Apparatus for providing a lightning protective vehicle surface
STATEMENT OF GOVERNMENT INTEREST 
     The government has rights in this invention. 
    
    
     DESCRIPTION 
     1. Technical Field 
     The present invention is directed toward materials for use as the surface of a vehicle body, and more particularly, toward an improved composite material to provide a lightning protective skin member for a vehicle body. 
     2. Background of the Invention 
     Lightning protection has long been an area of concern for moving vehicles and particularly for aircraft. In addition to providing a lightning protective outer surface, the material needs to meet other requirements of light weight and strength. Many materials have been proposed for providing a structural surface for aircraft which will meet the requirements for lightning protection as well as being strong and lightweight. Among these are composite materials. 
     Research and development with regard to composite materials have resulted in an increased usage of these materials in aircraft. Composite materials have a high strength-to-weight ratio and can provide a reduction in the empty weight of an aircraft, which results in increased payload capacity. However, the use of composite materials on exterior or lightning-accessible surfaces has presented a special problem with regard to lightning attachment. 
     Many presently suggested composite materials have electrical conductivities and therefore do not meet requirements for dissipating electrical current caused by a lightning strike. Others have suggested use of a wire mesh to overlay the composite material. While this approach improves the ability of the surface material to dissipate lightning currents, the increase in weight (and concomitant decrease in payload) does not justify use of these materials over more common surface materials, such as aluminum. 
     It is desirable, therefore, to provide a surface material for aircraft which is lightweight and strong and meets requirements for dissipating lightning currents. 
     DISCLOSURE OF THE INVENTION 
     The present invention provides improved lightning protective material for use as the skin member of a moving vehicle. The lightning protective skin member comprises first and second structural panels constructed of a fibrous graphite material in combination with an adhesive. A wire grid is also provided which is constructed of a plurality of wire members arranged at angles to create a grid-like pattern. The wire grid is disposed intermediate the first and second panels to create the structurally improved, lightning protective skin member. 
     In more preferred embodiments, each of the first and second panels comprises a multiplicity of graphite fibers aligned along the longitudinal axes thereof. The multiplicity of fibers are combined to form a plurality of graphite plies, each having its longitudinal axis aligned with the axes of the fibers which comprise the ply. The plurality of plies are combined in a laminate structure with the adhesive such that a portion of the adhesive is disposed intermediate consecutive ones of the plurality of plies. 
     In a still further embodiment, a conductive strip is positioned around the periphery of the lightning protective skin member and electrically coupled to the wire members of the wire grid such that all of the wires of the grid are electrically coupled together. 
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
     Subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the numbered paragraphs appended hereto. However, the invention may be best understood by reading the following detailed disclosure in conjunction with the drawings in which: 
     FIG. 1 is a sectional view of a graphite panel which comprises the novel lightning protective skin member; and 
     FIG. 2 is a plan view of a graphite panel. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a lightning protective skin member for aircraft which is the subject of the present invention. The lightning protective skin member comprises first and second panels 100 and 102, respectively, having a wire grid disposed therebetween. A plurality of skin members are typically combined to form structural components, which components are combined to provide the surface of an aircraft vehicle. It will be appreciated by those skilled in the art that although the subject invention is described by reference to aircraft vehicles, it is equally applicable to all moving vehicles. 
     Positioning the wire grid 104 intermediate the two panels 100 and 102 adds strength and elasticity to the overall structure of the skin member. This is in distinction to prior structures which have provided wire meshes and the like attached to the surface of a composite material. Such structures have failed to add mechanical strength or elasticity to the structural skin member. Techniques for fastening graphite structures with metal fasteners are not fully developed, but techniques for fastening metal with metal fasteners are well known. 
     As will be expected, the wire grid also adds lightning protection to the structural skin member. A lightning strike comprises intense current (approximately 200,000 amps at approximately 200,000 volts) which has a heating effect on the surface of the aircraft if the current is not conducted quickly. The increased heat causes quickly increasing pressures resulting in a shock-wave effect upon the surface of the aircraft. The shockwave stresses the aircraft surface and causes failure. 
     Further, if the current is not conducted by the surface of the aircraft, the current may be conducted through the passengers of the aircraft, resulting in injury and/or death due to electrocution. 
     In an effort to reduce increased heat caused by electrical resistance resulting from the intense current and to prevent the current from being conducted by passengers of the aircraft, the current must be quickly distributed over the surface of the aircraft by a less resistive means. If the current is conducted quickly, the current will not increase the temperature of the surrounding air and, accordingly, will not increase pressure or cause shockwaves or destruction of the skin member. 
     Additionally, providing a conduction path with a conductivity which is substantially higher than the conductivity of the aircraft passengers prevents the current from being conducted through the passengers. The current will be conducted over the surface of the vehicle via the wire grid, which is positioned about the periphery of the skin member. In theory, the current will be transmitted across the aircraft and exit from the side opposite from where it entered. 
     Although the effectiveness of the wire grid to dissipate current is somewhat decreased by positioning the grid intermediate the two structural panels, the decrease in effectiveness is minimal and far outweighed by the increase in strength and elasticity of the skin member which results by positioning the wire grid intermediate the two panels. 
     Each lightning protective skin member also includes a conductive strip 200 (FIG. 2) positioned about the periphery of the skin member and electrically connected to the wire grid positioned between the two panels by, for example, welds 202. The strip 200 may include attachment holes 204, or other type bores, for securing the skin member to other structural members of the aircraft. The attachment holes may be positioned at the intersection of the grid with the strip or elsewhere, as shown in FIG. 2. 
     The conducive strip 200 aids in conducting current over the entire surface of the aircraft such that the current is quickly dissipated. Additionally, by providing attachments at selected intersections of the wire grid, mechanical loads can be effectively transmitted to the strong graphite panels. 
     The wire grid comprises a plurality of wire members arranged at angles to form a grid-like structure. Preferably, the ends of the wire members are welded or electrically connected to the conductive strip (as shown in FIG. 2) to provide electrical conductivity between each of the wire members of the grid and the conductive strip. Alternatively, the ends of one or more wire members may be connected together and/or to the strip to provide a conduction path between the connected members. In practice, each individual wire in the grid is welded or otherwise electrically fastened to the strip 200. 
     Further, in the preferred embodiment, the ends of selected wire members are connected to the conductive strip in a manner to create a mechanical connection thereto such that mechanical loads can be readily transmitted to the sturdy graphite panels. It will be appreciated by those skilled in the art, however, that the ends of selected wire members may be connected to provide the mechanical connection without use of the conductive strip. 
     Each of the two panels 100 and 102 of the skin member is constructed from a plurality of graphite plies, 106, 108, 110, 112, 114 and 116. Each ply of each panel 106-116 comprises a multiplicity of graphite fibers aligned along the longitudinal axes thereof. The plurality of graphite plies each have a longitudinal axis which is aligned with the axes of its constituent fibers. The plurality of plies are combined in a laminate structure with an adhesive. The adhesive is disposed intermediate consecutive ones of the plurality of plies. 
     As illustrated in FIG. 1, the plurality of plies are also arranged in a manner such that the longitudinal axes of consecutive plies intersect at plus or minus 45-degree angles. In this manner, the strength of the composite graphite material is enhanced. 
     In a presently preferred embodiment, the panels are constructed of 8 plies laminated to 20 inch square panels. Each ply is constructed from graphite/epoxy tape as is known in the art. The wire grid is constructed of titanium having a diameter of 0.05 inch. The metal strip is also constructed of titanium to prevent corrosion problems which are encountered by using other metals in contact with graphite. The metal strip is approximately 1 inch by 0.05 inch in width and thickness and has a length equal to the perimeter of a panel. 
     The panels constructed as above survived a 200,000-amp, 200,000-volt strike with noncatastrophic damage. It is noted that with panels constructed in accordance with the above-described invention, the area of lightning contact can be repaired locally without repair of the entire panel. 
     While only several presently preferred embodiments of my novel construction for lightning protective skin members for vehicle surfaces have been described in detail herein, many modifications and variations may be made without departing from the true scope and spirit of the invention. It is my intent, therefore, to embody all such modifications and variations in the claims appended hereto.