Composite airfoil leading edge protection

Methods for improving the erosion resistance of composite airfoils are disclosed as are the resultant structures. Wire mesh materials are coated with an erosion-resistant coating, formed to the shape of the airfoil leading edge, and molded into the leading edge during airfoil fabrication.

TECHNICAL FIELD 
This invention relates to composites, and more particularly to a method for 
improving the erosion resistance of composite components employed in gas 
turbine engines. 
BACKGROUND ART 
Engine weight is an important factor when considering the overall cost and 
performance of a gas turbine engine. For many years attempts have been 
made to decrease the overall weight of the engine while maintaining or 
improving engine performance. One manner in which researchers have 
attempted to reduce the overall weight of the engine is by utilizing 
composite airfoils in place of the metal airfoils currently employed in 
most gas turbine engines. Composite airfoils offer a significant weight 
savings over metal airfoils, however, composite materials have inherently 
poor resistance to foreign object damage (FOD). 
Many types of foreign objects may be entrained in the inlet airflow of an 
aircraft gas turbine engine ranging from large birds, such as sea gulls, 
to hailstones, rain, sand, and dust. Damage from foreign objects generally 
takes two forms. Smaller objects can erode the blade material, causing the 
aerodynamic shape to change, and degrade the performance of the 
compressor. Impact by larger leading edge objects can dent or deform the 
blades. Portions of an impacted blade can also be torn loose and cause 
secondary damage to downstream blades and other engine components. 
The consequences of foreign object damage are greatest in the fan and low 
pressure compressor sections of turbine engines. However, these components 
offer the greatest potential in weight reduction due to their large tip 
diameters, as great as eight feet, and spans in the order of two or more 
feet. 
The vulnerability of composite blades to foreign object damage is due to 
two factors. First the lightweight matrix materials employed, generally 
polymeric resins or metals such as aluminum, are relatively soft and do 
not have high tensile strengths. Second, the high-strength filaments 
employed in such composites are relatively hard and brittle. As a result, 
the matrix material is subject to erosion and the fibers are subject to 
breakage upon foreign object impact. 
From this it would appear that some sort of protection system should be 
provided for these composite blades and vanes. Many such protection 
systems have been proposed. They include claddings of various compositions 
applied to the leading edge portion of the entire surface of the blade. 
One proposed cladding system involves fixing a solid metal sheath over the 
leading edge of the blade. This procedure, however, requires expensive 
forming operations and the sheath must ultimately be adhesively bonded to 
the airfoil as a secondary operation after airfoil manufacture. This 
process proves to be both costly and time consuming. In addition, solid 
metal sheaths require stringent surface preparation and priming prior to 
adhesive bonding, and are subject to environmental degradation of the 
adhesive bond when in operation. This naturally reduces the life of the 
protected composite airfoil. 
Another proposed method for protecting the leading edge of composite blades 
and vanes is disclosed and claimed in U.S. Pat. No. 3,892,612, Method for 
Fabricating Foreign Object Damage Protection for Rotor Blades, Carlson et 
al. The disclosed and claimed method of U.S. Pat. No. 3,892,612 is 
directed to a complicated method of applying a protective metal coating to 
a non-conductive substrate which comprises the steps of (i) incorporating 
a woven wire mesh into the substrate, by means of a bonding agent which 
fills the interstices of the mesh, and then abrading the outer surface of 
the mesh layer to remove the adhesive from its nubs; (ii) applying a thin 
conductive layer to the bonding agent in the interstices of the mesh with 
the mesh nubs free of the thin conductive layer; and (iii) 
electrolytically depositing a metal coating on the wire mesh/conductive 
layer surface to obtain an essentially uniform thickness coating forming a 
metallic strip. The above steps are both complicated and time consuming. 
In addition, it is noted in a later U.S. Pat. No. 4,006,999 entitled, 
Leading Edge Protection for Composite Blades, Brantley et al., that a 
metallic strip leading edge protection created by the aforementioned 
method has demonstrated problems with delamination when impacted by 
medium-sized birds. This problem, according to the assignee, can result in 
secondary engine damage as the leading edge protection strip is ingested 
through the engine and, in addition, engine imbalance at high speeds can 
cause further damage. 
DISCLOSURE OF INVENTION 
It is an object of the present invention to provide an improved method for 
protecting the leading edge of a composite blade against foreign object 
damage, such method being both cost-effective and easy to implement. 
It is another object of the present invention to produce an improved method 
for providing durable protection for the leading edge of a composite 
airfoil so as to increase both the reliability and the longevity of the 
composite blade. 
According to the present invention, a composite airfoil is made resistant 
to erosion and foreign object damage by a process which includes applying 
an erosion resistant coating to a mesh, conforming the coated mesh to the 
desired airfoil contour, and integrally molding the coated mesh into the 
composite airfoil so as to protect the aerodynamic shape and useful life 
of the airfoil. The mesh may be metal and should maintain an open 
construction at its interstices after the erosion resistant coating is 
applied, prior to molding. Keeping an open construction allows for resin 
infiltration from the composite lay-up during molding and provides a 
strong, durable, mechanical interlock between the composite and the mesh. 
The foregoing and other objects, features, and advantages of the present 
invention will become ore apparent in the light of the following detailed 
description of exemplary embodiments thereof, as illustrated in the 
accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the FIGURE, a composite airfoil 10, in accordance with the 
present invention, includes a coated mesh 12 adjacent with the exterior 
portion of leading edge 14, the coated mesh being contoured abut the 
leading edge 14 of the airfoil 10 to protect the leading edge 14 against 
foreign object damage. This invention comprises the method for making the 
composite airfoil 10 with the coated mesh 12 leading edge 14 protection. 
The method of this invention comprises the steps of applying an 
erosion-resistant coating to a mesh, conforming the coated mesh to the 
desired airfoil contour, and then integrally molding the coated mesh to 
the composite airfoil so as to protect the airfoil from erosion and other 
foreign object damage while also maintaining the aerodynamic shape of the 
airfoil. 
In carrying out this invention, the mesh is preferably made of a material, 
such as stainless-steel, which is strong enough to provide the leading 
edge with enhanced protection against erosion, yet sufficiently pliant, 
and ductile, so as to be formable about the airfoil's leading edge, and to 
which an erosion-resistant coating will adhere. However, any other 
sufficiently strong and pliant material, such as carbon fibers, to which 
the erosion-resistant coating will adhere, may also be utilized. 
One type of wire mesh useful with the present invention is a square, plain 
weave pattern mesh, although other types of mesh may also be utilized. In 
this type of mesh substantially parallel lengthwise wires are relatively 
perpendicular to substantially parallel crosswise wires. The lengthwise 
wires are typically referred to as warp wires, while the crosswise wires 
are typically referred to as weft wires. The warp wires pass over and then 
under successive weft wires and continue in an over one, under one 
pattern. A void area is formed by the intersection of two adjacent 
parallel warp wires with two adjacent parallel weft wires. This void area 
is useful because during the molding process resin infiltrates from the 
composite through the mesh voids to provide a strong, durable, mechanical 
interlock between the composite and the mesh. To provide the mesh with 
good mechanical strength without interfering with aerodynamic flow once 
the mesh is in place, the diameter of both the warp and weft wires should 
be in the range of about 3.0 mils to 5.0 mils, with about 4.5 mils being 
the preferred wire diameter. The above-described plain weave pattern mesh 
is known to those skilled in the art as a Plain Dutch Weave and may be 
purchased from a variety of companies, including INA Filtration 
Corporation of South Carolina. 
Prior to securing the mesh to the composite airfoil the mesh is formed to 
the approximate contour of the leading edge and is coated with an 
erosion-resistant coating. If the coating is sufficiently ductile the mesh 
may be coated first and then formed to the approximate contour of the 
leading edge. If the mesh contains more weft wires than warp wires then it 
is preferred that the mesh be formed so that its warp wires will lie 
longitudinally along the airfoil's leading edge when the mesh is 
integrally molded to the airfoil. 
The mesh is then coated on at least one side, with an erosion-resistant 
coating, such as electrolytic or electroless nickel, to a thickness of 
about 0.1 mils to 5.0 mils, with the preferred thickness being about 1.0 
mils to 2.0 mils. Both sides of the mesh may be coated, but the side of 
the mesh which is placed in contact with the airfoil need not be coated. 
Other erosion-resistant coatings, such as titanium nitride and titanium 
diboride, which can be applied to a thickness of 0.1 mils to 5.0 mils in 
order to improve the mesh's durability, without adding excess weight or 
impacting the aerodynamic shape of the airfoil. When coating the mesh it 
is important to keep the void areas open, because during the molding 
process resin infiltrates from the composite airfoil, through the mesh to 
secure the mesh to the airfoil. 
After the mesh is coated it is placed around the lay-up of a composite 
material which is impregnated with resin, and is formed and secured to the 
finished airfoil contour by compression molding. During compression 
molding resin infiltrates from the composite lay-up through the void areas 
of the mesh and provides a strong, durable interlock between the composite 
and the wire mesh. This eliminates the need for adhesive bonding which 
often involves stringent surface preparation and priming procedures of the 
mesh and/or airfoil. 
Although this invention has been shown and described with respect to 
detailed embodiments thereof, it will be understood by those skilled in 
the art that various changes in form and detail thereof may be made 
without departing from the spirit and scope of the claimed invention. For 
example, while the invention is described with respect to compression 
molding, it will be understood that other composite processing methods 
such as resin transfer molding (RTM) and autoclave molding may be 
utilized.