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From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-622548@logan.com>
Subject: Re: [FlyRotary] Re: FW: Re: Plugs and Mags
Date: Wed, 26 Jan 2005 08:12:26 -0500
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Some interesting points, Jim.

Both plugs in the rotary are Shrouded, the trailing one more so than the
Leading one.  In a piston engine the plug generally extends a bit into the
combustion chamber, however, if that happened in a rotary (which is fairly
easy to do with the wrong plug) the grounding electrode of the plug can clip
the apex seal as it comes around - not good.

Most of the time the SAG occurs with only one rotor, you can tell because
the EGT drops as much as 300F on one rotor and remains constant on the
other.  I presume this may be due to unburned fuel cooling the exhaust in
the one?  It is much more noticeable with you get a SAG in both rotors -
which generally occurs if you ignore the condition long enough.

I presume the reason it happens mostly under high power settings is that is
a tougher environment to generate a good spark and if the spark plug
condition has deteriorated sufficiently - then at some point it starts to
miss.

Several folks have suggested a "hotter" plug to burn off deposits as is done
in reciprocating engines.  I use stock plugs and Tracy confirms using a
lower cost plug and we both encounter SAG.  However, apparently based on the
latest report from Tracy he is now getting over 200 hours between SAGS
unless he uses 100LL which reduces the time interval. My interval using
100LL is between 20 - 30 hours.

After reading Paul Yaws experiment with sparkplug and the possibility of
some brands extending into the combustion chamber and clipping apex seals -
I have not been inclined to experiment.

Replacing the SAGging plugs with new plugs always cures the problem
immediately and that is what I have been doing.

Ed A
----- Original Message -----
From: "Jim Sower" <canarder@frontiernet.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, January 26, 2005 12:59 AM
Subject: [FlyRotary] Re: FW: Re: Plugs and Mags


> Ed,
> I've been hearing about SAG periodically.  I don't fell like I know all
> of the details or the finer points of the symptoms, but some brain farts
> have occurred to me:
>
> If as you say it's the plugs getting dirty enough around the porcelain
> shroud, to lay down a carbon arc (like in a damp distributor cap) I
> would doubt that trick electrodes would help.
>
> Since it's peculiar to rotary engines and is basically unheard of on
> recips, one might reasonably attribute it to the shrouded plug(s).
>
> I'm wondering if SAG can be replicated on the ground.  Like if you
> experienced it on takeoff, can you reasonably expect (after you've
> recovered) to make it happen on the ground, say during a prolonged high
> power turn up?
>
> I would intuit that since power SAGs but doesn't DISAPPEAR, that it only
> involves one plug.  I would furthermore intuit that it would be the
> shrouded plug.
>
> Is there an accessible primary wire that powers the leading or lagging
> plugs that might be switched for test purposes.  If you could replicate
> the symptoms by shutting down the leading or trailing plugs I think it
> would give you some insights into the problem.
>
> If I'm not mistaken, a very "cold" plug in a recip engine will start
> missing after a relatively short period, and a very "hot" plug can get
> hot enough to lead to detonation.
> What if you were to replace your shrouded plugs with plugs about 3 or 4
> heat ranges hotter and see how long they last.  I can't imagine how a
> shrouded plug could make a hot spot prominent enough to cause detonation.
>
> Or am I missing something REALLY important here ... Jim S.
>
>
> Ed Anderson wrote:
>
> >No, I'm not certain, Mark,
> >
> >However, the plugs are not necessary worn out at 20-30 or 100 hours on
MoGas
> >but they must have the center ceramic cleaned of lead or carbon deposits.
> >Not arguing that platinum and Iridium plugs won't give longer life - just
> >don't  believe they will go significant longer before the SAG hits.
> >
> >Those that have not yet hit the SAG have a thrill awaiting.  The first
time
> >your engine drops in rpm and the EGT drops 300F, you'll likely think some
> >major problem has occurred.  For months I though I had erratic fuel
> >injectors or some intermittent fuel problem.  Tracy is the one that put
me
> >onto the plugs - change to new plugs and the problem ends immediately.
> >Generally  backing off on the power seems to help temporarily and the
> >condition will go away on its on (after getting your heart beat
accelerated)
> >to recur at ever increasingly frequent intervals.  Seems to happened
mostly
> >at high power settings - like on takeoff where you really don't want
> >anything unusual from the engine.  Even though I have had it happen a
number
> >of times, I still have not gotten use to it {:>)
> >
> >Ed A
> >
> >
> >
> >
>
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