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Dynamic Propeller Balance Issues

mwjohn63

Member
Recently, after having a muffler tang break off, I decided to get the prop/spinner balanced. I checked the blade angle, float weights and synchronized the carbs. Everything looked good, so I flew over to a prop shop in Sellersburg, IN. After spending all day on it, they were unable to get a consistent balance check/correction. The good news is they didn't charge me, the bad news is they were unable to balance the Rotax setup. I'll look for another shop that is more familiar with the Rotax 912ULS after I complete the the SB-00026 (retainer plate install and re-synch the carbs). Any suggestions on doing the dynamic balance (rpm, etc.). Any recommendations for prop shops near the Cincinnati Ohio area.
Thanks,
MJ

Flying RV-12 (170+ hrs and Oshkosh 2019)
N418MJ
Warren County, OH (I68)
[email protected]
http://www.mykitlog.com/mwjohn63
https://www.youtube.com/watch?v=EJdXcjckN6M
2020 dues paid
 
I have seen problems completing a dynamic balance in instances where there was a gear box issue (low slipper clutch friction, etc.).
 
I have seen problems completing a dynamic balance in instances where there was a gear box issue (low slipper clutch friction, etc.).

What he said, blade angle, carb sync, gear box can all cause issues. I've done quite a few Rotax's but some were just not stable enough to balance.
I found that it help to keep the RPM down a bit as well which may be related to gearbox.
 
Setup?

Walt,
For prop balance Did you use some sort of bracket or adapter to mount a sensor to the Rotax Gearbox?
Thanks!
 
I mount my vib sensor to the open bolt hole on top of the gearbox with a small bracket, pretty simple.
 
Setup

I mount my vib sensor to the open bolt hole on top of the gearbox with a small bracket, pretty simple.

Thanks!
I suspected as much but hadn’t seen anyone mention it. I see they sell a variety of brackets, thin material, and wondered if stiffness could be an issue.
Apparently not.
Thanks again.
 
Thanks!
I suspected as much but hadn’t seen anyone mention it. I see they sell a variety of brackets, thin material, and wondered if stiffness could be an issue.
Apparently not.
Thanks again.

I use .125 alum for brkt., yes must be rigidly mounted.
 
What unit did they use to do the balancing? I am not fimilar with the prop balancing boxes. I use an industrial vibration alalysis box. It filters out all of the noise and will balance just the frequencies related to the RPM of the target and it's harmonics. It is quite a bit more expensive than the prop balancers, but it should handle your prop.
It will not eliminate other vibrations; however i imagine their amplitude is not enough to break things.
 
Stiff is good

Good to know. Is your bracket just an “L” shape?
Would you mind sharing pic of it?

Of course, I'm not Walt -- but here is a vibration sensor bracket I made and it does sound similar to what Walt described. Made of .125 aluminum and quite stiff.

i-Kw4c7T5-L.jpg


Two metric bolts secure it into unused tapped holes on the gearbox.
 
Of course, I'm not Walt -- but here is a vibration sensor bracket I made and it does sound similar to what Walt described. Made of .125 aluminum and quite stiff.

i-Kw4c7T5-L.jpg


Two metric bolts secure it into unused tapped holes on the gearbox.

Do you just drill a hole in the prop bulkhead when you figure out how much weight to add?
 
I don't have a pic of my sensor with mount but it does not look like the above, its simply a small 90 deg piece about .75"x.75", it bolts to the case and the sensor bolts to it, similar to the pic below mounted on a Bell gearbox. The one above looks like it would induce it own vibrations.

2-0.jpg
 
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I don't have a pic of my sensor with mount but it does not look like the above, its simply a small 90 deg piece about .75"x.75", it bolts to the case and the sensor bolts to it, similar to the pic below mounted on a Bell gearbox. The one above looks like it would induce it own vibrations.

Good point, I'll modify it.

N112DR -- yes, drill hole and use a/c hardware for weights.
 
What unit did they use to do the balancing? I am not fimilar with the prop balancing boxes. I use an industrial vibration alalysis box. It filters out all of the noise and will balance just the frequencies related to the RPM of the target and it's harmonics. It is quite a bit more expensive than the prop balancers, but it should handle your prop.
It will not eliminate other vibrations; however i imagine their amplitude is not enough to break things.

One item that confuses me is the presence of the reduction gearing. I understand balancing a big bore engine, where the prop is rigidly connected to the engine so the entire unit revolves at the same angular frequency.

In our setups, are prop balances really prop balances (targeting the RPM of the propellor) or are they engine balances (targeting the RPM of the engine)? By mounting the accelerometer on the engine, I would think the later, but... :confused:
 
One item that confuses me is the presence of the reduction gearing. I understand balancing a big bore engine, where the prop is rigidly connected to the engine so the entire unit revolves at the same angular frequency.

In our setups, are prop balances really prop balances (targeting the RPM of the propellor) or are they engine balances (targeting the RPM of the engine)? By mounting the accelerometer on the engine, I would think the later, but... :confused:

The balancer electronics "knows" the difference of the prop RPM and the engine RPM because the balancer strobes the propeller (there is a piece of reflective tape on the back of the prop). It counts every rotation, knows where 12:00 is and can relate each out of balance pulse to that clock position. Any other vibrations (engine) it feels are random since they don't occur with each revolution of the propeller.

I hope that makes sense, Its kinda hard to explain in a simple sentence.
 
I wonder what effect bugs have on prop balance? Maybe a few grams of bugs, more on one blade tip than the other, would be noticeable. Small weight differences out near the tip of the prop arc can have a large effect on dynamic balance. Couple that with interrupted airflow by some bug smeared on the leading edge and who knows what effect that has.

Just musing here while COVID sheltering…
 
I'm looking at Tony_T's photo of balance weight added to the spinner backing plate. It looks like the weight is almost 90 degrees to the prop blades. I would think that the weight would be located near one of the blades (lighter blade). This would make sense for static balance but maybe not for dynamic balance?

Another thought I have is dynamic balance along the axis of the prop shaft. The balance weight is not in line with the prop arc. This would be analogous to balancing a car tire with lead weight only on one side of the tire. Car tires split the balance weights (in/out) and even use different size weights to adjust for balance point across the width of the tire.

 
I would think that the weight would be located near one of the blades (lighter blade). This would make sense for static balance but maybe not for dynamic balance?

This would be an accurate assumption if the propeller has its mass exactly centered on the chord wise axis (perfect construction), and it is mounted perfectly centered on the prop shaft, and all the other spinning mass in exactly on centerline.

We can only statically balance the propeller off of the airplane, the bearings in the gearbox and their connection to the motor are not free enough to do a static balance. Dynamic balance allows all of the moving parts related to the propeller RPM to be balanced together.

The propeller is flat plate compared to a car tire. The reason car tire weights are split is to compensate for balance errors along the thickness / width of the tire. In comparison you don't split weights on a motorcycle tire.
 
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weight

I'm looking at Tony_T's photo of balance weight added to the spinner backing plate. It looks like the weight is almost 90 degrees to the prop blades. I would think that the weight would be located near one of the blades (lighter blade). This would make sense for static balance but maybe not for dynamic balance?

Another thought I have is dynamic balance along the axis of the prop shaft. The balance weight is not in line with the prop arc. This would be analogous to balancing a car tire with lead weight only on one side of the tire. Car tires split the balance weights (in/out) and even use different size weights to adjust for balance point across the width of the tire.



The amount of weight looks to be more than one ounce. Is there a limit on the amount of weight per location?
 
We’re just having a friendly conversation here so don’t jump down my throat…. It’s been over 40 years since I had a course in Dynamics and I guess it shows. It would seem to me that statically balancing a two-bladed airplane prop would entail adding weight to the lighter blade, or conversely, removing weight from the heavier blade. The dynamic part would be adding/removing weight from the right location on the blade so that both blades have the same “weighting” from the hub to tip. This is sometimes done on full-scale helicopters and is very common on radio-control helicopters. Unfortunately, not doable on a full-scale airplane. See… https://youtu.be/OUPUEvi7hRI

So, getting back to my post #19 above, it still seems to me that the balance weight would be placed on the backing plate (because it’s convenient) but directly behind the lighter weight blade. Maybe I’m just blowing smoke…
 
The static balance uses a different shaft than the airplane. If that shaft is only a few thousandths off center (the way it fits the propeller) compared to the actual airplane shaft then the static balance is no good. The shaft error can hold the prop off center span wise or chord wise.

Static balancing is not bad, but it is seldom perfect once on the plane..

FYI,The best balance on a car tire is spin balance while on the car.

If the mounting hole / prop shaft is not perfectly on center chord wise you will need to add the balance weight beyond the leading or trailing edge, as in the above picture.

Once you mount the prop on the engine or gearbox you have added other parts to the rotating mass, back plate, prop shaft, gears, etc. Their balance errors become factors in dynamic balancing.

In the model helicopters which I am VERY familiar with there is another factor. The blades freely lead / lag. They move into a position in the circle that is not always 180 degrees from each other (2 bladed rotor system). Their position is determined by the blade mounting / pivot hole and the blade mass location. Weight can be added onto the blade to balance the system span wise and sometimes if the weight is added to the leading edge or trailing edge the blade can be made to fly closer to 180. If there is a very slight error in the mounting hole or rotor head design they will not be 180 and there becomes more mass on one side of the main shaft. On a flybar rotor head weight can be added to the flybar to fix that. There is another problem caused when this happens where the center of lift is not on the center of the main shaft, that is hard to solve but not really an issue with airplane propellers.
 
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This has been terrific discussion and addressed some of the questions I have that I’ve not seen addressed elsewhere on VAF! So...
Tony, how have you stiffened your instrument mount(s)? Also, I wonder if the weights in your photo are the only ones, or are there others installed so as to “net out” to a result in between somewhere?
 
This has been terrific discussion and addressed some of the questions I have that I’ve not seen addressed elsewhere on VAF! So...
Tony, how have you stiffened your instrument mount(s)? Also, I wonder if the weights in your photo are the only ones, or are there others installed so as to “net out” to a result in between somewhere?

I have not yet tried to stiffen the mount shown. It could be stiffened by shortening the L holding the vibration sensor, but that would move the sensor off of the centerline of the prop shaft. Doing that could result in difficulty interpreting the phase angle displayed. The same picture was sent to the instrument manufacturer and they said the set-up looked OK. Also, if the vibration sensor is resonating the results after adding weights won't make sense. My readings all make sense. The weights installed in the picture brought the IPS down from an initial 0.76 IPS (rough) to 0.08 IPS (good). A vibration level of 0.76 can be felt pretty strongly on the rudder pedals and the throttle handle. Adding an additional AN960-10 washer brought it down to 0.03 IPS.

The weights shown are the only weights. You may notice another hole on the backplate, that was the initial balance years ago before the Sensenich front hub was replaced per SB.

Once you get down below about 0.08 with basic Dyna-vibe it takes some thoughtful work and more runs to get to the really low numbers.
Bottom line; the Basic Dyna-Vibe will get the balance to 0.10 IPS (considered good) pretty easily. If you want to get it perfect with this equipment -- good luck.
I think most RV-12s could be nicely improved using the basic Dyna-Vibe. But the Rotax can have it's own balance issues as Walt has already pointed out.

Now a Catto fixed pitch 3-blade, that is a whole different animal. That prop runs like a turbine, and yes I have ridden in a Catto propped 12. I didn't now what smooth was until that ride. Plus you can take it on and off the airplane effortlessly. But I digress, different topic.
 
I have not yet tried to stiffen the mount shown. It could be stiffened by shortening the L holding the vibration sensor, but that would move the sensor off of the centerline of the prop shaft. Doing that could result in difficulty interpreting the phase angle displayed. The same picture was sent to the instrument manufacturer and they said the set-up looked OK. Also, if the vibration sensor is resonating the results after adding weights won't make sense. My readings all make sense. The weights installed in the picture brought the IPS down from an initial 0.76 IPS (rough) to 0.08 IPS (good). A vibration level of 0.76 can be felt pretty strongly on the rudder pedals and the throttle handle. Adding an additional AN960-10 washer brought it down to 0.03 IPS.
.

The mount bracket we use is very small and stiff which makes it positioned slightly off center. We regularly get finished values at .02 ips.
We use the Aces 2020HR balancer.
 
Just a follow up from the original post. The muffler tang was fixed and I went with the Viking rental option using a dyna vibe g3x system. It came with everything I needed to mount it and I was able to correct it down to 0.02 IPS. Everything is running smooth now.
My guess on the original issue at the prop shop, after talking to some folks, was that they ran the balancer wires too close to the spark plug wires. They may not have been familiar with the Rotax set up, which has the wires run across the top vs. Lycoming which run down the sides. I followed the setup as shown by previous post here and made sure the wires were routed away from the spark plug wires. All is well.

Flying RV-12 (170+ hrs and Oshkosh 2019)
N418MJ
Warren County, OH (I68)
[email protected]
http://www.mykitlog.com/mwjohn63
https://www.youtube.com/watch?v=EJdXcjckN6M
2020 dues paid
 
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