Apparatus for Connecting a Rotor Blade to a Rotor Hub

The present invention relates to an apparatus for connecting a rotor blade to a rotor hub, more specifically, for connecting a helicopter rotor blade to the hub of the drive shaft. Such connecting apparatus conventionally includes a blade connector bolt and a dampened connection between the rotor blade and the rotor hub.

The forces effective on a rotor or rather on a rotating blade and on the anchoring means connecting the blade to the rotor hub, comprise centrifugal force components as well as static and dynamic shearing force components and bending moments. Especially the bending moments are caused by periodically recurring wobbling and swivel motions of the rotor blade.

German Pat. No. 1,531,675 discloses in connection with hingeless rotors to take up the wobbling and swivel motions substantially ahead of the blade root to avoid the effect thereof on the connections of the blade to the rotor shaft. This is accomplished by constructing the neck section of the rotor blade to be yielding to bending moments but stiff relative to torsion. However, in such a construction it is not possible to increase the length of the neck section, which has a low resistance to bending, to any desired degree, because such an increase would adversely affect the aerodynamic characteristics of the rotor blade. Furthermore, it is unavoidable that the bending and shearing deformations in combination with the effective centrifugal forces are transmitted to the connecting hardware between the root of the rotor blade and the rotor drive shaft. As a result, these transmitted deformations and forces generate continuous friction forces between the blade grommet made of fiber reinforced synthetic material and the metal hardware. Such friction forces must be taken up or absorbed by the inner or inherent damping of the rotor blade and the connecting hardware. Since these forces cannot be precisely ascertained and since they may vary, the interfaces between the blade and the connecting hardware are subject to a continuous destructive wear and tear effect.

In view of the above, it is the aim of the invention to achieve the following objects singly or in combination:

TO AVOID THE DRAWBACKS OF THE PRIOR ART BLADE CONNECTOR STRUCTURES FOR HINGELESS ROTORS, MORE SPECIFICALLY, TO EFFICIENTLY DAMPEN THE SWIVEL MOMENTS WHICH ARE EFFECTIVE AND TO CONTROL OR INFLUENCE THE TAKE-UP OF THE DAMPING;

TO CONTROL THE FRICTION COEFFICIENT OF THE DAMPING ELEMENTS IN SUCH A MANNER THAT THE DESIRED DAMPING EFFECT MAY BE TAKEN INTO ACCOUNT AS A SUBSTANTIALLY CONSTANT FACTOR;

TO CONSTRUCT A CONNECTING MECHANISM FOR A HELICOPTER ROTOR BLADE WHICH PROVIDES A TWO-FOLD SUPPORT FOR SUCH BLADE;

TO CONSTRUCT THE DAMPING ELEMENTS IN SUCH A MANNER THAT THEIR DAMPING CHARACTERISTICS ARE SUBJECT TO CALCULATION, SO THAT THEY MAY EASILY BE ADAPTED TO ANY REQUIREMENTS THAT MAY OCCUR IN CONNECTION WITH SECURING A ROTOR BLADE TO A ROTOR SHAFT;

TO CONSTRUCT THE CONNECTING APPARATUS IN SUCH A MANNER THAT THE ROTOR BLADES MAY EASILY BE FOLDED FOR THE PURPOSE OF TRANSPORTING OR STORING; AND

TO CONSTRUCT THE CONNECTING MECHANISM SO THAT ALL FOUR ROTOR BLADES MAY BE FOLDED BACK IN PARALLEL TO EACH OTHER.

According to the invention, there is provided a connecting apparatus for securing the rotor blades to a rotor hub, for example, in a helicopter, wherein the damping means between the rotor blade and the rotor hub comprise at least one pair of damping elements arranged symmetrically relative to the axis of the rotor blade and relative to the blade connector bolt and connected to the rotor blade, as well as to an inner sleeve of the rotor hub in a force transmitting manner.

According to a further embodiment of the invention the damping means comprise elastomeric elements including discs of metal and of an elastomeric material, wherein an inner metal plate is secured to the rotor blade in a manner to resist shearing forces, and wherein an outer metal plate is secured to an inner sleeve of the rotor hub, again in a force transmitting manner.

In a first or preferred example embodiment of the invention, a pair of elastomeric elements with discs extending perpendicularly to the direction of the rotor blade profile, is arranged between the rotor blade neck and two fork members of the inner sleeve of the rotor hub.

In order that the invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein:

FIG. 1 illustrates a top plan view partially in section and with parts broken away to simplify the illustration of the connection of a rotor blade to a rotor hub and showing a first embodiment of the invention, whereby the section line I--I extends as shown in FIG. 2;

FIG. 2 is a sectional view through the connecting apparatus according to FIG. 1, whereby the section extends along section line II--II in FIG. 1;

FIG. 3 is a view similar to that of FIG. 1, however, showing another example embodiment of a connecting apparatus according to the invention for connecting a rotor blade to an inner sleeve of a rotor hub;

FIG. 4 is a sectional view through the embodiment of FIG. 3 along the section line IV--IV in FIG. 3;

FIG. 5 is a partial sectional view along the section line V--V in FIG. 4;

FIG. 6 is again a view similar to that of FIG. 1, however, illustrating a third example embodiment of a connecting apparatus according to the invention;

FIG. 7 is a sectional view through FIG. 6 along the section line VII--VII in FIG. 6; and

FIG. 8 is a sectional view along the section line VIII--VIII in FIG. 6.

FIG. 1 shows a plan view of the rotor blade 1 which merges into a blade neck 2, the left hand end of which is secured to a fork member 4 by means of a blade connector bolt 3. The fork member 4 which is part of the connector hardware, is secured to an inner sleeve 5 of a rotor hub, the details of which are not shown, since the hub structure as such is not part of the invention. Two damping units or elements 6 are arranged to contact the neck 2 of the blade 1 at two opposite points. The damping units 6 are connected to the fork member 4 by means of bolts 7 extending through holes in the lobes 8 of the fork member 4.

Each damping unit 6 comprises, according to the invention, a number of metal discs and layers or sheets of elastomeric material arranged in alternate fashion. The elastomeric material may, for example, be rubber, which is arranged in such a manner that an inner metal plate 9 is secured to the blade neck 2 and that an outer metal member 12 including an eyelet for the bolt 7 is arranged on the outer side of the unit 6. Rubber discs 10 and metal discs 11 are sandwiched between the inner metal plate 9 and the outer metal member 12, as best seen in FIG. 2.

The swivelling moment 15 indicated by the respective arrow subjects the outer fibers of the rotor blade 1, especially in the region of the blade neck 2 to a lengthening, as well as to a compression. The swivelling moment 15 may be represented by a force pair 16 which is taken up by the blade connector bolt 3. The elastomeric dampening units or elements 6 are deformed by the swivelling moment and the deformation is of a shearing type, whereby the required damping is produced. The damping forces are represented by reference number 17. The following parameters for influencing the damping to be taken up as well as for influencing the natural swivel frequency of the rotor blade 1 may be taken into account. By varying the spacing 18 between the damping element 6 and the blade connector bolt 3, as well as the thickness and the cross section of the material of the elastomeric discs 10, the damping may be influenced and varied in a desirable manner. The wobbling motion of the rotor blade is not influenced, because the elastomeric elements are very soft or resilient in the wobbling direction.

As compared to prior art blade connecting devices which include metal hardware with several blade connector bolts, it is an advantage of the invention that it is possible to simply withdraw the bolts 7 and to tilt the rotor blade 1 about the axis of the single blade connector bolt 3, whereby all rotor blades may be folded, for example, to factilitate their transport or storage.

A further embodiment of the invention is illustrated in FIGS. 3 to 5 in which the blade neck 2 is also connected to elastomeric elements 6 at one end thereof, while the other end of the elastomeric elements 6 is secured to a frame 21 by means of pressure screws 19. In this embodiment two further elastomeric elements 20 are arranged at the sides of the frame 21 and perpendicularly to the elastomeric elements 6. The elastomeric elements 20 comprise the side members of the frame 21, elastomeric discs 22 and alternating plate members 23. The plate members 23 are secured to a fork 24 by means of bolts 25 and nuts 26. The fork 24 in turn is connected to the inner sleeve 5 of the rotor hub. The bolt 25 extends with free play all around through a bushing 27 which is inserted into a wedge shaped inner space 28 of the blade neck 2. The wedge shaped inner space 28 results from the manufacture of the neck end of the blade by placing strands of fiber reinforced synthetic material around the blade connector bolt 3, thereby forming a grommet or eye.

The swivelling moment 15 of the rotor blade is transmitted in this embodiment as a shearing force pair to the frame 21 through the elastomeric element 6 and further through the elastomeric element 20 into the fork 21, which in turn is connected to the inner sleeve 5 of the rotor hub. This embodiment has the advantage that a double damping effect of the swivel moments is accomplished. The elastomeric discs 22 of the elastomeric elements 20 are constructed as shown in FIG. 5 so that their sides form an angle 29 which originates on the axis 3a of the blade connector bolt 3. This feature of the invention has the advantage that the shearing stress in the elastomeric discs 22 is the same in all locations. As a result of this advantage an even larger damping effect and a longer useful life of the disc 22 is accomplished by the avoidance of a non-uniform stress distribution.

With the aid of the pressure screws 19 it is possible to precisely adjust the sweep back angle for each rotor blade 1. Moreover, by withdrawing the blade connector bolt 3, the rotor blade 1 may be tilted in both directions through an angle 135.degree. C which means that all four blades may be arranged in parallel to each other and backwardly, for example, to facilitate storage and/or transportation. Since the bolt 25 may be simply removed, it is possible to tilt the rotor blade through an angle of 90.degree..

FIGS. 6, 7 and 8 illustrate a further example embodiment according to the invention in which the swivel moment 15 is dampened by two large elastomeric ring elements 30. In this embodiment the blade neck 2 is surrounded on all sides in the region of the blade connector bolt 3 by a collar 31 having two extensions 32 provided with threaded bushings 33 engaged by pressure screws 34. Each of the elastomeric elements 30 comprises an inner metal disc 35 connected through the lands 36 to each other and contacted by the pressure screws 34. Each of these elements further comprises an elastomeric washer 37 and an outer metal disc 38. The cross section of the elastomeric ring washer 37 tapers from the outside inwardly by an angle having its origin or apex in the axis 3a of the blade connector bolt 3. The outer metal discs 38 are either connected to fork hardware members 39 or to the inner sleeve 5, which is constructed as a holding means capable of taking up centrifugal force. In addition, the elements 38 and 39 are connected to the bolt 42 by means of eyes 40 and 41, whereby the connection is screwed down to be rigid against relative rotation between the elements.

As in the previously described embodiment, the pressure screws 34 are also suitable for adjusting the sweep-back angle of the rotor blade 1 similarly to the pressure screws 19. The pressure screws 34 also serve for transmitting the swivel moment. For the just described purposes the head of the pressure screws 34 is provided with a number of holes 43 arranged along the periphery thereof. Locking or safety screws 44 extend through these holes 43 and into bores 55 in the collar 31. In this embodiment it is also possible to withdraw the bolt 42 and to then tilt the rotor blade 1 to the right or left through an angle of 45.degree., whereby two blades may be folded in pairs forwarldy and backwardly.

The advantages of the invention may be highlighted by the following comparison of the take up and damping of the swivel moments in a conventional apparatus and in a present apparatus. In the conventional apparatus according to German Pat. No. 1,531,375 the rotor blade constitutes a beam subject to a bending moment, one end of which is rigidly secured. In this type of structure the swivel moments must be taken up by the inner friction in the material of which the blade is made and by the friction in the connecting hardware. In this conventional construction the outer fibers of the blade in the region of the connecting hardware are subjected to tension stress by the alternating swivel moments. Thus, the friction between the blade and the connecting hardware must cause the required damping. However, the friction coefficient which substantially determines the size of the damping to be taken up between the blade and the hardware may vary substantially. Therefore, the damping of the swivel moments also varies considerably and it is not assured that the damping is sufficient.

Contrary to the foregoing the blade connector according to the invention replaces the friction damping in the hardware itself, as described above, by the friction in the elastomeric units, for example, units or elements 6. According to the first example embodiment described above with reference to FIGS. 1 and 2, the elastomeric elements of the invention are connected to both sides of the blade neck and through a lever arm in the form of a fork to the blade connector bolt. Compared to the rigidly connected blade connector neck, the structure of the invention has the advantage that the blade is actually supported by two separate means. According to the invention the swivel moment is taken up at the blade connector bolt and at the elastomeric elements. Under the influence of the swivel moment the outer fibers of the entire blade are subjected to tension and compression stress, whereby the elastomeric elements which are connected to the blade neck in a shearing stress resisting manner, are subjected to shearing stress, whereby the required damping is produced. The parameters for influencing the damping and the natural swivel frequency of the rotor blade include the spacing between the elastomeric elements and the blade connector bolt, the cross section, the number, as well as the material of the elastomeric discs. These elements may be thus calculated. As a result, the damping may be influenced in a relatively wide range so that the required friction is easily adapted to the requirements of the particular situation, especially since each parameter may be varied or influenced substantially independently of any of the other parameters.

Incidentally, the neck 2 of the rotor blade is made of fiber reinforced synthetic material, as mentioned above. Such fibers may, for example, be glass fibers or carbon fibers and/or synthetic fibers, for example, known under the tradename "KEVLAR." The embedding material may, for example, be an epoxy resin. The collar 31 is also manufactured from the same materials.

Although the invention has been described with reference to specific example embodiments, it is to be understood, that it is intended to cover all modifications and equivalents within the scope of the appended claims.