Helicopter rotor blade made from a multispar composite material with torsion compartments and a process for manufacturing same

The invention provides a helicopter rotor blade made from a composite material whose multistrut resistant framework adjacent the leading edge is disposed so as to form two superimposed compartments.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a blade made from a composite material for a 
helicopter rotor whose structure is particularly well adapted not only to 
bending but also to twisting forces and which is further-more redundant; 
this quality allows the operating integrity to be maintained after 
structural damage. This type of structure is also well adapted to blades 
whose root is "forked". 
2. Description of the Prior Art 
It is in fact known that for forming the root of the blades and fastening 
them to the articulations of the hub which allow angular movements of said 
blades in so far as the angle of attack, flapping and drag are concerned, 
particularly when these articulations are formed by an angularly 
deformable resilient body, known under the name of "laminated spherical 
thrust block" and such as described in French Pat. No. 2 427 251 filed on 
the June 2, 1978 in the name of the assignee, it is advantageous for the 
blade foot to comprise two branches forming a fork whose end is bolted 
directed to the internal frame of the laminated spherical thrust block. 
This type of construction provides on the one hand a simplification of the 
construction of the rotor and lightening thereof by reducing the number of 
constituent parts and, on the other hand, allows the blade/hub connection 
to be placed closer to the center of oscillation of the blade, which 
results in an appreciable reduction of the dynamic forces applied to the 
fastening. This gives to this member a better fatigue strength and an 
unlimited lifespan. 
Furthermore, compartment blades are known comprising either a single 
compartment in the vicinity of the leading edge or multiple compartments 
in the direction of the chord of the blade. The main advantage of such 
blades resides in their very high torsional rigidity for eliminating the 
vibrations due to coupling between the bending and twisting deformations 
of the blade. However, and more especially in the case of a blade having a 
fastening fork, construction thereof becomes complex while not giving 
entire satisfaction in so far as the strength of the connection zone of 
the current part of the structure to the forked root is concerned. 
In fact, a structure having a single compartment which must be divided into 
two compartments to form the two branches of the bork presents a 
discontinuity in the transition zone which is very unfavorable for the 
passage of torsional forces in particular. For structures with multiple 
compartments in the direction of the chord, it is impossible to extend 
them into forked branch compartments without a sudden structural 
discontinuity of at least one compartment, resulting also in a weaked 
transition zone. Of course, such weakening is offset by the positioning of 
reinforcements, ribs, hoops and other means to try to reconstitute a 
continuous structure, but that results in great complication of 
manufacture. It follows that blades with fork and compartment are 
practically impossible to manufacture in a simple and so economical way. 
SUMMARY OF THE INVENTION 
The present invention overcomes such drawbacks by providing a blade in 
which the resistant framework is formed as two compartments superimposed 
in the direction of the thickness of the blade so that they may be 
extended naturally and without solution of continuity into a frame for the 
branches of the fork fastening the blade to the rotor hub. 
More precisely, a first object of the invention is to provide a composite 
material blade for a helicopter rotor formed in the vicinity of its 
leading edge by a resisting framework of compartments centered 
substantially in the front quarter of its current cross section. 
According to one of the main characteristics of the invention, this 
framework comprises two superimposed longitudinal compartments, each 
compartment extending from the tip to the foot of the blade and each being 
formed by a front spar and a rear spar formed from a bundle of "rovings" 
made from fibers preimpregnated with resin, with a filling core made from 
a light material being disposed between the two spars and a casing 
enclosing the spars and the core comprising at least one layer of fabrics 
made from parallel fibers disposed slantwise with respect to the 
longitudinal axis of the blade so as to form a torsion compartment. The 
two compartments are joined together by simultaneous polymerization of the 
superimposed and jointing parts of their respective casing. At the rear of 
the framework is disposed a filling element and an outer covering 
envelopes the whole of the framework and the filling element. 
In addition to the advantage of such an arrangement concerning the simple 
construction of the branches of the fork which will be explained in detail 
hereafter, this structure has a second advantage in so far as safety is 
concerned in the case of damage or failure (fail safe character) of the 
blade caused by the impact of a projectile. This advantage is important 
for blades equipping military helicopters. In fact, a projectile such as a 
bullet will in practically all cases have a rising path since it is fired 
from the ground. The impact of the projectile will therefore be situated 
in the lower compartment. It is certain that, in the impact zone, the 
torsional strength of the low compartment will be very much reduced. There 
will however remain an appreciable resistance to torsion provided by the 
upper compartment which remains well distributed on each side of the pitch 
axis conventionally situated substantially at the front quarter of the 
section of the blade. This resistance is further strengthened by the fact 
that the structure has four spars or struts forming in any case a 
framework also well distributed about the pitch axis. This is not the case 
with a single compartment structure, where the torsional stiffness becomes 
practically zero in the impact zone, nor the case with a multicompartment 
structure in the direction of the chord of the blade where, although a 
residual rigidity is kept in the impact zone for the unaffected 
compartment, there is movement of the resistant zone with respect to the 
pitch axis and an important change of the distribution of stresses. 
Furthermore, considering the case of impact, the fracture of a compartment 
in the impact zone only causes displacement of the shearing center of the 
section in the vertical direction, towards the second compartment so that 
the aerodynamic forces always pass very close to the shearing center which 
avoids the creation of couplings which may generate dangerous vibrations. 
Generally, the "fail safe" character of the current part of a blade is 
improved by increasing the number of connections between compartments and 
struts and the quality of these connections which results from the 
compatibility of the materials used (fabrics, rovings, resins) allowing 
polymerization as a unit in a single mold, without using added bonding 
films. 
According to another characteristic of the invention, with the blade foot 
extended by a fork shaped root having two branches, the frame of each 
branch of the fork is formed by the end of the struts of each compartment, 
the end of the struts of the upper compartment diverging angularly from 
the end of the struts of the lower compartment to an appropriate distance 
from the ends of the branches of said fork, on each side of a wedge shaped 
filling material. The continuity of the framework right along the blade 
from its tip to its root is readily and naturally ensured without having 
recourse to added reinforcement elements. 
Advantageously, each strut is formed by a bundle of fibers folded back on 
itself about a socket for fastening the blade to the hub of the rotor, 
disposed at the end of each of the branches of said fork. 
Furthermore, to the extent that the blade comprises in the neighborhood of 
its foot a fork joint for fastening a control (pitch variation) or damping 
member, each leg of which is provided with an articulation socket, it is 
advantageous to provide a small strut, passing round said socket, formed 
from a bundle of suitably oriented fibers, applied to the rear strut and 
incorporated in the resistant framework of the compartment without having 
recourse to added parts. 
The socket of each of the legs of said fork joint may also be incorporated 
in the framework of the blade by draping the socket with a stack of 
fabrics (plies) forming a reinforcing about the socket, bearing on the 
rear strut of the corresponding compartment and covered by the casing of 
this compartment. 
The second object of the invention resides in a process for manufacturing 
the above blade in which the assembly of its different constituent 
elements and the polymerization of the resins which impregnate them are 
carried out in a single mold formed from two rigid half molds whose 
impression corresponds to the profile of the blade, each of them, prior to 
closure of the mold, being successively: 
coated with layers of skin of the blade and with a part of the layers of 
parallel fibers forming the casing of the compartment: 
equipped with the bundle of rovings situated at the leading edge, 
provided with the means for filling the compartment with a light cellular 
material, 
equipped with the bundle of rovings forming the rear strut of the 
compartment and with a second part of the fabrics of parallel fibers 
forming the casing of the compartment, one of them being also provided 
with the material for filling at the rear of the compartment framework, 
with the wedge shaped element ensuring divergence of the legs of said fork 
and with two molding tools for reserving the spaces separating 
respectively the legs of the fork and those of said fork joint. 
In accordance with the process and advantageously the mold is closed first 
of all for compacting the elements contained in the two half molds, then a 
metal cap is positioned for protecting the leading edge and the mold is 
closed and heated for polymerizing the impregnating resins.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first of all to FIG. 1 a helicopter blade 1 can be seen made from 
a composite material in accordance with the invention, a sectional view of 
the current part of which is shown in detail in FIG. 8. This blade 
comprises a resistant framework formed of two torsion compartments 2 and 3 
superimposed on the leading edge side 4. This framework is extended by a 
rear part adjacent the trailing edge 5 formed essentially by a filling 
material 6 (light and cellular such as a foam), the framework and the 
filling material being enclosed in a casing 7 forming the skin of the 
blade. For fastening it to a rotor hub, the foot of the blade is in the 
form of a fork 8 comprising two spaced branches 8a and 8b (see FIG. 2). It 
also comprises at its foot, a fork joint 9 formed of two legs (only one of 
which 9a is visible in FIGS. 1 and 2). It also comprises a plate 10 in the 
form of a cap for protecting its leading edge 4, and made for example from 
stainless steel. 
It can be seen in FIG. 8 that each of the compartments 2 and 3 comprises a 
front spar or strut 11a, 11b formed externally to the profile of the 
leading edge, and made from a bundle of fibers, called rovings, 
preimpregnated with polymerization resin, a rear spar or strut 12a, 12b 
made from the same material and an intermediate core 13a, 13b (a cellular 
foam). The assembly of struts and core of each compartment is wrapped in 
at least one resin preimpregnated layer of fabrics 14a, 14b made from 
parallel fibers (glass or other) and whose orientation is slanted with 
respect to the longitudinal axis of the blade. Said layers, in their 
jointing part, form after polymerization a single wall 15 which forms an 
extremely stable connection connecting the two compartments together. 
In FIG. 1, in the cut away part at the left, can be seen the leading edge 
strut 11a of the extrados compartment, the core 13a, the rear strut 12a, a 
portion of the casing 14a which rises as shown in FIG. 8 to the level of 
the skin of the blade for joining to another end of this casing. This 
assembly rests on said wall 15 under which is situated the strut 11b of 
the leading edge of the intrados compartment, the core 13b and the rear 
strut 12b. A rear end portion of casing 14b is also shown, which receives 
with the rear end portion of casing 14a the thickest part of the filling 
material 6. 
The right hand part of FIG. 2 shows the blade foot from which the skin 
covering has been partially removed. It can thus be seen that the 
framework of this blade foot, so the frame of the fixing fork 8 which it 
comprises, is formed, for each fork branch such as 8a, by the extension of 
said struts 11a, 11b and 12a, 12b of the corresponding compartment. It 
will be noted that these struts are in fact each formed by a bundle of 
rovings folded back on itself about a socket 16, 17 provided at the end of 
the branches of the fork. Each fork branch 8a, 8b (see FIG. 2) has a 
framework which is the natural extension of each of the compartments 2 and 
3 of the current part of the blade. There is therefore no discontinuity of 
structure between the current portion of the blade and its fastening 
portion, the transition zone being again in the form of a torsion 
compartment quite adapted to the passage of forces. 
In FIG. 2 it can be seen that bundles 12a and 12b which extend along the 
current portion, (like bundles 11a and 11b) adjacent the wall 15, diverge 
on each side of a wedge shaped element 18 to form the fork branches. The 
same goes for the casings 14a and 14b which form wall 15. 
The diagram of FIG. 3 illustrates the fact that each leg 9a, 9b of the fork 
joint 9, which has, in a known way an articulation socket 19, may be 
integrated in the blade foot compartment. In fact, an additional bundle 20 
formed of fibers bent around socket 19 is applied against strand 21 of the 
rear strut and integrates it with the assembly. Of course, draping with 
fabrics of slanted parallel fibers also covers the socket. 
FIG. 4 shows a variant of construction of FIG. 3 in which the legs of fork 
joint 9 are formed by a stack of preimpregnated fabrics formed as a thick 
plate 22 which forms the reinforcement about socket 19 and bears against 
the corresponding rear strand 21 of the bundle 12a of rovings. This stack 
is, as in the preceding Figure, enclosed in the casing of the compartment. 
FIGS. 5, 6 and 7, show schematically the different phases of the method of 
molding the blade of the invention. In two half molds 23 and 24 are placed 
successively: 
the preimpregnated external covering fabrics 25 of the blade forming its 
skin 7 with the layers 25 of fabrics made from parallel fibers orientated 
slant wise with respect to the longitudinal axis of the blade (forming a 
part of said casings 14a and 14b), 
the bundles of rovings 26 also preimpregnated forming respectively the 
front struts 11a and 11b, 
the slabs 27 of cellular foam shaped to the desired profile which form the 
cores 13a and 13b, 
the impregnated bundles of rovings 28 forming the rear struts 12a, 12b of 
compartments 2 and 3, 
the layers of preimpregnated parallel fiber fabrics 29 which form the 
second part of the casings 14a and 14b inside the blade and whose largest 
surfaces will be joined together by polymerization into a single wall 15 
forming a median flange for the blade, 
finally, in one of the half molds, a slab of foam 30 which forms the 
filling material 6. 
Before closing the mold all the other filling elements required will have 
been positioned, especially those disposed between the strands of each 
bundle in line with the sockets 16 and 17 of the fastening fork, the wedge 
element 18 on each side of which the bundles of rovings diverge, the 
elements forming the fork joint 9, the metal inserts and the mobile 
molding pieces (cores) forming the reserves required for forming the 
spaces separating the legs of the fork 8 and of the fork joint 9. One of 
these mobile assembly pieces is shown in FIG. 7 by the reference 31. It 
also serves for holding in place the wedge shaped piece 18 separating the 
two branches 8a, 8b of fork 8. 
The mold thus equipped is closed a first time as shown in FIG. 6 for 
compacting the different elements placed in position. It is then opened 
for positioning the shaped cap 10 for protecting the leading edge and is 
again closed for carrying out the polymerization by heating. 
The blade obtained by this method has numerous qualities in so far as its 
resistance to the forces to which it is subjected is concerned for its 
resistant framework has four struts disposed about the pitch axis (which 
is generally situated at the front quarter of its chord), three sole 
plates (extrados, intrados and median wall 15) and numerous continuous and 
homogeneous connections (free of any application of bonding agents or 
films of different natures of resins) between these resistant elements. 
The invention finds an interesting application in the field of aeronautic 
construction.