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Crazy Prop Vibration

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Crazy Prop Vibration

Old 09-23-2010, 07:15 PM
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LargeScale88
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Default Crazy Prop Vibration

Hello,

Well I put a saito with a 13x8 2 blade on my telemaster to break it in for my sport plane, and I had everything all secure, prop tight, mounted tight, so I start it, and I get the tach out, it doesn't hold an RPM. It kept going from 9300 to 9600 to 9400 to 9200 ect. So thats one problem I'm having, not sure what to do, the second problem is it was a prop balanced at the factory, so at idle its shaking like crazy, and if you look at the line of the prop, its way out of balance, but when I give it some juice and the plane moves I can see its not out of balance anymore, and my tail is shaking like crazy. The elevators are vibrating bad, I can hear it, and I'm nervous to take it in the air. Can figure out the problem. Any help?

Thanks
Old 09-23-2010, 07:38 PM
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da Rock
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Default RE: Crazy Prop Vibration

ORIGINAL: LargeScale88

Hello,

Well I put a saito with a 13x8 2 blade on my telemaster to break it in for my sport plane, and I had everything all secure, prop tight, mounted tight, so I start it, and I get the tach out, it doesn't hold an RPM. It kept going from 9300 to 9600 to 9400 to 9200 ect. So thats one problem I'm having, not sure what to do, the second problem is it was a prop balanced at the factory, so at idle its shaking like crazy, and if you look at the line of the prop, its way out of balance,
Depends on the factory whether they're actually balanced at the factory or not. I understand some have a sophisticated machine that holes blanks while comparing the weight of the two sides. By judging the two sides, the machine knows where the middle of the weight is and puts the hole there. They ignore the minor difference in radii that might create.

It's been stated that the casting process is so uniform that cast props are simply checked for imbalance (by machine) and those far enough out of balance are trashed because the difference identifies a poor cast.



.......... but when I give it some juice and the plane moves I can see its not out of balance anymore, and my tail is shaking like crazy. The elevators are vibrating bad, I can hear it, and I'm nervous to take it in the air. Can figure out the problem. Any help?

Thanks
Oh, it's still out of balance but it's not static balance that's the problem. You know those spin balancing machines at tire shops. They deal with dynamic balance and that's probably/possibly the problem with your prop.

Those elevators are vibrating in response to your props imbalance. The vibration from your prop shakes the plane at different frequencies, and the high speed shake gets to the tail. The low speed shake is absorbed up front and you see it there.

Some shake the plane at a frequency you don't see front or back. But bet your airplane on the probability your receiver feels all the frequencies, high low and invisible.

It's always worthwhile to take some time and check balance even if you just take the out of balance ones back to the LHS and don't try to balance them yourself. Although it's usually not too hard to do for the few you will find.
Old 09-23-2010, 07:40 PM
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abaser
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Default RE: Crazy Prop Vibration

Have you put it on a balancer? If not,Im betting that it needs balancing. But my honest opinion, if its making your plane shake that badly, I would toss it and get a new one. I also bet that will also be the problem with the rpm's. The vibration will put an unstable load on the motor.
Old 09-23-2010, 09:36 PM
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Default RE: Crazy Prop Vibration

LS88,

I agree with the above posts.

Rotate the propeller position on the shaft and give it a try.

Check if the problem happens with any propeller.

I have seen some propellers not having the hole and the plane of rotation perfectly perpendicular, which creates strong vibrations, regardless good balance.

Also check for a slightly bent shaft of the engine.
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Old 09-24-2010, 03:45 PM
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Default RE: Crazy Prop Vibration

I'm kinda thinking the hole is not centered as well.....
Old 09-24-2010, 08:27 PM
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Default RE: Crazy Prop Vibration

If he is breaking in a new saito it may be running to rich,they vibrate like crazy till leaned out on the bench,i would'nt break one in attached to an airframe.
Old 09-25-2010, 09:09 AM
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LargeScale88
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Default RE: Crazy Prop Vibration

Iapologize, but I meant to say its a 3 bladed prop, not 2 blade. Its a 13x8 3 blade.
Old 09-25-2010, 10:01 AM
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Default RE: Crazy Prop Vibration

Just in case you want to verify the balance:

http://www.rcaer.com/userfiles/file/...0Technique.pdf

Veifying the condition shown in Post #4 is relatively easy with a fixed dial indicator.
Old 09-25-2010, 04:19 PM
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Default RE: Crazy Prop Vibration

Thanks for the link. It helps alot. Its a master airscrew and the packag the prop came in said factory balanced, and I think thats bull crap.
Old 09-26-2010, 02:58 AM
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Default RE: Crazy Prop Vibration

You do have the wing on the fuse when you're running the motor right??

Smaller glow engines you can get away with just running them in the fuse, larger glow engines really need the whole plane put together, especially four-strokes, which is why running them on the bench for a tank or two is a good idea.

That being said, double check your prop balance first.

Old 09-26-2010, 06:41 AM
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Default RE: Crazy Prop Vibration

I believe "Old Fart" is right on track, as the problem may not be the prop. Is the engine a Saito 82 with a plastic back-plate?

If so, the vibrations will settle down once you run it in, adjust the high and low mixtures, and adjust the valves. Also moving the muffler in towards the engine body will reduce the vibrations. If you have the plastic backplate, they allow the intake manifold to vibrate, wear the o-ring, and change the air-fuel mixture. Horizon sells a metal backplate for the engine, which I highly recommend.
Old 09-26-2010, 04:09 PM
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LargeScale88
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Default RE: Crazy Prop Vibration

Thanks. I was looking at the metal backplate, better than plastic crap.
Old 09-26-2010, 05:52 PM
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Default RE: Crazy Prop Vibration

I don't suppose you might have forgotten to peel the warning sticker off it? I've seen people do that so many times its not even funny anymore.

If it needed reaming to fit the shaft, is there any chance you reamed the hole out a tad too much and its mounted off center as a result? This is another common theme I see all the time.

FWIW, I've seen very few MA's that were actually balanced to my satisfaction right out of the mold.
Old 09-27-2010, 03:49 PM
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LargeScale88
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Default RE: Crazy Prop Vibration

I did peal the sticker off, thats one of the first things I actually checked before I ran the prop on the engine.
Old 09-28-2010, 07:42 PM
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Default RE: Crazy Prop Vibration

Always be sure to the check the rotation of the prop is flat meaning the hole in the prop is 90 degrees to the hub face.   With your prop directly vertical... place someting just touching the front part of the tip at the bottom end then carefully turn the motor over not letting anything move (the plane or better the engine test stand) e and see how the prop lines up again to the fixed point.  Sometimes it's out of whack.

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