Abstract:
A leading edge structure for use in an aerospace vehicle includes a body having a flowpath surface which defines a leading edge adapted to face an air flow during operation; and a metallic icephobic plating comprising nickel applied to at least a portion of the flowpath surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to aerospace vehicle structures and more particularly to materials and designs for improving ice shedding characteristics from such structures. 
         [0002]    All aircraft include various “leading edge structures”, i.e. exposed surfaces that face the direction of flight. These surfaces include, for example, parts of the fuselage, wings, control surfaces, and powerplants. 
         [0003]    One common type of aircraft powerplant is a turbofan engine, which includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a flow of propulsive gas. A low pressure turbine driven by the core exhaust gases drives a fan through a shaft to generate a propulsive bypass flow. The low pressure turbine also drives a low pressure compressor or “booster” which supercharges the inlet flow to the high pressure compressor. 
         [0004]    Certain flight conditions allow for ice build up on the leading edge structures, and, in particular, the fan and booster flowpath areas of the engine. These areas include the blades, spinner cone, and static vane and fairing leading edges. The FAA requires certification testing at these flight points to demonstrate the ability to maintain engine thrust once the ice sheds from the various components and ingests into the engine. 
         [0005]    One particular leading edge structure of interest is the engine&#39;s fan splitter. The splitter is an annular ring with an airfoil leading edge that is positioned immediately aft of the fan blades. Its function is to separate the airflow for combustion (via the booster) from the bypass airflow. It is desired for the splitter and other leading edge structures to have mechanical, chemical, and thermal properties such that ice build up and shed volume is minimized during an icing event. This in turn minimizes risk of compressor stall and compressor mechanical damage from the ingested ice. 
         [0006]    Prior art turbofan engines have splitters made from titanium, which is known to provide favorable ice shed properties. The downside of titanium is the expense and weight when compared to conventionally treated aluminum. However, aluminum is believed to behave poorly in an aircraft icing environment. Examples of conventional treatments for aluminum include but are not limited to chemical conversion coatings and anodizing. 
         [0007]    Leading edge structures can also be protected with known coatings that are referred to as “icephobic” or “anti-ice” coatings, for example, polyurethane paint or other organic coatings. These coatings have the effect of lowering adhesion forces between ice accretions and the protected component. While these coatings can improve ice shedding characteristics, their erosion resistance may be not adequate to protect leading edge structures from the scrubbing effect of airflows with entrained abrasive particles which are encountered in flight. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    These and other shortcomings of the prior art are addressed by the present invention, which provides components having icephobic plating that reduces and/or modifies ice adhesion forces to promote ice release and reduce shedding of large ice pieces. 
         [0009]    According to one aspect, the invention provides a leading edge structure for use in an aerospace vehicle, including: (a) a body having a flowpath surface which defines a leading edge adapted to face an air flow during operation; and (b) a metallic icephobic plating comprising nickel applied to at least a portion of the flowpath surface. 
         [0010]    According to another aspect of the invention, a splitter for a turbofan engine includes: (a) an annular body having a flowpath surface which defines a leading edge adapted to face an air flow during operation; and (b) a metallic icephobic plating comprising nickel applied to a least a portion of the flowpath surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0012]      FIG. 1  is a perspective view of an aircraft powered by a pair of high-bypass turbofan engines, incorporating icing-resistant components constructed according to an aspect of the present invention; 
           [0013]      FIG. 2  is a schematic half cross-sectional view of an engine shown in  FIG. 1 ; 
           [0014]      FIG. 3  is a half -sectional view of a splitter shown in  FIG. 2 ; 
           [0015]      FIG. 4  is a view taken from forward looking aft at an alternative splitter; 
           [0016]      FIG. 5  is a view taken along lines  5 - 5  of  FIG. 4 ; 
           [0017]      FIG. 6  is a view taken from forward looking aft at another alternative splitter; and 
           [0018]      FIG. 7  is a view taken along lines  7 - 7  of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts a known type of commercial aircraft  10  which includes a generally tubular fuselage  12 , wings  14  carrying turbofan engines  16  mounted in nacelles  18 , and an empennage comprising horizontal and vertical stabilizers  20  and  22 . Each of these components includes one or more exposed surfaces having a curved or airfoil-like cross-section that faces the direction of flight (in other words, an aerodynamic leading edge). These components are referred to herein as “leading edge structures”. While the present invention will be described further in the context of a gas turbine engine, it will be understood that the principles contained therein may be applied to any type of leading edge structure. 
         [0020]    As shown in  FIG. 2 , the engine  16  has a longitudinal axis “A” and includes conventional components including a fan  24 , a low pressure compressor or “booster”  26  and a low pressure turbine (“LPT”)  28 , collectively referred to as a “low pressure system”, and a high pressure compressor (“HPC”)  30 , a combustor  32 , and a high pressure turbine (“HPT”)  34 , collectively referred to as a “gas generator” or “core”, various components of the nacelle  18 , and stationary structures of the engine  16 , including a core nacelle  36 , cooperate to define a core flowpath marked with an arrow “F”, and a bypass duct marked with an arrow “B”. 
         [0021]    A stationary annular splitter  38  is positioned at the forward end of the core nacelle  36 , between the bypass duct B and the core flowpath F. The splitter  38  may be a single continuous ring, or it may be built up from arcuate segments. While a variety of materials such as metal alloys and composites may be used, the splitter  38  is preferably constructed from an aluminum alloy to reduce weight and expense. For example, an aluminum-alloy splitter may have a lower weight and cost than a comparable titanium splitter. Various aluminum alloys and tempers are known for use in aerospace applications, and the particular alloy used for the splitter  38  is not critical so long as it has acceptable mechanical properties for the particular application (e.g. strength, fatigue resistance, corrosion resistance, etc.) One example of an alloy known to be suitable for constructing the splitter  38  is AL7075. 
         [0022]    As shown in  FIG. 3 , the flowpath surface  40  of the splitter  38  includes a radially-outward-facing portion  41  and a radially-inward-facing portion  43 . The two portions are demarcated by an aerodynamic leading edge  39 . The splitter  38  is an example of a leading edge structure as described above. At least a portion of the flowpath surface  40  has a metallic plating  42  applied thereto which has “icephobic” properties, that is, very low adhesion forces are generated between the plating  42  and any ice that forms thereon, as compared to the base material of the splitter  38 . The plating  42  is depicted with an exaggerated heavy line solely for the purposes of illustration. In the illustrated example, the splitter  38  has a chord length “C” in the axial direction of about 8.9 cm (3.5 in.). The exact dimensions are not critical and will vary with a particular application. The length of the flowpath surface  40  which is plated is denoted “L” in  FIG. 3 . The splitter  38  may be completely plated, in which case the length L would be 100% of the chord length C. However, in operation, ice typically does not cover the splitter  38  to this extent. Accordingly, the plating  42  may be restricted to any shorter length, which need not correspond to the extent of expected ice coverage. A practical example of an expected shorter length L is about 2% to about 20% of the chord length C. The radially inward-facing portion  43  of the flowpath surface  40  may be completely covered regardless of the extent of coverage on the radially-outward facing portion  41 . 
         [0023]    An example of a suitable icephobic metallic plating is nickel or a nickel alloy. One example of a known suitable nickel plating process is electroless nickel plating as described in AMS2404. In general, the smoother the surface finish of the plating  42 , the lower the ice adhesion forces are expected to be. Porosity of the plating  42  should be minimized to avoid corrosion and ice adhesion. Addition of phosphorous and/or boron in combined amounts of up to about 25% by weight may be helpful in reducing porosity. Use of a “high phosphorous” plating containing about 10% to about 13% percent by weight is known to minimize porosity of a nickel plating applied to aluminum. 
         [0024]    In this example, the finished plating has a thickness of about 0.04 mm (0.0015 in.) to about 0.15 mm (0.0060 in.). The thickness is not critical for icing reduction purposes, so long as the underlying substrate is not exposed (i.e. the plating is continuous) Component testing has demonstrated that nickel plating of this thickness provides erosion resistance comparable to a titanium part, and is suitable for use in a gas turbine engine. The thickness is only important for achieving desired durability and expected service life for the component. 
         [0025]    In operation, the engine  10  will be exposed to icing conditions, namely the presence of moisture in temperatures near the freezing point of water. Ice will naturally tend to form on the leading edge structures including the splitter  38 . As the ice mass builds up, it protrudes into the air flow and increasing aerodynamic (drag) forces act on it, eventually causing portions of it to shed from the splitter  38 . With the plating  42  described above, adhesion forces between the ice and the splitter  38  are substantially reduced as compared to conventionally treated aluminum and titanium. The result is that pieces of the ice break off and shed downstream when they are a smaller size than would otherwise be the case. This avoids excessive cooling and foreign object damage in the high pressure compressor  30 . 
         [0026]    In addition to lowering overall ice adhesion forces, it is also possible to improve ice shedding properties by varying the ice adhesion forces over the surface of the component. Specifically, by providing different sections of the splitter  38  or other leading edge structure with varying adhesion properties, stresses are generated within the ice itself as aerodynamic forces act on it. This causes the ice to break up into smaller pieces and in a more predictable fashion that if it were to shed naturally. 
         [0027]    For example,  FIGS. 4 and 5  show an alternative splitter  138  similar in construction to the splitter  38  described above. The flowpath surface  140  is “sectored” into separate surface areas in the circumferential direction. Areas  142 A have a metallic icephobic plating as described above, while the alternate areas  142 B have a coating or surface treatment, or are otherwise prepared so as to present a different ice-adhesion property (i.e. substantially greater or lesser adhesion than the plated areas  142 A).The boundaries between the different areas may be straight or curved, and may have varying alignments. Examples of materials for the alternate areas  142 B would include conventionally treated aluminum, a different metallic plating, organic coatings (e.g. polyurethane or epoxy paints), polytetrafluoroethylene (PTFE), etc. For practical purposes a suitable erosion-protective material should be used. The width “W” of the areas  142 A and  142 B in the circumferential direction may be selected to cause ice to breakup into relatively small pieces. 
         [0028]      FIGS. 6 and 7  show another alternative splitter  238 . Like the splitter  138 , the flowpath surface  240  is divided into areas  242 A with metallic icephobic plating as described above, and alternate areas  242 B with different ice adhesion properties. The boundaries between the different areas may be straight or curved, and may have varying alignments. In this example, the flowpath surface  240  is divided in both the circumferential and axial directions. As this example shows, any suitable combination of circumferential, axial, and/or radial sectoring may be used to reduce ice shed size. 
         [0029]    The foregoing has described materials and designs for ice shed reduction in aerospace structures. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only.