Starter magneto

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Puma257
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Starter magneto

Post by Puma257 »

The Spit is the first prop I encounter with a "starter" magneto.
Why the need for 3 magneto's?
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Killratio
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Re: Starter magneto

Post by Killratio »

As far as can be determined the Starting Magneto provided an isolate switch from the ground power source (Trolley Acc) and als acted in a similar fashion to the "Booster Coil" that was on later Spitfires,

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redberon2003
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Re: Starter magneto

Post by redberon2003 »

I had assumed it had something to do with Impulse couplings :lol:

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pj_3
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Re: Starter magneto

Post by pj_3 »

A starter mag is indeed, an impulse mag, but the Merlin had only two mags.
Here's a pretty fair (modified) definition:

The figures here are not those of the Merlin, necessarily, but do demonstrate the action as it will apply.

During starting of the engine, the crankshaft is rotating quite slowly (around 120 RPM) and the magnetos at half of that speed. The Merlin's magnetos had four lobes to each cam, and therefore ran at 1/3 crankshaft speed. Generated spark plug voltage is very low at that point. Ignition timing is fixed at 25° - 38° BTDC, and this is too early at low RPM, and if a cylinder would fire it could cause a violent kickback (rotates momentarily in the wrong direction) and probably damage the starter.
There are a number of methods used to modify the ignition system, and to prevent this happening the most common way is to use an impulse coupling to one of the mags, while temporarily grounding the other. It is achieved by retarding the ignition timing on the impulse mag to almost at TDC, and then 'flicking' the magnet inside the magneto, using a coiled spring to 'flick' the armature. This generates a higher voltage at the right timing to ignite the rich mixture at TDC. She will fire without a kickback. When the engine does fire, and its RPM rises - the timing is set back to 25° for normal operation (between 600 and 3000 RPM). The moment the engine fires and runs the impulse coupling detaches and timing is reset to 25° BTDC, with the second mag becoming operational at the same time.

http://www.sacskyranch.com/eng16.htm
http://www.sky-net.org.uk/kelvin/articl ... _dont.html
http://www.experimentalaircraft.info/ar ... nition.php
Last edited by pj_3 on 09 Feb 2011, 11:44, edited 1 time in total.
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redberon2003
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Re: Starter magneto

Post by redberon2003 »

pj_3 wrote:A starter mag is indeed, an impulse mag, but the Merlin had only two mags.
Here's a pretty fair (modified) definition:

The figures here are not those of the Merlin, necessarily, but do demonstrate the action as it will apply.

During starting of the engine, its crankshaft is rotating quite slowly (around 120 RPM) and the magnetos at half of that speed. Generated spark plug voltage is very low at that point. Ignition timing is fixed at 25° - 38° BTDC, and this is too early at low RPM, and if a cylinder would fire it could cause a violent kickback (rotates momentarily in the wrong direction) and probably damage the starter.
There are a number of methods used to modify the ignition system, and to prevent this happening the most common way is to use an impulse coupling to one of the mags, while temporarily grounding the other. It is achieved by retarding the ignition timing on the impulse mag to almost at TDC, and then with an acceleration - a 'kick' of the magnet inside the magneto, using a coiled spring to 'flick' the armature. This will generate a higher voltage at the right timing to ignite the rich mixture at TDC. She will fire without a kickback. When the engine does fire, and its RPM rises - the timing is set back to 25° for normal operation (between 600 and 3000 RPM). The moment the engine fires and runs the impulse coupling detaches and timing is reset to 25° BTDC, with the second mag becoming operational at the same time.

http://www.sacskyranch.com/eng16.htm
http://www.sky-net.org.uk/kelvin/articl ... _dont.html
http://www.experimentalaircraft.info/ar ... nition.php
It's the common starting method for nearly every modern Aviation engine with Magnetos.

The "Booster Coil" is something entirely different though. Makes one think of Carbon pile voltage regulators.

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pj_3
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Re: Starter magneto

Post by pj_3 »

Thar ya go!

http://www.thunderboats.org/history/history0324.html

Ignition: Ignition is by two high-tension rotating magnet magnetos mounted on each side of the wheelcase. The "A" bank (stbd bank - pj) magneto serves the intake spark plugs and the "B" side magneto serves the exhaust spark plugs. The magneto has an integral coil and breaker points operating off a four pole cam, each magneto is driven by a serrated coupling shaft turning at 1.5x engine speed. A distributor turning at 1/3 the magneto speed provides one complete cycle of 12 sparks every 2 revolutions of the engine. Provisions exist for an engine control mechanism to move the breaker base through 25 degrees of advance. In addition, the distributor features a two contacts on its rotor -- the leading finger is energized by the magneto and the trailing finger by an external terminal which can be connected to a booster coil to facilitate starting. The booster coils are essentially high tension coils with vibrating points which provide continuous spark -- when applied to the retarded finger they provide a high energy retarded spark which can greatly facilitate starting, especially with cold, slow turning (thick oil) engines which don't provide much speed for the magnetos. Merlins in aircraft only boosted the "A" bank, but I've seen boats with no boost, with booster coils on both banks, and both combinations in between.

An interesting tid-bit:-
http://www.maltaviationmuseum.com/hurricane.asp
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Jigsaw
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Re: Starter magneto

Post by Jigsaw »

Interesting info, thanks!
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pj_3
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Re: Starter magneto

Post by pj_3 »

@ RedBeron2003
Makes one think of Carbon pile voltage regulators.
Now we are showing our age, aren't we! :D :mrgreen: :wink:
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Scott - A2A
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Re: Starter magneto

Post by Scott - A2A »

pj_3,

We could actually model this kickback but it is a rather costly feature. The starting mag in our MkI pretty much accomplish the task by increasing the voltage at these lower RPM's. Thanks for posting the info.

Scott.
A2A Simulations Inc.

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pj_3
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Re: Starter magneto

Post by pj_3 »

Personally I can do without the kickback, Scott - but I'd really love to see the simulated results of a real wet start....

http://www.youtube.com/watch?v=BzVBlMXJDms
http://www.youtube.com/watch?v=frTA1PVmY_w
http://www.youtube.com/watch?v=Y76LnKuaYro&NR=1
:mrgreen:
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Jigsaw
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Re: Starter magneto

Post by Jigsaw »

Hehe, nice! That would be a nice touch.
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killerwatt
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Re: Starter magneto

Post by killerwatt »

I find I can trash an engine well enough without resorting to kickbacks :oops:
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Bleddyn
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Re: Starter magneto

Post by Bleddyn »

pj_3 wrote:Personally I can do without the kickback, Scott - but I'd really love to see the simulated results of a real wet start....
If that were simulated I am sure I would not be the only one over-priming it on purpose every time. (15°....? Lets give 'er 20!)
:twisted: :mrgreen:
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