Posted By: e57
Re: There is only three things I am afraid of... - 05/20/07 06:14 AM
Would they be hieghts, high-voltage and helicopter pilots named 'Shaky Jake'?
Posted By: Trumpy
Re: There is only three things I am afraid of... - 05/20/07 06:23 AM
Having been trained as a pilot with helicopters around EHV lines, this film put's it so succintly.
All nice and quiet no rotor wash, no comms between the pilot and the Line Mechanic.
Believe me, it's not like that at all.
I've flown choppers next to 500kV DC lines and 110 and 220kV AC lines checking the insulators.
Having a steady hand on your controls is imperative.
Inch one foot forward or sideways can be fatal to both you and the lineman sitting on the deck on your chopper.
One thing has to be said about choppers that are connected to High Voltages like this.
The body of the Chopper is bonded all over the place and even the different parts of the engine are bonded with heavy wire links.
To have a circulating (Eddy) current in a metallic engine would be death to it.
What's more is the fact that the Main Rotor and the Tail Rotor work on bearings.
Having bearing races weld through the Eddy currents, would have the same result.
Posted By: Alan Belson
Re: There is only three things I am afraid of... - 05/20/07 07:47 AM
We have already seen this clip [ posted 30 April '07 in 'General Topics' ]. I cannot add to what I posted at the time.
Stunning, an absolute treat, listening and watching a quiet man, methodically going about his work with no bravado or hysterics.
Posted By: Alan Belson
Re: There is only three things I am afraid of... - 05/20/07 11:08 PM
I have watched this clip several times now and there’s something I don’t understand.
When the helicopter approaches the line, the wand arcs as the potential between the line and ship are equalised, capacitance, well and understood. But why does an arc strike as the chopper leaves? The helicopter and the line are at equal potentials so why then? One would also suppose the helicopter to be charged at 500,000V after moving away and would need to discharge safely to ground potential before landing.
Posted By: renosteinke
Re: There is only three things I am afraid of... - 05/20/07 11:12 PM
Alan, you make the mistake of thinking that the line is the source of that arc.
Actually, the arc is static generated by the rotor moving through the air.
Under testing circumstances, I've actually seen fan blades discharge though dusty air. It's also why fan blades are dusty.
Posted By: e57
Re: There is only three things I am afraid of... - 05/21/07 12:55 AM
I have seen some arc's from much larger helo's (CH-53), and although they can be a hazard of several thousand volts that one would not want to have a step potential, or re-fueling fume across - they were nowhere near that large... I'll make the assumtion that the arc's seen in this video are a combination of both the aircraft and more so the lines. The other side of the main rotor is upwards of 30'+ away - and figure that is the (much lower) potential that needs to be equalized.
Posted By: joncon
Re: There is only three things I am afraid of... - 05/21/07 02:02 AM
It's also why fan blades are dusty.
Damn!! I learnt sumthin today!!!
Posted By: winnie
Re: There is only three things I am afraid of... - 05/21/07 03:21 PM
The same clip was discussed over on Mike Holt's board. I did some looking, and calculated that the _capactive_ current between the lines and the helicopter would be on the order of 50-60mA at 500KV and 60Hz. This would be continuous current, always flowing between the chopper and the line; you would see the arc whenever the two are not bonded but close enough for 500KV to arc over. (This by calculating the capacitance of a sphere somewhere between the size of the helicopter body and the rotor disk.)
On top of this you would have the 'static' current developed by the blades moving through the air. This would only be 1-2mA in the worst possible static charging conditions. (This from DOD studies of discharge devices to eliminate the static build up.)
Finally, I guess that there would be a certain amount of corona discharge from the sharp bits of the helicopter. I've no idea how to calculate the magnitude of this current, but it would again be continuous whenever the helicopter is electrically connected to the line, so you would see it both at approach and departure.