FYI Prop+Hub Balancing

Burkeam · Oct 15, 2015

Maddog said:
I agree- if it ain't broke, don't fix it. I also don't like placing anything on the leading edge of the prop as it will cause some disruption of the airflow. How much? Who knows, but I do everything I can to minimize vibration. I prefer to sand material off the center of the bottom of the blade.

I know the only thing I did to my airplane propellers leading edge is to dress it -to remove nicks and scratches. You want it smooth.

RichWest · Oct 15, 2015

I really don't think there is an issue having the tape on the leading edge regarding aerodynamics. At least from the hub out to about the half way point. The highest velocity air wise starts at about 1/3 out from the hub and increasing exponentially towards the tip.

Plenty of examples of protective metal leading edge on wings and propellers that are well fitted, but are far from perfect to remainder of the air foil profile.

Timmytimetravel · Oct 16, 2015

I can see my hubs appear out of balance using the vertical test, horizontal which I previously balanced is fine. I still experience some vibration, which could be anything.

I read a previous post on this forum talking about hub balancing and it seemed the consensus was it was doing much the same as balancing props horizontally but with more material since centrally located and may not be worthwhile? It got a little waylaid as the discussed was talking about removing the plastic material from near the hub instead of adding glue and the issue with possible weakening.

I've feel I've missed something in between when everyone now seems to be doing their hubs.

Has anyone run some before and after tests where they balanced horizontally, flew a test. Balanced the hubs, flew another test same conditions and noticed a discerning difference?

I think I'm ready to do some silcon just in case why not, just wondering if people are seeing good results from hubs or is it always better as peeps are balancing props horizontally at the same time?

Jubalr · Oct 16, 2015

Timmytimetravel said:
I can see my hubs appear out of balance using the vertical test, horizontal which I previously balanced is fine. I still experience some vibration, which could be anything.

I read a previous post on this forum talking about hub balancing and it seemed the consensus was it was doing much the same as balancing props horizontally but with more material since centrally located and may not be worthwhile? It got a little waylaid as the discussed was talking about removing the plastic material from near the hub instead of adding glue and the issue with possible weakening.

I've feel I've missed something in between when everyone now seems to be doing their hubs.

Has anyone run some before and after tests where they balanced horizontally, flew a test. Balanced the hubs, flew another test same conditions and noticed a discerning difference?

I think I'm ready to do some silcon just in case why not, just wondering if people are seeing good results from hubs or is it always better as peeps are balancing props horizontally at the same time?

I've always balanced my props for years, but never the hubs. While it certainly wouldn't hurt anything, I'm not convinced that adding a drop of anything that close to the center of a spinning object has an impact at this level just because of the other forces that are involved. I also prefer to sand the bottom of the blades to maintain airfoil as opposed to adding to them.

But to each their own; just my .02..

mcneely19 · Oct 16, 2015

I'm going to talk to one of our mechanical engineers today. I don't buy all this. Maybe I'll post up a freebody diagram later.

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Maddog · Oct 16, 2015

Timmytimetravel said:
I can see my hubs appear out of balance using the vertical test, horizontal which I previously balanced is fine. I still experience some vibration, which could be anything.

I read a previous post on this forum talking about hub balancing and it seemed the consensus was it was doing much the same as balancing props horizontally but with more material since centrally located and may not be worthwhile? It got a little waylaid as the discussed was talking about removing the plastic material from near the hub instead of adding glue and the issue with possible weakening.

I've feel I've missed something in between when everyone now seems to be doing their hubs.

Has anyone run some before and after tests where they balanced horizontally, flew a test. Balanced the hubs, flew another test same conditions and noticed a discerning difference?

I think I'm ready to do some silcon just in case why not, just wondering if people are seeing good results from hubs or is it always better as peeps are balancing props horizontally at the same time?

I don't think a test flight would reveal any difference. However, if you use a dynamic balancer (spinning the prop) you can see dramatic differences in balancing.

Hard to tell if this translates into a noticeable difference on the copter, but every little bit helps.

Several YouTube videos on this.

z-axis · Oct 16, 2015

Back before we had ready-made aerial filming rigs, balancing props and hubs was manditory to address vibration. Makes a huge difference. About leading edge, blah blah, it's so damn thin. It might matter for NASA but not at the hobby level. It works just fine this way. We could debate it all day long but you should be out flying, anyway.

Tape, sanding, silicone, hot glue, whatever. When it's balanced, it will always hold its position. Just do it to whatever level makes you happy, zero or all the way. Just sharing my relative experience of these props with respect to having done it for APC and Graupners in the past.

If you really want to do it right, balance your motors and your props with one of these. I have one but haven't had the bench time to do it yet.

Dynex Hobby Vortex Balancer

Marich · Oct 16, 2015

I use CA for my hubs as well.
Takes awhile with the glue alone, and accelerator can be kinda pricey, but take it from an old balsa RCer -
Before accelerator there was baking soda!

kysersoze · Oct 16, 2015

I have been using fingernail polish for the hubs. Black. Add a little of weight per application and dry. works well.

Marich · Oct 16, 2015

mcneely19 said:
I'm going to talk to one of our mechanical engineers today. I don't buy all this. Maybe I'll post up a freebody diagram later.

Sent from my VS985 4G using Tapatalk

Not quite sure what you mean - not sure about hub balancing, etc.?
Well, if you consider one main rule of prop design, that (as a metaphor) one balancing "ounce" of weight on the hub can eliminate the need for ten ounces and the blades, well... seems pretty obvious.

mcneely19 · Oct 16, 2015

http://rc-airplane-world.bmobilized...com/how-to-balance-a-propeller.html&width=360

This is what I always did with planes. As long as your hub isn't crazy unblanced there is no need to treat this as two bodies. Its one system. I got my solo props balanced with minimal sanding on the heavy blade. If you balance the blade then balance the hub you are affecting the centroid of the whole system. Maybe I just got good blades.

nuggetz · Oct 16, 2015

I used some resin that is cured via UV light. On amazon it's called bondic and sets almost instantly when you shine the UV led on it.

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jeeboont · Oct 17, 2015

Can someone show a tutorial on how to balance hubs? I've done prop balancing but never heard of hub balancing till now. Thanks in advance!!!

Lamda · Oct 18, 2015

jeeboont said:
Can someone show a tutorial on how to balance hubs? I've done prop balancing but never heard of hub balancing till now. Thanks in advance!!!

Here's a short video showing hub balancing. As other people have stated you can use tapes or other glues to add weight to the lighter side of the hub.

Acroduster · Oct 18, 2015

mcneely19 said:
If you balance the blade then balance the hub you are affecting the centroid of the whole system.

Of course you're changing the CG of the whole system. That's the point. If both the blades and hubs are balanced, that means the CG of the whole system passes through the center of the shaft.

Marich said:
Well, if you consider one main rule of prop design, that (as a metaphor) one balancing "ounce" of weight on the hub can eliminate the need for ten ounces and the blades, well... seems pretty obvious.

It's precisely the opposite. It takes much more of a change at the hub to equate to a small change on a blade. The forces go up by the square of the distance to the center of the shaft. A little bit of weight far from the shaft has a much greater effect than at the center of the shaft. It's the reason that a little bit of sanding will fix the blade balance, but it takes a dob of glue to fix the hub balance.

mcneely19 · Oct 18, 2015

Acroduster said:
Of course you're changing the CG of the whole system. That's the point. If both the blades and hubs are balanced, that means the CG of the whole system passes through the center of the shaft.

It's precisely the opposite. It takes much more of a change at the hub to equate to a small change on a blade. The forces go up by the square of the distance to the center of the shaft. A little bit of weight far from the shaft has a much greater effect than at the center of the shaft. It's the reason that a little bit of sanding will fix the blade balance, but it takes a dob of glue to fix the hub balance.

So how do you balance the hub and the blades separately as suggested in this post when they are 1 system? Not being argumentative just need a procedure. Agreed on the circular motion equation... Less effect at hub and more further out on the blade.

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nuggetz · Oct 18, 2015

jeeboont said:
Can someone show a tutorial on how to balance hubs? I've done prop balancing but never heard of hub balancing till now. Thanks in advance!!!

Here's a video I made a while ago questioning the hubs.

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Acroduster · Oct 18, 2015

To be technically accurate, you are not balancing them separately, you're balancing them both at the same time along two different axes. Let me set up a system so I can type this with a chance of being understood. Visualize the prop horizontal. Now think of it in three pieces - the left blade, the hub, and the right blade. Now:

Case 1: Let's say the left blade is "heavy." In that case, we'd sand a little behind the blade until it balanced out. Since everything else was OK, it will not spin in the balancer no matter what orientation it starts from - a perfectly balanced prop.

Case 2: Left blade heavy, bottom of hub heavy. We would need to sand the heavy blade until it stayed horizontal in the balancer. Then we would add some glue (or whatever your fav method is) to the top side of the hub so that when you orient the prop vertically in the balancer, it stays put.

Case 3: Left blade heavy, left side of hub heavy. In this case, you would sand extra on the blade to get it to stay level. You think that it's just a blade badly out of balance - but you have no way of knowing that the hub was contributing. It doesn't matter. It will be balanced and not vibrate due to imbalance.

Case 4: The bottom edge of the left blade is heavy, hub balanced. Again, you can't know what part of the blade is heavy. You sand the blade to fix it horizontally, then you add glue to the hub to fix it vertically, not knowing that it was the blade that was making it not stay vertical in the balancer. Again, it doesn't matter as far as the shaft is concerned.

Well, I've probably confused things tremendously, but all I'm trying to say is that "prop" then "hub" balancing is just making sure that the whole system is balanced about the shaft while rotating.

mcneely19 · Oct 18, 2015

Acroduster said:
To be technically accurate, you are not balancing them separately, you're balancing them both at the same time along two different axes. Let me set up a system so I can type this with a chance of being understood. Visualize the prop horizontal. Now think of it in three pieces - the left blade, the hub, and the right blade. Now:

Case 1: Let's say the left blade is "heavy." In that case, we'd sand a little behind the blade until it balanced out. Since everything else was OK, it will not spin in the balancer no matter what orientation it starts from - a perfectly balanced prop.

Case 2: Left blade heavy, bottom of hub heavy. We would need to sand the heavy blade until it stayed horizontal in the balancer. Then we would add some glue (or whatever your fav method is) to the top side of the hub so that when you orient the prop vertically in the balancer, it stays put.

Case 3: Left blade heavy, left side of hub heavy. In this case, you would sand extra on the blade to get it to stay level. You think that it's just a blade badly out of balance - but you have no way of knowing that the hub was contributing. It doesn't matter. It will be balanced and not vibrate due to imbalance.

Case 4: The bottom edge of the left blade is heavy, hub balanced. Again, you can't know what part of the blade is heavy. You sand the blade to fix it horizontally, then you add glue to the hub to fix it vertically, not knowing that it was the blade that was making it not stay vertical in the balancer. Again, it doesn't matter as far as the shaft is concerned.

Well, I've probably confused things tremendously, but all I'm trying to say is that "prop" then "hub" balancing is just making sure that the whole system is balanced about the shaft while rotating.

No that was a great description. I'm tracking now!

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Marich · Oct 18, 2015

Acroduster said:
Of course you're changing the CG of the whole system. That's the point. If both the blades and hubs are balanced, that means the CG of the whole system passes through the center of the shaft.

It's precisely the opposite. It takes much more of a change at the hub to equate to a small change on a blade. The forces go up by the square of the distance to the center of the shaft. A little bit of weight far from the shaft has a much greater effect than at the center of the shaft. It's the reason that a little bit of sanding will fix the blade balance, but it takes a dob of glue to fix the hub balance.

Yes, it occurred to me after I wrote the post it might seem that way. What you say is absolutely correct.
I was actually thinking about brushless motor design, particularly outrunners, dealing with intitial centrifugal force at startup. I made my own for RC airplanes, and many times that did entail using not only a prop but a simple counterweight. A bladeless prop? A single-bladed prop?
Anyhow, no harm done - and I bet you see what I meant now.