Abstract:
A method of forming a vibration stable airfoil by forming a composite blade having a metallic sheath thereon. The sheath has a head section extending out from the blade by a sufficient distance to permit deformation of the head section. The airfoil is tested to determine the vibrational stability thereof; and the head section is re-cambering to adjust the vibrational stability to a desired level.

Description:
BACKGROUND 
       [0001]    Composite materials offer potential design improvements in gas turbine engines. For example, in recent years composite materials have been replacing metals in gas turbine engine fan blades because of their high strength and low weight. Most gas turbine engine metal fan blades have been made from titanium. The ductility of titanium fan blades enables the fan to ingest a bird and remain operable or be safely shut down. The same requirements are present for composite fan blades. 
         [0002]    A composite airfoil for a turbine engine fan blade can have a sandwich construction with a carbon fiber woven core at the center and two-dimensional filament reinforced plies or laminations on either side. To form the composite airfoil, individual two-dimensional plies are cut and stacked in a mold with the woven core. The mold is injected with a resin using a resin transfer molding process and cured. The plies vary in length and shape. The carbon fiber woven core is designed to accommodate ply drops so that multiple plies do not end at the same location. 
         [0003]    Previous composite blades have been configured to improve the impact strength of the composite airfoils so they can withstand bird strikes. During use, foreign objects ranging from large birds to hail may be entrained in the inlet of the gas turbine engine. Impact of large foreign objects can rupture or pierce the blades and cause secondary damage downstream of the blades. 
         [0004]    In order to prevent damage from the impact of foreign objects such as birds, a metallic sheath has been used to protect the leading edge of rotor blades and propellers made from composites. Materials such as titanium and nickel alloys are fitted on the leading edge of the element to be protected. Examples of sheaths used for covering and protecting a component leading edge of an airfoil component are disclosed in U.S. Pat. No. 5,881,972 and U.S. Pat. No. 5,908,285. In both patents, the sheaths are formed from metal that is electroformed on the airfoil component on a mandrel. The sheath and mandrel are separated and the sheath is mounted on the airfoil. 
         [0005]    In more recent years, sheaths have been bonded on a molded composite blade by forming the blade, usually in a resin transfer molding (RTM) process. Once the blade has been formed, an adhesive is placed on the leading edge and a leading edge sheath is placed against the adhesive, heat and pressure are applied and the adhesive cures to mount the leading edge as needed. While this process is costly, it is also effective in producing airfoils capable of withstanding impact by birds and other debris that might otherwise damage or destroy the airfoil. 
         [0006]    There are, of course, a number of factors that control the dynamic properties of rotating blades, such as speed of rotation, range of speeds, blade deflection, blade flexibility. Current composite blades are manufactured to accommodate these conditions. Due to the fact that blades are tapered, inboard regions tend to be torsionally stiff relative to the outboard regions. Thus, the outboard torsional mass and stiffness properties are of major importance. 
         [0007]    During production of gas turbine engines, often it is necessary to revise the design of components such as 1 st  stage fan blades and the like due to an unacceptable aero structural response known as flutter. This phenomenon occurs when the aerodynamic loading acting on an airfoil combines with the vibrational response of the airfoil to create an unstable condition with negative damping, often due to the outboard components of the blade. This instability can and often does lead to unacceptable vibrational stresses and ultimately to failure of the airfoil. 
         [0008]    Due to a relatively long lead time in manufacturing an airfoil, one common practice with metallic airfoils is to locally re-camber the leading edge, generally in the outer span portions of the airfoil, to reduce the aerodynamic incidence and increase the flutter margin. However, when resin transfer molded composite fan blades are manufactured, the ability to locally reshape the airfoil is severely limited by the long lead time required for tooling and due to the inability to plastically deform composite materials. Essentially a whole new blade has to be made from the redesigned mold. 
         [0009]    Composite fan blades are also subject to erosion and soft body impact. As a result, these blades are fitted with a sheath on the leading edge. Typical sheaths are made from titanium, titanium alloys, nickel and nickel alloys. The sheaths are conventionally attached to the composite fan blade by an adhesive followed by a heat cycle to cure the adhesive. It is normally at this point when flutter is found during testing. In some instances the sheath is saved or repaired for use again. 
       SUMMARY 
       [0010]    A molded composite blade with a leading edge sheath and method of making the same reduces flutter and other aerodynamic design faults in molded composite fan blades. The method includes the steps of molding the fan blade, attaching or otherwise including a metallic sheath with a plastically deformable head section that extends from the fan blade to leading edge of the blade. The fan blade is then placed in service or in a test apparatus to see the aerodynamic and operational performance. If the operation is not satisfactory, such as if flutter is observed, the head portion of the metallic sheath is re-cambered by plastic deformation to reduce vibration instability of the airfoil. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of a metal sheath used on a composite blade, with the original shape shown in solid and a re-cambered shape shown in dash lines. 
           [0012]      FIG. 2  is a cut away, partial view of a leading edge metal sheath attached to a composite blade with the original shape shown in solid and a re-cambered shape shown in dash lines. 
           [0013]      FIG. 3  is a cut away, partial view of a leading edge metal sheath attached to a composite blade showing the change in thickness of the metal sheath in dash lines. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Fan blades normally have sheaths to protect against erosion, ingestion of birds, hail and ice, tire fragments and other objects encountered during operation of the aircraft. It is essential that each blade have adequate outboard torsional mass and stiffness properties to essentially eliminate flutter and other vibrational phenomena that interfere with smooth operation of the aircraft. 
         [0015]    In the present invention, the metal sheath is used as more than a protective device because an additional portion has been included to allow adjustment of the shape of the metal sheath by an amount sufficient to reduce or eliminate unwanted vibrational responses during aerodynamic loading acting on the airfoil.  FIG. 1  illustrates leading edge metal sheath  17  for attachment to a airfoil, not shown in  FIG. 1 , that has been found to have unwanted vibration and flutter. Rather than remold a new air foil, head section  19  of sheath  17  is re-cambered, such as by plastically deforming the sheath material. Re-cambering includes plastic deformation with or without heat, simple bending, and also includes replacing the sheath with a new sheath having an effectively re-cambered shape. Re-cambering means bending, curving or arching the head section to achieve a different curve or aerodynamic shape. Since sheaths are made from metal, in order to have the hard surface that resists impact by birds, hail and other ingested objects, they are capable of being deformed under heat. Plastic deformation, for example, is accomplished by placing the airfoil, or at least sheath head section  19 , in a hot re-camber die and locally deforming head section  19  to a new shape  19   a  that is estimated to eliminate flutter and other unwanted vibrations. This re-cambering of head section  19  can be done once or several times, depending on the results of further tests of the rotational stability of the blade to which it is attached. 
         [0016]      FIG. 2  illustrates an enlarged view of the plastic deformation of head section  19  of sheath  17  of airfoil  20 , generally, and is mounted on composite  21 . During the original manufacture of composite  21 , sheath  17  may be attached to the cured composite  21  that forms airfoil  20 . Sheath  17  may also be formed from metal that is electroformed for use on airfoil  20 . Any method for placing sheath  17  on composite  21  is within the scope of this invention as long as sheath head section  19  is formed to extend out for a sufficient distance to permit reshaping of the head section to correct vibrational properties. 
         [0017]    Composite  21  may be formed by a variety of methods. It has been found that composite blades may be made by placing a woven core in a mold, adding filament plies to fill the mold, and resin transfer molding the blade. A method of fabrication a composite blade  21  is disclosed in a U.S. patent application titled Core Driven Ply Shape Composite Fan Blade and Method of Making, filed Nov. 30, 2009, having Ser. No. 12/627,629, which is incorporated herein by reference in its entirety. Composite blade  21  may also be formed my molding a woven core without filament plies, or by molding a sufficient quantity of filament plies without a core. It is also contemplated that the composite  21  may be formed by pre-impregnation prior to insertion into the mold, rather than using the resin transfer mold method. 
         [0018]    Head section  19  of sheath  17  has a longer length L, the distance from forward edge  21   a  of composite  21 , than conventional designs. This extra length allows for the local re-camber or bending of the airfoil by plastic deforming. By having only head section  19  extend out distance L allows for utilization of the metal in the outer span regions of blade where it can be deformed or re-cambered while minimizing the weight in the inboard regions of airfoil  20 . 
         [0019]    Typical composite airfoil leading edge sheaths are thin metallic covers. In some instances they may have a solid portion of less then ˜1″ to improve the designs robustness to ingestion of a foreign object such as a bird. In this invention, head section  19  includes a more substantial portion  19   a  of more than the conventional portion of about one inch (2.54 cm) in length. Dash line  19   d  illustrates how sheath  17  can be altered in configuration to change the aerodynamic loading and subsequent vibrational characteristics of airfoil  20 , in this case by bending head section  19   a  down to head section  19   d . Composite  21  is unaffected by this modification, thus eliminating the need for re-molding a new composite. 
         [0020]    In the event that the amount of re-camber that is required to eliminate vibrational concerns exceeds the limit of plastic deformation available for head section  19 , a new sheath could be fabricated and installed. 
         [0021]      FIG. 3  illustrates another means by which the re-cambering of head section  19  is accomplished by a reduction in thickness T 1  between the inside  19   i  of head section  19  on the leading edge  21   e  of composite  21  and an increase T 2  between the trailing edge  19   t  of head section  19  and the trailing edge  21   t  of composite  21 . This change in thickness is accomplished by the fabrication of a new sheath detail versus the plastic deformation of the existing sheath. This alternative, although more costly, could increase the range of potential flexibility in changing the aerodynamic shape. 
         [0022]    The present invention allows for a much shorter time for local aerodynamic modifications of a blade during engine development programs and greatly reduces the cost of producing vibrationally stable airfoils for aircraft. 
         [0023]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.