This invention relates to shimmy dampers, in particular those shimmy dampers used on aircraft landing gears to damp gear vibrations.

PRIOR ART

Shimmy is a complex oscillatory phenomenon affecting some aircraft landing gears to such a level that the gear stability may be jeopardized. It has been a long aeronautic industry practice to equip landing gears with shimmy dampers to prevent vibrations evolving in a divergent way. Such a shimmy damper is illustrated in US5224668.

In several aircraft incidents, some very known shimmy dampers have been identified as unable to damp shimmy phenomenon. Those shimmy dampers were thought to suffer from at least one of the following drawbacks:

- very low damping coefficient, based on small hydraulic restrictions, because incorporation of higher restrictions would have required to protect the restricted orifice by a screen, which may be difficult to integrate in the known designs;

- the hydraulic restriction suffers from important geometric tolerance, because it is often provided with simple drilled holes as opposed to a precise calibrated hole;

the hydraulic restriction suffers from important oil temperature variations;

the anti-cavitation valves are often custom made, which make them hard to change if a high frequency response is required after tests; - the hydraulic chamber oil volume is small, and the oil quickly gets heated under intense shimmy, diminishing the damping capacity.

the hydraulic compensator is threaded into shimmy damper body, making a weak mechanical connection that may fail under vibration fatigue, like in previous incidents .

OBJECT OF THE INVENTION

An object of the invention is to provide a shimmy damper that is cavitation free, offering damping characteristics depending very little on oil temperature, and easy to tune .

BRIEF DESCRIPTION OF THE INVENTION

According to the invention, there is provided a hydraulic shimmy damper including:

- a hollow body having an inner cylindrical surface and two end walls defining an inner functional volume ;

- a piston having a piston head sealingly engaging said inner cylindrical surface so as to define two hydraulic active chambers within the functional volume, said piston having a rod projecting from said piston head, which sealingly traverses one of said end walls ;

- biasing means to bias said piston relative to said body towards a rest position so as to allow for fore and aft motion of the piston around said rest position under action of external forces ;

- an hydraulic compensator having a internal fluid volume that is significantly more important that a fluid volume expelled from the body when the piston moves fore and aft;

- an hydraulic manifold arranged between said body and said hydraulic compensator and defining a trans- fer channel between said chambers that is connected to said hydraulic compensator and further receiving interchangeable hydraulic cartridge-type valves for metering the fluid flowing from said chambers to said transfer channel, and allowing the fluid to freely enter said chambers from said transfer channel.

Said hydraulic cartridge- type valves have internal screens to protect them from big particle contaminants.

Thanks to the important volume of the compensator, the metered fluid expelled from the body generates only marginal temperature elevation of the total oil volume contained within the shimmy damper. This makes the shimmy damper of the invention much less sensitive to oil temperature variation under intense shimmy.

Furthermore, the use of standard hydraulic car- tridge-type valves makes it easy to change said valves to tune the metering characteristics of the shimmy damper. The shimmy damper of the invention can be very easily adapted to various landing gears, and may be modified during the life of the landing gear to account for shimmy characteristics changes of the landing gear due to ageing and wear.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be further understood in the light of the detailed description of one particular em- bodiment of the invention, with reference to the following figures: - figure 1 is a perspective view of a shimmy damper according to one embodiment of the invention;

figure 2 is a partial sectional view along line II- II of figure 1;

- figure 3 is a hydraulic diagram of the shimmy damper of figure 1.

DETAILED DESCRIPTION OF ONE EMBODIE ENT OF THE INVENTION

With reference to figure 1, the hydraulic shimmy damper comprises a body 10 in which a piston 20 is engaged for translational motion along a longitudinal axis X. Said body 10 is attached to an hydraulic manifold 30, which in turn is attached to an hydraulic compensator 40. The shimmy damper is to be installed at a joint between two relatively oscillating components of the landing gear, one component being affixed to said body 10 while the other component is affixed to said piston 20. As an example, the shimmy damper may be installed between the torque links extending between the gear cylinder and the gear piston.

With reference to figure 2, said body 10 comprises a hollow cylinder 11 having an inner cylindrical surface 12 and a first end wall 13. Said cylinder 11 is closed at the opposite side thereof by a second end wall 14 so as to define an internal functional volume. Said piston 20 has a piston head 21 that sealingly engages said inner cylindrical surface 12 to define two hydraulic chambers CI and C2 within said functional volume. Said piston 20 also encompasses a rod 22 that protrudes from said piston head 21 and sealingly traverses said first end wall 13. While not shown here, said body 10 and said piston 20 have respective end fixtures for being affixed to the relatively oscillating components of the landing gear.

Said piston 20 is hollow and a plunger 15 affixed to said second end wall 14 extends so as to sealingly engage an inner surface 23 of said piston 20. Said plunger 15 has a circumferential protrusion forming a stop 16 for a sleeve or spring carrier 17 mounted for sealingly slid- ing around said plunger 15. A first spring 18 extends between said first end wall 13 and said piston head 21 to urge said piston head towards said spring carrier 17. A second spring 19 extends between said second end wall 14 and said spring carrier 17 to urge said spring carrier 17 towards said piston head 21. As a result, said piston head 21 and said spring carrier 17 are urged against one another. Action of hydraulic pressure on both sides of said piston head 21 and action of said springs 18,19 results in said spring carrier 17 being abutted against said stop 16 while said piston head 21 abuts against said spring carrier 17, thus defining a rest, central position of said piston 20 relative to said body 10, as illustrated in figure 2. Said piston 20 is free to move fore and aft along longitudinal axis X around said rest posi- tion whenever an external force is sufficient to overcome the pressure and springs efforts. It will be observed that in said rest position, there is some available displacement between said piston head 21 and said stop 16 so as to allow for motion of said piston 20 in direction of said stop 16. As best seen on figure 3, said body 10 is in flu- idic communication with a manifold 30 by means of communication holes H1,H2. Said manifold 30 defines a channel between the two chambers CI and C2 through holes HI and H2, as hereafter explained. In figure 3, like elements have like references, though symbolically pictured.

One recognizes said body 10 and said piston 20. One also recognizes said hydraulic compensator 40 that comprises a housing 41 in which a piston 42 biased by a spring 43 is sealingly sliding into the housing for maintaining a positive pressure throughout the shimmy damper, so that no cavitation is to be experienced. Said spring 43 may be a mechanical spring, as pictured, or a much lighter gas spring. Said compensator 40 has an internal volume that is significantly more important than a volume expelled from the chambers CI, C2 during motion of said piston 20.

Between said body 10 and said compensator 40 extends said hydraulic manifold 30 which defines a transfer channel 31 extending between said holes HI and H2 to put said chambers CI, C2 and said compensator 40 into fluidic communication. At the outlet of each hole HI, H2 , said manifold 30 encompasses a parallel combination of a check valve and a restriction valve (respectively CV1,RV1 and CV2,RV2) arranged so that fluid expelled from one chamber is forced to flow through the corresponding restriction valve as associated check valve is urged into a no-flow state, and fluid entering into another one chamber flows through the check valve as the check valve is urged into a flow state. Thus, when said piston 20 moves one direction, it causes the volume of one of the chambers to diminish, therefore causing fluid to be expelled from that chamber into said transfer channel 31 against pressure maintained by said compensator 40, whereas the volume of the other chamber is increased, causing fluid to enter that chamber by virtue of positive pressure maintained by said compensator 40 into said transfer channel 31. Fluid expelled from one chamber is metered by the corresponding restric- tion valve, while fluid entering in the other chamber encounters only a marginal resistance, as the flow section of the check valve in the flow state is large. Any differential flow between the expelled fluid and the entering fluid is absorbed by said compensator 40.

It will be easily understood that when said body

10 and said piston 20 enter a relative fore and aft periodic motion under external forces, fluid is expelled from one of said chambers and therefore metered twice per period, generating damping of that periodic motion.

Also, said springs 18,19 define a spring rate by which the shimmy damper opposes resistance to external forces that is proportional to the amount of displacement of said piston 20 with respect to said body 10. It should be observed that in the illustrated embodiment, said spring rate has distinct values when said piston 20 is moving in one direction or the other with respect to said body 10.

Said valves CVl , CV2 , RV1 , RV2 are standard cartridge-type hydraulic valves and can easily be changed if needed for fine tuning of the shimmy damper. Said manifold 30 is provided with a supplementary check valve CV3 to connect said transfer channel 31 to a feed port 32 that allows for feeding the shimmy damper with oil if needed.

This invention is not limited to what has been detailed hereinbefore, but encompasses any variant falling within the ambit of the claims.

In particular, although it is preferred that said body and said compensator be affixed in a removable man- ner to said manifold, said manifold may be built as a unit with said body, still being adapted to receive standard cartridge-type valves.