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I am designing a dirigible, robotic drone. It is basically a polyurethane airship, approx 3.6 m length and 2.1 m diameter.

I am worried about lightnings: as far as I know, a Faraday Cage should be able to protect its interior from external electrostatic fields. Now my question is: a lightning is much more likely to strike on a conductive material mid-air rather than on its surroundings, right? Now, even if the Faraday Cage could protect it from the electrostatic field, there should be a huge amount of energy released in the Faraday Cage due to the Joule effect of the lightning streaming through it, enough energy to melt the structure. A lightning can bring a current up to 100 kA: say I have a wire in copper 5mm in radius and 2 m long to protect it: that lightning could release up to 4.3 MW on that lightning rod!

So I was wondering: is there any other strategy to protect the drone? On example, could it be possible to cover it with some dielectric (is there anything stronger than air?) so that lightning would rather strike somewhere else?

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  • \$\begingroup\$ I'd say that makin all its surface a conductor (Al foil?) is enough, but I can't support my idea quantitatively. Airliners don't have any particular apparatus and are struck by lightning quite often. \$\endgroup\$ – Vladimir Cravero Jul 22 '14 at 7:29
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    \$\begingroup\$ @Vladimir: Airliners with composite construction do have special provision for conduction of lightning strikes (usually inclusion of metal mesh in composite construction). Even all-metal hulled airliners do sustain skin damage that must be repaired. \$\endgroup\$ – RedGrittyBrick Jul 22 '14 at 8:35
  • \$\begingroup\$ Well thanks for pointing that out, I didn't know that. \$\endgroup\$ – Vladimir Cravero Jul 22 '14 at 8:55
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    \$\begingroup\$ If you're flying your dirigible near thunderstorms, I think lightning is the least of your worries. The wind, rain and hail will probably destroy it before lightning strikes it. \$\endgroup\$ – Dave Tweed Jul 22 '14 at 11:33
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    \$\begingroup\$ Is the dirigible tethered to the ground? If not I don't think a lightening strike is all that likely. (And like @DaveTweed I wonder why you are flying it in a thunderstorm.) \$\endgroup\$ – George Herold Jul 22 '14 at 12:42
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Imagine how much cable you'd need to conduct a lightning strike. How thick would that cable have to be to conduct the strike without taking significant damage? You'd have to sheathe your airship in an equivalent amount of copper. (The cross-sectional area of the copper cable would have to be roughly equivalent to the cross-sectional area of any path lightning could take through the skin of the ship.) Long story short, under all that weight, your new toy wouldn't so much float majestically as... plummet ballistically.

You won't have anything to worry about unless you actually intend to fly the thing in a thunderstorm, in which case, you should be far more concerned about how to protect yourself than your airship. Even so, it would be impossible to get enough power on board to counteract the winds in such weather.

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