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Hi Dave,
Its been know for quite some time (Tracy
Crook first identified the problem several years ago and cause) that the SAG is
definitely caused by the spark plugs. Change the spark plugs and the
problem goes away immediately. Up to this point there is no conjecture - it
works every time!
I have had it happen a number of times early-on and
initially though it was an intermittent problem with my fuel injection system
due to the dropping EGTs and lost of power - until Tracy mentioned the
cause and cure. In this case, It is definitely not caused by the
ignition system settings.
Now the rest is conjecture on my part - but is my
BEST GUESS of what is happening. Alternative explanations are
welcomed!
The SAG is apparently caused by the build up of
carbon/lead on the center ceramic electrode - which causes the electrical energy
to "bleed" off before the sufficient voltage potential can develop to cause the
spark to jump the gap at the electrodes. You can clearly see the ceramic
completely covered to the point it appears black with carbon/lead which we know
causes a "fouled" plug.
(as a side note - I should have changed the plugs
before the trip but got pressed for time and failed to do so, thinking I had a
few more hours on them.)
Stick in a new sparkplug set with clean ceramic
cone at you can fly more than 100 hours with no-lead/mogas or approx 20-30 hours
if you use 100LL avgas (at least that is my experience). Cleaning
the electrode should also work and I have a bunch of plugs that are waiting to
be cleaned, but since they are not $$ aircraft sparkplugs, just have not gotten
around to doing so.
My conclusion is that since the symptoms are the
same as a piston engine with excessive retarded ignition timing that is
essential what is happening in this case. However, in this NOT a case
of the ignition timing being excessively retarded by the ignition system
setting.
Here is my BEST GUESS of what may be
happening.
We know that higher compression causes a
tougher problem for the spark ignition - it takes more voltage to jump the gap
under high compression condition than lower compression (may sound
counterintuitive - but its been proven). So normally we expect the spark
to be timed such that it fires when the compression is high so we get best
combustion efficiencies. But, that is also the point that puts the most
demand on the ignition system. So when the ceramic cone becomes coated
with sufficient carbon/lead, it bleeds off the electrical energy precluding the
voltage build up sufficient to get the spark jumping the electrode gap at its
intended timing point (at/near the point of highest compression).
However, as the rotor continues to rotate past the point of intended ignition
the pressure now starts to drop considerably - it finally reaches a point where
the remaining electrical energy IS able to jump the gap (due to the lower
compression pressure) and the fuel mixture starts to burn.
However, because you are past the optimum point for
converting the burning fuel into mechanical energy the energy instead ends up as
additional heat which is not blasted out the exhaust as it is normally because
the pressures never get to the normal levels. The burn
fuel instead ends up adding to the heat load of the oil and coolant. (My
speculation about why my temps were elevated during the SAG and the exhaust
temps drop)
Could be wrong of course - but that is my best
guess about what is happening. While my explanation may be incorrect,
there is no question that the spark plugs are the source of the SAG problem and
the cure and it is NOT the settings of the ignition system timing (but it
does acts as if the ignition system setting is too
retarded).
The problem is probably compounded by the type of
spark plug used. The standard 13B plug is shrouded which makes it harder
for combustion gasses to have a cleaning effect on the ceramic cone - compounded
by the fact that the sparkplug is recessed into the rotor housing (to keep the
apex seals from being damaged by an electrode sticking down into the combustion
chamber) - so both factors probably amplify the problem. There are some
"unshrouded" spark plugs that have been mentioned and used by Racers which
may extend the interval before the SAG happens, but I have not yet tried
them.
Anyhow, Dave, that is my speculation about what is
happening.
Best Regards
Ed
----- Original Message -----
Sent: Sunday, March 13, 2005 7:12
PM
Subject: [FlyRotary] Re: SAG Report
So.. did the overtemp cause the SAG? or did the SAG cause the
overtemp? What is causing the over-retarded timing (programming I am
assuming)...
Best Guesses are fine as long as they are identified
as such...
About how many hours had these plugs been running since last
swap..?
Dave
Ed Anderson wrote:
Hi Folks
Just got back from Florida where spent weekend
with Tracy and Laura. Tracy and I flew to the Leeward Ranch RV Flyin
on Saturday. On the way there I encountered the worst case of spark
plug "SAG" ever. We were cruising at 4500 MSL when the old EGT started
south dropping from around 1650 to below 1400 F. First on the rear
rotor then on the front rotor. Most of the time it only happens on one
rotor at a time and will frequently go away after a few minutes - not
this time.
I began getting a little concerned when both
rotors were in SAG and seemed content to remain there. I tried backing
off the throttle, switching injector sets on and off with no avail.
This had been going on for several minutes (longer than normal) when I
noticed something I had never noticed before - either because the SAG had
never remained that long on both rotors OR simply had not scan the oil and
coolant temps in previous sags.
In any case, I was surprised to find the oil at
190F and the coolant a 210F when with the OAT and power setting I was
carrying they should have both been around 165-170F. So it clearly
looked like the same symptoms as overly retarded ignition
timing. This must have lasted approx 15 minutes or more.
Finally on let down to the Flying location the SAG went away and temps came
down.
So on the ground I took out the sparkplugs and
it was clear the ceramic white center was completely coated in lead/carbon -
you could see the lead crystal glint in the sun. Both leading and
trailing look about the same - there was little to no detectable wear on the
electrodes just the ceramic cone black.
Installed new spark plugs and flew it back to
Tracy's and on home today without further incident.
Just wanted to pass this on because it can sure
cause you to think you may have fuel system problems when in reality its the
spark plugs - especially for any of you using Avgas as I do.
Had a great weekend (as always) with the Crooks
and was happy to join them both at a birthday dinner for Laura's father on
Saturday evening.
Best Regards
Ed
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