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.
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>
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
>
>Ken Whiteley
<!DOCTYPE HTML PUBLIC "-//W3C//DTD W3 HTML//EN">
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All,
charge is measured in coulombs and a coulomb per second is an amp).
adequately.
white. These are aerospace products, used on Arianne, and are very
expensive (though we wouldn't need much). Ken Whiteley may know of
applied beneath another coat.
earth clip to fasten to.
the flap brackets on the fuselage bottom, and the anti-static filler cap
cap.
risk.
Dave Simpson
-----Original Message-----
href"mailto:ken@kenwhit.demon.co.uk";>ken@kenwhit.demon.co.uk
href"mailto:Gramin@aol.com";>Gramin@aol.com
href"mailto:europa@avnet.co.uk";>europa@avnet.co.uk
1999 11:05
Subject: Re: Static Charge
faceArial size2>
composite Europa is
static discharge.
static generated by
metal tank could
travel through the
fuel filling nozzle.
conduct through the
if there were
charge and
certain
fuel
the
fuel
solved
Whiteley
href"mailto:info@avnet.co.uk";>info@avnet.co.uk
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