Hi Steve,
Isn’t this stuff fascinating?
I’ve had many encounters with SAG
all the way from one memorable take off fully loaded on a hot summer day (but
not 9000 ft density altitude) in Tennessee.
Both rotors decided to go into SAG at the same time – really gets your
attention. Being fully loaded for my trip to Memphis and with a full load of fuel, my rate
of climb was about 500 fpm. I was really puckered until I recalled that
flying the old C-150, if I could GET 500 fpm on a hot day, I thought I was
doing great. So I relaxed and climbed to altitude over the air port,
whereupon pulling back on the power caused the SAG to go away and I flew
on to Memphis where Tracy was kind enough to provide me with a set of
non-sagged plugs. I now do not fly without at least two complete plug
changes. It’s tough to pull back on power during take off {:>)
But, it seems to happen most frequently at
high power settings but will gradually extend to every phase of flight
including cruise.
My theory is that at high power settings
your manifold pressure and therefore your combustion pressure is at its
highest. As you know a spark has a harder time of jumping the gap when
pressures are high. So I believe as the plug fouls over with lead on the ceramic
cone accumulating, it becomes easier for at least some of the charge to bleed
off before it builds high enough to jump the gap. Pull back on the
throttle, pressure decreases and the plug fires normally – at least for a
while.
I once flew back from Louisiana without having changed my plugs at
my nominal 25-30 hour point. I was encountering SAG symptoms about every
15 minutes. I discovered that if I use the ignition check switch to turn
off the leading plugs spark (firing on only the trialing) that the EGT would
increase from about 1575-1600F to over 1700F. After restoring the leading
spark system, I could fly another 15 or so minutes before the SAG symptoms
started again. I don’t really know why that made a difference –
did I change the combustion conditions and cause higher temps burning off some
of the deposits – I don’t know.
But typically, the first thing I notice is
a change in the drone of the engine, the second most noticeable thing is the
EGT on the affect rotor will be on its way down typically dropping around 300F
deg. The rpm will decrease as well somewhere around 150-200 rpm (this on
one affected rotor), don’t recall what it did during take off with two
sagging. Generally, if at full power, reducing the throttle seems
to help. Then frequently it will occur - last for a minute or so and then
disappear again. The intervals will generally become shorter and the
effects periods longer. You can easily think you have a fuel or injector
problem when its really not.
I never get use to the engine making
un-normal sounds, so to reduce seat cushion wear, I change my plugs around 25-30
hours, but remember I use 100LL almost exclusively – autogas seems to
give much extended plug use before SAG occurs.
YMMV
Ed
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
On Behalf Of sboese
Sent: Monday, July 06, 2009 6:48
PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Muffler
Some of you may remember that I returned home from the
Rotorfest at the end of May to find the internal baffle of my muffler
broken. I theorized that the broken part may have partially blocked the
outlet and caused a power drop compared to what I should have been
producing. Pictures of the broken parts and repair were posted on the
list last month.
When the weather and my time off coincided recently, I gave the
repaired muffler a flight test after a ground run to be sure things were
working normally. The flight lasted 9 minutes. The plane was much
lighter than when leaving Texas,
but the density altitude was about 9000 ft which is normal for here. The
plane would barely climb, would not accelerate over 110 kts, and the oil and
coolant temperatures were abnormally high. This was quite similar to the
data that I posted last month which was a record of the take off and climb upon
leaving the Rotorfest.
Since the repair to the muffler was not a fundamental change
in design, blaming the lack of performance on the damaged muffler was probably
incorrect. So what was happening? With the engine shut down and still
warm, turning it over slowly with the prop produced a faint clicking noise in a
regular pattern. All indications were that the noise was coming from
internal to the engine. Compression was fine on all compression strokes
when turning with the prop. I thought that maybe the engine had developed
chatter marks in the rotor housings to the extent that the apex seals would
make the clicking noise when turning slowly while also losing compression when
turning fast enough that the apex seal could not follow the uneven surface of
the housing. Pretty remote possibility, I know, but grasping at straws
makes one come up with some pretty weird ideas. This was investigated by
removing the exhaust headers and looking into the chambers. No chatter
marks were seen. Everything I could check short of a complete engine
teardown seemed to be normal.
To make a long story short, replacing the spark plugs
brought things back to normal as demonstrated by a subsequent flight
test. So the data posted last month is most likely not a record of
operation with increased back pressure as I had indicated, but rather one of
operation with spark plug SAG. This also is in line with the observation
during the trip to the Rotorfest at one point where I fully opened the throttle
for a short time and observed the RPM to drop off slightly. I was under
the impression that when SAG occurs, it is sudden and unmistakable.
Apparently with my system, at least, it can be somewhat insidious. The
SAGGED take off when leaving from Texas
didn’t seem unusual since I’m used to the lower power levels
produced at the high density altitudes here. The trip around the pattern
after SAG developing gradually during time right around liftoff was memorable.
Oh, the clicking noise? It turned out to be due to the
slight lash in the crank angle sensor gear when a sensor point was magnetically
pulled into alignment with the pickup. Being a little more hard of
hearing could have saved me a lot of work.
Steve Boese
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