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Note.  I haven't had my gearbox rebuilt in at least 9 years.

I started the engine on my Katana this past weekend and got some strange vibration.  The engine started ok, but it seemed like the prop was rocking back and forth as it rotated.  It caused a lot of vibration in the cockpit.  It almost seemed like as the prop was turning it was also bouncing back and forth if that makes sense.  I should have taken video.

I have had this happen before in cold weather (less than 32F) and chalked it up to low voltage.

I tested the gearbox friction using a spring gauge (not sure how good the calibration was), but I measured a little over 13 lbs at the near edge of the prop.  The Katana has a constant speed Hoffmann propeller.

Any ideas?

thanks.  John

  • Re: Torsional vibration

    by » 3 years ago


    Hi John,

    How far from the prop center did you measure? 26", 28" or 30"? This is important to multiply your 13 lbs. by. If it was 30" then that's only 390 in/lbs and that's on the low side and I'd have that gearbox checked especially after 9 years. If it was only 26" out on the blade that would be even worse. What I normally see is 440 in/lbs to 490 in/lbs. 460 - 485 is very normal. I would run the engine any more till you get the gearbox checked as it could cause other internal damage.

    p.s.

    What rpm was this problem at? Not too low I hope.

    p.s.s.

    Sometimes spending a little money up front even though we don't want to saves big bucks later from other damage.


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


  • Re: Torsional vibration

    by » 3 years ago


    Roger Lee wrote:

    Hi John,

    How far from the prop center did you measure? 26", 28" or 30"? This is important to multiply your 13 lbs. by. If it was 30" then that's only 390 in/lbs and that's on the low side and I'd have that gearbox checked especially after 9 years. If it was only 26" out on the blade that would be even worse. What I normally see is 440 in/lbs to 490 in/lbs. 460 - 485 is very normal. I would run the engine any more till you get the gearbox checked as it could cause other internal damage.

    p.s.

    What rpm was this problem at? Not too low I hope.

    p.s.s.

    Sometimes spending a little money up front even though we don't want to saves big bucks later from other damage.

    I measured it out about 1 inch from the prop tip as I didn't have a measuring tape handy.  So that should be around 32 inches.  Also, I didn't fully lock the crankshaft per the SB.  I moved the prop forward so it engaged with the engine, then moved it back about 10 degrees before measuring. Not sure if this invalidates my test.  I had expected the tension to be low which would explain the vibration.

    It was at fairly low RPM.  around 1000 prop RPM which should be about 2272 engine RPM. 

    My plan is to overhaul the gearbox anyway, but just wanted to double check that this was most likely the cause.  I did check both ignition circuits L,R, and Both which didn't change the behavior.

    thanks  John

     


  • Re: Torsional vibration

    by » 3 years ago


    Hi John,

    Yep the test wasn't any good. The engine rpm at 2200 is okay, but the prop test wasn't good on a few fronts. This is explained in the Line Maint. manual on how to do it at the end of the manual.


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


    Thank you said by: John Walsh

  • Re: Torsional vibration

    by » 3 years ago


    Gotcha.  Thanks!  I'll read up on it and perform the test again according to instructions.  Do you know what size allen key is required to unscrew the crankcase locking port?  I had imperial wrenches, but didn't get an exact fit.  Also, is it best to heat up the bolt first?


  • Re: Torsional vibration

    by » 3 years ago


    That bolt should come out pretty easily – it's just a plug.  But do you have the crankshaft locking pin (240 880)?  You'll need that; it costs about $7.

    It's also a little tricky finding the notch it goes into on the counterweight if you haven't done it before...


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