All,
Apologies - there is an error in the 5th, last paragraph of my previous text
on anti-static. The penultimate sentence in it should read:
"The route to the wheel needs to be considered carefully but with a little,
even sloppy, preparation it should be possible to make an electrical
connection of less than 20 M ohms between, say, the flap brackets on the
fuselage bottom, to the wheel axle via the flap operating arm".
Dave Simpson
-----Original Message-----
From: Dave Simpson <Dave_Simpson@londonweb.net>
<Gramin@aol.com>
<hlindsay@nmsu.edu>; europa@avnet.co.uk <europa@avnet.co.uk>
Date: 27 March 1999 20:52
Subject: Re: Static Charge
All,
I work at an aerospace company which designs and manufactures composite nose
radomes for civil and military aircraft. Static build up, or p static,
generated in flight by movement of the aeroplane through the air, dust
particles, hail, and snow, can reach such a high voltage that it can
puncture the composite's skin or result in a breakdown over the surface of
the composite to produce electrical interference in (mainly) VHF receivers.
Worst case rate of charge acretion is generally accepted as 150 microamps
per sq. m. (If you think those are funny units it's because charge is
measured in coulombs and a coulomb per second is an amp).
The standard treatment on both commercial and military aeroplanes is a
coating of anti-static paint which traditionally has a resistivity of 3 to
100 Megohms per square (110 Mohms if you work for Airbus). The resistivity
must be low enough to dissipate charging current without generating a
voltage, across the resistance, which is high enough to break down and
produce a flashover on the surface. It is a simple matter to calculate the
resistance from Ohm's Law and, as usual, the aerospace traditional value
works out to be grossly conservative. In fact in this case it is many
orders away from the actual minimum value needed. (11 Giga ohms or 11 x 10
to the 12th). Stay with me - the point is that the resistivity value of the
coating, and for that matter the total resistance to ground, actually
matters little. It can be very high and still perform adequately.
The anti-static paint usually consists of a carrier containing tiny chopped
fibres of carbon which partly conduct. In the case of the nose radome, it
is necessary to ground its rear to the rest of the (usually) metal
airframe - a facility we do not have with the Europa of course. This type of
anti-static paint is black though there are conducting polymers available in
white. These are aerospace products, used on Arianne, and are very
expensive (though we wouldn't need much). Ken Whiteley may know of cheaper,
commercial alternatives. It does not matter if the anti-static paint is
applied beneath another coat.
If builders are really keen to address the static problem properly, and
eliminate the risk of fuel ignition during refuelling, then there is only
one way to do it; that is to ground the charge before the fuel hose pistol
gets to it. Although desirable, it is impractical to tie a ground to all of
the surface of the Europa's fuselage. However it may be practical to ground
an area local to the filler neck by the application of anti-static paint.
We could coat the composite with anti-static around the filler cap over a
circular area of say, 0.5m dia on the outside of the skin, ensuring that
this coating was bled on to the surface of the metal filler neck. The top
coat would go on after. We then connect this patch, via the aluminium
filler cap ring, to the nearest (electrically speaking) ground, that is the
undercarriage leg. A fine wire is all that's needed here. Alternatively we
could provide a conventional grounding point for the bowser earth clip to
fasten to.
I have some concerns about the grounding effectiveness of the U/C leg. The
tyre's not usually a problem since it contains carbon black; the resistance
between rim and periphery on mine is less than 100 k ohms, point contacts.
The route to the wheel needs to be considered carefully but with a little,
even sloppy, preparation it should be possible to make an electrical
connection of less than 20 M ohms between, say, the flap brackets on the
fuselage bottom, and the anti-static filler cap patch. The flap brackets
are the nearest metal item to the filler cap.
Much of this is theory so proceed at your own risk.
Dave Simpson
-----Original Message-----
From: Kenneth S. Whiteley <ken@kenwhit.demon.co.uk>
<hlindsay@nmsu.edu>; europa@avnet.co.uk <europa@avnet.co.uk>
Date: 27 March 1999 11:05
Subject: Re: Static Charge
>I would say that the polyethylene fuel tank of the composite Europa is
>in a particularly favourable position with regard to static discharge.
>The hazard in refuelling a metal aircraft is that static generated by
>fuel rushing through pipe and being collected in a metal tank could
>build up a charge at a high voltage which is free to travel through the
>metal to discharge to a grounded object such as the fuel filling nozzle.
>In the case of a polyethylene tank the charge cannot conduct through the
>tank to discharge anywhere. The only hazard would be if there were
>sufficient metal parts in the tank which could collect the charge and
>then conduct back to the fuel cap. To ignite fuel vapour a certain
>discharge energy needs to be exceeded and I doubt whether the fuel
>filler cap has sufficient capacitance to store sufficient energy. If the
>metal parts of the fuel system are all connected back to the metal fuel
>cap then there would start to be a problem which would have to be solved
>by earthing the fuel nozzle (or metal fuel can) to the filler cap.
>
>Ken Whiteley
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