Ignition switch, what happen when Left or Right is selected

Greg Farris wrote:

In article ,
says...


Assuming just one magneto with an impulse coupling, the other magneto is
shorted and the starter is engaged (by electrical relay, generally...I'm not
aware of any airplane starter that runs the current straight through the mag
switch).

Note also that this is true only of systems that use an impulse coupling for
starting.

Of course, you could have just posted that information, rather than turning
into a pop-quiz.

Pete




What's the logic of grounding the non impulse-coupled mag during
starting?
(Not a "pop quiz" - I'd like to know).

G Faris



The non-impulse magneto will fire earlier, if at all, and it may
back-fire the engine, as the cranking speed is slow.

--

Tauno Voipio
tauno voipio (at) iki fi

"BTIZ" wrote in message
news:uhzie.7310$Db6.649@okepread05...
and now the other part of that question... for most key switches that have
"Off" "Left" "Right" "Both" "Start" positions.. what happens when you go
to the "Start" position.?.

Already answered. Why are 'L' and 'R' often reversed on the switch?

moo



BT

"Bill Zaleski" wrote in message
...
You are shorting the magneto that is not selected, therefore
inhibiting it's operation.


On Tue, 17 May 2005 22:49:09 -0400, Martin B
wrote:

Hello,

Here is my question:

On regular aircrafts, like C150, C172, PA28, when we perform the
magneto check where we select Left, than Right, while looking for RPM
drop... what is the switch doing?

When we select Left or Right, do we short that particular magneto or
are we shorting the other one?

I checked all my books and cannot find the answer.

Thanks in advance,

Martin

Guillermo wrote:
"Bill Zaleski" wrote in message
...

You are shorting the magneto that is not selected, therefore
inhibiting it's operation.


And that's one of the dangerous things of a magneto system. They are hot on
their own, unless shorted. So if the wiring gets loose in the key switch, or
something else along the circuit gets disconnected, actually you may have a
hot prop, which is very dangerous.
If when you do a magneto check you notice NO RPM drop, then either the
grounding is not correct, or the other magneto is not working at all.


Or things are severely mistimed.

But the fact there is a drop on L and R doesn't tell you things
are working properly in the OFF position.

BTIZ wrote:
and now the other part of that question... for most key switches that have
"Off" "Left" "Right" "Both" "Start" positions.. what happens when you go to
the "Start" position.?.

Depends on the switch. Some switches ground out the RIGHT mag, some
leave both on. It also provides a contact for the starter solenoid.

Some more thoughts on selecting the correct mag for starting -

The impulse function on a mag provides a both a substantial timing
delay and a velocity enhancement for starting purposes. Both are
important. The timing delay of the impulse prevents an engine from
kicking back. That's why starting is done on one (the impulse) mag
only. Most aircraft have only one mag with an impulse, but even a
non-impulse mag may generate a spark at cranking speeds if the mag is
in good condition.

Two things to consider are

1) Be sure the only correct mag is selected if hand propping - and
never put your fingers over the back of the blade. I didn't once on a
TriPacer and still have a fortunately small scar on the tip of my index
finger when it kicked back mightily.

2) Consider that possibly the mag p leads might get reversed in
maintenance. The starter may be able to handle the cranking on the
wrong mag but it could be a hand propping accident waiting to happen.

That's why I won't hand prop a strange airplane for the first time.

"Guillermo" wrote in message
...
And that's one of the dangerous things of a magneto system. They are hot
on
their own, unless shorted. So if the wiring gets loose in the key switch,
or
something else along the circuit gets disconnected, actually you may have
a
hot prop, which is very dangerous.
If when you do a magneto check you notice NO RPM drop, then either the
grounding is not correct, or the other magneto is not working at all.


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.

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

"Greg Farris" wrote in message
...

What's the logic of grounding the non impulse-coupled mag during starting?
(Not a "pop quiz" - I'd like to know).

G Faris



In article ,

says...

Assuming just one magneto with an impulse coupling, the other magneto is
shorted and the starter is engaged (by electrical relay, generally...I'm
not
aware of any airplane starter that runs the current straight through the
mag
switch).

Note also that this is true only of systems that use an impulse coupling
for
starting.

Of course, you could have just posted that information, rather than
turning
into a pop-quiz.

Pete

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

I would have sworn there was an excellent article on avweb.com, I thought it was
by Deakin. I can't find it.

Anyway, an impulse coupling is a device to assist starting. At low RPM
(starting), the spark is retarded, and the impulse coupling spring spins the
magnetic fields faster during part of the rotation to produce a stronger spark.
Usually only one of the two magnetos has an impulse coupling, by convention,
usually the left. When you pull the prop through by hand you can hear the
"sproing!..." as the impulse coupling spins around. You'll hear it twice per
revolution of the crankshaft.

Dave

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.

thanks

guillermo

"Tauno Voipio" wrote in message
...
Greg Farris wrote:

In article ,

says...


Assuming just one magneto with an impulse coupling, the other magneto is
shorted and the starter is engaged (by electrical relay, generally...I'm
not
aware of any airplane starter that runs the current straight through the
mag
switch).

Note also that this is true only of systems that use an impulse coupling
for
starting.

Of course, you could have just posted that information, rather than
turning
into a pop-quiz.

Pete




What's the logic of grounding the non impulse-coupled mag during
starting?
(Not a "pop quiz" - I'd like to know).

G Faris



The non-impulse magneto will fire earlier, if at all, and it may
back-fire the engine, as the cranking speed is slow.

--

Tauno Voipio
tauno voipio (at) iki fi

Just running down the list in my head, I am going to take the opposite
position. Most aircraft have BOTH mags with an impulse; a small minority
have only one with an impulse.

Jim


Most aircraft have only one mag with an impulse