In a message dated 6/11/2008 3:03:20 AM Eastern Daylight Time,
europa-list@matronics.com writes:
Not sure thats a terribly good idea.
Hi Carl, et al,
Apparently, the FAA and Aeromot think it is a good idea. The Ximango is a
certified aircraft, has aluminum fuel tanks and they, too, are bonded to the
rest of the metallic parts in the aircraft. In their parts manual, Aeromot
refers to these bonding straps installed as "lightning bonding."
As it has been explained to me by our resident composite Guru here at the
'Possum Werks, what happens in composite airplanes in a lightning strike to make
them explode is the super heating of residual moisture that is trapped
within the layers of epoxy and cloth at the time of manufacture. When the
strike
passes through the composite, trapped moisture rapidly heats, expands and the
structure comes apart explosively.
Having designed, installed and tested lightning protection systems on over
7000 cellular telephone sites, I've seen this phenomenon quite a bit with
fiberglass antenna masts on poorly grounded sites. When a cell antenna takes a
direct hit it explodes. The tower can take a direct hit and only slight
discoloration will result. The result of an exploded antenna mast is similar to
what
you see after one of those exploding cigars goes off. If the site is
properly grounded and bonded, this almost never happens, though.
What you are trying to do with bonding metallic parts in a plastic airplane
is give the lightning a low resistance path to pass through and exit the
airplane so that it doesn't take a high resistance (i.e., through the
composite)
path to exit and destroy the airplane. Low resistance is the key here. By
providing a low resistance path for the lightning to pass though you reduce or
eliminate the instantaneous heating that occurs in high resistance paths
during a discharge. You also want to prevent arcing that would occur at loose
metallic fittings, such as rod ends and any metallic parts that are bolted
together. These are points of high resistance and arcing (and welding) can
result.
What you are doing here is making all the metallic parts equal in resistance,
therefore equal in potential when charged. Ohm's law at work. I've heard of
more than one occurrence of gliders that did not have bonded control systems
being struck in flight and having their control rods welded to the point
where the aircraft was no longer controllable and the occupant(s) were forced
to
bail out.
These bolted connections are precisely where the bonding straps are
installed on the Ximango. The bonding straps that are installed, given their
size,
are designed to take a lot more current than a simple static discharge. They
are essentially the same thing we install to bond and protect towers,
transmission equipment, batteries, fencing, coax, antennae, etc., in cell sites
---From
lightning. The reason braided cable is used is surface area. The more surface
area, the more current it can handle. Braided cable has much, much more
surface area than typical Tefzel aircraft wire or a solid wire, thus more
current
handling capacity.
Is it a cure all? No. There is no such thing as perfect lightning
protection. Will it help save your bacon in a residual discharge situation?
Probably.
Will it save your bacon in a direct hit? Probably not.
Fred, as far as a source on the bonding straps, try Harger. _www.harger.com_
(http://www.harger.com) . If it has anything to do with lightning protection,
Harger carries it.
Hope it helps!
Regards,
John Lawton
Whitwell, TN (TN89)
N245E - Flying
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