Studies of Helicopter Charging. Task 1. Electrical Hazards to Airborne Operations

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Studies of Helicopter Charging. Task 1. Electrical Hazards to Airborne Operations

C. B. Moore / J. J. Jones / S. J. Hunyady / W. Rison / W. P. Winn

Helicopters hovering above the earth often acquire large electrical charges which can be dangerous to ground personnel who come in contact with the aircraft. In tests of ground-based CH-53E helicopters isolated from earth by insulating pads, potentials in excess of 40 kV relative to the earth were developed after the rotors began to turn. Although the common view is that the electrification is caused by collisions between the rotor and atmospheric dust particles, during this study strong electrification was observed in the absence of any appreciable dust. Charging currents of up to 12 uA were observed in clean air measurements when the helicopter rotor turned and caused strong downdrafts. Measurements in this study show that an important source of aircraft charging is the current that flows in the hot, electrically-conductive exhaust gases under the influence of the local electric fields. This current was successfully reduced by control of the electric fields acting on the exhaust gases immediately after their emergence from the engine while they were still conductive. Control of these fields required the use of electrostatic shields around the exhausts to reduce the strength of external fields together with the creation of the appropriate internal fields by application of voltages to electrodes within the shields. This technique, applied through a servo controller that was driven by a sensor of the helicopter potential, allowed the export (through the exhaust gases) of undesired charges on the airframe. It also worked well in eliminating charge when the helicopter operated in a dusty environment.