CA1284482C - Method of fabricating hollow composite airfoils - Google Patents

Abstract

HOLLOW COMPOSITE AIRFOILS WITH
CORRUGATED INTERNAL SUPPORT
STRUCTURE AND METHOD OF FABRICATING SAME.
ABSTRACT OF THE DISCLOSURE
A hollow composite airfoil with an integral internal laminated corrugated support structure is formed by disposing silicone rubber mandrels in the corrugations of the laminated support structure to form a core assembly having a desired aerodynamic shape, then stacking on both sides of the core assembly laminae of a composite material (of which the support structure may also be formed), with the stacks overlapping adjacent the leading and trailing edges of the core assembly. Heat and pressure are then applied to the core assembly with the laminae thereon to bond together in a single step the laminae of each stack, and the two stacks to each other and to the support structure to form a continuous shell around the core assembly. The mandrels are then removed.
One open end of the resulting hollow airfoil is plugged and that end is inserted into a recess in a mounting platform with a predetermined substantially uniform clearance space therebetween. An elastomeric material is then injected into the clearance space for filling it, and then cured to bond the platform to the airfoil. A polyurethane sheet may be wrapped around the airfoil and cured. A vibration damping polyurethane layer may also be disposed between the core assembly and the composite laminae before bonding thereof. Vibration damping polyurethane layers may also be interleaved with the laminae of the support structure.

Description

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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.

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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;

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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~

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.

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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 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.

Claims (8)

1. A method of fabricating a reinforced hollow airfoil of laminated shell structure comprising the steps of:
(a) providing a core assembly including an elongated laminated corrugated support structure of one material and a plurality of elongated mandrels of another material disposed in the corrugations of said support structure in contact therewith and cooperating therewith to define the core assembly, (b) then applying a laminated shell of stacked laminae of a material comprising a composite of carbon or graphite fibers and glass reinforcement fibers impregnated with epoxy resin on the corrugated support structure encompassing said core assembly except at the ends thereof, and (c) contacting said corrugated support structure and bonding the laminae to each other and to said support structure by heat and pressure, and (d) then removing said mandrels through an open end of said shell structure, leaving a hollow airfoil with an integral internal corrugated support structure.

2. The method of claim 1 wherein the corrugations of said support structure are generally trapezoidal and provide area contact with said shell at spaced-apart areas therealong.

3. The method of claim 2 wherein the corrugations of said laminated corrugated support structure are formed by aligning a first plurality of spaced apart mandrels, placing the material of said laminated corrugated support structure over said first plurality of mandrels and into the spaces between said mandrels and then interposing a second plurality of mandrels in between said first plurality and said core to form corrugations therebetween.

4. The method of claim 3 wherein the material of said laminated corrugated support structure comprises laminae of metallic foils bonded together.

5. The method of claim 4 which further includes the step of applying a layer of elastomeric vibration damping material to the inside surface of said shell in intermittent contact with said laminated corrugated support structure.

6. The method of claim 5 which further includes the step of applying an erosion-resistant sheath to the outer surface of said shell.

7. The method of claim 6 which further includes the steps of:
(a) providing a mounting platform having a recess therein shaped complementary to but dimensioned slightly larger than one end of the airfoil, (b) plugging one end of said hollow airfoil, (c) inserting said plugged end of the airfoil into the recess in the platform which a predetermined substantially uniform clearance space between the recess of the platform and the inserted end of the airfoil, then (d) injecting an elastomeric material into the clearance space for filling same, and (e) then curing the elastomeric material for bonding the inserted end of the airfoil to the platform.

8. The method of claim 7 which further includes the step of applying an erosion-resistant sheath to the outer surface of said airfoil after assembly thereof to the mounting platform.