Static electricity

[Flagrum]

I've seen some documentation on TV some time ago that mentioned static electricity as a threat for helos. The rotor blades moving through the air induce this static electricity that can cause a high voltage spark that could easily kill a man - for example if you climb into a hovering helo and thus are connected to the ground and to the helo at the same time.

 

Does anyone know more about this? How real is this threat, and what is done to prevent accidents and damage to happen?

 

(I think, the documentation was about the guys who maintain high voltage over land lines. They hover close to the cable while one technican leans out (on a small platform) and works on the cable/isolators/whatever. Before doing so, they have to connect the helo with the cable by a small wire so the static electricity can vanish (the high voltage cable is turned off and there is no current :o). Damn great flying btw, and damn scary at the same time! Hrm, I'll see if i find that on youtube ... great vid whatsoever)

 

edit:

(ok, connecting the helo with the cable has nothing to do with static electricty, it seems. It's rather because the cable does have current on it ... anyhow, my initial question remains.)

Edited January 26, 2014 by Flagrum

[javelina1]

I remember seeing something on TV, about the chopper pilots who are part of those maintenance crews for the power lines. as you note, they're one "heck" of a pilot.

[NeilWillis]

Static build up is a real issue for stuff like underslung load work. We always used a shepherds crook device to earth the chopper before we hooked anything on. I have seen a guy thrown several feet when he didn't connect the crook properly, and got a shock.

 

It's why you unknowingly earth your car every time you refuel too. The filler nozzle is grounded, so it discharges the static.

 

Beware when you fill up using jerry cans.

[Flagrum]

found the vid - see first posting :o)

[BIGNEWY]

I've seen some documentation on TV some time ago that mentioned static electricity as a threat for helos. The rotor blades moving through the air induce this static electricity that can cause a high voltage spark that could easily kill a man - for example if you climb into a hovering helo and thus are connected to the ground and to the helo at the same time.

 

Does anyone know more about this? How real is this threat, and what is done to prevent accidents and damage to happen?

 

 

Not sure about "kill a man" but static is a issue. I am ex infantry and one of my roles during that time was a rigger marshaler.

 

Basically we were tasked with attaching sling loads to various helicopters. One of the ways around static is for the rigger to carry a probe, a metal hook that has an insulated handle and a chain to the ground to earth the aircraft before attaching the load.

 

The static is real though, I was once knock onto my arse by a static charge from a sea king helicopter :)

Edited January 26, 2014 by BIGNEWY

[OutOnTheOP]

Come to think of it, that'd make a really cool visual effect in-game for night-time flight: the static discharges off the rotor blades are, in fact, visible as a faint purple-ish glow on the trailing edges of the rotor blades when flying in very dark areas.

 

You'd never notice it when flying over a well-lit city, but over the wilderness, sure.

[Flagrum]

Come to think of it, that'd make a really cool visual effect in-game for night-time flight: the static discharges off the rotor blades are, in fact, visible as a faint purple-ish glow on the trailing edges of the rotor blades when flying in very dark areas.

 

You'd never notice it when flying over a well-lit city, but over the wilderness, sure.

Is that what they call "St. Elmo's Fire" ( http://en.wikipedia.org/wiki/St._Elmo%27s_fire )?

[sobek]

(ok, connecting the helo with the cable has nothing to do with static electricty, it seems. It's rather because the cable does have current on it ... anyhow, my initial question remains.)

 

The cable carrying current is of no relevance. The whole reason for this helicopter repair thing is so they don't need to turn the current off while working on the line.

 

The helo just mustn't be on a different potential or the guy who climbs onto the cable will act as conductor until the potential has equalised on the helicopter and the cable. The reason is indeed static electricity, not the current flowing through the cable.

 

If you want to read more about the mechanism of static charge buildup, you can look up how a Van De Graaff generator works, it's the same principle.

 

Edit: Well actually, while thinking some more on this, the potential of the cable is changing constantly and the helo is indeed a capacitor, so you need to force the potentials together. So there is indeed a difference whether the cable is live or not.

Edited January 26, 2014 by sobek

[hitman]

Does anyone know more about this? How real is this threat, and what is done to prevent accidents and damage to happen?

 

 

(ok, connecting the helo with the cable has nothing to do with static electricty, it seems. It's rather because the cable does have current on it ... anyhow, my initial question remains.)

Static electricity is induced onto the airframe via friction from moving air. Ever notice that when you drive your car, you will sometimes shock yourself when you close your door after you get out and refuel your vehicle? Attaching cables to an aircraft usually just grounds the vehicle to dissipate the static electrical charge on the airframe. All aircraft have static wicks that help with this problem, but I dont recall seeing some on helicopters.

[Revan]

The movie "The Hunt for Red Octeber" showed that

[PLP]

The cable carrying current is of no relevance. The whole reason for this helicopter repair thing is so they don't need to turn the current off while working on the line.

 

The helo just mustn't be on a different potential or the guy who climbs onto the cable will act as conductor until the potential has equalised on the helicopter and the cable. The reason is indeed static electricity, not the current flowing through the cable.

 

If you want to read more about the mechanism of static charge buildup, you can look up how a Van De Graaff generator works, it's the same principle.

 

Edit: Well actually, while thinking some more on this, the potential of the cable is changing constantly and the helo is indeed a capacitor, so you need to force the potentials together. So there is indeed a difference whether the cable is live or not.

 

Well, if I remember my electrodynamics course correctly, static electricity can reach very high voltages because the capacity between the chopper and the power line is very small (you can define capacity between any two conductors). This means that a small charge (created by friction) will translate to a huge voltage difference. That is why static creates a impressive shock without being really dangerous (small charge hence little energy). When the helo is connected to the power line, this static electricity is instantly unloaded, and the whole helo is at the potential of the cable, be it grounded or oscillating at 110kV/50Hz. The point of this connection is to make sure the static electricity is not unloaded through the worker. Power in the cables should not be an issue as long as you stay clear of other cables or the ground, just like when birds land on power cables.

 

I'd attach a bunch of formulas and compute the whole stuff, but I'm far too lazy for that:).

[sobek]

I'd attach a bunch of formulas and compute the whole stuff

 

Mkay, i'd like you to analytically solve me the capacity of the helicopter please. When you're finished, come back and you'll get the nobel price in mathematics (i'm an electrical engineer, btw. ;))

 

The point is that the helicopter is a big heap of metal, so the capacity is not so small that it will give you just a small zap like the leyden jars on your typical lab wimshurst machine. That's a big ass capacitor and the zap might, if not causing serious damage, at least incapacitate someone temporarily, which is not something you'd want while sitting on a platform some 30ft off the ground (possibly having the chopper connected to the wire even, forcing him to hover indefinitely).

 

 

Well, if I remember my electrodynamics course correctly, static electricity can reach very high voltages because the capacity between the chopper and the power line is very small

 

That's just everyday static electricity. You can use any charge separation device (like a Van De Graaff generator) to build up a significant charge that would be able to insta-fry a human being. The point is, the helo is not a small capacity and there's a lot of charge separation going on.

 

They also have to ground helicopters when they land on ships because the static electricity buildup can zap the landing platform platform personnel.

 

The point of this connection is to make sure the static electricity is not unloaded through the worker. Power in the cables should not be an issue as long as you stay clear of other cables or the ground, just like when birds land on power cables.

 

Well, birds have a ridiculously low capacity, so when they equalize their potential to one of the live wires, the current will indeed be small enough to not present any harm. The chopper on the other hand is a reasonably sized capacitor, so to equalize its potential to the line, there could indeed be a significant current. You don't want that under any circumstances to flow through a human. Also since the potential of the line is constantly changing, it won't be just one zap but a continuously oscillating current.

[PLP]

Mkay, i'd like you to analytically solve me the capacity of the helicopter please. When you're finished, come back and you'll get the nobel price in mathematics (i'm an electrical engineer, btw. ;))

 

I'm from Physics - I was more thinking along the lines of a sphere near an infinite cable (that's what we do, simplify the problem until it becomes easy, and then think we know it all, and try to predict life, the universe and everything ;) ). Jokes aside, it should give you the right order of magnitude. The exact shape (sphere or complex chopper) should not dramatically change the capacity AFAIK, exept maybe for the rotorblades.

 

Well, birds have a ridiculously low capacity, so when they equalize their potential to one of the live wires, the current will indeed be small enough to not present any harm. The chopper on the other hand is a reasonably sized capacitor, so to equalize its potential to the line, there could indeed be a significant current. You don't want that under any circumstances to flow through a human. Also since the potential of the line is constantly changing, it won't be just one zap but a continuously oscillating current.

 

You're probably right, I underestimated the capacity that the chopper presented respective to the nearest "ground": the return cable in this case I guess. I'll do a back-of-the-envelope estimate if I find the time (with spheres and infinite lines of course). Although it should not be important if the helo is out of metal or not, as long as it is more or less a conductor. The problem should be shape-dependent before all afaik.

 

I think as you pointed the main problem is that you can build up to very big voltage (200kV was measured by the army in the cited reference) since the air is such a good insulator, and the main charge leak would be due to humidity. But then again as Bignewy mentioned, I'm not sure you can kill someone since the current*time is so small. You apparently need 1350 milijoules for static to be lethal (http://www.straightdope.com/columns/read/3044/can-static-electricity-kill-you).

 

It would be interesting to estimate what current you need to keep the chopper at the cable's potential and how that compares to the miliamp which is the current dose which you start to feel, according to:

http://www.physics.ohio-state.edu/~p616/safety/fatal_current.html

[Griffin]

Is that what they call "St. Elmo's Fire" ( http://en.wikipedia.org/wiki/St._Elmo%27s_fire )?

According to Wiki, it's a weather phenomenon so I guess aircraft static electricity discharge is not.

St. Elmo's Fire does happen on aircraft though:

http://www.airliners.net/photo/VietJet-Air/Airbus-A320-214/2283467/M/&sid=3f9bbe69ac6f58607e271e2a2f21d422

 

All aircraft have static wicks that help with this problem, but I dont recall seeing some on helicopters.

In this picture, you can see two grounding wicks under main landing gear:

http://sitelife.aviationweek.com/ver1.0/Content/images/store/14/11/3eaab8f1-8a8c-48d3-bbb4-b276be9e958f.Full.jpg

 

In this picture you can see two on the fin:

http://www.b-domke.de/AviationImages/NH90/Images/NH90_NFH_N-110_31666.jpg