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I can confirm Brent's experience without static wicks. While trying to set a
San Francisco to Denver speed record, we entered some light clouds at 27,000
feet. Not only did the radios check out, but the digital clock on the panel
flashed a couple of times, then reset to zero, and started counting up
again. It was very entertaining. So when he said "order of magnitude
improvement" with wicks, he wasn't kidding.
I installed the Bill Maddox kit with 15 wicks, and discussed the basis of
wick number and location with Dayton-Granger, the wick manufacturer. They
use an empirical formula that includes the air speed and the surface area of
the airplane that can intercept the droplets or ice crystals that are
carried into contact with the air frame by the air stream.
Generally the largest area intercepting the ice crystals consists of the
front half of the wings and the front portion of the fuselage , vertical
stabilizer, and horizontal stabilizer (essentially what you can see if you
stand directly in front of the airplane at a large distance). On the Lancair
IV, even though the airframe is carbon, the engine cowl is fiberglas (in
most cases) and failure to manage static build up on the cowl can cause
problems. Also, the wings must be electrically grounded to the fuselage (the
fiberglass fairings are great insulators as is the spar), as must be the
horizontal stabilizer. The vertical stabilizer is non-conductive and also
presents a challenge.
The Lancair IV rudder is fiberglas, yet Dayton Grainger specifies three
wicks at the top of the rudder. To make them work requires adding a wire
connecting them to the carbon airframe. But such a wire represents a
reflector for the com antenna mounted in the vertical stabilizer. (The
presence of the com antenna is the reason the vertical stabilizer and rudder
were made of fiberglas in the first place). So rather than string a wire
that might interfere with the antenna pattern factor, I installed the wicks
on the bottom of the rudder, and strung the wire from there, distant from
the antenna, and near to the carbon fuselage. Compromise, compromise,
compromise.
Non-conductive surfaces can be static controlled through the use of
conductive epoxy primer under the finish coat of paint. Several
manufacturers of air frame epoxy finishes make such primers, most commonly
for use on radomes specifically for static discharge. The conductive primers
bleed off the static charge to the conductive regions of the airframe for
subsequent transport to the static wicks for discharge. However, the
conductive primers are not conductive enough to interfere with radio
frequency transmissions in the frequency bands of choice (HF, VHF, and UHF
including airborne radar and transponder frequencies). I have concluded that
the prudent approach thus consists of the following:
1) Paint all non-conductive substrates (particularly engine cowl on Lancair
IV, and wings on an ES) with conductive epoxy primer. Cover this with a
primer system compatible with your finish coating system. The charge can
"seep" through the finish coat to the primer to be carried away much as it
seeps through to the aluminum skin of a metal airplane.
2) Make sure that all conductive structures (carbon wings and fuselage for
example) are electrically connected (grounded) to each other, and verify
with an ohm meter.
3) Mount the recommended number of static wicks. More is better, so it is a
question of aesthetics and money. Eight works for Brent. Dayton Grainger
recommends 15, but their recommendation is surely based on some testing in
truly ugly conditions. Make sure the static wicks are electrically connected
to the airframe ground with a separate wire. Don't count on controls,
hinges, etc. to do the job as these may or may not be good electrical
pathways.
4) And always stay a long, long way away from thunderstorms. Lightning and
composite airplanes are not a good combination.
Fred Moreno


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Please send your photos and drawings to marvkaye@olsusa.com.
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