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Balancing large propellers? - 12/16/2012 3:51 AM   
abufletcher



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Status: offline 		I've got two large XOAR props (24x10 and 22x10) that I need to check for balance. With my smaller models I've just used the TopFlight prop balancer:

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHY61&P=ML

But this obviously isn't up to the job for the XOAR props. A second question is how do you restore balance on a large prop, if it's out of balance. With the small Zingers, you can just add a spot or two of CA to the back of one of the blades. Or sand a bit and revarnish. But what about large props...and it would be a shame to have to sand and refinish the beautiful WWI XOAR props.

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RE: Balancing large propellers? - 12/16/2012 4:46 AM   
R/C Art


 

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Status: offline 		Don, rather than sand the heavy side, try spraying a coat of two of clear to the light side.

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RE: Balancing large propellers? - 12/16/2012 4:53 AM   
TFF


 

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Status: online 		Under your criteria the only thing you can do is paint the light blade with some clear paint. The tough part of really balancing a prop is getting the cord direction balanced; tip to tip is easy. The trick to know how good the cord balance is by placing the heavy blade straight down; if the prop moves any, the width of the prop is out; either by wood having variations or the mount hole is not centered. If the trailing edge moves up that side of the prop cord is the light side, and the opposite if the leading edge moves up. The bigger the prop the more important it becomes. http://www.thompson-aero.com/index.php?option=com_content&task=view&id=59&Itemid=32
http://www.eaa.org/experimenter/articles/2009-11_howto_props.asp is pretty neat.

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RE: Balancing large propellers? - 12/16/2012 5:06 AM   
Laird SS


 

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Status: offline 		This past summer I had the opportunity to visit the "factory" where the Biela carbon fiber props are made. They balance each prop (of course) using a balancer that has four wheels (two on the front and two on the back) to support the axle that holds the prop. Each wheel is approximately 1.5" in diameter. The heavy blade is lightened by drilling small holes in that side. The distance from the center of the prop to the hole is a measure of how much out of balance that blade is, eg, the closer the blade is to being in balance, the closer the hole is to the hub. None of the holes go completely through the blade. They do not add weight to a blade, nor do they sand one. They use this method for all of their props - 2-, 3-, and 4-blades.

Hope this helps a bit.

The US distributor for their props is Troybuilt Models (tbm).

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RE: Balancing large propellers? - 12/16/2012 5:07 AM   
abufletcher



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Status: offline 		Using the axle from the TopFlite balancer, an old cabane jig, and a couple of sewing bobbins for bearings, I put together a balancer that seems to work for these larger props. Both props are very close to the correct balance (and in fact XOAR states that they have been carefully balanced before leaving the factory). The prop shown here seems to stay in one place from one direction, but droops a bit when the prop is reversed on the stand.

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RE: Balancing large propellers? - 12/16/2012 5:12 AM   
abufletcher



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ORIGINAL: R/C Art
Don, rather than sand the heavy side, try spraying a coat of two of clear to the light side.


Thanks, Art. This approach entirely slipped my mind and seems like the best option.


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RE: Balancing large propellers? - 12/16/2012 5:22 AM   
TFF


 

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Status: online 		With your prop staying one way and dropping when turned 180 it is out of cord balance.

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RE: Balancing large propellers? - 12/16/2012 9:54 PM   
gabriel voisin


 

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Status: offline 		Don,
your balancer with the plywood is nothing.
The surface is to big and the friction to high, the friction must be reduced to almost zero.
The edge of our propeller balancers for the original full size propeller is only 0.8 mm.
For model props, you can do it with razor blades or with metal disks, two on each side in opposite directions.
At both constructions must mounted the propeller on a fixed axis.

nice day,....Matz

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RE: Balancing large propellers? - 12/16/2012 11:55 PM   
abufletcher



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Status: offline 		Matz, you're right. This was just put together quickly and I plan to improve it. For the axis, I'm using the one that comes with the TopFlite balancers, which works fine on the large props. Does anyone have any photos of the balancers you use?

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RE: Balancing large propellers? - 12/17/2012 12:18 AM   
abufletcher



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Status: offline 		The DuBro prop balancer looks like it can handle larger props:

http://www.espritmodel.com/precision-truspin-propeller-balancer.aspx

I don't really understand how the following one from Vogelsang works:

http://www.vogelsang-aeroscale.com/fema3dbalancer.html

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RE: Balancing large propellers? - 12/17/2012 12:32 AM   
abufletcher



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Status: offline 		But I can probably improve my current setup just by adding two thin washers on each side for the axle to rest on. And that'll cost me nothing! The next step up would be to find some small rollers for each side.

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RE: Balancing large propellers? - 12/17/2012 12:53 AM   
TFF


 

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Status: online 		The vogelsang is like a real airplane static balancer. The prop teeters on the rod. The hub has a bubble level. It will show how far out it is in 2 D.

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RE: Balancing large propellers? - 12/17/2012 2:10 AM   
abufletcher



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Status: offline 		Thanks, I see the bubble level now in the photo. At $60 plus shipping through, this is really the Mercedes of model propeller balancers (as are most of the Vogelsang products).

*****

I've improved my home-made balancer by replacing the sewing bobbins with two single-edged razor blades epoxied to the ply jig. This reduces the contact (and friction) to almost nothing. And, yes, one side of the prop does appear to be ever so slightly heavier than the other.

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< Message edited by abufletcher -- 12/17/2012 11:48 AM >

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RE: Balancing large propellers? - 12/17/2012 7:03 PM   
gabriel voisin


 

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Status: offline 		Of course Don:

Matz



http://www.produktinfo.conrad.com/datenblaetter/200000-224999/207163-an-01-de-SCHWERPUNKTWAAGE_PRO_BALANCER.pdf

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RE: Balancing large propellers? - 12/17/2012 7:58 PM   
Teus



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Status: offline 		Don, I use the Fema balancer (shown at Vogelsang). That balancer is worth every penny spent and if you have used it once you never want back But this one only works for the larger props, lets say from 14"and up I think.

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RE: Balancing large propellers? - 12/17/2012 8:06 PM   
John Redman


 

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Status: offline 		I did a video on balancing props to include the hub. Pretty easy and all is covered here to inculde the larger props.

http://www.horizonhobby.com/article/2256-balancing-your-prop

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RE: Balancing large propellers? - 12/18/2012 12:39 AM   
abufletcher



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Status: offline 		So a lot of good information here about the tools for finding out if a large prop is out of balance. But what techniques do you use to correct the balance. Art, mentioned spraying on clear. Then there's the idea of adding a spot of CA. In the video there's mention of drilling a small hole or holes in the hub. Do you all prefer to correct out on the end of the blades or in towards the hub?

< Message edited by abufletcher -- 12/18/2012 8:14 AM >

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RE: Balancing large propellers? - 12/18/2012 1:35 AM   
TFF


 

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Status: online 		I got one of these at work once you get the static done for perfect balance. http://www.dssmicro.com/products/prod_mv2_main.htm

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RE: Balancing large propellers? - 12/18/2012 4:10 AM   
John Redman


 

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Status: offline 		You will need to correct both the out of balance blade and the hub, not just one or the other. And when installing a screw in the much larger props, it is done on the side, not the back of the hub.

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RE: Balancing large propellers? - 12/18/2012 8:06 AM   
abufletcher



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Status: offline 		I guess I don't understand the difference between an out of balance blade and an out of balance hub. And I still don't get how "cord-wise" balance on the blades can be corrected.

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RE: Balancing large propellers? - 12/18/2012 12:20 PM   
sensei



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quote:

ORIGINAL: abufletcher

I guess I don't understand the difference between an out of balance blade and an out of balance hub. And I still don't get how ''cord-wise'' balance on the blades can be corrected.


The difference is an out of balance blade only needs to have the weight distributed evenly from one blade to the other. Now in more cases then we would like to admit the prop hub is out of balance due to the center hole in the propeller being off center in the 90 +-.000" degrees axis to the blades, and it only takes a couple of thousands of an inch to be a paint shaker on the front of your airplane because that center hole keys to the engine's output centering shaft, so now the entire propeller from tip to tip is off center line to the center line on the crank hub making balancing just the blades a secondary issue to the real problem at hand. So any time I balance a propeller, I check and hub balance them as needed.

Bob

< Message edited by sensei -- 12/18/2012 1:23 PM >

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RE: Balancing large propellers? - 12/18/2012 12:54 PM   
geezeraviation


 

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Status: offline 		A large prop with the hole not centered across the hub is a paint stirrer, an expensive one
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RE: Balancing large propellers? - 12/18/2012 1:00 PM   
abufletcher



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Status: offline 		OK. So that's all fine. But HOW do you balance a hub that isn't centered correctly? In fact, how do you discover this problem to begin with? I don't see how even the expensive Vogelsang balancer could show the difference between an out of balance blade and an out of balance hub.

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RE: Balancing large propellers? - 12/18/2012 1:01 PM   
sensei



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Status: offline 		A large prop with the hole not centered across the hub is a paint stirrer, an expensive one
Doc

But it can be balanced, I have seen many carbon fiber props that also needed hub balancing, it just take the time to do it.

Bob

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RE: Balancing large propellers? - 12/18/2012 1:06 PM   
sensei



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quote:

ORIGINAL: abufletcher

OK. So that's all fine. But HOW do you balance a hub that isn't centered correctly? In fact, how do you discover this problem to begin with? I don't see how even the expensive Vogelsang balancer could show the difference between an out of balance blade and an out of balance hub.


I first scuff the hub area both sides and I don't mean the drive and thrust washer areas with 320 grit sand paper, next I place the prop on the balancer and allow the prop to drop as it will, then I hold the prop level and favor one side low and then the other and allow it to drop again and again. The reason I do this is because most the time the prop will drop on the same side, however every now and then it will go the other way if just favored a little, what that tells me is sometimes I get a prop that the center hole is off just a tad and that takes a little different hub balancing. anyway if the prop drops the same blade when flipped then I begin to add a couple of drops of thick CA to the light side hub area, hit with kicker and re-balance I repeat this step flipping the prop over and adding drops to the light side of the hub until I achieve a level prop. Now as I stated; if the prop drops a blade when favored down either blade but will drop the same blade when turned over I must add CA drop to the light side of the hub first and get that heavy blade a little lighter, I also add a couple of drops to the center area of the hub on that side to counter the slight offset of the center hole, now I continue this until the prop is balanced. That’s it...

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