1970-02-01

Electrostatic Charging and Noise Quieting 700926

It is known that aircraft in flight charge electrostatically due to triboelectric contact with precipitation, cross fields, and ionization in engines. This charging can create broadband radio frequency noise due to streamer currents across nonconductive surfaces, corona discharge from the aircraft, and arcing between structural members.In the past, charging of an aircraft has been measured, noise sources identified, and corrective steps applied. However, such efforts have been principally confined to large commercial and military jet aircraft. Cost has prohibited extensive system treatment of all noise sources preceded and followed by extensive flight test verification of charging phenomena and noise reduction measures.Smaller general aviation aircraft occupy the same flight environment and utilize the same airspace as transport and military aircraft. These aircraft also charge electrically. Noise sources are frequently even more tightly coupled to receiving antennas due to the physically small dimensions of general aviation airplanes.A systems approach was taken to identification of noise sources due to electrostatic charging, to measurement of charging levels, to application of noise reduction techniques, and to flight test verification.Streamer currents to 40 μ amp were measured on plastic structural components, cross-field stresses exceeding 100 kV/m were recorded, and propeller corona currents were observed to exceed 50 μ amp/blade. Precipitation charging rates of 250 μ amp were encountered. A tip-type discharger reached a 400-μ amp discharge current on one occasion. Discharges of opposing polarities were observed from different aircraft extremities at the same time. A total discharge current of 2.5 mA was occasioned.Steps were taken to reduce noise generation when the aircraft became charged electrically. These included installation of quiet dischargers, conductive coating of plastic frontal surfaces, and use of d-c sealed antennas.Actual flight tests demonstrated that with proper application of corrective measures, usable navigation and communications could be retained in the most severe conditions encountered. Without the corrective measures, communications and navigation, through the VHF spectrum, were lost for extended periods of time in actual flight.

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