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Ultrathrust

Old 05-02-2002, 04:40 PM
  #1  
djlyon
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Default Ultrathrust

Hi Dave
What is the proper operating range of the Ultrathrust muffler for the OS91FX? The info sheet that came with the muffler only goes up to 60 size.
Here's my results for 2 props, 15% fuel, OS#8 and OS F plugs, all on the same day.

13/10 APC - OS muffler 10200 - Ultrathrust 10800
13/8 Master 3 blade - OS muffler 10400 - Ultrathrust 11600

These are peak ground rpms, not flight ready rpms.
I was having a little problem getting a clean needle setting with the ultra thrust but by the end of the session I had dertermined that I had a high speed needle problem and that clouds the results. I found that some times just touching the side of the needle would cause the rpm to jump up 300rpm.
I was surprised at the difference in gain between the 2 props and that's why the question. On the 13/10 near peak rpm the engine sounded like an engine just about to get on the pipe but not quite there. The 3 blade was clean from 11100 to 11600 but if fell below 11100 it would drop to about 10500. It did not seem to want to run between10500 and 11100. Again this is all clouded by the needle being little messed up

Denis
Old 05-02-2002, 06:36 PM
  #2  
Dave Shadel
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Default Ultrathrust

The 13x10 is a bit big for the muffler. I have a long chamber version if you want to run the bigger props. A 12x10 or 13x8 would be better. The numbers on the 3 blade look good.
Sounds like your needle assembly is worn out.
Good Flying,
DS
Old 05-07-2002, 07:02 PM
  #3  
djlyon
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Default Ultrathrust

Hi Dave
A new needle fixed the problem of getting a clean adjustment. I've flown the engine 3 times now with the ultrathrust and have excellent power, idle, throttle responce etc etc.
But I have an anomolie. The engine with the OS muffler behaved like any normal engine on this plane. I set the max rpm about 300 below peak and the idle a little rich to counter a tendency to run lean at the mid range. Everything worked fine.
With the ultrathrust the engine is going very rich in the air at full throttle. I first set the ground rpm at 11100, peak is 11600. In the air it got very rich at full throttle and would hold the rich setting thru big loops and long verticle up lines. it would come off the rich setting slowly on outside loops. I ran the first tank out with no change. I set the ground rpm to 11300 and tried again. No change except it was a little less rich in the air. I set the ground rpm to 11600 (peak) and tried again. Still the same except it will now clean up on long uplines and outside loops but it's still rich while flying straight and level at full throttle.
Is it possible that this is something peculiar to the ultrathrust? I'm a little reluctant to lean it further. I have found I can turn the needle another 1/4 turn in without further change to the rpm and before it starts to sound lean. I have had similar experience with a straight pipe and the fix was just to lean em out a little more even though they seemed at peak. They would cool in the air then go rich I guess.
Any short quick thoughts.
Denis
Old 05-07-2002, 07:09 PM
  #4  
Dave Shadel
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Default Ultrathrust

Richening up in the air would indicate that you don't have enough load on the engine. Try a bigger prop.
Dave

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