Static Elec or St Elmos Fire or both?

[Vfrpilotpb]

Being very interested in the Gulf Sky News syndrome as Mrs Vfr would say, I noticed several time that Helis filmed at night with a starlite lens seemed to be putting out quiet a lot of high voltage showers from both tail rotors and main rotors, but when the same shot was looked at without the starlite lens or whatever they actually call them the Helis appeared to be totally dark, could you more experianced folk out in Rotorhead land expand this for me, for it would seem all the bad guys need to do is to don their night viz goggles and they would see the good guys comming.

Many regards to you all

[NickLappos]

This was a thread a few months ago.

Those aircraft are hovering, and those flashes of light are the sand particles emitting bright glows as they are disturbed. It is the piezoelectric effect, where a crystal makes a big energy shift when its crystal lattice is mechanically disturbed. this is the way a diamond record needle drives the sound into the amplifier. The goggles amplify about 40,000 times, so these very dim glows look like sparks.

As an experiment, get fully dark adapted, then smash a sugar cube with a hammer. The glow is a shocking green!

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You don't have to be on goggles to see this, I was hovering over a beach the other night using white light to search for a missing person and the sand recirculating into the disc caused the same shower of 'sparks' - very spectacular but not St Elmos fire.

[Lu Zuckerman]

To: Crab

You don't have to be on goggles to see this, I was hovering over a beach the other night using white light to search for a missing person and the sand recirculating into the disc caused the same shower of 'sparks' - very spectacular but not St Elmos fire.

When I mentioned that when I was in Iran I saw a halo like formation around the tip path of several helicopter types I was told I was full of it.

According to Dayton-Granger maker of static precipitation wicks for airplanes and helicopters as the sand particles are struck by or impact on the spinning rotor there is an energy exchange resulting in what is described on this forum as a spark. This energy exchange or spark results in the accumulation of a static charge on the blades and this charge if not carried to the fuselage will result in corona discharge from the blade tips and this is in the form of visible light. (at night)

It is not necessary to have sand entrained in the air to generate the static discharge (corona effect) it can build up at a rapid rate in dry air as well as in humid air.

:cool:

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I don't think I told you you were full of it on this point Lu (others maybe but not this one;)
It is very noticeable on the Sea King, I have seen it whilst ground taxiing at St Mawgan. What causes it I leave to the experts!!!

[Vfrpilotpb]

To Nick, Crab and Lu,

Thank you for the answers, I remember it was aired a few months ago, but in all honesty I had never seen this static show, but the recent overkill by Sky news and co have shown many many shots of the night tasks that did show it very well, what I forgot to add to my first question was:- is the static discharge that can be seen any risk to the airmen or the ground crews, in other words does it have any sort of amperage value, I know that in refuel situations an earth strap is always used but what at the point of touchdown where do the amps or volts go to! :ooh: :ouch:

Edited twice, sorry, today is my birthday Gin & tonic don't allow the keys to be pressed very accurately

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VFR I think you have almost answered your own question ie it is the earthing that is the dangerous bit - you just have to make sure it is not you who links the aircraft with the ground/sea. Our winchmen attach a 'zapper snapper' to the winch hook; this is simply a wire that dangles below the level of their feet so it hits the ground/sea first and allows the static to discharge safely. Anyone who has had the misfortune to be a hooker up for underslung loads will tell you that if you don't use a static discharge lead/pole to touch the hook first then you will get an almighty belt as you place the shackle from the load onto the aircraft hook.

[ShyTorque]

I've flown many hours on NVGs and I've seen St Elmo's fire on my helicopter with the naked eye. The St Elmo's fire was a one off though and it didn't appear on the rotor blades, it was on the pitot heads, windscreen wipers and on the windscreen.

I think there are therefore separate effects causing light to be emitted from rotors, one electrical, one or possibly two mechanical, with the mechanical cause being most common.

Regarding the mechanical cause, on the Sikorsky S-76 we see rotor disc sparking whenever we get into ground effect over concrete, including ground taxying. We see momentary very bright white sparks occurring randomly along the rotor blade leading edges strips, with the naked eye, even above the aircraft white searchlight and landing light beams.

In the UK, concrete usually has a high granite content, which is a very hard rock. The sparking effect is always less when the ground is wet - simply because the grit dust doesn't fly so easily! It's not so marked over tarmac, grass or water, salt or fresh either, usually absent. That's why leading edges get pitted, grit's abrasive!

The most spectacular time for me was on NVG, landing a CH-53 in the desert in recirculating sand. The halo effect from the disc lit up the area around the aircraft like an NVG compatible floodlight.

I understand the piezo electric effect theory and I don't discount it at all, (my garage floor is covered in crushed sugar cubes) but IMHO the light spark is mainly from a tiny fragment of burning metal from the leading edge protection strip, ignited by the impact forces. I get the same effect whenever I use the grinding wheel in my workshop to shape a piece of steel.

If I get a quantity of steel filings from under my grinding wheel I can ignite them with a blowlamp and they burn with a very similar sparkle to what we see almost every night on the 76. :8