Precipitation-Static (P-Static) Overview of Composite Aircraft 2001-01-2933
Aircraft charging due to p-static results from two atmospheric conditions: 1) the vehicle’s presence in a thunderstorm, and 2) the triboelectric charging (frictional) caused by neutral snow, rain, or dust particle bombardment of the vehicle frontal surface. Both charging mechanisms can lead to p-static interference by the following mechanisms of charge redistribution: 1) corona discharges from sharp-edged extremities, 2) streamer discharges on dielectric surfaces, and 3) arc over between electrically isolated or intermittently grounded metallic sections.
Corona discharges have spectral energy primarily in the LF through HF region but can have potentially significant energy levels in the VHF/UHF bands. Streamer noise is caused by charge buildup on non-conductive vehicle frontal areas such as radomes and windshields. The spectral energy content of streamer discharges extends beyond the UHF band; but the noise amplitude is low and the potential for RF interference is only a threat if the receive antennas are located close to the streamer discharge source.
An aircraft charged or exposed to fields due only to triboelectric charging may develop corona and limited streamering at its extremities, but it will not trigger lightning. A vehicle such as the B-2 with its large effective area is vulnerable to static charging. The B-2 is thus vulnerable to significant surface static discharge damage if the static discharge system is not optimized. These in-service issues for the B-2 underscore the criticality of a comprehensive design approach with composite materials and finishes to minimize static discharge occurrences.
Materials can be characterized as insulators, static free, or conductor. These are rather broad categories that are defined in terms of ohm/square characteristics. Materials with surface resistivities > 1012 ohm/square are likely to develop electrostatic charges which will not bleed off by themselves due to the high electrical insulating property of the surface.
This paper addresses static free materials primarily. Static dissipative composites are, in general, composed of conductive materials such as carbon or metallic particles which are distributed within an insulating medium. The conductive elements are randomly distributed throughout the surface as well as in the bulk portion of the material so that a required amount of volume and surface electrical resistivity are realized. This paper will furthermore provide an overview of material characterization techniques prior to adaptation to a platform and verification techniques after installation.