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Author Topic: Initial static balance of the discrete component prop  (Read 389 times)
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Konrad
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« on: January 24, 2017, 01:27:31 PM »

I’ve noticed that there looks to be a rash of prop balance problems with the last few production runs from Freewing RC and  Flightline RC. Specifically the component prop (separate blades that are screwed onto the back plate of the spinner). What I’ll try to show is how to perform the initial static balance. This should be more than adequate for the low rpm (sub 10K rpm) we see in these “Scale” like props. And should be applicable to any component rotor (prop) system. It needs to be said that a dynamically balance system will be statically balance. But a statically balanced system may not be dynamically balance.

The key here is understand that the center of mass for the component blades must all be close to the same distance from the axis of rotation. This and their total mass should be close to the same. Otherwise the difference in the  angular momentum of each blade will cause vibration.

One should be able to find the tools needed in most hobby rooms or at least Hardware stores. You will need a caliper with a precision of 0.02mm, a gram scale with a precision of 0.1 grams, a razor blade, a fine tip felt tipped marker, sandpaper and CA glue.  

The first step is to insure that the spinner back plate is dimensionally true. With the caliper make sure the features are within 0.15mm of the center. Pay particular attention to the blade mounting holes.  If the features are off it can usually be traced to the center bore having been drilled or reamed off center. This can often be corrected with a bushing as shown. Note that many back plates now use a hex or proprietary shaped drive hole. If the back plate is out of tolerance and can’t be brought back into true, it must be replaced.

The second step it to find the span wise center of mass for each blade. This is done by placing the blade on the edge of a razor blade and moving the blade until the blade teeters on the razor blade. Mark this location with a felt tip pen. With the razor blade method I find I get a repeatability of less than a millimeter in finding the blade’s center of mass. Measure from the blade mounting holes to the center of mass mark. Note where the center of mass is relative to the mounting holes and to the other blades that will be used for the propeller.

The third step is to find the over all mass of the blade. I like to measure this to within .2 grams. Use the gram scale to find this number

Now comes the brains part of the solution. The goal is to move the center of mass so that they are all close to the same distance from the blade’s mounting holes. The blades over all mass will tell you if you are adding or removing weight and the direction you have to move the center of mass is telling on which end of the blade you will have to make the adjustment. (see attached)  Do not remove material on the blade’s mounting surface (under the mounting screws) this area controls the blades pitch! Should you get all the blades with the center of mass in the same location span wise and the total mass of the blades is still different. You will need to add or subtract weight from the whole length of the blade to keep the center of mass in the same location as you bring the blades all to the same weight.

I like to use a heavy hunting knife to scrape the back side of the blade to remove mass. I then finish with some sanding with 320 grit wet and dry paper (wet). To add mass I like to use this CA glue spread in thin layers as needed. I like CA as in dries fast and has little change in mass as it cures.

Assemble the blades onto the hub (spinner back plate) while keeping an outward pull on the blades as the tighten down. This is to keep the blade from shifting outwards should there be any clearance between the blade and the screw.

That is it, you now have a prop which is initially in static balance. The center of mass for the blades and the total mass of the blades have been adjusted.

Now I like to take the extra step and but the assembly of a High Point style balancer, and verify that tolerance from the above process haven’t stacked up against each other.  A statically balance prop with stop at any point  (360°). If the prop stops repeatably at the same point (even horizontally for a 2 bladed prop) it is out of balance.

I hope that you now see that just adding or taking away mass to the offending blade, as is often suggested elsewhere on the web, doesn’t begin to address the issue of a balanced prop.

This process is not limited to just the Freewing RC and  Flightline RC product line as I’m aware that FMS uses the same type of prop assembly. But to date I haven’t had to balance an FMS assembly.

Please note that I have been describing initial static balance. With todays cell phones that have accelerometer in them  we can actually perform some dynamic balancing. The cell phone process is still an iterative one but with the cell phone we can now have quantitative data as to how much the rotor system is vibrating. I have to admit that with the foam models from the previously mentioned vendors I have not seen the need to balance the props with the cell phone process. The cell phone process has been very effective for me in balancing heli rotor systems. But that’s another can of worms!

If you want to go down the rabbit hole on this, here is a start
https://en.wikipedia.org/wiki/Angular_momentum
https://en.wikipedia.org/wiki/Dynamic_balance

All the best,
Konrad
Attached files Thumbnail(s):
Initial static balance of the discrete component prop
Initial static balance of the discrete component prop
Initial static balance of the discrete component prop
Initial static balance of the discrete component prop
« Last Edit: January 24, 2017, 01:46:55 PM by Konrad » Logged

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Bill G
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« Reply #1 on: February 06, 2017, 05:01:06 PM »

Interesting info.  I think these "dynamic" problems are why we run into that occasional rotor or prop that seems impossible to balance.  I kinda shake my head when guys say "dynamically balanced" with EDFs.  True dynamic balancing should include placing the balance weight (glue or whatever) toward the front or rear of a blade or spinner.
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Konrad
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« Reply #2 on: February 06, 2017, 07:32:46 PM »

Very true. What you describe is known a 2 dual plane balancing.  What most guys are addressing with EDF is the resonance of the system. This often has to deal with the rigidity of the case rotor and bearing support structure.  They clock the impeller, spinner and the motor to get the lowest vibration at speed as measured by some kind of accelerometer (AKA, cell phone). Often time this results in the "System" really moving (vibrating) in the lower rpm bands.  
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Bill G
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« Reply #3 on: February 08, 2017, 03:25:39 AM »

I had to do the 2 plane balancing, with my M1c spinner.  Some guys said you'll never get it balanced.  It wasn't easy.  Determining the front to rear placement was as important as where it was placed around the perimeter.  It took a LOT of experimenting.  As far as I know, I haven't heard of a small balance machine that is like the quality tire balancing machines, that will tell you which side to place the weight on.
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Konrad
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« Reply #4 on: February 08, 2017, 09:32:45 AM »

The heart break of a nose over! Cry

Cantilevered systems like our props and spinners are particularly sensitive to balance issues

There are machines that can do light weight dual plane balancing. I've found the issue is in mounting the rotor ( spinner , prop impeller etc.) on the mandrel consistently.  You might be able to make these machines. I've used the accelerometers in old smart phones as the heart of the machine. Can't say as I've tried as light a rotor as your spinner assemblies.
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