth port 45 vents to the outside environment and is arranged to sealingly receive a tubular portion 48' of a second movable element 48. The sixth port 46 includes a lower rigid collar portion connected to flexible upper sleeve, to account for vertical travel of the second movable element 48. The second movable element 48 is in this embodiment a reservoir piston 48 having a hollow central portion coupled in a fluid tight matter to the tubular portion 48' that extends from a top face of the reservoir piston 48 through the sixth port 45. The combination of the reservoir piston 48, tubular portion 48' and the seal between the tubular portion and the sixth port 45 divides the internal space of the auxiliary oil reservoir 40 into a third subspace 42' and a fourth subspace 42". The third subspace 42' and fourth subspace 42" are thus sealed in a substantially air-tight manner from one another. Auxiliary oil within the auxiliary oil reservoir 40 can be replenished via the tubular portion 48'. A cap 54 is provided to seal the free end of the tubular portion 48'. The fluid replenishment device according to embodiments of the present invention including a dedicated auxiliary reservoir can thus provide a visible indication of when the auxiliary reservoir requires re-filling, as well as allowing a different fluid to be used in the shock absorber than is used in the aircraft hydraulic fluid system.

In the illustrated embodiment the pressure of the second supply of pressurised fluid is arranged such that the fluid pressure within the fourth subspace 42" is greater than the normal fluid pressure within the first subspace 12' when the oleo -pneumatic shock absorber is extended. This ensures that auxiliary oil will enter the first subspace 12' when the valve 30 is open. In some embodiments of the present invention the fluid replenishment device 10 is arranged such that the fluid pressure within the fourth subspace 42" is lower than the pressure of the gas spring of the oleo-pneumatic shock absorber when on the slider end stop 114', to prevent over-inflation of the shock absorber due to compression of the gas spring, thus allowing more oil into the oil chamber of the shock absorber than intended.

A second biasing device 50 in the form of a second compression spring 50 is provided to limit the upwards travel of the reservoir piston 48 when replenishing auxiliary oil. In some embodiments, the compression spring may be arranged to provide an additional force onto the reservoir piston 48 so that it increases the pressure in the fourth subspace 42" to slightly above that in the first subspace 12', such that the first and the second supply of pressurised fluid can be of equal pressure and in some embodiments common.

It should be noted that the auxiliary oil reservoir 40 can take any suitable configuration that provides a pressurised supply of auxiliary fluid to the valve 30. For example, in another embodiment, as shown in Figure 3, the auxiliary oil reservoir 40 could be the hydraulic system oil of an aircraft, in some embodiments passing through a pressure reducing valve before arriving at the valve 30. Such embodiments may enable the total weight of the fluid replenishment device to be reduced in comparison with embodiments including a dedicated oil reservoir 40.

In use, pressurised air is supplied to the second subspace 12" and third subspace 42' via pipes 24 and 52, respectively, such that the fluid pressure in the fourth subspace 42" is greater than that in the first subspace 12', when the shock absorber is extended. The auxiliary oil within the fourth subspace 42" is pressurised against the valve 30. Generally speaking, the system variable is the volume of oil within the oil chamber of the shock absorber, the first subspace 12' forming a part thereof. When a sufficient volume of oil is present within the shock absorber oil chamber, the recuperator functions in a normal manner, as described above with reference to Figures la to lc.

When the volume of oil within the shock absorber oil chamber drops such that the recuperator piston 26 approaches its uppermost travel limit, as the recuperator attempts to fully extend the shock absorber, as shown in Figure 2a, the piston 26 mechanically engages with the valve member 36, holding the valve 30 open against the closing force provided by the spring 38. With the valve 30 open, the pressurised auxiliary oil is able to flow into the first subspace 12' to top up the shock absorber oil. Providing the pressure of the supplied auxiliary oil is less than the pressure within the shock absorber gas chamber, there is no danger of over inflation due to compression of the gas spring allowing in more oil than is desirable.

Once the volume of oil within first subspace 12' is such that the oil pressure is greater than the air pressure within the second subspace 12", the piston 26 moves downwardly to equalise the pressure and in doing so moves and away from the valve 30 and the valve closes by virtue of the closing force supplied by the spring 38. The volume of oil within the compression chamber is thus now fixed.

Subsequent compression of the shock absorber results in the volume of the first subspace 12' reaching a maximum, as shown in Figure 2c. Thereafter, further compression of the shock absorber results in compression of the nitrogen gas within the gas chamber of the shock absorber (as illustrated in Figure la). In this embodiment, the valve 30 is inhibited from opening due to increased oil pressure by virtue of the fact that the valve is arranged such that net pressure from the oil loads the valve member 36 downwards onto the seat 32'.

The fluid replenishment device of a landing gear according to embodiments of the present invention including a recuperator thus enables the exchange of oil between an auxiliary supply and the shock absorber oil chamber whenever the volume of oil within the shock absorber oil chamber results in the recuperator piston reaching an upper limit that opens the valve. This enables the landing gear oil to be topped up to account for leakage and automatically corrects the volume of oil in the compression chamber to account for thermal contraction of the oil. Embodiments of the invention using a recuperator piston to open the supply valve may also have the advantage of enabling a low pressure oil reservoir to be used for topping up, which may be lighter and may allow a simpler filler cap arrangement to be used than if it were high pressure (since the recuperator is not exposed to peak shock absorber pressures due to the restriction of oil flow by pipe 118). Furthermore, the spring curve characteristics of a recuperated shock absorber mean that there is little, and in some cases no, spring resilience to oppose filling of the shock absorber, or to confuse volumes.

It should be noted that, whilst in the illustrated embodiments the piston 26 is arranged to directly contact the valve member 36 to open the valve 30, in other embodiments a different type of mechanical coupling may occur, such as the piston 26 indirectly coupling with the valve member 36 via a suitable linkage or the like.

In other embodiments the upwards force on the recuperator piston 26 and/or the downwards force on the auxiliary oil reservoir piston 48 may be provided by resilient means, such as a mechanical spring, instead of air pressure.

Figure 4 shows a conventional oleo -pneumatic shock absorber 110 that is not coupled to a recuperator. Similarly to Figures la to lc, the oleo -pneumatic shock absorber 110 has a casing 112 within which a slider 114 is telescopically housed. The travel of the slider 114 is limited by a slider end stop 114' contacting a casing end stop 112'. The slider end stop 114' defines a shoulder portion arranged to contact an outer shoulder portion of the casing end stop 112'. The slider end stop 114' also has ain inner shoulder portion. A shock absorber floating piston 116, which functions as a gas spring piston, is slidably constrained within the slider 114, so as to partition the shock absorber 110 into a shock absorber oil chamber 113, 115a and a shock absorber gas chamber 1 15b. The inner shoulder of the slider end stop 114' limits upward travel of the shock absorber floating piston 116. In normal use, when the shock absorber 110 is fully extended the shock absorber floating piston 116 is spaced from the inner shoulder of the slider end stop 114', to provide some allowance for oil loss. The pressure within the shock absorber oil chamber 113 and the shock absorber gas chamber 115b is approximately 1,379,000 Pa (200 psi) in the fully extended condition.

In the event of oil loss, the shock absorber floating piston 116 will move closer to the inner shoulder of the slider end stop 114', to equalise pressure. The gas within the shock absorber gas chamber 115 thus expands, which has the disadvantage of now giving a different spring rate on closure.

Figure 5 shows a partial schematic drawing, in cross section, of a landing gear 60 according to a third embodiment of the present invention. The landing gear includes an oleo-pneumatic shock absorber 110', similar to the shock absorber 110 shown in Figure 4, that is connected to the airframe at its upper end and connected to a wheeled assembly at its lower end and may be pivotally deployed, for landing, and retracted, for stowing (however, for clarity, only the shock absorber 110' and replenishment device of the landing gear 60 are shown in the accompanying Figures) .

The shock absorber 110' differs from the conventional shock absorber 110 in that it includes a port 64 to which is in fluid communication with a first side of a valve 66. A supply of pressurised oil is in fluid communication with a second side of the valve 66 via a supply pipe 68. The supply pressure of the oil in the supply pipe 68 exceeds the oil pressure within the shock absorber oil chamber 113 when the shock absorber 110' is fully extended. The supply pipe 68 may be fed directly from the aircraft hydraulic system, via a pressure reducing stage if required, or could be fed via an intermediate chamber similar to a recuperator. Feeding the supply pipe 68 from an intermediate chamber has the advantage of reducing the likelihood of cross contamination between the shock absorber 110' and the aircraft hydraulic system and may also limit the amount of fluid that could be added (or expelled should the valve 66 jam open).

In some embodiments the shock absorber 110' may include a valve disposed at the end region of the slider 114, to regulate oil flow between the part of the oil chamber 113 defined by the casing 112 and the part of the oil chamber 115a defined by the slider 114. In such a case, the valve may be provided on a plate that closes the top end of the slider 114 and the push rod 62 may pass through the plate in a sealing manner at a location remote from the valve.

The valve 66 is arranged to seal the fluid passageway between the supply pipe 68 and port 64 when in a closed configuration and permit fluid communication between them when in an open configuration. In the illustrated embodiment, the valve is biased towards its closed configuration by a spring 67. The landing gear 60 is arranged to inhibit the valve 66 from opening due to compression of the shock absorber 110' in use. In some embodiments this can be achieved by making the supply pressure supply pipe 68 greater than the maximum internal pressure of the shock absorberl 10' or by using a very strong spring 67, a small area valve. An alternative embodiment is to use a fully or partially balanced area valve that is the same or similar to the valve 30 of the first embodiment.

The shock absorber floating piston 1 16 moves in response to the volume of oil within the oil chamber 1 13, 115a. In this embodiment the movable element takes the form of a push rod 62 extending from the shock absorber floating piston 116 towards the valve 64. It will be appreciated that the shock absorber floating piston 116 is at its highest position when the shock absorber 11 ' is fully extended. The push rod 62 and valve 66 are together arranged such that a free end of the push rod 62 can be brought into mechanical coupling with the valve 66, to change the valve 66 from its closed to its open configuration, and moved out of mechanical coupling with the valve 66, such that the spring can return the valve to its closed configuration, in accordance with the position of the shock absorber floating piston 116. Whilst the illustrated embodiment provides a push rod as the means to form a mechanical linkage between the separator piston and supply valve, in other embodiments any suitable linkage may be provided to enable the supply valve to be opened and closed by virtue of the position of the separator piston. For example, a rod could extend downwards from the valve and be arranged such that the separator piston contacts a free end of the rod. Also, in further embodiments the push rod, or the like, may be configured to pull the valve closed as it moves away from it.

When the shock absorber 110' is in its fully extended position, as shown in Figure 5, and has a normal operating volume of oil in its oil chamber 113, the free end of the push rod 62 is spaced from the valve 66 by a small distance. As oil is lost, or contracts in volume, the shock absorber floating piston 116 and thus the free end of the push rod 62 will move closer to the valve 66. The length of the push rod 62 is selected such that the free end thereof then contacts and opens the valve 66 before the shock absorber floating piston 116 contacts the inner shoulder of the slider end stop 114'.

Once enough oil has passed from the supply pipe 68 into the shock absorber oil chamber 113, the shock absorber floating piston 116 will move downwards to equalise the pressure between the oil chamber 1 13 and gas chamber 115 and in doing so will move the free end of the push rod 62 out of mechanical coupling with the valve 66, such that the valve 66 can close.

Embodiments of the invention provide a valve arrangement that is mechanically activated by contact with a piston responsive to the volume of hydraulic fluid in a shock absorber (in the case of a "recuperated" shock absorber, the volume of hydraulic fluid in the combined shock absorber and recuperator system) Whereas the gas pressure in a shock absorber varies significantly with temperature, and thus a small variation in temperature would result in a large error in fluid volumes if filled according to pressure, the oil volume varies less than the variation in gas pressure. Hence there is much less error due to temperature when filling according to oil volume.

While specific types of hydraulic fluid, such as oil, have been referred to in the described embodiments other suitable types of hydraulic fluid can be used in embodiments of the present invention, at other suitable pressures, as will be appreciated by a person skilled in the art.