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Hi Brian,
SAG refers an decrease (or sag) in engine
performance - since we found this sag to be associated with the sparkplug,
ergo the SAG = "Sparkplug Attention Getter". This condition
generally happens - after approx 25-30 hr of flying with 100LL
(considerably longer with Mogas). But using stock Mazda plugs you
will eventually encounter SAG although the new expensive Irridium plugs
may last longer. The sparkplug appears to have normal wear but just stops
working properly.
The first thing I notice is that the drone of the engine
changes note - sure to get your attention. The next thing I notice is the
EGT on one (or both if its gotten bad) will drop several hundred degrees on the
affected rotor. The engine RPM will normally drop around 250-300 rpm -
sure to get your attention. Power is reduced.
It starts off with perhaps one time on a flight and has
you wondering whether you have an injector or ignition problem - once you learn
to recognize it - its no longer heart stopping - just distracting and
annoying. OK, it may happen once then go away for awhile. But, the
frequency of the eposodes will increase and the duration will be
longer.
Normally it happens at WOT - we have discovered that
throttling back will temporarily make it go away (for a while). But,
needless to say, you are most likely to first encounter SAG on take off at
WOT, one is NOT inclined to reduce the throttle during that phase of flight (at
least I'm not). However, It will continue to increase in duration and
frequency until you just don't want to fly you aircraft.
Tracy Crook was the first to my knowledge to make the
association between SAG and the sparkplug.
The worst case I ever had was on the way to a flyin in
Memphis TN when after refueling at a midway point, I took off on a hot day with
a fully loaded RV-6A. Both rotors went into sag right after lift
off. Needless to say I was griping the seat cushion and trying to decide
my next action - when I noticed I still had 500 fpm climb rate - Heck, I said,
when flying a Cessna 150, I thought I was doing great to get 500 fpm. So I
relaxed and circled the airport climbing at 500 fpm until I got to a couple of
thousand feet. Pulled back on the throttle and got my power back.
Fortunately talked to Tracy at the Flyin and found he had an extra set of plugs
which he contributed to the cause.
So I normally get 25-30 hours using mainly 100LL - others
using Mogas get well over 100 hours. So my theory was the lead was fouling
the plugs and until Steve's excellent analysis that was about the best theory we
had to go on.
Hope this helps
Ed
Sent: Thursday, September 30, 2010 12:15 PM
Subject: [FlyRotary] Re: New SAG Theory Discussions of
SAG
Ok ,
since I am incapable of public embarassment, what is SAG other than what I
see in the mirror?
Brian
-----Original
Message----- From: Ed Anderson < eanderson@carolina.rr.com> To:
Rotary motors in aircraft < flyrotary@lancaironline.net> Sent:
Thu, Sep 30, 2010 8:46 am Subject: [FlyRotary] Re: New SAG Theory Discussions
of SAG
Thanks for the clarification, Steve
However, that raises a new question. If the
porosity is even a significant contributing factor
to thermal cycling, I would expect the same thermal cycling
deterioration with Mogas or 100LL - not certain why it would be different
between the two? IF not significantly different, then that leaves the lead
enhanced corrosion as the major culprit - at least that is the way it appears to
me.
Agree, regardless of cause - the answer is replace the
plugs - however, if you discover the colder plugs last longer - then that would
permit me to reduce my bushel basket storage of old spark plugs - cleaning (even
if it were possible) does not appear to be a option any longer
{:>)
Ed
Sent: Thursday, September 30, 2010 11:14 AM
Subject: [FlyRotary] Re: New SAG Theory Discussions of
SAG
Ed,
You take good notes.
Just a couple of clarifications: The
decrease in the thermal conductivity of the copper core may be the result of
thermal cycling causing development of porosity within the copper.
This is independent of the corrosion going on between the Mn alloy
electrode sheath and the insulator. So there are two essentially
independent mechanisms contributing to the heat range change of the spark plug
with age.
The development of the porosity within the copper
is not particularly affected by the type of fuel whereas the corrosion between
the Mn alloy sheath and the insulator is accelerated by the use of
100LL.
The cure is still to install new
spark plugs.
I have colder spark plugs from
Racing Beat that are recommended for prolonged use in high RPM high power
applications. At best, the colder plugs will merely delay the onset of SAG
but won't prevent it from happening eventually. It will be some time
before the results of using those plugs will be known.
Steve Boese
In a nut shell, Steve's excellent data gathering (how many
of use have access to a scanning electron microscope or spectrum analyzer
{:>)) and analysis indicates a possible alternative reason for SAG other than
fouling.
It basically boils down to a theory of pre-ignition
based on the deteorating heat removal capacity of the plug.
There is a copper core (the photos showing the two cores)
in the center of the ceramic cone which is responsible for conducting 90% of the
heat away from the center electrode. Based on Steve's' electron microscope
viewing of two cores (one new and one having been used in a 100LL environment),
it appears that the plug using leaded gasoline has corrosion occurring on the
boundary between this copper core and the surrounding (hot) elements. This
layer of corrosion hinders heat removal from the plug leading to parts of the
plug becoming hot enough to trigger pre-ignition.
This SAG tends to happened at high power settings and
reducing power tends to make it go away (for a while). Steve's theory is
that the plug encounters max heat and temperature at high power settings and
that reducing the power permits the plug to cool off enough for the pre-ignition
to go away (for a while).
So we now have two theories - one is the fouled plug and
one of the overheated plug causing SAG. Preignition generally leads to
detonation which does not appear to be happening (at least no one is finding the
typical detonation damage to the rotary engine). A possible reason is the
quenching effect of the less than optimum shape of the rotary combustion space
may tend to delay or prevent the onset of detonation damage that one would
expect from continuous preignition caused by a too hot plug.
I believe Steve intends to see if he can find some colder
plugs (the ones that conduct heat better) and see if that extends the time
between SAG. Currently I find that roughly 25-30 hours is about the max I
can get using 100LL before SAG sets in, whereas Tracy and others have well over
150 hours using unleaded Mogas. So if the colder plugs extend the time
before SAG, I would say that pretty much would suggest Steve's theory is correct
- if Not, then perhaps the fouled Plug theory still has merit or it could be
some other phenomena happening.
Steve, my apologies if I did not present your case as well
as you could
Ed
.
Sent: Thursday, September 30, 2010 9:59 AM
Subject: [FlyRotary] Discussions of SAG
I understand that there
was a presentation on SAG at the meeting. Can someone go over the
alternative theory with the ones of us that were not in attendance? It
looked like someone had a scanning electron microscope photo that they were
showing! Wow! Are we getting sophisticated or what!! Was this
an explanation of the cause, or ideas for prevention, both??
Bill
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