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On Fri, 30 Dec 2005 19:31:17 GMT, Matt Whiting <whiting@epix.net>
wrote:
>RST Engineering wrote:
>
>> Matt --
>>
>> Because you want to get rid of the charge quietly, without any sharp
>> transients that will cause RFI. You can come at the answer from an RC time
>> constant angle, from a total energy (power dissipation) angle, from a couple
>> of more exotic angles.
>>
>> I myself like to think of it from the total energy point of view. That
>> charge starts out at some large potential. In travelling through the
>> resistor to get to the pin point, moving charge is the definition of
>> current, which by Ohm guarantees that there will be a power loss going
>> through the resistor, and hence a lower voltage to dissipate when it does
>> get to the pin point.
>>
>> If you want to come at it from the RC time constant point of view (being
>> that the airplane is the conductor and the atmosphere is the capacitor) the
>> R of the RC is going to round the edges of those transients nicely.
>>
>> YOu cannot use too high a value resistor or the charge won't even see the pin
>> point. You cannot use too low a value resistor or the charge won't lose any
>> power in gettint to the pin point.
>>
>> How did I come up with a value of 100K to 1M? Simple. Back when I worked
>> for the airline, there was a spec on the static wicks on all the Boeings and
>> on all the McDonnells that said the static wicks should measure between 100K
>> and 1M from the attach point to the pin point(s). I figured if it was good
>> enough for McBoeing at 500 knots, it was good enough for Jim at 100 knots.
>
>Thanks, Jim. I simply reasoned that with sufficient wicks and given the
>high conductivity of the airframe, I didn't think you would likely ever
>build up enough charge to get an "impulse" discharge. I figured the
>charge would quietly leak off into the ether, but I guess if the radius
>of the point is not small enough, it will take a fair bit of charge to
>ionize the air enough to get a discharge.
>
Even with a fine point and no resistor you are likely to hear
something like a whistle rising in pitch ending in a "pop!" over the
radio. Or, it just may be a hissing sound that continues, and
continues, and continues, with no pop.
In the Deb, which has no static wicks, I had every radio go deaf. ADF,
VOR, and communications. I shut the master off, counted to 10, turned
it back on and they all went back to working... for a few minutes.
Once out of the rain and well away from the storm everything worked
fine.
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
>
>Matt
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Re: static wicks
