AIRCRAFT NOSE LANDING GEAR ASSEMBLY

Abstract

An aircraft nose landing gear assembly is disclosed including two wheels, motors, brakes, and a controller. The wheels are separated by a steering axis and independently rotatable about a rotation axis in a rotation direction. The motors and brakes are each arranged to selectively engage a respective wheel. The motors and brakes supplement and resist rotation of the respective wheel in the rotation direction, respectively. On the basis of an indication to the controller of rotation of the two wheels in the rotation direction, the controller is arranged to: cause one motor to engage its respective wheel and supplement rotation, and cause the brake associated with the other wheel to engage the other wheel and resist rotation. Engagement of the motor and brake causes the wheels to pivot about the steering axis during a turning event.

Claims

1. An aircraft nose landing gear assembly comprising: two wheels separated by a steering axis and each independently rotatable about a rotation axis in a rotation direction; two motors each arranged to selectively engage and supplement rotation of a respective wheel of the two wheels in the rotation direction; two brakes each arranged to selectively engage and resist rotation of a respective wheel of the two wheels in the rotation direction; and a controller arranged to, based on an indication to the controller of rotation of the two wheels in the rotation direction, cause one motor of the two motors to engage the respective wheel for the one motor and supplement rotation, and the brake associated with the other wheel to engage the other wheel and resist rotation, such that said engagement of the motor and brake causes the two wheels to pivot about the steering axis during a turning event.

2. The aircraft nose landing gear assembly according to claim 1, comprising two electromotive devices each associated with one of the two wheels and selectively operable by the controller as one of the motors or one of the brakes.

3. The aircraft nose landing gear assembly according to claim 2, wherein the controller is arranged to cause at least a portion of an energy demand by the one motor to supplement rotation to be provided by energy recovered by the brake to resist rotation.

4. The aircraft nose landing gear assembly according to claim 3, wherein the controller is arranged to cause most of the energy demand to be provided by the energy recovered.

5. The aircraft nose landing gear assembly according to claim 3, wherein the controller is arranged to cause the energy demand to be provided by the energy recovered during the same turning event.

6. The aircraft nose landing gear assembly according to claim 3, comprising an energy storage to store the energy recovered as stored energy, wherein the controller is arranged to cause the energy demand during the turning event or a subsequent turning event to be provided by the stored energy.

7. An aircraft comprising the aircraft nose landing gear assembly according to claim 1.

8. The aircraft according to claim 7, wherein each of the two motors is rated to change a ground speed of the aircraft by no more than 5 knots when selectively engaged.

9. The aircraft according to claim 7, further comprising: a main propulsion system configured to propel the aircraft when taxiing, wherein the controller is arranged to cause a maximum energy demand by each of the two motors to supplement rotation of the wheel corresponding to the motor to be less than an amount of energy required to propel the aircraft by the main propulsion system when taxiing.

10. The aircraft according to claim 7, wherein the aircraft has a maximum take-off weight of at least 10,000 kg and the two motors are arranged to selectively engage and supplement rotation of a respective wheel of the nose landing gear have a combined maximum power output which is insufficient to cause the aircraft to taxi along flat and level ground in a manner that would maintain a typical taxiing speed, the typical taxiing speed being a speed in the range of 10 to 20 knots.

11. The aircraft according to claim 10, wherein the aircraft comprises one or more propulsion systems separate from the nose landing gear which are arranged to propel the aircraft along the ground when taxiing and capable of causing the aircraft to taxi along flat and level ground in a manner that would maintain a speed greater than or equal to the typical taxiing speed.

12. The aircraft according to claim 11, wherein the brakes and motors are so arranged that, during the turning event, the motor supplementing the rotation of one wheel uses energy recovered from the brake associated with the other wheel.

13. A method of operating an aircraft nose landing gear assembly comprising: while a port side wheel and a starboard side wheel are rotated in a same direction, supplementing rotation of one of the port side and starboard side wheels by a motor engaged with said one of the port side and starboard side wheels, and resisting rotation of the other wheel of the port side and starboard side wheels by a brake engaged with said other wheel during a turning event and while said port side and starboard side wheels are pivoted about a steering axis.

14. The method according to claim 13, wherein at least a portion of an energy demand by the motor that supplements the rotation of the one wheel comprises an energy recovered by the brake resisting the rotation of the other wheel.

15. The method according to claim 13, wherein the port side and starboard side wheels are rotated by an energy demand exceeding a maximum energy demand by the motor.

16. An aircraft nose landing gear assembly comprising: two independently rotatable nosewheels steerable about a steering axis; a motor and brake; and a controller arranged to, based on an indication to the controller of a forward or reverse movement of the nosewheels, cause the nosewheels to pivot about the steering axis in the same direction by engagement of the motor with an outer one of the nosewheels and providing a driving torque on the outer nosewheel, and by engagement of the brake with an inner one of the nosewheels and providing a retarding torque on the inner nosewheel.

17. The aircraft nose landing gear assembly according to claim 16, wherein the driving torque is additional to a driving force for driving the nosewheels forward or in reverse by an external energy source.

18. The aircraft nose landing gear assembly according to claim 16, wherein a maximum energy output of the motor is insufficient to solely propel the aircraft to a taxiing speed of at least 10 knots.

19. The aircraft nose landing gear assembly according to claim 16, wherein the controller is arranged to cause at least a portion of a braking energy that is generated by the retarding torque of the brake to be used in operation of the motor to provide the additional driving torque.

20. An aircraft with a maximum take-off weight of at least 10,000 kg comprising: a nose landing gear comprising at least one left wheel and at least one right wheel arranged to the left and right of a steering axis respectively; one or more propulsion systems separate from the nose landing gear which are arranged to propel the aircraft along the ground when taxiing; and a nose landing gear steering system comprising one or more motors arranged to provide differential driving power to the left and right wheels of the nose landing gear, the one or more motors having a combined maximum power output; wherein the aircraft is operable in a steering mode such that: the one or more propulsion systems deliver sufficient power to cause the aircraft to taxi along the ground and to maintain a speed that is above a threshold taxiing speed, the threshold taxiing speed being at least 10 knots; and the one or more motors of the nose landing gear steering system delivers power to at least one of the left and right wheels of the nose landing gear so as to cause a differential torque to be provided to the left and right wheels that is sufficient to enable steering of the aircraft by movement of the left and right wheels about the steering axis; and wherein the combined maximum power output of the one or more motors of the nose landing gear steering system is insufficient to cause the aircraft to taxi along flat and level ground in a manner that would maintain a speed above the threshold taxiing speed.

Description

DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0041] FIG. 1 shows a schematic rear view of a nose landing gear assembly according to a first embodiment of the invention;

[0042] FIG. 2 shows a schematic rear view of a nose landing gear assembly according to a second embodiment of the invention;

[0043] FIG. 3 shows a front view of an aircraft according to a second embodiment of the invention;

[0044] FIG. 4 shows a top view of the aircraft of FIG. 3; and

[0045] FIG. 5 shows an example method in accordance with the invention.

DETAILED DESCRIPTION

[0046] FIG. 1 shows a schematic rear view of a nose landing gear assembly 1 for an aircraft according to a first embodiment.

[0047] The assembly 1 comprises a wheel 10L for arrangement on a port side of the aircraft and a wheel 10R for arrangement on a starboard side of an aircraft. The wheels 10L, 10R are each independently rotatable about a rotation axis 2 and are spaced apart from each other along the rotation axis 2. This allows each wheel 10L, 10R to rotate about the rotation axis 2 without causing rotation of the other wheel 10L, 10R about the rotation axis 2. Independently rotatable wheels 10L, 10R allow the wheels to rotate with a differential rotational speed, such that one wheel rotates more quickly than the other wheel.

[0048] The wheels 10L, 10R are rotatable in a rotation direction 6. In FIG. 1, the rotation direction 6, shown by a circular arrow around the rotation axis 2, is indicative of a forward direction of the assembly 1. The rotation direction 6 is an anti-clockwise direction when looking into the rotation axis 2 on a left, port side of the rotation axis 2. The wheels 10L, 10R are also rotatable in a reverse direction, which is a direction opposite to the direction of the circular arrow.

[0049] The assembly 1 comprises a steering axis 3 arranged at a mid-point between the wheels 10L, 10R. The steering axis 3 is a substantially upright axis of rotation about which the wheels 10L, 10R rotate when the assembly 1 is deployed from an aircraft taxiing on the ground. The wheels 10L, 10R are spaced apart from each other on a respective left and right side of the steering axis 3 when looking at FIG. 1. The wheels 10L, 10R are arranged to pivot about the steering axis 3 at substantially the same time. That is, the wheels 10L, 10R are rotatable fixed with respect to the steering axis 3, such that the wheels pivot about the steering axis 3 at the same time and to the same degree. In FIG. 1, a steering direction 5, shown by a circular arrow around the steering axis 3, is indicative of steering towards a port side. The steering direction 5 is an anti-clockwise direction when looking into the steering axis 3 from the top of the steering axis 3. The wheels 10L, 10R are also steerable towards a starboard side, which is a direction opposite to the direction of the circular arrow.

[0050] The assembly 1 comprises two motors 21L, 21R and two brakes 22L, 22R. Although the motors 21L, 21R and brakes 22L, 22R are shown away from the wheels 10L, 10R, it is intended that each motor 21L, 21R and brake 22L, 22R is installed in each wheel 10L, 10R. One motor 21L and brake 22L is for arrangement on a port side of the aircraft and another motor 21R and brake 22R is for arrangement on a starboard side of an aircraft and are shown on respective left and right sides of the steering axis 3 when looking at FIG. 1. The port side motor 21L and brake 22L is to be located in an internal region of the port side wheel 10L, and the starboard side motor 21R and brake 22R is to be located in an internal region of the starboard side wheel 10R.

[0051] The port side motor 21L is arranged to selectively engage and supplement rotation of the port side wheel 10L about the rotation axis 2. The starboard side motor 21R is arranged to selectively engage and supplement rotation of the starboard side wheel 10R about the rotation axis 2. The port side brake 22L is arranged to selectively engage and resist rotation of the starboard side wheel 10R about the rotation axis 2. The starboard side brake 22R is arranged to selectively engage and resist rotation of the starboard side wheel 10R about the rotation axis 2.

[0052] The assembly 1 comprises a controller 30. The controller 30 is arranged to receive a signal 31 indicating to the controller 30 about information relating to the assembly 1. In this embodiment, the information comprises information about a rotation of each wheel 10L, 10R in the same rotational direction. The signal 31 in this example comprises a signal indicative of a wheel speed. In other embodiments, the indication may be different from a wheel speed.

[0053] The controller 30 is communicable with the two motors 21L, 21R and two brakes 22L, 22R via communication lines 4. Each communication line 4 comprises an input and output represented by the input and output arrow heads. The communication lines 4 comprise electrical connections between the controller 30 and the two motors 21L, 21R and two brakes 22L, 22R. The electrical connections allow for control information to be sent to the two motors 21L, 21R and/or two brakes 22L, 22R. The electrical connections also allow for feedback information to be received from the two motors 21L, 21R and two brakes 22L, 22R. The communication lines 4 therefore carry current and/or voltage signals.

[0054] The controller 30 is arranged to cause the two motors 21L, 21R and two brakes 22L, 22R to engage their respective wheel 10L, 10R depending on which direction the wheels 10L, 10R need to turn. During a port side turning event, for example, the wheels 10L, 10R pivot about the steering axis 3 in the direction shown by arrow 5. This causes the wheels 10L, 10R to turn from right to left as shown in FIG. 1. During the port side turning event, the port side wheel 10L acts an inner wheel and the starboard side wheel 10R acts as an outer wheel.

[0055] On receipt of the signal 31 indicating that both wheels 10L, 10R are rotating in the same direction, illustrated by arrow 6 in FIG. 1, the controller 30 causes a further signal to be sent to the starboard side motor 21R and the port side brake 22L to indicate that a turning event is required. The further signal causes the motor 21R and brake 22L to simultaneously engage with their respective wheel 10R, 10L. Engagement of each motor 21L, 21R causes a rotational speed of the respective wheel to increase. Thus, during the port side turning event, engagement of the starboard side motor 21R causes the starboard side wheel 10R to speed up. Engagement of each brake 22L, 22R is to cause a rotational speed of the respective wheel to decrease. Thus, during the port side turning event, engagement of the port side brake 22L causes the port side wheel 10L to slow down. Cooperative engagement of the motors 21L, 21R and brakes 22L, 22R on opposing wheels 10L, 10R therefore results in a differential rotational speed being produced between the wheels 10L, 10R.

[0056] During a starboard side turning event, the controller 30 causes the wheels 10L, 10R to pivot about the steering axis 3 in the direction opposite to that shown by arrow 5. The wheels 10L 10R pivot because the controller 30 causes the motor 21L and brake 22R to simultaneously engage and supplement or resist rotation of their respective wheels 10L and 10R. During the starboard side turning event, the starboard side wheel 10R acts an inner wheel and the port side wheel 10L acts as an outer wheel.

[0057] Each motor 21L, 21R is capable of exerting a driving torque on the respective wheel 10L, 10R to supplement rotation of the respective wheel 10L, 10R. The driving torque causes the rotational speed of the respective wheel 10L, 10R to increase. The driving torque is additional to a driving force provided by a main propulsion device to propel an aircraft to which the assembly 1 is incorporated, such as that shown in FIGS. 3 and 4. In contrast, each brake 22L, 22R is capable of exerting a retarding torque on the respective wheel 10L, 10R to resist rotation of the respective wheel 10L, 10R. The retarding torque causes the rotational speed of the respective wheel 10L, 10R to decrease.

[0058] Engagement of one motor 21L, 21R on one wheel 10L, 10R and one brake 22L, 22R on the other wheel 10L, 10R is to cause minimal, if any, increase in ground speed of an aircraft to which the assembly 1 is incorporated, such as that shown in FIGS. 3 and 4. In most instances, each motor 10L, 10R is rated to change a ground speed of the aircraft by no more than 5 knots when selectively engaged. Beneficially, this allows the motors 21L, 21R to be sized for only steering the wheels 10L, 10R. An impact of the motors 21L, 21R on a total weight of the assembly 1 is therefore limited to a motor suitable to only provide a steering function and not, alternatively or additionally, a propulsion function.

[0059] An amount of energy required to supplement rotation is insufficient to cause an aircraft to move at a maximum taxiing speed, typically in the range of 15 knots to 20 knots. An additional propulsion force resulting from the motor supplementing rotation is specifically arranged to steer the assembly 1 rather than to propel an aircraft. This enables the steering functionality of the aircraft for taxiing to be separate from the population system for taxiing. The motor 21L, 21R employed for supplementing rotation can therefore be sized to provide a steering function only. This helps to reduce a weight of the assembly 1.

[0060] FIG. 2 shows a schematic rear view of a nose landing gear assembly 101 for an aircraft according to a second embodiment. Common features between the first and second embodiments are incremented by 100 in FIG. 2. Only the differences between the first and second embodiments are discussed below.

[0061] The assembly 101 comprises two electromotive devices 120L, 120R, each for engaging a respective wheel 110L, 110R. The electromotive devices 120L, 120R are in-wheel devices, meaning that each electromotive device 120L, 120R is located within an internal region 111L, 111R of each respective wheel 110L, 110R.

[0062] Each electromotive device 120L, 120R comprises a stator 125 and a rotor 127. The rotor 127 is used herein to describe a passive (not energized) part of each electromotive device 120L, 120R that is moved, by an electromagnetic field of the motor 121L, 121R, or to induce current when the electromotive device 120L, 120R is operated as a generator. The stator 125 is the active (energized) part of each electromotive device 120L, 120R that generates the driving electromagnetic field in the motor 121L, 121R or in which current is induced in the generator. The relative motion between the stator 125 and the rotor 127 causes these effects. As is well known, when the rotor 127 is driven by an external means, and moved sufficiently quickly relative to its stator 125, the motor 121L, 121R will normally act as a generator of electricity. When electrical energy is supplied to the stator 125, the rotor 127 will normally move relative to the stator 125 to produce mechanical work. In view of that interchangeability of function, the term “electromotive device” is used for convenience herein, to refer interchangeably to motors and/or generators. Motors and generators are well-known and further discussion of their structure and functionality is omitted for brevity.

[0063] The wheels 110L, 110R are mounted on an axle 140 and are spaced apart from each other along the axle 140. The axle 140 comprises a rotation axis 102 about which the wheels 110L, 110R independently rotate. The axle 140 is mounted to a steering post 150. In this embodiment, the steering axis 103 is coaxial with the longitudinal axis of the steering post 150. The steering post 150 is rotatable about the steering axis 130 on a bearing 170. The steering post 150 is coupled to a torsional damper 152 to provide enhanced dynamic stability of the steering post 150. The torsional damper 152 is coupled to the steering post 150 by fasteners 154. The torsional damper 152 is coaxial with the steering post 150. In other embodiments, the torsional damper 152 may be omitted.

[0064] A torsional damper 142 is also mounted between the axle 140 and the steering post 150 to provide enhanced dynamic stability of the axle 140 about the steering post 150. The torsional damper 142 is coupled to the steering post 150 by fasteners 144. The torsional damper 142 is coaxial with the axle 140 and the rotation axis 102. In other embodiments, the torsional damper 142 may be omitted.

[0065] In this example, each electromotive device 120L, 120R is operable as a motor 121L, 121R or a brake 122L, 122R. When operated as a brake 122L, 122R, the electromotive device 120L, 120R functions as a generator, as described above. This allows the brake 122L, 122R to recover energy normally lost under braking. Electrical energy generated under braking is optionally stored in an energy storage 160L, 160R, which, in this example, is as a battery but can be a capacitor, for example. This allows energy recovered from one steering event to be used in the same or a subsequent steering event or even for other devices requiring electrical energy. Electrical energy is transferred between the controller 130, the electromotive device 120L, 120R, and optionally the energy storage 160L, 160R via the communication lines 104.

[0066] The electromotive devices 120L, 120R are sized to provide no more than the required energy to pivot the wheels 110L, 110R about the steering axis 103. Beneficially, when an aircraft, in which the assembly 101 is incorporated, is propelled during a ground manoeuvre—that is, when the aircraft is taxiing—a main propulsion system of the aircraft is not required to adjust to provide sufficient power to steer the aircraft. This enables the main propulsion system to stay within an efficient operating range. The impact (for example, a volume and weight) of the equipment required to provide a steering function can also be minimised by separating the steering and propulsion functions. This effect is enhanced when one of the electromotive devices 120L, 120R is used as a regenerative brake and energy recovered in that process can be supplied to the other one of the electromotive devices 120L, 120R for use in supplementing rotation of the respective wheel 110L, 110R.

[0067] As described in relation to the assembly 1 of the first embodiment, the controller 130 of the assembly 101 of the second embodiment is arranged to cause the two motors 121L, 121R and two brakes 122L, 122R to engage their respective wheel 110L, 110R depending on which direction the wheels 110L, 110R need to turn. In the second embodiment, the controller 130 selectively operates one of the electromotive devices 120L, 120R as the motor 121L, 121R and the other one as the brake 122L, 122R. During a port side turning event, for example, the wheels 110L, 110R pivot about the steering axis 103 in the direction shown by arrow 105. This causes the wheels 110L, 110R to turn from right to left as shown in FIG. 2. During the port side turning event, the port side wheel 110L acts an inner wheel and the starboard side wheel 110R acts as an outer wheel. In contrast, during a starboard side turning event, the starboard side wheel 110R acts an inner wheel and the port side wheel 110L acts as an outer wheel.

[0068] On receipt of a signal 131 indicating that both wheels 110L, 110R are rotating in the same direction, illustrated by arrow 106 in FIG. 2, the controller 130 causes a further signal to be sent along the communication lines 4 to electromotive devices 120L, 120R to indicate that a turning event is required. The controller 130 causes the starboard side electromotive device 120R to operate as a motor 121R and the port side electromotive device 120R to operate as a brake 122L. The causes the motor 121R and brake 122L to simultaneously engage with their respective wheel 110R, 110L. Engagement of the motor 121R causes a rotational speed of the starboard side wheel 110R to increase. Engagement of the brake 122L causes a rotational speed of the port side wheel 110L to decrease. The engagement causes the wheels 110L, 110R to pivot about the steering axis 103 from right to left when looking at FIG. 2.

[0069] During application of the respective brake 122L, 122R electrical energy is generated by slowing down the wheel 110L, 110R. Energy recovered in the form of electrical energy can be used by the motor 121L, 121R acting on the opposing wheel 110L, 110R. This allows the energy required to supplement rotation of the opposing wheel 110L, 110R to be provided by the energy recovered from the brake 122L, 122R. A power rating of the electromotive devices 120L, 120R can be sized based on the input of electrical energy during regenerative braking of an opposing electromotive device 120L, 120R. This helps to reduce the impact (for example, the volume and weight) of the electromotive device 120L, 120R compared to instances when regenerative braking is mot utilised. In other embodiments, a resistor may be used to dissipate energy during braking.

[0070] FIGS. 3 and 4 show an aircraft 200 having two wings 205L, 205R, each wing having a main landing gear 206L, 206R mounted thereon. The aircraft 200 comprises a nose landing gear 201 mounted on the fuselage of the aircraft 200. The nose landing gear 201 of FIGS. 3 and 4 may comprise the nose landing gear assembly 1, 101 according to the first or second embodiment. The aircraft 200 comprises a main propulsion system comprising an engine 206L, 206R mounted on each wing 205L, 205R. In this embodiment, the engines 206L, 206R are combustion engines. In other embodiments, the main propulsion system may comprise one or more electric motors capable of generating the thrust necessary for powering the aircraft 200 in-flight. In other embodiments, the main propulsion system may comprise a combination of combustion engines and electric motors suitable for hybrid-electric propulsion of the aircraft 200 in-flight.

[0071] The main propulsion system is for propelling the aircraft 200 in-flight and when taxiing on the ground for example in a forward direction 208F using forward thrust or in a reverse direction 208B using reverse thrust. To steer the aircraft 200 in a port side 209L direction, wheels of the nose landing gear 201 rotate about the steering axis of the nose landing gear 201 in an anti-clockwise direction when looking at FIG. 4 by engagement of the motor on the starboard side nosewheel and the brake on the port side nosewheel. In contrast, to steer the aircraft 200 in a starboard side 209R direction, the wheels of the nose landing gear 201 rotate about the steering axis of the nose landing gear 201 in a clockwise direction when looking at FIG. 4 by engagement of the motor on the port side nosewheel and the brake on the starboard side nosewheel. When the aircraft 200 turns, the aircraft 200 turns about an axis that is different to the steering axis of the nose landing gear 201.

[0072] The aircraft 200 is configured such that the main propulsion system provides an amount of energy required for taxiing the aircraft 200, whereas the motor and brake of the nose landing gear assembly 1, 101 provides an amount of energy required to steer the aircraft 200. Advantageously, the main propulsion system is operable under more steady-state conditions when steering the aircraft 200, rather than more transient conditions associated when steering by differential thrust. This may help to minimise cyclically loading the main propulsion system when taxing.

[0073] When selectively engaged, the motors of the nose landing gear 201 may provide marginal impact on a speed of the aircraft 200. This is because the motors are not suitable for propelling the aircraft 200 at nominal taxiing speeds. That is, each motor is rated to change a ground speed of the aircraft 200 by no more than 5 knots when selectively engaged. The motors are separate from the main prolusion device of the aircraft 200. This allows smaller motors to be used as opposed to motors that are required to be sized for driving the aircraft 200 at taxiing speeds, for example.

[0074] FIG. 5 shows a method of operating a nose landing gear assembly, for example using the nose landing gear assembly 1 according to the first embodiment. A ground manoeuvre 300 causes a port side and starboard side wheel to be rotated 310. A motor is engaged 320 with one of the wheels and rotation of that wheel is supplemented 330 by the motor. A brake is engaged 340 with the other wheel and rotation of the wheel is resisted 350 by the brake. Engagement of the motor and brake with the respective wheels to supplement and resist rotation of the respective wheel causes the wheels to be pivoted 360 about a steering axis in the same direction.

[0075] Optionally, when rotation of the wheel is supplemented 330 by the motor, at least a portion of an energy output of the motor by a controller comprises an energy input recovered by the brake that resisted the rotation of the other wheel. An example controller is one described in relation to the first or second embodiment above. Optionally, the wheels are rotated 310 by an energy input that exceeds a maximum energy output of the motor.

[0076] 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.

[0077] Although steering under a forward or reverse movement is described in relation to the above embodiments, the wheels 10L, 10R, 110L, 110R can be steered without such movement. In that instance, both motors 21L, 21R, 121L, 121R are engaged to rotate the wheels 10L, 10R, 110L, 110R in the same direction at the same time. Both motors 21L, 21R, 121L, 121R then drive the wheels 10L, 10R, 110L, 110R without further energy from a separate energy source rather than supplementing rotation of the wheels 10L, 10R, 110L, 110R.

[0078] In the above embodiments, the motors 21L, 21R, 121L, 122R and brakes 22L, 22R, 122L, 122R are described in an engaged state. For the avoidance of doubt, it should be understood that the motors 21L, 21R, 121L, 122R and brakes 22L, 22R, 122L, 122R can be selectively disengaged from the respective wheel. When the motors 21L, 21R, 121L, 122R and brakes 22L, 22R, 122L, 122R are disengaged, the wheels 10L, 10R, 110L, 110R can be free to caster. That is, the motors 21L, 21R, 121L, 122R and brakes do not resist pivoting movement about the steering axis 3, 103. Beneficially, the wheels 10L, 10R, 110L, 110R are provided with an inherent free caster capability. In the embodiments described above, a clutch or gearbox between the wheel and the motors may not be required.

[0079] In the above embodiments, the controller 30, 130 is configured to cause the wheels 10L, 10R, 110L, 110R to pivot about the steering axis 3, 103 such that a required slip angle is generated. The controller 30, 130 may receive an input informing the controller 30, 130 about a slip angle and a limit of slip angle dependent on a ground speed of the aircraft in which the assembly 1, 101 is incorporated. Engagement of the motors 21L, 21R, 121L, 122R and brakes 22L, 22R, 122L, 122R can be adjusted by the controller 130 depending on a current or desired slip angle.

[0080] 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.