Improved electrical resistance heaters for preventing the accum- ulation of ice on aircraft surfaces are undergoing development. The primary intended market for these heaters is that of small single- and twin-engine airplanes and helicopters, most of which have not been equipped with anti-icing heaters because the weights and costs of such heaters have made them impractical. The improved heaters are expected to add very little to the weights of aircraft and, when massproduced, to cost about half as much as do anti-icing systems of prior design. The aircraft could be equipped with high-output alternators to supply the additional electric power needed for the heaters.
In the previously developed electrical anti-icing heaters used on larger aircraft, the resistance heating elements are metallic. Power den- sities are zoned by use of multiple elements and multiple electrical termin- ations. A concomitant of multiple heating zones is cold spots and the consequent need for complex control mechanisms: most such systems include multiple timers.
In the present developmental systems, the heating elements are made of expanded- graphite foil, which is flexible, has an electrical resistivity between 6 × 10–4 and 10 × 10–4 Ω⋅cm, has a thermal conductivity approaching that of brass, and is available in a variety of thicknesses. Typically, the foil in a heater of this type is laminated between (1) an insulating rubber or plastic sheet in contact with an aircraft surface and (2) an outer thermally conductive and protective layer of polyurethane or polyamide with a thickness between 0.001 in. (≈0.03 mm) and 0.010 in. (≈0.25 mm). The heater laminate can be formed as a monolithic tape (see Figure 1) that can readily be bonded to an aircraft surface area where protection against icing is needed.
The heater laminate/tape for a given area need have no more than two electrical contacts, and there is no need for complex controllers for zoning: instead, the spatial variations of power density needed for most effective shedding of ice can be obtained through spatial variations of sheet electrical resistance effected by use of different thicknesses and/or different densities of expandedgraphite foil. For example, one preferred design calls for a heater laid out along the leading-edge area of a wing (see Figure 2). The heater would contain a single foil heating element comprising (1) a central parting strip of greater thickness along the stagnation line wherein the power density would be high enough to keep the temperature above freezing and (2) shedding zones on both sides (downstream) of the parting strip where the thickness of the graphite foil and power density would be lower by an amount that would make the power density at least 3 to 5 times lower than in the parting strip.