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912 ULS changes tone

in 912 / 914 Technical Questions 13 years ago
While cruising I have a situation that after flying for a while my engine will suddenly change tone to a deeper and more "throaty" sound. Sometimes it will switch back and forth but will ultimately stay in this state. I don't notice a change in rpm or engine power. Any ideas what might be causing this or things to check to determine the cause?

  • Re: 912 ULS changes tone

    by » 13 years ago

    I have a 3 blade Nueform prop that does the same thing, I was told its the prop adjusting to the rpms...
      
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  • Re: 912 ULS changes tone

    by » 13 years ago

    Prop loading from a very slight rpm change you may not see, wind changing through the prop at different angles, descending or ascending slightly.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: 912 ULS changes tone

    by » 13 years ago

    The batteries in your ANR headsets getting low? ;)

    (just a little humor)
      
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  • Re: 912 ULS changes tone

    by » 13 years ago

    Interesting... I've sent an email to my prop manufacturer (WarpDrive) to get their take. The interesting thing is that I've flow with this prop/engine for about 200 hrs before the issue started. At first it was less common but now has become more common.
      
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  • Re: 912 ULS changes tone

    by » 11 years ago

    I am having exactly this issue in my Sting Sport, right now. In my case, I am quite convinced that it is exhaust-tube (and pulse) resonnance (and sometimes, associated high-frequency vibration)from my #3 cylinder exhaust, which cracked about 15 hours ago. We welded it three times and each time the weld failed; most recently, we welded a sleeve-wrap around the bad area. The sleeve is holding, so far, much longer than just the welds alone, but it (and doing the welding, without gas-purge inside) has surely messed up the gas expulsion characteristics (like throwing boulders into a little stream) harmonics on this stack. I am waiting for a factory replacement of that part--but it is a long row to hoe (i.e., a real problem with the manufacturer and its US distributor that I better not go off on, here...).

    My point is: You may have a cracked exhaust part someplace--or some exhaust part has developed harmonics, getting ready to crack. These cracks can be very small and hard to see; on my #3 tube, you needed a magnifying glass to see it well...
      
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