(1) Field of the Invention
The present invention relates to the field of rotorcraft rotors, and it relates more particularly to ways of providing lead/lag damping for the blades of such rotors.
The present invention provides more specifically a rotorcraft rotor fitted with connection devices between lead/lag dampers and the blades of the rotary wing of said rotor. Such connection devices are more particularly of the type comprising a respective laminated ball joint that forms a stratified elastomer body incorporating blades of metal. Such a laminated ball joint is interposed in particular between a blade and a lead/lag damper associated therewith. This ball joint accommodates multidirectional relative movements between the members that are joined together by said ball joint.
(2) Description of Related Art
Rotorcraft are rotary wing aircraft in which at least lift is provided by at least one main rotor of substantially vertical axis. In the specific context of a helicopter, the main rotor provides the rotorcraft not only with lift, but also with propulsion in any travel direction. The attitude in flight of the helicopter can thus be modified by a human pilot or by an autopilot varying the pitch of the blades of the rotary wing of the main rotor cyclically and/or collectively.
Conventionally, the main rotor comprises a rotary hub driven in rotation by a power plant of the rotorcraft. Each blade has a root, referred to as the blade root, enabling it to be mounted individually on the hub in order to enable the blades to be driven jointly in rotation. In order to enable a pilot of the rotorcraft to cause the blades to perform said pitch variations, each blade is mounted on the hub with individual hinging at least about an individual axis for varying its pitch, referred to as the pitch variation axis, e.g. by means of a laminated abutment member having a spherical bearing surface.
The blades can thus be pivoted by the pilot of the rotorcraft about their respective pitch variation axes, collectively and/or cyclically, for the purpose of modifying the flight attitude of the rotorcraft.
Nevertheless, the individual behavior of blades moving on a hub is known to be complex. In addition to being movable in pivoting about their respective pitch variation axes, the blades are also commonly mounted to be individually movable relative to the hub in pivoting both about a flapping axis and about a lead/lag axis.
More particularly, each blade is generally mounted to be individually movable relative to the hub in vertical flapping about a flapping axis that extends orthogonally relative of the axis of rotation of the hub in a plane lying in the general plane of rotation of the hub. The term “vertical” should naturally be considered relative to the general direction in which the rotorcraft under consideration extends when on the ground. When the rotary wing is set into rotation, centrifugal force and lift forces typically cause the blades to move with vertical flapping motion, giving the rotary wing a shape that is slightly conical.
Still more particularly, the blades in general are also each hinged to the hub to perform individual lead/lag movement about a lead/lag axis extending substantially parallel to the axis of rotation of the hub, i.e. perpendicularly to the general plane in which the blade extends. Mounting the blades so that they are free to perform lead/lag movements and/or vertical flapping movements serves, when the rotor is set into rotation, to avoid the blades being subjected to bending moments in the general plane in which they extend.
Nevertheless, the individual oscillations of the blades about their lead/lag axes give rise to phenomena known as ground resonance and/or air resonance of the rotorcraft. Such resonance phenomena are known to be dangerous when the lead/lag movement of the blades about their lead/lag axes produces dynamic excitation in a stationary reference frame at a “regressive” frequency that is close to the resonant frequency of the aircraft on the ground or in flight.
In order to remedy this problem of resonance phenomena, it is known in particular to fit the blades of the main rotor with respective individual lead/lag damper devices suitable for damping the lead/lag oscillations of a given blade, referred to below as the blade “under consideration”, with said blade under consideration having a given lead/lag damper associated therewith for damping its lead/lag oscillations.
In particular, each such lead/lag damper is placed between the hub and the corresponding blade under consideration, either directly in a configuration known as a “blade-hub” configuration or else indirectly in a configuration known as an “interblade” configuration that passes via a blade adjacent to said blade under consideration.
The lead/lag dampers commonly used can be arranged in a variety of ways, e.g. they may be of an elongate shape and work in traction and compression, or indeed they may be cylindrical in shape and work in twisting, for example.
In this context, a lead/lag damper conventionally comprises damper means provided at each of its ends with a respective endpiece, each of the endpieces being fitted with a laminated ball joint. More particularly, each endpiece is provided with a lug having an eye surrounding the elastomer body that is stratified and that incorporates metal plates.
A proximal endpiece is dedicated to connecting the lead/lag damper to the hub, and a distal endpiece is dedicated to connecting the lead/lag damper and to said blade under consideration. The laminated ball joints are connected by respective fittings to the hub for the proximal endpiece and to the blade under consideration for the distal endpiece. In general, said fitting is a simple fitting for the hub, and by way of example a fitting that is fastened to the blade root of each blade for fastening the blade to the hub, such as a blade root that is arranged as a cuff, for example.
By way of example, Document GB 2 041 310 Lovera (1979) describes a hinged rotor for a helicopter in which each blade of a rotor of this type is connected to its hub so as to have freedom of movement enabling it essentially to pivot about three fundamental axes. In that rotor, a damper comprising a bar that is movable longitudinally in a casing is arranged between each U-shaped yoke and the hub of the rotor. The bar is connected by a ball joint to the appendix of the lever, while the casing is connected by a second ball joint to the hub. The center of one of the ball joints is arranged on the longitudinal axis of the yoke, while the center of the other ball joint is placed on an axis perpendicular to the preceding axis and passing through the center of the spherical surfaces of the bearing.
Document FR 3 005 631 or EP 2 803 571 describes a resilient hinge connection for connecting a lead/lag damper to a rotor blade of a rotary wing aircraft. The connection includes a resilient ball joint for connecting to the damper, the resilient ball joint having a preferred direction of rotation, a lug is secured to the blade and a rod is assembled in the lug and to the resilient ball joint so that the axis of revolution of the rod extends along the privileged direction. That axis of revolution is for being positioned in a plane perpendicular to the axis of rotation of the hub and the rod is connected by a first connection embedded in the lug and by a second connection embedded in the resilient ball joint.
Document US 2004/037701 describes a helicopter rotor hub having a main body and connection elements projecting radially from the main body in order to co-operate with the main body to define respective closed profile housings. These housings receive respective blades. Each connection element is defined by a component separate from the main body and connectable to the body by releasable fastener means. Each element is C-shaped and forms two opposite arms that are connected together by a transverse portion, with their free ends being fastened to said body by the releasable fastener means.
For information about a technological environment close to the present invention, reference may be made for example to the following documents FR 2 653 405 or U.S. Pat. No. 5,141,398, and FR 2 733 961 or U.S. Pat. No. 5,636,970. Those documents disclose various structural organizations and various mounting configurations on a hub for lead/lag dampers of blades forming the rotary wing of a rotorcraft main rotor.
The use of a laminated ball joint provides a robust connection between the lead/lag damper and the blade under consideration, while still allowing omnidirectional relative movement between the lead/lag damper and said blade under consideration. Furthermore, such a laminated ball joint provides the advantage of simplifying the structural arrangements for providing the looked-for connection with freedom to move omnidirectionally between the lead/lag damper and said blade under consideration. Moreover, such a laminated ball joint is commonly used for connecting together two members with freedom for omnidirectional movement relative to each other.
On this topic, reference may be made by way of example to the following documents FR 2 839 945 or US 2004/001754, and EP 0 901 580 or U.S. Pat. No. 5,902,050, which disclose such ways of using an elastomer body for connecting together two members that are to be assembled together with freedom for omnidirectional relative movement between each other.
More specifically, concerning a hinge connection between a lead/lag damper and a rotor blade of a rotorcraft making use of a laminated ball joint, reference may be made for example to Document FR 2 630 703 or U.S. Pat. No. 4,915,585.
More particularly, tested techniques that are traditionally used for connecting a lead/lag damper to a said blade under consideration are known that make use of a laminated ball joint. For this purpose, use is made in particular of a connection device between the lead/lag damper and said blade under consideration, in which a laminated ball joint fitted to the distal endpiece of the lead/lag damper is provided with a tube of inside recess that provides a passage for receiving a rod for fastening the laminated ball joint to said blade under consideration.
Said rod is inserted through the tube and is fastened, in particular by bolting, to the blade root of said blade under consideration. Conventional fastening of the rod to the blade root by bolting provides a connection between them that is easily undone in order to enable an operator to connect the lead/lag damper easily to said blade under consideration, but care must be taken to ensure that the quality of tightening is sufficient and reliable.
In this context, it should be recalled that each blade of a rotorcraft main rotor is commonly movable individually not only with lead/lag movement about a lead/lag axis oriented substantially parallel to the axis of rotation of the hub, but also in vertical flapping in its own general plane about a respective flapping axis that is oriented perpendicularly to the axis of rotation of the hub and that lies in its general plane of rotation, while also being capable of varying its pitch, by the blades, pivoting individually in its own general plane about its respective pitch variation axis that is oriented substantially perpendicularly to the axis of rotation of the hub along the direction in which the blade extends in its own general plane, where “perpendicularly” ignores the flapping movements of the blades.
In known manner, the laminated ball joint has a preferred direction of deformation. The rod fastened to the blade root of said blade under consideration then extends relative to the hub so as to facilitate making use of deformation of the laminated ball joint in said preferred direction of deformation.
Naturally, it is desirable to place the laminated ball joint under stress in its preferred direction of deformation as a function of choices made concerning the various stresses to which laminated ball joint is subjected under the effect of said blade under consideration moving relative to the hub about its various movement axes with the hub, including the pitch variation axis, the flapping axis, and the lead/lag axis.
By way of indication, the preferred direction of deformation of the laminated ball joint allows said blade under consideration to move angularly by an amount that may involve angular variation of about 30° at most, whereas in directions perpendicular to said preferred direction of deformation of the laminated ball joint, angular movements of the blade under consideration are allowed at most over about 10°.
The lead/lag movements of the blade under consideration are small in amplitude and they are damped by the lead/lag damper provided for this purpose. However, the movements of the blade under consideration in terms of pitch variation and vertical flapping transmit a large amount of torque to the lead/lag damper, which torque is then taken up by the laminated ball joint.
In order to preserve the rod as well as possible from the twisting stresses to which it is subjected under the effect of flapping of said blade under consideration, it is conventional to orient the rod perpendicularly to the general plane in which the blade under consideration extends, i.e. essentially parallel to the lead/lag axis or indeed essentially parallel to the axis of rotation of the hub, ignoring pivoting variations of the blade respectively about its pitch variation axis and about its flapping axis. However, given that the rod is in such an orientation, the laminated ball joint is strongly stressed about axes perpendicular to the orientation of the rod, i.e. about high-stress axes that are oriented essentially parallel respectively to the flapping axis and to the pitch variation axis.
In this context, and also because of known facts for specifying the concept of the essential orientation of the rod, in particular relative to the axis of rotation of the hub, the movements of the blade under consideration about its various movement axes (pitch variation axis, flapping axis, and lead/lag axis) are known to vary depending on the azimuth orientation of said blade under consideration relative to the forward direction of the rotorcraft along its pitching and/or roll axis. Consequently, it is also known that there is a difference between the respective orientations of the rods providing connections between the various lead/lag dampers and the blades under consideration that are respectively associated therewith.
On the basis of the above-specified observed facts that have led to the approaches of the present invention, the present invention provides a rotorcraft rotor having lead/lag dampers for the blades making up the rotary wing of the rotorcraft.
In conventional manner, the lead/lag dampers are fastened to respective ones of said blades under consideration that are associated therewith by means of respective connection devices involving a respective laminated ball joints as defined above.