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Vibration from what?

in 2 Stroke Technical Questions 11 years ago
Flying this morning when heard and felt a sound like a cable snapping. Then RotAX 582 blue head started vibrating aircraft. Thought I'd lost part of a prop blade, but didn't see anything hit the prop. Managed to land safely - engine still has power and undies are clean.

Ground run on landing - prop appears ok, engine runs to nearly full power (only did this briefly - in case) and all temps EGT on BOTH cylinders are normal and water are normal. But vibration is there and something is not right. Choke is off and throttle cable is normal. Both cylinders show a small drop in revs when ignition switches are cycled.

Totally confused - could this be a failed bearing ?? in the bowels of the engine. Why would that vibrate? Engine has 192 hrs on it and recently overhauled carbies, replaced rubber etc. Appreciate thoughts…...

  • Re: Vibration from what?

    by » 11 years ago

    Morming Jeffery. Have a very close look at your engine mount and attachment.
    Dennis
      
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  • Re: Vibration from what?

    by » 11 years ago

    Jeff,

    Could be a bearing. They don't always cause engine failure suddenly and can give you some warning. I wouldn't run it any more until you find the cause.

    Could also be the gearbox. a broken gear or bad bearing there would definitely vibrate. The gearbox is easiest to check. Drain the oil and look for bits of metal in the oil. Check the prop hub for up and down play. You can remove the front half of the gearbox and examine the gears for damage. With the gearbox off, you can also check for up and down play in the end of the crankshaft.

    You can drain the rotary valve shaft sump and check that oil also. Look there mainly for brass dust or shavings in the oil. also look for traces of water in the oil. Water will usually turn the oil into something resembling chocolate mousse. Also look in the coolant reservoir or radiator for oil floating on top of the coolant.

    Pull off the exhaust manifold and examine the top of the pistons for bits of metal. The bearing ball cage is usually the first part of the bearing to go and it will leave bits in the crankcase, in the cylinder, and even in the intake and exhaust manifold. Also check the pistons for any excessive up and down motion.

    Check engine compression. Not differential compression, but regular cranking compression. Usually, if a bearing is bad, there will be loose play in the piston rod resulting in lower compression in that cylinder.

    Check the weep hole under the magneto for any signs of an oil leak on the magneto end. A bad bearing on that end will usually wreck the oil seal as well.

    Pull the carbs off and check the rotary valve for excessive loose play, indicating worn or damaged valve shaft gears.

    This is about all you can do without tearing the engine down. I hate to say it, but my guess would be a bad crank bearing. They can vibrate excessively for a little while just before they go completely and destroy the engine.

    Good luck,

    Bill.
      
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  • Re: Vibration from what?

    by » 11 years ago

    Dennis is right. check the motor mounts first off.

    Bill
      
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  • Re: Vibration from what?

    by » 11 years ago

    Pulled the donk and sent it to Rotax - needed a TBO anyway. Guess what, no problem! Huh? Back to airframe check mounts so that leaves one thing - the Brolga 68" prop. This time a closer looker and... Whadya know. It's only small but a crack in the gap between hub and outer, and pitch blocks have moved allowing movement of a blade. It WAS the prop, I just didn't see it first time around. It was the loose blade vibrating about 5mm that caused it. New Bolly prop is in the post and I guess I'll get practice torquing up engine mounts and balancing the carbs--------again!
      
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