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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.
Matt
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Re: static wicks
