Helicopter rotor structure

The present rotor structure for rotary wing aircraft, such as helicopters, s constructed to minimize the mass unbalance which normally occurs due to the lead-lag of the rotor blades or wings. For this purpose all the bearings supporting the rotors on the rotor heads are constructed as bearings permitting an axial movement of the tension bars which interconnect the wings of a pair of wings. In addition, the wing interconnecting bars are bending-resistant and connected with each other at their crossing point by a centering device which is freely movable in the rotor plane and relative to the rotor head. The connection of the bars by the centering device is rigid in the longitudinal, axial direction of the tension bars.

BACKGROUND OF THE INVENTION 
The present invention relates to a rotor structure for rotary wing 
aircraft, such as helicopters having a rotor head on which the rotor 
blades or wings are supported for rotation without any lagging hinges or 
without any lagging hinges and without any flapping hinges, however in a 
non-rigid manner, whereby, in the case of a four wing rotor structure, to 
tension bars which cross each other, interconnect the rotor blades in 
pairs. The tension bars are resistant or substantially resistant against 
tension stress and bending moments. The tension bars are supported on the 
rotor head by a bearing means which are angularly movable in the lead-lag 
direction and which are radially spaced on both sides from the rotational 
axis of the rotor shaft. 
In connection with conventional rotor structures for rotary wing aircraft 
it is not avoided that the common mass center of gravity of two rotor 
wings arranged diametrically opposite each other, is displaced radially 
outwardly from its central position coinciding with the rotational axis of 
the rotor shaft. Such displacement or excursion takes place when the two 
wings do not lead-lag in the same direction simultaneously and/or when 
they do not lag by the same angle. Such an excursion of the center of 
gravity is largest when the lag angle is of equal size for both wings 
while the wings of a pair arranged opposite each other lag in opposite 
lagging directions. This condition is referred to as the so-called 
symmetric lag-bending form. As a result of such a lag-bending form, a 
temporary mass unbalance is produced which subjects the rotor and its 
bearing means to substantial loads. Besides, such lag-bending form causes 
rather very disturbing vibrations. 
OBJECTS OF THE INVENTION 
In view of the above it is the aim of the invention to achieve the 
following objects singly or in combination: 
TO BALANCE AT LEAST TIALLY THE MASS UNBALANCE WHICH IS CAUSED IN ROTARY 
WING AIRCRAFTS BY THE LAGGING MOVEMENT OF THE ROTOR WINGS; 
TO SUPPORT THE BLADE INTERCONNECTING TENSION BARS IN SUCH A MANNER THAT ALL 
BEARING POINTS PERMIT A MOVEMENT IN THE LONGITUDINAL AXIAL DIRECTION OF 
THE TENSION BARS; 
TO PERMIT A RELATIVE MOTION OF THE TENSION BARS IN THE DIRECTION OF THE 
ROTATIONAL AXIS OF THE ROTOR SHAFT; 
TO MAKE THE TENSION BARS ELASTIC RELATIVE TO TORSION LOADS; AND 
TO CONSTRUCT THE TENSION CARRIER BARS AND THEIR SUPPORT BEARINGS IN SUCH A 
MANNER THAT THE BARS MAY BE ROTATED IN UNISON WITH THE MOTION OF THE WINGS 
RESULTING FROM THE CYCLIC CONTROL OF THE PITCH ANGLE OF THE WINGS. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a rotor structure for a rotary 
wing aircraft which is characterized in that all support bearings between 
each carrier beam and the rotor head are constructed to permit an axial 
movement of the respective carrier bar and that the carrier bars are 
connected with each other by a centering device in a manner rigid in the 
longitudinal axial direction of the carrier bars, said centering device 
being arranged at the crossing of the carrier bars. In addition, the 
centering device is freely movable in the plane of rotation of the rotor 
wings relative to the rotor head. 
According to the invention, both rotor blades are supported by the rotor 
head for axial excursions in the longitudinal direction of the wings or 
blades because the bearings between the rotor head and the wings are 
constructed as axially loose bearings permitting such movement. Each pair 
of blades is secured against axial excursions relative to the other pair 
of blades by the centering device. 
In operation, if an excursion of the center of gravity takes place due to a 
lead-lag movement of the blades of a pair, the carrier bar interconnecting 
the members of such pair of blades bends in a direction opposite to the 
center of gravity excursion and in the range or area of the rotational 
axis of the rotor shaft. As a result of such bending the bar displaces the 
respective other pair of blades in the axial direction of said other pair 
of blades due to the coupling of the interconnecting bars by the centering 
device. As a result of such displacement the center of gravity of the 
other pair of blades is shifted in a direction opposite to the center of 
gravity excursion in the lead-lag direction of the first mentioned pair of 
blades. The just described motions achieve at least a partial mass 
balancing which has the advantage that vibration causing effects are 
diminished. This principle of mass balancing may be applied not only to 
rotors comprising four blades or wings, but also to rotors having a larger 
even number of blades. 
In an embodiment in which the rotor blades are supported on the rotor head 
in a non-rigid manner and without any flapping hinges as well as without 
any lagging hinges, the bearings are angularly movable also in the 
flapping direction. 
Further, in such an embodiment it is advantageous that the centering device 
permits a relative motion of the carrier beam in the longitudinal 
direction of the rotational axis of the rotor shaft, whereby the carrier 
beams may be bent in the flapping direction independently of each other. 
According to a further embodiment of the invention wherein the bearings 
which support the blades rotatably on the rotor head are constructed as 
blade angle bearings to permit the adjustment of the pitch angle, and 
wherein the rotor blades are connected to the tension bars in a manner 
rigid against rotation, the centering device comprises, as a preferred 
embodiment, two axial bearings, each cooperating with a respective carrier 
beam in such a manner that the carrier beams can be rotated in unison with 
the rotor blade connected to the carrier beams, said rotation taking place 
about the longitudinal axis of the blades or wings. In such an embodiment 
the axial bearings are preferably constructed to simultaneously function 
also as radial bearings. Moreover, the bearings are arranged so as to be 
displaceable relative to each other in the direction of the rotational 
axis of the rotor shaft, whereby a structurally simple assembly is 
accomplished which permits a bending of the carrier beams independently of 
each other in the flapping direction. 
The carrier beams are suitably rotatable between their ends through an 
angle corresponding to the movement of the wings which the latter perform 
for the collective adjustment of the pitch angle. This feature in 
combination with the rotatable supporting of the carrier beams on the 
centering device has the advantage that the carrier beams may be rotated 
in unison with the corresponding pair of wings. During the cyclical 
adjustment of the pitch angle two blades or wings opposite each other form 
a pair and are tilted in the same direction and by the same angular 
excursion, thus, merely during the collective adjustment of the pitch 
angle of all blades or wings is it necessary to twist the carrier bars 
between their ends. For this purpose the carrier bars are suitably made to 
be elastic against torsion loads and are constructed of fiber reinforced 
synthetic material, whereby a structurally simple lightweight assembly is 
achieved.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS 
Referring to FIG. 1, the rotor 2 comprises four blades supported on a rotor 
head, the ring 4 of which is supported by a rotor bearing not shown, but 
directly secured to the helicopter or rotary wing aircraft body to be 
rotatable relative to such body. The rotor is driven through the rotor 
shaft through conventional propulsion and gear means not shown. Two 
carrier bars 6 and 8, forming part of the rotor structure or assembly, are 
rotatably supported on the rotor head 4 at the ends of these bars 6, 8 by 
means of blade angle bearings 10 and 12, as well as 14 and 16. The bars 6, 
8 cross each other in the area of the rotational axis of the rotor shaft 
and at right angles relative to each other. These bars 6 and 8 are 
substantially resistant against tension forces and bending moment loads. 
Preferably, the bars 6, 8 are made of fiber reinforced synthetic material, 
whereby the direction of extension of the fibers is in the longitudinal 
direction of the bars. The two bars 6, 8 are spaced from each other in the 
direction of the rotational axis of the rotor shaft. Each of these bars 
extends in a plane which in turn extends substantially perpendicularly to 
the rotational axis of the rotor shaft. The end of each bar 6, 8 is 
connected to the respective neck section 20 of the rotor blades 22A, 22B, 
or 22C and 22D. The connection between the bar end and the respective neck 
is located radially outside of the respective blade angle bearings 10, 12, 
14 and 16. Each wing comprises a blade section 18 and the mentioned neck 
section 20 which is resistant against torsion moments, but sufficiently 
yielding relative to bending in the flapping direction and in the lead-lag 
direction. 
The blade angle bearings 10, 12, 14 and 16 are constructed as so-called 
loose bearings permitting an axial relative movement between the bearings 
and the respective end of the carrier bar 6, 8. Thus, the carrier bar 6 or 
8 and the respective pair of blades 22A, 22B or 22C, 22D form an integral 
structural unit capable of moving in unison in the direction of the 
longitudinal axis of the blades and the longitudinal axis of the 
respective bar relative to the rotor head 4. Further, these axially loose 
bearings 10, 12, 14 and 16 also permit angular movements in the manner of 
pivot or self-aligning bearings so that the forces effective at each blade 
pair 22A, 22B or 22C, 22D cannot be effective on the rotor head 4 as 
bending moments, but are rather transmitted to the respective carrier bar 
6 or 8, whereby they are compensated by the corresponding bending of the 
bar 6 or 8 in the flapping direction and in the lead-lag direction. 
According to the invention a centering device 24 is provided for centering 
the bars 6 and 8 and, with these bars, the blade pairs at the rotor head 
4. The centering device firmly holds the bars 6 and 8 relative to each 
other in the axial direction of the respective bars. The structure of the 
centering device 24 connecting the bars 6 and 8 to each other is described 
in detail with reference to FIG. 2. The centering device 24 permits a 
rotation of the bars 6 and 8 about their longitudinal axis so that each 
bar may rotate in unison during the cyclical adjustment of the pitch 
angle, whereby the bar and the respective blades forming a pair are 
adjusted as a unitary structure, said rotation about the longitudinal 
blade axis taking place in the centering device 24 and in the blade angle 
bearings 10, 12, or 14 and 16 located radially outwardly relative to said 
centering device. During the collective blade angle adjustment a twisting 
of the respective bar between its ends is necessary. This may be 
accomplished by making the bars 6, 8 to be elastic relative to torsion 
moments and within predetermined limits. In order to provide a good 
rigidity or strength against tension loads and bending moments while 
simultaneously providing a high torsional elasticity, it may be desirable 
or practical to construct each bar 6, 8 to have, instead of the 
illustrated circular cross section, an open hollow cross-section, for 
example, in the shape of a U-section or in the shape of an I-section. Said 
section would have high strength boom or marginal zones interconnected at 
least in certain areas by relatively thin webs, and made of superimposed 
belts of synthetic material reinforced by fiber materials, whereby layers 
of elastomeric materials are embedded between such belts of synthetic 
material. 
The rotation or twisting of the bars 6, 8 is accomplished by means of 
control members or horns 26A, 26B, 26C, and 26D connected radially 
inwardly of the blade angle bearings, to the bars 6 and 8. The steering or 
control horns are adjusted in a conventional manner by means of respective 
control rods 28A, or 28B, or 28C, or 28D by means of a wobble plate not 
shown for adjusting the angular movement of the respective blade. 
FIG. 2 illustrates the centering device 24 having upper and lower divided 
bearing shells 30 and 32. The upper bar 8 is connected with the bearing 
shell 30 by means of a bearing 34 which is effective in the axial and 
radial direction permitting rotation between the bar 8 and the shell 30, 
but preventing a relative tilting therebetween as well as an axial 
displacement. The lower bar 6 is connected in the same manner through a 
radially and axially effective bearing (not shown) with the bearing shell 
32 for cooperation therewith. Both bearing shells 30 and 32 are 
displaceable relative to each other by a bolt 38 in the direction of the 
rotational axis A of the rotor shaft, said bolt 38 extending into a bore 
36 of the upper bearing shell 30. However, the bearing shells are 
interconnected rigidly against tilting and against relative movement in 
the direction of a plane extending perpendicularly to the rotational axis 
A of the rotor shaft. Due to the relative displacement of the bearing 
shells 30, 32 in the direction of the axis A, it is possible for the bars 
6, 8 to bend independently of each other in the flapping direction. 
As long as the rotor blades 22A to 22D do not perform any lagging motion, 
the center of gravity of each structural unit comprising a beam 6, 8 and a 
pair of blades 22A, 22B or 22C, and 22D is centered in the rotational axis 
A of the rotor shaft, whereby the bars 6 and 8 support each other at the 
crossing by means of the centering device 24. However, if the rotor blades 
follow an excursion in the lead-lag direction, the common center of 
gravity travels out of the rotational axis A as shown by dash-dotted lines 
in FIG. 3 for the blade pair 22C and 22D, taking into account a symmetric 
lag-bending form, whereby the common mass center of gravity is displaced 
from the point A in FIG. 3, to the point S.sub.1 in FIG. 3. As a result, 
the interconnecting bar 8 which interconnects the respective blades 22C 
and 22D is also bent, however, in a direction opposite to that of the 
excursion of the center of gravity. As a result, the bending bar 8 
entrains the bar 6 through the centering device 24 and thus the blade pair 
22A, 22B secured to the bar 6 so that their common center of gravity is 
displaced in the direction contrary to the excursion of the center of 
gravity of the displaced blade pair 22C, 22D, radially away from the 
rotational axis A, of the rotor shaft to the point S.sub.2. This just 
described operation assures at least a partial balancing compensation of 
the center of gravity excursion occurring as a result of the lead-lag of 
the blades, whereby a reduction in the loads on the rotor caused by the 
unbalance, is accomplished. Simultaneously, vibration tendencies are also 
substantially reduced. This principle of displacing the center of gravity 
in opposite directions for the purpose of achieving a mass balancing may 
be employed in an equally effective manner in rotors having more than four 
even numbered blades. 
As illustrated, the bars 6, 8 are supported by anti-friction bearings. 
However, instead it is also possible to use in the same manner slide 
bearings or elastomeric bearings. Further, where the bars 6, 8 have 
different cross sections, such as an open U-cross section, the two 
radial-axial bearings of the centering device 24 may be arranged in the 
openings or recesses in the bars 6, 8 to reduce the relative spacing 
between these bars, whereby the bearings would be constructed as so-called 
inner bearings. 
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.