GB2621162A

GB2621162A - Aircraft landing gear shock absorber strut

Info

Publication number: GB2621162A
Application number: GB2211392.2A
Authority: GB
Inventors: Bennett Ian
Original assignee: Safran Landing Systems UK Ltd
Current assignee: Safran Landing Systems UK Ltd
Priority date: 2022-08-04
Filing date: 2022-08-04
Publication date: 2024-02-07
Also published as: WO2024028572A1; GB202211392D0

Abstract

An aircraft landing gear shock absorber strut 50 comprises an outer cylinder 52 defining an oleo chamber O for containing pressurised oleo-pneumatic shock absorber fluid. It further comprises a sliding tube 52 including a piston head 56 movably mounted within the outer cylinder it being movable between a compressed position and an extended position. An annulus portion 60 is defined between the outer cylinder and the sliding tube. An out-stop member 68 is located within the annulus portion such that the piston head contacts the out-stop as the sliding tube moves from the compressed to the extended condition to limit extension of the shock absorber strut. The out-stop member comprises a resilient portion configured to be resiliently compressed in the axial direction of the outer cylinder by at least 2mm by the oleopneumatic force provided by the oleo-pneumatic shock absorber fluid. The resilient portion may be of annular shape and can comprise one or more springs. The out-stop can comprise a collar in parallel with the resilient portion, the collar having flanges which extend to overlap the resilient portion to retain it in a partially compressed state.

Description

AIRCRAFT LANDING GEAR SHOCK ABSORBER STRUT

Background to the Invention

It is common for an aircraft landing gear assembly to include a main hydraulic shock absorber strut having an upper end arranged to be pivotally coupled to the underside of the aircraft and a lower end coupled to a wheel and brake assembly.

Such shock absorber struts can comprise an outer cylinder and a sliding tube arranged to telescope relative to the outer cylinder. The shock absorber strut can be compressed and extended as the sliding tube moves relative to the outer cylinder. The two portions are coupled together to define a chamber containing oil and in some cases a gas. As the shock absorber is compressed, oil within the chamber is forced through damping orifices and, where gas is also provided, the gas is compressed, in order to absorb landing loads. The compressed gas serves as a spring to lengthen the shock absorber as applied external load decreases. Recoil damping orifices can be provided to restrict the flow of oil to the annulus as the shock absorber extends.

"Weight-on-wheels" is a term used in the art to refer to an operational phase of an aircraft when the weight of the aircraft is being support by its landing gear. Weight-on-wheels switches or sensors, also known in the art as "squat switches", are commonly used to indicate to aircraft systems via an electronic signal that the aircraft has transitioned from air to ground mode or vice-versa. This signal, which will be referred to as a "weight-onwheels signal", can be used to enable lift dumpers or brakes to operate on fixed wing aircraft or can be used in the case of rotorcraft to indicate a change of control laws once in ground contact.

It is desirable to transmit the weight-on-wheels signal as soon as possible (consistent with reliability) so that lift dumpers for example can be deployed as early as possible to shorten landing distance.

A weight-on-wheels signal is typically triggered by proximity sensors or proximity switches, or by microswitches on older aircraft, driven by a mechanism related to strut closure i.e. extension state. Various mechanisms exist for the operation of the switch, but it is common to sense an initial angular movement of the upper torque link i.e. the upper link of the pivotable linkage which inhibits axial rotation of the lower cylinder or "sliding tube" of the shock absorber strut relative to the outer cylinder of the shock absorber strut.

Microswitches can be set to trigger at consistent movement but have several disadvantages. As such, microswitches have largely been replaced by proximity sensors. However, proximity sensors have a tolerance range on their sensing i.e. variation in position between guaranteed activation and guaranteed deactivation. For commercial wide body aircraft it is common for the weight-on-wheels switch to not be guaranteed to trigger before about 25 to 30 mm of shock absorber closure has occurred.

Due to the inflation pressure of a shock absorber there is a minimum "breakout load" below which the shock absorber will not move. Thus, a weight-on-wheels transition can occur before it is sensed and the weight-on-wheels signal is transmitted to the aircraft systems.

EP3069994B1 and W02021/019422 describe aircraft landing gear shock absorber struts.

The present inventor has devised a new type of aircraft landing gear shock absorber strut that can have one of more of the following advantages relative to known aircraft landing gear shock absorber struts: * faster and/or more reliable sensing of a weight-on-wheels condition upon landing * a reduction in mass of the aircraft landing gear shock absorber strut * a reduction in complexity of the aircraft landing gear shock absorber strut * a reduction in out-stop impact loads

Summary of Invention

According to a first aspect of the invention, there is provided an aircraft landing gear shock absorber strut comprising: an outer cylinder having an inner surface defining a cylinder bore extending into the outer cylinder from a first axial face of the outer cylinder, the cylinder bore defining an oleo chamber for containing pressurised oleo-pneumatic shock absorber fluid; a sliding tube including a piston head movably mounted within the cylinder bore so as to be movable along an axis of the cylinder bore between: a compressed position in which a free end portion of the sliding tube disposed outside of the bore is relatively close to the first axial face of the outer cylinder; and an extended position in which the free end portion of the sliding tube is relatively far from the first axial face of the outer cylinder; an annulus portion of the bore defined between the outer cylinder and the sliding tube which varies in length as the sliding tube moves between the compressed position and extended position; an out-stop member located within the annulus portion such that the piston head contacts the out-stop member as the sliding tube moves from the compressed condition to the extended condition to limit extension of the shock absorber strut, wherein the out-stop member comprises a resilient portion configured to be resiliently compressed in the axial direction of the bore by at least 2mm by the oleo-pneumatic force provided by the oleo-pneumatic shock absorber fluid.

Thus, the aircraft landing gear shock absorber strut according to the first aspect has an out-stop member including a resilient portion such as a spring that provides an operationally useful amount of force opposing the oleo-pneumatic force, which adds to compressive loads applied to the shock absorber strut upon landing. Thus, the force from the resilient portion attempting to extend can aid in a landing load to overcome the breakout force of the shock absorber, thereby increasing the likelihood of the shock absorber compressing enough to trigger a weigh-on-wheels sensor. Put another way, the first aspect of the invention considers the force balance on the sliding tube when fully extended or near fully extended and uses the resilient portion of the out-stop member to partially counteract the oleo-pneumatic force (gas pressure x area) keeping the shock absorber extended. By altering the static force balance it requires less force to gently close the shock absorber to the point when a weight-on-wheels switch/sensor triggers, making it able to detect touchdown earlier (i.e. lighter load) under light load conditions. In the event of a higher sink rate landing, when early sensing of weight on wheels has a greater chance of being detected early, the majority of the force resisting closure will be damping pressure. The overall difference in energy absorbed at medium and high sink rates will be negligible, meaning that the shock absorber strut according to the first aspect can provide the benefit of less force being required to gently close the shock absorber to the point when a weight-on-wheels sensor triggers, without a need for any increase in the total shock absorber stroke. Moreover, the resilient portion provides an additional benefit by reducing the impact on the out-stop due to inertial loads when reaching full extension i.e. the spring feature of the resilient portion cushions the extension load.

The resilient portion can be configured to be resiliently compressed in the axial direction of the bore by at least 5mm by the oleo-pneumatic force provided by the oleo-pneumatic shock absorber fluid and preferably by at least 20mm. This can increase the likelihood of a weight-on-wheels sensor triggering upon a light landing.

In any embodiment, the resilient portion can be configured to be resiliently compressed in the axial direction of the bore by no greater than 35mm by the oleo-pneumatic force provided by the oleo-pneumatic shock absorber fluid and preferably no greater than 30mm.

The resilient portion can be of annular shape. This can result in the resilient portion reacting the force provided by the oleo-pneumatic shock absorber fluid around the entire annulus portion of the bore.

The out-stop can comprise a collar arranged in parallel with the resilient portion within the annulus, the collar being less resilient than the resilient portion, the collar having radial flanges which extend to axially overlap the resilient portion to retain the resilient portion in a partially compressed state, and a push member having one end in contact with the resilient portion and a second end which extends axially beyond the collar such that movement of the sliding tube towards the extended position causes the piston head to push the push member to compress the spring. The collar can therefore limit extension of the resilient portion to retain some preload at full extension of the shock absorber strut.

The push member can be arranged to be contacted by the piston of the sliding tube and movably coupled to the collar to be restrained by the end of the collar furthest from the out-stop; for example by way of engaging flanges. Alternatively, the push member can form part of the piston head. Alternatively, the resilient member, the collar, and the push member can be reversed such that the piston head is coupled to the out-stop member which is then coupled to the push member.

Since the collar is rigid in comparison to the resilient portion, the collar can also be sufficiently rigid to reacting the force provided by the oleo-pneumatic shock absorber fluid, thus preventing resilient portion being loaded into a fully flattened condition. Other methods of preventing full flattening are also possible such as shaped spacing washers or shaped disc springs.

The resilient portion can comprise one or more springs. Where a plurality of springs is provided, they can be stacked in the axial direction of the bore and can be separated by washers to maintain stack alignment. The springs can for example comprise disc springs. Other types of spring can be employed instead of disc springs e.g. coil spring, wave spring (wave washer), cylinder with slots to form beam spring elements, channels or grooves (sulcated spring) or elastomeric material.

According to a second aspect of the invention, there is provided an aircraft landing gear assembly comprising: the aircraft landing gear shock absorber strut according to the first aspect, the shock absorber strut including a mounting bearing via which it is arranged to be movably coupled to an aircraft to move between a deployed condition for take-off and landing and a stowed condition for flight; and a wheel or other ground contacting assembly coupled to the shock absorber strut.

The landing gear assembly can comprise a side stay, drag stay or plunger lock arrangement coupled to the shock absorber strut and arranged to enable the shock absorber strut to be maintained in a deployed condition relative to an aircraft to which the landing gear assembly is movably coupled.

The landing gear assembly can comprise a main landing gear assembly.

According to a third aspect of the invention, there is provided an aircraft comprising one or more aircraft landing gear assemblies according to the second aspect.

Brief Description of the Drawings

Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a diagram of an aircraft; Figures 2a to 2e are diagrams of an aircraft landing gear assembly; Figure 3 is a diagram of an aircraft landing gear assembly; Figures 4A to 4C are diagrams of an aircraft landing gear assembly shock absorber strut according to an embodiment of the invention; Figure 5 is a diagram of an aircraft landing gear assembly shock absorber strut according to a further embodiment of the invention; Figures 6A to 6B are diagrams of a typical spring curve of an aircraft landing gear assembly shock absorber strut and a spring curve of an aircraft landing gear assembly shock absorber according to an embodiment of the invention, respectively; Figure 7 is a diagram of an aircraft landing gear assembly shock absorber strut according to a further embodiment of the invention; and Figures 8A-8C are diagrams showing the aircraft landing gear assembly shock absorber strut of Figure 7 at different extension states.

Description of Embodiments

Figure 1 is a diagram of an aircraft 10. The aircraft 10 includes assemblies such as a nose landing gear 12, main landing gear 14 and engines 16. The landing gear 12, 14 each includes a shock absorber strut for damping landing loads and supporting the weight of the aircraft 10 when it is on the ground. The term aircraft as used herein can include aeroplanes, helicopters and the like having mass in excess of 450Kg.

Referring now to Figures 2a to 2e, an aircraft assembly, namely an aircraft landing gear assembly, is shown generally at 14. Figures 2a to 2e are an example of an aircraft landing gear assembly which can include a shock absorber strut according to an embodiment of the invention. It will however be appreciated that shock absorber struts according to embodiments of the invention can be used in a range of types of aircraft landing gear.

The landing gear assembly 14 includes a foldable stay 18, a lock link 20 and a down lock spring assembly 22 mounted to the stay 18 and arranged to urge the lock link 20 to assume a locked state. The landing gear assembly also includes a main shock absorber strut 24, comprising an outer cylinder 26 and a sliding tube 28, as well as a wheel and brake assembly 30.

The aircraft landing gear assembly is movable between a deployed condition, for take-off and landing, and a stowed condition for flight. An actuator (not shown) is provided for moving the landing gear between the deployed condition and the stowed condition. This actuator is known in the art as a retraction actuator, and more than one can be provided. A retraction actuator can have one end coupled to the airframe and another end coupled to the outer cylinder such that extension and retraction of the actuator results in movement of the outer cylinder between deployed and stowed conditions.

The stay 18 serves to support the orientation of the outer cylinder 26 when the landing gear is in the deployed condition. The stay 18 generally includes a two bar linkage that can be unfolded to assume a generally straight or aligned, over centre condition in which the stay 18 is locked to inhibit movement of the outer cylinder, as shown in Figures 2c and 2e. When the stay is broken, it no longer prevents pivotal movement of the outer cylinder 26 and the outer cylinder 26 can be moved by the retraction actuator towards the stowed condition, as shown in Figure 2a. During flight the stay 18 is arranged in the folded condition, while during take-off and landing the stay 18 is arranged in the generally straight or aligned condition. Some main landing gear assemblies include a pair of stays coupled to a common shock absorber strut.

The stay 18 has an elongate upper stay arm 18a having a lower end defining a pair of lugs pivotally coupled via a pivot pin 32 to a pair of lugs defined at an upper end of an elongate lower stay arm 18b. The stay arms 18a and 18b can therefore pivotally move relative to one another about the pivot pin 32. The upper end of the upper stay arm 18a defines a pair of lugs that are pivotally coupled to a lug of a connector 34 which in turn is pivotally coupled to the airframe 11. The lower end of the lower stay arm 18b defines a pair of lugs pivotally coupled to a lug of a connector 36 which in turn is pivotally coupled to the outer cylinder 26.

The lock link 20 has an elongate upper link arm 20a having a lower end pivotally coupled to an upper end of an elongate lower link arm 20b via a pivot pin 38. The link arms 20a, 20b can therefore pivotally move relative to one another about the pivot pin 38. An upper end of the upper link arm 20a defines a pair of lugs that are pivotally coupled to a lug of a connector 40 which in turn is pivotally coupled to the outer cylinder 26. A lower end of the lower link arm 20b defines a lug that is pivotally coupled to lugs of the stay arms 18a, 18b via the pivot pin 32. Lugs of the upper stay arm 18a are in this example disposed between the lugs of the lower stay arm 18b and the lugs of the lower link arm 20b.

When the lock link 20 is in the locked condition, as illustrated in Figures 2d and 2e, the upper and lower link arms 20a, 20b are generally longitudinally aligned or coaxial, and can be 'over-centre', such that the lock link 20 is arranged to oppose a force attempting to fold the stay 18, so as to move the landing gear assembly from the deployed condition towards the stowed condition. The lock link 20 must be broken to enable the stay 18 to be folded, thereby permitting the outer cylinder 26 to be moved by the retraction actuator towards the stowed condition.

One or more down lock springs 22 are generally provided to assist in moving the landing gear assembly to the deployed condition and locking it in that state by making the lock link. Down lock springs 22 also inhibit the lock link accidentally being unlocked. Down lock springs 22 are generally metal coil springs, which can be coupled between the lock link and another part of the landing gear assembly, such as an arm of the stay assembly, as shown in Figures 2b and 2e.

The spring assembly 22 is arranged to bias the lock link 20 towards the locked condition by way of spring tension. A distal end of the spring 22a is coupled to the lower stay arm 18b via a lower engagement formation 22b which in turn is coupled to an anchor point defined by the lower connector 22c.

The coil spring of the spring assembly 26 is at its shortest when the landing gear assembly is in the deployed condition, as shown in Figure 2e, and at its longest when the landing gear assembly approaches the stowed condition, as shown in Figure 2b. As the landing gear assembly is retracted towards the stowed condition, the spring of each spring assembly extends, resulting in increased spring load and torsional stress.

Referring to Figure 2e, a lock stay actuator 42 is coupled between the upper stay arm 18a and lower link arm 20b and arranged to pivotally move the link arms 20a, b so as to 'lock' and 'unlock' the lock link 20, as illustrated in Figure 2c. The actuator 42 can break the lock link 20 against the down lock spring bias, allowing the landing gear assembly to be folded and stowed as described previously.

Figure 3 is a diagram of an aircraft landing gear assembly 14. The landing gear assembly 14 includes a torque link comprising an upper 34a and lower 34b torque link member. The torque link restricts relative rotation between the sliding tube 28 and outer cylinder 26 to reduce the rotation of the wheel assembly about the strut axis. As the torque link is pivotally coupled to the outer cylinder 26 and sliding tube 28, the angle at which the torque link forms with the strut assembly 24 will vary with the extension state of the shock absorber. Other configurations of torque link are known and torque links are applied to other types of aircraft landing gear, for example not having a bogie beam.

Weight-on-wheels signal is typically sensed by proximity sensors or proximity switches, or by microswitches on older aircraft, driven by a mechanism related to leg closure. There are a number of different mechanisms for the operation of the switch, but they usually relate to shock absorber closure and typically by sensing an initial angular movement of the upper torque link.

Figures 4A, 4B, and 4C show a diagram of an aircraft landing gear shock absorber strut 50 according to an embodiment of the invention.

The shock absorber strut 50 comprises an outer cylinder 52 having an inner surface 52a defining a cylinder bore B extending into the outer cylinder from a first axial face 52b of the outer cylinder 52. The cylinder 52 is elongate and in this embodiment is of circular cross section.

A sliding tube 54 is movably mounted within the cylinder bore B so as to be movable along the cylinder bore B between a compressed position in which a first free end portion 54a of the sliding tube 54 disposed outside of the bore B is relatively close to the first axial face 52b of the outer cylinder 52 and an extended position in which the first free end portion 54a of the sliding tube 54 is relatively far from the first axial face 52b of the outer cylinder 52.

A piston head 56 is located at the inner end 54b of the sliding tube 54 to slide against the inner surface 52a defining the bore B. The cylinder bore defines an oil and gas chamber 0. A space within the bore B between the cylinder 52 and sliding tube 54 defines an annular chamber which varies in size as the sliding tube 54 moves between the compressed and extended positions. The oil can comprise any suitable hydraulic liquid and the gas can comprise nitrogen for example. An out-stop member is provided within the annulus 60 and provides an abutment surface 62 for the piston head 56 when the shock absorbing strut 50 is in the extended position. The out-stop member can be any suitable structure which results in the piston head abutting said structure instead of an internal axial surface of the outer cylinder.

One or more dynamic seals (not shown) are arranged within the cylinder bore B between the inner surface of the outer cylinder 52 and an outer surface of the sliding tube 54 to inhibit oil within the oil chamber 0 passing the dynamic seals as the sliding tube 54 moves between the extended and compressed positions.

The piston head 56 can be provided with plurality of damping orifices at least some, but not all, of which are provided with one-way valves such that oil can flow relative easily through them as the shock absorber is compressed but flow is relatively difficult in the reverse direction, in order to provided recoil damping as oil flows from the annulus back into the main region of the oil chamber 0.

Figure 4A shows the shock absorber 50 in an extended position. The internal pressure present in the bore B produces an extension force 66 to sliding tube 54 via the axial face of the piston head 56 which faces the oil chamber 0. The internal pressure acting on the piston head 56, and thus sliding tube 54, is reacted by a contact force 68 on the out-stop 60.

Full extension of the landing gear, as illustrated in Figure 4A, can occur when there is no contact with the ground. Upon landing, the landing gear contacts the ground via a ground contacting assembly, producing an initial force 64 which acts on the first free end portion of the sliding tube 54a.This begins to provide an additional counteracting force against the force 66 produced from the pressure within the bore B, as shown in Figure 45. Typically, in known landing gear shock absorbers, it is this initial force 64 alone which results in movement of the sliding tube 54 to trigger the weight-on-wheels sensor.

The present inventor has realised that by forming the out-stop member 60 with a resilient portion, an additional counterforce against the pressure within the bore B is provided by the out-stop member 60 during the initial contraction of the shock absorber, as seen in Figure 45. The weight-on-wheels sensor can be tripped during this initial movement. The additional counterforce reduces the load required to obtain the same closure as a conventional arrangement in which the out-stop does not have a resilient portion. Therefore, as a smaller load can produce the same shock absorber contraction, meaning that a smaller landing load can trigger the weigh-on-wheels sensor. Thus, even a landing event with an initial lower-sink rate has a higher likelihood of triggering the weight-onwheels sensor in comparison to a conventional arrangement.

Figure 4C shows the shock absorbing strut after further closure due to an increased counterforce 70 acting on the sliding tube 54. The piston head 56 does not contact the out-stop 60 and the operation of the shock absorber 50 continues in a conventional manner.

Figure 5 shows an example of a suitable resilient portion 72 arranged in the annulus between the out-stop member 60 and the piston head 56 of the sliding tube 54. The resilient portion 72 can be formed from a disc spring pack comprising one or more disc springs. The disc springs can for example be stacked alternately, in groups of two, three or more, or may have plain washers between the spring discs. The disc spring pack may be allowed to 'float' axially within the annular confines of the shock absorber 54 between sliding tube outer diameter and main fitting inner diameter (defined by the inner surface of the outer cylinder 52a), or it may be constrained with an axial extension limit, as shown in Figure 7.

Alternatively, the resilient portion 72 may be formed of a coil spring, wave spring (wave washer), a cylinder arranged with slots to form beam spring elements, channels or grooves (sulcated spring), an elastomeric material, or any suitable material which can provide intentional resilience. Intentional resilience is defined as providing a deflection over 2 mm or 5mm and preferable at least 20 to 30 mm. It is preferred that the deflection is no more than 50mm, and preferably no more than 40mm.

Figure 6A shows a typical spring curve for a conventional landing gear assembly shock absorber. The gas spring curve is shown in a dashed line with the total load including hydraulic damping at high sink rates shown in an unbroken line. The compression of the shock absorber required to trigger the weight-on-wheels sensor is highlighted by the indicating arrow 74a.

Figure 6B shows a spring curve for a landing gear assembly shock absorber according to an embodiment of the invention. Again, the gas spring curve is shown in a dashed line with the total load including hydraulic damping at high sink rates shown in an unbroken line. The compression of the shock absorber required to trigger the weight-on-wheels sensor is highlighted by the indicating arrow 74b. As the resilient portion 72 of the out-stop member 60 provides an additional counterforce, a lower load due to ground contact is required to produce a compression of the shock absorber strut, which can be seen in the lower load portion of the spring curve 76. As hydraulic damping force is typically proportional to the square of speed of the oscillating object, there is negligible effect on the total force under high sink rate conditions. However, under lower sink rate conditions, when early sensing of weight-on-wheels can be more critical, there is less energy to be absorbed than the high-sink rate conditions. Therefore, the opposing spring force on the initial shock absorber travel (approximately the first 30mm) can have negligible effect on energy absorption or total stroke requirements.

Figure 7 shows a shock absorber assembly 80 according to a further embodiment. The assembly 90 is similar to the assembly 50 of Figures 4A-C and for brevity the following description will focus on the differences.

The shock absorber assembly 80 of Figure 7 includes an extension limiting device 78. Figure 7 shows the shock absorber strut 90 at the point of compression immediately prior to the point in which the sliding tube 94 stops contacting the resilient portion 82 of the out-stop member. The extension limiting device 78 can be arranged in the annulus around the resilient portion 82 to physically restrict the extension of the resilient portion 82. The extension limiting device 78 can be fixed with respect to the outer cylinder 92 such that the resilient portion 82 is free to compress and expand within the extension limiting device 78. To enable the expansion of the resilient portion 78 being translated to the sliding tube 94, a strut 80 forms a contact between the resilient member 82 and piston head 96. The strut 80 can be arranged such that a first end of the strut 80a contacts the sliding tube 94, for example, via the piston head 96. A second end 80b contacts the axial face of the resilient portion closest to the piston head 82a, between the resilient member and an upper portion of the extension limiting device. Therefore, the extension limiting device 78 extends around the resilient portion 82 and the second end of the strut 80b. The strut 80, which serves as a push member, can for example comprise a tube with an outwardly extending radial flange at the lower end, while the extension limiting device 78 can comprise a tube with inwardly extending radial flanges at one or both ends.

The extension limiting device 78 is arranged to limit the extension of the resilient member 82 and allow a spring stack of the resilient member to retain a preload at full extension, when the shock absorber 90 is compressed. The extension limiting device 78 can also provide a mechanism for resetting the resilient member 82 to a compressed condition when the shock absorber strut 90 extends. The extension limiting device 78 can be a sliding spring seat. The extension limiting device can comprise two semi-cylindrical shells to increase ease of assembly.

The extension limiting device 78 can also be configured to bear load, thus preventing the resilient member 78 being loaded into a fully compressed condition, if so desired. Alternative arrangements for preventing full flattening can be achieved via shaped spacing washers or shaped disc springs.

The shock absorber strut 90 can be provided with a conventional gland nut assembly (not shown), which can be coupled to or form part of the out-stop member 98.

Figures 8A, 8B, and 8C further demonstrate the operation of the extension limiting device. Figure 8A shows the shock absorber 90 in a fully extended position in which the sliding tube 94 abuts the resilient portion 82 of the out-stop member 98. The internal pressure within the bore B exerts a force on the sliding tube to push the sliding tube 94 such that the piston head 96 is closer to the first end of the outer cylinder 92b. As there is no load from ground contact, this force is reacted by the contact of the extension limiting device 78 and strut 80 and compresses the resilient portion 82 of the out-stop member 98. In this illustrated example, the resilient portion is not flat loaded and remains slightly conical.

Figure 8B shows the shock absorber 90 when a small load from ground contact has been applied. As the resilient member 82 is expanding, the force due to the internal pressure within the bore B is reacted by resilient member and the load due to ground contact. As the resilient portion 78 has expanded with respect to the arrangement shown in Figure 8A, the extension limiting device 78 does not contact the piston head 96 and the force from the internal pressure within the bore B is no longer reacted through the extension limiting device 78. Thus, the contact point between the resilient portion 82 and the sliding tube 94 is via the strut 80 and piston head 96. Thus, the shock absorber 90 begins to compress, but at a lower load than conventional landing gear arrangements. The weighton-wheels sensor can be triggered at this point meaning that a lower load, in comparison to known landing gear arrangements, is required to produce the motion which can trigger the weight-on-wheels sensor.

Figure 8C shows the shock absorber 90 experiencing an increased landing load with respect to Figure 85 due. The resilient portion 82 of the out-stop member 60 is fully expanded which has resulted in the second end of the strut 80b contacting the limiting device 78, restricting any further movement of the resilient portion 82. As the strut 80 is not providing a contact point between the resilient portion 82 and the sliding tube 94, the force from the internal pressure within the bore B is reacted by only the landing load. Therefore, typical operation of the shock absorber strut continues.

Extension of the shock absorber 90 will reduce the distance between the piston head 96 and the first end of the strut 80a of the extension limiting device 78 until eventually, the piston head 56 contacts the strut 80 at the first end 80a. The strut 80 will provide a reaction force against the internal pressure of the bore B and will provide a compression force to the resilient portion 82 of the out-stop member 60. If the ground load continues to be reduced, the compressive force on the resilient portion 82 will increase, compressing the resilient portion 78 until the sliding tube 94 contacts the extension limiting device 78 and the shock absorber 90 returns to the extended position shown in Figure 8A.

The skilled person would appreciate that the combination of the resilient member 82, collar 78, and push member 80 can be inverted. Therefore, in alternative embodiments, the stack of the resilient member 82, the collar 78, and the push member 80 can be coupled to the piston head 96 such that, as the shock absorber extends, the push member will come into contact with the out-stop member 98 to apply force to the resilient member 82.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parenthesis shall not be construed as limiting the claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. Parts of the invention can be implemented by means of hardware comprising several distinct elements. In a device claim enumerating several parts, several of these parts can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS1. An aircraft landing gear shock absorber strut comprising: an outer cylinder haying an inner surface defining a cylinder bore extending into the outer cylinder from a first axial face of the outer cylinder, the cylinder bore defining an oleo chamber for containing pressurised oleo-pneumatic shock absorber fluid; a sliding tube including a piston head movably mounted within the cylinder bore so as to be movable along an axis of the cylinder bore between: a compressed position in which a free end portion of the sliding tube disposed outside of the bore is relatively close to the first axial face of the outer cylinder; and an extended position in which the free end portion of the sliding tube is relatively far from the first axial face of the outer cylinder; an annulus portion of the bore defined between the outer cylinder and the sliding tube which varies in length as the sliding tube moves between the compressed position and extended position; an out-stop member located within the annulus portion such that the piston head contacts the out-stop member as the sliding tube moves from the compressed condition to the extended condition to limit extension of the shock absorber strut, wherein the out-stop member comprises a resilient portion configured to be resiliently compressed in the axial direction of the bore by at least 2mm by the oleo-pneumatic force provided by the oleo-pneumatic shock absorber fluid.
2. The aircraft landing gear shock absorber strut according to claim 1, wherein the resilient portion is configured to be resiliently compressed in the axial direction of the bore by at least 5mm by the oleo-pneumatic force provided by the oleo-pneumatic shock absorber fluid.
3. The aircraft landing gear shock absorber strut according to claim 1, wherein the resilient portion is configured to be resiliently compressed in the axial direction of the bore by at least 20mm by the oleo-pneumatic force provided by the oleo-pneumatic shock absorber fluid.
4. The aircraft landing gear shock absorber strut according to any preceding claim, wherein the resilient portion is configured to be resiliently compressed in the axial direction of the bore by no greater than 35mm by the oleo-pneumatic force provided by the oleo-pneumatic shock absorber fluid.
5. The aircraft landing gear shock absorber strut according to any preceding claim, wherein the resilient portion is configured to be resiliently compressed in the axial direction of the bore by no greater than 30mm by the oleo-pneumatic force provided by the oleo-pneumatic shock absorber fluid.
6. The aircraft landing gear shock absorber strut according to any preceding claim, wherein the resilient portion is of annular shape.
7. The aircraft landing gear shock absorber strut according to any preceding claim, wherein the out-stop comprises a collar arranged in parallel with the resilient portion within the annulus, the collar being less resilient than the resilient portion, the collar having radial flanges which extend to axially overlap the resilient portion to retain the resilient portion in a partially compressed state, and a push member having one end in contact with the resilient portion and a second end which extends axially beyond the collar such that movement of the sliding tube towards the extended position causes the piston head to push the push member to compress the spring.
8. The aircraft landing gear shock absorber strut according to claim 7, wherein the push member is arranged to be contacted by the piston of the sliding tube and is slidably coupled to the collar.
9. The aircraft landing gear shock absorber strut according to claim 7, wherein the resilient member is coupled to a body portion of the out-stop or coupled to a gland nut assembly.
10. The aircraft landing gear shock absorber strut according to any preceding claim, wherein the resilient portion can comprise one or more springs.
11. The aircraft landing gear shock absorber strut according to claim 10, wherein a plurality of springs is provided, stacked in the axial direction of the bore and separated by washers to maintain stack alignment.
12. An aircraft landing gear assembly comprising: the aircraft landing gear shock absorber strut according to any preceding claim, the shock absorber strut including a mounting bearing via which it is arranged to be movably coupled to an aircraft to move between a deployed condition for take-off and landing and a stowed condition for flight; and a wheel or other ground contacting assembly coupled to the shock absorber strut.
13. The aircraft landing gear assembly according to claim 12, further comprising a side stay, drag stay or plunger lock arrangement coupled to the shock absorber strut and arranged to enable the shock absorber strut to be maintained in a deployed condition relative to an aircraft to which the landing gear assembly is movably coupled.
14. The aircraft landing gear assembly according to claim 12 or 13, further comprising a main landing gear assembly.
15. An aircraft comprising one or more aircraft landing gear assemblies according to any of claims 12 to 14.