Hingeless and Bearingless Multiblade Rotor for Rotary Wing Aircraft

The invention relates to a hingeless and bearingless multiblade rotor for rotary wing aircraft, particularly for a helicopter tail rotor. In such rotors, the blade roots of the lift producing wing portions of the rotor blades are each connected to the substantially rigid rotor hub by means of several parallel acting spar sections which are elastically yielding relative to blade angle adjusting torsion loads and which are elastically bendable in the flapping direction as well as in the lead-lag direction. The mounting points of the spar sections are angularly displaced or staggered on the rotor hub in the direction of rotation of the rotor.

Rotors of this type are known wherein several spar sections, which diverge from one another from the airfoil or wing portion toward the rotor hub, are associated with each rotor blade. The spar sections are constructed less resistant to bending in the flapping direction than in the lead-lag direction. The spar sections become twisted between their ends during the blade angle adjustment. These known rotor types have the disadvantage that the fictive flapping hinges, and above all, the fictive lead-lag hinges are located undesirably far from the axis of rotation of the rotor and the freely twistable length of the spar sections is distinctly smaller than the radial length between the axis of rotation of the rotor and the radially outward, airfoil-side spar end, so that comparably high control forces must be applied for the cyclic and collective blade angle control movements.

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

to provide a hingeless and bearingless multiblade rotor for use in rotary wing aircraft, especially as a tail rotor;

to construct the multiblade rotor with flexible spar mounted blades so that the control forces required for the blade angle control are substantially reduced;

to shift the effective, fictive or virtual flapping and lead-lag hinges of the respective rotor blades appreciably closer to the axis of rotation of the rotor in comparison to prior art spar section mounted multiblade rotors;

to provide a hingeless and bearingless multiblade rotor with spar mounted blades in which the free length of the spar sections in the region of the rotor hub is increased without lengthening the spar sections at the outer ends where the blade roots are connected to the spar sections;

to provide a spar section mounting for a multiblade rotor with free clearance between the spar sections which allows mutually independent torsional, lateral, and rotational motions of the individual rotor blades withut mutual interference;

to eliminate undesirable forces and moments in the vicinity of the rotor hub in spar section mounted multiblade rotors;

to utilize the maximum amount of total cross-sectional surface of the rotor hub in multiblade rotors for the hub side lengthening of blade mounting spar sections, thereby to utilize the largest possible spar length for elastic twisting of the spar sections and to minimize the total mass required for propeller moment compensation and for static and dynamic balancing and to also minimize the forces necessary for steering such rotors; and

to construct the connecting spar sections as loops so shaped that tension load peaks are avoided at the connection points, preferably the loops and blades are made of fiber compound materials whereby the fibers run unidirectionally in the radial direction, whereby the fibers run all the way from the blade tip through the loop.

According to the invention there is provided a hingeless and bearingless multiblade rotor for rotary wing aircraft, wherein the hub side mounting points for the spar sections of each rotor blade are situated on the half side of the rotor hub facing away from the respective rotor blade and are arranged on the hub with mutual spacing in the direction of the rotor axis so that the rear spar section of each rotor blade, with reference to the direction of rotation of the rotor, overlaps with a free clearance the front spar section of the immediately following rotor blade in an arrangement having the same sense for all rotor blades.

The rotor according to the invention increases, as a result of the special position of the hub side mounting points, the free length of the spar sections in the region of the rotor hub without lengthening the spar section at the outer, airfoil side end. This increase is such that the spacing of the fictive or effective flapping hinges and of the fictive lead-lag hinges from the rotor axis may be reduced to a desired extent and at the same time the torsional stiffness of the spar sections which effectively determine the steering forces for the blade angle adjustment is noticeably reduced without impairing its elastic bending characteristics for the flapping and lead-lag hinges. A special feature resides in the mutual overlapping arrangement of the spar sections with a free clearance which allows mutually independent flapping and lead-lag movements of the individual rotor blades without mutual interference. It also allows an identical blade connection for all rotor blades, without the occurrence of a bothersome flow of force in the region of the rotor hub as a result of the effective loads and tilting moments. All in all, the rotor according to the invention shows a substantially better control characteristic because of the favorable position of the fictive or equivalent hinges and due to the small dimensions, and this rotor is particularly useful as tail rotor for a helicopter.

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 shows a schematic top plan view of a three-blade tail rotor according to the invention;

FIG. 2 shows schematically a partial view of the tail rotor according to FIG. 1 on an enlarged scale;

FIG. 3 shows schematically a section along the line III--III of FIG. 2;

FIG. 4 shows schematically a view of the spar sections of a rotor blade looking in the direction of the arrows IV--IV of FIG. 2;

FIG. 5 shows schematically a section along the line V--V of FIG. 2;

FIG. 6 shows a schematic top plan view, similar to FIG. 1, of a four blade tail rotor, however, with reverse rotor thrust direction; and

FIG. 7 shows an enlarged partial plan view of the tail rotor according to FIG. 6.

FIGS. 1 to 5 illustrate a three blade helicopter tail rotor 2, which produces a downward thrust K (FIG. 3) in the direction of the axis R of rotation of the rotor, that is, toward the helicopter tail, when the tail rotor 2 turns in the direction of rotation S. The rotor blades 4A, B, C of the tail rotor 2 are each connected to a substantially rigid rotor hub 14 formed at the upper end of the rotor mast 12, wherein the connection is accomplished by means of two V-shaped spar sections 8, 10 diverging from the airfoil section 6 of the rotor blades to the rotor axis R.

The spar sections 8, 10 are made of compound fiber material, for example glass reinforced fiber material, with the fibers extending unidirectionally in the longitudinal direction of the spar. The spar sections 8, 10 are constructed to be torsionally elastic as well as elastically bendable in fictive flapping hinges and in the fictive lead-lag hinges. Thus, the blade angle may be adjusted about the longitudinal blade axis L. More specifically, the spar sections 8, 10 are constructed less resistant to bending in the flapping direction than in the lead-lag direction to provide a better control characteristic of the rotor 2. The airfoil or blade ends of the spar sections are held together by means of a two piece clamp 16 and change into a single piece wing spar shown in FIG. 2 with dashed lines which reaches into the respective airfoil section 6 as far as the blade tip. The twisting of the spar sections 8, 10 and the changing the blade angle of the associated rotor blade about the respective longitudinal or radial axis L is caused by means of a control horn 18 attached to the clasp 16. The control horn 18 is connected to a control rod, not shown, for the cyclic and collective blade angle adjustment.

The hub side ends of the spar sections 8, 10 are connected to the rotor hub 14 by means of the mounting bolts 20, 22, 24 in such a manner, that in each instance two spar sections 8, 10 of neighboring rotor blades are secured by the same mounting bolt, and the mounting bolts associated with each rotor blade are situated on the half side of the rotor hub 14 facing away from this rotor blade. In this manner, the spar section 8 of the rotor blade 4A as well as the spar section 10 of the rotor blade 4C are held by the mounting bolt 20, and the mounting bolts 20, 22 associated with the rotor blade 4A are arranged in the peripheral region on the half side of the rotor hub 14 facing away from this rotor blade 4A.

The mounting bolts 20, 22, 24 are secured to the rotor hub 14 (FIG. 3) on three successive flanges 28, 30, 32 spaced in the direction of the rotor axis, whereby all of the rear spar sections 10, with reference to the rotating direction S of the rotor, are connected between the flanges 28 and 30 and all of the forward spar sections 8, with reference to the rotating direction S of the rotor, are connected between the flanges 30 and 32. That is, in each instance the respective spar sections are connected at the same height on the rotor hub 14 with reference to the axis of rotation R of the rotor. With this feature, a blade connection is obtained which is identical for all rotor blades and an undisturbed elastic deforming of the spar sections 8, 10 over the entire spar length is guaranteed when blade angle control movements, flapping movements, and lead-lag movements are performed.

As is shown most clearly in FIG. 5, the spar sections 8, 10 each have a rectangular cross-section with a substantially larger cross-sectional height "h" than a cross-sectional width "b". The spar sections 8, 10 have a mutual spacing in the direction of the rotor axis, which spacing becomes increasingly larger toward the mounting bolts. The forward spar section 8, with reference to the direction S of rotation of the rotor, is situated above the rear spar section 10 relative to the thrust direction K of the rotor. In addition, the center lines M of the spar sections 8, 10 extending in the direction of the cross-sectional height "h" are also inclined at an angle .alpha. of about 10.degree. relative to a plane perpendicular to the axis R of rotation of the rotor. Consequently, the spar sections 8, 10 are constructed substantially less resistant to bending in the flapping direction than in the lead-lag direction, and their center lines M have the same inclination in the untwisted state of the spar sections as does the airfoil, shown in FIG. 5 with dashed lines, in the center of the blade angle adjustment range. Consequently, the maximum torsion angle of the spar sections 8, 10 required during cyclic and collective angular changing of the blades is kept small compared with the untwisted state, and the cross-sectional center lines M of the spar sections 8, 10 extend in parallel to the wing chord in the transition region between the airfoil section 6 and the spar section proper.

The hub side ends of the spar sections 8, 10 are constructed as loops 34, 36, which embrace the associated mounting bolts whereby a sleeve 38 or 40 respectively is interposed between the loop and the bolt. The cylindrical outer surfaces of the sleeves 38, 40 are matched to the inclination of the loops 34, 36, while the central sleeve bore taking up the mounting bolts 20, 22, 24, extends in parallel to the rotor axis R. A filler piece 42 which is also made of (FIG. 2) fiber compound material is inserted between the loop legs of each spar section 8, 10. The cross-sectional width "b" of the spar sections 8, 10 in the loop region becomes increasingly larger toward the hub side mounting point, while the radial wall thickness decreases in the direction of the cross-sectional center line "M". Stress peaks on the hub side mounting points of the spar sections 8, 10 are reduced by these means. The mounting bolts 20, 22, 24 are secured at their upper and lower ends by means of axial stop disks 44, 46, so that the mounting bolts 20, 22, 24 do not fall out in the instance of a bending break and so that the spar sections 8, 10 stay connected to the hub by means of the then shear stressed bolts.

The propeller moment compensation is accomplished in the usual manner by means of compensating weights 48 which are attached to the clamp 16 and are adjustable in the direction of the moment compensating axis P. The trim weights 50 required for balancing the blades are housed in chambers 52 situated on both sides of the blade plane and concentric to the moment compensating axis P (FIG. 3). The weights 50 contribute to the propeller moment compensation and hence respectively reduce the weight of the compensating weights 48.

The example embodiment according to the FIGS. 6 and 7 shows a tail rotor with four rotor blades 4A, B, C, and D and with reverse rotor thrust K, that is, the rotor thrust is directed upwardly relative to the direction of the axis of rotation of the rotor R. The structural elements of the example embodiment of FIGS. 6 and 7 which correspond to those elements of the three blade rotor of FIGS. 1 to 5, are indicated by the same reference numbers. Thus the higher lying, forward spar sections 8 with reference to the direction of rotation S, are connected between the flanges 28 and 30, and the lower lying, rear spar sections 10, with reference to the direction of rotation S, are connected between the flanges 30 and 32 and connected to the rotor hub 14. Stated differently, the blade connection with reference to a plan perpendicular to the axis R of rotation of the rotor is reversely symmetrical to the blade connection of the first example embodiment. Two spar sections 8, 10 of different rotor blades are again supported each with a fitting bushing 38 or 40 respectively interposed between each of the mounting bolts 20, 22, 24 and 26 and the respective spar loop. The spar section connections are arranged with a uniform distribution in the outer, peripheral region of the rotor hub 14. The mounting bolts associated with each rotor blade are also situated in the region of the half side of the rotor hub facing away from this rotor blade, however in this instance, they are situated at the border of the rotor hub half facing toward the rotor blade. Thus, according to FIG. 7, the two mounting bolts 20 and 24, which are located diametrically to the axis R of rotation of the rotor and substantially perpendicular to the longitudinal axis L--L of the rotor blades 4A, C are associated with the spar sections 8, 10 of the rotor blades 4A and 4C. The other two mounting bolts 22 and 26 serve for connecting the blade pair 4B, D. Otherwise, the mutual inserting and the cross-sectional construction of the spar sections 8, 10 and the remaining structural and functional features of this example embodiment correspond to those of the tail rotor according to the FIGS. 1 to 5.

In the light of the above disclosure it will be appreciated that by locating the mounting points of the spar sections in the peripheral region of the rotor hub facing away from the respective rotor blade, substantially all of the total cross-sectional surface of the rotor hub is used for the hub side lengthening of the spar sections. Since the mounting points of corresponding spar sections of the rotor blades are situated in a common plane perpendicular to the rotor axis, and since all mounting points are of identical construction, a uniform load distribution is achieved on the rotor hub according to the invention. Especially in connection with rotors having more than two rotor blades it is suggested to construct the mutual overlapping of the spar sections in such a manner, that the trailing spar section of each rotor blade, with reference to the rotating direction of the rotor, and the leading spar section of the immediately following rotor blade overlap each other in a crossing manner with a mutual spacing in the direction of the rotor axis R.

The leading spar section of the rotor blade as viewed in the direction of rotation, is connected to the rotor hub by means of a mounting point located higher in the thrust direction K of the rotor than the trailing spar section of this rotor blade. This is especially advantageous in the instance when each rotor blade has two V-shaped spar sections diverging from the airfoil or wing means toward the rotor hub because when the spar sections are in the untwisted state a lift producing presetting of the rotor blades may be obtained in combination with an extremely simple construction of the connection or wing or transition region between the spar sections and the airfoil or wing sections. The hub side mounting points preferably each comprise a common mounting bolt for several spar sections of different rotor blades; this mounting bolt is secured on the hub, parallel to the rotor axis, whereby the assembly at the hub side mounting points is simplified and the number of structural components is reduced. In the present instance, a particularly secure supporting of the mounting bolts is obtained in that these bolts are preferably each held in position by vertically spaced flanges of the rotor hub, whereby the bolts are secured against falling out, if a bolt should break, because the flanges from axial stops. Consequently, the rotor blades are held secured to the rotor hub even in the event of a bending break of the bolts due to the still remaining shearing resistance of the bolts.

The spar sections, at their hub side ends, preferably have a cross-section which becomes, toward the associated mounting point, increasingly more resistant to bending in the flapping direction. Further, the spar sections are loop shaped in order to avoid the occurrence of tensile stress peaks at the mounting points and in order to produce an advantageous connection by means of loops having a small radial wall thickness.

The above mentioned dimensioning of the die spar sections so that the height "h" of the rectangular cross-section is greater than the cross-sectional width "b", preferably "h"=6b to 3b, has the advantage that in the nontwisted condition of the spar sections, the center lines M extend at an angle to a plane perpendicular to the rotor axis. This special cross-sectional geometry provides a substantially smaller flapping stiffness than the lead-lag stiffness of the spar sections with a minimal torsion resistance. Further, in the untwisted state of the spar sections these spar sections assume a position in the center of the blade pitch adjusting region, whereby blade angle adjustments may be made symmetrically relative to said center with a uniform force requirement for said blade angle adjustment.

The spar sections are preferably made of a compound fiber material with the fibers extending unidirectionally in the longitudinal direction of the spar, whereby the material characteristics are optimally utilized. Additionally, such structure makes possible an extremely simple connection of the spar sections to the associated wing blade and the connection is well suited for the proper load application in accordance with the material characteristics of such fiber compound material, particularly when the spar sections of each rotor blade are constructed as a single piece extending into the wing blade all the way to the blade tip.

By attaching the control horn 18, the clamp 16, the propeller moment compensating weights 48, and the balancing weights 50 in the region where the spar merges into the blade or wing and by placing the weights 48 and 50 along a common axis P, these elements are most effective. Accordingly, the largest possible spar length is utilized for twisting the spar sections, and the total mass of the additional weights required for the propeller moment compensation and the static and dynamic balancing of the rotor blades is substantially reduced, because the trim weights also become effective in the sense of a propeller moment compensation.

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