Abstract:
Radar absorbing electrothermal deicing apparatus for use in deicing an airfoil surface comprises: a heater element including a predetermined area pattern of conductive metallic material; and a layer of radar absorbing material disposed over the heater element. In addition, a method of absorbing radar signals in an electrothermal deicer comprises the steps of: disposing an electrothermal deicing element at a surface of an airfoil; and disposing a layer of dielectric material comprising a filler of magnetic material over the electrothermal deicing element.

Description:
[0001]     This utility application claims the benefit of the filing date of U.S. Provisional Application No. 60/506,126, filed Sep. 25, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention is related to electrothermal deicing systems, in general, and more particularly, to electrothermal deicing apparatus having radar absorbing characteristics.  
         [0003]     Electrothermal deicing apparatus is applied generally to the airfoils of aircraft to protect the surfaces thereof from accumulating ice that may disturb the airfoil aerodynamics or that may be dislodged from the surface and become a potential foreign object damage (FOD), especially in the case of the aircraft engines. Generally, electrothermal deicing apparatus as shown in the cross-sectional airfoil illustration of  FIG. 1  comprises a layer of electrically conductive circuits  10  which may be configured as metal foils, wires, conductive fabrics and the like, for example, disposed in a pattern over a surface  12  of an airfoil  14 . A deicing system  20  controls the voltage and current to the electrical circuits of layer  10  via a plurality of leads  16  to protect the airfoil surface  12  from accumulating ice. However, the metallic/conductive nature of the layer  10  renders present electrothermal deicing apparatus impractical to be used on aircraft where radar attenuation is of concern.  
         [0004]     The present invention overcomes this drawback of the present electrothermal deicing apparatus and permits application of the conductive layer  10  on surfaces requiring both radar attenuation and protection from ice.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with one aspect of the present invention, radar absorbing electrothermal deicing apparatus for use in deicing an airfoil surface comprises: a heater element including a predetermined area pattern of conductive metallic material; and a layer of radar absorbing material disposed over the heater element.  
         [0006]     In accordance with another aspect of the present invention, a method of absorbing radar signals in an electrothermal deicer comprises the steps of: disposing an electrothermal deicing element at a surface of an airfoil; and disposing a layer of dielectric material comprising a filler of magnetic material over the electrothermal deicing element. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a cross-sectional illustration of a portion of an aircraft airfoil on which electrothermal deicing apparatus is applied.  
         [0008]      FIG. 2  is a cross-sectional illustration of electrothermal deicing apparatus suitable for embodying the principles of the present invention.  
         [0009]      FIG. 3  is an exemplary area pattern for an electrothermal heater element suitable for use in the electrothermal deicing apparatus of  FIG. 2 .  
         [0010]      FIG. 4  is a graph of radar cross sectional area data in accordance with the present embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]      FIG. 2  is a cross-sectional illustration of an exemplary embodiment of electrothermal deicing apparatus  24  suitable for use on the airfoil surface  12  of  FIG. 1 . In the present embodiment, the electrical circuits of layer  10  are combined with one or more layers of radar absorbing materials (RAM) and together disposed in a layered prepreg or other such composite material using composite manufacturing techniques such as resin transfer molding, for example, to form a radar absorbing structure (RAS). The electrical circuits of layer  10  have a predetermined area pattern which will work well as a heating element for surface  12  within most resin systems, like epoxy and bis-moly imide (BMI), for example, depending on the temperature specifications.  
         [0012]     An exemplary area pattern for the heating element  12  is shown in the embodiment of  FIG. 3  which comprises wire runs in open ended, rectangular shaped patterns one within the other. The wire is conductive metallic which may be copper, stainless steel or an alloy of stainless steel, for example, and may be only a few mils in diameter. The wire may be coated with an insulating layer approximately a few mils thick. Electrical energy may be applied to the ends of wire runs to apply electrothermal heating for deicing purposes. The area pattern of the heating element  12  may be made small enough to be applied to a vane of an aircraft engine or large enough to span an aircraft wing or portion thereof. The airfoil surface may be made of a metallic material or a composite without deviating from the broad principles of the present invention. If the airfoil is made of a composite material, the radar absorbing, electrothermal deicing apparatus may be disposed within the composite material, preferably at the surface thereof.  
         [0013]     Referring back to  FIG. 2 , one or more layers of RAM  30  are disposed over the electrical circuit layer  10 . In the present embodiment, the RAM layer  30  may be comprised of a dielectric material, like ethyl acrylic or a VAMAC™ (a trademark of Dupont) material, for example. For good thermal conduction, the RAM layer  30  should be made as thin as possible, but not so thin that it will defeat the radar absorbing properties thereof. A suitable thickness of a VAMAC™ material for the present embodiment was found to be on the order of 0.037 inches, for example.  
         [0014]     The RAM  30  may be tuned to absorb a particular radar frequency or frequency range depending on the specifications of each individual application. This may be accomplished by mixing a filler in the form of a fine powder of a ferrite or magnetic material, like iron carbonyl, for example, into the layer of dielectric material. The absorption tuning according to specification may be effected by the percentage of ferrite filler material mixed into the dielectric material. The mixing of fine powder filler into the dielectric material may occur through a roller milling process, for example, and a resultant desired thickness of the RAM  30  may be controlled through the rolling process.  
         [0015]     An outer or erosion protective surface layer  32  may be disposed over the RAM layer  30  depending on the application. In addition, back side insulating plies  34  may be disposed between the electrothermal heater layer  10  and the airfoil surface  12  for electrical and thermal insulation. In some applications, the conductive heater layer of electrical circuits  10  may include dielectric insulating plies  34  on both sides thereof. The dielectric insulating plies  34  may be comprised of plies of glass, Quartz, Kevlar, Graphite and the like, depending on the structural specifications of the electrothermal deicing apparatus.  
         [0016]     Moreover, the elemental area pattern of layer  10  may be configured to aid in the ability to attenuate or minimize reflection of radar signaling. Depending on the amount of heat or power densities specified to be generated, the material of the heating element  10  could be considered part of the RAS  24 . Accordingly, the heating element layered embodiment  24  described in connection with  FIG. 2  is operated by the deicing system  20  to form a radar absorbing electrothermal de-icer for utilization in many different applications.  
         [0017]     A series of composite test parts were constructed using variants of the foregoing described technology with different electrothermal heater designs. The parts were tested on a compact radar range over the 2-18 gigahertz frequency spectrum and were rotated at four desired angles on incidence. A focused radar beam was used to determine radar cross sectional area against a standard baseline six inch diameter aluminum sphere in both horizontal and vertical polarizations.  
         [0018]     The foregoing described technology was effective in absorbing the radar signal reflected by the electrically conductive electrothermal heater. Different absorptances were achieved when the reflective signal was completely attenuated by such technology. The efficiency of collecting and absorbing by the technology was determined to be tunable by both different combinations of design and materials.  FIG. 4  is a graph of radar cross sectional area data taken of a sample of the technology at ten degrees angle of incidence and with vertical polarization. The dark line in the graph represents technology deicer data and the light line represents baseline testing data of the same deicer design.  
         [0019]     While the present invention has been described herein above in connection with one or more embodiments, it is understood that such description was merely by way of example. Accordingly, the present invention should not be limited in any way by the described embodiments herein, but rather construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.