Hydraulic shock absorber

1. A hydraulic shock absorber comprising:

an inner housing portion slidably coupled to an outer housing portion to define a variable size chamber for containing shock absorber fluid, the region where the inner and outer housing portions overlap defining an annulus between adjacent surfaces of the inner and outer housing portions which varies in size in accordance with the extension state of the shock absorber; and

a seal located within the annulus and arranged in contact with one of the inner and outer housing portions for confining shock absorber fluid to the chamber;

wherein the shock absorber fluid comprises an electro-rheological or magneto-rheological liquid and the shock absorber includes a device for generating a magnetic or electric control field in the region of the seal in order to increase the viscosity of the shock absorber fluid such that the fluid in the region of the seal changes from a liquid state to a gel-like or solid state to inhibit passage of the shock absorber fluid beyond the seal; and

wherein the control field is stronger in a region of the electro-rheological or magneto-rheological liquid relatively near the seal than in a region of the electro-rheological or magneto-rheological liquid relatively far from the seal.

2. The hydraulic shock absorber according to claim 1 , wherein the seal is a dynamic seal arranged in contact with the inner housing portion.

3. The hydraulic shock absorber according to claim 1 , wherein the field generating device is arranged such that the intensity of the magnetic field or electric field defined within the chamber is relatively high in the region of the seal and relatively low in a damping region of the shock absorber.

4. The hydraulic shock absorber according to claim 3 , wherein the damping region of the shock absorber comprises a region of one or more damping orifices.

5. The hydraulic shock absorber according to claim 1 , wherein the field generating device is arranged such that control field is localised in or substantially confined to the region of the dynamic seal.

6. The hydraulic shock absorber according to claim 1 , wherein the field generating device is mounted within the annulus, adjacent to the seal.

7. The hydraulic shock absorber according to claim 1 , wherein the field generating device is formed as part of the circumferential wall of the outer housing portion.

8. The hydraulic shock absorber according to claim 1 , wherein the field generating device comprises an array of one or more permanent magnets.

9. The hydraulic shock absorber according to claim 1 , wherein the field generating device comprises one or more electromagnets.

10. The hydraulic shock absorber according to claim 1 , wherein the field generating device comprises an array of permanent magnets and electromagnets, the electromagnets being arranged to substantially cancel the magnetic field defined by the permanent magnets when energised.

11. The hydraulic shock absorber according to claim 1 , wherein the damping characteristics of the shock absorber are substantially unaffected by a change in the strength of control field.

12. The hydraulic shock absorber according to claim 1 , wherein the electro-rheological or magneto-rheological liquid in a damping region of the shock absorber is substantially unaffected by the control field.

13. The hydraulic shock absorber according to claim 1 , wherein the seal is arranged to inhibit passage of the shock absorber fluid beyond the seal and out of the shock absorber.

14. An aircraft assembly including a shock absorber, the shock absorber comprising:

an inner housing portion slidably coupled to an outer housing portion to define a variable size chamber for containing shock absorber fluid, the region where the inner and outer housing portions overlap defining an annulus between adjacent surfaces of the inner and outer housing portions which varies in size in accordance with the extension state of the shock absorber; and

a seal located within the annulus and arranged in contact with one of the inner and outer housing portions for confining shock absorber fluid to the chamber;

wherein the shock absorber fluid comprises an electro-rheological or magneto-rheological liquid and the shock absorber includes a device for generating a magnetic or electric control field in the region of the seal in order to increase the viscosity of the shock absorber fluid such that the fluid in the region of the seal changes from a liquid state to a gel-like or solid state to inhibit passage of the shock absorber fluid beyond the seal; and

wherein the control field is stronger in a region of the electro-rheological or magneto-rheological liquid relatively near the seal than in a region of the electro-rheological or magneto-rheological liquid relatively far from the seal.

15. The aircraft assembly according to claim 14 , wherein the aircraft assembly comprises an aircraft landing gear assembly.

16. A hydraulic shock absorber comprising:

an inner housing portion slidably coupled to an outer housing portion to define a variable size chamber for containing shock absorber fluid, the region where the inner and outer housing portions overlap defining an annulus between adjacent surfaces of the inner and outer housing portions which varies in size in accordance with the extension state of the shock absorber; and

a seal located within the annulus and arranged in contact with one of the inner and outer housing portions for confining shock absorber fluid to the chamber;

wherein the shock absorber fluid comprises an electro-rheological or magneto-rheological liquid and the shock absorber includes a device for generating a magnetic or electric control field in the region of the seal in order to increase the viscosity of the shock absorber fluid such that the fluid in the region of the seal changes from a liquid state to a gel-like or solid state to inhibit passage of the shock absorber fluid beyond the seal; and

wherein the damping characteristics of the shock absorber are substantially unaffected by a change in the strength of control field.

17. A hydraulic shock absorber comprising:

an inner housing portion slidably coupled to an outer housing portion to define a variable size chamber for containing shock absorber fluid, the region where the inner and outer housing portions overlap defining an annulus between adjacent surfaces of the inner and outer housing portions which varies in size in accordance with the extension state of the shock absorber; and

a seal located within the annulus and arranged in contact with one of the inner and outer housing portions for confining shock absorber fluid to the chamber;

wherein the shock absorber fluid comprises an electro-rheological or magneto-rheological liquid and the shock absorber includes a device for generating a magnetic or electric control field in the region of the seal in order to increase the viscosity of the shock absorber fluid such that the fluid in the region of the seal changes from a liquid state to a gel-like or solid state to inhibit passage of the shock absorber fluid beyond the seal; and

wherein the electro-rheological or magneto-rheological liquid in a damping region of the shock absorber is substantially unaffected by the control field.