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Duchess_Driver
24th Nov 2008, 08:48
Some SEP's and light MEP's I've come across specify starting on one mag then once the thing is started switching to both.

Would have thought you'd get a better chance of start/ignition with the switch in both.....can one bright spark (pun intended) shed any light on this please?

Mark 1
24th Nov 2008, 09:23
Traditionally, the LH magneto has an impulse coupling but the RH one doesn't.

So starting on the impulse mag gives a delayed and stronger spark with less chance of a backfire and the potential of damaging vacuum pumps etc.

There may be other reasons too, but that's what I recall.

Capot
24th Nov 2008, 09:50
What Mark 1 said.

Henry VIII
24th Nov 2008, 14:14
Some SEP's and light MEP's I've come across specify starting on one mag then once the thing is started switching to both.
Never heard to now.
To which engine(s) do you refer ?
Tks

Flash2001
24th Nov 2008, 14:32
Let me add a little bit to the (correct) earlier responders. At aircraft engine cranking speeds, chances are that the right mag won't throw a spark at all. That's the reason the impulse coupling is there.

After an excellent landing you can use the airplane again!

Duchess_Driver
24th Nov 2008, 14:59
What's brought the question about is a Lycoming O-360 in an AA5B.

MAGENTO - On LEFT
STARTER - Push
MAGNETO - BOTH

(Abreviated.)

dont overfil
24th Nov 2008, 15:00
This was the correct technique in a Grumman traveller I used to fly. I believe the right mag was also slightly more advanced than the left which would increase the chance of kickback.
DO.

JW411
24th Nov 2008, 15:12
I think I am right in saying that the PA-28-161 Warrior only has an impulse on the LH magneto. Therefore, for starting purposes, the RH mag might just as well not be there and that in turn obviously means that the engine will not start if the LH mag is u/s regardless of the state of the RH mag.

Max Angle
24th Nov 2008, 16:44
Just to add the confusion quite a lot of aircraft with key starts have a system that cuts out the RH mag when the key is in the start position so the pilot is not actually aware that only one mag is being used for starting.

BelArgUSA
24th Nov 2008, 17:42
Standard procedure with the old Piper Cubs.
Mag selector on L MAG...
:ok:
Happy contrails

SNS3Guppy
24th Nov 2008, 17:48
Impulse coupling or shower of sparks.

bfisk
24th Nov 2008, 20:58
The Grumman Cougar is started in this fashion. Says so in the POH.

Moody Alex
24th Nov 2008, 21:20
Only "Max Angle" is partially correct!!!!!!!!!!!!!!

NONE of the other answers are entirely correct!

"Mark 1" is on the right track but not exactly right.

It`s all down to the way the magnetos are "timed", if you don`t start the engine on whichever mag the "impulse" is atatched to (can be either, usually the left) you can do damage!

Unusual an AA-5B with an O-360 instead of an IO-360!

Brian Abraham
25th Nov 2008, 03:52
Magnetos are timed to the engine so as to provide the spark at 20° to 25° before top dead center of piston compression stroke. This timing is designed to provide engine efficiency at normal operating speeds. Since starting occurs at very low engine turning speeds, magneto timing must occur closer to top dead center to push the piston down. The objective of the impulse coupling is to delay magneto firing from 20° before top-dead-center until about 5° before Top-Dead-Center by building in a 15° lag angle into the magneto impulse coupling. This happens only in theory.

The degree of impulse coupling retard is directly proportional to crank speed. Take for example the Lycoming O-235-L2C engine. The original starter cranked the engine too fast. Lycoming Service Instruction 1362 changed the lag angle of the impulse coupling from 15° to 5° to compensate for the fast turning starter and greatly improve starting on this engine. (with the 5° lag angle the engine may have a tendency to kick-back during hand propping. Don't hand prop!)

A later starter (Prestolite MMU-4001R) was specifically designed to turn the engine slower. But be careful now, this slower starter works with the original 15° impulse coupling so the magneto impulse coupling lag angle should remain at 15°.

Now lets say you purchase a starter or magnetos for your O-235L2C engine. To get optimum starting performance you need to match the starter with the impulse coupling.

The same hard starting problem may occur if you switch to a faster turning light-weight starter and don't change the impulse coupling on any impulse coupling magneto.

SNS3Guppy
25th Nov 2008, 04:28
It`s all down to the way the magnetos are "timed", if you don`t start the engine on whichever mag the "impulse" is atatched to (can be either, usually the left) you can do damage!


Not necessarily. You may have a harder time starting, however. Some engines using an impulse coupling make use of both magentos for start, and some don't. Engine damage is seldom the issue at all, but ease of starting is.

Magneto timing takes place in two ways. Magnetos are internally timed to what is known as "E gap," and are also timed to the engine. Both affect the operation of the engine.

Magneto-to-engine timing is the timing to which one generally refers, and to which you are referring; it may be fixed or variable, depending on the system in use. Most traditional magnetos use fixed timing at a set position before top dead center, and some use an impulse coupling or a shower of sparks system to set up a spark advance at low RPM's, or a series of sparks at low RPM's that blanket the advanced position to ensure a lightoff close to top dead center.

Not all engines utilizing one or the other will require a mag shut off for starting. Refer to the aircraft flight manual or operating manual for specifics. Even aircraft of the same make and model, but different vintage or different engine installations may have entirely different procedures; make sure you marry up your procedure to the specific airplane.

Tree
25th Nov 2008, 18:35
A mag in good shape will usually produce a weak spark even at cranking speed. You should turn off the one without the retard function; especially when hand propping.

Mike The pom
27th Nov 2008, 01:24
Won't most drop the right out anyway ? I'm just thinking in terms of what has been mentioned above (PA28) on the key OFF - LEFT - RIGHT - BOTH - START ??

Mark 1
27th Nov 2008, 09:32
Bob Nuckolls of "Aeroelectric connection" fame has written a good article at http://www.aeroelectric.com/articles/MagnetoSwitchOptions.pdf which should explain all.

Mach E Avelli
29th Nov 2008, 04:26
If you fly old Piper Cubs etc and have to hand swing, make sure you follow the procedure according to the engine fitted. A nasty kick back because you got it wrong is not nice with a prop still in your hand.

Graybeard
29th Nov 2008, 05:20
Both my 1963 Cessna 150 with Continental O-200 and my 1946 Bellanca Cruisair with Franklin O-335 had impulse couplings on both mags. Hand propping was quite doable. The mags on the Franklin were timed to 28 BTDC, with the impulse couplings delaying the spark to just ATDC. A cylinder firing with more than maybe 5 degrees advance is sure to kick back.

Automatic spark advance on cars was rare before about 1930, which meant having a spark advance control lever on the steering column. Forget to retard the spark for starting, and you get a broken arm if cranking with the wrong hand. In fact, there is a certain break that was well known to doctors of the era. It was called a Ford fracture, or some such, as the self starter only became an option on the Model T in 1919, some years after most other cars had them.

GB

Tree
4th Dec 2008, 02:28
Won't most drop the right out anyway ? I'm just thinking in terms of what has been mentioned above (PA28) on the key OFF - LEFT - RIGHT - BOTH - START ??


Well sort of except some use toggle switches (Agwagon, Twin Comanche etc.) and some don't have the "start" position (starter is activated by a separate device)

Toggle switches were always fun. Pilots on a runup would turn one mag switch to off, get a rough running engine and return complaining that the mag on that switch was bad. They were always puzzled when I would go to work on the other mag and fix the problem. Think about it!

cockney steve
4th Dec 2008, 11:30
Mr. Abraham, Post#14
[QUOTE][The degree of impulse coupling retard is directly proportional to crank speed./QUOTE]

Sir, this is a somewhat ambiguous statement...I would suggest that it's FIXED according to crank -speed.


for those who are less into the technicalities,-
The flame-propagation is, to all intents, at a constant speed. the advance is given to ensure the charge is burning BEFORE the piston reaches TDC , but expands AFTER tdc....once an impulse-coupled mag is set, it will always "release" at the same timing-point...flicking the Mag smartly as the spring is "tripped" each impulse-coupling has an "operating angle "between arresting mag-rotation and release......the salient point is that at slower cranking-speeds, there is less rotation and therefore the piston is further down the bore FOR A GIVEN AMOUNT OF TIME......faster crank= piston quicker to reach TDC = fire needs to get lit earlier. (otherwise gas expands before piston/crank goes over top,stops it, and pushes it back AKA "kickback" )
so, a given coupling will always give the same retard on impulse,irrespective of speed(as long as it's below the point at which the impulse disengages)


My very limited experience DOES extend to Armstrong-starting a vintage Aeronca....I was taught to "crease" no more than the first joint of the index -finger over the prop leading edge and NOT hook it....free hand behind back firm ,straight-arm pull and follow-through....great fun and I still have all my fingers:}