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