GB2287681A

GB2287681A - An aircraft steering arrangement

Info

Publication number: GB2287681A
Application number: GB9405458A
Authority: GB
Inventors: Robert Ian Thompson
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1994-03-19
Filing date: 1994-03-19
Publication date: 1995-09-27
Anticipated expiration: 2014-03-19
Also published as: GB2287681B; GB9405458D0

Abstract

An aircraft steering arrangement includes a main support 3 held fast in rotation with the aircraft, a steerable sub-assembly 5 including ground engaging wheels (6, 7 Fig 4) and a steering actuator 21, 22 the sub-assembly being mounted on the main support for steering movement in relation thereto and the steering actuator transmitting steering forces between the main support and sub-assembly. The steering actuator includes an overload protection device in the form of shear link 23 designed to snap, or a cam mechanism (24, Fig 9) designed to disengage, if steering feeds above a predetermined load are supplied, to protect an aircraft steering mechanism. The steering actuator may be in the form of two coaxial sub collars 22 and 21 joined in rotation by shear link 23 or cam mechanism 24. <IMAGE>

Description

AN AIRCRAFT STEERING ARRANGEMENT This invention relates to aircraft steering arrangements and in particular to such steering arrangements in which the aircraft is directed by steering movement of part of a nose wheel assembly comprised in the undercarriage for the aircraft.

Such nosewheel assemblies typically comprise a main support (often called a main fitting) which is held fast in rotation with the aircraft in use. Steering for the aircraft is then provided by mounting a steerable sub-assembly comprising a telescopic tube upon which the wheels are mounted for telescopic up and down sprung movement within the main support and with a steering linkage to conduct steering forces from the main support to the steerable sub-assembly.

Such steering arrangements are rigidly coupled to a steering mechanism for the aircraft which transmits steering inputs from a pilot control in the cockpit through powered actuating means to the steerable sub-assembly.

During towing of the aircraft on the ground a rigid tow bar has normally been attached to the steerable sub-assembly of the steering gear for both pulling and steering the aircraft.

Steering overload protection was provided by a shear device designed to cease transmitting steering loads upon a steering overload being applied to the sub-assembly. In this way the aircraft steering mechanism was protected from overload damage during towing.

The introduction of towbarless towing when transporting aircraft to the runway has introduced new problems however in protecting the aircraft steering mechanism from damage.

Towbarless towing is carried out by placing the wheels of the aircraft nosewheel assembly on a tow truck whereby steering forces are transmitted to the aircraft steering arrangement directly through the wheels to the steerable sub-assembly. It will be appreciated therefore that the provision of some sort of overload shear device on the steerable sub-assembly will no longer be of any use in protecting the aircraft steering mechanism as the steering inputs from the tow truck effectively bypass any such device by carrying the aircraft wheels directly on the tow truck.

It is therefore an object of the invention to provide means to protect an aircraft steering mechanism for use when the aircraft is subject to towbarless towing with the steerable wheels of a nosewheel assembly being carried on the tow truck.

According to the invention there is provided an aircraft steering arrangement including a main support held fast in rotation with the aircraft, a steerable sub-assembly including a ground engaging wheel and a steering actuator, the sub-assembly being mounted on the main support for steering movement in relation thereto and the steering actuator transmitting steering forces between the main support and sub-assembly, wherein the steering actuator includes an overload protection device designed to cease transmission of steering forces therethrough above a predetermined value, whereby to protect an aircraft steering mechanism.

By this means the aircraft steering mechanism can be protected from excessive steering loads transmitted into the steering arrangement via the nosewheels of the aircraft.

According to one embodiment the steering actuator comprises a collar held fast in rotation with the steerable sub-assembly and mounted for rotational movement on the main support, said collar including a steering input connection and a steering output connection with the overload protection device being operable between said connections in response to steering loads in excess of a predetermined amount being transmitted therebetween.

The collar may comprise an input sub collar to which the input connection is made and an output subcollar to which the output connection is made, and the overload protection device may comprise a severable linkage between the subcollars.

In one embodiment the steering input connection is made via a rack and pinion mechanism mounted between the main support and the input subcollar and in another embodiment the steering input connection is made via at least one hydraulic actuator mounted between the main support and the input subcollar.

The steering output connection may conveniently be made via torque links mounted between the steerable sub-assembly and the output subcollar. In this way telescopic relative movement between the steerable sub-assembly and the main support can take place at the same time as steering movement.

The invention will now be described by way of example with reference to the accompanying drawings of which: Figure 1 is a schematic side detail view of an aircraft's steering arrangement according to the prior art, Figure 2 is a section along the line II-II of Figure 1 showing transmission of towing loads into the steerable sub-assembly, according to the prior art, Figure 3 shows a schematic side details view of an aircraft towing arrangement according to the invention, Figure 4 is a section taken along the line lV-IV of Figure 3 showing transmission of steering loads into the steerable sub-assembly, Figure 5 is a section taken along the line V-V of Figure 3 with modifications, Figure 6 is a section taken along the line VI-VI of Figure 3, Figure 7 is an alternative section along the line V-V of Figure 3, Figure 8 is an enlarged side elevation of a main fitting and oleo strut showing an alternative overload protection device, and Figure 9 shows the device of Figure 8 in a released position.

Referring to figures 1 and 2 of the drawings an aircraft steering arrangement in the form of a nosewheel undercarriage assembly 1 in shown supporting an aircraft fuselage 2.

The undercarriage assembly 1 comprises a main support in the form of a main fitting 3 held fast in rotation with the fuselage 2. Fore and aft loads are transmitted into the fuselage 2 by a forward strut 4. The main fitting 3 is a hollow tube which receives part of a steerable sub-assembly in the form of a main oleo strut 5 to allow telescopic suspension movement.

The oleo strut S carries ground wheels 6, 7 rotatably mounted on a axle 8. Steering loads are transmitted between the oleo strut 5 and the main fitting 3 by a steering collar 9 and a pair of torque links 10, 11. Steering loads are transmitted between the main fitting 3 and steering collar 9 by means of either a pair of hydraulic actuators 12, 13 (see Figure 5) or a rack and pinion mechanism 14 (see Figure 7). The torque links 10, 11 transmit steering loads between the steering column 9 and the oleo strut 5 in a manner which permits relative telescopic movement the oleo strut 5 and the main fitting 3 by pivotal movement of the torque links 10, 11 about pivots 15, 16, 17.

A towing fitting 18 is rigidly attached to a forward part of the oleo strut 5. This towing fitting includes a shear pin 19 designed to snap upon the application of excessive towing or steering loads (indicated by the arrows in Figure 2) between a towbar (not shown) and the towing fitting 18.

Referring now to Figures 3 to 7, an aircraft steering arrangement according to the invention is shown in which like parts as compared to figures 1 and 2 are given like reference numerals for consistency. A nosewheel undercarriage assembly 1 is shown mounted on a tow truck 20 so that turning movement of the tow truck will be transmitted into a steering mechanism (not shown) of the aircraft via the wheels 6, 7 which are chocked in position on the tow truck 20.

The steering collar 9 of figure 1 is replaced by a pair of steering subcollars 21, 22.

These subcollars are rigidly joined together in rotation by a shear link 23 which is designed to fail upon the application of an excessive steering load to it. Steering input loads are transmitted between the main fitting 3 and the subcollar 21 by the hydraulic actuators 12, 13 (see Figure 5). Steering forces are transmitted between the subcollar 22 and the oleo strut 5 by the torque links 10, 11.

It will be seen that the towing fitting 18 is retained on this steering arrangement for conventional tow bar towing. It will also be readily apparent that the provision of a shear pin in the towing fitting 18 is of little use when the aircraft is being towed with the wheels on a tow truck. In this situation the steering loads are transmitted from the tow truck 20 through to the aircraft steering mechanism as follows: through the wheel axle 8 into the oleo strut 5, from the oleo strut 5 through the torque links 10, 11 into the lower steering subcollar 22, from the subcollar 22 into the subcollar 21 via the shear link 23, from the subcollar 21 to the main fitting 5 via the hydraulic actuators 12, 13. In the event that the aircraft steering mechanism subject to potential damage comprises the hydraulic actuators 12, 13, these will be protected by the intervening shear link 23.Any further linkage involved in the steering mechanism will be between the upper subcollar 21 and the cockpit hand controls and so will be protected by the shear link 23.

Thus it will be appreciated that the invention provides, via the shear link 23 designed to give way at a safe predetermined steering load, protection from excessive steering inputs from the tow truck.

Figures 8 and 9 show an alternative embodiment of an overload protection device according to the invention. In place of the shear link 23 acting to transmit steering loads between the upper and lower subcollars 21 and 22 there is provided a cam mechanism 24.

This mechanism comprises a drive pin 25 engaged, during the transmission of normal steering loads, in a recess 26 defined in a pivotable cam plate 27, mounted on the subcollar 21 and the drive pin 25 is mounted on the subcollar 22, pivotable about pivot axis 28. On an upper edge 29 of the cam plate are formed three detents 30, 31, 32 for receiving a sprung roller 33. The shape of and depth of the central detent 31 is fashioned such that the sprung roller 33 will not be displaced therefrom during transmission of all normal steering loads through the cam mechanism 24. If an excessive steering load is applied however, for example as shown by the large arrow 34when the tow truck 20 turns too sharply, the sprung roller 33 will be driven out of the central detent 31 by the drive pin 25 forcing itself out of the recess 26 whereupon the cam plate 27 will be caused to pivot about its axis. This pivoting movement will eventually result in the sprung roller 33 engaging either one of the detents 30 or 32 according to the direction of application of the excess steering torque. The cam plate 27 will then be held in this position by the sprung roller until a reverse steering load is applied which returns the drive pin 25 towards the central position shown in figure 8 and which in turn will cause the re-engagement of the drive pin in the recess 26 to return the cam plate 27 to its normal driving position as shown in figure 8.

It can thus be seen that the mechanism of figures 8 and 9 has the same effect as the embodiment of figures 3 to 7 in protecting the steering mechanism from steering overload.

However the cam mechanism of figures 8 and 9 has the added advantage that it is automatically resettable after an overload.

Claims

1. An aircraft steering arrangement including a main support held fast in rotation with the aircraft, a steerable sub-assembly including a ground engaging wheel and a steering actuator, the sub-assembly being mounted on the main support for steering movement in relation thereto and the steering actuator transmitting steering forces between the main support and sub-assembly, wherein the steering actuator includes an overload protection device designed to cease transmission of steering forces therethrough above a predetermined value, whereby to protect an aircraft steering mechanism.

2. An aircraft steering arrangement as in claim 1 in which the steering actuator comprises a collar held fast in rotation with the steerable sub-assembly and mounted for rotational movement on the main support, said collar including a steering input connection and a steering output connection with the overload protection device being operable between said connections in response to steering loads in excess of a predetermined amount being transmitted therebetween.

3. An aircraft steering arrangement as in claim 2 in which the collar comprises an input subcollar to which the input connection in made and an output subcollar to which the output connection is made, and the overload protection device comprises a severable linkage between the subcollars.

4. An aircraft steering arrangement as in claim 3 in which the steering input connection is made via a rack and pinion mechanism mounted between the main support and the input subcollar.

5 An aircraft steering arrangement as in claim 3 in which the steering input connection is made via at least one hydraulic actuator mounted between the main support and the input subcollar.

6. An aircraft steering arrangement as in claim 4 or 5 in which the steering output connection is made via torque links mounted between the steerable sub-assembly and the output sub collar.

7. An aircraft steering arrangement substantially as herein described with reference to figures 3 to 7 of the accompanying drawings.