Re: Vibration on OS-91FX!

i have a problem in the start of the season with the my os91FX with vibration , i have balanced the prop but i didn't help much until i remove the spinner and the vibrations stop . I checked the spinner for balanced and there was my problem.

Yvan "phi486" a écrit dans le message de news:8A7eb.9750$ snipped-for-privacy@newsread3.news.pas.earthlink.net...

Buy a Saito ;-)

A Saito four stroke will shake as much or more than a similar sized two stroke.

Hi Yvan, I'm not too sure if the spinner is the main problem here due to the fact that OS61FX with the same spinner will not make that vibration. I've researched and found out that Sullivan has a shock absorbor type of mounting that is designed to deal strictly with engine vibration. Has anyone ever tried this type of mounting?

In days of yore, we eliminated vibration on some of the old spark engines with an unbalanced prop. With the piston at top dead center, the prop was mounted with the heavy side down. I have not tried it on any newer engines, but just something you might try.

I experienced enough vibration when I replaced an OS 61SF with an OS 91FX on a Big Stik 60 ARF that it split the fuselage. Made sure the prop was balanced (it was). The problem ended up being the two-piece Great Planes adjustable engine mount and their directions to mount the engine as far forward as possible on the mount. I moved the engine as far back as the mount would allow and for the most part eliminated the worst of the vibrations and have been flying trouble-free for 9 months now.

Jim

Jim, Thanks for sharing your info here! Perhaps a one piece fiber-glass or metal mount would take care the vibration. In your opinion, does this method completely eliminate the vibration or just suppress it to a certain acceptable degree?

Thanks again!

I'm no mechanical engineer, but I think with my engine on the forward extreme tip that its vibration was on the same resonant vibration of my fuselage and so it shook the crap out of it. Imagine a moderately vibrating object in your hand - it doesn't seem that bad. Now attach it to the end of a 60" pole and the vibration may seem a lot more magnified. With a 50" or

No flames for the above paragraph please! I fully admit that what I said above probably has no scientific backing that I'm aware of (what do you expect for a rectal extraction?).

Moving my engine to the back of the mount (towards the fore-aft CG of the plane) changed the frequency of the vibration such that it's not that bad. As someone else said, there's always going to be some vibration without a counter-balancer in a single cylinder engine.

I did try a one piece mount on a test stand and it seemed to vibrate less than Great Planes' two piece mount. The problem I was unwilling to tackle though was that the ARF firewall came pre-drilled for the GP mount and I couldn't find a one piece mount that lined the holes up.

So I experimented with what I could and moved the engine and it "solved" my problem.

I've got an Ultra Stick 120 with a Saito 180 that shook pretty well too (my first 4-stroke so I didn't know what to expect). I ended up using the supplied mount but used JTEC's Snuff-Vibe vibration isolators on the mount and it dampened the vibration quite a bit. I think Sullivan also has some anti-vibe mounts with pretty beefy looking isolators.

Good luck.