Abstract
Retraction of a landing gear assembly on an aircraft is actuated by a hydraulic actuator. The actuator includes a piston that travels within a cylinder 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. The movement of the piston along its stroke length is snubbed at one end by a different amount according to the direction of travel, for example by use of an orifice plate that has a discharge coefficient that is greater in one direction than in the opposite direction. Asymmetric snubbing is thus provided, which enables the landing gear to retract faster.
Claims
1. 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, if any, is less when moving in the opposite direction, there thus being provided asymmetric snubbing, the asymmetric snubbing being provided by the flow of hydraulic fluid through a passageway, there being more snubbing for fluid flow in one direction than in the opposite direction, as a result of the shape of the passageway.
2. A landing gear system according to claim 1, wherein the shape of the passageway is asymmetric along its length, and the shape when there is snubbing with fluid flowing in one direction is the same as the shape when there is snubbing with fluid flowing in the opposite direction.
3. A landing gear system according to claim 1, wherein the passageway has a shape between its extreme ends which reduces to a minimum cross-sectional area in one direction along the passageway faster than it reduces to the same minimum cross-sectional area in the opposite direction.
4. A landing gear system according to claim 1, wherein the passageway has a cross-sectional shape which tapers in one direction.
5. A landing gear system according to claim 1, wherein the passageway has a cross-sectional shape which presents a sharp edge to the flow of fluid in one direction.
6. A landing gear system according to claim 3, wherein a portion of the passageway has a shape extending from a position at or proximate to one of its extreme ends having a constant cross-section which has an area equal to the minimum cross-sectional area.
7. A landing gear system according to claim 1, wherein a portion of the passageway comprises a structure with one or more perforations through which the hydraulic fluid flows.
8. A landing gear system according to claim 1, wherein the passageway has a shape corresponding to a discharge coefficient of 0.9 or more in a flow direction corresponding to the snubbing, if any, when the piston is moving in said opposite direction.
9. A landing gear system according to claim 1, wherein the passageway has a shape corresponding to a discharge coefficient of 0.7 or less in a flow direction corresponding to the greater snubbing of the piston.
10. A landing gear system according to claim 1, wherein the hydraulic actuator is arranged to actuate retraction of the landing gear assembly from its extended configuration to its retracted configuration.
11. A landing gear system according to claim 1, wherein the landing system comprises a main strut at an extreme end of which there are mounted one or more aircraft wheels.
12. A method of operating an aircraft landing gear which is movable between a deployed position suitable for supporting a portion of the weight of the aircraft on the ground and a stowed position, the method comprising the steps of moving the landing gear from the stowed position to the deployed position, the last 5% of the distance to be travelled by the landing gear as a percentage of the total distance from the stowed position to the deployed position being at an average speed, V.sub.1, that is lower than the average speed, V.sub.2, for the middle 90% of the distance, and then moving the landing gear from the deployed position to the stowed position, the first 5% 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 being at an average speed, V.sub.3, that is greater than V.sub.1 by at least 10%.
13. A method according to claim 12, wherein V.sub.1<50% of V.sub.2, and V.sub.2>V.sub.3>120% of V.sub.1.
14. An aircraft landing gear retraction actuator configured to provide asymmetric snubbing at an end of its stroke length with the use of an orifice plate that has a discharge coefficient that is greater in one direction than in an opposite direction.
15. A hydraulic actuator configured for use as the hydraulic actuator of the landing gear system according to claim 1.
16. An actuator according to claim 14, wherein the actuator has a stroke length of 300 mm or more.
17. An aircraft comprising the landing gear system of claim 1.
18. An aircraft comprising the landing gear retraction actuator of claim 14.
Description
DESCRIPTION OF THE DRAWINGS
[0023] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:
[0024] FIG. 1 is a front view of an aircraft according to a first embodiment of the invention, the aircraft including a retractable landing gear;
[0025] FIG. 2 is a side view of the aircraft shown in FIG. 1;
[0026] FIG. 3 shows the retractable landing gear of FIG. 1 in an extended configuration;
[0027] FIG. 4 shows the retractable landing gear of FIG. 1 in a retracted configuration;
[0028] FIGS. 5 to 8 show the hydraulic actuator of FIG. 1 during various stages of operation;
[0029] FIGS. 9 to 11 show examples of orifice plates for use in an actuator for the purposes of providing snubbing,
[0030] FIGS. 12 and 13 show fluid flow through the orifice plate of FIG. 10;
[0031] FIG. 14 is a flowchart illustrating the steps of a method of operating a landing gear in accordance with a second embodiment;
[0032] FIG. 15 shows the retractable landing gear of an aircraft according to a third embodiment of the invention, with the landing gear in an extended configuration; and
[0033] FIG. 16 shows the retractable landing gear of FIG. 15 in a retracted configuration;
DETAILED DESCRIPTION
[0034] 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 asymmetric snubbing such that at the start of moving the LG away from the retracted position (and/or extended position) there is less or no snubbing, compared to the case in reverse—i.e. at the end of the movement of the LG to the retracted position (and/or extended position). 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 takeoff weight to be increased up to 1% for critical airworthiness climb-out cases.
[0035] FIGS. 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 (“NLG”) 2 is mounted on the fuselage 3 and a main landing gear (“MLG”) 4 is mounted to each wing 5. Both NLG and MLG 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 MLG and its associated retraction actuator.
[0036] FIG. 3 shows a close 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.
[0037] 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 FIGS. 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 extended configuration when the LG is extended fully.
[0038] FIG. 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). For comparison, the position of some elements of the landing gear when the landing gear is extended is indicated using dashed lines in FIG. 4. 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 retracted configuration when the LG is retracted fully.
[0039] 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.
[0040] FIGS. 5 to 8 show the actuator 12 at various stages during the retraction process. FIG. 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 and the second supply line 20b acts as a return line; whereas, when the actuator is extending, 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 second fluid line 20b, and flow ports 30a, 30b, and the piston 28 moves to the left (as shown in FIGS. 5 to 8) through the position shown in FIG. 6 to the position shown in FIG. 7. At the position shown in FIG. 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×R{circumflex over ( )}2)/fluid flow rate, where R is the inner radius of the cylinder. The free flow portion of the actuator stroke may comprise about 75% 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 100 mm/sec.
[0041] As shown in FIG. 6, fluid flows into the cylinder 26 through ports 30a, 30b and out via ports 30c, 30d. At the point at which the piston 28 reaches the position shown in FIG. 7, port 30c is closed by the piston 28, thus forcing all return fluid to flow via port 30d. The port 30d includes a specially shaped orifice plate, which provides for asymmetric snubbing. During the extension phase, full snubbing is provided by the orifice plate in port 30d. In this fully snubbed portion of the stroke the piston travels at a reduced speed of about 30 mm/sec, covering the last 50 mm of the movement in about 1½ seconds. FIG. 8 shows the piston 28 in the full extended position corresponding to the extended configuration of the LG. When the LG is commanded to retract, reduced snubbing (as compared to the snubbing provided during the extension phase) is provided by the orifice plate in port 30d, as a result of its asymmetric shape. The piston travels at a speed of about 50 mm/sec, still slower than free-flow, but significantly faster than full snubbing in the reverse direction. At about 1 second, the time taken for the piston to move the first 50 mm of its 1,000 mm stroke length from the fully extended position is about 0.5 seconds quicker than the time taken for the piston to move the last 50 mm of its 1,000 mm stroke length on approaching the fully extended position. The time taken to retract the LG may thus be considered to have been shortened by ˜0.5 seconds. The time during which the actuator moves, when retracting the LG, is about 10 to 12 seconds or so, and the full process of retracting the LG (with unlocking/locking of locks, opening/closing of LG bay doors) takes about 15 seconds.
[0042] The asymmetric snubbing of the movement of the piston being provided by a fixed shape orifice plate will now be explained briefly with reference to FIGS. 9 to 13. The velocity of fluid and therefore of the piston, in the snubbed phase, is dominated by the discharge coefficient (or Cd) of the passageway through the orifice plate.
[0043] The discharge coefficient Cd can be calculated by using the formula:
Cd=Mf/(A√2×ρ×ΔP), where
Mf is the mass flow rate, ρ is the fluid density, A is the cross-sectional area of the narrowest point passage, and Δ P is the pressure drop across the narrowest point passage.
[0044] FIG. 9 shows a simple symmetrical sharp-edged orifice plate in the form of a disc with a hole in the centre. Such discs are very simple in design and can be ‘tuned’ to particular flow rates for a given fluid and temperature by adjusting the size of the hole. Thus, the orifice plate of FIG. 9 has a passageway (orifice) having a fixed cross-section and a symmetrical Cd of about 0.6 in both directions. FIG. 10 shows a first asymmetric passageway of the office plate of a type used in the first embodiment. The passageway comprises a section of tapering passageway with a linear chamfer, such that the cross-sectional area reduces linearly with increasing distance along the tapered passage from the widest point, at one side of the orifice plate, to the narrowest point near the middle of the passageway though the plate. The passageway comprises a section of passageway with a constant cross-section that extends from one side of the orifice plate to the narrowest point of the tapered passage. The narrowest part has a diameter of the order of 1 mm or so. The passageway is thus relatively easy to machine. Viewed in FIG. 10, the Cd of the orifice the Cd of the orifice plate for fluid flowing from left to right is about 0.90, whereas the Cd of the orifice plate for fluid flowing from right to left is about 0.60. FIG. 11 shows an alternative example of an asymmetric passageway of an orifice plate of a type suitable for use in an embodiment of the invention. The passageway comprises a section of tapering passageway with a curved chamfer (or a radius profile), such that the cross-sectional area reduces progressively slower with increasing distance along the tapered passage from the widest point, at one side of the orifice plate, to the narrowest point near the middle of the passageway though the plate. The passageway comprises a section of passageway with a constant cross-section that extends from one side of the orifice plate to the narrowest point of the tapered passage. Viewed in FIG. 11, the Cd of the orifice plate for fluid flowing from left to right is about 0.98, whereas the Cd of the orifice plate for fluid flowing from right to left is about 0.60.
[0045] FIGS. 12 and 13 show schematically why the Cd of the orifice plate can differ according to the direction of flow. The orifice plate of FIG. 10 is shown, firstly in FIG. 12 in a fully snubbing mode where fluid flows upwards (in FIG. 12) corresponding to Cd=0.60 and secondly in FIG. 13 in a reduced snubbing mode where fluid flows downwards (in FIG. 13) corresponding to Cd=0.90. In FIG. 12, the fluid initially “sees” a sharp edge and a very rapid reduction in cross-sectional area as the fluid enters the orifice plate. The cross-sectional area reduces almost immediately to its minimum value. The smooth low-friction laminar flow immediately preceding the orifice plate is immediately disrupted resulting in inefficient fluid flow through the plate in the direction of flow shown in FIG. 12. In FIG. 13, however, the fluid is initially presented with a smoother edge and a larger (wider diameter) hole, followed by a more gradual reduction in cross-sectional area to the minimum area which is not encountered until the fluid is almost halfway through the orifice plate. The smooth low-friction laminar flow immediately preceding the orifice plate is not affected so much, thus resulting in more laminar flow overall than in FIG. 12, and therefore better (less restricted) flow of fluid through the plate in the direction of flow shown in FIG. 13.
[0046] FIG. 14 shows a flowchart illustrating a method 100 (according to a second embodiment) of operating an aircraft landing gear, which could be of the type described in respect of the first embodiment. The method starts (represented by box 101) with the landing gear in its stowed position. Then (box 102) the LG is moved to the deployed position. The total stroke length is 1000 mm. The first 50 mm (snubbed) takes 1.5 seconds. The next 900 mm takes 9 seconds. The last 50 mm takes 1.5 seconds. Thus, the last 5% of the distance to be travelled by the landing gear is at an average speed, V.sub.1, of 33 mms.sup.−1. This is lower (less than half of) the average speed, V.sub.2, for the middle 90% of the distance, being 100 mms.sup.−1. The method then includes a step (box 103) of moving the landing gear from the deployed position to the stowed position, the first 50 mm of movement happening within 1 second, corresponding to an average speed, V.sub.3, of about 50 mms.sup.−1, thus being greater than V.sub.1 by at about 50%.
[0047] FIGS. 15 and 16 show a main landing gear (MLG) 204 for an aircraft of a third embodiment. The differences between the first and third embodiments can be discerned by comparing FIG. 3 with FIG. 15 and comparing FIG. 4 with FIG. 16, but will also now be briefly described. FIG. 15 shows the MLG 204 in the extended position. An actuator 212 is attached at one end of the MLG 204 to structure of the MLG that is above (in FIG. 15) the pintle 206 and at the other end to the aircraft at a point located within landing gear bay. The actuator 212 is connected to a hydraulic system 218 in a similar manner to the first embodiment. It will however be noted that the actuator is in a fully retracted configuration when the LG is extended fully. FIG. 16 (compare with FIG. 4) shows a schematic view of the MLG 104 in the fully retracted configuration. In FIG. 16, the main strut of the MLG is rotated by about 90 degrees relative to its position in FIG. 15. It will be noted that the actuator is in a fully extended configuration when the LG is retracted fully. Asymmetric snubbing with the use of an asymmetric orifice plate may thus be utilised for the motion immediately leading up to, and away from the LG being in its retracted configuration.
[0048] 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.
[0049] In FIG. 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.
[0050] 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.
[0051] The orifice plate could additionally or alternatively comprise a perforated disc with more than one perforation through which the hydraulic fluid flows.
[0052] 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.
