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Balancing Props

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From the desk of SleepyC

When flying multirotors, anything that you can do to minimize vibration is usually worth the effort. One of the simplest things that can yield the biggest results is properly balancing the propellers. Your rig's flying response will improve, your flight control will respond better and if you are filming with your multirotor, your video will look cleaner! There isn't a downside to having properly balanced propellers on your rig!

When speaking to a lot of multi pilots, many guys simply balance the blades to one another meaning that they put the propeller on a balancing rig and once the propeller remains level they consider it balanced. In reality, this propeller is only half balanced. You must balance the hub as well as the blades. Usually a properly balanced hub is where the most vibration reduction will come from. A quick signal that your propeller is perfectly balanced is to take it and put it at any position on your prop balancer. Imagine a clock, your properly balanced propeller should be able to lock and hold it's position at 1, 4, 8, 10 12, any hour you want!

So how do you balance the prop?

1) Balance laterally. Put your propeller on your favorite prop balancing machine. Find the heavy blade (the blade that always drops). With most carbon or plastic multirotor props, I will add small pieces of Scotch magic tape to the light side on the bottom of the prop blade. If it doesn't balance the first piece, I remove it, and try a bigger piece until I get it right. With wood blades, I will usually spray clear pant or sometimes I'll even sand the blade a little. Just BE CAREFUL if you choose this method not to remove too much material where the prop could lose structural integrity. Once you have the prop blades balanced, move on to the hub!

2) Balance the hub. Once your blades are balanced laterally, put it back on your balancer, move the blade to the 1 o'clock position and watch what happens. If the prop stays put, try 4o'clock. If it again stays put, you got lucky, your hub is balanced and you are done! But most likely the prop has rotated from the 1 o'clock position and started to spin around some. The prop should end up in the 9 - 3 position, and the bottom part of the hub facing the ground is your heavy side. You will want to add some material to the opposite side and try the 1 o'clock hold again. Do this until you feel the hub is balanced. Once it is, it will hold any position of the "clock." Depending on what prop size and material you are using, you can simply add CA to the lighter side of the prop hub and then use kicker to secure it (also you may want to slightly scuff the prop hub with sandpaper to give the CA some tooth to bite into). If you are dealing with larger propellers that may require quite a bit of weight, a popular trick is to use a piece of fuzzy velcro, and apply the CA to the fuzz. This locks in the CA to the velcro and makes a secure weight.

If you are still scratching your heard, our good buddy John Redman from Horizon Hobby has made a great video on how to balance an electric prop. So please check it out:

HorizonHobby.com How To - Prop Balancing (13 min 41 sec)

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Old Mar 29, 2013, 12:38 AM
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R Zielinski's Avatar
USA, NH, Madbury
Joined Dec 2007
166 Posts
Prop balancer cone position

Nice video. Forgive me if I missed it, but I don't think he discussed the positioning of the cones to secure the prop on the balancer shaft. This may be a minor point, but can occasionally make a difference. I usually orient the cones with only one pointy side into the prop, and the other reversed with the flat part contacting the prop's mounting surface.

Why? Problems can occur when the prop shaft hole is slightly off axis. The prop may then be in a different position in relation to the motor shaft than it is with the balancing machine shaft. (You'll find for example that when the cones are both oriented into the prop hole, you can often wobble the prop in various axes and snug it up various orientations). You may end up with a balanced prop on your balancer, but unbalanced on your motor because the prop mounts differently on the motor shaft than it did on you balancer machine.

Many E props use spacer rings to accomodate different motor shaft sizes. The prop hole is precision milled on the back side of the prop to accept these rings. This hole is milled perfectly perpendicular to the flat back surface of the prop which contacts the spinner back plate. However, this precision back hole may not be on the same axis as the (larger) less-precise molded hole on the front side of the prop.

This issue of the hole not being on the same axis front and back is even worse if you ream the prop with a tapered reamer (or worse yet a drill bit). The prop hole may not end up on the same axis it was molded at, and may wind up NOT perpendicular to the flat mounting surface at the back of the prop.

You need a snug motor shaft fit at only one point within the prop hole so that the prop lies flat on the spinner back plate and the off axis prop hole doesn't force the prop off axis.

I would argue it's best to ream the hole so that the prop is snug on the motor shaft, but at the same time can be "wobbled" very slightly because the entire prop hole is NOT snugly contacting the motor shaft. Use a tapered reamer from both sides of the prop, a little at a time until you get a snug fit on your motor shaft. Then the shaft only contacts the inside of the prop hole at one small area in the middle where the reaming from the front met the reaming from the back. That's all that's needed to keep the prop centered. The back plate of the collet or spinner keeps the prop perpendicular to the shaft. Your balancer cone should (optimally) contact that point inside the prop hole where the motor shaft snugs up.

For these reasons, I orient the cones so that only one pointy cone goes into the prop shaft. The other cone is reversed to contact the prop on its flat surface. The cone should contact the inside of the prop shaft in the precision milled side of the hole, or in the precision spacer ring supplied with the prop. (Thats the back side of APC E props). If I reamed the prop (no spacer ring), I may reverse this, inserting the cone into the front of the prop so the other cone's flat side contacts the precision-milled flat at the back of the prop.

This keeps the prop oriented perpendicular to the balancer shaft (the way it will be oriented on the motor shaft). Fussy? Yes. Usually overkill? Probably. Therapeutic for OCD modelers? You bet.
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Old Mar 29, 2013, 09:18 AM
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United States, CA, Huntington Beach
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I'm Confused!

WHAT?

I have read and re-read the comments about NOT using the two cones to balance props.

My confusion comes from the lack of ability to "center" the prop in relation to the balancers' shaft.
How does that work again?

IMHO without using the two cones as designed, it would be nearly impossible to aline the prop's mounting hole, around which the prop rotates, to the balancers' shaft negating the entry balancing process. Unless the props' through hole was drilled to very, very straight and the balancers' shaft was exactly the same size as the hole, with minimal tolerances to allow insertion, the balancer would NOT work as designed.
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Old Mar 29, 2013, 10:17 AM
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Personally I use the cones both facing inward towards the hole on the prop, unless that is not an option (prop too thin) . I seem to have best success that way. But if at the end of the day if your props are fairly vibration free when you are finished, use what ever method works best for you!
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Old Mar 29, 2013, 06:26 PM
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Russellton, PA
Joined Oct 2002
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JR's video rocks. He is really good at explaining things.

As you guys said,some props are too thin at the hub and you can only use one cone for centering/mounting.

Infact most of mine are. As JR says close counts.

Thanks the quick link to the video.

Regards,
GMRO
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Old Mar 30, 2013, 09:03 AM
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USA, NH, Madbury
Joined Dec 2007
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Quote:
Originally Posted by hoggdoc View Post
WHAT?

I have read and re-read the comments about NOT using the two cones to balance props.

My confusion comes from the lack of ability to "center" the prop in relation to the balancers' shaft.
How does that work again?
Sorry my post is confusing. Both cones are needed on the balancer shaft. One cone must be oriented so the pointy end goes into the prop hole. The other cone is used on the other side of the prop, but flipped around so that the flat end contacts the prop.

The goal is to center the prop on the cones (on the exact same axis that the motor shaft will carry the prop) AND keep the prop perfectly perpendicular to the balancer shaft (as it will be on the motor). This is sometimes difficult or impossible if both cones are pointed into the prop hole. The difficulties are due to thin prop hubs (as Sleepyc said) but also prop shaft holes that are manufactured or reamed into an off axis orientation.

I believe APC's website describes this issue, and explains that their prop holes are precision milled on the back side, but NOT on the front side where the hole is simply molded larger so as not to contact the prop shaft at all.
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Old Mar 30, 2013, 10:29 AM
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I do the same thing as R Zielinski. I arrived at it using much the same logic; then used repeatability studies to confirm the results. In any mechanical system it is important not to over constrain the part being measured (or balanced in this case). RZ's method uses a cone and a flat plane. The plane establishes the prop's perpendicularity as it would be if mounted to the motor, the cone pins the prop against the plane and establishes the centering. This works well for most props I use. Occasionally one will not sit flat and/or center well because the hole manufacturing is too far off. Those inferior props are exposed by this using method and thus become bashers.

Dan
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Old Mar 30, 2013, 02:17 PM
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A n00b question - DJI props haven't got a circular hole as they are keyed to the motor shaft - at least the props for DJI450 and 550s. Is it ok to use the cones facing inwards to align them to the shaft ior is there a better method?
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Old Mar 30, 2013, 05:34 PM
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USA, NH, Madbury
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Quote:
Originally Posted by macspite View Post
A n00b question - DJI props haven't got a circular hole as they are keyed to the motor shaft - at least the props for DJI450 and 550s. Is it ok to use the cones facing inwards to align them to the shaft ior is there a better method?
Cone(s) should work fine. Whatever shape the hole is (hexagonal I think), it is symmetrical so the cone will center in it. I think DJI's are a prime example of ones with such a shallow hub that you maybe can't point both cones in. If the cones hit each other in the middle and prevent the prop from snugging up, just flip one cone around.
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Old Mar 31, 2013, 09:10 AM
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Quote:
Originally Posted by R Zielinski View Post
Cone(s) should work fine. Whatever shape the hole is (hexagonal I think), it is symmetrical so the cone will center in it. I think DJI's are a prime example of ones with such a shallow hub that you maybe can't point both cones in. If the cones hit each other in the middle and prevent the prop from snugging up, just flip one cone around.
Thanks for that! They aren't hex as such - the shaft has 2 flats opposite each other so the hole in the prop is not circular like "O" but more like "0"
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Old Mar 31, 2013, 10:37 AM
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Quote:
Originally Posted by R Zielinski View Post
Cone(s) should work fine. Whatever shape the hole is (hexagonal I think), it is symmetrical so the cone will center in it.
No, do not use the cone on the nut end of the prop. If you do there is no way to ensure that the prop is centered on the shaft. Turn the cone around on the nut end of the prop and use the flat side. Use the cone on only one end with DJI props.
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Old Mar 31, 2013, 11:25 AM
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Quote:
Originally Posted by macspite View Post
Thanks for that! They aren't hex as such - the shaft has 2 flats opposite each other so the hole in the prop is not circular like "O" but more like "0"
My bad. Never saw those props and didn't imagine they'd design a hole like that. On looking at photos of those DJI props, I agree that cones will not center very reliably in those holes. (The hubs also sure look too thin for both cones to point in without hitting each other). I really don't know what you could do to center those props on a balancer shaft reliably.

My only idea is not a great one: use a tapered prop reamer (one that's totally cone shaped) or a cone-shaped diamond file and ream off both of the flats on one side of the prop only. Ream just until the reamer touches the round parts of the prop hole. You'd have to be careful to remove material as symmetrically and centered as possible. Then you should have a centered circular hole on one side that the cone will seat in. Might work while maintaining the "O" shaped hole at the other side of the hub.

Hopefully someone here has a better solution.
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Old Mar 31, 2013, 05:13 PM
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Can't assume holes are drilled perpendicular to back face, nor can you even assume pitch is consistent between the two blades or matched the label.
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Old Apr 01, 2013, 01:55 PM
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Joined Mar 2010
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Prop balancing headaches

I thought I knew how to balance props until I read about how the hubs need to be balanced also. Thats when after days of work I decided that the equipment we have to balance and mount our props is not machined accurately enough to balance the props on a balancer and then expect they are going to sit with the same balance once mounted on an individual motor. I also find that no matter how I mount the cones if I balance the prop then take it off and put it back on with different cone rotation what was previously balanced is now not balanced. This is obviously because the cones, the hub hole and other variables are greater than the errors in the prop that we are trying to fix.

I have some Gemfan composite props and the hub balance is so far off that you have to put blobs of hot glue on the hub or so much tape on the props that it ruins the aerodynamics. I read an article by a guy that balances all his props mounted on the motors individually by feeling the vibration as he moves tape around from one side to the other. He says also that if you measure vibration when the prop is mounted it will change if you rotate it to a different position on the hub just because of the inaccuracies in the prop shaft ,hub , nuts , prop hole etc. So you need to keep rotating it around until you find the place with the least vibration. I think I am going to try a hybrid of this by getting them as best I can on a prop balancer then using the tape trick once I put them on the motor. I am also going to try copper tape to see if that creates less aerodynamic problems.
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Old Apr 01, 2013, 05:40 PM
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I get much better results balancing the hub first.

Having found out which is the heavy blade, add more tape to it to make it really heavy.

Then put the prop horizontal and as it falls, measure how far up the other side it swings.

Rotate prop 180 degrees and do the same for the other side of the hub.

Add your preferred weight (glue, tape, varnish) to the light side of the hub and repeat until the heavy side swings up to the same height, in the same time, on both sides. Hub is now balanced and with more accuracy than it just not moving after the blades are balanced - especially on light props, where even light friction on a prop balancer can make it stop when it's not actually even.

Then just do the blades in the normal way.

Easy test. Take a prop you think is balanced, add tape to one blade and see if it swings up evenly on both sides. If it doesn't, the hub is not balanced.
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