CA2379351C - Zoned aircraft de-icing system and method - Google Patents

Abstract

An electrothermal zoned de-icing system for an aircraft employs a heat- conducting tape bonded to the leading edge of an aircraft structure subject to an impinging airstream during flight. The heat-conducting tape has a spanwis e parting strip area, and first and second ice accumulation and shedding zones (132, 134). The tape comprises a non-metallic electrical and heat-conducting layer consisting of flexible expanded graphite foil laminated to an outer he at- conducting layer, in which the thickness of the flexible expanded graphite foil layer in the parting strip area is always greater than the thickness of the foil layer in either of the ice accumulation and shedding zones. Therefo r, the parting strip area has a decreased electrical resistance, a greater flow of current, and becomes hotter than the zones in which the foil layer is thinner. Because the flexible expanded graphite foil is a monolithic structu re that may be shaped, sculptured or layered to form different thicknesses in different areas, only a single control mechanism for a single set of electri c terminals is necessary to produce temperature differentials in the parting strip and ice accumulation and shedding zones for a cycled zoned de-icing system.

Description

ZONED AIRCRAFT' Dlr-ICING SYS?EM AND MET~iOD
Robert H. Rutherfvrd and Richard L. Dudman BACKGROUND OF TTY DJVENTtON
Aircraft, during flight andJor wlu'1e on the ground, may encounter atmospheric conditions that cause the formation of ice on airfoils and other surfaces of the aircraft structure, including wings, stabilizers, rudder, ailerons, engine islets, propellers, rotors, fuselage and the like. Accumulating ice, if not removed, can add cxccssive weight to the aircraft and alter the airfoil configuration, causing undesirable and/or dangerous flying conditions. Genera! aviation aircraft are IS particularly susceptible to the detrimental consequences of ice formaticra because only small amounts of ice on structural members, such as wings, tail, propellers, and the l'kc, can significantly alter flight characteristics.
Since the cariiest days ~ of ~ flight, attempts have been made to overcome the problem of ice accumulation, and mechanical, chemical and thermal 2p de-ice andlor anti-ice systems have been developed for use in large commercial and military aircraft. Thermal systems include those in which bleed air or hot air from one of the compressor stages of a turbine aircraft are diverted to beat the airfoil leading edges. Other thermal systems employ electrically conductive rssistanee heating elements, such as those contained in heating pads bonded to the leading edges of the aircraft, or on the propeller or rotor blades, or incorporated into the structural members of the aircraft. Heating pads of this type usually consist of a thermally insulating material in contact with win or other metal heating elements dispersed throughout the insulating layer: Because heat must be transferred from the metal heating elements to the surrounding insulating areas, these heaters are 30 in~ff'~ient insofar as the energy and time required for heat up to a required _2_ temperature, and the time required for cool-down when the current is removed.
Electrical energy for the heating elements is derived from a generating source driven by one or more of the aircraft engines or an auxiliary power unit. The electrical energy is continuously supplied to provide enough heat to prevent the formation of ice, or intermittently supplied to loosen accumulating ice. However, such systems art only usable where sufficient wattage is available to raise and/or maintain the temperature of the airfoil surface above the freezing point at typical aircraft speeds in icing conditions.
Electrothermal anti-ice and de-ice systems are classified as either evaporative or "naming wet". Anti-ice evaporative systems supply enough heat to evaporate substantially all water droplets impinging upon the heated surface.
The running wet de-icing systems, however, provide only enough heat to prevent freezing of the water droplets. The water then flows afi of the heated surface where it freezes, resulting in what is commonly known as runback ice, In "zoned" de-icing systems, runback ice is removed periodically by rapid application of sufficient heat to melt and loosen the ice bonded at the surface-ice interface; the bulk of the ice is then removed by aerodynamic or centrifugal forces.
In many heating pads used for ciectrothermal zoned de-icing systems, metal heating elements are configured as serpentine ribbons that form interconnected conductive segments. Because of the low electrical resistivity of metal heating elements, such as copper, aluminum, and the like, the serpentine configuration is designed to provide sufficient length to the element to achieve a high enough resistance to generate energy. Each ribbon is individually electrically energized by a pair of contacts, one on each end of the ribbon, and a current is transmitted through 2. the ribbon by establishing a voltage differential between its corresponding pair of contacts, resulting in heating of the element. , Heating pads such as these are described in U.S. Patents 5,475,204 (issued December 12, 1995) and 5,657,951 (issued May 26, 1998). One of the problems described in association with zoned de-icing systems employing heating pads of this type is that cold spots tend to develop at intersegmental gaps between the electrically conductive segments and at interheater gaps between adjacent zones. lee formed at these cold spots can be very difficult to melt v~rithout the consumption of excessive current. Funher, in this type of heating pad, each metallic heating clement requires ,~ 27175-90 its own electrical tenninations or contact strips. Because they are not heated, the melting of ice on or around the contact strips can also be very difficult.
Accumulation of ice at the intersegmental and interheater gaps and atotmd the contact strips is particularly undesirable since the accumulated ice can serve as an "anchor" far additional ice formation. In an attempt to address the problem of "cdd sports", an elec~otl~xnal de-icing pad descn'bed in U.S. Patent No. 5,475,204 (issued December 12, 1995) provides at least two hears having ca~Ctive heating elemer>ts that are positioned relative to each other so that the marginal ponions are overlapped in an attempt to eliminate gaps. However, as with other previously described electrothermal de-icing systems, these heaters have multiple heating Zones containing a phtrality of metallic heating elements, including a plurality of electrical terminations, requiring the use of complex control mechanisms that rely on multiple timers to control multiple zones.
As discussed above, the use of electrothermal heating pad systems is 1 S only feasible where sufficient wattage is available to raise andlor maintain the temperature of the airfoil surface above the freezing point at typical aircraft speeds in icing conditions. Because of the configuration of the metal elements in these pads, the watt densities are not uniformly distributed, resulting in substantial heating inefficiencies in terms of the average watt density provided. The power requirements for the anti-ice andJor de-ice systems using these metallic heating pads are large. ?herefore, electrothermal systems that have been successfully used in large aircraft have been impractical for general aviation light aircraft, such as single engine and light twin airplanes and helicopters, because of power requirements that are in excess of the electrical power available. Moreover, auxiliary on-board power generating units for de-icing systems have not been employed in light aircraft because of the substantial weight and expense penalty that would be inctu~red.
Thus, there is a need for an ice protection system for all aircraft, including general aviation light aircraft, that has sufficient operating efficiency to protect aircraft structural members, such as the wings, tail structures, propeller, rotor blades, and the like, against the accumulation of ice, that is light weight, that does not interfere with aircraft flight characteristics, and that is economical.
Morc particularly, there is a nerd for an efficient thermoelectric heater system that can be 4~
. zoned" to provide as effective de-ice system in which elecvical energy can be intermittently ~ or continuously supplied to . provide hat sufficietrt to prev~
the formation of ice or to laoscn accumulating ice.
Recently, technology has been developed to allow a light atnaa~t to be fitter with a 150 ampere to 200 ampere alternator producing 40 to 60 voltr, without s significant weight penalty. It is now possible that a combination of a very efficient thermoelectric heater, in terms of the watt densit~r provided by the heater, and such as alternator, could also allow general aviation aircraR to utilize rzliabk electrotherrnal io-flight anti-ice/de-ice systems.
St~~D~IARY OF TIC ~1VF.N'IION
It is an object of the invention to provides an ~ elecr~emnal >x~
that can be used in a de-ice andlar anti-ice system for all ai~a$ including general aviat~ai light sircraR, because the watt densities provided by the heater arc su8icient to meh or loosen accumulating ice using the power avau7able in large airzraft or is light aircraft augmented with, if necessary; an oa-board lightweight, high-output, auxiliary alternator, such as that described above. <>> ~ p~cular, the invention provides a lightweight heat-conducting tape bonded to the surface of an aircraft structim that includes a leading edge, for electrotbermally removing ice from or preventing the formation of ice on the surface during in-flight andlot on the ground icing conditions. Although the heat-conducting tape is herein described for use on aircraft surfaces, the tape may be used for any surface which requires anti-ice or de-ice capability and where a power source is available. Such applications include roofs, gutters, pipes, automobile hoods and trunks, sad the flce.
The invention employs such a heat-conducting tape for use as an anti-icelde-ice system as disclosed e.g, in our U.S. Patent No. 5,934,617.
The heat-conducting tape is used in a zoned aricraft electrothermal de-icing system for an aircraft structural member that includes a leading edge subject to an impinging airstream during flight, said airstream passing over an outer surface of the structural member in a fore to aft direction, the system comprising a heat-conducting tape bonded to the outer surface t This Object is solved by an electrotberrasl heater according to claim 1.

of the structural member, the heat-conducting tape comprising a first area that forms a parting strip having a length disposed spanwise along the leading edge, a second area disposed spanwise above and aft of the parting strip forming a first ice accumulation and shedding zone, and a third area disposed spanwise below and aft of the parting strip forming a second ice accumulation and shedding zone. As used herein, the terms "disposed spanwise", "disposed above" and "disposed below"
are used in reference to a substantially horizontal aircraft structure (e.g., a wing, a horizontal tail stabilizer, a helicopter rotor blade, and the like); but these terms are intended herein to be fully inclusive of a parting strip disposed along the length of a non-horizontal leading edge (e.g., a vertical tail stabilizer, an aircraft propeller blade, and the like) and ice accumulation and shedding areas which are disposed on one side and/or an opposite side of the parting strip.
The heat-conducting tape comprises at least two layers laminated to each other under heat and pressure, i.e., a non-metallic electrical and heat-conducting layer consisting of flexible expanded graphite foil, also known as vermiform graphite, laminated to an outer heat-conducting layer that is an electrical insulator and seals the interior of the tape against penetration and water damage. The flexible expanded graphite foil sheet of the heat-conducting tape has a first thickness in the parting strip, a second thickness in the first ice accumulation and shedding zone, and a third thickness in the second ice accumulation and shedding zone, and the thickness of the flexible expanded graphite foil sheet in the parting strip is greater than the thickness of the foil sheet in either of the first or the second ice accumulation and shedding zones.
The tape is bonded (e.g., by an adhesive) to an electrically insulating layer, such that the flexible expanded graphite layer is disposed between the heat-conducting outer layer and the insulating layer. The electrical insulating layer may be directly bonded to the tape to form a third layer before application to the aircraft surface. Alternatively, the electrical insulating layer may be a component of the aircraft surface to which the two-layer tape is applied.
A feature of this embodiment of the invention is that the flexible expanded graphite foil comprises a single, monolithic heating element that can be shaped, sculptured or layered in such a way as to vary the watt densities in the .6-parting strip and the ice accumulation and shedding zones of the heating element is a predetermined manner. It is known that varying the length, width and/or thickness of flexible expanded graphite foil changes the electrical resistance along the length of the foil by a large magnitude and, as is known, the electrical resisraaa of a material determines the amount of electric current that w111 ' flow through the material. Consequently, for a given length and width, a secti~ of the heat.
conducting tape comprising a greater thickness of flexible expanded graphite foil has .
a lesser electrical resistance, a greater flow of ciurent, and becomes, hotter than sections in which the foil layer is thinner. In the zoned deicing embodiment of the invention, the flexible expanded graphite foil layer of the heater has a thickness in the parting strip that is greater. than the thickness of the foil in each of the fast and the second ice accumulation and shedding zones. Therefore, the temperature of the parting strip exceeds the temperature of either of the two ice accumulation and shedding zones at all given power settings.
In a preferred embodiment, the flexible expanded graphite foil sheet is a continuous shaped sheet comprising a decreasing gradient of . thicknesses bctw~en the parting strip and each' of the ice accumulation and shedding zones. In another embodiment, the parting strip comprises at least two layered flexible expanded graphite foil sheets, and the layers are arranged to form a decreasing gradient of thicknesses between the parting strip and each of the ice accumulation and shedding zones. In yet another embodiment, the parting strip and one or both of the ice accumulation and shedding zones may be separate sections of the foil, but the ice accumulation and shedding sections are separated from the parting strip by s gap of no greater than about 60 mils (1.52 mm~. In this embodiment, one or more of the sections may be shaped to form a decreasing gradient of thicknesses between the parting strip section and the ice accumulation and shedding zones.
Regardless of the configuration of the flexible expanded graphite layer of the heat-conducting tape, a further feature of the coned de-icing embodiment of the invention is that the flexible expanded graphite foil layer is preferably ' connected to a power source by a single set of two electrical terminals, A
entreat is transmitted through the foil by establishing a voltage differential between ins corresponding pair of terminals, resulting in heating of the foil. A
given'amount of '' 27175-90 _ 7.-power supplied to the flexible expanded graphite layer results in watt densities that differ between the parting strip and the ice accumulation and shedding zones, as governed by the predetermined thicknesses in these areas, and the temperature of the parting strip always exceeds that of the ice accumulation and shedding zones. Therefore, only a single control mechanism far a single set of electric terminals is necessary to produce desired watt densities and temperatures in the parting strip and ice accumulation and shedding zones, resulting in zoned de-icing system that is greatly simplified compared to previously known systems.
Moreover, the use of only two terminals results in substantially fewer termination points or contact strips as potential cold spots that could detrimentally become anchor points for ice accumulation.
Tt is another object of the invention to provide a method for electrothermal de-icing. This object is solved by a method for electrothermal de-icing of an aircraft structural member that includes a leading edge subjected to an impinging airstream during flight, said airstream passing over an outer surface of the structural member in a fore to aft direction, the method characterized by comprising the steps of: bonding a heat-conducting tape to the outer surface of the structural member, wherein the heat-conducting tape comprises a first area that forms a parting strip having a length disposed spanwise along the leading edge, a second area disposed spanwise above and aft of the parting strip forming a first ice accumulation and shedding zone, and a third area disposed spanwise below and aft of the parting strip forming a second ice accumulation and shedding zone, wherein the heat-conducting tape comprises at least two layers laminated to each other under heat and pressure, the layers comprising (i) an outer heat-conducting '' 271?5-90 -7a-layer that is an electrical insulator, and (ii) a non-metallic conductive layer connected to a power source by a single set of two terminals, the non-metallic conductive layer consisting of a flexible expanded graphite foil sheet having a first thickness in the parting strip, a second thickness in the first ice accumulation and shedding zone, and a third thickness in the second ice accumulation and shedding zone, wherein the thickness of the flexible expanded graphite foil sheet in the parting strip is greater than the thickness of the foil sheet in either of the first or the second ice accumulation and shedding zones; supplying power to the flexible expanded graphite foil layer for a first period of time to maintain a first temperature of the outer heat-conducting layer at the parting strip to prevent freezing of impinging water droplets and to allow water droplets to flow aft from the parting strip to the first andlor the second ice accumulation and shedding zones, wherein the power supply also maintains, in the first period of time, a second temperature of the outer heat-conducting layer at the first andlor second ice accumulation and shedding zones which is freezing or below freezing, to allow formation of ice and an ice-to-surface bond on the outer heat-conducting layer at the first and/or second ice accumulation and shedding zones; and subsequently increasing the power supply to the flexible expanded graphite foil layer for a second period of time, wherein the increased power supply melts the ice-to-surface bond at the first andlor second ice accumulation and shedding zones and allows formed ice to be shed into the impinging airstream.
When, or preferably prior to, encountering in-flight icing conditions, a programmable controller directs a supply of power from the power source to the flexible expanded graphite foil layer of the heat-conducting tape for '' 27175-90 -7b-a first period of time to maintain a first temperature of the outer heat-conducting layer at the parting strip that is sufficient to prevent freezing of impinging water droplets and to allow water droplets to flow aft from the parting strip to the first and/or the second ice accumulation and shedding zones of the heat-conducting tape. The first temperature is above 32°F (0°C), preferably about 32°F
(0°C) to about 50°F (10°C), more preferably about 35°F
(2°C) to about 45°F (7°C). The power supply is also sufficient in the first period of time to~maintain a second temperature of the outer heat-conducting layer at the first and second ice accumulation and shedding zones which does not exceed 32°F
(0°C). The water droplets flowing aft from the parting strip form ice and/or an ice-to-surface bond on the outer heat-conducting layer at the first and/or second zones. The first time period allowed for ice accumulation at the first and second ice accumulation and shedding zones is about 10 seconds to about 5 minutes, depending on the rate of ice accretion under the icing conditions encountered.
Subsequently, at the end of the first time period, the controller directs an increased power supply to the flexible expanded graphite foil layer wherein the increased power supply is sufficient to maintain a third temperature of the outer heat-conducting layer at the first and/or second ice accumulation and shedding zones at greater than 32°F for a second time period sufficient to melt the ice-to-surface bond and to allow formed ice to be shed '~ 27175-90 .8-into an impinging ~airstream. Preferably the third temperature is about 34°F 11°Cl to about 40°F~4°C) and the second time period is about S seconds to about 60 seconds.
The application of power for the second time period also increases the temperature of the parting strip. Due to the efficient thermal conductivity of the flexible expanded graphite foil, heat f3om the parting strip is conducted aft, into the ice accumulation and shedding zones, thus reducing the power required to raise the temperature in these zones. The cycle of runback freezing of the water droplets and intermittent ice removal is then repeated as oRen a necessary until icing conditions no longer exist.
The beat-conducting outer layer may comprise nay heat-conducti~
material that is essentially hn electrical noes-conductor. Preferably, the hoat-conducting outer layer comprises a thermoplastic or thenmosot material containing a heat-conducting inorganic filler. More preferably, the heat-conducting layr comgriscs a thermoplastic material, such as polyurethane, with a filler selected from aluminum Ditride, boron nitride, alumina, and the l~7ce.
Flexible expanded graphite foil suitable for use is the heat-conducting tape of the invention is a headily commercially available material that is relatively . inexpensive. The flexible expanded graphite foil has an electrical rcsistivity along its length and width of about 2.? x 10-'4 ohm-in.(6,8.6x10"4ohm-cm) to about 3.2 x 10.4 ohm-in.(8.13x 10-4ohm-cml, with an average of about 3.1 x 10-4 ohm-in.
(7.9x10"'lohm-cml. and is fully electrically conducting without the necessity of including wire elements, metals or heat-conducting fibers within or through the layer.
Because of the continuous, monolithic configuration of the flexible expanded graphite foil, the watt density throughout the heat-conducting tape is uniformly distributed. Thus, the 251 heat-conducting tape of the invention is much more efficient than heating pads employing metal elements, and the measured watt densities are true watt densities rather than . average watt densities. Because flexible expanded graphite foil is well suited, in terms of surface area to volume, for conducting heat and electricity, the wattage required for producing a rapid rise in temperature from ambient to that required for anti-ice andhx de-ice systems over a large surface area, is far less than that required by known electrical wire heating systems.
In addition, flexible expanded graphite has a low thermal mass, enabf~ng a rapid heat up and a rapid cool down when the current is removed.

-g-The two- or three-layer heat-conducting tape is easily applied, using commercially available adhesives such as rubber-based adhesives, to any surface of the aircraft, including the fuselage, wings, ailerons, propeller or rotor blades, tail sections, including stabilizers and rudder, engine cowling, oil pan, and the like. In the zoned de-ice system described above, the tape is applied to an aircraft structure that includes a leading edge. The combination of the flexibility of the expanded graphite, the heat-conducting outer layer, with or without a flexible electrically insulating layer, provides a heat-conducting tape that is easy to die cut to size and configure to a variety of aircraft structural shapes, including control surfaces and other irregularly shaped removable and movable components. The heat-conducting tape is also light in weight and inexpensive compared to existing de-ice and anti-ice systems.
Thus, in a broad aspect, the invention provides an electrothermal de-icing system for an aircraft structural member that includes a leading edge subject to an impinging airstream during flight, said airstream passing over an outer surface of the structural member in a fore to aft direction, the system comprising a power source and being characterized by further comprising a heat-conducting tape bonded to the outer surface of the structural member, the heat-conducting tape comprising a first area that forms a parting strip having a length disposed spanwise along the leading edge, a second area disposed spanwise above and aft of the parting strip forming a first ice accumulation and shedding zone, and a third area disposed spanwise below and aft of the parting strip forming a second ice accumulation and shedding zone, wherein the heat-conducting tape comprises at least two layers laminated to each other under -9a-heat and pressure, the layers comprising (i) an outer heat-conducting layer that is an electrical insulator, arid (ii) a nonmetallic conductive layer connected to the power source, the non-metallic conductive layer consisting of a flexible expanded graphite foil sheet having a first thickness in the parting strip, a second thickness in the first ice accumulation and shedding zone, and a third thickness in the second ice accumulation and shedding zone, wherein the thickness of the flexible expanded graphite foil sheet in the parting strip is greater than the thickness of the foil sheet in either of the first or the second ice accumulation and shedding zones; an electronic connection for connecting the flexible expanded graphite layer to the power source;
and a programmable controller configured (i) to direct a supply of power from the power source to the flexible expanded graphite foil layer of the heat-conducting tape for a first period of time, to maintain a first temperature of the outer heat-conducting layer at the parting strip to prevent freezing of impinging water droplets and to allow water droplets to flow aft from the parting strip to the first and/or the second ice accumulation and shedding zones of the heat-conducting tape and wherein the power supply also maintains, in the first period of time, a second temperature of the outer heat-conducting layer at the first and/or second ice accumulation and shedding zones which is freezing or below freezing to allow formation of ice and an ice-to-surface bond on the outer heat-conducting layer at the first and/or second ice accumulation and shedding zones, and (ii) to subsequently direct an increased power supply to the flexible expanded graphite foil layer of the heat-conducting tape for a second period of time, wherein the increased power supply melts the ice-to-surface bond at the first and/or second ice accumulation and shedding zones and allows formed ice to be shed into the impinging airstream.

" 27175-90 -9b_ In another aspect, the invention provides a method for electrothermal de-icing of an aircraft structural member that includes a leading edge subjected to an impinging airstream during flight, said airstream passing over an outer surface of the structural member in a fore to aft direction, the method characterized by comprising the steps of: bonding a heat-conducting tape to the outer surface of.
the structural member, wherein the heat-conducting tape comprises a first area that forms a parting strip having a length disposed spanwise along the leading edge, a second area disposed spanwise above and aft of the parting strip forming a first ice accumulation and shedding zone, and a third area disposed spanwise below and aft of the parting strip forming a second ice accumulation and shedding zone, wherein the heat-conducting tape comprises at least two layers laminated to each other under heat and pressure, the layers comprising (i) an outer heat-conducting layer that is an electrical insulator, and (ii) a non-metallic conductive layer connected to a power source by a single set of two terminals, the non-metallic conductive layer consisting of a flexible expanded graphite foil sheet having a first thickness in the parting strip, a second thickness in the first ice accumulation and shedding zone, and a third thickness in the second ice accumulation and shedding zone, wherein the thickness of the flexible expanded graphite foil sheet in the parting strip is greater than the thickness of the foil sheet in either of the first or the second ice accumulation and shedding zones; supplying power to the flexible expanded graphite foil layer for a first period of time to maintain a first temperature of the outer heat-conducting layer at the parting strip to prevent freezing of impinging water droplets and to allow water droplets to flow aft from the parting strip to the first and/or the second ice accumulation and shedding zones, wherein the power .. 2~m 5-90 - 9c-supply also maintains, in the first period of time, a second temperature of the outer heat-conducting layer at the first and/or second ice accumulation and shedding zones which is freezing or below freezing, to allow formation of ice and an ice-to-surface bond on the outer heat-conducting layer at the first and/or second ice accumulation and shedding zones;
and subsequently increasing the power supply to the flexible expanded graphite foil layer for a second period of time, wherein the increased power supply melts the ice-to-surface bond at the first and/or second ice accumulation and shedding zones and allows formed ice to be shed into the impinging airstream.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic illustration of a single engine aircraft having the heat-conducting tape of the invention bonded to the leading edges of the wings, rudder and stabilizers.
Figure 2 is a schematic illustration of a propeller blade having the heat-conducting tape bonded to a leading edge.
Figure 3A is a schematic illustration of a jet aircraft having the heat-conducting tape bonded to the leading edges of the wings, rudder and stabilizers.
Figure 3B is a schematic illustration of further areas to which the heat-conducting tape may be bonded to an aircraft, such as the leading edges of the wings, rudder, stabilizers, engine air inlets, auxiliary air inlets, propellers, antennas, balance horns, essential instruments and radome.

-9d-Figure 4 is a schematic illustration of the components of the heat-conducting tape, including the flexible expanded graphite layer, the heat-conducting outer layer and the electrically insulating layer.
Figure 5 is a cross section of the three-layer heat-conducting tape illustrating the flexible expanded graphite layer disposed between the heat-conducting outer layer and the insulating layer.

Figure 6 is a schematic cut-away illustration of a wing section having the heat-conducting tape of the invention bonded to a portion of the leading edge surface.
Figure 7 is a schematic illustration of a power source and a programmable power control electronically connected to the flexible expanded graphite layer of the heat-conducting tape and a temperature sensor, respectively.
Figure 8 is an electrothermal de-icer according to the prior art mounted on a structural member in the form of a wing.
Figure 9 is a schematic illustration of the heat-conducting tape of the zoned electrothermal de-icing system embodiment of the invention, bonded to the leading edge of an aircraft structure, such as a wing.
Figure 10 is a schematic illustration of one embodiment of the flexible expanded graphite foil layer of the heat-conducting tape of the de-icing system embodiment.
Figure 11 is a schematic cross-sectional view of the heat-conducting tape of Figure 10, bonded to the leading edge of a wing.
Figure 12 is a schematic cross-sectional view of another embodiment of the heat-conducting tape of Figure 10, bonded to the leading edge of a wing.
Figure 13 is a schematic illustration of another embodiment of the flexible expanded graphite foil layer of the heat-conducting tape of the de-icing system embodiment.
Figure 14 is a schematic cross-sectional view of the heat-conducting tape of Figure 13, bonded to the leading edge of a wing.
Figure 15 is a schematic illustration of another embodiment of the flexible expanded graphite foil layer of the heat-conducting tape of the de-icing system embodiment.
Figure 16 is a schematic cross-sectional view of the heat-conducting tape of Figure 15, bonded to the leading edge of a wing.
Figure 17 is a schematic illustration of an embodiment of the invention wherein the heat-conducting tape is bonded to the leading edge of a helicopter rotor blade or a propeller blade.

'' 27175-90 Figure 18 is a schematic cross-sectional view of an embodiment of the heat-conducting tape comprising an outer erosion-resistant layer, bonded to the leading edge of a helicopter rotor blade or a propeller blade.
Figure 19 is a graph of the resistivity of Grafoil~ Brand Flexible Expanded Graphite versus density.
Figure 20 is a graph of the resistance of Grafoil~
versus thickness.
Figure 21 is a histogram illustrating the resistance of Grafoil~ versus thickness and density.
Figure 22 is a histogram illustrating the time to heat Grafoil~ of various thicknesses and densities to 125°F
(51.7°C).
Figure 23 is a histogram illustrating the time for Grafoil~ of various thickness and densities to cool down from 125°F to 82°F (from 51.7°C to 27.8°C).
Figure 24 is a histogram comparing the time to heat up and time to cool down of Grafoil~ versus a Veratec~
material.
Figure 25 is a three-dimensional schematic illustration of the power requirements to heat Grafoil~
having different thicknesses bonded to a composite laminar flow wing from a Vantage~ aircraft.
Figure 26 is a histogram illustrating the heat transfer between in a simulated parting zone and ice accumulation and shedding zones, when the parting zone only was powered.
Figure 27 is a histogram illustrating the temperatures of the simulated parting zone and ice -11a-accumulation and shedding zones, when all zones were powered.
Figure 28 is a figure reproduced from Heinrich, A., et al., Aircraft Icing Handbook, Technical Center Publication #DOT/FAA/CT-88/8-2, United States Federal Aviation Administration, 1991, illustrating atmospheric icing design conditions for stratiform clouds.
Figure 29 is a schematic illustration of the location of seven thermocouples on the leading edge of a Cessna 182-R wing to which the heat-conducting tape was bonded.
Figure 30 is a schematic illustration of the location of six thermocouples on the leading edge of a Lancair IV wing to which the heat-conducting tape was bonded.

Figure 31 is a graph of the watt densities versus the temperature of the outer surface of the heat-conducting layer of the tape under four different sets of atmospheric conditions.
Figure 32 is a histogram of the watt densities versus the temperature of the outer surface of the heat-conducting layer of the tape under four different sets of atmospheric conditions.
Figure 33 is a graph illustrating the time to heat and time to cool the outer surface of the heat-conducting layer of the tape under a set of atmospheric conditions.
Figure 34 is a graph illustrating the watt densities required to raise the temperatures of the parting strip zone, and the first and second ice accumulation and shedding zones.
Figure 35 is a graph illustrating a power off condition in a single de-icing sequence.
1 S Figure 36 is a digital photograph showing runback ice forming on an ice accumulation and shedding zone of the heat-conducting tape bonded to the leading edge of a Lancair IV wing.
Figure 37 is a graph illustrating a power on condition in the single de-icing sequence.
Figure 38 is a digital photograph showing the initial shedding of the runback ice of Figure 36.
Figure 39 is a graph illustrating time following the power on condition of Figure 37.
Figure 40 is a digital photograph showing the removal of the runback ice of Figure 39.
Figure 41 is a block diagram illustrating components of the electrothermal de-icing system of the invention.
Figure 42 is a block diagram illustrating the zoned electrothermal de-icing system connected to the components of Figure 41.

DESCRIPTION OF THE INVENTION
In a first embodiment of the invention disclosed in our U.S. Patent No. 5,934,617, an electrothermal system for removal of ice from or prevention of the formation of ice on an outer surface of an aircraft structure comprises a heat-s conducting tape that comprises at least two layers laminated to each other under heat and pressure, the layers comprising (i) a non-metallic electrical and heat-conducting layer consisting of a flexible expanded graphite foil sheet disposed between an outer heat-conducting layer and an electrically insulating layer that is bonded to or is a component of the aircraft surface.
The heat-conducting tape of the first embodiment of the invention may be bonded to any surface or partial surface of any aircraft structure that is subject to the formation of ice. For example, as illustrated in Figures 1 and 2, structural areas to which the heat-conducting tape 2 of the first embodiment may be applied to a light aircraft 1 include, but are not limited to, the leading edges of the wings 3, rudder 4, stabilizers 5 and propeller blades 6, as well as other structures, such as the oil pan. As illustrated in Figure 3A, the heat-conducting tape 2 of the first embodiment may also be applied to any structural area subject to icing in a commercial aircraft 10 including, without limitation, the leading edges of the wings 12, stabilizers 14 and rudder 16 sections, as well as other surfaces, including ailerons, flaps, engine cowling, and the like. The tail structures on commercial aircraft have historically been the most susceptible to in-flight ice hazards.
As illustrated in Figure 3B, further structural areas to which the heat-conducting tape 2 of the first embodiment may be applied to a light aircraft, such as a twin engine aircraft 100 include, but are not limited to, the leading edges of the wings 102, empennage leading edges 104 of the rudder and stabilizers, balance horns 103, propeller blades 105, engine air inlets 106, as well as other structures, including auxiliary air inlets 107, essential external instruments 108, antennas 109 and oil pan (not shown).
In the zoned de-icing system embodiment of the invention, the aircraft structure to which the heat-conducting tape may be applied includes, without limitation, any leading edge of the aircraft such as those illustrated in Figures 1, 2, 3A and 3B including the leading edges of the wings, empennage leading edges of ., 27175-90 the rudder and the vertical sad-horuonts) tail s~tabillze=a, and propeller blades, incladi~
beUco~tet rotor blades.
T6e' atructursl components of the heat-caaducting tape are gaieraDy illustrated is Figures 4 sad .3. The heat-condnctiag taps 2 comprises ,a flexl'W~
3 expanded graphite foil layer 20 laatinated to as outer he~atcoadutting layer ?Z and disposed between the outer layer 22 and an electrically insulating layee ?r4.
Tba flexible expanded graphite foil lays 20 it connectable to , a p~ sotuce by eltetrical tsrminal 40 p~hich can be, foe e;xataple, art edge connector o~t bets bar, a the like; and wiring system 36. Another edge eonnecttir or bns bar 3a is avai'labk 14 ~ f~ grounding the electrical circuit In the fast cmbodimetrt of the inventiesl, the flcxi'blo expanded graphite foil is a sheet having a thiclmest of abort 0.003 to about 0.
125 inches (about 0.08 to about 3.2 mm), preferably about 0.025 to about 0.125 inches (about 0.63 to about 0.32 mm), more preferably about 0.003 to about 0.070 inehesfabout 0.08 to about 1.8 rorol, and especially about 0.003 to about 0.030 15 ipchcs(about 0.08 to about 0.8 rrun~. In the zoned de-icing aysteat embodiment of the invention, the thickness of the flexible expanded graphite foil layer 20 _irt the hear-conducting tape 2 varies is a pr~--determined manner in difkrcat areas of the tape, as dcscn'bed further below.
The insulating layer 24 msy be bonded to the flexible expanded graphite foil layer 20 by as adhesive, such is any rubber-based adhesive that maintains its bonding capability 20 over a wide range of temperatures. An example of a readily ava~7abk suitable adhesive is the rubber.based conuct adhesive 1300-L (3M , Company), Altcrnativcly, the thief Isycrs of the heat-conducting tape may be laminated to each other under heat and pressure. For example, the area of outer brat-condutting layer may be larger than the area of the flcxiblt expanded graphite foil sheet layer 20, and may be, laminated directly to the insulating layer 24. The insulating lays 24 is bonded directly to an airc:aR surface such as a leading edge section of an aluminum wing 30, as illustrated in a cut-away schematic in FiEut~e 6, avo by means of an adhesive, such as the foregoing rubber-based contact adhesive.
When the aircraft stivctute already has an electrically insulating 30 ~mpanent, s two-part heat-conducting tape may be used that comprises the flt:xbk expanded graphite layer 20 and the heat-conducting outs layer 22. Foe example, tbt aircraft surface may be painted or otherwise coated with an electrically insulating material, such as a polyurethane paint or an aluminized paint. The flexible expanded graphite layer 20 and the heat-conducting outer layer 22 are then bonded directly to the insulating component of the aircraft structure, with a rubber-based adhesive.
Alternatively, the aircraft structure itself may be manufactured of an electrically non-conducting composite, such as fiberglass reinforced plastic, or the like. In this case, the flexible expanded graphite foil sheet 20 may be embedded in the composite during its manufacture, by methods known to those skilled in the art of composites, and a heat-conducting outer layer may be later painted or sprayed on or bonded to the composite surface with an adhesive. In this embodiment, the flexible expanded graphite foil sheet is in close proximity to or in contact with the heat-conducting layer in order to transfer heat to this layer. Examples of suitable paint or spray-on heat-conducting layers include polyurethane-based or aluminized paints, that contain inorganic fillers, such as aluminum nitride.
Flexible expanded graphite foil for use in the heat-conducting tape may be prepared, as is well known, by expanding graphite flakes many times and 1 S then compressing the expanded flakes to form a cohesive structure. The expansion of graphite flakes can be readily achieved by attacking the bonding forces between the layers of the internal structure of graphite, such as by the use of an acid. The result of such an attack is that the spacing between the superimposed layers can be increased so as to effect a marked expansion in the crystalline structure. By means of an intercalation or "between the layers" compound formation, subsequent high temperature heating effects a 100-1000 fold greater expansion, producing a wonn-like or vermiform structure with highly active, dendritic, rough surfaces which may then be formed under pressure into a foam material, since the particles have the ability to adhere without a binder due to the large expansion. Sheets, and the like, are formed from the expanded graphite particles by simply increasing the compressive pressure, the density of the formed graphite being related to the applied formation pressure. A more complete description of the method of forming such flexible expanded graphite sheets can be found in U.S. Patent No. 3,404,061.
The flexible expanded graphite sheet product is essentially pure graphite, typically 90-99.9% elemental carbon by weight, with a highly aligned structure. Only naturally occurring minerals (from the natural raw graphite materials) remain as impurities in the product in the form of essentially inert, chemically stable metal oxides and sulfate. The presence of these impurities is not essential to and dots not contn'butt to the cleccical and heat-conducting capabilities of the expanded graphite.
Although any suitablt flexible expanded graphite foil short may ba used in tbt present invention, it is preferred that the charatteri5'tics of the flextbk expanded graphite sheet be equivalent to that provided as Grafoil~ Braid Fleactbk Graphics, mBnu~ctvr-~ed by UCAR Carbon CompBtty. In the first embodiment of the invention, the density of the preferred flcxibk expanded graphite is about $0 to about 90 lbs./~t.3 (about 0.77 to about 1.4 g/cm~, preferably about 70 lbsJfl.3 (about 1.1 g/cm3). In this embodiment, the preferred flexible expanded graphite fofl has an electrical resistivity of about 1 x 10~ to about 10 x 10'~ ohm-in.(about 2.54 x 10~ to about 25.4 x 10'° ohm-cm), preferably about 2.8 x 10'~ to about 7.5 x 10~ ohm-in. (about 7 x 10'~
to about 19 x 10'" ohm-cm) and, more preferably, about 3.1 x 10~ to about 6.5 x 10~ ohm-in (about 7.9 x 10~ to about 16.5 x 10'' ohm-cm), and has a them~al conductivity of about 140 WlM°K at 70°F (21°C) and about 44 W/M°K at 2000°F (1093°C). Because of its excellent heat-conducting properties, flexible expanded graphite has been toed in other applications, such as gaskets, valve stern of pump packings, end high temperature applications, such as thermal radiation shielding, furnaes linings, sad the 1'ke.
The preferred densities, electrical resistivity and resistanCCS; and length, width and thicknessca of the flexible expanded graphite foil layer for use is the zoned de-icing system embodiment of the invention arc disc,issed further below.
'fhc outer heat-conducting layer 22 is preferably a thermoplastic or tbermbsctting material, including rubber or other elastomene materials, that is a thermal conductor and en electrical insulator and is durable and abrasion-resistant Suitable maurials include pol3roretbane,. polyethylene, polyvinyl chloride, polyamides, polystyrenes, and the like. The preferred material is essentially non-efectric~lly conducting, having a volume electrical resistivity of about 103 ohm-in.(2.54 x I 03 ohm-cm) to about 10'2 ohm-in (2.54 x 10'z ohm-cm). The preferred outer heat-conducting layer has a thermal conductivity of about 0.1 wattshnetet°K
(W/M°K) to about 5 W/M°K and, more preferably, about 0.5 W/M°K to about 4 W/M°K. 1n 90 order for the material to be heat-conducting and electrically non-conducing, a heat-conductive inorganic compound or mixrturc of heat-conductive inorganic compounds is typically added as a filler during the manufactwe of the material. Examples of inorganic compounds '' 27175-90 ~17-employed as fillers to confer these properties oa a thermoplastic or thermosetting material are nitrides, such as aluminum nitride and boron nitride, alumina, silicon compounds, and the IOcc. The manufacture of such thermoplastic and thermosetting materials containing these frllers is known ' to those skilled is the art of plastics compounding. Preferably, the heat-conducting outer layer an the heat-conducting tape of the invention comprises a thermoplastic material cantainiag aluminum nitnde, boron nitride, slumina, or mixtura .of these, ~ such as a Staysd'!c"' thermoplastic adhesive, available from Alpha Metals, Inc., Craastoa, RI The thermoplastic material may be in liquid, film or paste form. More preferably, the b~-~aducting outer layer comprises a polyurethane film containing boron nitride or aluminum nitride. Most preferably, the polyurethane contains boron nitride.
The heat-conducting outer layer is preferably laminated under heat and pressure to the flexible expanded graphite layes . to ensure the integrity of the heat transfer between the flexible expanded graphite , foil sheet layer and the protective heat-conducting outer layer. Moreover, lamination of the outer layer to the foil substantial prevents the flaking off of pieces of the foil to assure a continuous foil layer for optimum electrical and heat conductivity. For example, a thermoplastic adhesive (Stayst~7c"') may be suitably laminated to the flexible expanded graphite layer under heat end pressure conditions of 125°C to 200°C and 0 to I 0 psi (0 to 690 hPa), respectively. The thiclmess of the heat-conducting outer layer in the heat-conducting tape is about 0.001 inches to 0.030 inches (about 0.025 mm to 0.76 mm), preferably about 0.001 inches to about 0.010 inches (about 0.025 mm to about 0.254 mm) and, more preferably, about 0.005 inches (about 0.13 mm).
The insulating base Isyer 24 may be comprised of any of a number of materials that arc insulating for electricity, that are flexible enough to facilitate the installation of the heat-conducting tape onto irregularly shaped aircraft surfaces, and can be bonded, such as by as adhesive, to the heat-conducting outer Layer arsdlor to the flexible expanded graphite layer. The electrically insulating material may or may not be also heat insulating. For example, it may be desirable to heat the aircraft surface under the heat-conducting tape. In this case, the sekction of the electrically 3p insulating layer includes a material that is not heat insulating. Suitably insulating base layers include, but are not limited to, elastomers, such as chloroprene, isoprene and neoprene, or thermoplastic or thermosetting materials containing inorganic heat-conducting fiIlcra, alone ar in combination ui'rth insulating fabrics, such as fiberglass cloth. The insulating Isyer has a thickness of about 0.005 inches to O.Z50 inches (about 0.13 mm to 6.35 mm).
As further illustrated in Figure 6, is order to assess, is veal brae, the amount of heat generated on the aircraft surface 30 created by the beat-conducting tape 2, a temperature sensor, such as ane or more thermocouples 32 of known types, may be includod in one or more areas of the beat conducting tape 2.
Temperafiue sensors other than thermocouples may be used is the invention and these are well known to those skilled in the art. In the embodiment of the invention illustrated in Figurx 6, a bondable foil thermocouple 32 is incorporated into the heat-conducting spa at sa outer surface of the outer heat-conducting layer and thermocouple control wires 34 may be .muted with the electrical wires to a coatml system (see below).
Temperature sensors can also be bonded within the heat-conducting tape (not shown). The temperature sensor is ideally thin and flat and can sense temperatures up to 150°C. A suitable thermocouple sensor for use in the invention is a self:
adhesive Omega iran/canstantaa thermal couple (Omega Engineering, Inc., Stamford, .
In order to selectively control the temperature of a surface to which the heat-conducting tape is applied, the flexible expanded graphite layer is connected to s source of electrical energy 50. For example, as illustrated in Figures 6 and 7 ~a flexible expanded graphite layer 20 is connected to the power source SO
using an edge coru~ector or bus bar 40 and wiring system 36. The pliable wiring and bus arrangemem connects the flexible expanded graphite layer to a main grid (not shown). Another edge connector or bus bar 38 is riveted to the metal aircraft section to provide a ground for the electrical circuit Thus, the flexible expanded graphite foil layer is preferably connected to the power source by a single set of two efeetrical terminals. A current is transmitted through the foil by establishing a voltage differential between the terminals, resulting in heating of the foil.
The power source 50 may be one of several types on the aircraft, as is known to those skilled in the art. Light aircra$ may be fitted with a 150 ampere ~ 200 ampere alternator producing 40 to 60 volts, without a significant weight penalty. Suitable alternators are available from ~GC enterprises, ine., Chardon, Ohio. These alternators produce 150 amps at 50 volts and the voltage is selectable.

~ 27175-90~
.19-Tests of the heat-conducting tape of the present invention have determined that an alternator having 3 to 35 volts available can provide a flexible expanded graphite foil heater producing 2 to 15 wattsrm2f031 to 233 watt~/cm2l.
In the first embodiment of the invention, the temperature of the aircraft surface may be controlled by varying the voltage applied to the flcxtble expanded graphite layer of the heat-conducting tape over a fixed or varied amourn of time or by providing a constant voltage for a series of fixed intervals or time.
The amount of wattage supplied to the flexible expanded graphite layer can be varied is response to the outer surface temperature using a prog~~aramable power control logic system 60, such as a microprocessor. as illustrated in Ffgurt 7.
The aircraft may also comprise other sensors, known to those skilled in the art, for sensing icing conditions when ice-producing combinations of ambient ~tcmperature, humidity and dew point exist in the atmosphere encountered by the aircraft, the sensors) being progammed to signal the power source to provide electrical energy to the flexible expanded graphite foil layer of the heat-conducting tape when such icing conditions are sensed.
The invention is now described in detail with regard to a preferred embodiment of the invention employing the heat-conducting tape as a zoned de-icing system for aircraft surfaces including a leading edge. Special requirements for e~cient coned de-icing protection, published by the Federal Aviation Administration (FAA) in Aircraft Icing Handbook, Report fDOT/FAA/CT 88/8-2, 1112-3, include a high specific heat input applied over a short time period;
immediate cessation of heating and rapid cooling of the , surface after ice shedding occurs to greatly reduce runback ice; a minimum size of the heated area so that the heat is applied only under the ice and not dissipated to the sirstream; a proper distribution of heat to produce clean shedding and to avoid runback icing, such that the melting of the ice bond occurs uniformly over the surface; prevention of anchorage of ice by bszdging from one none to another, and a cycle "o$ time" which is controlled to permit adequate ice accretion for the best shedding characteristics. The "off time"
depends upon the thermal capacity of the shedding zone and the rate at which the surface cools to 32°Ff0°C), as well as the icing rate, so that the ice thickness accumulated is the best for shedding when de-icing occurs. It is known from icing tonne) testing that thermal ice protection systems must be on before entering icing conditions, due to the "bridging" characteristics of ice formation and the amount of energy required to remove ice which has already formed on a structural member. It will be appreciated from the following description, that each of these special requirements is met or exceeded by the zoned de-icing system of the invention.
As illustrated in Figure 8, an electrothermal de-icer 70 according to prior art is shown mounted on a structural member 72 in the form of a wing. As is known, the structural member 72 includes a chordwise axis and a spanwise axis.
During flight, the airflow impinges on a leading edge 74 of the structural member 72 and, when icing conditions are encountered, a number of icing stagnation points can develop, forming an ice stagnation line or axis, the position of which vanes with the angle of attack of the leading edge during flight.
In the zoned electrothermal de-icing system embodiment of the present invention, a heat-conducting tape 76 is bonded to the leading edge 74 of an aircraft structure such as a wing 78, as illustrated in Figure 9. The heat-conducting tape comprises at least two layers (not shown) laminated to each other under heat and pressure, i.e., the outer heat-conducting layer described above, and the non-metallic electrically conductive layer consisting of flexible expanded graphite foil.
However, as described below, in this embodiment the flexible expanded graphite foil does not have a uniform thickness. Otherwise, the heat-conducting tape 76 is the same as that illustrated in Figures 4 and 5, and the bonding of the tape to the aircraft structure is the same as that illustrated in Figure 6, including the insulating layer, the thermocouple(s), electrical contacts and connection to a power supply.
The heat-conducting tape may further comprise an additional outer erosion-resistant layer covering the outer heat-conducting layer, such as a layer of titanium, nickel, aluminum, stainless steel, and alloys thereof. Such erosion-resistant layers are known in the art.
In the zoned de-icing system embodiment illustrated in Figure 9, the heat-conducting tape 76 comprises a first area that forms a parting strip 80 having a length disposed spanwise along the leading edge 74 of the structure and having a width sufficient to accommodate a change in the position of the ice stagnation line.
The heat-conducting tape 76 further comprises a second area disposed spanwise ~~ 27175-90 ~l-above and aft of the parting strip 8D forming a first ice accumulation and shedding zone 82, and a third area disposed spanwise below and aft 'of the parting strip 80, forming a second ice accumulation and shedding zone 84. As described in detail below, the thickness of the flexible expanded graphite foil in the parting strip is s always greater than the thickness of the graphite foil is each of the ice accumulation and shedding zones. When a predetermined amount of electric current is transmitted' continuously through the tape, the outer surface of the heat conducting outer layer at the parting strip is heated continuously to a temperature above 32°F
f0°C} to maintain a continuous ice~frec (running wet) condition. At intermittent intervals, as increased electric current is applied to the tape to raise the temperature of the outer surface of the beat-conducting outer layer at the spanwise ice accumulation and shedding Zones above 32°F (0°Cl to melt or loosen accumulated runback ice, which is then aerodynamically removed from the structural member by the airstzram passing aver the aircrai't or by centrifugal forces when the aircraft structure is a propeller or rotor blade.
One embodiment of the flexible expanded graphite foil layer of the heat-conducting tape is illustrated in Figure 10 and in cross-section on the leading edge of a wing 9D in Figures 11 and 12. In this embodiment, the flexible expanded graphite foil 120 is a monolithic, continuous structure which is shaped or "sculptured" to form a first tbickness in the parting strip 122, a second thickness is the fast ice accumulation and shedding zone 123, and a third thickness in the second ice accumulation and sbedding zone 124, wherein the thickness of the foil sheet in the parting strip 122 is greater than the thickness of the foil in each of the first or the second ice accumulation ' and shedding zones. Preferably, the continuous foil shit comprises a decreasing gradient Of thiCkneSSes 125 and 126 between the parting strip 122 and the first ice accumulation and shedding zone 123 and between the parting strip 122 and the second ice accumulation and shedding zone 124, respectively.
The decreasing gradient of thicknesses may be shaped to form an angle or a series of "steps", as illustrated in Figures 10 and 11, or may be smoothly contoured, as illustrated, in Figure 12. Calculations of the thicknesses of the foil in the decreasing thickness gradients represented by the angle, steps or smooth contour are discussed below. The flexible expanded graphite foil layer 120 is disposed between tire ~~ 27175-90 .32-insulating layer 128, which is bonded to the outer surface of the wing 90, and the outer heat-conducting layer 130. In each of Figures 1 l, 12 and Figures 14 and below, the ice stagnation Line 92 is indicated as a broken horizontal line.
In eaother embodiment of the flexible expanded graphite foil Layer . of the zoned de-icing system embodiment illusbatcd in Figure 13 and i~a cross-sectioa on tbc leading edge of a wing 9Q in Figure 14, the parting strip 122 comprises at least two layered flexible expanded graphite fob sheds 132 and 133, having thickacsses that are the same as or different from each other. In this embodiment, bottom layer 134 of foil may have a thiclouss that is the same as or dent from the thickness of the foil in the ice accumulation and shedding areas 123 and 124. The layers are shaped to provide a decreasing gradient of thicknesses 125 and 126 between the parting strip 122 and the fu~st ice accumulation and shedding zone 123 and between the parting strip 122 and the second ice accumulation and shedding zone 124, respectively.
In another embodiment of the flexible expanded graphite foil layer 120 of the zoned de-icing system embodiment illustrated in Figure 15 and in cross-section on the leading edge of a wing 90 in Fi~nre 16, the foil comprises at least two separate sections selected from the group consisting of a parting strip section 142, a first ice accumulation and shedding sertioa 143, and a second ice accumulation and shedding section 144, wherein the first andlar the second ice accumulation and shedding sections are separated from the parting strip section 142 by a gap 140 of no greater than 60 mils f 1.524 mm). A gap of this dimension is acceptable because it allows heat transfer between the separated sections, and it allows the three sections of the foil to be connected to a power supply by a single set of two electric terminals.
In ~o~~r embodiment which may be desirable for testing purposes, cacti of the three sections of the foil may have its own set of two electric terminals for connection to the power supply.
In the illustrated embodiment of Fgnres 15 and lb, both the fast and second ice accumulation and shedding sections are separated from the parting strip section; f~owever, in another embodiment, only one ice accumulation and shedding section may be separated from the parting strip section. The parting strip section 142, or at Least one of the ice accumulation and shedding z~ona sections 143 and 144, " 27175-90 respectively, may comprise a decreasing gradient of thicknessex 145 or 146 between the parting strip and the first ice accumulation and shedding zone or between the paving strip and the second ice accumulation and shedding zone, respectively.
In another embodiment of the zoned de-icing system, the heat-conducting tape 76 is bonded to a leading edge 131 of a helicopter rotor blade or a propeller blade 136, as illustrated in Figure 17. Because of the speed of rotation of the blades during flight, and the centrifugal forces produced, these blades are more susceptible to erosion by the force of the impinging water droplets.
Therefore, it is preferred that, in this embodiment illustrated in Figure 18, an additional outer erosion-resistant layer 135 is bonded to the outer heat-conducting layer, such as by an adhesive. Preferably, the outer erosion-resistant layer is selected from titanium, nickel, aluminum, stainless steel, and alloys thereof. It is to be appreciated that this additional outer erosion-resistant layer may be present when any of the embodiments of the heat-conducting tape illustrated in Figures 10 through 16 is employed on a helicopter rotor blade or a propeller blade.
Any of the embodiments of the tape described above may be produced by calendering, such as that described in U.S. Patent No. 5,198,063 (issued March 30, 1993), or by vertical pressing, as known to one of ordinary skill in the art.
To, determine the desired thicknesses of the flexible expanded graphite foil in the paring zone and in the ice accumulation and shedding zones, the electrical resistance of the foil must be considered. It is known that the electrical resistivity and resistance of flexible expanded graphite along the length and width of the foil varies with both the thickness and the density of the foil.
Therefore, the thickness of flexible expanded graphite foil layer having a known resistivity, density, length (L) and width (W) may be calculated, as follows, where A is the cross-sectional area (W x thickness, T) of the foil:
x resistivity = resistance (R) (1) A
Power (watts) _. I2 (amps)_R (2) Power = I E (voltage) (3) ~' 27175-90 ~24-I ~ FJR and R ~ FJI (4) The value of R may then be used in equation (1) to detenaitu the 3 thickness of the foil by calculating the cross-sectional area A.
For example, a flexible expanded graphite, foil has a density of 90 Ibs,li~
(1.4 g/cm'), a resistivity of 3.1 x 10'' ohm-in. (7.87 x 10'' ohm-cm), a length of 60 inclars (152.4 cm) and a parting strip width of one inch (2.54 cm). From exemplary testing data, such as that descn'bed below, it may be determined that Grafoil~ having an area of 60 in' 1 Q (387.1 cmz) requires 5 wattslin? (0.775 wattslcm~ to maintain a temperature in the parting strip above 32°F (0°G~, e.g., about 34~ (1°G~ to about 37°F (2.8°C). The total wattage necessary is 300 watts. Employing power from a high output attunator, sucl' as that descn'bcd above, an exemplary 25 volt output is selected From the above equations, 300 = I E where E = 25 volts 15 I = 12 amps 300 =122 8 R = 2.08 ohms 60/A x 3.1 x 10~ = 2.08 A = 60 x 3.1 x 10'x/2.08 = 0.009 in2 (0.05806 cm~) A=WxT
T = ~ = 0.009 inches (= 0.0229 cm) The above calculations can also be used to determine the thickness' of the flex5ble expanded graphite foil layer in the ice accumulation and shedding ants. The calculations assume a uniform thickness of the foil, excluding decreasing thickness gradients. In the embodiment of the invention illustrated is Figure 12, wherein the flexible expanded graphite layez is contoured, similar calculations can be used ~to determine optimum thickness gradient using calculus, as known to one of ordinary skill is the art.

As illustrated in Figures 19 and 20, 1" (2.54 cm) wide x 19" (48.26 cm) long test strips of Grafoil~ Brand Flexible Expanded Graphite of 50 (50#), 70 (70#) and 90 (90#) lbs/ft3 (0.77[50], 1.08[70] and 1.39[90] g/cm3) density and varying thicknesses were subjected to a selected power from a DC power supply providing 2 to 6 volts and 4 to 17 amperes. The values for the electrical resistivity and resistance of the Grafoil~ were calculated by known methods, using the equations described above and voltmeter and ammeter measurements. As illustrated in Figure 19, the resistivity of the foil having a density of 90 lbslft3 was substantially constant at between about 3.1 and about 3.4 x 10-4 ohm-in (about 7.87 and about 8.64 x 10-q ohm-cm) over a range of thicknesses of at least 0.005 inches (0.13 mm) to 0.030 inches (0.76 mm). The resistivity of the 70 lbs/ft3 (1.08 glcm3) foil was also in the range of about 3.1 to about 3.7 x 10-4 ohm-in (about 7.87 to about 9.40 x 10-~ ohm-cm) at thicknesses of about 0.005 to about 0.015 inches (about 0.13 to about 0.38 mm). As is apparent from the foregoing calculations for obtaining thickness values for each of the parting strip and ice accumulation and shedding zones, it is preferred that the resistivity of the flexible expanded graphite comprises a substantially constant value, such as about 3.1 x 10-4 ohm-in (7.87 x 10-9 ohm-cm). As illustrated in Figure 20, the resistance of the flexible expanded graphite foil along its length and width decreases with the thickness and the density of the foil. Foil having a density of 70 or 90 lbs/ft3 (1.08 or 1.39 g/cm3) showed a lower resistance over all thicknesses than the 50 lbsfft3 foil, with the 90 lbs/ft3 (1.39 g/cm3) foil showing the greatest decrease in resistance with increasing thickness. The resistance of various thicknesses of Grafoil~
having a density of 70 lbs/ft3 (1.08 g/cm3) and a resistivity of 3.1 x 10-4 ohm-in (7.87 x 10-4 ohm-cm) at various thickness is illustrated in Figure 21. In view of the foregoing " 27175-90 measurements of resistivity and resistance, the density of the flexible expanded graphite foil used for the zoned de-icing system embodiment of the invention may be about 50 to about 95 lbs/ft3 (about 0.77 to about 1.46 g/cm3) but is preferably about 70 to about 95 lbs/ft3 (about 1.08 to about 1.46 g/cm3), more preferably about 80 to about 95 lbs/ft3 (about 1.23 to about 1.46 glcm3) especially about 90 to about 95 lbs/ft3 (about 1.39 to about 1.46 g/cm3).
The heat-up and cool-down times of the flexible expanded graphite foil used in the heat-conducting tape of the invention were compared to heat-up and cool-down times of the conductive ply of an electrothermal heating pad employing metal elements for aircraft de-icing disclosed in U.S. Patent No. 5,344,696 (issued September 6, 1994). The disclosed conductive ply comprises a mat of nickel-coated carbon fibers, VERATEC Grade Number 80000855, available from VERATEC, a division of International Paper Co., Walpole, Mass. Strips of the VERATEC mat (0.003 inches or 0.076 mm thick) or Grafoil~ Brand Flexible Expanded Graphite of varying densities and thicknesses, measuring 2" (5.08 cm) wide x 20" (50.8 cm) long were subjected to 20 watts (0.5 Watts/in2 or 0.0775 Watts/cm2) of power. The time to heat to 125°F (51.7°C) from ambient temperature (82°F or 2?.8°C) in air, with no convection, was measured. When power was removed, the time to cool down from 125°F to 82°F
was measured. As demonstrated by the data in Figures 22, 23 and 24, Grafoil~ having a thickness of 0.005 inches (0.13 mm) and a density of 70 lbs/ft3 (1.08 g/cm3) heats as quickly to 125°F (51.7°C) as 0.003 inch (0.076 mm) thick VERATEC, but cools down about five times faster. Moreover, Grafoil~ having a thickness of 0.005 inches (0.13 mm) and a density of 90 lbs/ft3 heats almost as quickly as the VERATEC, but cools down about twice as quickly.

" 27175-90 -26a-Preliminary bench tests were performed to determine the power requirements for heating Grafoil~ having a density of 70 lbs/ft3 (1.08 g/cm3) or 90 lbs/ft3 (1.39 g/cm3) bonded to a composite laminar flow wing from a Vantage~ aircraft (supplied by VisionAire Corporation, Ames, Iowa). The tape was divided into three 2" (5.08 cm) wide x 20" (50.8 cm) long sections having different thicknesses simulating a parting strip area having a thickness of 0.020 inches (0.51 mm), and each of two ice accumulation and shedding areas having a thickness of 0.005 inches (0.13 mm). The tape was subjected to DC voltage at various amperages, and the temperature of the outer heat-conducting layer of the tape was measured with a thermocouple. The test was conducted at ambient temperature (75°F or 23.9°C) in air with no convection. The results of this test, illustrated in Figure 25, show that for each of the given voltages and amperages, the thicker "parting strip" (checkered ribbon) was hotter than either of the two thinner ice accumulation and shedding areas (hatched ribbons). Moreover, at a power setting of less than 2.55 volts and 10 amps, the "parting strip" was about 20°F
(11.1°C) hotter than the flanking areas. As the power setting increased to 2.78 volts and 12 amps, the difference in temperature widened to about 32°F (17.8°C).
A further bench test was conducted to determine the heat conducted from the parting strip zones to the ice accumulation and shedding zones. In this test, Grafoil~
having a density of 90 lbs/ft3 (1.39 g/cm3) was divided into three 2" x 20" (5.08 cm x 50.8 cm) sections that were separately connected to electrical terminals. The sections were apposed to each other on the Vantage composite wing with a gap of no greater than 60 mils (1.524 mm), with a ~7-parting strip section having a thickness of 0.020 inches (0.51 mm) along the leading cdge, flanked by an ice accumulation and shedding zone section having a thickness of 0.005 inches (0.13 mm) on each side. Each section of the tape was separately connected to electrical terminals. The test was conducted at ambient temperat~ire in air with zero convection. The data in Fignre 26 illustrate the heat transfer test with only the parting strip powered; whereas the data in Flgnre Z7 illustrate the heat transfer test with both the parting strip and the ice accumulation and sheddiag~ zones powered. At each wattage tested, there was significant heat transfer from the parting strip to each of the ice accumulation and shedding zone, when the parting strip only was powered.
Moreover, , when all zones are energized, minimal energy was required for achieving desired temperatures, such as during a short de-icing cycle.
Prcfiminary tests to determine the power requirements for heating the heat-conducting tape having various thieknesses of flexible expanded graphite foil, above 32~ (0~) Wider various environmental conditions, were performed at the Icing Research Tunnel (IRT) at NASA Glenn Research Center, Cleveland, Ohio.
Atmospheric icing conditions were simulated to fall within the FAA guidelines for aircraft de-ice testing (Aircraft Icing Handbook, Report, #lD0?/FAA/CT-8818-2, Appendix C, Figure 3-2, FAR 25 Atmospheric Icing Design Conditions - Stratiform Clouds). The conditions ~e for continuous (s~atifarm clouds) atmospheric icing conditions, with a pressure altitude range of sea level to 22,000 feet (6705.6 m); a maximum vertical extent of 6500 feet (1981.2 m); and a horizontal extent at a standard distance of 17.4 nautical miles (32.2 Ian).
The published Figure 3-2 is reproduced as Figure 28. Environmental conditions in the IRT were selected to fall within approximately the middle of the illustrated envelope.
. The heat-conducting tape comprised Grafoil~ a laminated to a layer taf mineral filled polyurethane material (StaystikTM) having a thickness of 0.003 inches (0.08 mm). The tape was bonded with a rubber backing to one of two aircraft wing models, i.e., a Cessna 182-R wing or a Lancair IV carbon composite wing. For the Cessna wing, the tape was a single sheet, and the density of the Grafoil~ was 70 lbsJft.3 (1.08 glcm'). For the Lancair wing, the tape was divided into three sections, as described below, and the density of the Grafoil~ was 90 lbsJtt.3 (1.39 g/cm3), .28-The location of the seven thermocouples on the leading edge of the Cessna 182-R wing and the six thcsmocouples (at arrow heads) on the Lancait N wing are indicated i~
Figures 29 and 30, respectively. On the Cessna 182-R leading edge, temperature probes #3 and #6 (153 and 156, respectively) are located along the icing stagnation line;
temperature probe ~1 (151) is located inside the leading edge behind the aluminuru;
tempcratun probes #2 (152) and ~l3 (153) are located 2-3/8 inches (6 cm) to the right of and 1-3/8 inches (3.5 cm) to the left of the icing stagnation line, respectively, and 7 inches (17.8 cm) from the bottom of the wing section; and tempeta~re probes #5 (155) and #7 (157) are located 2-3/8 inches (6 cm) to the right of and 1-1/4 inches (3.2 crn) to the left of the icing stagnation line, respectively, and 7 inches (17.8 cm) from the top of ~a wig section. On the Lancair N wing, sets of three thermocouples a1, bl, cl;
a2, b2, cZ; and a3, b3, c3 are located in parting strip zone l, ice accumulation and shedding zone 2 and ice accumulation and sbcdding zone 3, respectively.
The results of initial testing of the heat-conducting tape to determine the Watt densities provided at the location of the thermocouple ~'3 on the Cessna 182-R wing under various environmental conditions are presented fn Fgvre 31. The dimensions of the tape were 6" (15.24 cm) wide x 20" (50.8 cm) long. The Gntfoil~ had a thickness of 0.005 incbes (0.13 mm). . Water droplets were sprayed on the wing at 0°F (-17.8°C) or 20°F (-6.7"C),. at a simulated aircraft speed of 150 miles per hour (mph) (241.4 km/h), to produce a liquid water content (LWC) of 0.75 glm3 or 1.5 g/m3, respectively, and a mean volumetric water drop diameter (MVD) of 40 microns. The conditions of 20°F (-6.7°C), no spray are represented by graph line 1;
0°F are represented by graph line 2; 20°F, 0.75 LWC, 40 MVD arc represented by graph line 3; and 0°F, 1.5 LWC, 40 MVD are represented by graph line 4.
2$ In another test, using the same tape and the Cessna wing, further tests were conducted to determine the power rcquirementt to vise the temperature of the outer surface of the heat-conducting layer of the .tape to various Levels under a set of environmental conditions selected to fall well within the envelope of FAA
regulations in FAR 25, Appendix C of Figure 26.13e results are illustrated in Fwre 3I. For example, under icing tunnel conditions of 26°F (-3,3°C)~ a simulated aircraft speed of 150 mph (241.4 km/h), an LWC of 0.75 glm3, and an MWD of 20 microns, 8 wattsrn2 (1.24 Watts/cm2) were uquired to raise the temperature of the outer surface of the tape to about 62°F (16,7 °C);

whereas 6 Watts/in2 (0.93 Watts/cm2) produced a temperature of about 35°F. Similarly, under icing tunnel conditions of 10°F
(-12.2°C), a simulated aircraft velocity of 150 miles per hour (mph) (241.4 km/h), a liquid water content (LWC) of 0.50 g/m3, and a mean volumetric water drop diameter (MVD) of 20 microns, 13 Watts/in2 (2.02 Watts/cm2) were required to raise the temperature of the outer surface of the heat-conducting layer to about 75°F (23.9°C). whereas 8 Watts/in2 (1.24 Watts/cm2) produced a temperature of about 45°F (7.2°C).
A further test was conducted on the Cessna wing and tape of the previous example to determine the time to heat up (at 7 Watts/in2 or 1.09 Watts/cm2) and cool down when power was turned off, under tunnel conditions of 19°F (-7.2°C), aircraft velocity of 151 mph (241.4 km/h), LWC of 0.75 g/m3 and MVD of 20 microns. As illustrated in Figure 33, the time to heat to about 50°F (10°C) was about 50 seconds, and cool-down to 32°F (0°C) occurred in 5 seconds when power was turned off.
Another test was conducted to determine the watt densities required to raise the temperatures of the first and second ice accumulation and shedding zones, labeled zones 2 and 3, respectively, in Figure 30. Zone 1 is the parting strip. The heat-conducting tape was bonded with a rubber backing to the Lancair IV carbon composite wing. The tape was divided into the three zoned sections, each 2" x 20" (5.08 cm x 50.8 cm), separated from each other by a gap not exceeding 60 mils (1.52 mm). The thickness of the Grafoil~ in zone 1 was 0.020 inches (0.51 mm); and the thickness of the Grafoil~ in each of zones 2 and 3 was 0.005 inches (0.13 mm). Electrical power was applied to each zone separately so that tests using different watt densities in different zones could be accomplished. Three separate environmental conditions were selected, representing conditions within the specified FAR 25, Appendix C, envelope of Figure 28. All tests were conducted at 20 MVD and a simulated aircraft velocity of 150 mph (241.4 km/h). Test 1 was conducted at 26°F (-3.3°C) and 0.75 LWC; test 2 at 10°F (-12.2°C) and 0.50 LWC; and test 2 at 10°F (-12.2°C) and 0.44 LWC. The results, shown in Figure 34, illustrate that watt densities of 4, 8 or Watts/in2 (0.62, 1.24 or 1.55 Watts/cm2) were sufficient to raise the temperature of all zones well above 32°F (0°C).
10 From the accumulated test data described above, it was determined that the flexible expanded graphite foil layer in the heat-conducting tape used in a zone de-icing system embodiment has a thickness in the parting strip, excluding the decreasing thickness gradients, of about 0.005 to about 0.060 inches (about 0.13 to 0.15 mm). Similarly, it was determined that the thicknesses of the flexible expanded graphite foil layer in the first and second ice accumulation and shedding zones, excluding the thickness gradients, are the same as or different from each other and range from about 0.001 to about 0.050 inches (about 0.025 to about 1.27 mm), typically about 0.001 to about 0.030 inches (about 0.025 to 0.76 mm). In the embodiment of the invention, wherein the foil sheets are layered to form a decreasing gradient of thicknesses between the parting strip and the first ice accumulation and shedding zone and between the parting strip and the second ice accumulation and shedding zones, the parting strip comprises at least two layered flexible expanded graphite foil sheets that may have thicknesses that are the same as or different from each other and range from about 0.0025 to about 0.047 inches (about 0.064 to about 1.2 mm). As described above, the maximum thickness of the flexible expanded graphite foil in the parting strip is always greater than the maximum v " 27175-90 -30a-thickness of the foil in both of the ice accumulation and shedding zones.
The width of the heat-conducting tape will be sufficient to accommodate a change in location of an icing stagnation line along the leading edge, will vary with the airfoil leading edge size, configuration, impingement area and expected angle of attack during icing conditions, and will be different for each aircraft. For example, the heat-conducting tape on a horizontal tail stabilizer on a Beechcraft Baron B55 may be 60 inches (152.4 cm) in length and 4 inches (10.2 cm) in width, including a parting strip having a one inch width.
The width of the ice accumulation and shedding areas may be the same or different from each other. Other aircraft, such as those with a very thin leading edge and impingement area (e.g., Lancair IV horizontal tail stabilizers), may require a parting strip 0.25 inches (6.4 mm) in width and a total width of the heat-conducting tape of about 2 inches (5.08 cm).
Moreover, aircraft having a very large leading edge and impingement area (e.g., Cessna 421 wings), may require a parting strip of about 3 inches (7.6 cm) in width and a total width of the heat-conducting tape of about 8 inches (20.32 cm). Thus, the parting strip width may vary from about 0.25 to about 3 inches (about 6.4 mm to about 7.62 cm), typically about 0.5 to about 2.5 inches (about 1.27 cm to about 6.35 cm), and more typically about 0.75 to about 1.5 inches (about 1.91 to about 3.81 cm). Similarly, the widths of the first and second ice accumulation and shedding zones may range from about one to about 6 inches (about 2.54 to about 15.24 cm), typically about 1.5 to about 5 inches (about 3.81 to about 12.7 cm), more typically about one to about 3 inches (about 2.54 to about 7.62 cm). Using the calculations and information provided herein, one skilled in the art will be able to determine the length, width and thicknesses of the heat-conducting tape and the flexible expanded graphite layer for any airfoil leading edge, without undue experimentation.
Regardless of the configuration of the flexible expanded graphite layer of the heat-conducting tape, a further feature of the zoned de-icing embodiment of the invention is that the parting strip and each of the ice accumulation and shedding zones (Zone 1 and Zone 2) of the flexible expanded graphite foil layer are preferably connected to the power source by a single set of two electrical terminals 212 and 214, as illustrated in Figure 42. A suitable electrical connection between the terminals and the power source is illustrated above in Figures 6 and 7. A
current is transmitted through the foil 210 by establishing a voltage differential between the corresponding pair of terminals 212 and 214, resulting in heating of the foil 210. A
given amount of power supplied to the flexible expanded graphite layer results in watt densities that differ between the parting. strip and the ice accumulation and shedding zones, as governed by the predetermined thicknesses in these areas, and the temperature of the parting strip always exceeds that of the ice accumulation and shedding zones. Therefore, only a single control mechanism for a single set of electric terminals is necessary to produce desired watt densities and temperatures in the parting strip and ice accumulation and shedding zones, resulting in zoned de-icing system that is greatly simplified compared to previously known systems.
Moreover, the use of only two terminals results in substantially fewer termination points or contact strips as potential cold spots that could detrimentally become anchor points for ice accumulation.
A system for electrothermally de-icing an aircraft structural member that includes a leading edge is illustrated in Figure 41. A power source 201 on the aircraft energizes the electrothermal de-icing system, as is known to those skilled in the art. In light aircraft, an auxiliary alternator driven by the aircraft engine may serve as the power source 201. As described above, suitable auxiliary alternators for light aircraft are 150 ampere to 200 ampere alternators producing 40 to 60 volts, such as those available from EGC enterprises, inc., Chardon, Ohio. These alternators produce 150 amps at 50 volts, and the voltage is selectable. The alternator is connected to a programmable logic controller (microprocessor) which controls the voltage output of the alternator by a voltage regulator 203. The controller 202 further contains a power sequencer 202 connected to the voltage regulator 203. The voltage regulator 203 is programmed to control the field voltage which, in turn controls the alternator output voltage. The voltage regulator comprises a temperature feedback thermostat (not shown) that controls the output voltage to insure that the impingement zones (parting strips) of the de-icing system maintain a temperature slightly above freezing during or prior to an encounter with icing conditions. As described further below, the controller 202 continually samples static outside air temperature from a thermocouple 205, and compares the impingement zone temperatures and static temperatures to regulate the power output.
As schematically illustrated in Figure 42, a first temperature sensor 218 is in communication with an outer surface of the heat-conducting layer in the parting strip area, and is in communication with the controller 202 for real time transmitting to the controller 202 of a first temperature value representing the real 1 ~ time temperature of the outer surface in the parting strip area of the heat-conducting layer. The controller 202 further comprises a receiving unit for receiving the first real time temperature value. As described further below, the controller is programmed to store a first predetermined reference temperature range for the first temperature; the controller is further programmed to compare the received real time first temperature value with the first predetermined reference temperature range; the controller is further programmed to indicate an acceptable first temperature when the received temperature value falls within the first predetermined reference range; and the controller is further programmed to signal the power source to provide more or less electrical energy to the flexible expanded graphite layer of the heat-conducting tape when the received first temperature falls outside the first predetermined reference temperature range.
Optionally, in addition to the first temperature sensor 218, there may be provided an optional second temperature sensor 216 and/or an optional third temperature sensor 220 in communication with an outer surface of the heat-conducting layer in at least one of the first and second ice accumulation and shedding zones, and in communication with the controller for optional real time transmitting to the controller of an optional second value representing the temperature of the outer surface of at least one of the first and second ice accumulation and shedding zones, wherein the controller further comprises a receiving unit for receiving the second real time temperature value. As described above for control of the temperature of the outer surface of the heat-conducting layer in the parting strip, the controller is programmed to store a second predetermined reference temperature range for the second temperature; the controller is further programmed to compare the received real time second temperature value with the second predetermined reference temperature range; the controller is further programmed to indicate an acceptable second temperature when the received second temperature values fall within the second predetermined reference range; and the controller is further programmed to signal the power source to provide more or less electrical energy to the flexible expanded graphite layer of the heat-conducting tape when the received second temperature falls outside the second predetermined reference temperature range.
The controller may optionally also be programmed to store a third predetermined reference temperature range for a third temperature, which is the temperature of the outer heat-conducting layer at the first and/or second ice accumulation zones as a result of an increased power supply to the flexible expanded graphite foil layer for a period of time sufficient to melt the ice-to-surface bond; the controller is then further programmed to compare the received real time third temperature value with the third predetermined reference temperature range;
the controller is also further programmed to indicate an acceptable third temperature when the received third temperature value falls within the third predetermined reference range; and the controller is also further programmed to signal the power source to provide more or less electrical energy to the flexible expanded graphite layer of the heat-conducting tape when the received third temperature falls outside the third predetermined reference temperature range.
The sequencer 204, illustrated in Figure 41, is within the microprocessor 202 and comprises a voltage sequencer and a timer. The sequencer 204 directs the output of the voltage regulator 203 to aircraft structural members subject to de-icing such as, but not limited to, the tail section 208 and the wing controller section 206. As described further below, the sequencer 204 directs a ' 27175-90 constant voltage during a fast period of time to the flexible expanded graphite foil layer of the heat-conducting tape until a de-iciag sequence takes place. Duriag the de-icing sequence, the sequencer 204 is prograauned to command the voltage regulator 203 to increase the controlled feedback voltage to the auxiliary altcnnator 201, thereby increasing the output voltage, according to a prs-programmed timid sequence.
The sequencer 204 then directs the higher voltage from the voltage regulator 203 to s structural member to lx de-iced, such as the tail section 208, for a regulated period of time. The power is then sequenced to saother structural. member, such as the wing controller 206, for s preselected time to de-ice the wiag: 207.
l0 The wiag controller 206 is programmed to cycle the voltage between the left and right wings 207. This enables the wing surfaces to receive maximum power during the de-ice sequence. The wing controller 206 has a preselected timer which controls the cycles between the left and right wings during the de-icing sequeaee.
While, or preferably prior to, encountering icing conditions, suffcierrt power is supplied from the power source 241 to the flexible expanded graphite foil layer of the heat-conducting tape to maintaia the temperature of the outer heat-conducting layer at the parting strip arcs above freezing for a first time period to prevent the formation of ice on the parting strip. Preferably the temperature is maintained at about 33°F (0.6°C) to about 50°F (10°C), more preferably about 35°F~(1.7°C) to about 45°F (7.2°C). When the heat-conducting tape is to be used in an anti-icing system embodiment, tht temperature of the outer heat-conducting layer is sufficient to produce evaporation of substantially all water droplets impinging on the tape and may thus exceed 45°F
(7.2°C). In the zoned de-icing system embodiment, the temperature of the outer beat-conducting layer at the parting strip during the first time period is sufficient to prevent freezing of substantially all water droplets impinging on the outer heaf-conducting layer of the parting strip, and the water droplets are allowed to flow aft from the parting strip to the fast and/or the second ice accumulation and shedding zones of the heat-conducting tape. In this coned de-icing embodiment, the power supplied to the flexible expanded graphite layer in the first time period also maintains . a temperature of the outer heat-conducting layer at the first and/or the ' ' 27175-90 .35-second ice accumulation and shedding zones that is freezing or below freezing, to allow the water droplets to form ice and an ice-to-surface bond in one or more of these zones.
The length of the first time period will dcpcnd on the rate of icing which, in turn, depends on the environmental conditions, including the temperature, liquid water content, mean volumetric droplet diameter, the velocity of the aircraft, and the lOce. The first time period may be about 5 seconds to about 5 minutes, typically about 15 seconds to about 2 minutes and, more typically, about 30 seconds to about 1.5 minute.
At the end of the first lane period, whey a desired amount of ice has accumulated on the surface of the ice accumulation and shedding zones of the heat conducting tape, the power supplied to the flexible expanded graphite foil layer is subsequently increased and directed to the foil layer for a second period of timt, whet~sio the increased power is suffcient to melt the ice-to-surface bond and to allow formed ix to.be shed into an impinging sirstreasrr. During the second time period, the temperature is typically maintained in a range of about 37°F (2.8°C) to about 40°F (4.4°C), but may be ~ low ~ 34°F (1.1°C). The second time period is about 5 seconds to about 2.5 minutes, typically about 15 seconds to about 1.5 minutes, especially about 30 seconds to about one minute.
Thus, a method for electrothermal de-icing of an aircraft structural member that includes a leading edge subjected to an impinging airstream 'during fligbt, the airstream passing over an outer surface of the structural anember in a fore to aft direction, comprises the steps of (a) bonding a heat-conducting tape to the outer surface of the structural member, wherein the heat-conducting tape comprises a first area that forms a parting strip having a length disposed spanwise along the leading edge, a second ~~ disposed spanwise above and aft of the parting strip forming a first ice accumulation and shedding zone, and a third aJea disposed spanwise below and aft of the parting scrip forming a second ice accumulation and shedding zone, wherein the heat-conducting tape comprises at least two layers laminated to each other under heat and pressure, the layers comprising ~y an outer heat-conducting layer that is an electrical insulator, and (ii) a non-metallic conductive layer connected to a power source by a single xt o~
two terminals, the non-metallic conductive layer consisting of a flexiblt expanded graphite foil sheet " 27175-90 having a first thickness is the parting strip, a second thic>atcs': is the fast ice accumulation nod shedding zone, cad a third thidcn~ss is the second ice accu~mviatioa and shulding Zone, whercia the thiclrne:: oaf the fladble acpaaded graphite foil sheet in the parting strip is gtcatex' than the thick of ~e fob sheet in citbrr of the first or the second is accumulation cad shcdd'tng zones; (b) supplying power to the flcx~'ble expanded graphite fob layer for a first period of time to maintain a fu~st temperature of the outer heat-condactiag lays at the parting strip that is sufficient to prevesrt froezing of imping'mg .water droplrts and to allow water droplets to flow aft from the parting strip ~so the fast andlos tht second ix accumulation and shedding .zones, wherein the power supply also roaiatains, in the first period of time, s second tcmpcnture of tht outer heat-conducting layer at the first andlor second ice accumulation and shedding zones which is freeing as below frecang, to allow formation of ice and as iccto-surface hood on the outtc 6e.~
conducting layer at the first andlor second ics accumulation and .shedding zones; and (e) subsequently increasing the power supply to the flexible expanded graphito fob layer for a second period of time, whcrtin the increased power supply is su8icient to melt the ice.to-.urface bond at the fast andlor second' ice acctunulation sad shedding zones and to allow formed ice to be shed into the impinging sitstream.
Preferably, the method further comprises the step of sensing the presence of atmospheric icing conditions prior to the supplying power atop and, more prcfuably, further comprises tht Mep of sensing tae presence of atmospheric icing conditions during the supplying power and the increasing the power supply steps. The method also preferably comprises' repeating the supplying pmw~r step and the subsequent increasing power step until atmospheric, icing conditions are no longer sensed.
It is a further feature of the embodiments of the invention, that when the temperature is raised to above 3Z°F (0°C) in the ice accumulation and shedding areas, the watt density in the parting strip is also raised, t.g, to ?2.4 wattsfm.2.
(3,d7 Wattslcm2 from 5 wattsfm Z (0.775 W/emz) with a concomitant rapid rise ~in temperature (t.~., to about 80°F (26.7°C) to about 90°F (32.2°C) ) in the parting strip. Because of the high thermal conductivity of the flexible expanded graphite foil, heat transfer from the parting strip to the ice accumulation '' 27175-90 nerd shedding zones occurs, resulting is a decrEased requirement for power to raise the temperature in these zones, than would ordinary be expected The heat transfer fr~p the parting strip to the iu accumulation and ahcdding zones is also eahancod because of the gradient of current flowing through the thidaness gradients bctvvtea the parting strip and S these zones.
The following example t7lustratcs the de-icing capabilities of the toned de-icing system ecabodimcat of the heat conducting tape of the invention, The exempla is not to be considered limiting, however, as other heat conducting outer Lyecs, adhcs'rves, inaulatiag laycra, thiclaocasc: of these layers, and densities and dimensions of the flexible expanded graphite foil layer, and the lOce, may be nsod is the pracxice of the is invention.
The de-icing test was conducted of the king Research lbnael of NASA
Glenn Research is Cleveland, Ohio.
Example 1 15 The heatconductiag tape comprised Grafoil~ baying a deruity of 9Q
IbsJft.3 (1.39 g/cm3) larriinatcd to a layer of minual filled polyurethane material (Staysulc~) having a thickness of 0.003 ieches (0.076 mm). The tape was bonded with a rubber bscldng by a rubbcr~based contact adhesive (l 300-L, 3M Computy) forming an adhesive layer of about 0.010 inches (0.25 mm) ~i~: to the Laacair IV carbon .
20 . composite wing. The tape was divided into the three zoned sections, each 2" x 20" .(5.08 x 50.8 cm), separated from each other by a gap not exceeding b0 mils (1.52 mm).
Zone 1 was the parting strip, having a Cxrafoil~ thicloness of 0.020 inches (0.5 mm).
Zones 2 and 3 were the lower and upper ice accumulation and shedding zones, respectively, having a Grafoil~ thickness of 0.005 inches (0.13 mm). Electrical power was applied to cash zone 25 . separately so that tests using different watt densities in different zones could be accomplished. Three separate environmental conditions were selocted, representing conditions within the specified FAR 25, Appendix C, envelope of Figure Z8.
A single de-ice sequence ~~aa run oa the zoned deicing system. Tho 30 electrical current provided was 8 wad: The environmcnuf conditions vueTe 10°F
(-12.2°C), a simulated aircraft velocity of 150 mph (241.4 lan/h), an LWC of 0.50 and an MVD
of 20.61. The power on-off sequence is illustrated in Figures 35, 37 and 39, with the graphs 38~
indicating the temperature of the outer heat conducting layer of the tape and wattrJ'ta. s at the parting strip (zone 1) and at ice accumulation and shedding zone (zone 3).
The corresponding actual runback icing accumulation and ice shedding on the outer surface of the heat-conducting tape bonded to the wing segment arc illustrated is Figures 36, 38 and 40. 'The parting strip zone h is to the right of the picture and the impinging airstr~eam is moving from right to leR Zone 3 is in the center of the picture.
At the beginning of the de-icing sequence, at tune 14 hours, 20 minutes, zero seconds (14:20:00), shown in Figure 35, the parting strip and zone 3 have already been heated to 70°F (21.1°C) and 35°F (1.7°C), respectively. At time 14:20:00, the power is turned off is zone 3 and the watt density in zone 3 is zero. (T'he power is maintained at 8 watts in the parting strip, zone 1, at all times). During the 15 second duration of power o$ the temperature of zone 3 falls to below 32°F (0°C), (i.e., about 11°F (-11,7°C). At time 14:20:15 runback ice has begun to freeze on the outer surface of zone 3, as is seen in Figure 36. In Figure 37, the power is turned on in zone 3 at time 14:20:30 to achieve 5 wattslin.2 (0.78 Watts/cm2). By time 14:21:13, ice has already begun to shed from zone 3 (Figure 38) and by time 14:21:26 (Figure 39), the ice has been shed from zone 3 into the ianpingiag airstrcam (Figure 40).
While the invention has been descn'bed herein with reference to the preferred embodiments, it is to be understood that it is not intended to limit the invention to the specific forms disclosed.

Claims (55)

CLAIMS:

1. An electrothermal de-icing system for an aircraft structural member (72,78,106,108,136) that includes a leading edge (74,104,134) subject to an impinging airstream during flight, said airstream passing over an outer surface of the structural member (72,78,106,108,136) in a fore to aft direction, the system comprising a power source (50,201) and being characterized by further comprising a heat-conducting tape (2,76) bonded to the outer surface of the structural member (72,78,106,108,136), the heat-conducting tape (2,76) comprising a first area that forms a parting strip (80,122,142) having a length disposed spanwise along the leading edge (74,104,134), a second area disposed spanwise above and aft of the parting strip (80,122,142) forming a first ice accumulation and shedding zone (82,123,143), and a third area disposed spanwise below and aft of the parting strip (80,122,142) forming a second ice accumulation and shedding zone (84,124,144), wherein the heat-conducting tape (2,76) comprises at least two layers laminated to each other under heat and pressure, the layers comprising (i) an outer heat-conducting layer (22,130) that is an electrical insulator, and (ii) a nonmetallic conductive layer (20,120) connected to the power source (50,201), the non-metallic conductive layer consisting of a flexible expanded graphite foil sheet (132,133) having a first thickness in the parting strip (80,122,142), a second thickness in the first ice accumulation and shedding zone (82,123,143), and a third thickness in the second ice accumulation and shedding zone (84,124,144), wherein the thickness of the flexible expanded graphite foil sheet (132,133) in the parting strip (80,122,142) is greater than the thickness of the foil sheet in either of the first or the second ice accumulation and shedding zones;

an electronic connection (40) for connecting the flexible expanded graphite layer to the power source (50,201); and a programmable controller (60,202) configured (i) to direct a supply of power from the power source (50,201) to the flexible expanded graphite foil layer (20,120) of the heat-conducting tape (2,76) for a first period of time, to maintain a first temperature of the outer heat-conducting layer (22,130) at the parting strip (80,122,142) to prevent freezing of impinging water droplets and to allow water droplets to flow aft from the parting strip (80,122,142) to the first and/or the second ice accumulation and shedding zones (82,84,122,123,143,144) of the heat-conducting tape (2,76) and wherein the power supply also maintains, in the first period of time, a second temperature of the outer heat-conducting layer (22,130) at the first and/or second ice accumulation and shedding zones (82,84,122,123,143,144) which is freezing or below freezing to allow formation of ice and an ice-to-surface bond on the outer heat-conducting layer (22,130) at the first and/or second ice accumulation and shedding zones (82,84,122,123,143,144), and (ii) to subsequently direct an increased power supply to the flexible expanded graphite foil layer (20,120) of the heat-conducting tape (2,76) for a second period of time, wherein the increased power supply melts the ice-to-surface bond at the first and/or second ice accumulation and shedding zones (82,84,122,123,143,144) and allows formed ice to be shed into the impinging airstream.

2. The electrothermal de-icing system of claim 1, wherein the flexible expanded graphite foil sheet (132,133) is connected to the power source by a single set of two terminals.

3. The electrothermal de-icing system of claim 1, wherein the first temperature is 0.6°C to 10°C.

4. The electrothermal de-icing system of claim 3, wherein the first temperature is 1.7°C to 7.2°C.

5. The electrothermal de-icing system of claim 1, wherein the first time period is 5 seconds to 5 minutes.

6. The electrothermal de-icing system of claim 5, wherein the first time period is 15 seconds to 2 minutes.

7. The electrothermal de-icing system of claim 6, wherein the first time period is 30 seconds to 1.5 minutes.

8. The electrothermal de-icing system of claim 1, wherein the second time period is 5 seconds to 2.5 minutes.

9. The electrothermal de-icing system of claim 8, wherein the second time period is 15 seconds to 1.5 minutes.

10. The electrothermal de-icing system of claim 9, wherein the second time period is 30 seconds to one minute.

11. The electrothermal de-icing system of claim 1, further comprising:
a first temperature sensor (218) in communication with an outer surface of the heat-conducting layer (22,130) in the parting strip area (142) and in communication with the controller (60,202) for real time transmitting to the controller (60,202) of a first value representing the temperature of the outer surface in the parting strip area (142) of the heat-conducting layer (22,130), wherein the controller (60,202) further comprises a receiving unit for receiving the real time first temperature value; wherein the controller (60,202) is programmed to store a first predetermined reference temperature range for the first temperature; the controller (60,202) is further programmed to compare the received real time first temperature value with the first predetermined reference temperature range;
the controller (60,202) is further programmed to indicate an acceptable first temperature when the received temperature value falls within the first predetermined reference range;
and the controller (60,202) is further programmed to signal the power source (201) to provide more or less electrical energy to the flexible expanded graphite layer (20,120) of the heat-conducting tape (2,76) when the received first temperature falls outside the first predetermined reference temperature range.

12. The electrothermal de-icing system of claim 11, further comprising:
a second temperature sensor (216) in communication with an outer surface of the heat-conducting layer in at least one of the first and second ice accumulation and shedding zones (82,84,122,123,143,144), and in communication with the controller (60,202) for real time transmitting to the controller (60,202) of a second value representing the temperature of the outer surface of at least one of the first and second ice accumulation and shedding zones (82,84,122,123,143,144), wherein the controller (60,202) further comprises a receiving unit for receiving the real time second temperature value, wherein the controller (60,202) is programmed to store a second predetermined reference temperature range for the second temperature; the controller (60,202) is further programmed to compare the received real time second temperature value with the second predetermined reference temperature range; the controller (60,202) is further programmed to indicate an acceptable second temperature when the received temperature value falls within the second predetermined reference range; and the controller (60,202) is further programmed to signal the power source (50,201) to provide more or less electrical energy to the flexible expanded graphite layer (20,120) of the heat-conducting tape (2,76) when the received second temperature falls outside the second predetermined reference temperature range.

13. The electrothermal de-icing system of claim 12, wherein the second temperature sensor (216) transmits to the controller (60,202) a third real time temperature of the outer heat-conducting layer (22,130) at the first and/or second ice accumulation zones (82,84,122,123,143,144) in the second period of time; the controller (60,202) is programmed to receive the real time third temperature value; the controller (60,202) is programmed to store a third predetermined reference temperature range for the third temperature; the controller (60,202) is programmed to compare the received real time third temperature value with the third predetermined reference temperature range; the controller (60,202) is further programmed to indicate an acceptable third temperature when the received third real time temperature value falls within the third predetermined reference range: and the controller (60,202) is also further programmed to signal the power source (50,201) to provide more or less electrical energy to the flexible expanded graphite layer (20,120) of the heat-conducting tape (2,76) when the received third temperature falls outside the third predetermined reference temperature range.

14. The electrothermal de-icing system of claim 13, wherein the third real time temperature is 1.1°C to 4.4°C.

15. The electrothermal de-icing system of claim 1, wherein the flexible expanded graphite foil sheet (132,133) is a continuous sheet comprising a decreasing gradient of thicknesses between the parting strip (80,122,142) and the first ice accumulation and shedding zone (82,123,143) and between the parting strip (80,122,142) and the second ice accumulation and shedding zone (84,124,144).

16. The electrothermal de-icing system of claim 15, wherein the thickness of the flexible expanded graphite foil sheet (20,120) in the parting strip (80,122,142), excluding the decreasing thickness gradients, is 0.13 to 1.52 mm.

17. The electrothermal de-icing system of claim 15, wherein the thicknesses of the flexible expanded graphite foil sheet (20,120) in the first and second ice accumulation and shedding zones (82,84,123,124,143,144), excluding the thickness gradients, are the same as or different from each other and range from 0.025 to 1.27 mm.

18. The electrothermal de-icing system of claim 17, wherein the thicknesses of the foil sheet in the ice accumulation and shedding zones (82,84,123,124,143,144) range from 0.025 to 0.76 mm.

19. The electrothermal de-icing system of claim 1, wherein the parting strip (80,122,142) comprises at least two layered flexible expanded graphite foil sheets (132,133).

20. The electrothermal de-icing system of claim 19, wherein the foil sheets have thicknesses that are the same as or different from each other and range from 0.064 to 1.19 mm.

21. The electrothermal de-icing system of claim 19, wherein the layered foil sheets form a decreasing gradient of thicknesses between the parting strip (80,122,142) and the first ice accumulation and shedding zone (82,123,143) and between the parting strip (80,122,142) and the second ice accumulation and shedding zone (84,124,144).

22. The electrothermal de-icing system of claim 1, wherein the flexible expanded,graphite foil sheet (132,133) comprises at least two separate sections selected from the group consisting of a parting strip section (142), a first ice accumulation and shedding section (143), and a second ice accumulation and shedding section (144), wherein the first or the second ice accumulation and shedding sections are separated from the parting strip section by a gap of no greater than 1.524 mm.

23. The electrothermal de-icing system of claim 22, wherein the parting strip section (80,122,142) or at least one of the ice accumulation and shedding zone sections of the flexible expanded graphite foil sheet (132,133) comprises a decreasing gradient of thicknesses between the parting strip (80,122,142) and the first ice accumulation and shedding zone (82,123,143) or between the parting strip (80,122,142) and the second ice accumulation and shedding zone (84,124,144).

24. The electrothermal de-icing system of claim 1, wherein the parting strip (80,122,142) has a width to accommodate a change in location of an icing stagnation line along the leading edge.

25. The electrothermal de-icing system of claim 24, wherein the width of the parting strip (80,122,142) is 6.35 to 76.2 mm.

26. The electrothermal de-icing system of claim 25, wherein the width of the parting strip (80,122,142) is 12.7 to 63.5 mm.

27. The electrothermal de-icing system of claim 26, wherein the width of the parting strip (80,122,142) is 19 to 38.1 mm.

28. The electrothermal de-icing system of claim 1, wherein each of the first and second ice accumulation and shedding zones (82,84,122,123,143,144) comprises an area approximately equal to its respective area of ice accumulation.

29. The electrothermal de-icing system of claim 1, wherein the first and the second ice accumulation and shedding zones (82,84,122,123,143,144) comprise widths that are the same as or different from each other and range from 2.54 to 15.24 cm.

30. The electrothermal de-icing system of claim 29, wherein the widths of the first and the second ice accumulation and shedding zones (82,84,122,123,143,144) range from 3.8 to 12.7 cm.

31. The electrothermal de-icing system of claim 30, wherein the widths of the first and the second ice accumulation and shedding zones (82,84,122,123,143,144) range from 2.54 to 7.62 cm.

32. The electrothermal de-icing system of claim 1, wherein the flexible expanded graphite foil sheet (132,133) has a density of 0.77 to 1.46 g/cm3.

33. The electrothermal de-icing system of claim 32, wherein the flexible expanded graphite foil sheet (132,133) has a density of 1.08 to 1.46 g/cm3.

34. The electrothermal de-icing system of claim 33, wherein the flexible expanded graphite foil sheet (132,133) has a density of 1.23 to 1.46 g/cm3.

35. The electrothermal de-icing system of claim 1, wherein the flexible expanded graphite foil sheet (132,133) has an electrical resistivity of 6.86×10 -4 to 8.13×10 -4 ohm-cm.

36. The electrothermal de-icing system of claim 35, wherein the flexible expanded graphite foil sheet (132,133) has an electrical resistivity of 7.87 × 10 -4 ohm-cm.

37. The electrothermal de-icing system of claim 1, wherein the outer heat-conducting layer is selected from electrically insulating materials having a volume resistivity of 2.54 × 10 3 ohm-cm to about 2.54 × 10 12 ohm-cm.

38. The electrothermal de-icing system of claim 37, wherein the outer heat-conducting layer (22,130) comprises a thermoplastic or a thermosetting material and an inorganic filler that conducts heat.

39. The electrothermal de-icing system of claim 38, wherein the inorganic filler is selected from the group consisting of aluminum nitride, boron nitride, alumina, silicon nitride, and mixtures thereof.

40. The electrothermal de-icing system of claim 39, wherein the material comprises polyurethane.

41. The electrothermal de-icing system of claim 1, wherein the outer heat-conducting layer (22,130) has a thermal conductivity of 0.1 W/M°K to 5 W/M°K.

42. The electrothermal de-icing system of claim 41, wherein the outer heat-conducting layer (22,130) has a thermal conductivity of 0.5 W/M°K to 4 W/M°K.

43. The electrothermal de-icing system of claim 1, wherein the thickness of the outer heat-conducting layer (22,130) is 0.025 mm to 0.76 mm.

44. The electrothermal de-icing system of claim 43, wherein the thickness of the outer heat-conducting layer (22,130) is 0.025 mm to 0.25 mm.

45. The electrothermal de-icing system of claim 44, wherein the thickness of the outer heat-conducting layer (22, 130) is 0. 13 mm.

46. The electrothermal de-icing system of claim 1, wherein the heat-conducting tape (2,76) further comprises an electrically insulating layer (128), wherein the flexible expanded graphite layer (20,120) is disposed between the outer heat-conducting layer (22,130) and the insulating layer (128).

47. The electrothermal de-icing system of claim 46, wherein the insulating layer is a component of the heat-conducting tape (2,76) or is a component of the aircraft surface.

48. The electrothermal de-icing system of claim 46, wherein the insulating layer (128) is a heat insulator.

49. The electrothermal de-icing system of claim 46, wherein the insulating layer (128) has a thickness of 0.13 mm to 6.35 mm.

50. The electrothermal de-icing system of claim 1, wherein the structural member (72,78,106,108,136) comprises a rotor blade or a propeller blade and the heat-conducting tape (2,76) further comprises an outer erosion-resistant layer bonded to the outer heat-conducting layer (22,130).

51. The electrothermal de-icing system of claim 50, wherein the outer erosion-resistant layer comprises a selection from the group consisting of titanium, nickel, aluminum, stainless steel, and alloys thereof.

52. A method for electrothermal de-icing of an aircraft structural member (72,78,106,108,136) that includes a leading edge subjected to an impinging airstream during flight, said airstream passing over an outer surface of the structural member (72,78,106,108,136) in a fore to aft direction, the method characterized by comprising the steps of:
bonding a heat-conducting tape (2,76) to the outer surface of the structural member (72,78,106,108,136), wherein the heat-conducting tape (2,76) comprises a first area that forms a parting strip (80,122,142) having a length disposed spanwise along the leading edge (74,104,131), a second area disposed spanwise above and aft of the parting strip (80,122,142) forming a first ice accumulation and shedding zone (82,123,143), and a third area disposed spanwise below and aft of the parting strip (80,122,142) forming a second ice accumulation and shedding zone (84,124,144), wherein the heat-conducting tape (2,76) comprises at least two layers laminated to each other under heat and pressure, the layers comprising (i) an outer heat-conducting layer (22,24,130) that is an electrical insulator, and (ii) a non-metallic conductive layer (20,120) connected to a power source (50,201) by a single set of two terminals (40,212,214), the non-metallic conductive layer (20,120) consisting of a flexible expanded graphite foil sheet (132,133) having a first thickness in the parting strip (80,122,142), a second thickness in the first ice accumulation and shedding zone (82,123,143), and a third thickness in the second ice accumulation and shedding zone (84,124,144), wherein the thickness of the flexible expanded graphite foil sheet (132,133) in the parting strip (80,122,142) is greater than the thickness of the foil sheet in either of the first or the second ice accumulation and shedding zones (82,84,123,124,143,144);
supplying power to the flexible expanded graphite foil layer (20,120) for a first period of time to maintain a first temperature of the outer heat-conducting layer (22,130) at the parting strip (80,122,142) to prevent freezing of impinging water droplets and to allow water droplets to flow aft from the parting strip (80,122,142) to the first and/or the second ice accumulation and shedding zones (82,84,123,124,143,144), wherein the power supply also maintains, in the first period of time, a second temperature of the outer heat-conducting layer (22,130) at the first and/or second ice accumulation and shedding zones (82,84,123,124,143,144) which is freezing or below freezing, to allow formation of ice and an ice-to-surface bond on the outer heat-conducting layer (22,130) at the first and/or second ice accumulation and shedding zones (82,84,123,124,143,144); and subsequently increasing the power supply to the flexible expanded graphite foil layer (20,120) for a second period of time, wherein the increased power supply melts the ice-to-surface bond at the first and/or second ice accumulation and shedding zones (82,84,123,124,143,144) and allows formed ice to be shed into the impinging airstream.

53. The method of claim 52, further comprising the step of sensing the presence of atmospheric icing conditions prior to the supplying power step.

54. The method of claim 53, further comprising the step of sensing the presence of atmospheric icing conditions during the supplying power and the increasing the power supply steps.

55. The method of claim 53, further comprising the step of repeating the supplying power step and the subsequent increasing power step until atmospheric icing conditions are no longer sensed.