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Old Mar 23, 2012, 11:09 AM
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Magnetic prop balancer, questions about balancing hubs

I got this at my LHS:
http://www.ebay.com/itm/Top-Flite-Po...item256c9ea18e

I had a bag of props balanced with my home made balancer, so I started checking those. Most of them will hang perfectly level horizontally, but only on one side of the hub. Given any opportunity it will always flip over so the same side of the hub is on the bottom, with the blades still perfectly horizontal. This tells me the blades are balanced, but the hub is either drilled off-center or heavier on one side(I bet I can guess which). It should park at any angle, either where I place it, or when it finishes spinning on its own. The instructions say never to remove material from a hub, but to counter balance it by adding weight. This is a safety precaution. But I'm not balancing TF wooden props, I'm balancing black Cox props, the toughest freakin' stuff ever molded into a propeller.

Any thoughts on taking one to the drill press and punching just the point of a small bit(not the body of the bit) into a Cox hub to balance it? It makes a nice clean cone shaped divot. I botched one, by drilling a small hole, and then another and another. I threw it away. My next one came out perfectly after making one tiny divot. It will park any place I stop its rotation on the balance shaft. If a hub needs more material removed, I figure I should use the point of the next size larger bit, rather than making more divots. But I haven't done that yet, I figured I should stop and ask first.

I don't think tape is going to stay on a hub. Nor is CA glue heavy enough. Any thoughts y'all can offer a novice prop balancer? I like the Cox props because they are easy to cut down to the sizes I need, mainly 5.5x3 and 5.25x3.
Thanks,
Rusty
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Old Mar 23, 2012, 06:57 PM
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Since I tried working on some more, the drill turns out to be a bad idea. So I answered that question myself.

Plan B. I also have a table top belt sander, so I tried sanding off some material on the heavy side of the hubs. I did it on old ones I can toss if I botch it. I'm surprised at how much plastic has to come off. Trying to keep the curve neat, but it's obvious where it's been ground down. But they will finally balance before I get down to the drive plate and spinner facings. I'm going to go with this. As tough as the rubber duckies are, I don't think they are in danger of self destructing.

I have some old Windsor props, I see on their site, that's the same as MA props I've heard about. They were almost perfect right off the bat. I think I'll get some of those. They look more brittle, so I'll be more careful with them, but their shape lends to easy cutting down.

So if I don't get a blade through my eye and out the back of my head, I'll come back to see any warnings or advice that might pop up.
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Old Mar 23, 2012, 07:50 PM
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You may also see a situation where one blade will be trailing edge heavy while the other is leading edge heavy. This would result in the problem you observed.
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Old Mar 23, 2012, 08:45 PM
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Although I religiously balance All my edf rotors as well as my 'lectric props.
For 1/2A all I ever do is determine the heaviest blade and position it opposite to the piston. Typically, all of these wee cranks have nowhere near the counterbalance that they should have.. using the heavier prop blade thusly helps on running smoothness.. more so than a balanced blade does.
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Old Mar 23, 2012, 08:53 PM
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Quote:
Originally Posted by Gnu4FF View Post
You may also see a situation where one blade will be trailing edge heavy while the other is leading edge heavy. This would result in the problem you observed.
Quote:
Originally Posted by Bare View Post
Although I religiously balance All my edf rotors as well as my 'lectric props.
For 1/2A all I ever do is determine the heaviest blade and position it opposite to the piston. Typically, all of these wee cranks have nowhere near the counterbalance that they should have.. using the heavier prop blade thusly helps on running smoothness.. more so than a balanced blade does.
Neither of those ideas would have occurred to me. Thanks for the tips, Gnu and Bare.
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Old Mar 23, 2012, 10:16 PM
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I shouldn't admit this but I rarely balance my Cox props, especially those that I run on reedie .049s. I spend tons of time getting the props to balance on my larger birds or high-revving 1/2As, but tend to run the slower stuff "out of the box". There, I feel better. Flame away.
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Old Mar 24, 2012, 02:33 AM
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I never worried much about it either until lately when I started putting together engines that turned enough RPM to suffer from off-balance props. My new Black Widow build vibrated so badly at 18.5K that you couldn't see the spring tip. And on one test run with a Cox 5x3 it wouldn't even turn 17.5k. I put a Top-Flite nylon on it and it ran steady over 19k and the spring was dead still. I worried about tearing up the front bearing.

Then I built a Killer Bee(except for the stock Bee crank) for my PT-19 last week that hit 22K RPM on the bench and I thought a bad prop could really hurt it at that speed. I can't afford to wear them out in one season, so I decided I should start balancing.

After reading what Bare said, I'm going to balance the blades and put the heavy side of those hubs opposite the piston's TDC.
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Old Mar 24, 2012, 07:42 AM
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We used to put the heavy blade on the counterbalance side in the Foxberg control line racers to balance the motors. I agree props should be balanced especially for high revving stuff, but the newer magnetic balancers are maybe showing more out of balance conditions than needed for most planes that are used at the field. (unless you fly FAI Speed or something at 40,000 rpm) Most plastic props are balanced fairly well at the factory, although wood and fiberglass ones sometimes are off a fair bit. For the above post If you don't want to wear out the main bearing, it would be better to rev a bit less with a balanced prop. I don't use 5-3" props on the plain bearing .049's anymore because all my mouse race cases are worn out. I put brass bushings in some but it is a lot of hassle.
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Old Mar 24, 2012, 09:06 AM
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Thanks aspeed, I don't think we've crossed paths since you joined RCG. Welcome, this forum is a great resource with helpful people.
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Old Mar 24, 2012, 10:49 AM
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You need to be careful with that balancer. The shaft is rather soft and easily bent, which could give you misleading results.
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Old Mar 24, 2012, 11:07 AM
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Hmmm significant vibration out of a BW suggests a serious issue.. are you sure it's just the prop? 19k is not exceptionally high for those.

I wonder ( suspect actually :-) If it's got some goofy crank fitted.

Suggest you do not balance the prop just stick a wee screw driver thru it's hole to determine the heavier blade then stick it on the engine.... try it for one run.. then decide.
Typically the downside is that the pesky prop is not in optimum position for finger flip starting. Bit of a nuisance initially to hold the model at odd angles for starting. Never used the springs.. except on my v first baby bee, for a few days..
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Old Mar 24, 2012, 10:06 PM
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Single cylinder motors are really only balanced at one speed anyway, so some rpms will just vibrate if the harmonics are just wrong, or the fuselage frame is a little weak. The Cox stuff does have a heavy steel piston too. An .051 cyl. will shake a lot on a reedy and it is only a few thou. bigger than an .049.
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Old Mar 25, 2012, 12:57 AM
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Quote:
Originally Posted by GliderJim View Post
You need to be careful with that balancer. The shaft is rather soft and easily bent, which could give you misleading results.
Quote:
Originally Posted by RknRusty
I have some old Windsor props... They were almost perfect right off the bat.
Yes I know, Jim, it needs to be softer in order to conduct the magnetic flux better than hard metal. The booklet said as much. I have checked it with only the cones and it's fine

Quote:
Originally Posted by Bare View Post
Hmmm significant vibration out of a BW suggests a serious issue.. are you sure it's just the prop? 19k is not exceptionally high for those.
I wonder ( suspect actually :-) If it's got some goofy crank fitted...
It was the prop.
It does not have any unusual vibration.
Quote:
Originally Posted by RknRusty View Post
... I put a Top-Flite nylon on it and it ran steady over 19k and the spring was dead still..
It doesn't have a goofy crank fit. It runs smooth. That's what led me to learning how to balance props. It's been running that way ever since, hauling my Li'l Satan around. It still doesn't have any unusual vibration. I just looked at the tach test video again and it sat on 19.3k. And the much maligned starter spring is dead still. I think that's fine for a 5.25x4 prop. Certainly not a sign of trouble. Do you think it should be turning another k?
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Old Mar 25, 2012, 11:07 AM
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Quote:
Originally Posted by RknRusty View Post
Yes I know, Jim, it needs to be softer in order to conduct the magnetic flux better than hard metal. The booklet said as much.

It was the prop.
Never heard that one before re the mag balancer.. but then I use a pair of quarter sized Neo magnets. Couple of bux at Lee Valley tools
There's not much that those can't grab serious hold of

New prop is an easy fix.. Good! Odd that one would be so far out..
What maker is responsible for such precision product ?

Never actually had one .. over too many years .. that vibrated.. badly.
Was just saying that 19 k isn't that unique for a BW, they did pretty well actually
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