prop balancing question

I have an (overpriced) Dubro prop balancer. I have a nylon prop, 6x3, that I believe to be out of balance because of the vibration that occurs when the engine tops out. I took an old wood prop, balanced it, mounted it on the engine, and it was smooth as can be. I put the nylon prop in the balancer, and could not balance it - it seemed to be in balance already. I suspect the Dubro prop balancer just can't balance a prop this light. Any suggestions (other then thro the Dubro in the trash and get a Top flite magnetic balancer)?

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Sometimes the problem is prop flexure..and there isn't much you can do about that.

You did make sure that you didn't have an oversize hole in the prop?

Wrapping tape around the shaft before fitting the prop helps get it centered OK.

Reply to
The Natural Philosopher


You're correct! The DuBro prop balancer IS overpriced. In that case you paid for the name.

Someone on this rcgroup made the statement that ONLY the DuBro could balance a prop larger than 12" dia. NOT SO! The Top-Flite balancer ($20) can balance a prop of ANY diameter. The prop doesn't have to swing all the way in either direction to indicate it's out of balance. One or two inches is enough to indicate it's out of balance.

The Top-Flite balance holds the prop suspended between two magnetics with NO RESISTANCE whatsoever. If the prop drops one or two inches in either direction, it's out of balance.

EarlOfTroy AMA 40329

Ook wrote:

Reply to
Earl Scherzinger

Ahh, that could indeed be it. It's a lightweight, thin plastic/nylon prop, and it would most definitely flex. I'm running on a teedee .051, and it's only when I peak out the engine at high rpm that it vibrates, and the vibration is moderately severe. I could not put it in an airplane with that much vibration.

I'll check the hole. It mounts between cones that go into the hole and hold it centered, I'll have to look into it some more.

On the subject of vibration, what other things can cause vibration at high rpm? I balanced the prop on my OS Max 35, and it smoothed out quite a bit. But my .049/.051 spin at a relatively high rpm, so I imagine it would not take much to get them to vibrate. A bent prop screw could. The spinners are too small to balance, and so small that I don't know if they could cause vibration if they were off a bit.

What I see is that it appears to run fairly smooth, but when I lean it out enough to peak the rpm, a very noticeable vibration starts to occur. Back it off a bit, vibration goes away. My non-teedee .049s can't spin that fast, so I don't notice the vibration in them.

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If the vibration doesn't increase smoothly with increasing speed you're either flexing the prob into an out-of-balance state, or you're causing the prop to flutter, or you're hitting a resonance in your stand.

If it weren't for the evidence of the wooden prop I'd say the most likely cause is that you're hitting a resonance in your stand. A stand resonance won't change the balance condition of your motor, but it'll sure amplify whatever vibration you see. If the wooden prop doesn't allow the engine to spin up quite as fast as the nylon prop does -- I'm still voting for a resonance. If you've never spun an engine quite that fast on that stand with that weight of thing hanging on that end, then you could have easily missed that resonance.

A good way to rule it out is to clamp a big weight very close to the engine mount on your stand -- just about anything sturdy and heavy that you can clamp on with a C-clamp or vice grips will do. It'll lower the resonance and soften it; you'll either dodge it completely or notice the excessive vibration happening at a different speed.

Reply to
Tim Wescott

"Earl Scherzinger" wrote

That is all well and good for balancing the tips, but it does not balance the prop side to side, which is another way to get it in perfect balance.

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Does such a balancer exist? And since the thickness of props is so small, does an imbalance, side to side, make any noticeable difference?

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Maybe simply a too light a prop?

The motor itself may vibrate, and hit resonance with the mountings etc..which a heavier prop would help with.

I balanced the prop on my OS Max 35, and it smoothed out quite a bit.

Is this in a model or on a test bench?

Reply to
The Natural Philosopher

Test bench. And with a heavier prop that is balanced, I don't see any vibration. I seem to have the best luck with wood props. They don't flex much, and I can get smooth runs with them. I have a set of brand new master airscrew 6x3 props I'm going to try and see how they do.

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It can show that the prop is thinner on one side than the other. This might cause flex to be uneven from one blade to the other.

"And since the thickness of props is so small, does an imbalance, side to side, make any noticeable difference? "

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You could try "tracking" the prop. Secure the engine/aircraft so it will not move. Rotate the prop til one blade is straight down. Place an object to mark the exact location of the blade tip. Rotate the prop

180 degrees. See if the second blade tip is in the exact same location. This tells you whether both blades are the same length, and are turning in the same plane. If one blade is bent, the prop won't track correctly. It may help to remove the ignitor plug when turning the prop.

What is being discussed here is "static" balancing, which is not always effective. When anything rotates, there are centrifugal forces generated, which vary geometrically with the RPM. That's the reason automobile wheels are dynamically balanced while being spun on a balancing machine. I haven't heard of props being balanced in that way, although it may be done.

Chuck wrote:

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Exactly. It's called chordwise imbalance, and a good balancer will find it. The prop should stay just about anywhere you place it; if you position it vertically and it wants to fall to a horizontal position, it might be off chordwise. Drilling the hole off centre will do it. Dynamic balancing shouldn't be necessary with small props. Tracking is definitely something to watch, often a problem with bent crankshafts.


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