Patent Publication Number: US-2016230774-A1

Title: Fan blade assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of and incorporates by reference herein the disclosure of U.S. Ser. No. 61/883,750 filed Sep. 27, 2013. 
    
    
     TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure is generally related to rotating assemblies for turbomachinery and, more specifically, to a fan blade assembly. 
     BACKGROUND OF THE DISCLOSURE 
     In a turbofan engine, lighter components generally lead to more efficient performance. If less energy is expended to move internal engine parts, more energy is available for useful work. At the same time, the components themselves must be strong enough to withstand operational forces, and types of failure typical for the operating environment of the engine. Safety considerations and regulations based on the frequency and/or severity of possible failure will often dictate that the engine components also be able to withstand other atypical, yet foreseeable events. Because stronger and lighter components are often more expensive, a balance must be struck between efficiency, safety, and cost. 
     Few locations in an aircraft are more representative of efforts to optimize the balance between efficiency, safety, and cost than the engine. While lighter materials are preferable to improve efficiency, the high risk of severe consequences from engine damage will require that the engine be made of components having additional margins of safety. Combining parts having both high strength and low density greatly restricts material choices and increases costs. Not infrequently, processing these strong and light materials such as titanium or composites is also complex and expensive. 
     Being designed to pull vast quantities of air through the bypass section to generate thrust, blades in the fan section of the engine are the first line of defense for the engine and are highly susceptible to both small and large scale damage from objects pulled in with the surrounding air, including bird impact damage. 
     Small scale blade damage causes performance deterioration and increases the number of potential crack initiation sites, while large scale damage includes blade deformation and failure. Small impacts can also lead to large scale damage by serving as crack initiation sites. Larger impacts, such as ingestion of birds can cause one or more blades to deform or break in a single event. Regulations are in place to limit the frequency and severity of single event failures because of the increased risk of emergency landings and catastrophic failure. 
     Blades made entirely from high-strength materials, such as titanium or titanium alloys to name just two non-limiting examples, have been proven to offer sufficient hardness to resist erosion and foreign object damage. But titanium alloys are often expensive to purchase and manipulate into a finished blade. And while titanium has a relatively low density compared to a number of metals, the weight of titanium fan blades are significant contributors to overall engine weight. Fiber composites offer significant weight savings relative to titanium and its alloys, but are far more expensive and do not offer the same resiliency. 
     One technique of reducing the weight of a blade is to use a lower-density metallic material for the airfoil body. As described earlier, composite blades are extremely light, but are far more complex and expensive to produce relative to titanium blades. Small composite blades do not generally achieve sufficient weight savings to merit the additional complexity and cost. 
     Forming the blade from a lightweight metallic material can reduce cost and weight over a titanium blade. But without additional support or reinforcement, airfoils made solely from most lightweight metals or alloys do not offer sufficient strength and longevity for long-term use. 
     Multi-material assembled fan blades, consisting of, but not limited to, a sheath and a blade body made of dissimilar conductive materials, such as metals and/or composites, create a galvanic potential. Currently, a non-conductive adhesive is used to bond the sheath to the blade. The non-conductive adhesive therefore provides an insulative layer that prevents the flow of electrons in the potential galvanic current. This adhesive can have gaps in coverage allowing electrons to flow between the two dissimilar materials, which can potentially lead to corrosion. 
     Various designs for providing a sheath for use on a fan blade have been proposed, but improvements are still needed in the art. 
     SUMMARY OF THE DISCLOSURE 
     In one embodiment, a fan blade assembly is disclosed, comprising: a conductive airfoil including a sheath receiving surface, the sheath receiving surface coated with a nonconductive material; a conductive sheath; and an adhesive disposed on at least a portion of the nonconductive material to bond the conductive sheath to the conductive airfoil at the sheath receiving surface. 
     In another embodiment, a gas turbine engine is disclosed, comprising in serial flow communication: a fan section including a fan blade assembly, the fan blade assembly comprising: a conductive airfoil including a sheath receiving surface, the sheath receiving surface coated with a nonconductive material; a conductive sheath; and an adhesive disposed on at least a portion of the nonconductive material to bond the conductive sheath to the conductive airfoil at the sheath receiving surface; a compressor section; a combustor section; and a turbine section. 
     Other embodiments are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments and other features, advantages and disclosures contained herein, and the manner of attaining them, will become apparent and the present disclosure will be better understood by reference to the following description of various exemplary embodiments of the present disclosure taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematic cross-sectional view of a gas turbine engine. 
         FIG. 2  is a schematic perspective view of a fan blade assembly in an embodiment. 
         FIG. 3A  is a schematic cross-sectional view of the fan blade assembly of  FIG. 2  in an embodiment. 
         FIG. 3B  is a schematic cross-sectional view of the fan blade assembly of  FIG. 2  in an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to certain embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated device, and further applications of the principles of the invention as illustrated therein are herein contemplated as would normally occur to one skilled in the art to which the invention relates. 
       FIG. 1  illustrates a gas turbine engine  10  of a type normally provided for use in a subsonic flight, generally comprising in serial flow communication a fan section  12  through which ambient air is propelled, a compressor section  14  for pressurizing a portion of the air (the gas path air), a combustor  16  in which the compressed air is mixed with fuel and ignited for generating a stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. Although a gas turbine engine is discussed herein as an illustrative example, the presently disclosed embodiments are applicable to sheathed blades in other applications, such as sheaths for helicopter rotors, to name just one non-limiting example. 
     A side view of exemplary fan blade assembly  30  is shown in  FIG. 2 , which includes cross section  3 - 3 . As seen in  FIG. 2 , three parts are joined to form fan blade assembly  30 : airfoil  32 , sheath  34 , and root  36 . Blade  30  has leading edge  38 , trailing edge  40 , and suction surface  42 . Fan blade assembly  30  also includes platform  46 , tip edge  48 , sheath head section  50 , sheath flank  52 A, and forward airfoil edge  54 . Pressure surface  44  and sheath flank  52 B are at the rear of blade  30  (not visible; shown in  FIGS. 3A and 3B ). It will be appreciated that platform  46  may be formed integrally or non-integrally to the remainder of the airfoil  32 . 
     Leading edge  38  and trailing edge  40  extend generally spanwise in a curved manner from platform  46  to tip edge  48 . Air flows chordwise from leading edge  38  over suction surface  42  and pressure surface  44 , meeting at trailing edge  40 . Root  36  links fan blade assembly  30  at platform  46  to a disk or rotor (not shown) in fan section  12 . Here root  36  is shown as a “dovetail” root; however, such an arrangement is not required in the present embodiments. Alternatively, fan blade assembly  30  can have a different configuration of root  36 , or root  36  can be incorporated with the disk in what is known in the art as an integral rotor blade configuration. 
     Sheath  34  covers a portion of airfoil  32  proximal forward airfoil edge  54 , extending spanwise over at least a part of the length of leading edge  38  between platform  46  and tip edge  48 . Forward airfoil edge  54  is represented by a broken line extending spanwise along sheath  34 . It has been found that adding protective sheath  34  over forward airfoil edge  54  of lightweight airfoil  32  can prevent a significant amount of such damage and slow degradation of fan blade assembly  30 . 
       FIG. 3A  depicts a partial cross-section of fan blade assembly  30  in an embodiment, taken across line  3 - 3  of  FIG. 2 . Fan blade assembly  30  includes airfoil  32 , sheath  34 , leading edge  38 , suction surface  42 , pressure surface  44 , sheath head section  50 , sheath flanks  52 A and  52 B, airfoil forward edge  54 , and sheath receiving surface  58  on the airfoil  32  and a corresponding airfoil contact surface  60  on the sheath  34 . 
     Sheath receiving surface  58  is located on airfoil  32  proximate leading edge  38  and includes a portion of suction surface  42  and pressure surface  44 . Flanks  52 A and  52 B extend back from head section  50  over portions of suction surface  42  and pressure surface  44  proximate leading edge  38 . A nonconductive adhesive covers the sheath receiving surface  58 /airfoil contact surface  60  to bond the sheath  34  to the airfoil  32 . 
       FIG. 3B  depicts a partial cross-section of fan blade assembly  30  taken across line  3 - 3  of  FIG. 2 . It is at the sheath receiving surface  58 /airfoil contact surface  60  that the possibility of a galvanic potential arises. If there is a gap in coverage of the nonconductive adhesive that covers the sheath receiving surface  58 /airfoil contact surface  60 , then a galvanic potential will be created between the dissimilar materials of the airfoil  32  and sheath  34 . Therefore, as shown in  FIG. 3B , at least the sheath receiving surface  58  of airfoil  30  is coated in an embodiment with a nonconductive material  70 , such as a ceramic or other isolating material, prior to bonding the sheath  34  to the airfoil  32 . Adhesive may still be used to bond the sheath  34  to the airfoil  32  during the fan blade assembly  30  assembly process, but the adhesive would not need to be relied on as the sole insulator between the dissimilar conductive materials of the sheath  34  and the airfoil  32 . In some embodiments, a conductive adhesive could be used to bond the sheath  34  to the airfoil  32  because the nonconductive coating would ensure that no electrical current is passed between the dissimilar materials. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.