FR2736890A1 - Helicopter rotor blade hydro-elastic damper, mounted between rotor blade and hub - has external tubular casing with coaxial rod supported by elastomer sleeves, piston between sleeves delimiting two hydraulic fluid chambers communicating through laminar flow passage - Google Patents

Abstract

The rotor blade damper comprises a body (11) with an external metal tubular casing. A central rod (12) is partly engaged coaxially in the body. Two elastomer sleeves (13,14) support the rod in the body. A piston (15c) is located between the flexible sleeves and is fixed to the body or the rod. Two chambers (20a,20b) are delimited in the body around the rod either side the piston. The chambers are closed on the side opposite the piston by the elastomer sleeves which provide stiffness and ensure elastic return to the initial relative position of the rod and the body. The two chambers, which are filled with hydraulic fluid, have variable volumes in opposite directions during relative displacement of the rod and body. They are in communication via a laminar flow passage (22) delimited radially outwards by the body wall. The hot laminar fluid flow in contact with the body assists in the heat transfer to the outside.

Description

"HYDROELASTIC DAMPER
FOR ROTOR OF GIRAVION "
The invention relates to improvements made to hydroelastic drag dampers for rotor blades of rotorcraft, in particular helicopters.

 To control the drag movement of each of the blades of a rotorcraft rotor, such as a main or auxiliary rotor of a helicopter, it is generally necessary to mount a component most often called drag damper, and sometimes called a drag adapter. frequency, between, on the one hand, each blade or a member, such as a sleeve, of connection of each blade to the hub of the rotor, and, on the other hand, the hub of the rotor or a blade close to the blade considered , or the connecting member of this blade adjacent to the hub.

The function of such a component called drag damper in the remainder of this specification, is twofold
- this involves positioning the first mode of drag of the blade or natural frequency in drag of the blade, to avoid coupling between the movement of the blades around their drag articulation and the structure of the helicopter or its train landing, i.e. avoiding the phenomenon known as ground resonance,
- it is also a question of damping the movement of drag, also to limit the effect of possible couplings, as well as to dampen the movements of the blades subjected to excitations of the wind gusts type or others.

 To fulfill these two functions, the dynamic characteristic of the shock absorber has a stiffness term, thanks to which the shock absorber opposes a resistant force proportional to the amplitude of the relative displacement of the two members, such as a blade and the hub. a rotor, between which the drag damper is mounted, as well as a damping term, thanks to which the damper opposes a resistant force proportional to the speed of this relative displacement. On certain helicopters (Alouette models from the French company AEROSPATIALE Société Nationale Industrielle), these two functions were fulfilled by a hydraulic shock absorber of conventional structure, combined with so-called "trimming" cables connecting each blade to the neighboring blades, in an inter-blade assembly, while on other helicopters these two functions have been fulfilled by a shear damper of at least one layer of viscoelastic material between two rigid reinforcements , each connected to one respectively of the two members between which the damper is mounted, and in which the shearing of the viscoelastic elastomeric material provides both stiffness and damping. On other helicopters finally, these two functions have been fulfilled by a hydraulic damper combined with at least one return spring to an initial equilibrium position, the spring possibly consisting of a block of elastomeric material, or comprising such a block, and also providing a (minority) share of damping, most of which is provided by the hydraulic damper.

 The specificity of a drag damper for a helicopter rotor blade is to have to dampen the movements of the drag blade on its natural frequency, which are movements of small amplitude and relatively slow (for example of + 1 mm, in extension or in compression from its initial equilibrium position, at a frequency of 1.5 Hz), while the blade and, for example, a drag damper which connects it to the hub of the rotor are subjected to forced displacements of greater amplitude and faster (for example from + 8 mm at 4.5 Hz). However, these faster and larger displacements are not annoying and do not have to be damped because, as they result from a forced excitation at the frequency of the rotation speed, the center of gravity of all the blades does not move not.

 The difficulty in producing a drag damper therefore lies in the definition of a shock absorber structure which must damp a movement of low amplitude at low speed, in the presence of a rapid superimposed movement and of greater amplitude. The general problem therefore arises of the evacuation of the energy dissipated (in thermal form) by the drag damper requested by these rapid and large displacements.

 To ensure precise positioning of the stiffness and damping characteristics presented above, while producing the damper with a simple and economical structure, drag dampers have already been proposed, in which the damping function, or at the very least the essential thereof, is produced by rolling a hydraulic fluid between two working chambers internal to the shock absorber, these chambers being closed by elastomer parts, elastically deformable, simultaneously fulfilling the function of stiffness and the dynamic sealing function, hence the absence of a dynamic seal, and therefore no risk of hydraulic fluid leaking.

 An example of such a drag damper, combining damping essentially by rolling a hydraulic fluid and parts made of elastomeric material to ensure the stiffness, the watertight closing of the hydraulic fluid chambers and the elastic return of the damper in a initial equilibrium position, is shown in Figure 1 and described below.

 The hydroelastic damper of Figure 1 comprises an outer casing body 1, which is tubular, metallic and of substantially cylindrical shape with a circular section. A central rod 2 is engaged, over most of its length, coaxially in the body 1 and supported in this body 1 by two elastic supports 3 and 4, constituted by two elastomeric sleeves, elastically deformable, of axial section substantially in shape trapezoidal and arranged symmetrically to each other with respect to the center of the shock absorber. Each of the elastomer sleeves 3 and 4 is integral, on the one hand, with the body 1 by being adhered, for example by vulcanization, by its external radial face against the internal wall of a corresponding axial end portion Sa or 5b of an external tubular spacer 5, fitted sliding in the body 1 for mounting reasons, and fixed in the latter by a nut 6 externally threaded, screwed into a threaded bore at one end of the body 1, the nut 6 being in axial abutment against one end of the tubular spacer 5, the other axial end of which is in abutment against an internal radial shoulder 1a of the corresponding axial end of the body 1. On the other hand, each elastomer sleeve 3 or 4 is integral with the central rod 2 while being adhered, for example by vulcanization, by its internal radial face against the outer wall of one respectively of the two thickened axial end parts 7a and 7b of an internal tubular spacer 7 mounted by sliding and fixed around the part of the central rod 2 engaged in the body 1, the fixing being ensured using a nut 8 screwed around the threaded end 2a of the part of the rod 2 internal to the body 1, the nut 8 coming into axial abutment against the end of the part 7a of the spacer 7 , whose opposite end of the other part 7b is in abutment against an external radial shoulder of the rod 2, at its end 2b external to the body 1 and by which this rod can be connected and articulated by any known means, such as '' a ball joint, s ur one of the two rotor members, for example a blade and the hub, between which the damper must be mounted. The end of the part 5a of the external spacer 5 which substantially surrounds the nut 8 is extended towards the outside of the body 1 by two branches or two diametrically opposite parts of a part, for example turned, with axial symmetry of 5d revolution forming a yoke or connecting piece of the shock absorber to the other of the two bodies between which the drag damper must be mounted. As a variant, this connecting piece may be directly secured to the body 1. Between its two end portions 5a and 5b, secured to the elastomer sleeves 3 and 4, and with which it constitutes an assembly, the external spacer 5 has a central part 5c of revolution around the central axis and projecting radially inwards towards the rod 2, up to a small radial distance from the external wall of the thin central part 7c of the internal tubular spacer 7. This central part 5c, in the form of a straight circular cylinder trunk, in the bases of which frustoconical recesses converging towards each other are formed, constitutes a piston, disposed between the two elastomer sleeves 3 and 4, and integral in movement of the body 1. This piston 5c delimits, between its internal radial face and the wall of the central part 7c of the internal spacer 7 an annular rolling orifice 9 opening through its axial ends in two chambers s working lOa and lOb. These chambers are thus delimited in the body 1, around the rod 2, and more precisely between the spacers 5 and 7, and on either side of the piston 5c, and, on the side opposite to the latter, each of the chambers lOa and lOb is sealed by one of the elastomer sleeves 3 and 4 respectively. The two chambers lOa and lOb, in communication with one another by the laminating orifice 9, are filled with a hydraulic fluid (an aircraft-grade hydraulic oil, for example), and have volumes which vary in opposite directions during relative displacements, essentially axial, of the rod 2 and of the body 1, when the damper is stressed in compression or in traction, which respectively tends to bring closer or separate from each other the two ends 2b and 5d of the damper. The elastic deformation of the elastomer sleeves 3 and 4 allows these relative displacements and, in cooperation with the piston 5c, causes the reduction in volume of one of the chambers 10a and 10b, the hydraulic fluid of which is compressed and expelled by the rolling orifice 9 towards the other chamber, the volume of which increases and in which the hydraulic fluid is expanded.

 In this structure, the elastomer sleeves 3 and 4 ensure, in addition to the sealed closure of the chambers 10a and 10b, the elastic return of the body 1 and of the rod 2 to an initial relative position of equilibrium, and provide the stiffness of the shock absorber. The damping of the drag movement is essentially provided by the rolling of the hydraulic fluid in the annular orifice 9, a minority fraction of this damping being possibly ensured by the deformation of the elastomer sleeves 3 and 4, if the elastomer which constitutes viscoelastic characteristics.

 This conventional structure has the major drawback that the rolling of the fluid through the orifice 9 dissipates a large amount of heat between the piston 5c and the central spacer 7, and that the evacuation of the calories produced by the damping effect is difficult because, axially, the elastomer of the sleeves 3 and 4 is a very poor thermal conductor, while radially, the heat transfers are difficult to effect from the piston 5c of the external spacer 5 to the damper body 1, in due to the crossing of the sliding metal-metal interface sliding for mounting reasons between the body 1 and the external spacer 5.

 The drawbacks encountered with this structure originate from the evacuation of the calories dissipated by the rolling of the hydraulic fluid at the heart of the shock absorber, because the dimensioning of the components of the shock absorber is generally determined by the shear fatigue of the elastomeric sleeves 3 and 4 vis-à-vis the large amplitude and high frequency displacements to which the drag damper is subjected. Indeed, it is in this stress configuration that the stresses are the highest, and it is also in this case that the temperature of the hydraulic fluid, in contact with the elastomer sleeves, is the greatest.

 However, it is well known that the fatigue strength of elastomers is greatly reduced when their temperature increases. In addition, these elastomeric materials have very poor thermal conductivity, and they do not contribute to cooling, and can even, on the contrary, participate in the heating of the damper when they provide part of the damping by visco effect. -elastic.

 The problem underlying the invention is to improve the fatigue strength of hydroelastic drag dampers of this type, in particular by reducing the level of heating of the various constituent elements, by improving the heat transfers to the outside.

 The main object of the invention is therefore to propose a hydroelastic drag damper overcoming the drawbacks resulting from the production of heat by rolling the hydraulic fluid in the central part of the damper, and, moreover, better suited than the dampers of known drag of this type to the various desiderata of the practice.

To this end, the invention provides a hydroelastic drag damper for rotorcraft rotor blade, intended to be mounted between two members of a rotor, of which a first member is a rotor blade or a connecting member of said blade to a hub of the rotor, and the second member is said hub, or a blade adjacent to the rotor, or a member for connecting said blade adjacent to said hub, to control the drag movement of said blade relative to said second member, and of the type including
- at least one body of an outer envelope, tubular, metallic and substantially cylindrical,
- at least one central rod, at least partially engaged substantially coaxially in said body,
- at least two elastic supports, supporting said rod in said body while ensuring an elastic return of the body and of the rod towards an initial relative position,
at least one piston, disposed between the two elastic supports, and integral in movement with one of the two elements that are said body and said rod, and
- At least two chambers delimited in said body, around said rod, on either side of said piston, and each between said piston and one respectively of the elastic supports, each comprising at least one elastically elastically deformable sleeve secured to the body and rod so as to provide a stiffness and ensure a tight closure of the corresponding chamber, the two chambers filled with hydraulic fluid having variable volumes in opposite directions during essentially axial relative displacements of the rod and the body , and being in communication with each other by at least one rolling orifice of the hydraulic fluid to provide damping of the drag movement, and is characterized in that it further comprises at least one passage delimited radially towards the outside directly by the wall of said metallic body and traversed by laminated fluid circulating between the two chambers during relative movements stem and body.

 By the improvement thus proposed, the heat transfers to the outside of the damper are improved by the movement of the laminated hydraulic fluid, therefore hot, directly against the wall of the metal body, which makes it possible to improve the evacuation of the dissipated calories. by rolling in this type of damper.

 The passage or passages thus delimited by the metallic body and traversed by the laminated fluid may or may be in communication with at least one of the chambers by at least one rolling orifice, but, as a variant or simultaneously, this passage or at least one of them can delimit at least one rolling orifice of the hydraulic fluid. In the latter case, the improvement proposed consists in bringing the heat source, namely rolling, as close as possible from the outside of the damper, and not in keeping it at the heart of the latter, which also promotes thermal transfers to the outside.

 Advantageously, to ensure effective cooling, this passage, or each of them, is delimited over most of its length between the body, towards the outside, and the piston, towards the inside.

 To take account of the shape of the piston, favorable to an advantageous delimitation of the working chambers of the shock absorber, in particular when the piston is integral in movement with the body, it is advantageous that the passage or each of them is in communication with at least one of said chambers by at least one orifice made in the piston, this orifice being able to be only a feeding orifice, if the passage defines a rolling orifice, or to be simultaneously a feeding and rolling, otherwise.

 In known manner, the piston can be secured to an external tubular spacer, fixed in the body, and to which the elastomeric sleeves are secured by external radial faces, the piston protruding from the external spacer radially inward, in which case it is advantageous, according to the invention, for the piston to slide with sealing by at least one dynamic seal around the central rod, or the central part of a tubular spacer surrounding the latter and the parts of which 'axial end are secured to the elastomer sleeves ,.

 But it is also possible that the piston is integral with such an internal tubular spacer, fixed around the central rod, and to which the elastomeric sleeves are secured by internal radial faces, the piston protruding from the internal spacer radially towards outside and towards the body, in which case it is advantageous that the piston delimits with the internal wall of the body said passage or each of them, for the circulation of the laminated fluid, and each of whose ends opens directly into one respectively of the two working chambers of the shock absorber.

 In both configurations, the circulation passage of the laminated fluid, or each of them, is preferably axial and, optionally, annular.

Other advantages and characteristics of the invention will emerge from the description given below, without implied limitation, of two exemplary embodiments described with reference to the appended drawings in which
FIG. 1 schematically represents, in axial section, a hydroelastic drag damper of the state of the art and described above,
FIG. 2 represents, in a similar axial section, a first example of a damper of the same type with a piston secured to the body, and
- Figure 3 shows, in similar axial section, a second example of the same type of piston damper attached to the rod.

 The hydroelastic drag damper of Figure 2 has the same general architecture as that of Figure 1, so that the same reference numerals increased by 10 are used to designate similar elements. We find a shock absorber having a piston 15c, constituting the central part of an external tubular spacer 15 slidingly fitted in the central bore of the metallic tubular body of external envelope 11, of which the piston 15c is integral in displacement, in the same way. as in the example of FIG. 1. The end part 15b of the external spacer 15 is in axial abutment against the internal radial shoulder 11a of the corresponding axial end of the body 11, and the fixing of the external spacer 15 in the body 11 is ensured by the screwing of the nut 16 in the threaded bore of the other axial end of the body 11, with the only difference, concerning this fixing, compared to the embodiment of the figure 1 that the axial locking on the other end part 15a of the spacer 15 and ensured by means of the sleeve 15e, metallic like the spacer 15, and at the end internal to the body 11 of the yoke or piece 15d link, to be axially stacked between the nut 16 and the end part 15a. However, in the example of FIG. 2 as in that of FIG. 3 described below, the fixing of the external spacer in the body can be insured exactly as in the example in Figure 1.

 There is also a central rod 12, the end 12b of which, external to the body 11, on the side opposite to the fixing yoke 15d, is arranged in articulated connection means, or is equipped with such means, for connection to the one of the two members between which the drag damper must be mounted, the other member being connected to the end yoke 15d. This rod 12 also includes an end portion 12a threaded and internal to the body 11, and onto which is screwed a nut 18 for the axial fixing of the internal tubular spacer 17 around the part of the central rod 12 engaged in the body. 11.

 As also in the known embodiment of FIG. 1, two elastomer sleeves 13 and 14, of the same constitution as the sleeves 3 and 4 of the previous example, are vulcanized by their external radial face against the internal walls of the parts of end respectively 15a and 15b of the external spacer 15, and by their internal radial face against the external walls of the end portions respectively 17a and 17b of the internal spacer 17 also metallic. Thus these sleeves 13 and 14 are integral, d on the one hand, of the rod 12, and, on the other hand, of the body 11 in order to ensure an elastic return of these two elements towards an initial position of equilibrium, and provide the stiffness of the damper and also close sealingly chambers 20a and 20b filled with hydraulic fluid, similar to chambers 10a and 10b of the previous example, and, like them, delimited in the body 11, around the rod 12, between the spacers 15 and 17 and of pa rt and other of the piston 15c, each of the chambers 20a and 20b extending axially between the piston 15c and one respectively of the two elastomer sleeves 13 and 14.

 As in the previous example, the piston 15c extends radially towards the thinned central part 17c of the internal spacer 17, from which it is separated only by a slight radial clearance, delimiting a central annular channel 19 of small section.

 However, unlike the previous example, the circulation of the hydraulic fluid from one to the other of the chambers 20a and 20b with variable volumes in opposite directions, during essentially axial relative displacements of the body 11 and the rod 12, does not can be carried out by the central annular channel 19, because the latter is closed off by a dynamic O-ring seal Shiite 21 retained in an annular groove in the internal radial face of the piston 15c, and allow the piston 15c to slide with sealing around of the central part 17c of internal spacer 17 during the aforementioned relative displacements of the rod 12 and of the body 11.

 According to a structure specific to this example of FIG. 2, the circulation of the hydraulic fluid between the two chambers 20a and 20b is effected by an axial and annular passage 22, delimited, radially inwards, by the external radial face of the piston 15c, and, radially outward, directly through the internal wall of the central part llb of the tubular body 11, this central part llb having an internal diameter greater than the internal diameter of the body 11 in its other parts. This passage 22 is in communication with each of the two chambers 20a and 20b by two sets of orifices 23a and 23b, the orifices 23a and 23b of each assembly communicating one respectively of the axial ends of the passage 22 with one respectively of the chambers 20a and 20b, by being distributed in rings of orifices drilled in the piston 15c and inclined radially outwards and from the corresponding chamber 20a or 20b towards the transverse median plane of the horn ps 11 and the piston 15c (transverse median plane passing substantially through the seal 21). It is thus possible to delimit the axial passage 22 over its entire length between the central part llb of the body 11, towards the outside, and the piston 15c towards the inside.

 The orifices 23a and 23b may be rolling orifices ensuring damping, in which case the heat source which they constitute is close to the metallic body 11, a good conductor of heat, and the hydraulic fluid heated by rolling at their crossing passes through the passage 22, and is directly in contact with the body 11 forming the outer shell of the damper, and itself directly in contact with the outside. The movement of the hot fluid in this passage 22 thus significantly improves the heat transfers to the outside.

 It is also possible that the orifices 23a and 23b are simply supply orifices for the annular passage 22, which constitutes the rolling orifice ensuring the damping. In this case, the heat is produced by the rolling of the fluid between the piston 15c of the external spacer 15 and the damper body 11. In this case also, the hottest fluid is directly in contact with the body 11, and the movement of the fluid in the passage 22 promotes heat transfers to the outside.

 As a variant, the annular passage 22 can be replaced by a plurality of axial passages each delimited between the body 11 and the piston 15c of the external spacer 15, each axial passage communicating with each of the two chambers 20a and 20b by orifices provided in the piston and opening at the axial ends of the passage.

 The example of a hydroelastic drag damper in FIG. 3 differs from that of the previous example, in terms of the basic structure, in that the piston 35c is no longer integral in movement with the body of the external envelope. 31, by means of an external spacer, but is fixed around the thin central part 37c of an internal tubular spacer 37, similar to the internal spacer 17 of the example in FIG. 2, of the same structure as this spacer 17 and fixed in the same way around a central rod 32, with the same structure as the central rod 12 of the example in FIG. 2. There is therefore a nut 38 screwed onto a threaded end 32a of the rod 32 to be in abutment against the end portion 37a of the spacer 37, the opposite end portion 37b of which abuts against the end portion with radial shoulder 32b which the rod 32 presents outside the body 31 This shock absorber also includes two sleeves the lastomer 33 and 34, similar to the sleeves 13 and 14 of FIG. 2, and like them vulcanized around the end parts 37a and 37b of the internal spacer 37. On the other hand, by their external radial face, the sleeves 33 and 34 are each vulcanized to the internal wall of one respectively of two spacer sleeves 35a and 35b, not integral with one another, and each mounted fitted and sliding in one respectively of two end bores 31a of the tubular body 31, until it comes into axial abutment against the central part 31b of this body 31, which has at this level an internal diameter smaller than that of its end bores 31a. This is obtained by maintaining the sleeve d 'spacer 35b in axial abutment against the central part 31b of the body 31 by a nut 36 screwed into an end bore 31a and directly in abutment against this spacer sleeve 35b, while the other spacer sleeve 35a is applied against the central part 31b of the body 31 by screwing a second nut 36 into the other end bore 31a, with interposition of the end sleeve 35e of the yoke or turned fixing part 35d, so as to obtain the same axial stacking as in the example of figure 2.

 In this example, the piston 35c, integral with the internal spacer 37, projects radially outwards on the latter and extends as far as near the internal wall of the central part 31b of the body 31, the piston of which 35c is only separated by an annular axial passage 42, opening through each of its axial ends directly into one respectively of the two chambers 40a and 40b of hydraulic fluid, which are, in this example, each delimited between the piston 35c, the corresponding elastomer sleeve 33 or 34, and the internal wall of the central part 31b of the body 31.

 The annular passage 42 constitutes the rolling orifice of the hydraulic fluid circulating between the two chambers 40a and 40b, and the heat source constituted by the hot fluid in this passage 42 is directly in contact with the metallic body 31, which favors the heat exchanges to the outside. These heat transfers are also favored by the fact that the chambers 40a and 40b of hydraulic fluid are directly delimited by the central part 31b of the body 31, and, as in the previous example, the movement of the hot fluid circulating in the chambers 40a and 40b and in the annular passage 42 promotes heat transfers to the outside of the body 31.

 The generation of heat is thus brought directly against the part of the damper in contact with the outside, namely the metal body 31, and advantage is taken of the dynamic conditions of movement of the hot hydraulic fluid to improve the thermal conductivity.

 It should be noted that this example in FIG. 3 saves the dynamic seal 21 and the production of the orifices 23a and 23b in the piston 15c of the example in FIG. 2.

 In the two examples of FIGS. 2 and 3, the fatigue strength of the shock absorbers is considerably improved by the reduction in the level of heating of the various components of the shock absorber, resulting from an optimization of the heat transfers to the outside.

Claims (9)

 1. Hydroelastic drag damper for rotorcraft rotor blade, intended to be mounted between two members of a rotor, a first member of which is a rotor blade or a member for connecting said blade to a hub of the rotor, and the second member is said hub, or a blade adjacent to the rotor, or a member for connecting said blade adjacent to said hub, to control the drag movement of said blade relative to said second member, and comprising  - at least one body (11, 31) of outer casing, tubular, metallic and substantially cylindrical,  - at least one central rod (12, 32), at least partially engaged substantially coaxially in said body (11, 31),  - at least two elastic supports (13, 14; 33, 34), supporting said rod (12, 32) in said body (11, 31) ensuring an elastic return of the body and the rod to an initial relative position,  - at least one piston (15c, 35c), disposed between the two elastic supports (13, 14; 33, 34), and integral in movement with one (15c, 32) of the two elements that are said body (15c, 35c) and said rod (12, 32), and  - at least two chambers (20a, 20b; 40a, 40b) delimited in said body (11, 31), around said rod (12, 32), on either side of said piston (15c, 35c), and each between said piston (15c, 35c) and one respectively of the elastic supports, each comprising at least one elastomeric sleeve (13, 14; 33, 34) elastically deformable and integral with the body (11, 31) and the rod (12 , 32) so as to provide a stiffness and ensure a tight closure of the corresponding chamber (20a, 20b; 40a, 40b), the two chambers filled with hydraulic fluid having variable volumes in opposite directions during essentially relative displacements axial of the rod (12, 32) and of the body (11, 31), and being in communication with each other by at least one rolling orifice (22, 23a, 23b, 42) of the hydraulic fluid to provide damping of the drag movement, characterized in that it comprises at least one passage (22, 42) delimited radially outwards directly through the wall of said metal body (11, 31) and traversed by laminated fluid flowing between the two chambers (20a, 20b; 40a, 40b) during relative movements of the rod (12, 32) and the body (11, 31).  2. Hydroelastic damper according to claim 1, characterized in that said passage (22) is in communication with at least one of the chambers (20a, 20b) by at least one rolling orifice (23a, 23b).  3. Hydroelastic damper according to one of claims 1 and 2, characterized in that said passage (42) delimits at least one rolling orifice of the hydraulic fluid.  4. Hydroelastic damper according to any one of claims 1 to 3, characterized in that said passage (22, 42) is delimited, over most of its length, between said body (11, 31), towards the outside, and said piston (15c, 35c), inward.  5. Hydroelastic damper according to any one of claims 1 to 4, characterized in that said passage (22) is in communication with at least one of said chambers (20a, 20b) by at least one orifice (23a, 23b) formed in said piston (15c).  6. Hydroelastic damper according to any one of claims 4 and 5, characterized in that said piston (15c) is integral with an external tubular spacer (15), fixed in the body (11), and to which the elastomeric sleeves (13, 14) are joined by external radial faces, said piston (15c) projecting from the external spacer (15) radially inwards and sliding with sealing around the central rod (12) by at least one seal dynamic seal (21).  7. Hydroelastic damper according to claim 6, characterized in that the elastomeric sleeves (13, 14) are secured by internal radial faces to an internal tubular spacer (17) fixed around the central rod (12), and the piston ( 15c) slides with sealing by said dynamic seal (21) around a central part (17c) of said internal spacer (17).  8. Hydroelastic damper according to any one of claims 4 and 5, characterized in that said piston (35c) is integral with an internal tubular spacer (37) fixed around the central rod (32) and to which the elastomeric sleeves (33, 34) are secured by internal radial faces, said piston (35c) projecting from the internal spacer (37) radially outward and towards said body (31), with the internal wall of which the piston (35c ) delimits said passage (42) each of whose ends opens directly into one of the two chambers (40a, 40b) respectively.  9. Hydroelastic damper according to any one of claims 1 to 8, characterized in that said passage (22, 42) is axial and annular.