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My Rotax 912 UL is swinging a two bladed wooden 68x48 Newton propeller on a Reality Escapade homebuilt.
T/O RPM is only 5000 rpm, in the cruise at 4750 rpm, there is an occasional knock which can be felt through the rudder pedals.
I am wondering if this could be overload causing detonation, although I haven't experienced the knock on T/O.
any thoughts on this. the CHT is normal, fuel is new mogas, needles are set as per normal Rotax position.
  • Re: Possible detonation on Rotax 912 UL

    by » 11 years ago


    Hi Malcom,

    It is hard to determine with only the info you provided, but it doesn't sound like it's the engine. Take off rpm is good, WOT rpm should be around 5500-5600 rpm for a fixed pitch or ground adjustable prop. You shouldn't be cruising at such a low rpm. Your engine wasn't designed with that mid range rpm cruise in mind and you will get more vibration, usually a little warmer temps and it isn't in a good torque range for the engine. Your much better off for the sake of the engine to be 5000 - 5300 rpm for a standard cruise rpm and the engine was designed to actually run as high as 5500 all day long. From the info you gave I don't think it is the engine your are feeling in the peddles, but a bump, flutter or vibration in the frame or tail section (the tail is a common offender). Many planes can get those vibrations as you describe in the peddles.

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


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