GB1277429A - Improvements in or relating to static electricity discharger systems - Google Patents

Abstract

1277429 Automatic control of static electricity discharge systems DEFENCE SECRETARY OF STATE FOR 1 Dec 1969 [3 Dec 1968 12 Nov 1969] 57433/68 Heading G3R [Also in Division H1] Build up of electrostatic charge on a structure is prevented by a discharge system comprising a transducer supplying a signal indicative of the magnitude and polarity of the electrostatic field gradient at the structure, means to inject electric charges into a stream of vapour-containing gas travelling at or near the speed of sound and means to make the stream expand sufficiently to cause condensation of vapour to form particles or droplets in the stream having greater inertia than ions of nitrogen in the atmosphere, the charge injection means being controlled by the transducer signal to eject charges from the structure into the stream to reduce the electrostatic field gradient. As applied to maintaining the field gradient at the surface of a helicopter 1 within tolerable limits, the transducer comprises a rotating-vane electro-meter 8 producing an A.C. signal of amplitude dependent on the field gradient, this signal being applied to a detector circuit 9 which derives a D.C. signal whose polarity and magnitude are dependent on that of the field gradient which may include a limiting or non-linear (e.g. logarithmic) amplifier, After passage through an amplifier 4, the D.C. signal is used to control charge dispersal apparatus 5 which receives gas from a gas turbine engine 6 and comprises a polytetrafluoroethylene tube 10 formed with a throat 11 and nozzle 12, the throat incorporating an annular metallic electrode 13 co-operating with a needle-like electrode 14 mounted on a support 15. Members 13, 15 receive the output of amplifier 4 so that a corona discharge occurs from the point of needle 14 and the ejected charges are swept away in the gas stream 17, whose condensation is produced by adiabatic cooling occurring in the expansion region beyond the throat 11. The gas flow reaches the speed of sound at the throat 11 and is supersonic between the throat 11 and nozzle 12. Charge carriers in the region of the electrode 14 are entrapped and swept away by the condensed particles due to the low mobilities of the latter. A discussion is given of the disposition of the electrode 14 relative to the throat 11 to maximize the removal of the charges. Mention is also made of increasing the humidity of the gas in the duct 7, or adding further quantities of vapour or particulate additives to it, or adjusting its temperature. Other modifications relate to the provision of automatic gain control for the amplifier 4 responsive to the discharge current sensed by a resistor in the line 19; to variation of the shape and number of the electrode(s) 14; and to duplication of the system to provide selective operation according to the polarity of the discharge required.