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912UL Engine vibration on throttle back

in 912 / 914 Technical Questions 12 years ago

  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi,

    To be fair I did the sync at about 3000rpm mainly because I was stood next to the engine with the gauges whilst my assistant operated the throttle. I don't think that it is a sync problem as the fault is not always present at the same load/speed. I feel it to be more of something sticking and not returning to its correct position.

    On the subject of carb sync what would be safer would be cable adjustment at the cabin end of the the throttle cable. You could then sit in the comfort and safety of the cockpit and carry out the sync without going anywhere near the engine.

    I have no idea of the friction torque values of my gearbox. I would be interested in any links to methods of measuring this as well as how to carry out dynamic balancing. The latter I believe requires some accelerometers and test kit.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin,

    we had exactly the same problem with our formers club plane (a 1998 model EV-97 with a 912 UL (80 hp) w/o slipper clutch and a Woodcomp SR2000). The problem did only appear in flight while throtteling back. Carb balancing didn't fix it and we ruled out electrical problems also. Replacing the engine mount rubber grommets did help either. It came down to a propeller issue. The last thing we were investigating before I left the club was either a blade warping (wood core) or a slip within the blade fixing at the hub (this issue I have seen with several props, either through wear or excessive tolerances: just put a small piece of paper around the blade root before tightening the bolts).

    Just my two cents...
    Thank you said by: Rodrigo Saldarriaga
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    How to test friction torque of the gearbox can be found by going to the top of this page , click support, then engine manuals , I believe it is in the line maintenance manual but I could be wrong . I know it is one of the manuals,either line or heavy maintenance , as I follow the manual when I do it on my engine.I just don't have them available right now , there at the hangar.

    Most aircraft mechanics have access to a dynamic prop balancer, some clubs even have there own or one can be had from aircraft spruce. you could try a google search of dynamic balancing as well for all kinds of info.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Claus,

    Thanks for your information, The prop is the one thing that I haven't looked at yet.

    I have spoken to another Eurostar owner at my airfield and he has the same problem. He has a different prop to me, a Kiev 3 blade fixed pitch which is virtually new.

    I probably won't be able to carry out any further investigation until the weekend.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin S,

    You have a UL so you most likely don't have a slipper clutch in the gearbox to test. The exception would be if one had been added.

    If someone has a slipper clutch in their 912ULS or UL if it was added it is an easy test. Some pre 2003 ULS engines don't have slipper clutches either so you will need to check if you don't know.
    Here is an overview of the slipper clutch check.
    Just measure from the center of the prop shaft out along the prop blade between 26" - 30" and put a little mark.
    Take the crankshaft locking pin screw out, you can look down inside the screw hole and rotate the prop until the "V" notch comes into view which is located on the shinny part of the shat lobe. Insert the crankshaft locking pin. When inserted correctly it will be sticking out 3/4" and seated in the center of the "V" notch. The prop will now have a 15 - 30 degree free movement with some friction before it stops. This friction is what you are going to measure. Put a piece of cord around the blade where you marked it. Then using a fish scale (I like the digital fish scales) attach the scale and pull the prop through the free play. Read the scale during the pull. It will read in foot pounds. Take that number and multiply it times the number of inches you marked out on the prop.

    i.e.

    28" x 16.3 ft/lbs = 456 in/lbs That would be the slipper clutch friction check.

    You can read this in more detail in the manuals, but this should give you a good overview.

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

      
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