a. Field of the Invention
This invention relates to heating elements, and more specifically to an electrically conductive laminate heater element for use as an ice protection system to withstand repeated mechanical stresses and thermal cycles in extremely harsh aerospace applications.
b. Description of Related Art
Under certain flying and weather conditions, aircraft may be troubled by accumulations of ice on aerodynamic and structural components of the aircraft. If the ice is not properly eliminated from the aircraft, the aircraft's flying capabilities may be severely limited. The ice may alter the airfoil configuration of the wings to cause an unflyable condition; or accumulated ice may separate from the aircraft during take-off or during flight. The flying ice may jeopardize the mechanical integrity of the aircraft or be ingested into the engines and possibly cause engine failure.
Typical ice protection systems include hot gas chemical fluid, mechanical and electro-thermal systems. Hot gas systems are usually designed directly into the structure of the aircraft and utilize hot air bled from the engine as a source of heat. The practicality, however, of hot gas systems is diminishing with the introduction of high efficiency engines.
Chemical fluid ice protection is accomplished by dispensing a fluid onto the surface of potential ice buildup. The fluids reduce adhesion forces associated with ice accumulation upon the aircraft or lower the freezing temperature of the water below 32.degree., so ice is not formed. Chemical fluid systems are deficient in that they are time limited, expensive and present potential environmental concerns.
Typical mechanical systems employ some type of pneumatic device installed on the leading edge of a wing or strut that expands to crack accumulating ice. Mechanical systems require high maintenance, have a limited life and may effect the aerodynamics of the aircraft.
The use of electro-thermal heating systems as a means of protecting aircraft from harmful buildups of ice is well-known and is becoming increasingly more attractive. Most electro-thermal systems available today use a conductive metal heater element which, due to its small effective cross-section, converts electrical energy into heat energy. Unfortunately, the materials which have traditionally been used in aerospace heaters are not always the best suited to the application. Much of the aerospace environment is harsh and extreme in its treatment of these heaters. The most common metal used in heaters, copper, is not particularly strong or corrosion resistant.
In the most common case of copper heating elements, long narrow strips of very thin copper foil are laid down in a pattern so as to attempt to create a region of heating. The area covered by the strip is small in comparison to the total heated area. The result is a wide range of temperatures and heat flows. Between heater strips there is no heat generated. In some applications, this is one of the primary reasons for covering the outer surface of the heater with metal sheathing-to allow the heat to more easily spread.
The copper foil heater is also prone to catastrophic failure due to localized damage. Since the foil strip is continuous over a large area of the heater, any break at any location would result in total heater failure. This is a common problem among the currently manufactured aerospace heaters, especially when used on propellers. The propeller application submits the heater to a high degree of foreign object damage due to the suction effect of the blades. Stones, small objects, and sand are continually being drawn into the propeller arc, damaging the blade heaters.
Flame sprayed metals have also been used in a number of applications to produce a workable heater element. These heaters depend on the high temperature deposition of very thin layers of metal onto non-metallic surfaces. The nature of the process makes it very difficult to control the thickness of the deposited layer. This presents considerably difficulty in controlling the resistance of the heater, and thus the total power generated. Also, the tendency for delamination between metal and plastics is well documented.
An alternative to the aforementioned ice protection systems are fiber-based heating elements as disclosed in U.S. Pat. No. 4,250,397 to Gray et al., U.S. Pat. No. 4,534,886 to Kraus et al. and U.S. Pat. No. 4,942,078 to Newman et al.
Kraus and Gray disclose methods of processing these materials into finished products using high volume/low cost manufacturing. The intended applications for these fiber heaters focus upon consumer, industrial and agricultural areas. While these concepts of heater design may prove to be adequate for simple low-cost applications, they have been found to be less than satisfactory for applications that demand reliability. Further, the materials discussed in the above cited disclosures are not suitable for extended use in harsh environments.
Because of the shortcomings in existing fiber heater technologies, considerable research effort has been expended in developing improved material combinations. Research conducted by NASA has shown that conductive fiber heaters can suffer from problems of corrosion. The corrosion found during this research occurred at the junction between the fibers and the conducting materials used to apply power to the heater. Kraus et al. and Gray et al. disclose the use of non-conductive materials in the joining of the fibers to conductive strips and the use of weaving or stitching fibers into conductive strips. These methods contribute to corrosion and low reliability of connections.
The primary concern with the previously proposed methods relates to the lack of controlled contact between the conductive strips and the non-woven element. To guarantee that an electrical connection will remain low in resistance and high in reliability, the primary requirement is to maintain a gas-tight contact surface. If air is allowed to come in contact with the connection surface, oxidization will occur, increasing the contact resistance and lowering the reliability of the connection. Normally, in electrical connectors, this gas-tight requirement is met by using a clamping force to maintain an undisturbed connection surface. In the case of an ice protection heater assembly, no such clamping may be used as any such device would significantly increase the overall thickness of the heater assembly, making the heaters unsuitable for external aircraft applications. Additionally, the fibers are too brittle to clamp in place since the exclusion of air is dependent on plastically deforming both of the clamped materials.
Thus, methods of excluding air and other oxidizing gases has focused on adhesive bonding. Adhesive bonding was suggested by Kraus et al., but the adhesives cited were all non-conductive, leading to less than suitable results. It was found during testing that non-conductive adhesives would allow individual carbon fibers to contact the conductive strip (usually copper foil) in discrete locations. These small contact points, having very high current density, create "hot spots" and have been seen to burn out at relatively low power levels. The fibers were observed to "burn out" with a flash or spark. Initial attempts at using conductive adhesives were also disappointing. The first adhesives used contained a relatively small number of relatively large conductive particles, mixed with an epoxy adhesive. Some particles were inert materials with conductive coatings. In each case, the localized visible effect was observed as individual fibers burned out. Some of these adhesives used copper-based particles which would have been unsuitable from a corrosion perspective in any case. Electrically conductive pressure-sensitive tapes resulted in similar fiber failure. The focus of some of the developmental efforts has been in finding appropriate adhesives which will increase the contact surface area and still be compatible with the fiber materials.
High performance ice protection systems also require highly controlled electrical and thermal characteristics. The power density (watts per square inch) requirements for deicing or anti-icing applications vary significantly from application to application. Even within a specific application there is often the need to locally increase the power density. The ability to control these characteristics is dependent on controlling the material characteristics and manufacturing processes of the fiber. Existing work in this field has suggested that specific combinations of fibers and conductive particles are required to produce even heating. By improving the methods of manufacture, however, it has been found that the consistency of heating can be obtained without the need to include conductive particles.
The resistance of a fibrous heating element is based on the summation of each fiber resistance and the resistance of the fiber-to-fiber contact points. In a non-woven conductive fabric, the fibers are of identical composition and evenly dispersed through the fabric. If the fiber-to-fiber connections had zero resistance, the resistance of the fabric would be highly predictable, based only on fiber composition and content. If this were the case, it would be possible to produce consistent resistive heaters using any of the methods proposed by Kraus et al. and Gray et al. in their respective patents.
Unfortunately, the contact points between the fibers are not well controlled prior to fabrication into a finished heater element. The resistance of a fiber-to-fiber connection is similar to the resistance of traditional electrical connections. The ideal electrical connection, for minimum resistance and maximum reliability, must have fairly high contact stresses. These contact stresses are normally termed Hertzian stresses. They relate to the degree of intimate contact between the two conductors. When the fabric is in the dry, as-manufactured state, the fiber-to-fiber contacts vary from no connection to intimate connection. The binding agent on the fabric, required for mechanical handling, does hold some fibers in contact, preventing extremely high resistance to occur. The addition of conductive particles increases the number of contact points by acting as electrical bridges between fibers. This results in a more even resistance across the heater fabric.
Proposed methods of manufacture for fiber-based heaters have included the immersion in, or dispensing of, liquid adhesives. These adhesives are intended to bind the heater element to a resilient mechanical carrier sheet or insulator. Since these methods do not control fiber-to-fiber contact, they have been found to be unacceptable.