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Date: Mon, 05 Sep 2005 10:42:52 -0500
From: Jim Sower <canarder@frontiernet.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Another great flying day = another day of troubleshooting
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<... very small plug gap can help the problem. I use .010" to .012" on 
both ...>
Wouldn't using hotter plugs help?  Wouldn't that "burn" the lead 
deposits off?  How about CD ignition?  doesn't that have a really short 
rise time?
There's gotta' be a better way than changing plugs more often than oil 
... Jim S.

Lehanover@aol.com wrote:

> In a message dated 9/5/2005 12:31:18 AM Eastern Daylight Time, 
> wdleonard@gmail.com writes:
>
>          Regarding SAG - my analysis is that susceptibility to SAG is
>         proportional to combustion chamber pressure at ignition and
>         the condition of the plugs.  The more pressure - the harder it
>         is for the spark to jump the gap, so if your plugs are getting
>         coated with lead/carbon, I would expect to see more SAG with
>         More manifold pressure.  Tracy and I have found the same thing
>         (unturboed).  At high power setting (higher manifold pressure)
>         the problem becomes more aggravated.  Reducing throttle
>         setting helped - but, replacing (perhaps cleaning the spark
>         plug center ceramic cone) is the cure.  I must have a fortune
>         in little-used spark plugs setting around waiting for me to
>         have nothing else to do but clean spark plugs {:>).
>
>      
>     Ok, that fits except for the fact that at increasing altitudes,
>     the MAP at which the SAG occurred was lower (eg. 36" at s.l. but
>     only 26" at 10k').   So something else is involved but I don't
>     know what.  Time to order more spark plugs.  :-)
>
> You folks are not old enough to remember this, but, you used to take 
> your plugs to the local gas station where there was a Champion plug 
> tester/cleaner. This thing had a high voltage source and a window in 
> it so you could watch the plug gap while increasing air pressure 
> around the plug. When dirty, the pressure could not be increased much 
> at all before the arc would go out.
>  
> You then let out the pressurized air and pushed a button that would 
> sand blast the plug tip.
> You then repeated the test to find that full available pressure would 
> not stop the arc.
>  
> In the rotary there is another problem. The mixture is moving past the 
> leading plug at the speed of a rifle bullet. The fact that it lights 
> at all is amazing. The trailing plug is in a little room by itself, 
> with the fuel air mixture filling in around it much like a piston 
> engine. This is much more likely to work than the leading plug situation.
>  
> When you loose a leading plug, you know right away. About a 15% to 20% 
> power loss. You can loose a trailing plug and not notice it. About 5% 
> power loss.
>  
> In a Kettering style system, the coil is charged up to 12 volts and to 
> fire, the primary voltage is removed (electronically or by a set of 
> points opening) to allow a field collapse an a high voltage (based on 
> windings ratio) to be generated.
>  
> It will soon be the case that there will be no vehicle left on earth 
> that uses the Kettering system, because of the pollution and/or fuel 
> consumption problems.
>  
> The field collapse system in electrical terms is very slow. The 
> secondary (high) voltage develops slowly, and this allows time for a 
> path of flow to develop around the plug gap.
> This what you are feeling in the SAG. Once a path of carbon is 
> established, there is not sufficient voltage left in the parallel path 
> to jump the plug gap.
>  
> A high energy system that has a very short rise time, eliminates the 
> problem.
>  
> Also a very small plug gap can help the problem. I use .010" to .012" 
> on both leading and trailing plugs with an MSD on both systems. One 
> set of plugs per year. No problems up to 10,000 RPM.
>  
> Lynn E. Hanover
>  
>  
>  
>