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Did you know:
The capacitor(condenser) across the
points is not there to "protect" them! It forms a resonant circuit with the
primary inductance. Its job is to keep the current flowing through the primary
in a reduced manner so that the voltage generated by this current collapse,
across the points, does not exceed the breakdown voltage across the points as
they open. Don't believe me? Take out the capacitor and see if you can get the
engine to run.
The typical dielectric breakdown
voltage of air at STP is about 50,000V/in. across smooth, rounded terminals.
Pointed terminals reduce the required breakdown voltage. When the
points become pitted, with little points, the breakdown voltage decreases to the
point where the primary voltage across them will arc, sustaining primary
current. So the current change necessary to generate the secondary (high)
voltage is reduced. This causes a weak spark and occasional missing or rough
running. Keep the points smooth, just as you used to in your car! Don't ignore
magneto maintenance!
The "P" (points) lead from the magneto
has all of the primary voltage across it. With wide plug gaps and high manifold
pressure, this can get into the hundreds of volts across the
points. On my old Cadillac, with 4 Amps primary current and primary
reactance of 60 ohms, the peak, open-loop voltage would get to 240V! The
"P" lead should be shielded and the shield connected to the body of the magneto
and the ground connector on the switch. Grounding the shield of this lead to the
airframe or the ground bus makes a new path for the current to return to the
magneto, generating lots of radio interference. Ground the shield at the
magneto, and at the switch, don't connect it to the airplane ground! Wired
correctly, there is no need for a filter in the "P" lead.
Widening the plug gap, as was pointed
out in a previous ms., increases the voltage that has to rise on the primary and
secondary of the coil. This can lead to breakdown across the points, the
coil, or at the rotor and cap, or in the ignition leads, or, as someone
wisely pointed out, across cigarettes in the plugs that were handled and not
cleaned. Even though a wider gap will give a larger flamefront and promote
better ignition, it is better to stick with the miniscule gap recommended by the
magneto and plug makers. Keep in mind, too, that as you go up in altitude, the
lower density decreases the breakdown voltage across these places, so that an
ignition that operates well at lower altitudes may show roughness up in the
'teens or twenties. Turbo-ed systems pretty much should have pressurized
magnetos and the wider plugs which are available. "Course if you operate above
90,000' to 120,000', the breakdown voltage starts back up. Our x-band slot
antennas on the Atlas booster, fed by a 2kW peak transponder, used to
show breakdown across the narrow dimension in this altitude range, muddying-up
the pulse waveform.
If you haven't started installing
conductors from a rear-mounted battery to the starter, see if you can get a
download of the article in the Feb. 1995 Kitplanes which discusses lighter, less
expensive conductors for installation in non-conductive airframes. Also, if
you're interested in inexpensive flashers you can make for your
landing or recognition lights, go to the EAA Ch. 170 web-site and download the
article there. Sorry, for you of that persuasion, this is not the
raincoat/overcoat type of flasher!
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