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912 S heavy vibratios as putting less power.

in 912 / 914 Technical Questions 13 years ago
Dear Rotax users:
I fly a Zenith Zodiac ch 601 with a rotax 912 ULS engine and a 3 blade ground adjustable Warp drive propeller. I have about 350 hours flown with this engine and propeller.
When engine is under effort during take off and in a nose up attitude when getting heiger, engine runs
very very smoth, but when leveling and putting less power reducing throttle, or putting nose down atittude, begins a very rough vibration. I have trayed diferent blade angles, but problem is the same.
Has somebody expiriensed something like this.
I would very apreciate and be grateful if somebody can help me.
Thanks,
Jorge Wenke

  • Re: 912 S heavy vibratios as putting less power.

    by » 13 years ago

    Hi Jorge,

    It could be several things. Your mag drop during the check should be some where around 50-90 rpm (using 3400-4000rpm) for a normal drop. It can be a little higher depending on the mag rpm when you check, but they should be close. Then you need to make sure your carbs are synced. Start at 3500 rpm and when they are balanced there run on up to 4000 and just double check.(Idle rpm for the ULS is close to 1800 +/- rpm) Never balance down at 2500 rpm for the higher rpm sync because you don't fly there and if you balance the carbs there, they will most likely be out of sync at the 3500-4000 rpm range. I have been discussing this with another person here on the forum on the phone. I would make sure your carb vent tubes are in the proper place, also pull the carbs and check the float arm level. They need to be checked because the floats control the fuel flow and if one carb float level is different it can cause this issue. Pitching the plane at different angles can affect the float movement and any misalignment in the carb bowl will cause issues. Having a poorly synced engine that vibrates badly will cause the floats to not meter properly, too. Make sure the carbs set straight up for an orientation. Make sure that the throttle arms move together and from idle stop to full open. Even a little off can make a huge difference.

    The other person found his carbs 5" of vacuum out of sync.

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

    Thank you said by: jorge wenke, YEN NIEN YU
      
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  • Re: 912 S heavy vibratios as putting less power.

    by » 13 years ago

    A few thoughts...what about reduced fuel flow rate causing a vapour lock? Leaking fuel pump diaphragm which is more noticeable at lower power settings?
      
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  • Re: 912 S heavy vibratios as putting less power.

    by » 13 years ago

    Jorge

    I add the following comments only under the assumption the "very rough vibration" you are experiencing is felt through the entire airframe?

    It is remotely possible you could have an engine mount problem starting to show it's ugly face. This is sometimes caused by softening rubber isolators which over time lose their ability to isolate the first order natural frequency of the engine from the rest of the airframe. A contributing factor can also be other airframe components wearing looser over time adding to the negative influence!

    When the 912ULS was first introduced without slipper clutch, this was a common problem on many aircraft types that originally worked well with the standard 80 hp 912. When everything was new and tight it wasn't a problem, but on some aircraft, airframe vibration issues evolved in a very short time. The real issue is the engine mount design was marginal at isolating this very different first order natural frequency of the 912ULS engine.

    Because it was very burdensome for airframe manufactures to redesign the entire engine mount to correct this, it was discovered by installing a slipper clutch on the engine as an addition to it's many other benefits it also changed the first order natural frequency of the engine to a level that most engine mount types could handle.

    That all said, the addition of a slipper clutch can't always totally eliminate the problem forever as you may be now experiencing(assuming your engine already has one?). With the different wear points as I mentioned all adding up to a threshold that again causes that first order natural frequency of the engine to become an issue the engine mount design can't handle! In many cases by getting all the wear points tight again, the problem can be eliminated. In your search for all the contributing factors and wear points that ultimately might have added up to potentially create your airframe/engine vibration, you need to consider almost everything, for example: exhaust system, intake system, rad brackets, engine mount to firewall connections, wings to fuselage connections, control push pull tubes, actual engine mount gussets and tubes etc. only to mention a few!

    Rotax has published a very specific SB on this topic with instruction on how to actually qualify an engine mount design and isolate troublesome vibrations, that's an entire new discussion for another time.

    Good luck, hope this info helps...
    Thank you said by: jorge wenke
      
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  • Re: 912 S heavy vibratios as putting less power.

    by » 13 years ago

    I am experiencing the exact same bad vibration in my RANS S7LSA. I take off full power 5500 rpm level off and retard power to 5000, all is smooth untill I drop the nose down 10 degrees and count to 5 and then the bad vibration starts. I do not touch the power and start a climb and within 10 seconds everythig is smooth. I can be at 5500 rpm and maintain level flight and retard the power very slowly and get back to about 3700 rpm before it starts to vibrate, I must not let the nose go down below level flight during this operation or it will imediately start to vibrate. Thanks Bob Garrabrant
      
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  • Re: 912 S heavy vibratios as putting less power.

    by » 13 years ago

    Robert:
    That is exactly what also happens to me. What propeller do you have?
    Now I am defenitivly taking out my Warp Drive propeller( 3 blades) and installing an in flight adjustable Woodcomp Varia 170 propeller ( 2 blades) to my Zenith Zodiac CH 601 HD. I have a friend who expirienced same vibrations whith the 3 blade Warp Drive propeller. I think Warp Drive propeller blades are to thin, and they twist each diferently depending on propeller effort and rpm.
    Later on I can tell you my results with the new propeller!
    Good luck,
    Jorge Wenke
      
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