Rotor structure for a rotary wing aircraft

The wings of a rotary wing aircraft comprise, for example, four wing blades orming two pairs of wing blades whereby the blades of a pair are located diametrically opposite each other. The blades of a pair are connected to each other by a spar extending from wing blade to wing blade. The spar is operatively secured to the rotor head. Each spar has between the rotor head and the respective wing blade a zone with a torsion yieldability larger than in the spar outside said zone. A beam stiff against bending, bridges the zone, referred to as a torque soft zone, from the wing blade toward the rotor head except for a small flexible section between the rotor head and the radially inner end of the beam. The flexible section permits the blade flapping movements and the blade lead-lag movements.

BACKGROUND OF THE INVENTION 
The present invention relates to a rotor structure for a rotary wing 
aircraft. More specifically, the invention relates to a rotor having an 
even number of rotor blades secured to the rotor head in pairs so that the 
rotor blades of a pair are arranged diametrically opposite each other. The 
members of a pair are interconnected by a common spar extending from one 
blade root of a pair to the other blade root of a pair, as an integral 
structure. The spar is sufficiently yielding to torsion loads resulting 
from blade angle movements. In other words, the spar is torsion soft. 
Similarly, the spar is sufficiently yielding against bending loads 
resulting from blade flapping movements and from blade lead-lag movements. 
German Patent Publication (DE-OS) 2,755,557 discloses a rotor without any 
defined flapping hinges and without any lead-lag hinges for the rotor 
blades. In such a structure the bending moments resulting from the blade 
flapping and from the blade lead-lag movements are transmitted to the 
rotor head through the spar member. Relatively soft zones of the spar 
contribute to relieving the rotor blades of these moments. In such rotor 
structures it cannot be avoided that the blade angle adjustments cause a 
twisting of the spar member because the latter necessarily has a different 
height and width due to stiffness considerations. As a result, such 
twisting causes a change in the bending stiffness of the spar member in 
the zone that is twisted. Such zone extends from the respective blade wing 
to the connecting point of the spar to the rotor head. As a result, in 
this type of rotor having a common spar member for each pair of rotor 
blades, the blade angle adjustment movements affect the blade flapping 
movements and the blade lead-lag movements in such a manner that the 
ficticious flapping hinge spacing and the ficticious lead-lag hinge 
spacing from the rotor head vary in response to the blade angle 
adjustment. Accordingly, this type of prior art rotor has the disadvantage 
of an undefined vibration characteristic. Strong vibrations may occur with 
the result that the rotor and the aircraft cabin or fuselage are subjected 
to high loads. 
German Patent Publication (DE-OS) 2,701,519 describes a rotor structure in 
which the wing blades are also interconnected in pairs by spar members 
made of fiber reinforced synthetic material whereby the fiber strands 
extend in the longitudinal direction of the blades and spar members. 
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 provide a rotor of the type described above which is so constructed that 
a mutual influence of the blade angle adjustment movements on the one hand 
and of the blade flapping movements as well as the blade lead-lag 
movements on the other hand, is eliminated without the use of mechanical 
blade flapping hinges and without any mechanical blade lead-lag hinges; 
to control the twisting of the interconnecting spar as a result of blade 
angle movements to such an extent that it will not influence the bending 
characteristics of the spar in response to flapping movements and lead-lag 
movements; and 
to reinforce a spar zone without undesirably influencing the bending 
characteristics of the spar between the blade root and the rotor head 
proper. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a rotor structure of the type 
described above which is characterized in that the spar means which 
interconnect two wing blades arranged diametrically opposite to each other 
are constructed as a single piece, the center of which is rigidly 
connected to the rotor head. Between the rotor head and the respective 
wing blades two spar members are formed. Each spar member is divided into 
a flexible section adjacent to the rotor head and a torque soft zone 
extending between the respective wing blades and the respective flexible 
section. The torque soft zone is reinforced by beam means against bending 
but remains substantially yielding against torque loads to permit the 
blade angle adjustment movements. The flexible section of each spar member 
on both sides and adjacent to the rotor head remains flexible relative to 
bending loads but has a torsion yieldability smaller than the torsion soft 
zone which is bridged by the bending stiffness increasing beam. In other 
words, the flexible section has a higher torsion stiffness than the zone 
reinforced against bending. 
It is an advantage of the invention that the described structure is 
separated into two zones so that the blade angle adjustment movements on 
the one hand and the blade flapping and blade lead-lag movements on the 
other hand are practically effective in these separated zones or sections 
of the spar member. Thus, the influences of these movements are 
effectively decoupled from one another. 
Further, the adjustment forces required for the collective blade angle 
movements may be reduced because the torsion softness of the zone which is 
bridged by the bending stiff beam may be developed to an optimal extent. 
This is so because the bending stiff beam completely takes up the moments 
and shearing forces that may result from the lift and the resistance 
encountered by the respective rotor blade or wing blade. The bending stiff 
beam transmits the resulting moments and shearing forces to the rotor head 
through the spar section which remains soft relative to bending between 
the rotor head and the bending stiff beam. Thus, this section functions 
substantially exclusively as a flapping hinge and as a lead-lag hinge 
whereas the bridged torsion soft zone permits the blade angle adjustments 
without affecting the just mentioned functional hinging section. This type 
of structure as disclosed by the invention obviates the structural 
reinforcement of the spar in the form of giving the spar a special profile 
which takes into account the blade flapping moments and the blade lead-lag 
moments. This in turn has the advantage that the manufacturing of the 
rotor blades with their spars is substantially simpler than in the prior 
art.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE 
OF THE INVENTION 
The present rotor as shown in FIG. 1 has four rotor wing blades of which 
the wing blade 1 is shown in FIG. 1. The blade wing is adjusted relative 
to a plane extending normal to the plane of the drawing in accordance with 
a blade angle adjustment angle. A spar 2 which is common to both blades or 
rather, to both wing blades, operatively interconnects these wing blades 
to form an integral component. For example, the rotor blades 1 and the 
respective spar 2 may be made of fiber reinforced synthetic material, 
whereby the fiber strands extend in the longitudinal blade direction. The 
fiber strands may extend from the tip of one blade of the structural 
component having two blades, to the tip of the other blade of the same 
component. Intermediate the blades of a pair the strands form the 
intergral, continuous spar 2. Such a structure is shown in the above 
mentioned German Patent Publication (DE-OS) 2,701,519. The common spar 2 
is tortion soft relative to blade angle movements and it is also flexible, 
soft against bending relative to blade flapping movements and relative to 
blade lead-lag movements. Hence, no separate mechanical hinges are 
necessary for accommodating these blade movements. 
FIG. 1 further shows that the common spar 2 is rigidly secured to the rotor 
head 3 for example by means of a screw connection 4 which simultaneously 
connects the flange 5.1 of the rotor shaft 5 to the rotor head. The rotor 
head 3 may comprise stiff plates 3.1 of synthetic material which may be 
reinforced by a fiber webbing. Further, spacer means 3.2 also comprising 
synthetic material plates reinforced by a fiber webbing may be used in the 
rotor head as is conventional. 
According to the invention a stiffening member 6 also made of fiber 
reinforced synthetic material, bridges a portion 2.2 of the spar 2 between 
the blade 1 and the rotor head 3. Thus, the stiffening member 6 separates 
the spar member between the rotor head and the blades into two zones. One 
zone 2.2 is bridged by the stiffening member 6. The other section or zone 
2.1 of the spar member 2 extends between the rotor head and the left hand 
end of the stiffening member 6. Thus, the stiffening member 6 which may be 
constructed as a sleeve which may have one or two open sides limits the 
radial extension of the bending soft section 2.1 of the spar 2. 
Simultaneously, the stiffening member 6 constitutes a carrier beam for the 
torsion soft zone 2.2 of the spar 2. Such carrier beam or stiffening 
member 6 is connected in parallel, so to speak, to the torsion soft zone 
2.2. The left hand end of the beam member 6 is spaced from the rotor head 
3 by the length desired for the flexible zone or section 2.1 and secured 
to the spar 2, for example, by a screw connection not shown for 
simplicity's sake but extending through two intermediate layers 6.1 of 
fiber reinforced synthetic material, thereby providing a rigid connection 
between the left hand end of the beam member 6 and the spar 2. The right 
hand end of the beam member 6 extends all the way to the blade root of the 
blade wing 1 and is also connected to the spar or blade by a suitable 
connection shown in FIG. 3 as a radial bearing 7. 
FIG. 3 shows a sectional view through the connection of the right hand ends 
of the beam member 6 to the spar or blade by means of the radial bearing 7 
made of elastomeric material. While the first mentioned connection with 
the intermediate layer 6.1 is substantially rigid in the radial direction, 
which is the longitudinal direction of the blades and of the spar, as well 
as in the direction perpendicularly to the radial direction, the second 
connection with the radial bearing 7 is rigid only in the radial direction 
but yielding in a direction circumferentially around the radial direction. 
Thus, the beam member 6 provides a stiffening of the zone 2.2 against 
bending loads but permits, with the aid of the bearing 7, an optimal 
torsion yielding of the spar zone 2.2. Thus, the zone 2.2 has a 
substantially higher torsion yielding or torsion softness than the section 
or zone 2.1 which is not bridged by the beam member 6. Accordingly, the 
beam member 6 keeps the zone 2.2 substantially free of any bending moments 
resulting from the blade flapping and from the blade lead-lag movement. On 
the other hand, the second section or zone 2.1 remains substantially 
uninfluenced by any blade angle adjustment movements while simultaneously 
being able to transmit bending moments resulting from blade flapping 
and/or blade lead-lag movements to the rotor head 3. 
The above mentioned separation of each spar member into a bending moment 
transmitting zone or section 2.1 and into a torsion soft zone 2.2 makes it 
possible to construct the latter zone 2.2 in an optimal manner relative to 
the forces necessary for accomplishing the blade angle adjustment. Thus, 
as shown in FIG. 2 the spar zone 2.2 may be divided into individual 
strands 2.2.1 thereby forming a total of four such strands 2.2.1 separated 
by two gaps 2.2.2 extending substantially at right angles relative to each 
other and intersecting substantially along the longitudinal, radial axis 
of the spar. Keeping in mind that an optimal torsional softness is to be 
achieved for the zone 2.2, the gaps 2.2.2 are suitably of such a width 
that even if the strands 2.2.1 are twisted as a result of a blade angle 
adjustment, the strands will not contact each other. 
As shown, the gaps 2.2.2 may extend from the connection at 6.1 to the 
radial bearing 7. However, it is also possible to extend the gap from one 
blade to the other through the spar and fill the gaps with a filler 
material 2.2.3 except in the zone 2.2. The construction of the spar 2 from 
separate strands with gaps therebetween is especially advantageous and 
simple with regard to its production and the filler material 2.2.3 does 
not make the spar sections completely stiff. The filler material 
interposed between the strands 2.2.1 may, for example, comprise synthetic 
resin impregnated webbing layers. 
Referring to FIG. 3, the connection between the spar 2 and the beam member 
6 is not totally rigid in the radial direction because the radial bearing 
7 of elastomeric material permits for an axial play of the spar 2 relative 
to the bending stiff beam member 6. This feature is advantageous with 
regard to the so-called centrifugal force lengthening of the spar 2 when 
the blades are subject to centrifugal forces in operation. Incidentally, 
the radial bearing 7 may be divided in the horizontal plane 10 or in the 
vertical plane 11 as shown in FIG. 3. 
Forming the beam member 6 as a sleeve which is open at least along one side 
as shown in FIG. 3 has the advantage that the sleeve may be slipped over 
the spar 2 without any difficulties. 
A further improvement can be achieved by making the sleeve so that it is 
open along two opposite sides as shown in FIG. 2 except at the points of 
connection to the spar 2, namely, at 6.1 and where the bearing 7 is 
located. It has been found that the beam member 6 is substantially free of 
centrifugal forces since these are taken up by the spar 2. Moreover, the 
construction of the sleeve with two open sides substantially reduces the 
size of the control forces necessary for the collective adjustment of the 
blade angle. 
Another advantage of the present structure is seen in that the lever for 
adjusting the blade angle may be a component of the respective rotor blade 
1. Such component for the blade angle adjustment is not shown in the 
figures, however, it would be connected to the bending stiff beam member 6 
in a manner permitting the blade angle adjustment of the blade 1 relative 
to the beam member 6. 
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.