Quoted: I suppose that a charge builds up on the skin of the airplane. As long as there is no path to ground, no potential, no current flow. |
but consider that a moment later the airplane is now charged at a potential which can ionize the air in the gap to the ground. there is another discharge, originating at the plane and ending on the ground.
www.youtube.com/watch?v=w69vfMvpeBcso there is current flow (and a high current flow at that)
through the plane, specifically through the skin of the plane. if the surface area of the plane is sufficiently conductive, and there is enough conductive area to avoid localized heating (i.e., melting of aluminum), there will be no damage to the aricraft.
what we've talked about so far is a situation where a large current passes through an ungrounded structure. let's take this onward. suppose the bolt does not hit the plane, but instead passes some 100yards to one side of it. what happens then?
we know that any current flow (especially a big current flow like a lightning bolt) produces electromagnetic (EM) energy -- turn on an AM/FM radio during a thunderstorm and you can hear it (much more so on AM than FM, btw, for the simple reason that FM is much more resistant to atmospheric disturbances). the current flow in the bolt creates an electric field (conceptually a wave front, sometimes referred to as a "flux") and a magnetic field. these two are referred to as an E-field and an H-field respectively. for simplicity here, take my word that the E-field and H-field travel together but at a right angle to one another. if you want to learn more about EM energy, google for Maxwell, Gauss, Faraday, and Michelson.
the lightning discharge can be thought of as an impulse; that is, a very short duration high energy pulse. again, for reasons a bit complicated to type up, such an impulse generates a broad range of electromagnetic energy -- which i'll translate to say that the lightning produces a wide bandwidth "bump" of powerful EM energy that spans many frequencies.
as the invisible electromagnetic wave produced by the lightning passes by the airplane, a current is generated on the surface of the plane. this is conceptually no different than the current that is produced on the antenna of your car -- an EM wave is transmitted by your favorite radio station, and your car antenna picks up this signal and converts it to audio.
the magnitude of the current on the skin of the airplane is a product of many factors, including the E- and H-field strengths, the spectral content of the wave, the angle of incidence, and the conductivity of the aluminum skin. nevertheless, because of a principle put forth by Faraday, assuming the skin is reasonably electrically contiguous up to the highest frequency impinging on it, the inside of the structure will not suffer any effects of the wave. this is true whether the structure is grounded or not.
Quoted: It's the same reason birds can sit on a 50kV HV line, the bird is at 50kV in respect to ground, but at 0V with respect to the line. BSW |
this is correct. the protein nonsense written above is just that, nonsense.
ar-jedi