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912 ULS 3000 RPM Vibration

in 912 / 914 Technical Questions 11 years ago

  • Re: 912 ULS 3000 RPM Vibration

    by » 11 years ago

    Hi Douglas,

    If one or more items that have been listed as hypotheses are present then one carb will not function properly and cause an effect as if the carbs are not in balance. The range of vibs are typically when carbs are not synched or mounts having such a stiffness that it generates a resonance. Mind you it is a 4 piston boxer engine which exhibits excitation in these ranges.

    Cheers Jan
      
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  • Re: 912 ULS 3000 RPM Vibration

    by » 11 years ago

    Jan:

    Thanks for the explanation. If this is true, then should I see the carbs out of balance at this RPM range during a synchronization? That is easy to check and could help me determine which carb is the problem.
      
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  • Re: 912 ULS 3000 RPM Vibration

    by » 11 years ago

    Hi Doug,

    What do you sync your carbs at for RPM? Do you use gauges or and electronic sync tool? The gauge can diagnose problems and the electronic type can't because you can't see what's going on between carbs. What type of prop? If it is ground adjustable are all the blades equal pitch and is it tracking correctly? Is your mag drop fairly equal? Engine temps equal?
    The problem with this particular issue is it can be a lot of things. The vibration right in this range is not that uncommon.



    p.s.
    Just finishing up a 5 year rubber replacement today on a Tecnam. This is one of the easier planes to do a rubber replacement.

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

      
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  • Re: 912 ULS 3000 RPM Vibration

    by » 11 years ago

    Roger:

    I sync the carbs at 2500 rpms and use an electronic sync tool. I agree with using guages and will get some to do future syncs. I have a 2-blade composite Sensenich ground adjustable prop, which I recently checked for pitch (pitch is about 0.3 degrees diff between blades)]and tracking as part of this investigation. I have not had the prop dynamically balanced as Sensenich told me that balancing of the composite props in not necessary. I can only read temp on one side of the engine in flight with my present monitoring equipment, although I could check temperature of each cylinder when the plane is on the ground. Mag drop is approximately the same on both sides, but when I have tried dropping a mag at 3000 rpm it greatly increases the vibration.

    The vibration is not a critical problem in that knowing that it will occur, I just set RPMs above or below to avoid it. However, I know it is not right and would like to get in front of it before it becomes a major problem. At RPM settings at/above 3200 RPMs the engine is smooth and the power is normal.

    The engine vibration started at about 500 hours. I thought by doing the rubber goods change, rebuilding the carbs and changing the engine mounts that I would get rid of the problem, but that has not proved to be true. In addition to what I have mentioned above, I have changed the plugs, and trimmed the plug wires and re-attached the connectors to be sure of good contact. Plugs don't show anything abnormal.

    A friend who has a lot of experience on Rotax engines suggested the issue might be the gearbox. It is not due to be rebuilt as the engine only has 600 hours on it. I also was wondering if this could be a problem with a slightly plugged carb jet which might not show up in an engine sync.

    Thanks for your help.

    Doug
      
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  • Re: 912 ULS 3000 RPM Vibration

    by » 11 years ago

    Hi Doug,

    Try syncing at 3500. To me 2500 is too low. It is possible to be the gearbox even at 600 hrs.

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

      
    Report
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