Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Wed, 11 Jun 2003 22:31:52 -0400
Message-ID: <redirect-2412292@logan.com>
From: "Charles R. Patton" <charles.r.patton@ieee.org>
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Subject: Re: [LML] Re: lightning strikes in composite planes -- a note about
 power dissipation
References: <list-2411843@logan.com>
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earl_schroeder@juno.com wrote:

>...I came away scared to death to fly my
>glass Lancair in a sky shared by thunderstorms. ...
>

I fully concur. I hope I didn’t give the false impression that a 
glass/epoxy composite is safer than the carbon. I was trying to explain 
the difference in the modes of destruction. To elaborate further. The 
likelihood is that in the glass/epoxy case, the lightning is likely to 
strike-through – probably through joint, crack or minimum thickness to 
the wiring, hard tubing, etc., then follow those paths until it exits 
through some other similar point. The entry and exit will be burned from 
the arc but the real worry is the path carrying the current which is 
sure to fry electrical equipment at the minimum and potentially able to 
weld moving joints or vaporize steel cables. The carbon composite 
potentially will carry the current outside to some extent, but as 
figured previously, the laminate itself can potentially “explode.” One 
other effect to mention. Many have seen the science museum “Faraday 
Cage” demo where the demonstrator steps inside the grid-like cage and 
artificial lightning strikes the cage, but the man inside is unaffected. 
This scenario DOES NOT apply to the composite plane including the 
conductive carbon. The problem is two-fold. One the plane has openings 
and doorways that don’t electrically seal, and second, but more 
important is back to that 5 ohms or so. Lets hypothesize that the 
lightning strikes the nose and exits the tail. 5 ohms times 10K amps is 
50K volts from tail to nose. Now assume your antenna coax runs about the 
same distance and the shield attaches to the fuselage. You have just 
involved your electrical system with a 50KV, 10KA electrical generator. 
This is NOT a good thing! The fallout from this will be extensive and 
expensive.

I think what I was trying to do is show the magnitude of the problem and 
to emphatically point out that I certainly would not fly in lightning, 
and that to be extremely wary with imagined fixes to this problem like 
light screen or thin foil.

I leave you with one final anecdotal example. Years ago I was back east 
in McGraw Edison’s hi-voltage test labs where they could test 
transmission line fault current breakers, over voltage protection MOV’s 
etc. The lab included a large lightning bolt generator. Some years 
earlier they had been contracted by GM to test a new car with 
electronics in it (I think it was electronic ignition but I’m not sure.) 
Nevertheless, here is the classic Faraday cage, the car. However they 
ran the testing through several episodes as the car design was tweaked 
to improve its performance which during first tests destroyed the 
electronics. During final stages of testing, the artificial lightning 
bolts welded the car doors shut! Among other things, this tells you that 
the lightning does not always flow where you think it will go, but jumps 
on it own “path of least resistance” which may include gaps or 
insulation in the way. A few million volts in a few microseconds is a 
very difficult phenomenon to tame. The other problem here is that I’m 
sure the designers did a careful initial design job to the best of their 
ability – but multiple testing showed the truth. I don’t think any of us 
want sign-up and risk the potentially destructive testing that process 
requires. Do you want to build a couple of planes and submit them to the 
lab? And I sure don’t want to be in the (un-tested) plane running those 
empirical tests.

Regards,
Charles R. Patton
LNC2 360JM