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Posted for Earl Schroeder <earleschroeder@yahoo.com>:
Scott Krueger wrote:
PS Static wicks are functional if the skin is carbon fiber where
there is
skin conductivity (such as aluminum) and the wicks are useful to
discharge
static buildups, like lightning. For us prprerpregolk, avoid
excessive
electron
exchanges by Mother Nature - the bolt just gets confused and when
your wing
is hit on its way to the ground the natural thing is to just
blast right on
ththru
Then Marv commented:
[Jim FrFrantzoined the term "flying dielectric" for the e-glass
composite airframe sometime back. I think he had a message in there. <M>
The 'lighting' subject pops up (pun intended) from time to time and I
marvel at how lightly it is treated. Over the years, I have attended
nearly all the lightning forums at Oshkosh which were given by very
qualified (in my opinion) presenters. To sum up my acquired
knowledge: I am scared to death of lightning to the point my IFR
ticket is mostly useless flying my eglass Lancair the Midwest. In
the Mustang II we flew between streaks of lighting frequently and
give it little thought. If I had my metal airplane, I would launch
and just dodge the darkest spots enjoying the light show. I won't do
that now. One NASA presentation showed a Cherokee's fiberglass tip
blown to bits because 80,000 amps tried to squeeze through the 18 ga
wire previously connected to the nav light. And this was just a
'leader', not a full strike. Probably nothing could survive direct
strike. As I understand it, lightning (a huge difference in electric
potential) sends out thousands of leaders looking for a path to
equalize the electrical pressure. If your airplane has less
resistance than the air, ZIP it goes in one point and then exits with
a bang. Guess what is the best conductor in my airplane? The push
rods. My previous acquired electrical knowledge (30+ years at GE)
tells me that my push rods will conduct better than the 18 ga wire
but still will struggle with the 80,000 amps. To possibly compound
the problem: I have a two inch al tube in my gas tank to seal off the
gas from the aileron tube.. a dandy capacitor.. can we spell SPARKS.
BOOM! is next. You ask: why doesn't a metal airplane suffer this
also. As most know, electrical current travels mostly on the outer
part of a conductor so the metal skin can pass the current quickly
and usually with minor damage as documented.
OK, I have my nomex flying suit on. Show me the errors of my
thinking so I can get more utility from my plastic airplane. <G> Earl
[Great points about lightning, Earl, but the static wicks aren't intended to shed a lightning strike. Their sole purpose is to provide static buildup a path for discharge. While it's probably not a problem for folks flying in the high humidity lower altitudes of the midwest and southeast, for people flying over the desert where dust can build up tremendously, the amount of static that can be deposited on a plastic airframe can be quite large, and where static wicks can help. I wish I had better news about the lightning issue, but unless you add a conductive skin to your airplane, I'm afraid there is no $200 solution. At least this is my understanding, and I am always happy to be contradicted when I'm wrong... it's just another path to knowledge. <Marv> ]
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