1 0r 2 mags?

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bandkiller
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1 0r 2 mags?

Post by bandkiller » Sun Aug 06, 2006 1:23 am

Do you guys run your aircraft engines with both mags on or just run one at a time? which one gets better fuel economy 1 mag or 2? I assume both mags should be ran at the same time but just wondering what every body else is running.

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Whitebear
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Post by Whitebear » Sun Aug 06, 2006 2:52 am

Yup run them both.

An aircraft has a mag switch for applying the ground to each mag when you want to run the engine. The aircraft switch is normally labled (L) or (R), left or right, and the purpose of even having that is to allow a quick mag-health check. Once the engine is warmed some at just before takeoff ya "kill" one mag at a time and look for a slight RPM drop compared to running both. anything in the neighborhood of 1k RPM is grounds to terminate the takeoff and return for maintenance. Seems I remember the Skylane O-470 dropping around 3-400 RPM at mag check, been a long time though. The same can be done in you airboat. There is no reason to not start and run the engine on "BOTH" mags.

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Des Moines Boater
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Post by Des Moines Boater » Sun Aug 06, 2006 8:44 am

I always start and run on both mags in the airboat and on my airplanes. The mag check is to check the timing of each mag. Most allow a 75 RPM difference. To check the mags just turn one off and if the engine is still running then the other mag is good. Now turn them both back on and now shut off the other one. If the engine is running then the other mag is good also. Now shut them both off. If the engine is still running, you have a hot mag and need to fix the defect.
Mags are hot unless they are grounded. The switch goes from the P-lead to a ground.
In aircraft we usually kill the engine with the mixture control, so, you need to check the mags for a bad switch every now and then. Nothing like turning a prop by hand and have the engine start when the switch is off. It happens.
Killing the engine with the mixture control also prevents dieseling on hot engines.

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dblj006
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Post by dblj006 » Sun Aug 06, 2006 9:18 am

The purpose of dual magneto design is redundency. For pilot-passenger safety, if one mag fails in flight you can still make an emergency landing
on engine power. Full engine power is available only with both mags switched on. Toggling between mags on startup will give you magneto
and system info-plug wire or plug failure also.

On my airboats I run with both mags on. I also use the mixture control to shut down. You can also use an inline fuel shut off solenoid, but I like the mixture control better. Using mags to shut down is not reccomended. The engine will still get fuel, hot plugs and cylinder compression will
act like a diesel with glow plugs-it will want to run on or "diesel".

Just my opinion.

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Mike Thurman
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Post by Mike Thurman » Sun Aug 06, 2006 4:33 pm

Run on both mags, thats why they are there. You should only start on your impulse magneto if you only have one. If you try to start on your straight mag the motor will kick back and tear up the starter or ring gear. It depends on your motor and mag set up. Some motors have two impulse mags, some only have one.. The impulse coupling retards the mag until after top dead center to fire. If your ignition timing is around 25 BEFORE top dead center and you are trying to start the motor you may have the motor kick back. The impulse coupling will retard the timing so when the mag fires the piston will be past TDC and on a downward stroke and the spark will continue to push the piston that way.
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