GB2587193A

GB2587193A - Aircraft landing gear system

Info

Publication number: GB2587193A
Application number: GB1913267.9A
Authority: GB
Inventors: Sharpe Peter
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2019-09-13
Filing date: 2019-09-13
Publication date: 2021-03-24
Also published as: GB201913267D0

Abstract

Retraction of a landing gear assembly (4, fig 3) on an aircraft (1, fig 3) is actuated by a hydraulic actuator 12. The actuator 12 includes a piston 28 that travels within a cylinder 26 along a stroke length between a first position corresponding to the landing gear assembly when extended and a second position corresponding to the landing gear assembly when retracted. Movement of the piston 28 along its stroke length is snubbed at different levels, for example by use of a snubbing spear 40 having one or more fluid channels 46, 48 formed herein. Differential snubbing is thus provided, which enables the landing gear to retract faster.

Description

AIRCRAFT LANDING GEAR SYSTEM

BACKGROUND OF THE INVENTION

100011 The present invention concerns a landing gear system for an aircraft including (a) a retractable landing gear assembly and (b) a hydraulic actuator for actuating retraction of the landing gear. The invention also concerns a method of operating a retractable aircraft landing gear, a hydraulic actuator configured for actuating retraction of a landing gear, and a method of manufacturing a snubbing device.

[00021 The present invention has particular application in relation to large commercial passenger aircraft, but may have application in relation to other types of aircraft having one or more landing gear assemblies which are arranged to extend (for example, to be deployed) and to retract. Retraction of the landing gear is typically performed with the use of a hydraulic actuator. A hydraulic actuator is the common term used for a hydraulic piston, ram or jack. Once retracted, the landing gear is typically stowed in a landing gear bay, which is closed off with the use of one or more landing gear bay doors. If the time taken for the landing gear to retract after take-off could be shortened that could enable drag to be reduced, by means of reducing the amount of time the landing gear is exposed in the airflow around the aircraft. Fully retracting the landing gear as soon as possible after take-off also has the potential advantage of providing greater clearance between the aircraft and any obstacles on the ground. Reducing the time a landing gear remains fully or partially deployed immediately after takeoff, even by a fraction of a second, can have significant benefit.

[00031 Enlarging an actuator to increase the load it can exert can reduce the speed, and conversely shrinking the swept volume of an actuator to increase the speed can reduce the load. Speed control of rate of hydraulic actuation for a fixed size depends on flow supply, and flow metering. Landing gear actuators are required to reach the end of their travel without imparting large forces, or impacting at high speeds. There are limits therefore on the speed of travel at either end of the motion of the landing gear between the extended and retracted positions. The maximum speed of action of an actuator is typically governed by the saturation flow rate of the actuator, the rated flow of the hydraulic supply, and the load that the actuator is overcoming; these are factors which cannot easily be changed once an aircraft design has been certified. -2 -

10004] The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to reduce the time taken for landing gear retraction. Alternatively or additionally, the present invention seeks to provide an improved landing gear system for an aircraft and/or an improved hydraulic actuator for a landing gear system.

SUMMARY OF THE INVENTION

100051 The present invention provides, according to a first aspect of the invention, a landing gear system for an aircraft, wherein the system includes a retractable landing gear assembly, and a hydraulic actuator for actuating retraction of at least a part of the landing gear assembly from an extended (e.g. deployed) configuration to a retracted (e.g. stowed) configuration. The landing gear system typically comprises a main strut at an extreme end of which there are mounted one or more aircraft wheels. The hydraulic actuator includes a piston that travels within a cylinder along a stroke length between a first position corresponding to the extended configuration of said part of the landing gear assembly and a second position corresponding to the retracted configuration of said part of the landing gear assembly. Movement of the piston along its stroke length is snubbed for a portion of the stroke length when proximate to, and moving in a direction towards, one of the first position and the second position. Snubbing may for example be desirable to decelerate the movement of the landing gear ("LG") immediately before it comes to a stop, so that the moving parts do not stop abruptly potentially causing undesirable wear or damage. The snubbing along that portion of the stroke length is greater at a position further along the portion in the direction of movement. In embodiments of the invention, different levels of snubbing may thus be provided as the piston moves towards at least one end of its stroke length. This makes it possible to have a first phase of relatively less snubbing (and relative fast piston travel speed when snubbed, despite being significantly slower than the speed during its unsnubbed movement) and a second phase of greater snubbing (and relative slow piston travel) for a relatively short part of the travel time. Thus, the proportion of time spent by the piston travelling at (very) low speed, as a result of the snubbing immediately before coming to a stop, may be reduced, leading to a shorter time required for the piston to move between its first and second position. The amount of snubbing provided -3 - (and, for example, how the levels of snubbing provided change) in one direction of movement may be the same as, or similar, to the amount of snubbing provided (and how the levels of snubbing provided change) in the opposite direction. The way in which snubbing is provided may make this an inevitable consequence of the levels of snubbing intended by design in one direction. In certain embodiments, this means that the snubbing provided to ensure that the landing gear when it extends (deploys) does not come to an abrupt stop at the end of its travel has a direct impact on the time taken for the landing gear to be retracted, because providing snubbing at the end of the movement when extending the LG results in there being snubbing at the start of the movement when retracting the LG.

[0006] Providing an actuator which is able to retract the landing gear quickly is desirable. Providing adequate snubbing requires a certain part of the stroke length to be snubbed. Simply shortening the amount of stroke length dedicated to snubbing for the purpose of shortening the travel time of the piston between its first and second positions is difficult to achieve. It is difficult to design and manufacture an actuator in which the snubbing portion of the stroke length is much reduced and which has repeatable and reliable snubbing performance, in view of the achievable tolerances of manufacturing, the natural expansion/retraction/settling and/or wear of parts during use and/or the sensitivity of the piston performance and snubbing characteristics to the smallest of changes in relative shape, size and position of the components of the actuator. The design of such an actuator must also take into account the variations in temperature to which an aircraft LG actuator might typically be exposed, e.g. ranging from below -40 to above +35 degrees Celsius, particularly as the flow characteristics of typical hydraulic fluids changes significant at extremely cold temperatures. Embodiments of the invention utilise differential snubbing, in both directions of travel, achieving a slow stop at the end of travel, but requiring less time for the piston to reach its unsnubbed phase of movement on retraction of the landing gear; as such, a notional time saving on landing gear retraction of the order of 0.5 seconds or more may be achieved.

[0007] It may be that movement of the piston along its stroke length is snubbed for a portion of the stroke length when proximate to, and moving in a direction towards, the first position, the snubbing being greater at a position closer to the first position (e.g. differential snubbing at both the end of the movement of the landing gear to its extended position, and at the movement of the landing gear from its extended position when retraction is initiated). Alternatively or additionally, it may be that movement of the piston along its stroke length is snubbed for a -4 -portion of the stroke length when proximate to, and moving in a direction towards, the second position, the snubbing being greater at a position closer to the second position (i.e. differential snubbing at both the end of the movement of the landing gear to its retracted position at the movement of the landing gear from its retracted position when extension is initiated).

100081 The actuator may be so arranged that the landing gear extends as the actuator extends. Alternatively, the actuator may be so arranged that the landing gear extends as the actuator retracts.

10009] It may be that one or more valves and/or a variable flow control device, for example a shuttling-flow control device or similar, could be used to provide differential snubbing. The snubbing of the movement of the piston is preferably provided by a snubbing device (for example a snubbing spear) which, in use, is received within a passageway. In use, snubbing may thus be provided in dependence on whether and how hydraulic fluid flows through such a passageway. Relative movement between the snubbing device and the passageway is dependent on the movement of the piston. For example, the snubbing device may be mounted on the end of the piston head, or being formed integrally therewith, and thus moves with the piston. Flow of hydraulic fluid may be restricted as the piston moves along the portion of its stroke length corresponding to a snubbing phase. It may be that the shape of the snubbing device and the shape of the passageway for receiving the snubbing device provide the different levels of snubbing; for example snubbing being provide as a result of the flow of hydraulic fluid being altered (e.g. restricted or prevented) as a result of the interaction between the shape of the snubbing device and the shape of the passageway, as the snubbing device enters and moves into the passageway.

100101 The snubbing device preferably has a fixed shape. The passageway associated with the snubbing device preferably has a fixed shape. Embodiments of the invention may thus provide differential snubbing without introducing new moving parts in the actuator (i.e. parts that are required to move relative to both the piston and cylinder during the movement of the piston). Using moving parts for snubbing movement in an aircraft landing gear actuator is not ideal, because the moving parts may stick, move out of positon, be susceptible to blockage and/or fail in other ways. If such a moving part were to fail open, such that it produces no snubbing, damage may be imparted to the actuator and associated structure which may lead to knock-on failures. If such a moving part were to fail closed, such that it continuously snubs or, worse, prevents the flow of hydraulic fluid by becoming obstructed, it may be that landing gear -5 -extension is compromised. A fixed shape snubbing device (for example in the form of a snubbing spear) does not have a fail open/closed failure mode due to its simple nature and thus does not involve as much risk as would using certain moving parts to provide variable snubbing. The use of additional moving parts in an actuator also complicates its design and manufacture and would increase cost.

100111 In use, there may be snubbing during a first phase of snubbing, greater snubbing during a second phase of snubbing, subsequent to the first phase of snubbing, and yet greater snubbing during a third phase of snubbing, subsequent to the second phase of snubbing.

[0012] The snubbing device may include a fluid channel arranged such that hydraulic fluid flows via the channel during a first phase of snubbing. It may be that the flow of hydraulic fluid via the channel is restricted or prevented during a second phase of snubbing. Snubbing during the second phase of snubbing may be greater than during the first phase. The channel may be formed in or on the snubbing device. Preferably, at least part of the channel is contained within the body of the snubbing device (e.g. the channel may be a closed surface channel rather than an open channel). A further fluid channel may be arranged such that hydraulic fluid flows via the further fluid channel during the first phase of snubbing and during the second phase of snubbing, but is restricted or prevented during a third phase of snubbing. Snubbing during the third phase may include snubbing at a level greater than the level of snubbing during the second phase. The level of snubbing may be judged by the average travel speed of the piston, when in situ, over a certain period of time (for example, a steady state speed of travel). The level of snubbing may be judged by a measure of the resistance to fluid flow in the piston, for example through the passageway which receives the snubbing device.

[0013] It may be that the snubbing device has one or more the fluid channels which extend within the body of the snubbing device from a hole (e.g. an outlet and/or an inlet) on an external surface of the snubbing device (e.g. also to a further hole, inlet/outlet on an external surface of the snubbing device). There may be multiple fluid channels each leading to a respective hole, for example there may be three or more outlets/inlets. It may be that there are multiple fluid channels which merge into fewer channels -there may for example be only a single inlet or only a single outlet. There may be a hole (e.g. an inlet and/or outlet) at or proximate to the end of the snubbing device (the "free end") first received by the passageway for receiving the snubbing device. (It will be understood that the term inlet and outlet presuppose a direction of fluid flow, and that the direction of flow may depend on whether the actuator is extending or -6 -retracting, but that in this case -when determining whether something is an outlet or an inlet -the direction of travel may be defined as that corresponding to the movement proximate to, and towards, the end position, whether that is the first or the second position). The snubbing device may comprise a central bore. The central bore may exit the snubbing device at or proximate to the free end of the snubbing device. The central bore may be in fluid communication (from within the snubbing device) with one or more holes on the exterior surface of the snubbing device that are axially spaced apart from the free end of the snubbing device and radially spaced apart from the central axis of the bore.

[00141 The exterior shape of the snubbing device may be generally cylindrical and therefore easier to machine than tapered shapes that might otherwise achieve differential snubbing. It will be appreciated that the tip of the snubbing device may be tapered, whilst the rest of the exterior shape of the snubbing device is cylindrical; and for the exterior shape of the snubbing device to still be considered as generally cylindrical (the tapering of the tip not being for the purpose of differential snubbing, but instead to ensure that the snubbing device is correctly received in its associated passageway in instances where play between parts and manufacturing tolerance might otherwise cause the tip to catch on the entrance to the passageway). Alternatively, or additionally, the entrance to the passageway may be flared, for similar reasons. It may be that the passageway for receiving the snubbing device has an interior surface which is generally cylindrical.

[00151 The present invention has particular application to a hydraulic actuator which is arranged to actuate retraction of the landing gear assembly, for example the whole / entire landing gear, from its extended configuration to its retracted configuration. The invention may have other applications however, for other hydraulic actuators for use on the landing gear assembly or other aircraft structures.

[00161 According to a further aspect of the invention there is provided a method of moving an aircraft landing gear from a deployed position to a stowed position. The aircraft landing gear may for example form part of the landing gear system, in accordance with any aspect of the present invention as claimed or described herein, possibly including any or all optional features relating thereto. There may be a first retraction phase, with snubbing. There may be a second retraction phase, subsequent to the first retraction phase, with reduced snubbing compared to first retraction phase. There may be a third retraction phase, subsequent to the second retraction phase, with reduced snubbing or no snubbing compared to second retraction phase. For example, the third retraction phase may correspond to an unsnubbed phase. It may be that the distance travelled by the landing gear (or the distance travelled by a piston of a retraction actuator which causes the landing gear motion) as a percentage of the total distance from the deployed position to the stowed position during the first retraction phase is about 1% or more of the total distance. It may be that the distance travelled by the landing gear (or the distance travelled by a piston of a retraction actuator which causes the landing gear motion) as a percentage of the total distance from the deployed position to the stowed position during the second retraction phase is about 2% or more of the total distance. At least 75% of the distance to be travelled may occur during the third retraction phase. The average speed, VI, of travel (for example of the LG or of the piston) during the first retraction phase, may be less than 75% of the average speed, V2, (possibly about half the speed of V2, or slower) during the second retraction phase. The average speed, V3, of travel during the third retraction phase may be twice Vi or more, optionally three times or more. It may be that there is a final (snubbed) extension phase, which corresponds to the same portion of travel as the first retraction phase, during which the average speed is within +/-40% (possibly within about +1-30%) of Vi (when measured under the same temperature conditions, those preferably being typical of average conditions). It may be that there is a further (snubbed) extension phase, which corresponds to the same portion of travel as the second retraction phase, during which the average speed is within +/-40% (possibly within about +1-30%) of V2. It may be that there is an unsnubbed extension phase, which corresponds to the same portion of travel as the third retraction phase, during which the average speed is within +/-40% (possibly within about +/-30%) of V3.

[00171 Vi may be about 20mms1 or less. V2 may be about 30mms-I or more. V3 is greater than V2: e.g. V3 may be greater than 150% of V2. V3 may be about 50mms4 or more It may be that V3 is greater than twice of V i.

[00181 It may be that the snubbing reduces during the second retraction phase such that, the average speed, V2a, of travel during the first half of the second retraction phase is less than the average speed, V2b, of travel during the second half of the second retraction phase, for example V2a may be 90% of V2b or lower (optionally 75% of V2b or lower).

[00191 The invention also provides a landing gear actuator. The actuator may be configured for use in a landing gear system, in accordance with any aspect of the present invention as claimed or described herein, possibly including any or all optional features relating thereto. The actuator may be configured for use in a method in accordance with any aspect of the -8 -present invention as claimed or described herein, possibly including any or all optional features relating thereto.

[0020] The actuator may be an aircraft landing gear retraction actuator. The actuator may be configured to provide differential snubbing at an end of its stroke length, for example with the use of an elongate snubbing member (e.g. a snubbing spear, lance or the like) having at least one fluid channel provided therein. The snubbing member may be so arranged that during a first snubbing phase, snubbing is provided by the snubbing member and hydraulic fluid flows via the fluid channel and so that during a second snubbing phase, for example immediately following the first snubbing phase, snubbing is provided by the snubbing member but the flow of fluid via the fluid channel is restricted or prevented (e.g. by the channel being closed off by a fixed surface of the actuator), thus providing greater snubbing than in the first snubbing phase. The snubbing member may be so arranged that is a third snubbing phase, with greater snubbing than in the second snubbing phase. The snubbing member may be so arranged that there is a phase in which no snubbing is provided by the snubbing member. There may be more than one fluid channel. There may be a first channel which extends between a first hole of the snubbing member and a second hole on the exterior of the snubbing member, the first and second holes being positioned at different positions along the length of the snubbing member (the length being parallel to the axis of the snubbing member). There may be a second channel which extends between a hole (for example the first hole of the first channel) and a hole (for example, a third hole -the third hole being positioned at a different position along the length of the snubbing member from both the first and second holes) on the exterior of the snubbing member. There may be at least one channel which links three or more holes located at the same (or substantially the same) distance along the length of the snubbing member. For example, a single channel may split into multiple channels at a location along the length of snubbing member. There may be a first ring of holes (for example three or more) at one location along the length of snubbing member and a second ring of holes (for example three or more) at different location along the length of snubbing member. The one or more channels in the snubbing member preferably include a section (e.g. a pipe-section) that is wholly contained within the body of the snubbing member (i.e. not an open channel).

[0021] The actuator may have a stroke length of 300mm or more (possibly 500mm or more). The actuator may include an associated accumulator; for example its own dedicated -9 -accumulator. The actuator may be provided separately from the hydraulic fluid that would normally be accommodated therein during use.

[0022] The landing gear system may be suitable for use on a commercial passenger aircraft, for example an aircraft suitable for transporting at least 50, for example at least 100, for example at least 200 passengers. For the purposes of the present specification the term commercial passenger aircraft also covers aircraft of an equivalent size configured for cargo and/or used on a non-commercial basis. The aircraft may have a maximum take-off weight (MTOW) of at least 20 tonnes, optionally at least 40 tonnes, and possibly 50 tonnes or more. The aircraft may have an operating empty weight of at least 20 tonnes, optionally at least 30 tonnes, and possibly about 40 tonnes or more.

[0023] The invention also provides an aircraft, or part therefor, comprising an apparatus in accordance with any aspect of the present invention as claimed or described herein, possibly including any or all optional features relating thereto.

[0024] The aircraft may comprise a power system, for example a hydraulic, electrical or other power system. The actuator may be connected to the power system to receive power therefrom. The power system may be connected to other aircraft systems for example one or more of the primary flight controls, secondary flight controls, braking system, cargo doors and others. In the case of a hydraulic power system, the system may comprise a liquid reservoir, one or more pumps, a manifold for distributing the liquid and a plurality of distribution lines connected to aircraft systems requiring hydraulic power.

[0025] The actuator, for example an end of the actuator, may be connected to the aircraft, for example to a mounting point, for example to a pin, located on the aircraft (i.e. not forming part of the landing gear assembly), for example located in a landing gear bay of the aircraft. Thus, a first end of the actuator may be connected to the landing gear (e.g. the main strut of the LG or structure attached to the main strut of the LG) and a second end of the actuator may be connected to the aircraft such that extension and/or retraction of the actuator causes the landing gear assembly to extend and/or retract. Movement, for example extension, of the actuator may cause the main strut to move away from the extended position towards the retracted position. Controlling the movement, for example the retraction, of the actuator may limit the speed at which the main strut moves away from the retracted position towards the extended position. [0026] The present invention also provides a method of manufacturing a snubbing device/member for use in the hydraulic actuator in accordance with any aspect of the present -10 -invention as claimed or described herein, possibly including any or all optional features relating thereto. The method may comprise machining a cylinder of material to form the main body of the snubbing device / member. The machining may comprise drilling a central bore. The machining may comprise drilling at least one side bore. It may be that a central bore and at least one side bore are machined to form a fluid channel extending within the cylinder of material between two holes on the exterior surface of the cylinder of material (for example from a hole aligned with the axis of the cylinder and one or more holes on the exterior circumference of the cylinder. The process of machining a cylinder of material (i.e. comprising a region of substantially constant external cross-section along its length) and/or drilling channels/holes therein is repeatable, allows for good tolerances and is a relatively straightforward manufacturing process to perform. It may also allow for a snubbing device design to be tuned to the snubbing characteristics desired, by means of for example choosing a particular diameter of drilling and/or the length and/or number of the bore(s).

100271 It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa

DESCRIPTION OF THE DRAWINGS

100281 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 is a front view of an aircraft according to a first embodiment of the invention, the aircraft including a retractable landing gear; Figure 2 is a side view of the aircraft shown in Figure 1; Figure 3 shows the retractable landing gear of Figure 1 in an extended configuration; Figure 4 shows the retractable landing gear of Figure 1 in a retracted configuration; Figures 5 to 11 show the hydraulic actuator, or part thereof, of Figure 1 during various stages of operation; -11 -Figure 12 is a cross-section along the length of a snubbing device used in the first embodiment; Figure 13 is a schematic sectional view across the length of the snubbing device of Figure 12; Figure 14 is a cross-section along the length of a snubbing device used in a second embodiment; Figure 15 is a schematic sectional view across the length of the snubbing device of Figure 14; Figure 16 is a flowchart illustrating the steps of a method of operating a landing gear in accordance with a third embodiment; Figure 17 is a graph showing displacement over time of a piston in an actuator used in the third embodiment; Figure 18 shows the retractable landing gear of an aircraft according to a fourth embodiment of the invention, with the landing gear in an extended configuration; Figure 19 shows the retractable landing gear of Figure 18 in a retracted configuration; and Figure 20 is a flowchart illustrating the steps of a method of making a snubbing spear in accordance with a fifth embodiment.

DETAILED DESCRIPTION

[0029] The embodiments generally relate to an aircraft landing gear ("LG") retracted by an actuator which is configured to provide snubbing (i.e. essentially damping / decelerating the motion) at the end of its stroke length at each end, so that there is snubbing both immediately before the LG is fully extended (i.e. fully deployed) and immediately before the LG is fully retracted (i.e. fully stowed). Such snubbing ensures that the LG does not come to an abrupt stop when it reaches its intended position when being retracted or extended. The embodiments utilise differential snubbing such that when coming to a stop, after unsnubbed motion, there is initially snubbing at a first level followed by snubbing at a greater level (more damping / slower speed). Similar snubbing characteristics are provided in reverse such that, on initiating retraction of the LG, there is snubbing at a greater level followed by snubbing at a lower level, -12 -which is then followed by unsnubbed motion. By reducing the amount of time the actuator travels whilst been subjected to maximum snubbing when coming to a rest, the time spent by the actuator at maximum snubbing at the start of the process of retracting the LG can be reduced, thus reducing the time taken to retract the LG. It is thus possible to reduce the time it takes to retract a landing gear into its bay, yet retain snubbing at the end of the actuator's movement on deployment. This can all be achieved without needing to affect the design of the rest of the actuator or landing gear. It is estimated that, for certain commercial aircraft, reducing retraction time by one second could enable maximum take-off weight to be increased up to 1% for critical airworthiness climb-out cases.

[0030] Figures 1 and 2 show an aircraft in accordance with a first example embodiment. The aircraft 1 comprises a fuselage 3 and wings 5. A nose landing gear (INLG") 2 is mounted on the fuselage 3 and a main landing gear ("1\TLG") 4 is mounted to each wing 5. Both NLG and IVILG are retractable into respective landing gear bays on the aircraft. All LG on the aircraft could utilise the benefits of the present embodiment, but the description that follows will refer by way of example only to a IVILG and its associated retraction actuator.

[0031] Figure 3 shows a dose up of a schematic view of a main landing gear 4 of the first embodiment in an extended configuration (i.e. deployed). For comparison, the position of some elements of the landing gear when the landing gear is retracted is indicated using dashed lines in Fig. 3. The main landing gear 4 is mounted on the aircraft 1 via a pintle 6 located at the upper end of a main strut 8. Two pairs of wheels 10 are mounted at the lower end of the main strut 8 on a bogie.

[00321 An actuator 12 is attached at one end to the main strut 8 and at the other end to the aircraft 1 at a point located within landing gear bay 13. Other links and support struts are provided to react loads sustained by the landing gear when extended and supporting the weight of the aircraft, and there are various lock and unlock mechanisms for locking/releasing the LG during use, but are not shown in Figures 3 and 4 for the sake of clarity. The actuator 12 is connected to hydraulic system 18 via hydraulic supply lines 20. It will be noted that the actuator is in a fully retracted configuration when the LG is extended fully.

[00331 Figure 4 shows a schematic view of the main landing gear 4 in the retracted configuration (i.e. fully stowed with its wheels 10 located within landing gear bay 13). In Fig. 4, the main strut 8 is rotated by about 90 degrees relative to its position in Fig. 3. It will be noted that the actuator is in a fully extended configuration when the LG is retracted fully.

-13 - [0034] In use, the landing gear 4 is released from the retracted position by unlocking a locking mechanism (not shown), for example prior to landing. The main strut 8 and the wheels 10 attached thereto drop under the action of gravity with the motion being determined by the flow of hydraulic fluid through the actuator 12. The landing gear 1 is locked in the extended position, for example using a locking actuator (not shown). The landing gear remains in the extended position during landing, taxiing and take-off. Following take-off the landing gear is unlocked and the actuator 12 moves the main strut from the extended position to the retracted position using force generated from pressure provided by the hydraulic system 18.

[0035] Figures 5 to 11 show the actuator 12 (in Figures 8 and 9 parts only of the actuator) at various stages during a deployment process (with retraction being essentially the same steps / stages performed in reverse). Figure 5 shows the actuator 12 and the ends of first and second hydraulic fluid lines 20a, 20b and two double-headed arrows 22, 24, which are included to show that fluid may be caused to flow in one direction or the other. Thus, the first fluid line 20a acts as a supply line when the actuator is retracting (LG extending) and the second supply line 20b acts as a return line; whereas, when the actuator is extending (LG retracting), the first fluid line 20a acts as a return line and the second supply line 20b acts as a supply line. The actuator 12 comprises a cylinder 26, in which a piston 28 travels. When the actuator is commanded to extend the LG, pressure is supplied via the first fluid line 20a, and flow ports 30b, 30c, and the piston 28 moves to the left (as shown in Figures 5 to 11) through the positions shown in Figure 6, 7, and 10 to the position shown in Figure 11 (with the actuator fully retracted and the LG fully extended). At the position shown in Figure 6, the rate of flow of hydraulic fluid is at its maximum (often termed 'free flow', unsnubbed flow' or 'unrestricted flow'). In this phase of extension, the piston speed may be calculated as ( Pi x R^2) / fluid flow rate, where R is the inner radius of the cylinder. The free flow portion of the actuator stroke may comprise about 75s71) to 80% of the total time of movement of the piston. The remaining motion is snubbed with the piston travelling over a proportionally much shorter distance. In free-flow, the piston travels at a speed of about 100mm/sec.

[0036] As shown in Figure 6, fluid flows into the cylinder 26 through ports 30b, 30c and out via port 30a. At the point at which the piston 28 reaches the position shown in Figure 7, a first phase of snubbing commences. The piston head 28a comprises a snubbing spear 40 which extends axially from the piston head 28a on the side opposite the piston rod 28b. The snubbing spear is received in passageway 42 which is connected to port 30a. At the positions shown in -14 -Figures 7 and 8 the snubbing spear 40 is partially received in the passageway 42. Figures 8 and 9 show magnified views of the piston head 28a and the snubbing spear 40. It will be seen that the snubbing spear 40 has formed in it a central bore, which extends from the tip of the spear and connects to a side bore which extends to a hole on the outer circumference of the spear (further explanation being provided below with reference to Figures 12 and 13). At the position shown in Figure 8, some fluid can pass along the gap between the exterior circumferential surface of the spear 40 and the internal circumferential surface of the passageway 42 and some fluid can flow through the side bore and central bore formed in the spear. As the piston retracts further (see the position shown in Figure 9), fluid can still pass along the gap between the exterior circumferential surface of the spear 40 and the internal circumferential surface of the passageway 42 but the fluid flow through the side bore is restricted as a result of the restriction provided by the internal circumferential surface of the passageway 42 effectively closing over the opening to the side bore. If the snubbing spear 40 and its associated passageway 42 are machined to be close-fitting then the effect of snubbing by the side bore being closed/restricted by the interior wall of the passageway 42 will be more pronounced. By the time the piston 28 has reached the position shown in Figure 10, full snubbing is in effect. Figure 11 shows the actuator 12 once fully retracted (LG fully extended/deployed).

100371 Figure 12 shows the snubbing spear 40 of the first embodiment in cross-section. The central bore 46 is shown extending from the tip (the top of Fig 12) along the axis of the spear. Three side bores 48 extend from the end of the central bore 46 opposite the tip of the spear, each side bore extending in a respective direction with both an axial component and a radial component emerging at holes 50 spaced equiangularly around the circumference of the spear. Only one side bore 48 is visible in Figure 12. Figure 13 is a schematic sectional view of the spear looking down the axis of the spear from a cone-shaped section represented by lines A-A in Figure 12. The three bores 48 linking to the central bore 46 can thus be seen in Figure 13. Whilst not shown in Figure 12 the diameter of the central bore 46 is larger than the diameter of each of the side bores 50.

100381 Figure 14 shows a snubbing spear 140 of a second embodiment in cross-section (see section X-X in Figure 15) and Figure 15 shows a down-the-axis view of section B-B of Figure 14. Figure 14 can be compared with Figure 12 and Figure 15 can be compared with Figure 13. The differences of the second embodiment as compared to the first embodiment will now be -15 -described. It will be seen that the spear 140 of the second embodiment includes three extra side bores 152 extending from the central bore 146, the three extra side bores 152 extending to holes 154 in the circumference that are spaced in the circumferential direction between the holes 150 of the other bores 148. The holes 154 of the three extra side bores 152 are positioned closer to the tip of the spear than holes 150. There is thus a first set of side bores 152 and a second set of side bores 148. The diameter of the first set may be different from the second set. The central bore 146 has a larger diameter than the first set and the second set of side bores. Thus, there is an extra level of snubbing provided: as the spear 140 enters its corresponding passageway (not shown in Figures 14 or 15) there is a first snubbing phase where fluid flows around the spear 140, via the first set of side bores 152 and via the second set of side bores 148. As the spear 140 moves further into its corresponding passageway, holes 154 are closed off by the passageway such that there is a second snubbing phase (with increased snubbing) where fluid flows around the spear 140 and via the second set of side bores 148, but not significantly via the first set of side bores 152. As the spear 140 moves yet further into the its corresponding passageway, holes 150 and 154 are closed off by the passageway such that there is a third snubbing phase (maximum snubbing) where fluid flows around the spear 140, but not significantly via either the first set of side bores 152 or via the second set of side bores 148.

100391 The actuator of the second embodiment thus enables LG retraction time to be reduced by means of an actuator that provides three phases of snubbing, each phase of snubbing being progressively less (the snubbing thus being such that movement is progressively more heavily damped on extension of the LG, from one snubbing phase to the next). This is achieved by a snubbing mechanism that avoids the use of separately moving parts to achieve snubbing (i.e. no more moving parts than snubbing using a convention snubbing spear). Snubbing characteristics of a snubbing phase and the snubbing profile (when one or more phases of snubbing start/stop) can be varied by varying the size, geometry and/or number of the channels (e.g. bores) within snubbing spear. A snubbing spear with internal channels can be tuned therefore (by designing the number of, and geometry of the internal channels accordingly) to meet a desired snubbing profile.

[0040] Figure 16 shows a flowchart illustrating a method 200 (according to a third embodiment) of operating an aircraft landing gear, with an actuator which could be of the type described in respect of the first embodiment. The method starts (represented by box 201) with -16 -the landing gear moving from its deployed position during a first retraction phase, with snubbing. There is then a second retraction phase (represented by box 202), subsequent to the first retraction phase, with snubbing but at a reduced level compared to first retraction phase. There is then (represented by box 203), a third retraction phase, subsequent to the second retraction phase, with no snubbing (i.e. free flow of fluid in the actuator). Figure 17 shows the steps, in graph form, performed in reverse (with the landing gear moving from its extended position to its deployed position), with the x-axis representing time, and the y-axis representing displacement (distance travelled) of the piston. The total stroke length is 1000mm. The first 950mm of motion (which may include snubbing at the start of the movement -not shown in Figure 17) takes about 11 seconds (from time to to ti). The maximum speed of travel of the piston is at about 100mm s-1. The next 25mm, from time ti to t2, has snubbing (and corresponds to the second retraction phase) -the speed being reduced to about 70mms-1. The last 25mm, from time t2 to t3, has increased snubbing, the average speed being reducing to about 35mm/sec. The snubbing phases together last about a second. This can be compared with a time of almost 11/2 seconds, if all of the snubbing were to be performed at a piston speed of about 35mm/sec (shown by the dashed line extending from time ti to time t4). When the steps are perform in the order shown in Figure 16 (i e. LG retraction) the time saved may be about second. Even a fraction of a second of time save when retracting a landing gear can have a significant impact on efficiency. Achieving such a time saving with little in the way of modification to the existing design of LG retraction actuators could provide a significant benefit to existing aircraft designs. The full process of retracting the LG (with unlocking/locking of locks, opening/closing of LG bay doors) takes about 15 seconds.

100411 Figures 18 and 19 show a main landing gear (MILG) 304 for an aircraft of a fourth embodiment. The differences between the first and fourth embodiments can be discerned by comparing Figure 3 with Figure 18 and comparing Figure 4 with Figure 19, but will also now be briefly described. Figure 18 shows the IVELG 304 in the extended position. For comparison, the position of some elements of the landing gear when the landing gear is retracted is indicated using dashed lines in Fig. 18. An actuator 312 is attached at one end of the IVILG 304 to structure of the N/ILG that is below (in Figure 18) the pintle 306 and at the other end to the aircraft at a point located within landing gear bay 313. The actuator 312 is connected to a hydraulic system 318, via lines 320, in a similar manner to the first embodiment. It will however be noted that the actuator is in a fully extended configuration when the LG is extended -17 -fully. Figure 19 (compare with Figure 4) shows a schematic view of the MLG 104 in the fully retracted configuration. In Fig. 19, the main strut of the IVILG is rotated by about 90 degrees relative to its position in Fig. 18. For comparison, the position of some elements of the landing gear when the landing gear is extended is indicated using dashed lines in Fig. 19. It will be noted that the actuator is in a fully retracted configuration when the LG is retracted fully. Differential snubbing with the use of a snubbing spear with internal flow channels may thus be utilised for the motion immediately leading up to, and for the motion immediately leading away from, the LG being in its retracted (e.g. stowed) configuration.

[0042] Figure 20 shows a flowchart illustrating a method 400 (according to a fifth embodiment) of manufacturing a snubbing device, for example one as shown in Figures 12 and 13. Thus, as a first step (box 401) a block of metal material (e.g. aluminium, stainless steel, alu-nickel-bronze alloy, or titanium) is provided and machined to form a cylindrical body, optionally with a tapered tip. As a second step (box 402) a central bore is drilled into the end of the generally cylindrical body. Then, as a third step (box 403) there are drilled three side bores, such that the central bore and each side bore forms a fluid channel extending within the body of material between a hole at the tip and a hole on an exterior circumference of the cylinder of material. The snubbing spear thus created may then be fixed to the piston head of the actuator in a conventional manner, for example by means of a bolted flange at the base attaching to the head with screws.

[0043] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

[0044] In Figure 2 two wheels 10 are shown in a diablo type arrangement, but in other embodiments further wheels may be included and/or may be used.

[0045] The embodiment(s) described above, relating to retraction of the entire landing gear, may have application to other types of LG actuators on the aircraft, or indeed actuators used to operate other moving parts on the aircraft [0046] Different arrangements of holes, bores and the like may be used to suit different snubbing requirements.

-18 - [0047] The speed of travel of the piston could decelerate gradually during the snubbing such that there are not separately discernible phases of snubbing, but that snubbing gradually increases the closer the piston is to its end of travel.

[0048] It will be appreciated that in certain embodiments, the extension/retraction speeds of the LG will be different. For example, landing gear retraction may be performed by pressurising the 'rod end' of the cylinder of the actuator, using the smaller area to exert a retraction force (a smaller volume to fill than if the 'earth end/piston end' of the cylinder were pressurised). Thus, it may be possible to retract a LG faster than it is extended, when considering the volume to fill divided by flow rate. An LG actuators might operate to extend the LG by pressurising both chambers (either side of the piston head) at the same time and allowing hydraulic fluid to recirculate, the recirculation of the fluid slowing the extension. In such arrangements, retraction times may be of the order of 12-15 seconds, while extension times might be slower, and of the order of 15-20 seconds.

[0049] Other methods of manufacture of the snubbing spear might be used. The snubbing spear may be integrally formed with the piston head of the actuator, for example by being milled or forged from block. It could be integrally formed with the piston rod, the rod being screwed onto/into the piston head and passing through the centre of the piston head, so that the tip of the rod forms the snubbing spear. Alternatively, the spear could be affixed into a slotted recess e.g. a bayonet type fixing, and retained with a screw, circlip or retaining ring; or be inserted through the opposite end of a hollow piston head and be retained by a mating jam-nut on the 'wet' side of the piston, the base of which would be sealed.

[0050] The bores need not necessarily be drilled.

[0051] 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. The term 'or' shall be interpreted as 'and/or' unless the context requires otherwise.

Claims

-19 -CLAIMS1. A landing gear system for an aircraft, wherein the system includes a retractable landing gear assembly, and a hydraulic actuator for actuating retraction of at least a part of the landing gear assembly from an extended configuration to a retracted configuration, the hydraulic actuator including a piston that travels within a cylinder along a stroke length between a first position corresponding to the extended configuration of said part of the landing gear assembly and a second position corresponding to the retracted configuration of said part of the landing gear assembly, and movement of the piston along its stroke length being snubbed for a portion of the stroke length when proximate to, and moving in a direction towards, one of the first position and the second position, and wherein the snubbing along said portion of the stroke length is greater at a position further along the portion in the direction of movement, whereby different levels of snubbing are provided as the piston moves towards at least one end of its stroke length.
2. A landing gear system according to claim 1, wherein the snubbing of the movement of the piston is provided by a snubbing device which is arranged to be received within a passageway, the relative movement between the snubbing device and the passageway depending on the movement of the piston, such that the flow of hydraulic fluid through the passageway is restricted as the piston moves along said portion of its stroke length.
3. A landing gear system according to claim 2, wherein the snubbing device is in the form of a snubbing spear.
4. A landing gear system according to claim 2 or claim 3, wherein the shape of the snubbing device and the shape of the passageway for receiving the snubbing device provide the different levels of snubbing.
-20 - 5. A landing gear system according to any of claims 2 to 4, wherein each of the snubbing device and the passageway for receiving the snubbing device has a fixed shape.
6. A landing gear system according to any of claims 2 to 5, wherein the snubbing device includes a fluid channel arranged such that hydraulic fluid flows via the channel during a first phase of snubbing, and wherein the flow of hydraulic fluid via the channel is restricted or prevented during a second phase of snubbing, which includes the snubbing at the greater level.
7. A landing gear system according to claim 6, wherein the snubbing device includes a further fluid channel arranged such that hydraulic fluid flows via the further fluid channel during the first phase of snubbing and during the second phase of snubbing, but is restricted or prevented during a third phase of snubbing, which includes snubbing at a level greater than the level of snubbing during the second phase.
8. A landing gear system according to claim 6 or claims 7, wherein at least one of the fluid channels of the snubbing device extends within the body of the snubbing device to a hole on an external surface of the snubbing device.
9. A landing gear system according to claim 8, wherein there are at least three such holes.
10. A landing gear system according to claim 8 or claim 9, wherein at least one of the fluid channels of the snubbing device extends within the body of the snubbing device from a hole at or proximate to the end of the snubbing device first received by the passageway for receiving the snubbing device.
11. A landing gear system according to any of claims 2 to 10, wherein the snubbing device comprises a central bore that extends from a hole at the end of the snubbing device first received by the passageway for receiving the snubbing device.
-21 - 12. A landing gear system according to any of claims 2 to 11, wherein the exterior shape of the snubbing device is generally cylindrical.
13. A landing gear system according to any preceding claim, wherein the snubbing device is arranged to provide snubbing during a first phase of snubbing, greater snubbing during a second phase of snubbing, subsequent to the first phase of snubbing, and yet greater snubbing during a third phase of snubbing, subsequent to the second phase of snubbing.
14. A landing gear system according to any preceding claim, wherein the hydraulic actuator is arranged to actuate retraction of the landing gear assembly from its extended configuration to its retracted configuration.
A hydraulic actuator configured for use as the hydraulic actuator of the landing gear system according to any preceding claim.
16. An aircraft landing gear retraction actuator configured to provide differential snubbing at an end of its stroke length with the use of an elongate snubbing member having at least one fluid channel provided therein and being arranged such that during a first snubbing phase, snubbing is provided by the snubbing member and hydraulic fluid flows via the fluid channel and such that during a second snubbing phase, immediately following the first snubbing phase, snubbing is provided by the snubbing member but the flow of fluid via the fluid channel is restricted or prevented, thus providing greater snubbing than in the first snubbing phase.
17. An aircraft landing gear retraction actuator according to claim 16, further being arranged such that during a third snubbing phase, immediately following the second snubbing phase, greater snubbing than in the second snubbing phase is provided by the snubbing member.
18. An aircraft landing gear retraction actuator according to claim 16 or claim 17, wherein said at least one fluid channel for providing differential snubbing includes a first channel which extends between a first hole of the snubbing member and a second hole on the exterior -22 -of the snubbing member and a second channel which extends between the first hole and a third hole on the exterior of the snubbing member, and wherein the first, second and third holes are positioned at different positions along the length of the snubbing member.
19. A method of manufacturing a snubbing device for use in the hydraulic actuator of claim 15 or for manufacturing an elongate snubbing member for use in the aircraft landing gear retraction actuator of any of claims 16 to 18, the method comprising machining a cylinder of material to form the snubbing device / member, the machining comprising drilling a central bore and drilling at least one side bore, the central bore and at least one side bore forming a fluid channel extending within the cylinder of material between a first hole on an exterior surface of the cylinder of material and a second hole on an exterior surface of the cylinder of material.
20. A method of moving an aircraft landing gear from a deployed position to a stowed position, the movement of the landing gear having at least a first retraction phase, with snubbing, a second retraction phase, subsequent to the first retraction phase, with reduced snubbing compared to first retraction phase, and a third retraction phase, subsequent to the second retraction phase, with reduced snubbing or no snubbing compared to second retraction phase.
21. A method according to claim 20, wherein at least 1% of the distance to be travelled by the landing gear as a percentage of the total distance from the deployed position to the stowed position occurs during the first retraction phase, at least 2% of the distance to be travelled by the landing gear as a percentage of the total distance from the deployed position to the stowed position occurs during the second retraction phase, and at least 75% of the distance to be travelled by the landing gear as a percentage of the total distance from the deployed position to the stowed position occurs during the third retraction phase, -23 -the average speed, VI, of travel during the first retraction phase, being less than 75% of the average speed, V2, during the second retraction phase, and the average speed, V, of travel during the third retraction phase being at least twice VI.
22. A method according to claim 21, wherein snubbing reduces during the second retraction phase such that, the average speed, V2a, of travel during the first half of the second retraction phase is less than 90% of the average speed, V2b, of travel during the second half of the second retraction phase.
23. An aircraft on which the landing gear system of any of claims 1 to 14 has been installed and/or on which a landing gear incorporating an actuator of any of claims 15 to 18 has been installed.