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Why Two Mags?

Old 18th Dec 2010, 01:04
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Why Two Mags?

I was going to put this in Tech Log but feel the answer is to be found in history.

The modern day FARs require two mags for redundancy, but I have vague memories of reading that two mags were originally the answer to extracting more power out of the big bore aero engines. Using a single spark plug the problem lay in the speed of the flame front not achieving full combustion of the charge in the required amount of time. Redundancy was merely a useful byproduct of the arrangement. Of course the two plugs needed to be on opposite sides of the head to achieve the desired effect of speeding up the combustion process.

Any thoughts? Be interested in any documentation that may illuminate.

Interestingly the DB601 had the two plugs on the same side of the head and allied analysis thought a power loss in the region of 10% was the result (don't quote me as I am on the road at the moment and the exact details are home on the library shelf).
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Old 18th Dec 2010, 01:43
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Brian, the well-modified Porsche 911SC that I used to take to the track had twin plugs/twin coils/twin ignition sources though not mags--a modification that I made to the engine--and it was solely for the purpose of allowing a substantial compression ratio on what were comparatively wide combustion chambers (the engine was also upgraded from 3.0 liters to nearly 3.4) without risking detonation. I needed to be able to light off flame fronts from two sides of the combustion chamber, not just one.

And after all, a 911 engine is just a roadworthy light-airplane engine...
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Old 18th Dec 2010, 03:42
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I recall when I was a CFIIMEI I always mentioned the reason there were two magnetos.

there were two reasons as discussed by the porche driver above.

I also recall later on how easily the plugs fouled on certain types of 100 octane gas and one set always seemed to foul worse than the other in certain lycoming engines.

one funny story...I recall how a magneto had failed and was removed from one of our planes...a different mechanic was told to fix it and he said...which one? the answer was the one that was ''missing''.

well, he started the plane up to see which mag missed during a runup

but the MAGNETO WAS PHYSCIALLY MISSING FROM THE PLANE

what an awfuol oily mess!!!! be careful what you say.
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Old 18th Dec 2010, 04:32
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From the book Aircraft Propulsion Powerplants, third edition of 1962 by Cargino and Karvinen.
'The Civil Aviation Regulations require that all engines be provided with dual ignition'.........and.......'In most cases, the two ignition systems are timed to fire both spark plugs simultaneously. However, due to variations in conbustion chamber design which affect the turbulence characteristics of the gases and the location of the spark plugs relative to the valves, it may be necessary to employ staggered instead of synchronised firing. With staggered timing the spark plug nearest the exhaust valve is fired from 4 to 10 degrees [crankshaft travel] before the other plug. This is necessary because the mixture around the exhaust valve is contaminated by residual exhaust gases and requires a longer time to burn'.
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Old 18th Dec 2010, 12:13
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If true redundancy is required for magnetos, then why are they driven by one chain around one sprocket?

The 1400 cc VW engine with two plugs pushes out the same hp as with a single source of spark.

Quite often hand swung engines are started on one mag which is modified to ease starting.

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Old 18th Dec 2010, 12:35
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Two magnetos?
Firstly: redundancy. A magneto is a completely self contained ignition system, complex and at least in the early days, not very reliable. It doesn't weigh very much or in the context of a whole aircraft, cost much either.

Secondly, two magnetos allow two plugs per cylinder. In a large cylinder there are real advantages in having the flame front propagate from two spark plugs. Plugs themselves were not very reliable in the early days so redundancy there was useful also.

Thirdly, having two entirely independent systems allows one system to be turned off so as to test the performance of the other. ("Mag drops")

It is the electrical side of the magneto which is considered less reliable. Whilst the drive can, and has failed, most mag. failures will be electrical. The Griffon magneto had two completely separate magnetic and electrical circuits within one unit. If either should fail, the other worked normally.
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Old 18th Dec 2010, 15:05
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hand swung engines are started on one mag which is modified to ease starting.
The words "impulse starter" spring to mind but I never did fully understand exactly what happened with that; did it work on one mag and one cylinder, or did it feed a fat spark to all cylinders.....?
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Old 18th Dec 2010, 16:44
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Two Mags

According to the 'Parts that Give Trouble' section of my copy of "Practical Flying" (1918), "the parts of an engine most liable to give trouble are the sparking plugs .... magneto ''. It then goes on to describe how to check a magneto by touching a live terminal whilst turning the prop. If you get a shock, it's working!

Move forward 52 years to Church Fenton and Chipmunk ground school. We were very clearly told that 2 mags were to provide redundancy, and the importance of doing the check for mag drop. was stressed If there was a significant drop in RPM (I can't remember how much - tempus has fuggited!), the rule was clear - taxi back and shut down.
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Old 18th Dec 2010, 16:52
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I seem to remember that before going back it was wise to select weak mixture and open up to max revs for a while - one minute springs to mind - and check for a mag drop again. 90% of the time you would have "burnt off" the problem.
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Old 18th Dec 2010, 17:48
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weak mixture and open up to max revs for a while - one minute springs to mind - and check for a mag drop again. 90% of the time you would have "burnt off" the problem.
The procedure for a Percival Prentice, if you wanted to get anywhere, like your next port of call, was to carry out a mag check at 1000 ft on the climb out, because if you did it on the ground you ALWAYS got a "return to the apron" mag drop. Gipsy Queens did not like being at an angle, and the back-end plugs oiled up on the ground, even on a full throttle run.

It was, one felt, best to do it within gliding distance of the upwind threshold. ATC never did really understand why a circle around the airfield was necessary on departure.
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Old 18th Dec 2010, 20:32
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I never wanted to get anywhere in a Prentice, except out of it.
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Old 18th Dec 2010, 22:02
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Prentice !!!

This is very unfair to the Prentice,an aircraft that it is possible to have a very nice lunch in during take off.
Also has those nifty wing tips that allow very low turns after t/off.
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Old 18th Dec 2010, 23:57
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Alfa Romeo brought out a 'Twin Spark' engine in the 1980s. At the Birmingham Motor Show I asked one of their engineers on the stand what the reason was, and he said it was so they could get bigger valves in. In this case the diameter of each valve was almost the radius of the cylinder. Offsetting the spark plug would reduce the efficiency, but adding a second one facing it restored it.

A friend of mine has a Mosquito powered hang glider harness with a Radne Racket two stroke engine. He added a second plug to this to help with inflight re-starts. The usual plug runs off the flywheel magneto, whilst the second uses a battery and coil which does not need a high RPM for a good spark.

I believe there is a possibility that if you have two spark plugs, if one fouls but the cylinder keeps firing on the other plug, the first will sometimes clear itself, whereas it is unlikely to do so if the cylinder is just pumping without combustion. Could anyone confirm this?
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Old 19th Dec 2010, 10:53
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[I]I believe there is a possibility that if you have two spark plugs, if one fouls but the cylinder keeps firing on the other plug, the first will sometimes clear itself, whereas it is unlikely to do so if the cylinder is just pumping without combustion. Could anyone confirm this?
Mechta, See post 8 above
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Old 19th Dec 2010, 12:40
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Capot - Magnetos with an impulse starter (often referred to, as an impulse coupling) have a setup consisting of small spring-loaded flyweights located on a spring-loaded, rotatable, driven plate within the magneto housing.
The aim of an impulse coupler is to retard the spark by about 15 at cranking RPM, as well as deliver a hotter spark, than would otherwise be available at cranking speed.

The magneto is flexibly connected through the impulse coupling by means of a spring on the driven plate, so that at low speed the driven plate is temporarily held by stop pins that engage with the flyweights, while the magneto shaft is rotated until the piston to be fired reaches approximately TDC position.

At this point the driven plate is released by other pins or lugs on the magneto shaft, running up ramps on the flyweights, which releases the flyweights from the stop pins... and the spring on the driven plate, kicks the driven plate back to its original position, resulting in a quick twist of the rotating magnet.
This, being equivalent to high-speed magneto rotation, produces a hot spark.

After the engine fires enough to sustain running (around 150-400 RPM), the flyweights in the coupling fly outward due to centrifugal force, and lock the two coupling members together. This makes it a solid unit, returning the magneto to a normal timing position relative to the engine.

The presence of an impulse coupling is identified by a sharp clicking noise as the crankshaft is turned at cranking speed past TDC on each cylinder.
Often the flyweights become magnetized and do not engage the stop pins, resulting in a malfunctioning impulse coupling. Additionally, congealed oil on the flyweights during cold weather may produce the same results.

Single cylinder engines or twin engines that fire 360 apart, have single flyweights for a single impulse every revolution of the magneto shaft.
In engines where a cylinder firing is required every 180 of the magneto shaft rotation, there are twin flyweights, which produce an impulse every 180 of rotation of the magneto shaft.

Last edited by onetrack; 19th Dec 2010 at 13:06. Reason: addendum...
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Old 19th Dec 2010, 15:21
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Gipsy impulse mags have a habit of sticking (detected by an absence of that 'click' during hand-swinging). It amuses onlookers when the hairy old Chippie pilot opens the cowling, whacks the mag with a lump of wood, closes the cowling, and the engine then starts on the first swing.
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Old 19th Dec 2010, 20:26
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Gipsy impulse mags have a habit of sticking (detected by an absence of that 'click' during hand-swinging). It amuses onlookers when the hairy old Chippie pilot opens the cowling, whacks the mag with a lump of wood, closes the cowling, and the engine then starts on the first swing.
Scenario. Flight in a Tiger for a birthday present, his wife in attendance. Engine would not start so got a large adjustable spanner* out of the car to hit the offending mag. Several thumps later engine still not starting, so give spanner to wife who puts it in her handbag and go to find anyone who may be able to help. Come across one of our regular Tiger pilots and request his help. His diagnosis is a sticking mag and has anyone got a large heavy object. Passengers wife immediately retrieves large spanner from her handbag and gives it to the helper. Somewhat surprised he says (I think I am quoting verbatim here but it was a long time ago). "Ah, adjustable spanner in the handbag...............are you married!!!!!

(*For our transatlantic cousins, I think its what you call a wrench )
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Old 20th Dec 2010, 07:29
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New Ford trucks in the US, the 6 litre gas engine has a shortened stroke and big bore and two plugs per cylinder. Not a new idea and still useful.
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Old 21st Dec 2010, 11:02
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It really is that simple.
When it comes to BIG engine performance it really isn't. Redundancy is good, certainly, but its only part of the reason.

We used to get a very "electrical" smell throughout the avionics workshops whenever the magneto test rig was in use - which, with a resident Shackleton squadron was most of the time. There was also a separate side shop in the engine workshops where they cleaned, adjusted and tested the Griffon spark plugs. As stated in earlier posts, magnetos and spark plugs are not the most reliable devices and they require constant and detailed preventive maintenance - and don't forget the ignition harnesses!

Did you hear about the Shackleton captain who arrived back at base with all four engines running? He had to shut one down as he didn't know how to do a four engine approach.

The old ones are best... Ta-taa!
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Old 21st Dec 2010, 18:59
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Why two Mags

Comments about Prentice performance take me back
At BFTS at Burnaston using Chipmunks, the RAFVR on the same station had Prentice.
If you follwed one out, it was wise to wait until it cleared the airfield boundary before starting to roll, otherwise you caught the thing before leaving the circuit..
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