GB2415169A

GB2415169A - Aircraft landing wheel rotation aid

Info

Publication number: GB2415169A
Application number: GB0413373A
Authority: GB
Inventors: James Mcbride
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-06-16
Filing date: 2004-06-16
Publication date: 2005-12-21
Anticipated expiration: 2024-06-16
Also published as: GB0413373D0; GB2415169B

Abstract

A landing gear assembly comprises a wheel 2, a skid 7 mounted on a support member 5 which is mounted coaxially with the wheel 2, and a mechanical drive means 13, 14, the skid 7 being positionable in a first position relative the wheel 2, but not in contact with a tyre 1 thereof, wherein on landing the skid 7 contacts the ground prior to the wheel 2 at which point the skid 7 moves to a second position relative to the wheel 2 which allows the wheel 2 to come into contact with the ground and which also imparts rotational movement to the wheel 2 via the mechanical drive means 13, 14, the assembly further comprising means 15 for holding the skid 7 in the second position. The mechanical drive means 13, 14 may comprise a pawl 13 and ratchet 14 assembly. The means 15 for holding the skid in the second position may be a braking plate. The skid 7 may have a contact portion 7a comprising a resilient material.

Description

Aircraft Landing Gear The invention relates to a means of protecting

aircraft tyres from the high transient forces existing at initial ground contact, thereby reducing the wear on the tyres and extending the tyre service interval.

The landing of conventional fixed-wing aircraft imposes exceptional loads on the tyres of the landing gear upon initial contact with the ground surface. The forces on the tyre act in three directions. The two main forces are a longitudinal component acting in the direction of the aircraft's forward velocity, and a vertical component proportional to the aircraft's rate of descent. The third force is a lateral component resulting from any sideways drift due to crosswinds.

In regard to the longitudinal force, since the tyre is not rotating at touchdown, there is a brief initial period when it is skidding along the ground surface prior to being accelerated to landing speed. This results in scrubbing of the tyre tread, producing rapid wear. This wear tends to be unevenly distributed, leading to 'flat spots' around the tyre surface, and areas where the tyre carcass is exposed. A consequence of this is extended maintenance downtime caused by aircraft Lyres having to be replaced on a regular basis.

Many previous inventions have sought to address this problem by providing a means whereby the tyre is rotated prior to touchdown. The majority of these suggest various forms of wind vanes, attached to the wheel or tyre. These have proved impractical in overcoming the high inertia and initial rotational resistance of aircraft wheels. A major factor contributing to rotational resistance is the disc brakes normally fitted to aircraft wheels, which have a tendency towards binding. Other suggested systems involving motor drives have also proved to be unduly complex, adding significant weight to the airframe.

A few inventions, of which U.S. Patent 4,155,523 and U.K. Patent GB550101 are examples, suggest interposing a skid or shoe between tyre and ground surface at the instant of touchdown, with a means of retracting same after landing.

Both these inventions depend on friction contact between skid and tyre to bring the wheel up to the desired speed. Such an arrangement has the potential disadvantage of wear being generated on the tyre by slippage between the tyre and the skid, in the same fashion as wear is produced between tyre and ground in a conventional undercarriage arrangement.

Furthermore such slippage can result in insufficient pre- rotation being imparted to the tyre, leading to skidding and further wear when the tyre contacts the ground. Such difficulties act against the economic justification for fitting the known devices.

It is an object of the present invention to provide an aircraft landing gear assembly that overcomes or minimises one or more of these disadvantages, by obviating the requirement for direct physical contact between skid and Lyre.

According to a first aspect of the invention there is provided a landing gear assembly for an aircraft, the assembly comprising an aircraft wheel, a skid member, a support member on which the skid member is mounted, and mechanical drive means, said support member being pivotally mounted substantially co-axially with the wheel, wherein the support member is formed and arranged such that said skid member is supported in a first position in which the skid member is closer to the ground than the load bearing surface of the tyre of said wheel but not in contact with said Lyre, when said landing gear assembly is deployed for landing, and is moved to a second position clear of the ground when said skid member contacts the ground on landing, and the support member is also formed and arranged to engage the mechanical drive means when the skid member is in said first position, said mechanical drive means being formed and arranged to impart forward rotation to said wheel, with respect to the direction of travel of the aircraft on landing, during movement of said skid member to its second position, and to disengage with the support member when the skid member reaches said second position; and wherein the assembly further includes arrestor means for capturing and holding said skid member in said second position thereof.

Preferably said support member defines a generally arcuate operating path for movement of said skid member between said first and second positions thereof.

We shall hereinafter refer to said first position as the deployed' position and to said second position as the retracted' position.

Preferably the skid member has a contact portion made of a resilient material, preferably a durable elastic material with a high coefficient of friction such as, for example, tyre rubber, so that at least a portion of the vertical force on initial ground contact is partially absorbed by the skid member with the remainder being transmitted via the skid, support member and wheel axle to the aircraft suspension.

The contact portion of the skid member may conveniently be mounted to a rigid supporting portion of the skid member. On landing, the longitudinal force resulting from the aircraft's forward velocity drives the skid member and the support member about the wheel axle, thereby operating the mechanical drive means to rotate the wheel until the skid member is captured and held by the arrestor means, whereupon the drive means automatically disengages having accelerated the aircraft wheel and Lyre to a rotational speed commensurate with the aircraft's ground speed immediately prior to the first ground contact of the wheel tyre.

There are significant advantages to be gained from the 1S application of such a device. Aircraft wheels and tyres can be of a smaller and lighter construction commensurate with the less stressful operating conditions encountered after first landing contact of the aircraft, thus providing a reduction in weight to offset against the additional weight of the skid member, support member and associated fittings.

The wear on the Lyres is greatly reduced, leading to much greater intervals between their replacement, and less maintenance downtime. Wear due to landing forces will instead be mainly confined to the contact portion of the skid member, which is preferably designed to be easily and quickly replaceable while in the retracted (second) position (and while the rest of the assembly remains mounted to the aircraft).

For example the skid member may be attached to the support member by two bolts, which are readily removable when the skid member is in the retracted position, to allow replacement of a worn skid member with a new or refurbished skid member.

The skid member is preferably of arcuate shape following generally the curve of the wheel Lyre around which it pivots.

The skid member is preferably tapered so as to narrow in thickness towards the front end thereof, as most of the wear will occur to the rear of the skid member. The terms 'front' and 'rear' are herein used with reference to the portions of the skid closest to the front of the aircraft and closest to the rear of the aircraft respectively, when the aircraft landing gear assembly is deployed for landing.

Preferably the support member is pivotally mounted on an extension of the wheel axle by means of a suitable bearing, which allows free rotation of the support member about the axle. The support member may comprise at least a portion in the shape of a sector, or any other shape which supports the skid member adequately.

Preferably, the mechanical drive means between support member and wheel comprises a pawl and ratchet arrangement, preferably of a heavy-duty nature, forming a unidirectional drive similar to a bicycle freewheel. In such an embodiment at least one pawl is preferably provided on the sectorshaped portion of the support member and is arranged to engage with a circular ratchet arrangement mounted on, or integrally formed with, the wheel and disposed concentrically around the rotational axis of the wheel. When the skid member is in its retracted (i.e. second) position the pawl or pawls are arranged to remain clear of the ratchet, allowing the aircraft to be reversed on the ground. It will be readily understood by the reader that "heavy duty" in this context means of a suitable strength and resilience to sustain the forces incurred in repeated landing operations over a reasonable period of time.

In an alternative embodiment, which allows the invention to be fitted as a modification to existing aircraft landing gear, without replacing the wheels, the mechanical drive means comprises a circular assembly comprising a ratchet arrangement, said circular assembly further comprising a stub axle at its centre. The circular assembly may be attached to the aircraft wheel of a conventional existing aircraft landing gear assembly with its stub axle being used as the axis about which the support member is pivotally mounted.

The arrestor means and other components of the assembly of the invention can also be readily fitted to existing landing gear assemblies.

Preferably, a spring or other biasing means is provided between the support member and the wheel axle of the aircraft to bias the skid member towards its first position, so as to assist the skid member in moving from its retracted (second) position to its deployed (first) position, and to hold it there against pressure from the slipstream upon deployment of the assembly for landing. A mechanical stop arrangement may conveniently be provided on the support member and wheel axle for ensuring that the support member does not move beyond its deployed (first) position.

The arrestor means preferably comprises a braking plate, which is preferably designed to dissipate the rotational energy of the skid member and support member upon capture of the skid member once it has swung clear of the ground surface. Preferably, the braking plate is also of arcuate shape, corresponding approximately to the shape of the skid - 9- member, and may be movably attached to a cantilever, the cantilever being fixed to the wheel axle mounting or other unsprung part of the aircraft suspension unit. The action of the braking plate is preferably controlled by an actuator mounted on the cantilever. The skid member will be held securely in its upper retracted position by the friction between skid and braking plate. However, a simple mechanical interlock between skid and plate may additionally be provided as an additional safety measure. A stop is preferably provided at the end of the braking plate to prevent the skid from overrunning it.

A temporary means of retaining the support member in the retracted position when the braking plate is released may also be provided. This will facilitate the removal and replacement of the skid member, and can be accomplished by means of, for example, a simple mechanical clamp between the support member and the wheel.

Alternative arrestor means can be envisaged, for example in an alternative embodiment the arresting mechanism may comprise a hydraulic damper connected by means of a mechanical drive to the support member, said mechanical drive being formed and arranged so as operate the damper only after the wheel has been accelerated to a rotational speed commensurate with the aircraft landing speed.

According to a second aspect of the invention there is provided a kit-ofparts for assembly to a landing gear of an aircraft for aircraft landing operations, the kit-of-parts comprising a skid member, a support member, a mechanical drive means, and arrestor means, wherein said skid member is formed and arranged to be mounted, in use, on said support member, said support member being formed and arranged to be, in use, pivotally mounted, substantially co-axially with a wheel of the aircraft landing gear and said mechanical drive means is formed and arranged for mounting, in use, to said wheel, said support member also being formed and arranged such that, in use, said skid member is supported in a first position in which the skid member is closer to the ground than the load bearing surface of the Lyre of said wheel, but not in contact with said Lyre, when said landing gear is deployed for landing, and is moved to a second position clear of the ground when said skid member contacts the ground on landing, said support member being further formed and arranged such that in use it engages the mechanical drive means when the said skid member is in the first position, said mechanical drive means being formed and arranged to impart, in use, forward rotation to said wheel, with respect - 11 to the direction of travel of the aircraft on landing, during movement of said skid member to its second position and to disengage with the support member when the skid member reaches said second position, and wherein said arrestor means is formed and arranged for capturing and holding, in use, said skid member in said second position thereof.

Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is an oblique view of an aircraft landing gear assembly showing a wheel outer face, a skid member, a support member and a braking plate thereof.

Figure 2 is an oblique view of assembly of Figure 1, showing the wheel inner face, simplified aircraft suspension, cantilever and braking plate components.

Figure 3 is a side view of the assembly of Figs 1 and 2 showing the wheel outer face, the skid member, support member and braking plate, in a landing configuration.

Figure 4 is a side view of the assembly of Figs l and 2 showing the skid member and support member in a retracted position, captured by the braking plate.

Figure 5 is a side view of the assembly of Figs l and 2 showing the skid member and support member being released from the braking plate and rotating towards a deployed (landing) position.

Figure 6 shows an alternative embodiment of the aircraft landing gear assembly in which a circular assembly is bolted to the aircraft wheel; Figures 7, 8 and 9 are oblique views of a further alternative embodiment of the aircraft landing gear assembly incorporating an arresting mechanism in the form of a lever- arm hydraulic damper; and Figure lo shows an alternative version of the embodiment of Fig. 6.

In Figures l to 5, a Lyre l is fitted to a wheel assembly 2, rotating about a fixed axle 3. The inboard end of the axle is attached to the lower component of the aircraft suspension, which is typically an oleopneumatic suspension strut 4. A disc brake mechanism is normally mounted to the inboard side of the wheel, but has been omitted here for clarity. This arrangement of parts, represented here in simplified form, is entirely conventional and need not be described further.

A support member 5, in the shape of a sector, is mounted to an extension of the wheel axle 3 by means of a bearing 6, the support member being free to pivot about the axle. An arcuate skid 7 is fitted to the end of the support member. Both support member and skid are in close proximity to the Lyre with neither coming into direct contact with it. The skid is designed to be readily detachable from the support member, accomplished in the example here by two bolts 8. The skid has a resilient contact portion 7a, of high coefficient of friction, which is tapered in thickness, being thinner at the front. This contact portion 7a is bonded to a rigid metal supporting portion 7b of the skid 7.

When deployed in the landing configuration, the skid is positioned with the thicker rear end lying under bottom dead centre of the tyre.

The support member and skid are biased towards the deployed position by means of a spring 10, which acts on the support member and wheel axle. A stop 11 on the support member contacts a projection 12 on the end of the axle to prevent the support member overrunning its deployed position.

A pawl 13 is mounted on the support member 5 and is free to swivel about an arc on the support member, engaging with the teeth of a ratchet 14. The ratchet teeth are disposed in a circle centered on the wheel axle, and the ratchet is securely attached to the body of the wheel, in this case to the inside of the wheel rim.

A braking plate 15, equal or greater in width to that of the skid, is located at a suitable point around and clear of the tyre crown, in the same rotational plane as the skid. A stop 16 is located at the forward end of the braking plate. The braking plate is pivotably attached at point 20 to a cantilever 17.

An actuator 18, in this case hydraulically operated, is mounted on the cantilever and applies leverage to the braking plate by a simple cam arrangement 19, moving the plate about its pivot inwards or outwards with respect to the crown of the Lyre. The cantilever 17 is mounted to the lower unsprung component of the suspension strut 4, and consequently fixed in position relative to the wheel axle.

The operation of these components is as follows: Referring to Figure 3, the aircraft is moving from right to left, represented by arrow V. As the landing gear approaches the ground surface A, the first part to make contact is the contact portion 7a of the skid 7. The vertical force, proportional to the aircraft's rate of descent, is partially absorbed by the resilient contact portion 7a, and transmitted upwards via skid 7 and support member 5 to the wheel axle 3, thence to the aircraft suspension 4.

The longitudinal force, resulting from the aircraft's forward velocity, also acts on the contact portion of the skid, accelerating the skid rearwards and causing the support member 5 to rotate about the wheel axle 3, in the direction represented by arrow X. The rotational moment of the support member is transmitted by the action of pawl 13 on ratchet 14 to the wheel 2.

The wheel 2 and tyre 1 are thereby rotated (in the forward direction) to a speed commensurate with the aircraft's ground speed. As the support member swings rearwards, the skid clears the ground surface, allowing the Lyre to contact the ground and the landing to proceed as normal.

It will be noted that in Figures 1 and 3, the pawl 13 is seen to be fully engaged with the ratchet 14. When deployed in practice, the probability is that the pawl would come to rest at an intermediate point on the slope of the ratchet teeth.

It would engage fully within the first few degrees of support member movement as the skid contacts the ground.

Referring to Figure 4, the skid continues round the periphery of the Lyre and is captured by the braking plate 15, its kinetic energy being dissipated there as heat. As the skid is braked and captured, the ratchet 14 overruns the pawl 13, and the pawl retracts. In the example here the pawl remains retracted in the position shown by the action of gravity and a suitable stop on the support member (not shown in the diagram). This is necessary to allow the wheel to move in the reverse direction during ground maneuvering. It will be seen from Figures 3 and 4 that the distance between the braking plate undersurface and the tyre crown lessens with increasing distance from the braking plate pivot point 20. As the contact portion 7a of the skid wears with every landing, the skid will be captured by the braking plate at a position gradually approaching the end stop 16. This facility serves as a useful indication as to when the skid is due for renewal.

Figure 5 represents the part of the operating cycle following the lowering of the aircraft landing gear, preparatory to landing. The actuator 18 applies leverage to the cam arrangement 19, which lifts the braking plate 15 about its pivot 20, in the direction of arrow U. This releases the pressure on the skid contact portion 7a. The skid 7 and support member 5, under the action of spring 10, move in the direction of arrow Z to the deployed position shown in Figures 1 and 3. As the support member deploys, the pawl 13 re-engages with the ratchet 14. Spring 10 provides sufficient torque to assist the support member 5 to its deployed position and retain it there against pressure from the slipstream during the landing approach, but is not so strong as to prevent the support member from retracting to the braking plate under its own momentum following the landing.

Figure 6 shows an alternative embodiment which comprises a circular assembly 21, with the ratchet 14 mounted internally around its periphery, and a stub axle 22 fixed to the centre of the assembly as shown. (Like reference numerals are used to reference like parts in Figs. 1 to 6). The stub axle is of hollow open-ended construction and is separate from, but co-axial with, the main wheel axle 3 of the aircraft. The assembly is bolted to a frame of the wheel assembly 2 with studs 23. The existing wheel studs, or extended versions of same, could alternatively be employed to fix the assembly to the wheel. As in the previous embodiment, the support member 5, on which the skid 7 is mounted, is pivotally mounted to S the stub axle by means of bearing 6. A pawl 13 is pivotably attached to the support member 5 to engage with the ratchet 14. Since the stub axle rotates with the wheel, a stationary reference point is required for the spring/stop mechanisms acting on the support member. This is provided by a fixed shaft 24, an extension of, or an attachment to, the main wheel axle 3, which projects through the hollow stub axle 22.

In a similar arrangement to the previous embodiment, the support member and skid are biased towards the deployed position by means of a spring 10, which acts on the support member 5 and the shaft 24. A stop 11 on the support member 5 contacts a projection 12 on the shaft 24 to prevent the support member overrunning its deployed position. Apart from these differences, the operation of this embodiment is identical in principle to the previous embodiment described in Figures 1 to 5.

Figures 7, 8 and 9 show a further alternative embodiment in which the skid 7 and support member 5 are pivotally mounted, as in previous embodiments, either to an extension of the wheel axle or to a separate stub axle, co-axial with the wheel axle 3 (again, like reference numerals are used to reference like parts in Figs. 1 to 9). The pawl/ratchet drive mechanism between support member and wheel is similarly arranged. An extension 27 to the support member 5 has a pin 28 fitted near its end. The pin engages with a slot 29 in an arm 30, the pin being free to pivot and move along the slot.

The arm 30 is bent to clear the crown of the tyre 1 and is pivotably attached to a lever-action hydraulic damper 31.

The damper 31 is mounted to the lower unsprung component of the suspension strut 4.

Figures 7 and 8 show the support member 5 and skid 7 in the deployed position preparatory to landing. Similar to previous embodiments, a spring and a mechanical stop arrangement (not shown) is fitted between the arm 30 and the damper 31 to ensure that the support member 5 is biased towards the deployed position and is prevented from overrunning this position. Figure 9 shows the skid and support member retracted after landing. The drive shaft mechanism (not shown) fitted between the arm 30 and the damper 31 is so arranged that the mechanical drive from arm to damper only connects once the skid clears the landing surface. Further rotational movement of the skid and support member is thereby arrested by the damper. A further mechanical stop (not shown) ensures that the arm does not overrun the retracted position.

A releasable mechanical catch (not shown) is also fitted between arm and damper to hold the arm in the retracted position.

S It will be appreciated that various modifications and variations to the above embodiments are possible within the scope of the invention. For example, in the embodiment of Fig. 6 instead of using a hollow stub axle 22, the stub axle could be closed at its outer end but still mounted separate! from, but co-axial with, the main wheel axle 3. In this embodiment, shown in Fig. 10, the stationary point of reference required for the spring/stop mechanisms acting on the support member are provided by an angled extension 26 to the cantilever 17. One end of the spring 10 is attached to this extension, the other acting on the support member 5 to assist the support member to its deployed position. The support member 5 has a projection 24 on which is attached a stop 25, this acting on the extension 26 to ensure that the support member does not over-run its deployed position. In all other respects this embodiment operates in the same manner as the embodiments of Figs. 1 to 6.

Claims

A landing gear assembly for an aircraft, the assembly comprising an aircraft wheel, a skid member, a support member on which the skid member is mounted, and mechanical drive means, said support member being pivotally mounted substantially co-axially with the wheel, wherein the support member is formed and arranged such that said skid member is supported in a first position in which the skid member is closer to the ground than the load bearing surface of the tyre of said wheel but not in contact with said tyre, when said landing gear assembly is deployed for landing, and is moved to a second position clear of the ground when said skid member contacts the ground on landing, and the support member is also formed and arranged to engage the mechanical drive means when the skid member is in said first position, said mechanical drive means being formed and arranged to impart forward rotation to said wheel, with respect to the direction of travel of the aircraft on landing, during movement of said skid member to its second position, and to disengage with the support member when the skid member reaches said second position; and wherein the assembly further includes arrestor means for capturing and holding said skid member in said second position thereof.
2. A landing gear assembly according to claim 1, wherein said support member defines a generally arcuate operating path for said skid member between said first and second positions thereof.
3. A landing gear assembly according to claim 1 or claim 2, wherein the skid member comprises a contact portion made of resilient material, and a rigid supporting portion bonded to the contact portion.
4. A landing gear assembly according to claim 3, wherein the contact portion of the skid member is made of durable elastic material with a high coefficient of friction so that at least a portion of the vertical force on initial ground contact is partially absorbed by the skid member.
5. A landing gear assembly according to any of claims 1 to 4, wherein the skid member is replaceable when in said second position, whilst the remainder of the assembly remains mounted to the aircraft.
6. A landing gear assembly according to any of claims 1 to 5, wherein the skid member is of arcuate shape following generally the curve of the aircraft tyre around which it pivots and is tapered so as to narrow in thickness towards the front end thereof.
7. A landing gear assembly according to any of claims 1 to 6, wherein the support member is pivotally mounted on an extension of the wheel axle by means of a bearing which allows free rotation of the support member about the axle.
8. A landing gear assembly according to any of claims 1 to 7, wherein the mechanical drive means comprises a pawl and ratchet arrangement.
9. A landing gear assembly according to claim 8, wherein the pawl and ratchet arrangement comprises at least one pawl provided on the support member and arranged to engage with a circular ratchet arrangement provided on a wheel of the aircraft and disposed concentrically around the rotational axis of the wheel, said pawl or pawls being formed and arranged such that when said skid member is in its said second position the pawl or pawls remain clear of the ratchet, allowing the aircraft to be reversed on the ground.
10. A landing gear assembly according to claim 8 or 9, wherein the mechanical drive means comprises a circular assembly comprising the said ratchet, said circular assembly being formed and arranged for attachment, in use, to an aircraft wheel, said circular assembly further comprising a stub axle at its centre, said stub axle being used as the axis about which the support member is pivotally mounted.
11. A landing gear assembly according to any of claims 1 to 10, wherein a spring is provided between the support member and the wheel axle of the aircraft to bias the said skid member towards its first position, so as to assist the skid member in moving from its second position to its first position, and to hold it there against pressure from the slipstream, upon deployment of the assembly for landing.
12. A landing gear assembly according to any preceding claim, wherein the said arrestor means comprises a braking plate to dissipate the rotational energy of the skid member and support member upon capture of the skid member once it has swung clear of the ground surface.
13. A landing gear assembly according to claim 12, wherein the braking plate is of arcuate shape, corresponding approximately to the shape of the skid member, and is movably attached to a cantilever, said cantilever being fixed to a wheel axle mounting or other unsprung part of the aircraft suspension unit, said braking plate being controlled, in use, by an actuator mounted on said cantilever.
14. A landing gear assembly according to claim 13, wherein a stop is provided at the end of the braking plate to prevent the skid from overrunning it, in use.
15. A landing gear assembly according to any of claims 1 to wherein the arrestor means comprises a hydraulic damper connected by means of a mechanical drive to the support member, said mechanical drive being formed and arranged so as operate the damper only after the wheel has been accelerated to a rotational speed commensurate with the aircraft landing speed.
16. A kit-of-parts for assembly to a landing gear of an aircraft for aircraft landing operations, the kit-of-parts comprising a skid member, a support member, a mechanical drive means, and arrestor means, wherein said skid member is formed and arranged to be mounted, in use, on said support member, said support member being formed and arranged to be, in use, pivotally mounted, substantially co-axially with a wheel of the aircraft landing gear, and said mechanical drive means is formed and arranged for mounting, in use, to said wheel, said support member also being formed and arranged such that, in use, said skid member is supported in a first position in which the skid member is closer to the ground that the load bearing surface of the tyre of said aircraft wheel but not in contact with said Lyre, when said landing gear is deployed for landing, and is moved to a second position clear of the ground when said skid member contacts the ground on landing, said support member being further formed and arranged such that in use it engages the mechanical drive means when the said skid member is in the first position, said mechanical drive means being formed and arranged to impart, in use, forward rotation to said wheel, with respect to the direction of travel of the aircraft on landing, during movement of said skid member to its second position and to disengage with the support member when the skid member reaches said second position, and wherein said arrestor means is formed and arranged for capturing and holding, in use, said skid member in said second position thereof.