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912UL vibrations...

in 912 / 914 Technical Questions 12 years ago
I have a 912UL engine with 670 hrs on it. I have a new 2-blade 72" IVO prop on my 2002 Merlin aircraft. Vibrations at cruise rpm (5400). I had the carbs syncronized at 500 hrs and even after that it had vibrations. Now I wonder if I should have the carbs synced again or could it be my imagination. (dumb question) Could the problem be harmonic vibrations from the IVO prop? It idles fine and smooth but when i throttle up it vibrates.
Anyone?

  • Re: 912UL vibrations...

    by » 12 years ago

    Hi Andy,

    Carbs should be synced every 100 hrs and they will almost always need a little tweaking then.
    First balance the carbs. If the vibration is still there then it could be one of these:
    Old engine mounts (rule this in or out),
    What is the mag drop, is it equal more or less on 1 & 2,
    The prop may need to be tracked,
    The prop needs to be static balanced then it needs to be dynamically balanced. If this wasn't done when you installed the prop it absolutely needs to be done. If the prop is light weight carbon fiber you may get away with only a dynamic balance.

    You can not always and you can rarely tell if a prop is balanced just by a human feel unless it is so far out it shakes every thing. The vibration frequency also changes when at idle verses 5400 rpm.

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

      
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  • Re: 912UL vibrations...

    by » 12 years ago

    Thanks Roger for the info.
    The reason for me thinking the vibrations are too much is that I rode with a friend in his Sport Star which has a 3-blade prop and a ring mount engine mount which I do not have. It was so smooth and therefore also very quiet. Other pilots have flown with me in my Merlin and they say that it is supposed to vibrate. Maybe their engines are also vibrating and think that it is normal. Well, I still think it should be smoother than it is and I will get to the bottom of this. I will syncronize the carbs as the first step and go on from there. The Rotax 912 is known for its smoothness and my engine should be just that. Thanks again Roger for steering me in the right direction.
    Andy in Calgary.
    IMG_0030.JPGWinterflight.JPG (You do not have access to download this file.)
      
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  • Re: 912UL vibrations...

    by » 12 years ago

    Hi Andy

    Do as Roger says, but also don't compare yours to your friends.

    They are in totally different aircraft, with different engine mounts and most importantly totally different props.

    The two blade prop will have a different vibration than a three blade.

    Mark
      
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  • Re: 912UL vibrations...

    by » 12 years ago

    Thanks Mark. Most of the times a 3 blade prop is smoother than a 2 blade. I'll also have the builder of the kit to go for a ride in my Merlin. He could probably tell me if the vibrations are excessive.

    Andy
      
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  • Re: 912UL vibrations...

    by » 12 years ago

    912 vibrations... Good news.
    Well, syncronization of the carbs did the trick. I did not realize that only 1" of mercury would make a difference. There is still vibrations but that is of course from the fact that the engine is running? One can not eliminate all vibrations. Prop is checked for balance (good), carbs balanced, new lord mounts installed (actually before carb sync) and the engine is firing on all 8 plugs. So there we have it. Balancing of the carbs is very important and I'm rigging up my own vacuum gauges so that I can check the carbs regularly. Thanks all.
      
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