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Re: [Phys-L] sparks from dust + helicopter blades



On 08/01/2013 09:28 AM, Forinash III, Kyle wrote:
Someone sent me this and I was wondering if anyone had an opinion about what might cause such a phenomena:
http://www.npr.org/blogs/krulwich/2013/07/30/206946740/mysterious-dancing-lights-in-afghanistan

Do you really want opinions? Opinions are a dime a dozen. Rational
analysis is somewhat scarcer.

All available evidence indicates that we are seeing a "grinding wheel"
effect. The dust is grinding off tiny bits of metal from the rotor,
with such violence that the bits catch fire and burn in the air. The
key evidence is the fact that you can see disconnected specks of light
in the pictures.

Note that you can use the color and structure of such sparks to
determine what the blade surface is made of.
http://www.google.com/search?q=spark-test+nickel+titanium

You might think the chemistry of the sand would also be significant,
but it usually isn't because typical minerals such as feldspar and
silica are already as oxidized as they could possibly be.

Further evidence for this, not mentioned in the article, is the fact
that after the aircraft lands, you can observe that the rotors have
been eroded.
http://en.wikipedia.org/wiki/Helicopter_rotor#Abrasion_in_sandy_environments

=====

Due diligence requires that we consider competing explanations:

In the original article and in the online comments, there is a heavy
emphasis on "static electricity" including some mention of St.
Elmo's fire.
*) Undoubtedly static electricity can accumulate on aircraft,
sometimes to a significant degree. Aircraft are equipped with
"static wicks" to bleed off the accumulated charge in a controlled
manner. Movable control surface are actually /wired/ to the rest
of the aircraft, so that we don't need to rely on the hinges to
carry the current.
*) The conditions that produce the grinding-wheel effect are a
subset of the conditions that produce static electrification,
so it is likely that static electricity is present in the
situations in question ...
*) OTOH electrical discharge is not prominently visible in
these pictures, and cannot possibly explain what we are seeing.
This is not a matter of opinion.
-- It would take the form of a diffuse bluish-white glow.
All published accounts of real St. Elmo's fire agree as to
its color and form. The color is determined by the laws of
physics, via the spectroscopy of oxygen and nitrogen.
-- The laws of electrostatics guarantee that the discharge is
strongly concentrated at the extremities, in this case the
tips of the rotors.
-- It would not take the form of disconnected glowing specks
Again this is guaranteed by the laws of physics. The
capacitance of a dust particle is not large enough to store
a significant amount of energy, and even if it were, there
would be no pathway for discharging the energy into a visible
form.

Note the contrast: Friction can cause erosion and combustion,
and friction can cause static electrification ... but these
are *not* the same thing.