Model Forum / Radio Controlled / Air Models / April 2004

My experience has been with converted weedie engines that use magneto style
ignition. I have found that if you use a non-metallic throttle pushrod, and
keep the throttle servo a good 6" or more away from the engine, that I have
no interference problems with a standard FM receiver. I don't know how an
electric ignition would play into this.

Not important at all. I use a US 41 engine and have never had a minutes
problem. Use a nyrod for the throttle.

Fred

One of the things PCM will hide is ignition interference.

Using PCM is a reasonable approach once you're sure there's no
on-board interference, and the way you find out is to test the
model/engine with an FM system first.

OTOH, since AM is the most susceptible to ignition-generated RF noise,
test with one of those if you can find one.  When the AM system is no
longer affected by ignition noise, you've got the stuff well and truly
killed.
Cheers,
Fred McClellan
The House Of Balsa Dust
http://home.mindspring.com/~the-plumber

Conventional practice is to keep _everything_ electronic in your radio
system, including wiring, 10-12" away from the engine and any associated
electronics (electronic ignition, its batteries and wiring). A lot of
folks cheat on this, because it's not easy to do, and get away with it.
This means using nylon for throttle linkage (or non-metallic push/pull
cable).

Use of resistor spark plug, and a RF shielded plug connector (usu. metal
jacket) helps.

   I've flown PCM for years but now fly all PPM, and in neither case have I
ever had any ignition interference problems.
   The type of modulation, PCM or PPM (they're both FM), contrary to what
some might tell you, have very little to do with protecting you from RF
interference. The IMPORTANT stuff are things like using a resister spark
plug, not using a metal pushrod for the throttle, keeping the receiver,
antenna, and servo wiring as far away from the engine as practical, using
proper grounded servo wire for long leads (you ground the shield to the
ground wire ONLY at the receiver end of the extension and leave the servo
end open), avoiding ANY metal on metal contact ANYWHERE on the aircraft
unless it's bolted TIGHT together and won't vibrate.
   Stuff like that. That's all I can think of on the fly but I know there
are other things to watch for.
   I've used the different types of ignitions (magneto, points, and
electronic) and I have never had a problem with any of them due to RF
interference. I'm now using a synthesized transmitter so NONE of my
receivers are PCM. The are all FM/PPM and some are positive shift and some
are negative shift... no problems.
   That tells me that a carefully thought out installation of the engine
and radio gear is the most important thing for preventing RF interference
from ANY source.

MJC