Hi Steve,

 

It just dawned on me that flying at a density altitude of 9000 MSL, you have only approx ½ the pressure than we folks at sea level have.  That would of course reduce pressure in the combustion chamber and would perhaps give you more time between plug changes before SAG sits in.  So that might be one reason you did not encounter it until you ventured to the low lands.

 

Ed

 

 

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

 

 

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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