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OS FS-91SII-P Extreme Vibration

Old 08-29-2006, 03:58 PM
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4StrokeBirdy
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Default OS FS-91SII-P Extreme Vibration

I bought a used OS FS91 and finally put it in my airframe. Once started it'll shake like there's no tomorrow! I've checked prop balance and have tried different props. The faster you rev it the worse the vibration is. It shakes so bad that I wouldn't even try to put it to the air. It is mounted in a Kat 70 profile plane. Any suggestions?
Old 08-30-2006, 11:30 AM
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Bax
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Default RE: OS FS-91SII-P Extreme Vibration

We'd want to tear the engine down and check the crankshaft bearings. Bad bearings can cause a lot of vibration.

...And it may not be the engine...it may be the airframe. Profile models are not known for their ability to absorb vibration...there's just not enough structure, and large 4-stroke engines can produce some powerful impulses when they fire. They may be strong enough to cause the airframe to flex. For lightweight models that are intended to have very high vertical performance, the structure starts getting a bit marginal for larger engines.

What size engine was the airplane originally intended to have? If the FS-91 is outside of the airplane's intended engine range, then you may need to go with a different engine.

Check the balance of the propeller and spinner, if you're using one. Out-of-balance components are also sources of vibration, and then when you add a lightweight airframe.....well, we think you get the picture.

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