Aircraft landing gear

1. An aircraft landing gear comprising:

a shock-absorbing main leg having a sprung part for attachment to an aircraft and an un-sprung part including an axle carrying at least one wheel, the wheel having a hub and a toothed ring gear mounted to the hub;

a drive transmission mounted externally on the sprung part, or on the un-sprung part, of the main leg, the drive transmission having at least one motor and a drive pinion for meshing with the toothed ring of the wheel; and

an actuator for lifting the drive transmission into and out of driving engagement with the toothed ring and for maintaining the driving engagement as the landing gear deflects during a ground taxiing operation.

2. An aircraft landing gear according to claim 1 , wherein the drive transmission is detachably mounted.

3. An aircraft landing gear according to claim 1 , further comprising an articulating mechanism pivotally mounted on the sprung part, or on the un-sprung part, of the main leg, wherein the articulating mechanism is connected to the actuator and supports the drive transmission.

4. An aircraft landing gear according to claim 3 , wherein the articulating mechanism pivot axis is configured to tilt relative to the main leg.

5. An aircraft landing gear according to claim 1 , wherein the toothed ring and the drive pinion have associated matching follower surfaces.

6. An aircraft landing gear according to claim 1 , wherein the drive pinion includes a constant-velocity joint.

7. An aircraft landing gear according to claim 1 , having at least two of the driveable wheels, the wheels each having a toothed ring gear, and the drive transmission including two drive pinions each for meshing with a respective one of the toothed rings.

8. An aircraft landing gear according to claim 7 , wherein the drive transmission includes two motors, each for driving a respective one of the drive pinions.

9. An aircraft landing gear according to claim 7 , wherein the drive transmission includes a differential drive connected between the or each motor and the drive pinions.

10. An aircraft landing gear according to claim 1 , wherein the toothed ring is disposed on the outer diameter of the wheel hub.

11. An aircraft landing gear according to claim 1 , wherein the toothed ring has teeth facing radially inwardly or radially outwardly.

12. An aircraft landing gear according to claim 1 , wherein the motor is hydraulic or electric.

13. An aircraft landing gear according to claim 1 , wherein the drive transmission includes a toothed drive belt.

14. An aircraft landing gear according to claim 1 , wherein the toothed ring has a larger diameter than the drive pinion to develop a significant torque magnifying gear ratio.

15. An aircraft landing gear according to claim 1 , wherein the drive transmission is mounted externally on the sprung part of the main leg, and the actuator is adapted to maintain the driving engagement between the drive pinion and the toothed ring as the sprung and un-sprung parts of the main leg move relative to one another during the ground taxiing operation.

16. An aircraft including the landing gear according to claim 1 and a power and control system for supplying power to, and controlling operation of, the drive transmission.

17. A method of operating the aircraft landing gear according to claim 1 , the method comprising driving the wheel in rotation by supplying power to the motor and engaging the drive transmission with the wheel via the toothed ring as the landing gear deflects during a ground taxiing operation.

18. A method according to claim 17 , further comprising disengaging the drive transmission from the wheel.

19. An aircraft landing gear according to claim 1 , wherein the drive transmission is mounted externally on the un-sprung part of the main leg.