Jerry,
Although I haven't done my electrical work yet, I have a plan. For what
it's worth:
Simple electrical earths, ie. those which serve only as the return (-ve)
line for electrical consumers, could be attached to a ground strip, bus bar
or single (big( terminal) more or
less anywhere - the bulhead is a convenient place. However the proper
grounding
of electrical screens to attenuate Radio Frequency Interference (RFI) is
another matter.
The inductance of even a few feet of thin wire presents high impedances to
very high frequencies; that is to say long wires cannot effectively work as
screen grounding lines if they are very long. So if you want to, say,
screen HT leads, or other ignition lines which carry high frequency (HF)
components and/or high voltages, (and it's always best to try to screen
noise at source rather than at the receiving end) you must ground each
screen as near to one of its ends as possible. Classic, lay theory, says
you should not ground a screen at both of its ends for fear of generating a
ground loop. ie. a circulating current passing through the screen which
itself induces unwanted RFI on to the signal lines it contains. However,
almost as often as not, you can achieve best results by doing just that -
terminating screens at both ends. This tends to be the case where the lead
is long. Eliminating electrical noise is a bit of a black art.
I admit that on composite aircraft with no metal airframe, to which we can
connect screens locally, all this is a bit academic. But we can achieve
good quiet radio and intercom installations with a little care. Virtually
all the electrical noise comes from the engine, some radiated from spark
plug and ignition system leads and some from the Rotax rectifier/regulator
(which generates its own HF and RFI) - this latter can be both radiated and
transmitted along the lines. Make your -ve terminal as near to the
regulator/rectifier as possible and connect there two capacitors: a large
electrolytic, say 2,200mfd and a second non-electrolytic in parallel, value
say 0.1mfd.
Certainly on the Rotax (and Robin) 2 strokes the ignition switch lines
carry noisy signals and act as transmitting antennas. I'm not sure if this
is the case with the 912 but if so, then it would be wise to screen these
lines too; particularly since they run behind the firewall into the panel.
(I've just remembered that there is a Rotax bulletin on this demanding that
they be screened; this after someone flew over a microwave transmitter in a
Katana and the engine missed once due to interference with the electronic
ignition system).
I'm currently operating a Pelican (Canadian kit plane) with a 912, and
there is some radio interference. I would like to try grounding one side
of the alternator output which is currently floating, and carry this ground
through to the engine, dc -ve and screens' earth point. Has anyone out
there tried this?
I've seen some stuff from Bob Nuckolls (Aeroelectric Connection) which is
sound; you may like to ask his advice.
Dave Simpson
----------
> From: Jerry <jerry@flyinghi.demon.co.uk>
> Subject: Electric earthing
> Date: 10 January 1999 14:02
>
> I am interested to know what people have done with their earth wires? I
have
> so many it would be difficult to put them all on one bolt on the fire
wall
> plus if I have to take one off I could have to take them all off. Has any
> body used fast-on crimps and some sort of earthing strip?
>
> I wait in anticipation.
>
> Jerry
>
>
> Flying Hi - Wish I was
> jerry@flyinghi.demon.co.uk
>
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