Note: Descriptions are shown in the official language in which they were submitted.
~2~34~
HOLLOW COMPOSIT~ AIRFOILS WITH
CORRUGATED INTERNAL SU PORT
STRUCTURE AND METHOD OF FABRICATING SAME
BACKGROUND OF THE INVENTION
The present invention relates to airfoils such as
blades, vanes, struts or the like with aerodynamic
surfaces, and to a method of fabricating such blades,
vanes or struts. The invention has particular
application to vanes of the type utilized in gas
turbines used for aircraft propulsion.
Blades, vanes and struts of various airfoil design
are commonly used in gas turbine engines. Typically,
such blades, vanes or struts are solid members, since
this affords the greatest combination of strength and
ease of fabrication. However, a critical consideration
in aircraft engine construction is weight reduction,
which militates against the use of solid structural
members. Accordingly, it is known to provide hollow
blades, vanes or struts for such applications.
Since hollow airfoils do not have the same
structural strength or stiffness as solid airfoils, it
is necessary to provide hollow airfoils with some type
of support such as stiffening ribs or the like.
Heretofore, hollow airfoils with internal support
structures have been disclosed, for example, in U.S.
Patent Nos. 3,365,124 - J.L. Burge et al issued January
3~
23, 1968; No. 3,627,443 - L. Pirzer issued December 14,
1971; and No. 4,221,539 - C. E. Corrigan issued
September g, 1980. The construction of such hollow
airfoils is relatively costly and complex. Typically,
the airfoil is formed in two parts or halves, with the
internal ribs being formed unitarily with one or both
halves and joined together by suitable bonding
techniques. Alternatively, the hollow airfoil shell
would have to be fabricated first and then the internal
rib structure inserted thereinto and bonded thereto.
Another important consideration in airfoils for
turbo machinery is vibration damping. Such damping has
been provided, for example, by external sheathing of the
airfoil, as disclosed in U.S. Patent No. 3,357,850 -
J. E. Baker issued December 12, 1967. Such external
sheathiny necessitates additional manufacturing steps
and can significantly increase the cost of the finished
airfoil.
SUMMARY OF THE INVENTION
__
It is a general object of this invention to provide
an improved hollow airfoil construction and method of
fabricating same, which avoids the disadvantages of prior
airfoil constructions and methods of fabrication while
affordin~ additional structural and operating
advantages.
An important object of the invention is the
provision of a novel hollow airfoil which is of
relatively simple and economical construction.
Another object of the invention is the provision of
a hollow airfoil of the type set forth, which has
adequate structural strength while affording good
vibration damping.
In connection with the foregoing objects, it is
another object of this invention to provide a method of
fabricating such a hollow airfoil which is simple and
$~
economical.
In connection with the foregoing object, it is yet
another object of the invention to provide a method of
the type set forth which minimizes fabrication steps.
These and other objects of the invention are
attained by providing an airfoil construction comprising:
a hollow shell, a corrugated support structure disposed
in the shell and in area contact therewith at spaced-
apart areas thereon, the support structure cooperating
with the shell to define hollow cavities therebetween.
These and other objects of the invention are
further attained by providing a method of fabricating a
hollow airfoil comprising the steps of: providing a core
assembly including an elongated corrugated support
structure of one material and a plurality of elongated
mandrels of another material disposed in the corrugations
of the support structure in contact therewith and
cooperating therewith to define the core assembly, then
applying a shell around the core assembly encompassing
the core assembly except at the ends thereof and
contacting the support structure, then bonding the shell
only to the support structure, and then removing the
mandrels through an open end of the shell, leaving a
hollow shell with an integral internal corrugated support
structure~
The invention consists of certain novel features
and a combination of parts hereinafter fully described,
illustrated in the accompanying drawings, and
particularly pointed out in the appended claims, it being
understood that various changes in the details may be
made without departing from the spirit, or sacrificing
any of the advantages of the present invention.
~28~
BRIEF DESCRIPTION OF THE DRAWINGS
For the purpose of facilitating an understanding
of the invention, there are illustrated in the
accompanying drawings preferred embodiments thereof,
from an inspection of which, when considered in
connection with the following description, the
invention, its construction and operation, and many of
its advantages should be readily understood and
appreciated.
FIG. 1 is a simplified cross-sectional view, in
partial cutaway, of an aircraft gas turbofan, including
outlet guide vane assemblies incorporating the features
of the present invention;
FIG. 2 is an exploded perspective view of a vane
assembly constructed in accordance with and embodying
the features of the present invention;
FIG. 3 is an enlarged sectional view taken along
the line 3-3 in FIG. 2,
FIG. 4 is a perspective view of the vane assembly
of FIG. 2 in assembled condition;
FIG. 5 is a fragmentary sectional view taken
along the line 5-5 in FIG. 4;
FIG. 6 is an enlarged side elevational view of the
end plug of the vane assembly of FIG. 2;
FIG. 7 is a further enlarged view in vertical
section taken along the line 7-7 in FIG. 6;
FIG. 8 is an enlarged fragmentary view of the
upper portion of the outlet guide vane assembly of
FIG. 2, illustrating the manner of attachment to the fan
cowl;
FIG. 9 is a perspective exploded view illustrating
a preform, assembly of which is the first step in the
fabrication of the vane assembly of FIG. 2;
FIG. 10 is an enlarged, fragmentary perspective
view illustrating the formation of the preform of FIG.9;
~2~A~
FIG. 11 is a sectional view of a mold assembly for
joining the parts of the preform illustrated in FIGS. 9
and 10;
FIG. 12 is a fragmentary sectional view of an
apparatus for bonding the vane to a mounting platform;
FIG. 13 is an enlarged sectional view of a press
mechanism for applying a sheath to the vane;
FIG. 14 is an enlarged fragmentary sectional view
of an alternative embodiment of the vane of the present
invention; and
FIG. 15 is a further enlarged fragmentary sectional
view of a further embodiment of the vane of the present
invention.
DÉSCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 of the drawings, there is
diagrammatically illustrated a gas turbofan engine,
generally designated by the numeral 20. While it is
recognized that turbofan engines are well known in the
art, a brief description of the operation of the engine
20 will enhance appreciation of the interrelationsip of
the various components by way of background for the
invention to be described below. Basically, the engine
20 may be considered as comprising a core engine 21, a
fan 22 including a rotatable stage of fan blades 23, and
a fan turbine 24A downstream of the core engine 21 and
which is interconnected to the fan 22 by a shaft 25. The
core engine 21 includes an axial flow compressor 26
having a rotor 27. Air enters inlet 28 from the left of
FIG. 1, in the direction of the solid arrow, and is
initially compressed by the fan blades 23.
A fan cowl or nacelle 29 circumscribes the core
engine 21 and is interconnected therewith with a
plurality of radially outwardly extending outlet guide
vane assemblies 30, (one shown) substantially equi-
angularly spaced apart around the core engine cowl. The
*~
prime purpose of the outlet guide vane assemblies 30 isto redirect the helical air flow exiting the fan blades
23 into a predominantly truly axial direction. A first
portion of the relatively cool compressed air exiting
the fan blades 23 enters a fan bypass duct 31 defined
between the core engine 21 and the fan cowl 29, and
discharges through a fan nozzle 32. A second portion
of the compressed air enters core engine inlet 33, is
further compressed by the axial flow compressor 26, and
is discharged to a combustor 34 where it is mixed with
fuel and burned to provide high energy combustion gases
which drive a core engine turbine 35. The turbine 35,
in turn, drives the rotor 27 in the usual manner of gas
turbine engines. The hot gases of combustion then pass
through and drive the fan turbine which, in turn, drives
the fan 22. A propulsive force is thus obtained by the
action of the fan 22 discharging air from the fan bypass
duct 31 through the fan nozzle 32 and by the discharge
of combustion gases from a core engine nozzle 37
defined, in part, by a plug 38 and the cowl 39 of the
core engine 21.
The present invention relates to the outlet guide
vane assemblies 30 of novel polymeric composite
construction and to a novel method of fabrication
thereof. Referring now to FIGS. 2 through 8 of the
drawings, each vane assembly 30 includes an elongated
airfoil vane 40 which comprises a hollow shell 41 having
walls 42 and 43 which are spaced apart to define a
cavity 44 therebetween (FIG. 3), and which are inter-
connected along the leading edge 45 and the trailingedge 46 of the vane 40. Disposed in the cavity 44 and
extending the longitudinal length of the shell 41 is an
elongated laminated composite corrugated support
structure 47 having generally trapezoidal corrugations
with flattened lands 47a which are integral with the
walls 42 and 43. Preferably, the lands 47a are bonded
to the walls 42 and 43, the support structure 47
serving as a stiffening member to provide internal
support for the walls 42 and 43. It can be seen that
the cavity 44 remains open between the corrugations of
the support structure 47, and the hollow shell 41 is
open at both ends thereof, as at 48 (FIG. 2).
Preferably, a polyurethane sheath 49 covers the outer
surface of the hollow shell 41 and serves to provide an
erosion-resistant covering for the vane 40.
One open end 48 of the shell 41 is closed with an
end plug 50 which includes an insert portion 51 having
a concave inner end 52. Integral with the insert
portion 51 at the outer end thereof and extending
laterally outwardly therefrom is a cap flange 53
dimensioned to bear against the distal end edge of the
shell 41 and be substantially flush with the peripheral
surface thereof. The other end of the vane 40 is
adapted to be received in a boot 55, which is mounted in
core engine cowl 39. More specifically, the boot 55 has
a socket insert 56 defining a cavity 57 in which the end
of the vane 40 is inserted. Integral with the socket
insert 56 at the upper end thereof and extending
laterally outwardly therefrom is an attachment flange 58.
Mounted on the plugged end of the vane 40 is a
mounting platform 60 to facilitate mounting of the vane
assembly 30 in the associated turbofan engine 20. The
mounting platform 60 has a substantially rectangular base
plate 61 provided with an upstanding peripheral wall 62
integral therewith around the perimeter thereof. Also
integral with the base plate 61 and projecting upwardly
therefrom is an arcuate body 63 defining a recess or
cavity 64 which is shaped complementary to but
dimensioned slightly larger than the plugged end of the
vane 40. The plugged end of the vane 40 is received in
the cavity 64 with a predetermined substantially uniform
clearance space therearound, which space is filled with
an elastomeric encapsulant 65 which serves to bond the
vane 40 to the mounting platform 60. Preferably, the
5 encapsulant 65 is injected into the clearance space
through an injection bore 66 in the arcuate body 63, as
will be explained more fully below. Also integral with
the base plate 61 and with the arcuate body 63 are two
mounting lugs 67, each provided with a bore for
10 reciving a complementary fastener, such as a bolt 68 and
nut plate 68a (FIG. 8). Both the platform 60 and the
plug 50 are preferably formed of a nylon filled with
carbon fibers.
In use, the vane assembly 30 is mounted in place
15 by inserting the free end of the vane 40 into the boot
55, which is mounted in a complementary recess (not
shown) in the cowl 39 of the core engine 21, being
secured in place by suitable means. The mounting
platform 60 is secured by bolts 68 to the inner surface
20 of the fan cowl 29, as illustrated in FIG. 8.
The vane assembly 30 offers the advantage of a
preformed assembly which is ready for mounting in the
gas turbofan engine 20 by the application of a few
fasteners, and has the advantage of low weight by reason
25 of its hollow construction. The corrugated support
structure 47 supports the outer aerodynamic shell 41
internally.
Referring now also to FIGS. 9 through 13 of the
drawings, the method of fabrication of the vane assembly
30 30 will be described. The vane 40 is first constructed
from a vane preform, generally designated by the numeral
70l which includes a core assembly 71 and shell preforms
75 and 76. The core assembly 71 comprises the uncured
laminated corrugated support structure 47 and a
35 plurality of elongated removable mandrels 73 which are
respectively disposed in the spaces between the
corrugations of the support structure 47 on both sides
thereof, as illustrated in FIG. 10. More specifically,
the laminae of the support structure 47 are stacked and
the mandrels 73 are interposed to form the corrugations
in the support structure 47. The mandrels 75 are shaped
and dimensioned to cooperate with the uncured support
structure 47 to form the core assembly 71 which is
substantially in the aerodynamic shape of the finished
vane 40. The support structure 47 may be formed of thin
laminae of a composite material, preferably a composite
of graphite or carbon fibers and glass fibers, such as
unidirectional hybrid 80-graphite/20-glass, impregnated
with a thermosetting epoxy resin, available from the 3M
Company, St. Paul, Minnesota. Alternatively, the support
structure 47, or the pre~orms 75 and 76, or both, could
be forme~ of a composite consistiny of, for example,
laminae of metallic foils bonded together by a suitable
adhesive. Each of the mandrels 73 is formed of a
material with release characteristics so that it will not
adhere to an epoxy resin during cure, the material
preferably being a silicone rubber, such as that sold by
General Electric Company under the trademark TUFEL.
Each of the shell preforms 75 and 76 comprises a
plurality of thin laminae 77 of a composite material,
preferably the same composite as the support structure
47. The shell preforms 75 and 76 are respectively laid
over the convex and concave surfaces of the core assembly
71, each of the shell preforms 75 being dimensioned to be
longitudinally coterminous with the core assembly 71, but
extending beyond the core assembly 71 along the leading
and trailing edges thereof so that these extending
portions of the shell preforms 75 and 76 overlap each
other. Thus, it will be appreciated that the inner ones
of the laminae 77 are in area contact with the lands 47a
3~
-- 10 --
of the support structure 47.
After the vane preform 70 is assembled, it is
placed in a molding machine 80 (FIG. 11) which includes
heated matched male and female dies 81 and 82. Heat and
pressure are simultaneously applied to the vane preform
70 by the molding machine 80 to cure the vane preform 70,
including the corrugated support structure 47, in one
step. More specifically, the laminae 77 of each of the
shell preforms 75 and 76 are bonded together, the laminae
of the support structure 47 are cured, and the over-
lapping portions of the shell preforms 75 and 76 are
bonded together along the leading and trailing edges of
the vane 40~ The inner ones of the laminae 77 are
simultaneously bonded to the lands 47a of the support
structure 47, but they are not bonded to the mandrels 73
because of the latterls inherent release characteristics.
For the preferred materials described above, the cure
cycle includes a cure of about one hour at 2300F.,
followed by post-curing at 275F., for four hours.
However, it will be appreciated that the curing cycle
could change in the event alternate materials are used.
After the vane preform 70 has been cured in the molding
machine 80, the mandrels 73 are removed through one end
of the hollow shell 41 by simply pulling them out.
There remains the hollow vane 40 with integral, internal,
longitudinally extending support structure 47.
Next, the vane 40 is assembled to the mounting
platform 60. Preerably, the inner surface of the cavity
64 and the outer surface of the end of the vane 40 to be
inserted therein are abraded, as by grit blasting, the
remaining surfaces of the vane 40 and the platform base
plate 61 first being appropriately masked. It will be
appreciated that alternative abrading techniques, such
as etching, could also be used. A suitable primer is
then applied to the abraded surfaces. The primer may,
for example, be a mixture of primers such as those sold
by the Dayton Coatings and Chemical Division and
Whittaker Corporation under the trademarks THIXON 300
and THIXON 301. Primer is applied to achieve a dry
film thickness of approximately .0003 to .0004 inch.
The injection bore 66 is then drilled in the platform
60 or, in the altérnative, is premolded into the
platform 60.
The primed vane 40 and platform 60 are then pre-
heated for about lS minutes at a temperature of about
320F., then loaded into a transfer mold assembly 85
(FIG. 12) which is maintained at a temperature of about
350 F. More specifically, the vane 40 is supported in
a suitable support fixture (not shown) and the insertion
end is clamped in a retaining plate 84. The platform 60
is received in a complementary cavity in a mold tool 86.
The retaining plate 84 is secured to the mold tool 86 so
that the ~braded end of the vane 40 is received in the
cavity 64 of the platform 60 with a predetermined
substantially uniform clearance space therearound.
Preferably, the depth of insertion of the vane 40 into
the cavity 64 is approximately 0.8 inch and a clearance
space approximately 0.08 inch is established between the
tip of the vane 40 and the bottom of the cavity 64 by not
bottoming the vane 40 in the cavity 64. Also the sizing
of the vane 40 and the cavity 64 is such that a clearance
space of about 0.08 inch is established between the sides
of the vane 40 and the sidewalls of the cavity 64.
The mold tool 86 has an injection sprue 87 which
is disposed in alignment with the injection bore 66
through the platform 60. The sprue 87 communicates with
a transfer cylinder 88 in which is disposed a piston 89.
Uncured elastomer, preferably a fluoroelastomer rubber
such as that sold under the trademark ~ITON by E. I.
DuPont de Nemours & Co. Inc., is loaded into the transfer
cylinder 88, which is maintained at a temperature of
about 3500F. The elastomer is then injected under about
3,500 psi maximum transfer pressure through the sprue 87
and the injection bore 66 into the clearance space
between the vane 40 and the platform 60. The vane/
platform assembly is retained in the transfer mold
assembly 85 for about 75 minutes at a temperature of
about 350 F~, which serves to cure the VITON elastomer
65 and securely bond the vane 40 to the platform 60. The
bonded assembly is then removed from the transfer mold
assembly 85 and post-cured for about 16 hours at a
temperature of about 300 F., after which surplus VITON
flash is removed from the platform 60 and from the vane
40.
The vane 40, after molding and the po5t-cure cycle
has low resistance to erosion caused by debris such as
sand, ~ravel and the like, to which aircraft gas turbine
engines may be exposed. Thus, the polyurethane sheath 49
is applied to the outer surface of the hollow shell 41 to
provide the necessary erosion resistance. First the
outer surface of the hollow shell 41 is lightly abraded,
as by grit blasting, the surfaces of the mounting
platform 60 and the encapsulant 65 being masked to
prevent erosion thereof during the grit blasting process.
Polyurethane film, approximately .010 inch thick with an
approximately .001 inch thick coating of an adhesive
resin on one surface thereof, is then cut into an
elongated strip of the desired size and shape. The film
strip is then wrapped around the hollow shell 41, being
worked down into intimate contact with the surface of the
shell 41 by use of a suitable tool, such as a spatula or
the like, to revent entrapment of air or the formation of
resin-rich pockets.
When the outer surface of the hollow shell 41 has
been completely covered by the polyurethane sheath 49,
~?~
the vane 40 is placed in a press fixture 90 (FIG. 13)
for curing the adhesive. The press fixture 90 includes
a convex lower member 91 and a concave upper member 92.
Before insertion of the vane 40 into the press fixture
90, a pressure-intensifier envelope 93 is wrapped around
the sheathed vane 40. Preferably, the envelope 93 is
formed of silicone rubber and is arranged in a single-
fold configuration having two flaps which respectively
lie along the convex and concave surfaces of the vane 40
and overlap, as at 94, beyond the trailing edge of the
vane 40~ Then the assembly of the sheathed vane 40 and
the pressure-intensifier envelope 93 are placed in the
press fixture 90 and cured for about 60 minutes at a
temperature of about 230F. The pressure-intensifier
envelope 93 serves to increase and evenly distribute the
pressure applied to the sheath 49 to assure uniform
curing thereof and uniform adherence to the outer surface
of the shell 41. The support structure 47 should provide
sufficient internal support during the pressing operation
but if nécessary. the hollow cure 44 could be
~ressurized for this operation. The polyurethane
sheathed vane 40 is then removed from the press fixture
90, the envelope 93 is removed and the sheathed vane 41
is post-cured in an oven for about four hours at 270F.
Excess polyurethane film is then trimmed from the vane
41.
There results a vane assembly 30 which is of
extremely light weight and inexpensive manufacture, and
has improved fatigue strength and erosion resistance.
Furthermore, the vane assembly 30 is characterized by
excellent dimensional uniformity and an improved surface
finish, as well as improved fatigue resistance compared
to comparable metallic airfoils. All of these
advantages are obtained without the use of potentially
strategic materials.
_`.L~
-- 14 --
In mounting the vane assembly 30 to the turbofan
engine 20, the free end of the vane 40 is inserted in
the boot 55 and the platform 60 is then bolted in place
on the fan cowl 29, as described above.
Referring now to FIG. 14, there is illustrated an
alternative vane construction, generally designed by
the numeral 100, which is essentially the same as the
vane 40 except that it includes a vibration damping
layer. More specifically, the vane 100 has laminated
10 composite outer shells 101 and lOla comprised of laminae
lOlb and having walls 102 and 103 which are spaced apart
to define an internal cavity 104, the walls 102 and 103
being joined together along the leading and trailing
edges of the vane 100. A corrugated laminated support
15 structure 105 is disposed in the cavity 104, the
corrugations being generally trapezoidal and having
flattened lands 106~ A layer 107 of elastomeric
vibration damping material lines the inner surface of
the shell 101 so as to be in area contact with the lands
20 106 of the support structure 105, the layer 107
preferably being formed of polyurethane. If desired, a
polyurethane sheath (not shown~ like the sheath 49 may
also be applied to the outer surface of the shell 101.
The method of fabrication of the vane 100 is
25 substantially the same as that described above for the
vane 40, with the exception that the .polyurethane layer
107 is applied between the core assembly 71 and the
shell preforms 75 and 76 during the assembly of the vane
preform 70. The epoxy resin in the shell laminae 77
30 provides the bonding medium for the polyurethane layer
107.
Referring now to FIG. 15 of the drawings, as a
further embodiment, the laminated support structure 105
may have additional vibration-damping layers interleaved
35 therein. ~lore specifically, the support structure 105
- 15 -
comprises laminae 105a of composite material. In this
embodiment layers 107a of elastomeric material, similar
to the layers 107, may also be interleaved with the
laminae lOSa, the laminae 105a and the layers 107a all
being co-cured simultaneously with the laminae lOla of
the shell 101 during the molding operation.
From the foregoing, it can be seen that there has
been provided an improved hollow vane construction with
an internal support structure which provides mechanical
support and vibration damping, as well as a unique
method of manufacturing such a vane. There have also
been disclosed a method for assembling the vane to a
mounting platform, resulting in an extremely light
weight and low cost vane assembly with improved
structural and operating characteristics.
