Patent Abstract:
An electrothermal deicing apparatus includes at least one heater conductor formed into a predetermined pattern for application to a structure. Each conductor of the pattern is coated with at least one layer of a material active to absorb electromagnetic energy. The pattern of the at least one coated heater conductor is operative to attenuate radar wave transmissions incident thereon to reduce the radar cross-section of the structure. A heater controller is coupled to the pattern of at least one coated heater conductor for controlling the deicing of the structure.

Full Description:
This utility application claims the benefit of the filing date of U.S. Provisional Application No. 60/667,982, filed Apr. 4, 2005. 

   RELATED PATENT APPLICATIONS 
   U.S. patent application Ser. No. 10/947,671, filed Sep. 23, 2004, entitled “Radar Absorbing Electrothermal Deicer” and assigned to the same assignee as the instant application, which related application being incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   Electrothermal heating has become an effective choice for airfoil and structure deicer heaters, especially when composite materials are used for the airfoils and/or other structures being deiced. An electrothermal heater may be used wherever icing conditions exist, including applications such as: airfoil leading edges of wings, tails, propellers, and helicopter rotor blades; engine inlets; struts; guide vanes; fairings; elevators; ships; towers; wind turbine blades; and the like, for example. In electro-thermal deicing systems, heat energy is typically applied to the surface of the airfoil or structure through a metallic heating element via electrical power supplied by aircraft or appropriate application generators. Typical heating elements are made from foil, wire, or metallic-coated fabrics. 
   Generally, the electro-thermal heater deicers may be implemented in a conductive pattern over or under the skin of the airfoil or other structure, or embedded in the composite material itself. The electrothermal deicer pattern, being conductive, has a tendency to give off a larger than desired cross-sectional radar image in response to radar illumination. This has become a particular problem when such deicer heater patterns are applied to military aircraft that may be illuminated by enemy radar systems. Accordingly, it is desired to keep the radar cross-section of an aircraft as small as possible. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, a heater conductor for electrothermal deicing of a structure is coated with at least one layer of dielectric material filled with magnetic particles. 
   In accordance with another aspect of the present invention, electrothermal deicing apparatus comprises: at least one heater conductor, each conductor coated with at least one layer of dielectric material filled with magnetic particles; and a heater controller coupled to the at least one heater conductor for controlling the heating thereof. 
   In accordance with yet another aspect of the present invention, electrothermal deicing apparatus comprises: at least one heater conductor formed into a predetermined pattern for application to a structure, each conductor of the pattern coated with at least one layer of a material active to absorb electromagnetic energy, the pattern of the at least one coated heater conductor is operative to attenuate radar wave transmissions incident thereon to reduce the radar cross-section of the structure; and a heater controller coupled to the pattern of at least one coated heater conductor for controlling the deicing of the structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a layered, isometric perspective illustration of an exemplary coated heater conductor. 
       FIG. 2  is a layered, isometric perspective illustration of an exemplary coated heater wire having a plurality of coating layers. 
       FIG. 3  is a layered, isometric perspective illustration of an alternate coated heater wire having a plurality of coated layers. 
       FIG. 4  is a layered, isometric perspective illustration of an exemplary coated heater conductor of rectangular cross-section. 
       FIG. 5  is an exemplary heater element pattern of coated heater conductors. 
       FIG. 6  is a cross-sectional schematic illustration of an exemplary electrothermal deicing apparatus utilizing a pattern of coated heater conductors. 
       FIG. 6A  is a cross-sectional schematic illustration of an alternate electrothermal deicing apparatus utilizing a pattern of coated heater conductors. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In order to alleviate the cross-sectional radar image issue of electro-thermal heater patterns for aircraft airfoils and/or other structures, the electromagnetic properties of the resistance heater conductors thereof may be modified with coating materials to provide a high magnetic and electrical energy loss which is designed to occur when an applied electromagnetic wave of energy, like radar illumination, for example, is applied at a desired frequency of utilization (Mhz to Ghz) and over a broadband range to maximize absorption of electromagnetic energy and thus, reduce the radar cross-section of the aircraft. The conductor and coating materials are designed primarily to act as an element of an electrical resistance heater for the preferred use of integration into composite non-metallic structures. In such structures, the electro-thermal heater element may be protected from physical damage and environmental corrosion by embedding it in a dielectric polymer within the composite material of the airfoil or structure. 
   An exemplary embodiment of a heater conductor  10  is shown in the layered, isometric perspective illustration of  FIG. 1 . Referring to  FIG. 1 , in the present embodiment, the heater conductor  10  comprises a wire  12  which may be made from any metal of desired electrical resistance with the preferred metals being ferromagnetic and made up of transition metals iron, cobalt, nickel, chromium and rare earth metals, for example. Preferably, the heater wire  12  may be a single strand 33 AWG wire of alloy Nickel 60%, Chromium 16%, Iron 24% with properties of 675 ohms per CMF at 20° C., for example. The heater wire  12  may be coated as shown with at least one coating layer  14  of dielectric material filled with magnetic particles, not previously known to the wire coating industry. 
   The filled coating layer  14  may comprise any dielectric insulating material such as elastomers, ceramics, or polymeric films like polyimide films, polyester films or amide imide films, for example. The magnetic filler particle, which may be carbonyl iron, iron silicide, ferrite or rare Earth magnetic particles and combinations thereof, for example, may be selected and processed to reduce particle size to less than 10 microns to allow the coating process through wire die applicators and still maintain effective electromagnetic/radar absorbing performance. Optionally, the wire  12  may be coated with one or more unfilled insulating layers  16  prior to being coated with the magnetic particle filled layers  14 . In the exemplary embodiment shown in  FIG. 1 , an initial coating of an unfilled polyester layer  16  is applied to the wire  12 , then one or more coatings of a magnetic particle filled amide imide layers  14  are applied over the unfilled layer  16 . 
   The coating layers  14  and  16  may be applied to the wire in various designs of multiple coating layers and thicknesses, which may total approximately 0.0018-0.0020 inches thick or greater on an approximate 0.0070 inch diameter wire, for example. One exemplary coated wire design is shown in the layered, isometric perspective illustration of  FIG. 2 . Referring to  FIG. 2 , a wire conductor  20 , having an outside dimension of 0.006954 inches, for example, undergoes five (5) passes of coating layers of an unfilled polyester material to produce an initial five dielectric coating layers  22 , and then undergoes three (3) passes of coating layers of a magnetic particle filled material as described infra to produce the final three coating layers  24 . The film build up of all eight (8) coating layers  22  and  24  for this example may be approximately 0.002018 inches, for example, yielding an overall coated wire  26  with an outside dimension of 0.008972 inches. 
   Another exemplary coated wire design is shown in the layered, isometric perspective illustration of  FIG. 3 . Referring to  FIG. 3 , a wire  30 , having an outside dimension of 0.006959 inches, for example, undergoes one (1) pass of coating layer of the unfilled polyester material to produce an initial single dielectric coating layer  32 , and then undergoes seven (7) passes of coating layers of the magnetic particle filled material as described infra to produce the final seven coating layers  34 . The film build up of all eight (8) coating layers for this example may be approximately 0.001875 inches, for example, yielding an overall coated wire  36  with an outside dimension of 0.008834 inches. 
   The geometric cylinder shape of a round wire conductor as shown in  FIGS. 1 ,  2  and  3  is inherently reflective to radar illumination and provides additional specular design characteristics to the electromagnetically active coating layers. However, it is understood that the heater conductor of the present embodiment need not be round in cross-section, but rather take upon many different cross-sectional shapes without deviating from the broad principles of the present invention. An example of a heater conductor having a rectangular cross-sectional shape is shown in the layered, isometric perspective illustration of  FIG. 4 . Referring to  FIG. 4 , a rectangular cross-section conductor  40  may be coated with a layer  42  of dielectric material filled with magnetic particles in a similar manner as the wire conductor embodiment of  FIG. 1  described herein above. An initial coating of an unfilled layer  44  of dielectric material is optional. 
   One or more conductors coated with the electromagnetically active layers, as described in connection with the embodiments of  FIGS. 1 ,  2 ,  3  and  4  above, may be disposed in a heater element pattern for use in an electrothermal deicing apparatus. An exemplary heater element  50  patterned with a plurality of conductors having electromagnetically active layers is illustrated in  FIG. 5 . The lines of  FIG. 5  represent the coated heater conductors.  FIG. 6  is a cross-sectional, schematic illustration of an exemplary electrothermal deicing apparatus  60  utilizing the heater element  50 . 
   Referring to  FIG. 6 , the deicing apparatus  60  comprises the pattern of heater conductors  50  which may be disposed at, under or over a surface  62  of a structure  64  which may be any of the structures enumerated in the Background section hereof. The coated conductors of heater element pattern  50  may form electrical circuits when coupled to a deicing system  66  over connecting leads  68 . The deicing system  66  may control the voltage and current to the electrical circuits of the heater element  50  via leads  68  to heat and protect the surface  62  of structure  64  from accumulating ice while the electromagnetically active coated heater conductors of the heater element  50  offer an attenuation to radar wave transmissions which may be focused thereat, thus reducing its radar cross-section. If the structure  64  is a composite, non-metallic structure, the heater element  50  may be embedded or integrated into the structure  64  of an alternate electrothermal deicing apparatus  70  as shown by the cross-sectional, schematic illustration of  FIG. 6A  in which reference numerals of like elements remain the same. 
   The radar absorption method of the present embodiments is new and unique employing a hybrid technique of radar absorbing material (RAM), circuit analog absorber (CA) and graded dielectrics on a ferromagnetic electrical conductor with design flexibility to produce light weight radar absorbing characteristics. The coated heater conductor of the present embodiment is designed to have the dual use of electrical heating and radar wave or electromagnetic energy absorbing applications. It has a secondary use for electromagnetic interference (EMI) shielding applications and may be useful where heater applications in close proximity to electrical controls or sensors may require EMI shielding. 
   While the present invention has been described herein above in connection with a plurality of embodiments, it is understood that such description is presented merely be way of example. Accordingly, the present invention should not be limited in any way by the embodiments described herein above, but rather construed in breadth and broad scope in accordance with the recitation of the appended claims.

Technology Classification (CPC): 7