On Fri, 30 Dec 2005 19:31:17 GMT, Matt Whiting
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 can't use too high a value resistor or the charge won't even see the pin
point. You can't 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 isn't 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