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helicopter static charge

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DrZoidberg

Tue Jun 17 2008, 08:26PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

I read that helicopters charge up due to friction with the air. But when I take a piece of metal and blow against it, I can't detect any current. The smallest current I can measure is about 0.1 nA. I get several nA just by moving the leads of the meter but blowing at it doesn't do anything. Why is that?

Solidacid

Tue Jun 17 2008, 08:37PM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

try using a hairdryer instead, your breath is filled with moisture.
i have gotten sparks from blowing into a pvc pipe attached to a leyden jar(with a modifyed hairdryer/compresser)

DrZoidberg

Tue Jun 17 2008, 08:56PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

With a hair dryer it doesn't work either. It also doesn't matter if I use hot or cold air. I blow the air against aluminum foil.

Dave Marshall

Tue Jun 17 2008, 10:55PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

How are you attempting to detect the current? If you ground one lead of the multimeter and attach the other to the metal sitting on something insulating, you could probably detect it.

Keep in mind also that the rotors on helicopters have alot of surface area, and they're moving damn fast. A helicopter in flight also has virtually perfect insulation from ground, and the atmosphere itself can often contribute to the availability of electrons. All conditions that a hair dryer blowing on a bit of aluminum foil can't duplicate.

Other examples of static due to friction with air can be seen in St. Elmo's fire atop the masts of tall ships, and on aircraft flying through heavy clouds and/or thunderstorms. For an interesting visual example, check youtube for a video of a KC-135 Stratotanker flying through a thunderstorm in Iraq. I can't access youtube right now to get an address(overseas and the internet is heavily filtered) but its fairly easy to find.

Dave

Solidacid

Wed Jun 18 2008, 07:00AM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

i don't know if it means anything but isn't helicopters usually coverd in paint?
according to This it should

also, i think you need something bigger than a piece of aluminum foil to store a measurable amount of electricity, like a leyden jar

GeordieBoy

Wed Jun 18 2008, 09:03AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

...check youtube for a video of a KC-135 Stratotanker flying through a thunderstorm in Iraq

Here:

Dr. Dark Current

Wed Jun 18 2008, 04:54PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Dave Marshall wrote ...

Other examples of static due to friction with air can be seen in St. Elmo's fire atop the masts of tall ships, and on aircraft flying through heavy clouds and/or thunderstorms.

I think St Elmo's fire is a corona discharge, it has something to do with atmospheric voltage gradient I believe (like before a thunderstorm the gradient is much higher, sometimes corona can be seen from tall objects).

Avi

Thu Jun 19 2008, 12:40AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

hm, so it should be possible to read a voltage if you put 2 electrodes on each side of a pole and stand it upright?

Dave Marshall

Thu Jun 19 2008, 08:17AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Absolutely. Its actually possible to create a couple of different varieties of circuits that measure the voltage gradient of the atmosphere.

Perhaps one of the simplest and easiest (precautions must be taken) involves coating a razor blade with a mildly radioactive material like Thorium ash from lantern mantles (or Americium-231 *if precautions are taken*). The physics of this are somewhat vague to me but my understanding is you create a small area of ionized air around the razor blade's edge, which increases the effective aperture size. Sort of inflating the razor blade to extend across a more substantial bit of the electrical gradient.

I hate to point to a website as...well questionable, as this one , but I've tried this particular experiment, and it works (enough to prove the point, any way). I wouldn't use it for predicting lightning strikes with any accuracy, but the principle can be adapted to some interesting ends. Fire up a TC anywhere in the vicinity, and watch it go nuts.

Far more effective methods of measuring the atmospheric charge gradient exist, such as Electric Field Mills. I'm currently living in lightning rich Georgia, and am considering constructing one of these for a little foray into monitoring actual lightning strikes versus local electrical conditions.

As for St. Elmo's Fire, it indicates an intense difference of potential over a short distance(steep electric field gradient). It is the ionization of air molecules to such an extent that they begin to glow (precisely the same phenomenon you see with a spark gap as it approaches its firing voltage). It's generally described as occurring around grounded objects such as radio towers and ship's masts, but as a career aviator I can attest to it *frequently* occurring around aircraft flying through heavy clouds or near thunderstorms. Aircraft aren't grounded, so logic suggests that this is indicative of a difference in potential between the aircraft itself, and the surrounding atmosphere.

I've got a fair bit of training with helicopters, particularly in a Search & Rescue context, and the first rule for working with them is to never, ever, be the first object a hoist cable or rescue harness contacts. It must always ground out before being handled. I've asked several instructors and pilots, and nobody can share more than vague third person anecdotes regarding actual fatalities from a helicopter's static discharge, but I *have* gotten more than your average 'dry day in a wool sweater' jolts from them. It could certainly be unpleasant to say the least.

Essentially, think of the whole thing as a big spark gap. The electrical field may not be substantial enough to bridge the gap from cloud to ground on its own, but give it a giant, hovering, electrically yummy object in between (say, a helicopter dangling a metal hoist cable through 100' of electric field) and you basically just get a really big chicken stick. It's will to find a way to ground.

Dave

Avi

Thu Jun 19 2008, 11:31AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

Dave Marshall wrote ...

Aircraft aren't grounded, so logic suggests that this is indicative of a difference in potential between the aircraft itself, and the surrounding atmosphere.

how come in that video the sparks are occurring from the base of the (glass?) window to somewhere further up on the surface of the window. Shouldn't it then be from the metal body out into the atmosphere.

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