Patent Description:
The present invention relates to an airfoil for a gas turbine engine, to a gas turbine engine and to a method of forming a composite airfoil for a gas turbine engine.

A gas turbine engine typically includes at least a compressor section, a combustor section and a turbine section. The compressor section pressurizes air into the combustion section where the air is mixed with fuel and ignited to generate an exhaust gas flow. The exhaust gas flow expands through the turbine section to drive the compressor section and, if the engine is designed for propulsion, a fan section.

The fan section includes an array of airfoils carried by a fan hub. Some airfoils are made of one or more layers of a composite material. During operation, the airfoils may be subjected to impact by foreign objects, such as during a bird strike event. An impact may cause the layers to delaminate, which can result in loss of structural capability and liberation of plies.

<CIT> discloses an airfoil according to the preamble of claim <NUM>, a gas turbine engine according to the preamble of claim <NUM>, and a method according to the preamble of claim <NUM>.

<CIT> discloses a method of making a composite fan blade, and <CIT> discloses airfoil structures.

According to a first aspect there is provided an airfoil for a gas turbine engine at set forth in claim <NUM>.

In an embodiment of the above, the airfoil section is free of the core for at least a majority of span positions of the airfoil section.

In a further embodiment of any of the foregoing embodiments, the three-dimensional network of woven fibers is formed from a dry fiber preform.

In a further embodiment of any of the foregoing embodiments, the composite core and the first and second skins are formed together by resin transfer molding or by resin pressure molding to define the airfoil section and the root section.

In a further embodiment of any of the foregoing embodiments, the two-dimensional network of fibers is formed from a pre-impregnated fabric or a pre-impregnated tape.

In a further embodiment of any of the foregoing embodiments, the composite core terminates in the spanwise direction along the airfoil section prior to about <NUM>% span.

In a further embodiment of any of the foregoing embodiments, the core is skewed toward the leading edge.

According to a further aspect there is provided a method as set forth in claim <NUM>.

In an embodiment of the above, the step of forming includes curing the core and the first and second skins in the resin subsequent to delivering the resin into the mold.

In a further embodiment of any of the foregoing embodiments, the step of forming is performed using a resin transfer molding process to define the airfoil section and the root section.

In a further embodiment of any of the foregoing embodiments, the step of fabricating the first and second skins is performed by an automated fiber placement process.

In a further embodiment of any of the foregoing embodiments, the method includes partially curing the core prior to the step of arranging.

In a further embodiment of any of the foregoing embodiments, the step of forming is performed using a resin pressure molding process to define the airfoil section and the root section.

In a further embodiment of any of the foregoing embodiments, the two-dimensional network of fibers is formed from a dry fiber tape.

In a further embodiment of any of the foregoing embodiments, the two-dimensional network of fibers is formed from a pre-impregnated fabric or pre-impregnated tape.

In a further embodiment of any of the foregoing embodiments, the step of arranging is performed such that the airfoil section is free of the core for at least a majority of span positions of the airfoil section.

The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of an embodiment.

"Low corrected fan tip speed" is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram °R) / (<NUM> °R)]<NUM> (where °R = K x <NUM>/<NUM>) The "Low corrected fan tip speed" as disclosed herein according to one non-limiting embodiment is less than about <NUM> ft / second (<NUM> meters/second).

Referring to <FIG>, the fan <NUM> includes a fan hub <NUM> that carries a plurality of fan blades or airfoils <NUM> (one shown for illustrative purposes) that are rotatable about the engine longitudinal axis A. The airfoil <NUM> includes a root section <NUM> and an airfoil section <NUM>. The fan hub <NUM> defines a plurality of circumferentially spaced apart slots <NUM>. The root section <NUM> of each airfoil <NUM> is dimensioned to be slideably received within a respective slot <NUM> to mount the airfoil <NUM> in the engine <NUM>. The root section <NUM> has a generally dovetail geometry that mates with walls of the slot <NUM>.

<FIG> illustrate an example composite airfoil <NUM>. In this disclosure, like reference numerals designate like elements where appropriate and reference numerals with the addition of one-hundred or multiples thereof designate modified elements that are understood to incorporate the same features and benefits of the corresponding original elements. The airfoil <NUM> can be a fan blade incorporated into the fan <NUM> of <FIG> and <FIG>, for example. Other portions of the engine can benefit from the teachings herein, such as fan exit guide vanes <NUM> or airfoils in the compressor or turbine sections <NUM>, <NUM> of <FIG>. Other systems can also benefit from the teachings disclosed herein, including ground-based power generation systems.

The airfoil <NUM> includes a root section <NUM> and an airfoil section <NUM>. The airfoil section <NUM> extends in a spanwise or radial direction R from the root section <NUM> to a tip <NUM>. The tip <NUM> is a terminal end of the airfoil <NUM>. The airfoil section <NUM> generally extends in a chordwise or axial direction X between a leading edge L/E and a trailing edge T/E. The airfoil section <NUM> defines a pressure side P and a suction side S separated in a thickness direction T. Generally, the airfoil section <NUM> provides an aerodynamic surface for guiding airflow to downstream portions of the engine in response to rotation of the airfoil <NUM>. The root section <NUM> of the airfoil <NUM> is mounted to a rotor, such as the fan hub <NUM> of <FIG> and <FIG>.

Referring to <FIG>, span positions of the airfoil section <NUM> are schematically illustrated from <NUM>% to <NUM>% span in <NUM>% increments to define a plurality of sections <NUM>. Each section <NUM> at a given span position is provided by a conical cut that corresponds to the shape of segments of the bypass flowpath B or the core flow path C (<FIG>), as shown by the large dashed lines. The <NUM>% span position corresponds to the radially innermost location of the airfoil section <NUM> that meets the root section <NUM>. A <NUM>% span position corresponds to a section of the airfoil <NUM> at the tip <NUM>.

Referring back to <FIG>, the airfoil <NUM> includes a composite core <NUM>, a first skin <NUM> and a second skin <NUM>. The core <NUM> defines the root section <NUM> and at least a portion of the airfoil section <NUM>, with the core <NUM> terminating at an edge or boundary <NUM> (shown in dashed lines in <FIG>). The core <NUM> can extend along a mean camber line <NUM> (shown in dashed lines in <FIG>) that bisects a thickness of the airfoil <NUM> for at least some span positions of the airfoil section <NUM>.

The core <NUM> can be dimensioned to provide increased rigidity to localized portions of the root and airfoil sections <NUM>, <NUM>. In the illustrated example of <FIG>, the core <NUM> defines a portion of the airfoil section <NUM> such that the boundary <NUM> of the core <NUM> is spaced apart from the tip <NUM>. In the illustrated example of <FIG>, the airfoil section <NUM> is free of the core <NUM> for at least a majority of span positions of the airfoil section <NUM>. In some examples, the boundary <NUM> of the core <NUM> terminates in the radial direction R along the airfoil section <NUM> prior to about <NUM>% span, or more narrowly between about <NUM>% span and about <NUM>% span. For the purposes of this disclosure, the term "about" means ±<NUM>% of the respective quantity unless otherwise stated. In other examples, the core <NUM> defines a portion of the tip <NUM>, as illustrated by boundary <NUM>' (<FIG>).

In some examples, core <NUM>' is skewed toward the leading edge L/E of the airfoil <NUM>, as illustrated by boundary <NUM>" of <FIG>. A reference plane <NUM> is established along the airfoil section <NUM> between <NUM>% and <NUM>% span. The reference plane <NUM> is equidistant between the leading and trailing edges L/E, T/E for each respective span position. Core <NUM>'' is situated such that a center of mass CM" of the core <NUM>" is defined along the mean camber line <NUM> (<FIG>) at a location between the leading edge L/E and the reference plane <NUM>, whereas a center of mass CM of the core <NUM> may be situated along the reference plane <NUM>, for example. Situating the core <NUM>" relatively closer to the leading edge L/E can provide improved localized impact resistance, while skins <NUM>, <NUM> can provide relatively greater in-plane strength and stiffness to regions adjacent to the trailing edge T/E, for example. Terminating the core <NUM> prior to the tip <NUM> can provide the airfoil <NUM> with better in-plane strength and stiffness in regions where the core <NUM> is not present, but may have less delamination resistance.

The skins <NUM>, <NUM> extend along the core <NUM> to define external surfaces of the airfoil <NUM>. In the illustrated example of <FIG>, the skins <NUM>, <NUM> extend along opposed sides of the core <NUM> to define the root section <NUM>. The root section <NUM> defines a dovetail geometry that interfaces with a respective slot <NUM> of a rotor (shown in dashed lines in <FIG>). The skins <NUM>, <NUM> extend along the dovetail geometry to space apart the core <NUM> from walls of the respective slot <NUM>. The skins <NUM>, <NUM> can be relatively more compliant than the core <NUM>, and can provide improved absorption of impacts and load transfer between the airfoil <NUM> and the fan hub <NUM> (<FIG>) during engine operation, which can improve durability of the airfoil <NUM>. In alternative examples (outside the wording of the claims), the root section <NUM> is free of the skins <NUM>, <NUM> such that the core <NUM> abuts the walls of the respective slot <NUM>.

The skins <NUM>, <NUM> extend along opposed sides of the core <NUM> for at least some span positions of the airfoil section <NUM> and join together to define the tip <NUM>. The boundary <NUM> of the core <NUM> can be contoured or tapered to provide a relatively smooth interface between the skins <NUM>, <NUM>. The skins <NUM>, <NUM> are joined together along the leading and trailing edges L/E, T/E and define the pressure and suction sides P, S. In some examples, the core <NUM> defines at least a portion of the leading and/or trailing edges L/E, T/E of the airfoil <NUM>. In other examples, the core <NUM> is spaced apart from the leading and/or trailing edges L/E, T/E.

The core <NUM> and skins <NUM>, <NUM> can be made of various composite materials to define the airfoil <NUM>. The core <NUM> and skins <NUM>, <NUM> can be constructed from fibers arranged in various orientations and in one or more layers based on structural requirements. In some examples, the core <NUM> and/or skins <NUM>, <NUM> include carbon fibers. The core <NUM> and/or skins <NUM>, <NUM> can be constructed from other materials, including fiberglass, an aramid such as Kevlar®, a ceramic such as Nextel™, and a polyethylene such as Spectra®.

In the illustrated example of <FIG>, the core <NUM> includes and is constructed from a three-dimensional network of fibers <NUM> (<FIG>), which can be woven or interlaced. The three-dimensional network of fibers <NUM> can be formed from a dry fiber preform, for example. Each of the skins <NUM>, <NUM> is a composite skin that includes and is constructed from a two-dimensional network of fibers <NUM> (<FIG>), which can be woven or interlaced.

The two-dimensional network of fibers <NUM> can be formed from a pre-impregnated ("prepreg") fabric or a pre-impregnated tape, for example. Pre-impregnating the fibers with resin can provide relatively greater strength and toughness to the composite article. Matrix (resin) materials used for prepreg can have greater toughness compared to matrix materials used with resin infusion methods, such that a part made from prepreg is relatively stronger, as measured by delamination resistance.

In other examples, the skins <NUM>, <NUM> include a plurality of relatively thin uni-tape plies having a plurality of fibers oriented in the same direction. In some examples, the core <NUM> and/or skins <NUM>, <NUM> can include a network of biaxial braids <NUM> (<FIG>). Other configurations for the core <NUM> and/or skins <NUM>, <NUM> can include a network of tri-axial braids <NUM> (<FIG>), for example. In other examples, the skins <NUM>, <NUM> include a network of stitched or non-crimped fabrics. The example fiber constructions are known in the art, but the incorporation of the fiber constructions into airfoil <NUM> to define the core <NUM> and skins <NUM>, <NUM> utilizing the teachings herein are not known.

The core <NUM> and/or skins <NUM>, <NUM> can include different fiber types in the fiber directions to tailor the strength and stiffness of the core <NUM> and/or skins <NUM>, <NUM>. For example, high modulus carbon fibers may be used in conjunction with low modulus carbon fibers. In yet another example, fiberglass or aramid fibers may be used in combination with carbon fibers.

Incorporating a three-dimensional network of fibers into the core <NUM>, including the three-dimensional network of woven or interlaced fibers <NUM> (<FIG>), provides reinforcement in localized portions of the airfoil <NUM> that may be more susceptible to degradation due to impact from foreign objects. Incorporating a two-dimensional network of fibers into the skins <NUM>, <NUM>, including the two-dimensional network of fibers <NUM> (<FIG>), can provide relatively greater in-plane strength and stiffness to other portions of the airfoil <NUM>.

Various techniques can be utilized to form a composite article for a gas turbine engine such as the airfoil <NUM>. <FIG> illustrates an example process in a flow chart <NUM> for forming a composite article or airfoil, such as airfoil <NUM>. Reference is made to airfoil <NUM> for illustrative purposes. The techniques disclosed herein can improve structural properties of the airfoil <NUM>, which can improve durability, and can reduce manufacturing cost.

At step <NUM>, a core <NUM> is fabricated or otherwise prepared. The core <NUM> can be fabricated from a dry fiber preform to form or otherwise define a three-dimensional network of woven fibers. The core <NUM> may be fabricated on a loom using a weaving process, for example. A tackifier can be applied to the preform to prepare the core <NUM>. The core <NUM> can be dimensioned according to expected stresses or impacts that may be observed during use of the composite article.

At step <NUM>, first and second skins <NUM>, <NUM> are fabricated or otherwise prepared. The skins <NUM>, <NUM> can be prepared from a pre-impregnated fabric or pre-impregnated tape to form or otherwise define a two-dimensional network of fibers.

The core <NUM> and skins <NUM>, <NUM> can be arranged to establish a layup. The layup can be formed on a tool. Steps <NUM> and/or <NUM> can be performed by an automated fiber placement (AFP) process, or the steps can be performed manually by hand placement of a three-dimensional woven core and hand placing of prepreg plies to form the skins. AFP is generally known, and includes placement of narrow strips of unidirectional material or "tows" to build up the composite layers that constitute the article according to a predefined geometry. Forming the three-dimensional woven core <NUM> using the techniques disclosed herein can eliminate the need to form relatively small plies or layers that may otherwise be needed to fill a volume of the airfoil to define the airfoil geometry but that may not be practical to use in an AFP process.

At step <NUM>, the skins <NUM>, <NUM> are assembled or otherwise arranged relative to the core <NUM> in a mold. Surfaces of the mold can be dimensioned according to an external profile or contour of the airfoil <NUM>. In some examples, step <NUM> includes partially curing the core <NUM> at step <NUM> prior to step <NUM> and prior to positioning the core <NUM> in the mold, which may be referred to as a "b-staged" core. Step <NUM> can include infiltrating the core <NUM> with a resin that is chemically compatible with a resin used for subsequent injection. Once infiltration occurs, the core <NUM> is partially cured or "b-staged". As known, prepreg is already b-staged.

Steps <NUM>, <NUM> and/or <NUM> can be performed such that the airfoil section <NUM> is free of the core <NUM> for at least a majority of span positions of the airfoil section <NUM>, including the core <NUM> terminating at any of the span positions disclosed herein.

At step <NUM>, a composite airfoil such as airfoil <NUM> is formed. Step <NUM> can include forming the core <NUM> and the skins <NUM>, <NUM> together with resin in the mold to define the airfoil <NUM>, and such that the core <NUM> defines the root section <NUM> of the airfoil <NUM> and the skins <NUM>, <NUM> extend at least partially along the core <NUM> to define at least a portion of the root section <NUM> and the airfoil section <NUM> of the airfoil <NUM>. Resin materials can include a thermoset epoxy, for example, and can infuse the dry fiber preform.

The process <NUM>, including step <NUM>, can be performed using a closed-molding process. For example, step <NUM> can be performed using a resin transfer molding (RTM) process or a resin pressure molding (RPM) process to define the airfoil <NUM> including forming the core <NUM> and skins <NUM>, <NUM> together to define the root and airfoil sections <NUM>, <NUM>.

Resin transfer molding (RTM) is generally known for manufacturing composite articles. RTM is a closed-molding process that typically includes fabricating a fiber preform by laying up plies of fiber sheets in a stack, placing the fiber preform in a closed mold, and then saturating the fiber preform with a liquid thermoset resin. The resin is typically mixed with a catalyst or hardener prior to being injected into the closed mold, or can be previously mixed together in a one-part resin system. One-part resin systems already have the catalyst mixed with the resin. The article is heated in the mold to a desired temperature to cure the article. The mold can be heated using a liquid heating system, for example. In some examples, the mold is heated by direct contact with heated platens such as in a compression press or free-standing in an oven. A variation of RTM is vacuum-assisted resin transfer molding (VARTM). In a VARTM process, a vacuum is used to draw the resin into the mold. The RTM process generally results in a part with a slightly lower volume percentage of fiber compared to a part made from prepreg and processed in an autoclave.

Resin pressure molding (RPM) is generally known for manufacturing composite articles. RPM can be considered a variation of an RTM process. RPM is a closed-molding process which includes delivering a liquid resin into a closed mold in which some, or all, of the fiber reinforcement has been pre-impregnated with a resin. Thereafter and similar to RTM, a combination of elevated heat and hydrostatic resin pressure are applied to the mold to cure the article.

Step <NUM> can include curing the core <NUM> and skins <NUM>, <NUM> in the resin at step <NUM>. Step <NUM> can occur subsequent to injecting or otherwise delivering the resin into the mold. Step <NUM> can include heating the mold to a predetermined temperature for a set period of time to at least partially or fully cure the core <NUM> and skins <NUM>, <NUM>. One would understand how to determine the temperature and time period to cure the core <NUM> and skins <NUM>, <NUM> utilizing the teachings herein. Step <NUM> can include applying resin pressure concurrently with heat to the core <NUM>, skins <NUM>, <NUM> and resin at step <NUM>, such as during an RTM or RPM process. The RPM process can include utilization of a b-staged core <NUM> and prepreg skins <NUM>, <NUM>, or a dry fiber core <NUM> and prepreg skins <NUM>, <NUM>, for example. Forming the airfoil <NUM> with an RPM process can close any gaps in the composite article and can yield relatively better dimensional control of the cured article. This technique can eliminate separate injection and curing steps for the core <NUM>.

Pre-impregnated fabric or tape can provide the ability to use resin systems with a higher toughness and impact resistance as compared to resin that may be used in a transfer molding (RTM) process to infuse the dry fiber tape. The dry fiber preform used to form the core <NUM> is infused with resin, but the relatively lower toughness resin that may used in the RTM process is adequate since the three-dimensional network of fibers used to form the core <NUM> can provide relatively greater toughness and impact resistance. In other examples, the skins <NUM>, <NUM> including a two-dimensional network of fibers is formed from a dry fiber tape. A binder material can be added to the dry fiber tape to hold the layers of the fiber tape together and promote adhesion.

One or more finishing procedures can be performed on the cured composite article defining the airfoil <NUM> at step <NUM>. Example finishing procedures can include one or more grinding operations to remove excess material at parting lines caused by the molding process, or final dimensioning of the airfoil <NUM> geometry.

In alternative examples, the core <NUM> is staged on a tool. The core <NUM> is integrated with the skins <NUM>, <NUM>. The core <NUM> and skins <NUM>, <NUM> are then cured together in an autoclave.

Claim 1:
An airfoil (<NUM>) for a gas turbine engine (<NUM>) comprising:
an airfoil section (<NUM>) extending between a leading edge (L/E) and a trailing edge (T/E) in a chordwise direction and extending between a tip and a root section (<NUM>,<NUM>) in a spanwise direction (R), the airfoil section (<NUM>) defining a suction side (S) and a pressure side (P) separated in a thickness direction (T), wherein a composite core (<NUM>;<NUM>") defines the root section (<NUM>) and a portion of the airfoil section (<NUM>) such that the composite core (<NUM>;<NUM>") is spaced apart from the tip (<NUM>), the composite core (<NUM>;<NUM>") includes a three-dimensional network of woven fibers (<NUM>), first and second skins (<NUM>, <NUM>) extend along opposed sides of the composite core (<NUM>;<NUM>") for at least some span positions of the airfoil section (<NUM>) and join together to define the tip (<NUM>), characterised in that:
each of the first and second skins (<NUM>,<NUM>) is a composite skin (<NUM>,<NUM>) including a two-dimensional network of fibers (<NUM>), the fibers of the two-dimensional network of fibers (<NUM>) are woven or interlaced, the root section (<NUM>) defines a dovetail geometry that is configured to interface with a slot (<NUM>), and the first and second skins (<NUM>,<NUM>) extend along the dovetail geometry and are configured to space apart the composite core (<NUM>;<NUM>") from walls of the slot (<NUM>).