In article ,
"Jim Knoyle" wrote:

"MikeM" wrote in message
...
Dan Jacobson wrote:

GIF of plane hit by lightning:
http://bm6aak.myweb.hinet.net/file/456.gif
(Lightning hits planes everyday and is no big deal.)

What it clearly shows is that airplanes do not get "hit
by lightning". What actually happens is that a lightning
bolt already headed from ground to cloud sometimes
makes a small detour through a conductive object (airplane)
if it happens to be where the lightening bolt may have gone
anyway.

There are billions of volts cloud-to-ground before the strike.
Once the air in the lightning bolt path is ionized, the
current that flows is only a few thousand Amps.
A metallic aircraft, if it becomes part of the current path,
has a max voltage drop across it of only a few hundred volts.

The airplane is self-protected in the same way as installing
a #8awg copper wire from a "lightening rod" from the roof of
a barn, around the outside of the barn, to a ground rod.
During a strike, the potential from tip of the lightening
rod to the ground under the barn is constrained to a few
hundred volts... This keeps the destructive current path out
of the wood; it flows along the copper wire instead of in the
wood.

Dont try this with a plastic, composite or wood aircraft.
The current pulse instantly turns absorbed moisture into steam,
literally blowing the aircraft apart.


Some occasional exceptions. One DC-10 I worked
had a real interesting bit of artwork after a strike. Along
the left side there was a splotch about three feet long as
if some artist had painted a jagged line with his paintbrush.
On closer exam, the splotch consisted of pitted and melted
aluminum.
Back when the Weather Radar (AVQ 10/30) used a parabolic
dish for a scanner, the scanner bearings could get welded solid.
The scanner probe consisted of an approx. 4 inch long 1/4 inch
diameter alloy rod for a probe. This rod was encapsulated into
a solid 3/4 inch Teflon cylinder. After a strike this Teflon could
look like a bit of Swiss cheese or a bad case of termites.

JK

OK, but all of these verify what I think I know about lightning - it is
a very high frequency phenomena and will tend to only flow through the
skin of the conductor. So while it may mark or weld exterior
attachments, interior "stuff" - instrumentation; people; etc. will be
quite safe.

(And as for the wooden or plastic aircraft, wouldn't the qualities of an
aircraft to attract lightning in the air be mostly to be a better
conductor than non-ionized air? And mostly for cloud to cloud strikes?
So I'm unclear on how good an electrical conductor this stuff would be.
And how about titanium - like in "Blackbird"?)

HR.