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Re: How to dissipate charges from aeroplanes?



On Sun, 8 Feb 1998, GEORGE PANAGIOTAKOPOULOS wrote:

Hi to all,

I want to know if there is a system on the airplane which operates as static
dissipater for the charges that collect an airplane when it is moving, to
avoid lightning strikes. If so, how does it operate?

Thanks in advance for the answer.

George Panagiotakopoulos
Teacher of physics
physiart@ath.forthnet.gr
ICQ:6880122



Having been a pilot for the Marine Corps in my younger days, this is an
easy question for me to field, so here goes.

Facts:

* the capacitance of many airframes (fixed-wing and helicopter) is
reasonably high;

* due to its interaction with its environment, an airframe can build up
and hold 20 to 30 kV especially when traveling through significant
moisture; {yes, greater moisture does make matters worse for retained
static charge}

* the energy stored in such a capacitor is easily enough to KILL --- if
not by "frying" the victim, at least by fibrillating the heart.

Note: That's why helicopters drop the rescue gear on cable into the water
away from a downed pilot and drag it towards them --- to discharge the
A/C(aircraft) --- not because we are worried about hitting the person to
be rescued "on the head" as we drop gear. Just think about the only
possible current path if we were to lower slowly the gear to the pilot in
the water and they were to reach up to grab it with their hand. There
goes the heart, and we have to drop a SEAL or UDT swimmer into the water
to hook up our now unconscious pilot.

Speculations:

Various reasons given by the "brains back in the lab" for the level of
static build up are

1) rotation of conducting objects (props, rotor blades, engine parts,
etc.) in the magnetic field of the earth {Always an explanation I enjoyed
since it seems to be the explanation that everyone wants to believe.
Frankly, any numbers that I can calculate for this source just don't seem
to generate enough energy!};

2) simple charging by friction, just like the rubber rod on "cat's fur or
glass rod on silk {To me, this seems to be the only choice for the
"primary source" of generated power. Plus, it is in keeping with the
observed facts that the problem gets worse in situations in which there is
a lot of moisture in the air.}.

Solutions:

* As I said, helicopter pilots always "discharge the gear either by
dragging it in the water or (as in Hunt for Red October) by having someone
on a boat "hook us up with a grounding pole or snare" to discharge our
A/C.

* The trailing edge of the rotor blades of helicopters and the wings of
fix-winged A/C have "static discharge wicks" which are basically 10-20
pieces of thin wire bundled together by heavy duty shrink wrap with the
tip ends exposed. As the A/C moves through the air, these wicks are able
to deplete the excess charge due the high E-fields at the tip of each wire
in the bundle. Not 100% effective, but it helps a lot. In good weather,
this will lower the voltage to 3 to 4 kV or less.

* There is yet another static discharge wick (sometimes more than one)
"trailing" under each wheel mount so that the A/C is grounded as it
touches down on the carrier or runway.

* Lastly, every A/C "parking area" either on land or on a carrier has
"metal "tie-down rings" to which the A/C is connected through "chain
dawgs" (tensionable chains with hooks at either end).

I hope this info is of some help and answers some questions in this area.


ERTEL SENDS.
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