Tension torsion strap of a rotor head of a rotary wing aircraft

This disclosure describes a tension torsion strap for part of rotor head of a rotary wing aircraft. The tension torsion strap includes, in a direction of a longitudinal axis, a first peripheral area with a connection eye, a central section, and a second peripheral area with a connection eye. The tension torsion strap may be of one-piece made of a multiplicity of joined layers. Improved production and installation is of the tension torsion strap may be achieved by that disclosed in this disclosure, resulting in, among other advantages, more simplified maintenance because the tension torsion strap has an overall asymmetric shape relatively to a transverse axis due to two unequally shaped peripheral areas which cannot be brought into alignment when folding around the transverse axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an US national phase of PCT/EP2021/060689, filed Apr. 23, 2021, which claims priority to Swiss patent application 00539/20, filed on May 6, 2022, the content of both of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a tension torsion strap which operates as part of rotor head of a rotary wing aircraft. The tension torsion strap includes, in a direction of a longitudinal axis a first peripheral area with a connection eye, a central section and a second peripheral area with a connection eye. The tension torsion strap may comprise one-piece and be made of a multiplicity of joined layers. The present invention further relates to a rotor head of a rotary wing aircraft, comprising at least a rotor hub, a rotor drive train hub, and a multiplicity of blade holder, wherein the multiplicity of blade holder is fixable with a multiplicity of associated tension torsion straps.

Description of Related Art

Tension torsion straps as part of a rotor head of a rotary wing aircraft are known. The rotor head comprises at least a rotor hub, a rotor drive train hub and a multiplicity of blade holder, wherein the multiplicity of blade holder is fixable with a multiplicity of associated tension torsion straps at the rotor drive train hub. The tension torsion strap or member as part of a rotorcraft counter torque device or rotor head absorbs centrifugal forces during flight operations.

From EP3315403 single tension torsion straps and twistable beams are known, wherein each of the twistable beams is fixed to a blade and to the hub, in particular to absorb the centrifugal forces undergone by the blade. Each tension torsion strap has a sufficiently low torsional stiffness to allow rotation of the blade around its pitch axis. The tension torsion strap is built of layershaving an overall symmetrical shape with identically formed connection eye sections in the peripheral areas as set out in EP3315403.

The tension torsion straps of U.S. Pat. No. 8,834,128 include a layered structure and are generally thicker than other tension torsion straps known in the prior art. Connection eyes or eyelets in the peripheral areas are identically formed and both peripheral areas are congruent, if the peripheral areas were to be folded around a transverse axis. Inclined planes in the peripheral areas are also congruent on both sides so that there is mirror symmetry of the tension torsion strap relative to the longitudinal axis and a transversal axis. Therewith, an overall symmetric shape relatively to a transverse axis, or in other words a rotation symmetry of prior art tension torsion straps, results.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to create, for a rotor head of a rotary wing aircraft, an optimized kind of tension torsion strap along with a rotor head of a rotary wing aircraft with a multiplicity of such tension torsion straps, whereby an improved production and installation is sought. A simplified maintenance may follow and optional weak points may become visible before destruction. A more simplified maintenance work of the tension torsion strap of the rotor head may result, thereby leading to a more efficient maintenance of the rotary wing aircraft.

As the manufacturing of the tension torsion straps is optimized and errors are prevented and/or reduced during assembly, a possibility arises to extend maintenance intervals.

An additional result, namely, easier detection of cracks or imminent fractures, time-consuming dismantling of many components may be avoided.

DETAILED DESCRIPTION OF THE INVENTION

As used throughout the present disclosure, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, the expression “A or B” shall mean A alone, B alone, or A and B together. If it is stated that a component includes “A, B or C”, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. Expressions such as “at least one of” do not necessarily modify an entirety of the following list and do not necessarily modify each member of the list, such that at least one of “A, B, and C” should not be understood as including only one of A, only one of B, only one of C, or any combination of A, B, and C.

FIG.1sets out an example embodiment of the herein described invention. For purposes of clarity and/or ease of understanding, a possible shroud and a multiplicity of rotor blades of the rotor have been omitted. At a rotor drive train, which is connected to a gear box, a rotor head3can be connected. The rotor head3comprises a rotor hub4, a rotor drive train hub5in the form of a coupling element which is separated from rotor hub4or integrated in rotor hub4, and a multiplicity of blade holder6with associated tension torsion straps7which are detachably fixed with mounting bolts8. A pitch control unit9is rotatably connected at least to the multiplicity of blade holder6and the whole rotor drive train hub5, and all can be covered with a hub cap10. The rotor head3can be rotated around the dotted central axis.

As depicted, the blade holder6comprises two parts, a strap holding part60and a blade holding part61. For purposes of brevity, further description of the blade holding part61is herein omitted. Both parts are integrally moulded here. The strap holding part60includes a connection bore600through which a strap holding bolt601can be inserted to hold one side of the tension torsion strap7. An integrally moulded pitch horn602or a fixed pitch horn602provides for a coupling to the pitch control unit9for every blade holder6and indirectly for every rotor blade. When assembled, the blade holder6is partly located in holes inside the rotor hub4. At least the blade holding part61of each blade holder6protrudes from the holes.

The tension torsion strap7essentially includes three sections: a first connection eye70, a central section71, and a second connection eye72. The first connection eye70merges into the central section71and the central section71merges into the second connection eye72. All sections may be made of planar or flat layers.

With the first connection eye70, a connection at the blade holder6becomes possible. Both components are connected by means of a screw connection or socket pins after the strap holding bolt601has been passed through the connection bore600. Thus, a linearly unchangeable fastening is achieved. When fastened, the tension torsion strap7protrudes through openings in the rotor hub4up to the rotor drive train hub5. On the other side, the tension torsion strap7is connected to the blade holder6, and more specifically to the strap holding part60mounted inside the circumference of the rotor hub4. Other design options for and/or of the blade holder6may be implemented here, the other designs being appreciated by a person skilled in the art.

With the second connection eye72the tension torsion strap7, a detachable attachment to the rotor drive train hub5can be made. For this purpose, corresponding openings are provided in the rotor drive train hub5, through which the mounting bolt8can be pushed, thereby holding the second connection eye72. Different types of connection, including screw connections or plug connections, are available under the invention. The whole tension torsion strap7must have a certain flexibility in order to allow the pitch control unit9to deflect the rotor blade with blade holder6.

In the rotor head3as depicted inFIG.2a, the multiplicity of tension torsion straps7, each connected one-sided at the rotor drive train hub5, reaching through openings in the rotor hub4, and radially projecting to corresponding blade holders6, are depicted. At the inner side of the tension torsion straps7, the mounting bolts8are fixed and may be secured by a nut. At the strap holding part60, the strap holding bolt601holds the tension torsion strap7inside the blade holder6. Here, too, a detachable connection by means of a pin connection may be used. After connection of each pitch horn602at the pitch control unit9, each tension torsion strap7can be lengthwise pivoted by a minimum angle. Some rotor blades are graphically indicated with dotted lines, wherein, additionally, the longitudinal axis of the tension torsion straps7are marked.

A corresponding sectional view through a mounted tension torsion strap7is shown inFIG.2b. The tension torsion strap7is clamped and detachably connected between the rotor hub4and the rotor drive train hub5in a direction of the longitudinal axis L of the tension torsion strap7. An outside area700of the tension torsion strap7is connected at the rotor hub4and an inside area720is connected at the rotor drive train hub5.

The shape of the tension torsion strap7supports advantageous results of the presently described invention.

The tension torsion strap7includes along its longitudinal axis L a first connection eye70in the outside area700, a central section71, and a second connection eye72in the inside area720. The tension torsion strap7has a layered structure and comprises at least two kinds of layer areas having different material layer. It is advantageous to use a layered structure to achieve desired torsional properties.

As depicted inFIG.3a, the overall shape along the longitudinal axis L of the tension torsion strap7is asymmetrical. By the asymmetry, a different shape of the peripheral areas700,720, or a non-concurrent design of the peripheral areas700,720is meant. The degree of asymmetry must be so great that the differences in the peripheral areas700,720are correspondingly visible to the naked eye.

With such an asymmetrical tension torsion strap7, an indication of the correct orientation for installation may be reached and determined with the naked eye. Beside this visual help, the asymmetric shape is adapted to their counterparts, thus making it impossible to install the part incorrectly. Therewith, an essentially failure proof system is obtained.

Such asymmetry, especially in the peripheral areas, can additionally lead to peripheral areas with different properties. Here the outside area700is a stronger section in terms of strength than the inside area and weaker section720. If a weaker section720is reached as part of the tension torsion strap7, this end will first show signs of fatigue failure. This makes inspection easier because only the weaker section720need be checked. Therefore, associated cut-outs should be placed in a blade pitch control spider or rotor drive train hub5or a cover of the rotor head3.

The typical layer structure arrangement for tension torsion straps7is depicted inFIG.3b. As tests have shown, preferable metallic sheets of a first material or different metals are used, which could be alternately provided or fixed with plastic layers. The use of several materials in different layers73,74,74′ results in structures that have as low as possible torsional stiffness while still maintaining sufficient strength in axial tension. A result in using multiple materials is less stiffness in torsion than when using a single material. Preferred results may be reached when using different metals/plastics compounds in a first layer area73, a second layer area74and a third layer area74′. Especially when these different layers73,74,74′ are alternately arranged as shown inFIG.3b, minimum torsional stiffness can be achieved. An alternated arrangement of at least two different layer areas73,74may also produce preferred results. Some layers may comprise plastics and than favourably reinforced by fibres. However, they are single layers and not a complete strap in one. Accordingly, they can be cut from a 2D plate thereby easing certain manufacturing requirements.

As depicted inFIG.3c, the asymmetry of the tension torsion strap7, and in particular peripheral areas700,720, can be achieved by different curvature forms. The outside area700includes a curvature of stronger section cs, while the inside area720includes a curvature of weaker section cw. The curvature cs in the outside area700in a longitudinal direction away from the longitudinal axis L is significantly less than in the inside area720. Accordingly, the shape of the first eyelet70is larger in longitudinal direction than the shape of the second eyelet72. Both eyelets70,72are easy to distinguish from each other and the alignment of such tension torsion strap7can easily be done correctly. The individual used layers of the tension torsion strap7must be adapted to the overall asymmetry of the tension torsion strap7to be achieved.

Another feature to ensure the correct arrangement of the tension torsion strap7during manufacturing and during placement between rotor hub4and rotor drive train hub5is the arrangement of an index75. With the index75, at at least one peripheral area700,720, an asymmetrical tension torsion strap7with asymmetrical peripheral areas700,720can also be achieved.

Such index75can be located on one of the end faces or side faces, in the area of the outer area700or the inside area720. In this case the index75is preferably located in the area of the inside area720, protruding from the outer face of the inside area720and protruding away from the side face of the inside area720in longitudinal direction L.

Accordingly, even only with the index75, an overall asymmetry of the whole tension torsion strap7described above can be reached.

For visibility of the correct preassembly of the tension torsion strap7, the index75, for example, formed as an arrow as depicted here, is added on one side of the tension torsion strap7. The arrow75protrudes from the outer surface of a peripheral area700,720, here at the side of the second connection eye72, indicating a direction with its arrowhead.

Due to asymmetry of the tension torsion strap7reached by means here described, with or without an index75at the tension torsion strap7, a wrong installation of the whole tension torsion strap7in the rotor head3, respectively in the rotor hub4and/or the rotor drive train hub5, can be prevented.

Here it is essential to allow the stronger section700and the weaker section720to be at the correct position. Therefore, the tension torsion strap7surrounding parts in the rotor head3are designed in such a way that a protruding index75or another form of an index75′, as described below, would collide with them. This disables the possibility of installing the tension torsion strap7in the rotor head3in the wrong way.

To improve the torsional behaviour the different layers73,74,74′ can be of different shape and material.

The asymmetric tension torsion strap7is asymmetrical relatively to a transverse axis Q with two unequally shaped peripheral areas700,720. These peripheral areas700,720cannot be brought into alignment when folding around the transverse axis Q.

A main intention of an asymmetric tension torsion strap7, with or without an index75is to make it impossible to install the part incorrectly. For higher stability of the asymmetric tension torsion strap7with or without an index75, a wrapping76is used and wrapped around the central section71. Such wrapping76is a synthetic piece, preferably made of a transparent or translucent elastomer. Preferably the wrapping76is designed to be endless or hose-like. The wrapping should be at least partly in direct contact with the outer surface of the central section71.

Possible elastomers or polymers for the wrapping76are sufficiently flexible polymers, in particular in a form of a heat shrinking tube. Preferred is a wrapping76of a very soft material in order not to provide additional unintended stiffness or to attract stress injection, the material including a moulded elastomer such as silicone.

The wrapping76is tightly wrapped and linearly immovably arranged. The wrapping76could be attached via heat shrinking and surrounding all layers73,74,74′ in the central section71of the tension torsion strap7.

In the perspective view of a tension torsion strap7according toFIG.5a, the layer-like structure can be easily recognized. Beside improving absorption of the centrifugal forces and still providing sufficient torsional properties, the layer-like structure can be used as index75′.

Here as an index75′ an indexed cross section Si of the tension torsion strap7is used, adjacent layers have different shapes or different layer outlines and are designed in such a way as to create a prescribed shape while visualizing correct assembly of the tension torsion strap7in the rotor head3. The indexed cross section Si includes a hexagonal shape with a multiplicity of layers73,74, with different layer outlines. Direct neighbouring layers include different layer outlines, leading to the overall hexagonal index cross section Si. The correct assembly of the tension torsion strap7is easy to recognize because an incorrect sequence of the individual layer73,74arrangement can be read off directly.

The indexing along with the index cross section Si of the tension torsion strap7can correspond to the shape of surrounding parts like rotor hub4, rotor drive train hub5and/or blade holder6. The cross-section Si is formed so as to optimize torsional stiffness and drive the stress in the tension torsion strap7.

The index cross section Si may comprise shapes other than hexagonal shapes, such shapes including polygons, star polygons, and in particular regular polygons, each being built with direct neighbouring layers with different shapes.

As indicated inFIG.5b, the layers73,74could also comprise groups of layers with different materials and/or shapes, here at least two groups of different shapes and materials in the layers73,74, may be used for strength and stiffness tuning. While this has so far been done by the shape of the different layers, here an additional tuning method by selecting different materials is added.

The material of the inner layers73in the cross-section Si is in particular stiffer or more rigid than the material used of the outer layers74. Additionally, the wear behaviour between the layers may be improved with the different materials.

In general, the tension torsion strap7needs no attachment of additional or external components at the body of the tension torsion strap7and in particular the joined layers73,74,74′, in order to reach an overall asymmetric body. Accordingly, no components can be attached in the wrong way or can fall off unwantedly.

All the tension torsion straps7according to the invention can be described as one piece of a layered structure. The shaping of the tension torsion strap7and in particular of the shapes of each layer73,74,74′, results in certain technical advantage and solves certain problems. Due to the shaping the herein described tension torsion straps7, a rotation symmetry to the transverse axis is avoided.

Since the devices and methods described in detail above are examples of embodiments, they can be modified to a wide extent by the skilled person in the usual manner without departing from the scope of the invention. In particular, the mechanical arrangements and the proportions of the individual elements with respect to each other are merely exemplary. Some preferred embodiments of the apparatus according to the invention have been disclosed above. The invention is not limited to the solutions explained above, but the innovative solutions can be applied in different ways within the limits set out by the claims.