Ignition switch, what happen when Left or Right is selected

"Guillermo" wrote in message
...
I was wondering what is exactly the function of the impulse-coupled
magneto.
I've read about them a little bit on the web, but I'd like somebody to
explain it in simple terms.

A regular non impulse magneto has the engine rotation directly attached to
the rotating part of the magneto, so at low cranking speeds, it may not be
going past the windings fast enough to generate a sufficient spark.

An impulse magneto has the generating part of the magneto designed just the
same as the regular magneto, but the trickery comes in, at how the magneto
is coupled to the engine.

In the impulse type, when the engine is being turned very slowly, there are
little counterweights that do not allow the generating portion to turn, but
instead, the rotation winds up a powerful spring. When it gets all the way
wound up, and the time has come to make a spark, it releases, and spins the
generating gizmo fast as the devil, creating a powerful spark, which sends
your piston scurrying down it's travel. The next time around, the mag will
be going fast enough to not wind up the spring, and it will behave just like
a regular magneto.

The winding up of the spring also serves to delay the spark until the piston
has barely gone past top dead center, rather than well before top dead
center. That is the reason you shut off the other regular mag; so it does
not possibly fire early, and make the starter have to work against the
attempted backwards combustion.

The impulse magneto is why rotating a prop can be so dangerous. If it has
the mags turned off, but the impulse mag has had it's ground path fail, it
will still wind up the spring. No matter how slowly the prop turns, if it
has a sufficient fuel air mix left in the cylinder and the impulse snaps
over center, the windings will send the spark on the way, thus starting the
engine.

By the way, at OSH, in the HB show area, the young men and women (Aviation
Explorers) helping you park your plane are NOT ALLOWED to touch your prop,
due to safety rules, because of this characteristic.

Clear enough? Ask if you need more clarification.
--
Jim in NC

RST Engineering wrote:


I suppose we can go down the list of every aircraft that was ever
made and
start a table. I'm not going to waste my time doing that.


My old straight tailed 172 had dual impulse mags. Made for very easy
handpropping, which was often required because of the cheesy 20 amp
generator.

Most of the later model Lycoming powered Cessnas and Pipers that I've
seen have the impulse mag on the left only. Piper started doing this
in the Cherokee model right after they quit using shower of sparks.
Standard start procedure was : key switch on the left mag and hit the
starter button.

John Galban=====N4BQ (PA28-180)

Assuming lawyers drive every technical decision nowdays, I wonder why
they did that (recommend switching to single impulse mags)?

Guillermo wrote:
what is a non-impulse coupled magneto? (also a question, not a quiz)

As has been explained elsewhere, an impulse coupling is a feature of some
magnetos which enhances the spark and retards the timing at low revolutions. A
"non-impulse coupled magneto" does not have an impulse coupling.

To give an example, the left magneto on the Lycoming O-320-B2D has an impulse
coupling. The right one does not. When the ignition switch is placed in the
"start" position, the left mag is hot and the right one is grounded out. If the
left magneto fails for any reason, it is impossible to start the engine with
either the starter or by hand-propping.

With that engine, the right magneto is a non-impulse coupled magneto.

George Patterson
"Naked" means you ain't got no clothes on; "nekkid" means you ain't got
no clothes on - and are up to somethin'.

My point was that if it gets past the point where it's going to keep
running, just leave it off. A quick flick of the switch to OFF and back ON
it
will certainly quit firing and lose a few rpm, but it won't die. I've been
doing these as long as I've been flying (the 70s) and I think it's
important to know the mags aren't hot. It's part of my shut down.
You should simply use any method you feel comfortable with to
check the P leads.


wrote in message
ups.com...

It WILL start to shudder and die if you turn it off, unless there's a
bad P-lead connection or mag switch. You want it back on so you can
shut the engine off with mixture, not ignition.

Dan

Exactly. I learned to fly in the early '70s and they didn't
teach us to look for a hot mag. I learned that when upgrading some
years ago. Doing this, I found that one of our 172's mag switches would
allow the mags to keep firing if the key was forced against the "Off"
stop, and upon investigation found an AD against any of those switches
that did that (76-07-12). Bet there are still a lot of them out there.
As far as the discussion about a single impulse mag rather than
two: Lycoming may have recommended a change to one to get rid of the
impulse coupling on one mag. The springs in these things have been
known to fail, and with two mags one might fail and not be readily
noticed if the pilot is a careless twit who doesn't do a decent runup;
if the other spring then failed as well the timing on both would go to
near zero BTDC. Power output would be low indeed. Corrosion has been a
problem in engines seldom flown or flown on very short flights, and the
springs, being a high-carbon steel, corrode quickly. The Bendix dual
mag (one housing, one gear and impulse coupling, two mags) had an AD
against it to replace that spring with a better one, since its failure
could and did cause accidents.

Dan

DT - Good explanation. I'd think a broken impulse spring would cause
the timing to lag so much that the runnup would make it obvious
something is grossly wrong. Maybe not?

wrote in message
ups.com...
Exactly. I learned to fly in the early '70s and they didn't
teach us to look for a hot mag. I learned that when upgrading some
years ago. Doing this, I found that one of our 172's mag switches would
allow the mags to keep firing if the key was forced against the "Off"
stop, and upon investigation found an AD against any of those switches
that did that (76-07-12). Bet there are still a lot of them out there.


Well, I certainly had occasion to turn the key to "OFF" on a 172 and hand it
to my instructor in the right seat while the prop kept spinning. We
reported it (and it was returned "fixed") at least 3 times on that aircraft
before somebody finally managed to isolate and *really* fix the problem.

Otherwise the RPM drop for each mag was consistent, so both mags were
"grounding", but not in the OFF position.

OtisWinslow wrote:
snip

That's a good thing to pay attention to. You can also check it at
idle before shut down by briefly turning the switch to "OFF" and
see if the engine starts to shut down.

I've been taught this shtudown procedure, too.
But, what is the magic about turning them BOTH off, simultaneously?

If, at idle, you turned off the left mag, and noted an rpm drop,
then turned off the right one, and also noted an rpm drop,
then doesn't that confirm the same thing as turning
both of and feeling for the engine to start to shut-down?

Although, maybe it's just me remembering my student fubar
of once turning off both mags during runup, then turning
them back on, and KA-BLAM!

"Wade" wrote

I've been taught this shtudown procedure, too.
But, what is the magic about turning them BOTH off, simultaneously?

As someone else said, it catches a faulty "off" position, in the switch.
--
Jim in NC