Patent Description:
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 <CIT> single tension torsion straps are known, wherein each of such 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 build of layers, which are showing an overall symmetrical shape with identically formed connection eye sections in the peripheral areas as explicitly mentioned and depicted in <CIT>.

The tension torsion straps of <CIT> are showing a layered structure and are generally thicker as tension torsion straps of other prior art. The connection eyes or eyelets in the peripheral areas are identically formed and both peripheral areas are absolutely congruent, if the peripheral areas would 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 is resulting.

The object of the present invention is to create for a rotor head of a rotary wing aircraft an optimized kind of tension torsion strap and a rotor head of a rotary wing aircraft with a multiplicity of such tension torsion straps, whereby a flawless production and installation is guaranteed. A simplified maintenance can follow and optional weak points can become visible before destruction. A more simplified maintenance work of the tension torsion strap of the rotor head is the result, 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 during assembly, it may even be possible to extend the maintenance intervals.

A preferred exemplary embodiment of the subject matter of the invention with some additional optional features is described below in conjunction with the attached drawings.

It should be noted that in the figures, which are not always representing different embodiments of the invention, the same parts are provided with the same reference symbols or the same component names.

As example for description of the invention a part of a tail rotor of a rotary wing aircraft is shown in <FIG>. For the sake of simplicity a possible shroud and a multiplicity of rotor blades of the rotor have been left out. At a rotor drive train, which is connected to a gear box, a rotor head <NUM> can be connected. The rotor head <NUM> comprises a rotor hub <NUM>, a rotor drive train hub <NUM> in form of a coupling element, which is separated from rotor hub <NUM> or integrated in rotor hub <NUM>, a multiplicity of blade holder <NUM> with associated tension torsion straps <NUM>, which are detachably fixed with mounting bolts <NUM>. A pitch control unit <NUM> rotatably connected at least to the multiplicity of blade holder <NUM> and the whole rotor drive train hub <NUM> and all can be covered with a hub cap <NUM>. The rotor head <NUM> can be rotated around the dotted central axis.

Here the blade holder <NUM> comprises two parts, a strap holding part <NUM> and a blade holding part <NUM>. Because the rotor blades are not of interest here, the blade holding part <NUM> is not further described here. Both parts are integrally moulded here. The strap holding part <NUM> shows a connection bore <NUM> through which a strap holding bolt <NUM> can be inserted to hold one side of the tension torsion strap <NUM>. An integrally moulded pitch horn <NUM> or a fixed pitch horn <NUM> provide for a coupling to the pitch control unit <NUM> of every blade holder <NUM> and indirectly of every rotor blade. When assembled, the blade holder <NUM> is partly located in holes inside the rotor hub <NUM>. At least the blade holding part <NUM> of each blade holder <NUM> protrudes from the holes.

The tension torsion strap <NUM> essentially shows three sections, a first connection eye <NUM>, a central section <NUM> and a second connection eye <NUM>. The first connection eye <NUM> merges into the central section <NUM> and the central section <NUM> into the second connection eye <NUM>, while all sections are made of planar or flat layers.

With the first connection eye <NUM> a connection at the blade holder <NUM> is possible. Both components are connected by means of screw connection or socket pins, after the strap holding bolt <NUM> has been passed through the connection bore <NUM>. Thus a linearly unchangeable fastening is achieved. When fastened, the tension torsion strap <NUM> protrudes through openings in the rotor hub <NUM> up to the rotor drive train hub <NUM>. On the other side the tension torsion strap <NUM> is connected to the blade holder <NUM>, more specifically to the strap holding part <NUM> mounted inside the circumference of the rotor hub <NUM>. That the design of the blade holder <NUM> can also be done in different ways is clear to the person skilled in the art.

With the second connection eye <NUM> the tension torsion strap <NUM>, a detachable attachment to the rotor drive train hub <NUM> can be made. For this purpose, corresponding openings are provided in the rotor drive train hub <NUM>, through which the mounting bolt <NUM> can be pushed, holding the second connection eye <NUM>. Different types of connection, also here screw connections or plug connections, are possible. The whole tension torsion strap <NUM> must have a certain flexibility in order to allow the pitch control unit <NUM> to deflect the rotor blade with blade holder <NUM> accordingly.

In the insight view in the rotor head <NUM> according to <FIG>, the multiplicity of tension torsion straps <NUM>, each connected one-sided at the rotor drive train hub <NUM>, reaching through openings in the rotor hub <NUM> and radially projecting to corresponding blade holders <NUM>, is depicted. At the inner side of the tension torsion straps <NUM> they are fixed with the mounting bolts <NUM>, which are secured by a nut. At the strap holding part <NUM> the strap holding bolt <NUM> holds the tension torsion strap <NUM> inside the blade holder <NUM>. Here, too, a detachable connection by means of a pin connection is used. After connection of each pitch horn <NUM> at the pitch control unit <NUM>, each tension torsion strap <NUM> can be lengthwise pivoted by a minimum angle. Some rotor blades are graphically indicated with dotted lines, wherein also the longitudinal axis of the tension torsion straps <NUM> are marked.

A corresponding sectional view through a mounted tension torsion strap <NUM> is shown in <FIG>. The tension torsion strap <NUM> is clamped and detachably connected between the rotor hub <NUM> and the rotor drive train hub <NUM> in direction of the longitudinal axis L of the tension torsion strap <NUM>. An outside area <NUM> of the tension torsion strap <NUM> is connected at the rotor hub <NUM> and an inside area <NUM> is connected at the rotor drive train hub <NUM>.

The shape of the tension torsion strap <NUM> guarantees the advantageous results of the here described invention.

The tension torsion strap <NUM> shows along its longitudinal axis L a first connection eye <NUM> in the outside area <NUM>, a central section <NUM> and a second connection eye <NUM> in the inside area <NUM>. The tension torsion strap <NUM> has a layered structure, comprising at least two kinds of layer areas, comprising different material layer. It is advantageous to use a layered structure to achieve the desired torsional properties.

The overall shape along the longitudinal axis L of the tension torsion strap <NUM> has to be asymmetrical, as depicted in <FIG>. By asymmetry a different shape of the peripheral areas <NUM>, <NUM> or a non-concurrent design of the peripheral areas <NUM>, <NUM> is meant. The degree of asymmetry must be so great that the differences in the peripheral areas <NUM>, <NUM> are correspondingly visible to the naked eye.

With such an asymmetrical tension torsion strap <NUM>, an indication of the correct orientation for installation is reached, what can be easily determined by eye. Beside this visual help, the asymmetric shape is adapted their counterparts, which makes it impossible to install the part wrongly. Therewith a really failure proof system is reached.

Such asymmetry especially in the peripheral areas, can additionally lead to peripheral areas with different properties. Here the outside area <NUM> is a stronger section <NUM> in terms of strength, than the inside area <NUM>, which is a weaker section <NUM>. If a weaker section <NUM> is reached as part of the tension torsion strap <NUM>, this end shows signs of fatigue failure first. This makes inspection more easy, because only the weaker section <NUM> can be checked. Therefore associated cut-outs should be placed in a blade pitch control spider or rotor drive train hub <NUM> or a cover of the rotor head <NUM>.

The typical layer structure arrangement for tension torsion straps <NUM> is depicted in <FIG>. 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 layers <NUM>, <NUM>, <NUM>' results in structures that show lower torsional stiffness as far as possible, but keeping strength in axial tension. The result using multiple materials is less stiff in torsion than using a single material. Best results were reached, using different metals/plastics compounds in a first layer area <NUM>, a second layer area <NUM> and a third layer area <NUM>'. Especially when these different layers <NUM>, <NUM>, <NUM>' are alternately arranged as shown in <FIG>, minimum torsional stiffness can be achieved. We found, that an alternated arrangement of at least two different layer areas <NUM>, <NUM> shows good results. Some layers could be from plastics and than favourably reinforced by fibres. However they are single layers and not a complete strap in one. Thereby we can say that they can be cut from a 2D plate, which is easier for manufacturing.

As clearly depicted in <FIG>, the asymmetry of the tension torsion strap <NUM> particularly of the peripheral areas <NUM>, <NUM> can be achieved by different curvature forms.

The outside area <NUM> shows a curvature of stronger section cs, while the inside area <NUM> shows a curvature of weaker section cw. The curvature cs in the outside area <NUM> in longitudinal direction away from the longitudinal axis L is significantly less than in the inside area <NUM>. Accordingly, the shape of the first eyelet <NUM> is larger in longitudinal direction than the shape of the second eyelet <NUM>. Both eyelets <NUM>, <NUM> are easy to distinguish from each other and the alignment of such tension torsion strap <NUM> can easily be done correctly. The individual used layers of the tension torsion strap <NUM> must be adapted to the overall asymmetry of the tension torsion strap <NUM> to be achieved.

According to the invention, to ensure the correct arrangement of the tension torsion strap <NUM> while manufacturing and during placement between rotor hub <NUM> and rotor drive train hub <NUM> is the arrangement of an index <NUM>.

With the index <NUM> at least one peripheral area <NUM>, <NUM>, an asymmetrical tension torsion strap <NUM> with asymmetrical peripheral areas <NUM>, <NUM> can be achieved.

Such index <NUM> can be located on one of the end faces or side faces, in the area of the outer area <NUM> or the inside area <NUM>. In this case the index <NUM> is preferably located in the area of the inside area <NUM>, protruding from the outer face of the inside area <NUM> protruding away from the side face of the inside area <NUM> in longitudinal direction L.

Accordingly, even only with the index <NUM> an overall asymmetry of the whole tension torsion strap <NUM> described above can be reached.

For visibility of the correct preassembly of the tension torsion strap <NUM> the index <NUM>, for example formed as an arrow as depicted here is added on one side of the tension torsion strap <NUM>. The arrow <NUM> protrudes from the outer surface of a peripheral area <NUM>, <NUM>, here at the side of the second connection eye <NUM>, indicating a direction with its arrowhead.

Due to asymmetry of the tension torsion strap <NUM> reached by means here described, with an index <NUM> at the tension torsion strap <NUM>, a wrong installation of the whole tension torsion strap <NUM> in the rotor head <NUM> respectively in the rotor hub <NUM> and/or the rotor drive train hub <NUM> can be prevented.

Here it is essential to allow the stronger section <NUM> and the weaker section <NUM> to be at the correct position. Therefore the tension torsion strap <NUM> surrounding parts in the rotor head <NUM> are designed in a way that a protruding index <NUM> or another form of an index <NUM>' as described below, would collide with them. This disables the possibility to install the tension torsion strap <NUM> in the rotor head <NUM> in the wrong way.

To improve the torsional behaviour the different layers <NUM>, <NUM>, <NUM>' can be of different shape and material.

The asymmetric tension torsion strap <NUM> is asymmetrical relatively to a transverse axis Q with two unequally shaped peripheral areas <NUM>, <NUM>. These peripheral areas <NUM>, <NUM> cannot be brought into alignment when folding around the transverse axis Q.

The main intention of an asymmetric tension torsion strap <NUM> is to make it impossible to install the part incorrectly.

For higher stability of the asymmetric tension torsion strap <NUM>, a wrapping <NUM> is used, wrapped around the central section <NUM>. Such wrapping <NUM> is a synthetic piece, preferably made of a transparent or translucent elastomer. Preferably the wrapping <NUM> is designed endless or hose-like. The wrapping should be at least partly in direct contact with the outer surface of the central section <NUM>.

Possible elastomers or polymers for the wrapping <NUM> are sufficiently flexible polymers, in particular in form of a heat shrinking tube. Preferred is a wrapping <NUM> of a very soft material in order not to provide additional unintended stiffness or attracting stress injection, a moulded elastomer as silicone.

The wrapping <NUM> is tightly wrapped and linearly immovable arranged, could be attached via heat shrinking, surrounding all layers <NUM>, <NUM>, <NUM>' in the central section <NUM> of the tension torsion strap <NUM>.

In the perspective view of a tension torsion strap <NUM> according <FIG>, the layer-like structure can be easily recognized. Beside the improving absorption of the centrifugal forces and still providing sufficient torsional properties, the layer-like structure can be used as another index <NUM>'.

Here as another index <NUM>', an indexed cross section Si of the tension torsion strap <NUM> is used, wherein adjacent layers have different shapes or different layer outlines, designed in a way to create a prescribed shape, visualizing correct assembly of the tension torsion strap <NUM> in the rotor head <NUM>. The indexed cross section Si here shows a hexagonal shape with a multiplicity of layers <NUM>, <NUM>, with different layer outlines. Direct neighbouring layers show different layer outlines, leading to the overall hexagonal index cross section Si. The correct assembly of the tension torsion strap <NUM> is easy to recognized, because an incorrect sequence of the individual layer <NUM>, <NUM> arrangement can be read off directly.

Again, the indexing, the index cross section Si of the tension torsion strap <NUM> can correspond to the shape of surrounding parts like rotor hub <NUM>, rotor drive train hub <NUM> and/or blade holder <NUM>. The cross section Si is formed to optimize torsional stiffness and drive the stress in the tension torsion strap <NUM>.

Of course the index cross section Si could have others than hexagonal shapes, like polygons, star polygons, in particular regular polygons, each are build with direct neighbouring layers with different shapes.

As indicated in <FIG>, the layers <NUM>, <NUM> could also comprise groups of layers with different materials and/or shapes, here at least two groups of different shapes and materials in the layers <NUM>, <NUM>, for strength and stiffness tuning are used. This was done so far by the shape of the different layers, but we add here one additional tuning method by selecting different materials.

The material of the inner layers <NUM> in the cross section Si is in particular stiffer or more rigid than the material of the outer layers <NUM> used. Additionally the wear behaviour between the layers can be improved with the different materials.

In general the tension torsion strap <NUM> needs no attachment of additional or external components at the body of the tension torsion strap <NUM> respectively the joined layers <NUM>, <NUM>, <NUM>' in order to reach an overall asymmetric body. Accordingly no components can be attached in the wrong way or can fall off unwantedly.

Claim 1:
Tension torsion strap (<NUM>) for a rotor head (<NUM>) of a rotary wing aircraft, comprising in direction of a longitudinal axis (L) a first peripheral area (<NUM>) with a connection eye (<NUM>), a central section (<NUM>) and a second peripheral area (<NUM>) with a connection eye (<NUM>), wherein the one-piece tension torsion strap (<NUM>) is made of a multiplicity of joined layers (<NUM>, <NUM>), characterized in that
the tension torsion strap (<NUM>), without attachment of additional components at the body of the tension torsion strap (<NUM>) respectively the joined layers (<NUM>, <NUM>) shows an overall asymmetric shape relatively to a transverse axis (Q) due to two unequally shaped peripheral areas (<NUM>, <NUM>), which cannot be brought into alignment when folding around the transverse axis (Q), wherein at one of the peripheral areas (<NUM>, <NUM>) an index (<NUM>) is formed.