You actually run on both. Part of the pre-take-off check is an engine run
up, at which time you check both mags by isolating to one, then the other,
then back to both. RPM drop is specified by engine - and remember these
engines max RPM is usually 2500. Right after overhaul, typical rpm drop may
be 25 rpm. Drop increases as things age. With a drop of 125 rpm or so, time
to polish up the points and clean and gap the plugs.
Important thing is the difference in drop between mags, not the total drop.
Bear in mind that these are generally fairly low (relative) compression
engines running usually fuel with a high lead content. Also typical bore of
5 inches or so - a spark plug on each side of the combustion chamber makes a
noticeable difference. These engines are almost always air-cooled as well.
Another also, there is a wire that 'grounds' the primary to kill the mag -
so the switch essentially disconnects (turns off) the path of this wire to
ground. So if the wire falls off or breaks, the engine keeps running. (you
kill these engines with a mixture (fuel) control by starving them of fuel)
I think the original point was that aircraft didn't have electrical systems
(too heavy). Then followed by reliability and redundancy. If the mag in
your car quits, you pull over to the side of the road. If the engine in you
airplane quits, you have a bigger problem (you just became the only single
engine glider in the area - what goes up must come down).
Thanks,
Jim.
-----Original Message-----
From: ardunbill@webtv.net [mailto:ardunbill@webtv.net]
Sent: Thursday, June 20, 2002 4:09 PM
To: land-speed@autox.team.net
Subject: Ardun Vertex Ignition
Hi Folks, why two mags on a plane engine? Same reason two solid state
ignition systems on a Nascar racer. Backup. One quits, you flip to the
other.
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