GB2040391A

GB2040391A - Valving in aircraft undercarriage shock absorbers

Info

Publication number: GB2040391A
Application number: GB8000968A
Authority: GB
Original assignee: Messier Bugatti SA; Messier Hispano SA
Current assignee: Messier Hispano SA; Safran Landing Systems SAS
Priority date: 1979-01-18
Filing date: 1980-01-11
Publication date: 1980-08-28
Also published as: GB2040391B; FR2472699A1; IT1129524B; IT8019236A0; FR2472699B1; DE3001169A1

Abstract

In a combined shock absorber jack assembly for an aircraft undercarriage, a sleeve 32 fixed within the end of a hollow piston rod 3 which is slidable within a cylinder 2 has a radial passage 50 opening outwardly of the sleeve 32 into an annular hydraulic fluid chamber 52 defined between the piston rod 3 and the sleeve 32 and communicating with the hydraulic fluid chamber 9 of the cylinder 2. A tubular valve member mounted within the sleeve 32 has two portions 60, 58 of different outside diameters for cooperating with two portions 49, 48 of the sleeve 32 of corresponding different inside diameters to form a differential piston and is acted upon by resilient means 64, the radial passage 50 being closed inwardly of the sleeve 32 by the portion 58 of the tubular valve member 56. In the event of a heavy landing of the aircraft producing forces driving the piston rod 3 into the cylinder 2 which are higher than the normal, the pressure applied to a shoulder 66 of the differential piston displaces the tubular valve member 56 against the force of the resilient means 64 to permit a supplementary flow of hydraulic fluid from the chamber 9 towards a gas chamber 15 in the hollow piston rod 3 such supplementary flow being additional to that which occurs through a permanent compression-mode throttle orifice 44. The cylinder 2 is also the rod of the undercarriage retraction jack cylinder (not shown). <IMAGE>

Description

SPECIFICATION Improvements in damper jack assemblies The invention relates to a damper jack assembly for an aircraft undercarriage particularly for a light aircraft or helicopter, and which are more particularly suited for use on undercarriage trains which are of the type referred to as the pivoted lever type. The damper jack assembly can be disposed either in a substantially vertical position, in which case the damper function is provided in a compression mode under the load applied, or in a substantially horizontal position, in which case the damper function is provided in an expansion mode under the applied loading.

Such arrangements make it possible simultane only to provide for a damper function, when the aircraft lands and/or taxies on the ground, the arrangement carrying the static load formed by the weight of the aircraft, when the latter is stationary on the ground, and also providing for the raising (or lowering) function for the undercarriage, when the aircraft is in flight.

In the specification of our British Patent Application No 3597/77, (Serial No 1563273), we proposed a design for a damper jack assembly which, in addition to the two functions described above, also provides the function of "dropping" of the aircraft, and return from the dropped position to the normal position in which the undercarriage is down, by means of a hydraulic actuating arrangement. Dropping is an operation comprising lowering the aircraft, while resting on its wheels, below its normal position, so as to facilitate certain operations or so as to improve certain characteristics of the aircraft. Dropping makes it possible for example to facilitate loading and transporting of a helicopter in the body of a cargo aircraft.It also makes it possible to improve the stability of a helicopter when resting on the deck of a helicopter carrier, an aircraft carrier or other ship, or even on a sea or off-shore platform. It can also be used to position a helicopter on the ground in a configuration which reduces the dangerous effects of ground resonance. It has been found that it is highly attractive for a helicopter to be provided with an undercarriage train which permits lowering movement to a dropped position.

The above-mentioned patent specification discloses a shock absorber unit of the piston-andcylinder kind for use in the landing gear of an aircraft, by means of which unit the landing gear can adopt a "retracted", "down" or a "dropped" position, and comprising a cylinder, a piston rod and a shock absorber rod which encloses a chamber for compressed gas and is provided with a valve having a throttling orifice for limiting the rate of flow of damping fluid, wherein the shock absorber rod is mounted to slide, in a fluid-tight manner, in a first chamber, filled with the damping fluid, of the cylinder or in an alternative mode of the piston rod, to form an oleo-pneumatic shock absorber; the piston rod is mounted to slide, in a fluid-tight manner, in the cylinder and is solidly connected to a piston head which, with the cylinder, defines a descent chamber which is full of fluid when the landing gear is in the "down" position but which is empty of fluid in the "dropped" and "retracted" positions; the unit also comprises a lifting piston to load the shock absorber and mounted to slide within a second chamber of the cylinder or in said alternative mode of the piston rod, so defining a lifting chamber which is full of fluid in the "retracted" position but empty of fluid in the "down" and "dropped" positions; a hydraulic locking valve having a fluid pressure controlled opening, for controlling flow of fluid to and from the lifting chamber and a locking device for locking the piston rod in relation to the cylinder in the "down" position, said locking device being releasable by application of a hydraulic pressure to allow retraction of the landing gear by hydraulic pressurisation of the lifting chamber and also allowing voiding of the descent chamber if the landing gear was previously in the "down" position, thereby to move the landing gear into the "retracted" position; application of hydraulic pressure to release the locking device, when the landing gear is in the "down" position with the aircraft on the ground, causing the descent chamber to be voided by the action of the weight of the aircraft to move the landing gear into the "dropped" position; and movement of the landing gear from both the "dropped" position and the "retracted" position to the "down" position being effectable by applying pressure simultaneously to the descent chamber and to said hydraulic locking valve, so as to open the hydraulic locking valve.

The patent specification further discloses that the locking device for the piston rod relative to the cylinder in the "down" position may be either mechanical of the kind comprising pawl members which are capable of engaging behind a projection and, optionally, a cover means for locking the pawl members, or the locking device may be purely hydraulic of the kind comprising a hydraulic locking valve which has a controlled opening action, of any suitable known kind, which provides for pressurising then isolating the lowering chamber of the shock absorber unit.Where the locking device is purely hydraulic, it is possible to provide, parallel to said controlled opening hydraulic locking valve, or on a supplementary orifice which opens into the lowering chamber, an over-pressure valve for the unit, acting as an "anti-crash" valve by opening in the event of a landing under emergency conditions, so that the piston can expel over its entire travel the hydraulic fluid which fills the lowering chamber. The unit then operates as a damper with extended travel movement, in the event of an emergency landing, by absorbing a high degree of energy upon impact, by means of its jack.

The unit can operate in a compression mode under the applied load, by providing the first and second chambers are provided in the piston rod, and that the movements of the raising piston under the effect of the raising pressure load the unit by means of a rod which is held in abutment on the damper rod. In order to ensure that the oscillations in load to which the damper rod is subjected are not transmitted to the raising piston, this arrangement provides that said rod carries an abutment which is displaced in a chamber containing free air, which is provided within the damper rod.

Finally, so that the unit acts like an accumulator for expansion of the hydraulic fluid, the second chamber can be in communication with the free air by means of a passage provided in the rod and in the raising piston.

The above-mentioned patent also seeks to simplifythe structure of such unit, while improving the efficiency of the arrangements provided for absorbing expansion of the hydraulic fluid and the characteristics in the damping mode.

For this purpose, the unit disclosed in the patent specification may comprise a supplementary chamber which is adjacent to the lowering chamber from which it is separated by means of at least one separating piston which is returned by resilient means against a first abutment so as to limit the volume of the lowering chamber, and which is capable of being displaced against the force of the resilient means under the effect of expansion of the hydraulic fluid of the lowering chamber, so that said supplementary chamber forms a chamber for accumulating hydraulic fluid expansion.

In one embodiment, the expansion accumulating chamber contains a gas under a pressure, which forms a pneumatic spring, for applying the separating piston against the first abutment, and, at the same time, a supplementary damping chamber whose effects are added to those of the oleopneumatic damper of the unit. The unit of this embodiment withstands only a maximum excess pressure in the hydraulic fluid, which corresponds to a vertical landing speed of from 5 to 6 metres per second.

It is among the aspects of the present invention to provide a damper jack assembly for an aircraft undercarriage which is capable of withstanding an over-pressure in the hydraulic fluid, which corresponds to a vertical landing speed of up to 12 metres per second, while providing a good damping action for normal vertical landing speeds, which are of the order of 3 metres per second. For this purpose, the damper must comprise an over-pressurevalve wherein the section of the flow orifices must be controlled by the pressure obtaining within the hydraulic fluid.

According to the invention there is provided a damper jack assembly for an aircraft undercarriage comprising a cylinder, a jack rod, a damper rod which contains a pressurised gas chamber and a valve provided with at least one compression mode throttle orifice, the damper rod is mounted slidably and sealingly in a chamber, filled with hydraulic fluid, of the jack rod to form an oleo-pneumatic damper, the jack rod is mounted slidably and seal ingly in said cylinder and is fixed with respect to a jack piston which, with said cylinder, defines a lower- ing chamber which can control lowering of an air craft undercarriage when the lowering chamber is supplied with hydraulic fluid, the damper jack assembly includes a sleeve which is generally cylin drical, is fixed within the end of the damper rod which is within the jack rod and has a radial passage therein, the radial passage opens outwardly of the sleeve into an annular hydraulic fluid chamber defined between the damper rod and the sleeve and communicating with the hydraulic fluid chamber of the jack rod, the assembly includes a tubular valve member mounted within the sleeve, having at least two portions of different outside diameters for cooperating with two portions of the sleeve of corresponding different inside diameters to form a differential piston and acted upon by resilient means, the radial passage is closed inwardly of the sleeve by the portion of the tubular valve member of smaller outside diameter, so that, when forces driving the damper rod into the jack rod are higherthanthe normal damper operating forces, the pressure applied to the differential piston displaces the tubular valve member against the force of the resilient means to permit a supplementary flow of hydraulic fluid from the chamber of the jack rod towards the gas chamber, such supplementary flow being additional to that which occurs through the compression-mode throttle orifice.

In a first embodiment of the invention, the sleeve, at its end which is towards the jack rod, has an end portion which, with the sleeve, forms the piston of the damper rod, and in which end portion the permanent throttle orifice is provided, an expansionmode braking valve comprising an apertured disc diaphragm being retained between said end portion and the smaller inside diameter portion of the sleeve, which portion forms an abutment for limiting the lifting movement of the expansion-mode braking valve above the compression-mode throttle orifice.

In accordance with a second embodiment of the invention, the sleeve is apertured with a second radial passage which opens outwardly of the sleeve into said annular chamber and inwardly of the sleeve onto a facing portion of the tubular valve member, having a first radial aperture forming the permanent compression-mode throttle orifice, the tubular valve member also having a second radial aperture which is larger in section than said radial aperture forming the permanent throttle orifice, so that, when the tubular valve member is displaced, the second radial aperture and the permanent throttle orifice are both in communication with said second radial passage in the sleeve.

It is desirable to reduce the dangerous effects of ground resonance which generallyoccurwhen the aircraft is at the lift limit, either at the moment that the undercarriage trains are about to leave the ground or when they have only just touched the ground. The pressure of the hydraulic fluid within the damper jack assembly then corresponds to aver- tical speed of the order of 0.3 metre per second, that is to say, it is much lower than the normal pressure upon landing, which corresponds to a vertical speed of 2 to 3 metres per second.

For this purpose, the sleeve is apertured with a second radial passage which opens outwardly of the sleeve into the annular chamber and opens inwardly of the sleeve onto a facing portion of the tubular valve member having a fine aperture forming a throttle orifice for the hydraulic fluid at pressures corresponding to low vertical speeds which occur at the lift limit of the aircraft, the tubular valve member also having a second radial aperture which is larger in section than the fine aperture and which forms the permanent compression-mode throttle orifice, and a large radial apertu re, i.e. larger in section than the second aperture, the second and the large apertures being so positioned that, when the tubular valve member is displaced under the action of a pressure higher than that which occurs at the lift limit, the second aperture moves into a position facing the second passage in the sleeve, and that, when the tubular valve member is displaced under the action of a pressure higher than the normal landing pressures, the second and large apertures are both facing the second passage in the sleeve.

The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Figure 1 shows a view in longitudinal section of the damper portion of a damper jack assembly for an aircraft undercarriage according to a first embodiment of the invention, in the position which it occupies when the undercarriage train is down and the damper is not loaded; Figure 2 shows a second embodiment of the damper portion of a damper jack assembly for an aircraft undercarriage according to the invention; and Figure 3 shows a third embodiment of the damper portion of a damper jack assembly for an aircraft undercarriage according to the invention.

Only the damper portion of the damper jack assemblies will be described hereinafter. As regards the jack portion of the assembly, reference may be made for example to Figure 7 of the abovementioned patent specification.

The damper jack assembly of the embodiment of Figure 1 comprises a jack rod 2 which is capable of sliding sealingly in a jack cylinder (not shown), and a damper rod 3. The jack rod 2 is fixed with respect to an extension sleeve 2' which is provided to guide the damper rod 3. A rod 19 which is connected to a piston for raising the undercarriage train and retracting a damper (not shown) is retained by an upper abutment 21 and is capable of sliding within a chamber 15 which contains a gas under pressure. A chamber 9 within the jack rod 2, the lower portion of the chamber 15, and the annular volume between said two chambers, contain a hydraulic fluid.At its end which is inside the jack rod 2, the damper rod 3 is provided with an annular abutment 30 which comes to bear against the end of the extension sleeve 2' which is within the jack rod 2, to limit outward movement of the damper rod 3.

Within the end of the damper rod 3 which is within the extension sleeve 2' is fixed a sleeve 32 which is generally cylindrical in shape. Extension of the rod 19 is limited by the cooperation of an annular abutment 34 fixed in the upper end of the sleeve and a cup-shaped abutment 36 fixed by means of a nut 38 to the end of the rod 19 which is outside the jack rod 2. Fixed to the other end of the sleeve 32 is an end portion 40 forming the piston of the damper rod 3.

The damper rod 3 is secured with respect to the annular abutment 30 by means of bolts 42 so that the damper rod 3, the sleeve 32, the annular abutment 30 and the end portion 40 form a unitary assembly which is slidable relative to the unitary assembly formed by the jack rod 2 and the extension sleeve 2'.

Provided in the end portion 40 is a permanent compression-mode throttle orifice 44 behind which an expansion-mode braking valve 46 is freely slidably mounted, being formed by an annular apertured disc forming a calibrated diaphragm. The valve 46 can move between the end portion 40 and an internal annular shoulder 48 which is defined on a portion of the sleeve 32 of smaller inside diameter.

The inside end of the sleeve 32, above the shoulder 48, comprises a bearing surface 49 which is of a greater inside diameter than the diameter of the shoul der48.

The sleeve 32 is apertured with a radial passage 50 which opens, outwardly of the sleeve, into an annular hydraulic fluid chamber 52 defined between the damper rod 3 and the sleeve 32 and in communication with the hydraulic fluid chamber 9 of the jack rod 2, by means of a longitudinal aperture 54 provided in the sleeve 32.

A tubular valve member 56 is mounted slidably within the sleeve 32 and has a lower or tail portion 58 whose outside diameter is equal to the inside diameter of the shoulder 48, and a central portion 60 whose outside diameter is equal to the diameter of the bearing surface 49. The tubular valve member 56 is urged into a position of abutment against a shoulder 62 which is directed towards the outer end of the damper rod 3, by a helicoidal compression spring 64 which extends between the tubular valve member 56 and the annular abutment 34.

The portions 58 and 60 of the tubular valve member 56 cooperate with the inside wall surface of the portions 48 and 49 of the sleeve, forming a differential piston to which the hydraulic fluid pressure is applied, through the radial passage 50.

The damperjack assembly shown in Figure 1 operates in the following manner: when the undercarriage is subjected to normal forces, for example in the event of a landing at a vertical speed which does not exceed 3 metres per second, the pressure obtaining in the chamber 9 is insufficient to remove the tubular valve member 56 from its seat on the shoulder 62. The radial passage 50 therefore remains closed by the tail portion 58 of the tubular valve member 56. The hydraulic fluid therefore flows from the chamber 9 to the chamber 15 by way of the permanent throttle orifice 44 and then by way of a gap between the annular abutment 34 and the cupshaped member 36, following movement of the rod 19 into the chamber 15.The permanent throttle orifice 44 is so calibrated as to provide for a correct flow of fluid and therefore a good damping action on the force applied to the damper jack assembly.

In the event of an emergency landing, the overpressure in the chamber 9 which results therefrom acts on a shoulder 66 of the tubular valve member 56 of the above-defined differential piston and displaces the tubular valve member 56 against the force of the spring 64, opening the radial passage 50. The hydraulic fluid can then flow more easily through the permanent throttle orifice 44 and the radial passage 50.

This can provide a good damping action in regard to vertical landing speeds of up to 12 metres per second.

In the embodiment of Figure 2, like references are used to denote like parts to those described with reference to Figure 1. The sleeve 32 is apertured with a second radial passage 70 which opens outwardly of the sleeve 32 into the annular chamber 52 and inwardly of the sleeve 32 onto a facing portion of the tubular valve member 56 which has a first radial aperture 72 therein of relatively small diameter, forming the permanent compression-mode throttle orifice, so that there is no requirement for forming such an orifice in the end portion 40 of the sleeve.

The tubular valve member 56 also has a second radial aperture 74 therein which is larger in section than the first radial aperture 72. It will be noted that the expansion-mode braking valve 46 is positioned in this embodiment above a longitudinal aperture 75 formed in an inwardly directed annular portion 77 of the abutment 30 and in which the rod 19 sealingly slides.

The damper jack assembly of Figure 2 operates in the following manner: At normal landing speeds, hydraulic fluid in the chamber 9 passes into the chamber 15 by flowing through the longitudinal aperture 54 which is formed in the end portion 40 of the sleeve 32 and the permanent throttle orifice formed by the first radial aperture 72.

In the event of an emergency landing, the pressure applied by way of the radial passage 50 to the differential piston, defined between the portions 58 and 60 of different outside diameters of the tubular valve member 56 and the sleeve 32, displaces the tubular valve member 56 against the force of the spring 64 so that the permanent first radial apertures 72 and the second radial aperture 74 are both in communication with the second radial passage 70 of the sleeve 32, thus permitting higher rate of flow of hydraulic fluid.

The damper jack assembly shown in Figure 3 differs from that in Figure 2 insofar as the sleeve 32 is apertured with a second radial passage 70 which opens outwardly of the sleeve into the annular chamber 52 and inwardly of the sleeve onto a facing portion of the tubular valve member 56 which has a fine aperture 80 forming a throttle orifice for the hydraulic fluid at low vertical speeds (of the order of 0.3 metre per second) which occur at the limit of lift of the aircraft. The tubular valve member 56 also has the second radial aperture 72 which is larger in section than the fine aperture 80 and which forms the permanent compression-mode throttle orifice, and a large radial passage 74 i.e. larger in section than the second radial aperture 72.

It will be noted that, in this embodiment, there is no expansion-mode braking valve corresponding to the valve 46 of the embodiments of Figures 1 and 2 since the aperture 80 can act as the braking valve.

At its upper part, the tubular valve member 56 has an upwardly directed annular shoulder 82 and, on its inside face, the sleeve 32 has an annular shoulder 84 which is also directed upwardly. An annular support member 86 is held applied against the shoulder 84 by a strong spring 88 which bears also against the abutment 34 fixed to the outer end of the sleeve 32.

A second spring 90 which is weaker than the spring 88 extends between the annular abutment member 86 and the shoulder 82 of the tubular valve member 56. The spring 90 is so calibrated that it is compressed when the tubular valve member 56 is subjected to a pressure corresponding to vertical landing speeds of the order of 3 metres per second, while the spring 88 is compressed when the tubular valve member 56 is subjected to a pressure corresponding to vertical landing speeds of the order of 12 metres per second.

The damper jack assembly of Figure 3 operates as follows: For vertical landing speeds of the order of 0.3 metres per second, corresponding to the forces which are applied to the damper at the limit of lift of the aircraft, the tubular valve member 56 is in the position shown at the lefthand side of Figure 3 i.e. in abutment with the end portion 40 of the sleeve 32 and the annular abutment member 86 is in the position shown i.e. in abutment with the shoulder 84.

The hydraulic fluid therefore can flow through the passage 54 and the fine aperture 80.

For normal vertical landing speeds, of the order of 3 metres per second, the tubular valve member 56 is displaced against the force of the spring 90 by the pressure which is applied through the radial passage 50 to the differential piston formed between the lower portion of the tubular valve member 56 and the facing portion of the sleeve. In such movement, only the spring 90 is compressed. The second radial aperture 72 comes to a position facing the radial passage 70 of the sleeve, as shown at the righthand side of Figure 3, which permits a more substantial flow of hydraulic fluid.

In the event of an emergency landing, the pressure which results therefrom within the hydraulic fluid contained in the chamber 9 acts through the radial passage 50 in the sleeve on the above-defined differential piston and further displaces the tubular valve member 56 against the force of the spring 88. Once the spring 90 has become fully compressed the upper end of the tubular valve member 56 abuts the annular abutment member 86 and transmits the movement of the tubular valve member 56 to the annular abutment member 86. Compression of the spring 88 allows the large radial aperture74to communicate with the radial passage 70. The additional flow of hydraulic fluid which flows towards the chamber 15 can ensure that a correct damping action is obtained.

Claims

1. A damperjack assembly for an aircraft under carriage comprising a cylinder, a jack rod, a damper rod which contains a pressurised gas chamber and a valve provided with at least one compression-mode throttle orifice, the damper rod is mounted slidably and sealingly in a chamber, filled with hydraulic fluid, of the jack rod to form an oleo-pneumatic damper, the jack rod is mounted slidably and sealingly in said cylinder and is fixed with respect to a jack piston which, with said cylinder, defines a lowering chamber which can control lowering of an air craft undercarriage when the lowering chamber is supplied with hydraulic fluid, the damperjack assembly includes a sleeve which is generally cylindrical, is fixed within the end of the damper rod which is within the jack rod and has a radial passage therein, the radial passage opens outwardly of the sleeve into an annular hydraulic fluid chamber defined between the damper rod and the sleeve and communicating with the hydraulic fluid chamber of the jack rod, the assembly includes a tubular valve member mounted within the sleeve, having at least two portions of different outside diameters for cooperating with two portions of the sleeve of corresponding different inside diameters to form a differential piston and acted upon by resilient means, the radial passage is closed inwardly of the sleeve by the portion of the tubular valve member of smaller outside diameter, so that, when forces driving the damper rod into the jack rod are higher than the normal damper operating forces, the pressure applied to the differential piston displaces the tubular valve member against the force of the resilient means to permit a supplementary flow of hydraulic fluid from the chamber of thejack rod towards the gas chamber, such supplementary flow being additional to that which occurs through the compression-mode throttle orifice.

2. A damper jack assembly according to claim 1, in which the supplementary flow of hydraulic fluid takes place through said radial passage in the sleeve, the inner end of which said radial passage is uncovered by the smaller outside diameter portion of the tubular valve member when the tubular valve member is displaced against the force of the resilient means.

3. A damper jack assembly according to claim 2, in which, at its end which is directed towards the jack rod, the sleeve has an end portion which, with the sleeve, forms the piston of the damper rod, and in which end portion the permanent compression mode throttle orifice is provided, an expansionmode braking valve comprising an apertured disc diaphragm being retained between said end portion and the smaller inside diameter portion of the sleeve, which said portion of the sleeve forms an abutment for limiting the lifting movement of the expansion-mode braking valve above the compression-mode throttle orifice.

4. A damper jack assembly according to claim 1, in which the sleeve is apertured with a second radial passage which opens outwardly of the sleeve into said annular chamber and inwardly of the sleeve onto a facing portion of the tubular valve member having a first radial aperture forming the compression-mode throttle orifice, the tubular valve member also having a second radial aperture which is larger in section than said first radial aperture forming the permanent throttle orifice, so that, when the tubular valve member is displaced, the second radial aperture and the permanent throttle orifice are both in communication with said second radial passage in the sleeve.

5. A damper jack assembly according to claim 4, in which, at its end which is directed towards the jack rod, the sleeve comprises an end portion which, with the sleeve, forms the piston of the damper rod, and to which an abutment for limiting the outward movement of the damper rod is fixed, said abutment being in the form of a disc which comes into a position of bearing against the end of the damper rod and which has at least one large diameter flow orifice behind which is disposed the expansionmode braking valve which is formed by an apertured disc diaphragm, the movements of which are limited between said end portion and said abutment.

6. A damper jack assembly according to any one of claims 1 to 5, in which, when there is no load, the tubular valve member is held applied by a single helicoidal spring against a shoulder formed on the inside wall surface of the sleeve, said spring also bearing against a fixed abutment mounted at the end of the sleeve which is outside the jack rod.

7. A damper jack assembly according to claim 1, in which the sleeve is apertured with a second radial passage which opens outwardly of the sleeve into the annular chamber and inwardly of the sleeve onto a facing portion of the tubular valve member having a fine aperture forming a throttle orifice for the hydraulic fluid at pressures corresponding to low vertical speeds which occur at the limit of lift of the aircraft, the tubular valve member also having a second radial aperture which is larger in section than the fine aperture and which forms the permanent compression-mode throttle orifice, and a large radial aperture i.e. larger in section than the second aperture, the second and the large apertures being so positioned that, when the tubular valve member is displaced under the action of a pressure higher than that which occurs at the limit of lift, the second aperture comes into a position facing the second passage in the sleeve and that, when the tubular valve member is displaced undertheaction of a pressure higher than the normal landing pressures, the second and large apertures are both facing the second passage in the sleeve.

8. A damper jack assembly according to claim 7, in which, when there is no load, the tubular valve member is held applied against a first shoulder formed on the inside wall surface of the sleeve by a first helicoidal spring which also bears against an annular sleeve and which, when the load is lower than a first predetermined value, is itself held applied against a second shoulder formed on the inside wall surface of the sleeve and directed towards the upper end of the sleeve, by a second helicoidal spring which is responsive to a load higher than a second predetermined value which is higher than said first value, said second spring also bearing against a fixed abutment mounted at the end of the sleeve outside the jack rod.

9. A damper jack for an aircraft undercarriage substantially as hereinbefore described and illustrated with reference to Figure 1, Figure 2 or Figure 3 of the accompanying drawings.