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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
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