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Prop balancing using the DynaVib

waterboy2110

Well Known Member
Finally got around to making the fixtures and testing out the DynaVib prop balance tool. Pretty impressed with the results.

There was a point when you pulled the throttle out a vibration would set up and resonate. We went through everything from mounts to carb balance. My buddy was still thinking he could tune it out but I pretty much knew that wasn't it.

I put the DynaVib on and measured in the high 0.6's at 360 degrees and began adding weight. When it was all said and done there's 29 grams split between 180 and 170 degrees. Final numbers are 0.01 IPS for three consecutive runs that randomly clock the prop, basically just noise.

It's a noticeable difference. The only vibration now is the pulse of the engine and the shudder while decelerating is gone. it's most notable during idle once warmed up.

An interesting aspect of this testing was that as the engine warms up the pulse clocks around the hub. I wasn't expecting this but it's consistent over the course of a couple days of testing. I assume that as the gears and everything forward of the gear box warm up they settle in to a place they like to be.

If you're flying a Rotax and just want to know what you're IPS is I'd be happy to come by and set it up any where between say KFAT and KRDD. Five minutes after the cowl is removed and the engine is warm and you'll have the data. Just an opportunity to meet folks and use the tool for me.
 
$1600 and change. As soon as I needed some washers I found a guy who had one - go figure.

I'm glad to have it. Spent years in the aerospace industry flying FFT's. I'm a data fan and these things help me visualize what's going on with engines.
 
I wonder how difficult it would be to build something like that, looking at some videos on prop balancing I see that they are using a laser trigger (which run about $250) and a single axis accelerometer. Then you are just looking for the "phase" between the two signals, I'm going to have to stick an accelerometer on my engine and see how messy the signal is and if it can be converted in software to a straight frequency and ignore the noise using a frequency domain algorithm. It would be kind of cool to do all of this on a raspberry pi.
 
Pretty sure there are spectral analysis programs for your laptop now. The trigger would be pretty simple as you say. The accelorometers used look to be off the shelf MEMS devices that come in chips now. When we did it back in the day it was HP FFT's for big bucks and Endevcos.

I'm sure the algorithms Fast Fourier Transforms would be easy to port to a raspberry.
 
Would it be possible to get some photos of the setup? Hardware can unduly influence the vibration readings and it seams you have gotten the correct setup. Thanks
 
I wonder how difficult it would be to build something like that, looking at some videos on prop balancing I see that they are using a laser trigger (which run about $250) and a single axis accelerometer. Then you are just looking for the "phase" between the two signals, I'm going to have to stick an accelerometer on my engine and see how messy the signal is and if it can be converted in software to a straight frequency and ignore the noise using a frequency domain algorithm. It would be kind of cool to do all of this on a raspberry pi.

You probably don't need a laser trigger either, just pick up the alternator output and trigger on the highest peak, or magneto grounding signal, or perhaps heat-shrink a teensy antenna onto one of the spark plug wires. It's easy to find electrical noise synchronized to the engine--we usually take extra steps to fight it. From there all it takes is a little bit of trial and error with washers to find out at what angle that electrical peak occurs.

A few months ago I picked up a MEMS accelerometer breakout board from Sparkfun with the idea bolting it to the front of the engine permanently. From the specs it looks like these things are quite rugged, wonder why Lyc doesn't bolt one to all their engines at a few bucks a pop now that we all have data acquisition systems built in on the Dynons and Garmins.

Will report back when I find some time to play with it.
 
You probably don't need a laser trigger either, just pick up the alternator output and trigger on the highest peak, or magneto grounding signal, or perhaps heat-shrink a teensy antenna onto one of the spark plug wires. It's easy to find electrical noise synchronized to the engine--we usually take extra steps to fight it. From there all it takes is a little bit of trial and error with washers to find out at what angle that electrical peak occurs.

A few months ago I picked up a MEMS accelerometer breakout board from Sparkfun with the idea bolting it to the front of the engine permanently. From the specs it looks like these things are quite rugged, wonder why Lyc doesn't bolt one to all their engines at a few bucks a pop now that we all have data acquisition systems built in on the Dynons and Garmins.

Will report back when I find some time to play with it.

Triggering off the plug would work for direct drive. Just use a strobe light trigger. The Rotax has gear reduction thus the need to sync to the prop.
 
Triggering off the plug would work for direct drive. Just use a strobe light trigger. The Rotax has gear reduction thus the need to sync to the prop.

Ah, forgot about that little twist. Odd reduction ratio too. Yup, need optical trigger on Rotax.
 
Dynavibe

Santa gave me a Dynavibe this year. How did you attach weights to prop and did you balance with spinner on or off?
 
I balanced my RV-12 with the spinner ON (you really have to since the spinner is part of the balance equation and the pitot tube needs to be supported). I added a very small amount of weight under the outer rim of the rear spinner bulkhead -- the weight is held on by one of the spinner's rear screws. This is very easy to do but does mean that you have to remember where the weight goes if you subsequently remove/reinstall the spinner (also the top cowling has to be removed to access the nut holding the weight on). I guess I could figure out some way to attach (glue? Ugh.) the weight/MK nut to the spinner bulkhead but that seems a tad finicky.
 
Yes, you balance with the spinner on. Temporary weights should be added on the outside of the spinner (under the aft spinner mounting screws). Once a acceptable balance is achieved, the weights have to be moved and bolted to the rear spinner bulkhead (no glue!!!). The operators manual of the balancer should explain the math required to relocate the temporary weights to the spinner bulkhead.
Alex
 
jeterjim said:
I just got a dynavibe and would like to know how you attached the weights to the spinner.?

You will be drilling a hole through the rear spinner bulkhead for the balance weight.

Once you determine the angle where the maximum IPS exist weight will be added 180 degrees from that point.

Make sure the engine is up to operating temp. You will be running the test full throttle to gather the data. I found that adding weight to the spinner screws didn't do anything.

Get some AN hardware before you start. You'll need a couple AN3 bolts and all metal locks. Get some AN 970 washers so you don't get caught up stacking up a half dozen 960’s. To remove 0.68 IPS at 360 degrees I added 29 grams at a 5 inch radius at 180 degrees.

There should be a label placed on the spinner bulkhead that states that the prop / spinner / bulkheads are balanced as a system to notify anyone working on the plane that it must be reassembled the same as it was taken apart and of course a log book entry.
 
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The standard method of doing a propeller balance is to add temporary weights to aft spinner attach screw locations.
I (and others) have been doing it this way for years and the procedures manual for the Asus prop balancers recommends this method.

It usually requires using a longer screw but that doesn't matter because you are calculating the Delta (weight difference) between the original screw and the weight used for a balance run anyway.
Once a good balance result is attained calculations are made to install a slightly higher amount of weight (because of the different polar moment of inertia) further inboard on the spinner pack plate.
 
The standard method of doing a propeller balance is to add temporary weights to aft spinner attach screw locations.
I (and others) have been doing it this way for years and the procedures manual for the Asus prop balancers recommends this method.

It usually requires using a longer screw but that doesn't matter because you are calculating the Delta (weight difference) between the original screw and the weight used for a balance run anyway.
Once a good balance result is attained calculations are made to install a slightly higher amount of weight (because of the different polar moment of inertia) further inboard on the spinner pack plate.

Scott is correct in that the manual asks to use the spinner screws to add weight during initial runs. The procedures go on to tell the mechanic to add weight at the 180 degree radial of the measured IPS. Placing weight on the spinner screws is a good ball park place to start to see if you are moving in the right direction but experience has shown that simply moving the weights from the radial of a spinner screw inboard to the backing plate with a simple weight and balance equation would rarely lead to optimum results. Spinner screws are generally 30 degrees apart and in this application do not exist on the propellers radial.

Consider the photo below:
Screen%2BShot%2B2017-12-29%2Bat%2B8.52.48%2BAM.png


Here we see our balance weights directly under the #2 blade on a typical prop and hub assembly (if you haven't described each blade by SN you will need to do that so the assembly, once balanced, can be returned to the balanced state during any subsequent service).
The placement of the weight at or near a blade would be expected since the blades arm under acceleration generally would exhibit the greatest moment.
In this application the #1 blade was heavy coming in at 0.68 IPS at 360 degrees. As you can see from the photo there are no spinner screws in the vicinity of the angle the instrument is calling out to add weight. The procedure also asks that you make small corrections to the weights and work towards a solution. In this case 10 grams was added at 180 degrees which brought down the IPS into the 0.4's at 170 degrees. Another 10 grams was added there and after a few more runs the total weight came out to 29 grams split between 170 and 180 degrees. This was achieved by using two AN3-4 bolts with all metal locks, two AN 960 washers on the bolt head (each 1 gram) and four AN 970 washers each weighing 4 grams.
At each run four measurements were made using the averaging function on the balance tool. The final test run for this propeller assembly is 0.01 IPS clocking randomly around the prop. Basically noise at this point.

Did it make a difference? You bet it did. Rotax SL-912-010 describes inherent vibrations in the 912 engines that can be excited by external inputs to the power plant. In the SL they describe what appears to be extensive vibration analysis on the power plant and discovered that between 3600 and 4800 rpm harmonics could cause a low frequency vibration to occur. I actually experienced this on the new engine (wasn't present on the first engine) in that when the throttle was retarded through the described RPM the plane would shudder.
After exhausting all listed items in SL -912-010 I determined that the prop needed to be surveyed. The IA that I use to work with who regularly provided propeller balance service (and taught me how to balance props) had moved on so I went ahead and bought the DynaVib since it was affordable and I felt that it was the instrument I needed to hunt down the source of the vibration during deceleration.

Once the propeller system was balanced the engine resonance could no longer be excited and the plane is as smooth as silk now. Flew it 1200 miles the other day and it the engine performed flawlessly.

A word about balancing:
In certificated aircraft, unless balancing is described by the manufacture, dynamic propeller balancing is considered a major alteration. The FAA and those of us licensed by the FAA take this stuff seriously and so should you. Be careful and by all means work within the constraints of your certification or ask for help. There are plenty of folks that can guide you through the process so that you end up happy with the work and safe when all is said and done.
 
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I have never used a Dynavibe balancer so it may not have the capability, but the ASUS balancers do the weight adjustment and repositioning calculations for you.

During the balancing process, if the called for weight location doesn't coincide with a spinner screw it will calculate the amounts to split the weight between two screw locations that you choose.

Once the balancing process has been completed using temp weights on the spinner screws, it will convert that to a location further inboard on the spinner for attaching permanent weights.

There is no guessing or approximating.
I routinely achieve a balance down to only .01 IPS with the temp weights and then still have .01 IPS after the permanent weights are installed.
 
If you're flying a Rotax and just want to know what you're IPS is I'd be happy to come by and set it up any where between say KFAT and KRDD. Five minutes after the cowl is removed and the engine is warm and you'll have the data. Just an opportunity to meet folks and use the tool for me.


Not a Rotax, but a Subaru with a 4 blade electric CS prop.

I have a beat frequency that I'm trying to sort out... so If you would like to play with your new toy, I would like to experiment as well.

(I've done a dynamic balance before, about 15 hours later I noticed the beat frequency.)

I'm at Yolo county, KDWA.
 
Not a Rotax, but a Subaru with a 4 blade electric CS prop.

I have a beat frequency that I'm trying to sort out... so If you would like to play with your new toy, I would like to experiment as well.

(I've done a dynamic balance before, about 15 hours later I noticed the beat frequency.)

I'm at Yolo county, KDWA.

No problem. PM me and we can set a date to meet
 
A word about balancing:
In certificated aircraft, unless balancing is described by the manufacture, dynamic propeller balancing is considered a major alteration. The FAA and those of us licensed by the FAA take this stuff seriously and so should you. Be careful and by all means work within the constraints of your certification or ask for help. There are plenty of folks that can guide you through the process so that you end up happy with the work and safe when all is said and done.

Just to set the record straight, AC 20-37E is clear that prop balancing is not considered a major repair or alteration.

(c) For aircraft or propeller manufacturers that provide procedures for dynamic balancing of the propeller in their maintenance manuals or instructions for continued airworthiness, propeller balancing is not considered a major airframe alteration.

(d) When approved aircraft or propeller manufacturer?s procedures are not available, there are other acceptable dynamic propeller balancing procedures. These include, but are not limited to the Chadwick-Helmuth Publication No. AW-9511-2, entitled ?The Smooth Propeller?, and ACES Publication No. 100-OM-01, entitled ?ACES Systems Guide to Propeller Balancing?. Dynamic balancing of propellers using FAA-approved or -accepted dynamic propeller balancing procedures is not considered a major propeller repair unless the propeller static balance weights are altered or when using the Chadwick-Helmuth or ACES type documents on propeller installations of 500 horsepower or more.
 
Just to set the record straight, AC 20-37E is clear that prop balancing is not considered a major repair or alteration.

(c) For aircraft or propeller manufacturers that provide procedures for dynamic balancing of the propeller in their maintenance manuals or instructions for continued airworthiness, propeller balancing is not considered a major airframe alteration.

(d) When approved aircraft or propeller manufacturer’s procedures are not available, there are other acceptable dynamic propeller balancing procedures. These include, but are not limited to the Chadwick-Helmuth Publication No. AW-9511-2, entitled “The Smooth Propeller”, and ACES Publication No. 100-OM-01, entitled “ACES Systems Guide to Propeller Balancing”. Dynamic balancing of propellers using FAA-approved or -accepted dynamic propeller balancing procedures is not considered a major propeller repair unless the propeller static balance weights are altered or when using the Chadwick-Helmuth or ACES type documents on propeller installations of 500 horsepower or more.

Well - I guess I took this statement from the Aces-Guide-Propeller-Balancing document Revision 2.0 June 1996 1000-OM-01,which is data approved by the administrator, literally...

"Modification of the spinner bulkhead, either by drilling or adding weights in the form of nuts, bolts, washers, or any combination thereof, requires the completion of FAA Form 337 for return to service"

A 337 is a major alteration / repair form for certificated aircraft. I was taught that when in doubt fill it out .. the 337 that is.

Since the AC states less than 500 HP I would agree that statement could override the one made in the ACES doc.

Walt - I see by your signature that you're quite experienced in this arena. I'm not challenging you or the work you do. I'm simply providing the data that led me to my conclusion. Thanks for the pointer Walt. Good data is always welcome.
 
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Balancing near Denver

Does anyone know of someone who does dynamic balancing near Denver? After 3 frustrating days playing with a borrowed Dynavibe and no success, I give up, we must be doing something wrong! I?d be happy to pay a reasonable fee to get my 7 smoothed out a bit; o360, Sensenich FP prop..
Thanks, Jim
 
I bought a balancer and loaned it to another RV12/Rotax owner, he has had some interesting times learning to use it. Something that has puzzled me is that while trying to balance the prop, an imbalance in the engine would be constantly changing the indications of the prop due to the odd reduction ratio.How is this compensated for?
He has come up with another thought, that the nosewheel on the floor is dampening out vibrations, and things might work better if you tied the tail down to the floor.
Anyone care to comment on this? Keep in mind that as is probably shown by the above comments, I know NOTHING about balancing a prop.
 
I bought a balancer and loaned it to another RV12/Rotax owner, he has had some interesting times learning to use it. Something that has puzzled me is that while trying to balance the prop, an imbalance in the engine would be constantly changing the indications of the prop due to the odd reduction ratio.How is this compensated for?
He has come up with another thought, that the nosewheel on the floor is dampening out vibrations, and things might work better if you tied the tail down to the floor.
Anyone care to comment on this? Keep in mind that as is probably shown by the above comments, I know NOTHING about balancing a prop.

The first question I always ask my self when something isn't working as expected, is "why does it work fine for LOTS of other people using this same procedure?"
It is rare that a new/different procedure needs to be invented when one has already been proven to work......
 
Uh Scott, does that mean you don't know the answer to my questions either?:D

The first question I always ask my self when something isn't working as expected, is "why does it work fine for LOTS of other people using this same procedure?"
It is rare that a new/different procedure needs to be invented when one has already been proven to work......
 
I'll take a shot. My disclaimer is, I have absolutely zero personal experience with dynamic prop balancers. I didn't stay at a Holiday Inn last night, but I have spent a few decades screwing around with electronics, microprocessors, sensors, embedded controls and stuff like that... so the concepts are not entirely unfamiliar to me. Occasionally I guess something close to the truth.

The tire on the ground thing shouldn't be an issue. The accelerometer gets bolted to the engine case, which is on rubber mounts, and I don't think the vibration is going to be damped much by the tire at the end of a long-ish flexible arm. I'm sure there is some effect, but is it measurable? Given that it's not common to do dynamic balancing in flight, I would guess it's not enough to cause problems.

One could isolate vibration caused by propeller imbalance simply by filtering out any vibration occurring at frequencies that don't match the prop rotation. In other words, if you detect that the prop is spinning at 2000 RPM, and you see a vibration that occurs at a rate of, say, 4860 Hz, then it's a good bet that vibration is not coming from the prop.

Some (maybe all?) of the balancers are smart enough to identify vibration that doesn't occur in sync with prop rotation. I recall seeing plots from someone that showed a plot of vibrations at different frequencies, meaning they were coming from the engine rather than the prop. But -- if your engine has enough vibration that it skews the results, I'd say that needs to be fixed first.
 
I bought a balancer and loaned it to another RV12/Rotax owner, he has had some interesting times learning to use it. Something that has puzzled me is that while trying to balance the prop, an imbalance in the engine would be constantly changing the indications of the prop due to the odd reduction ratio.How is this compensated for?
He has come up with another thought, that the nosewheel on the floor is dampening out vibrations, and things might work better if you tied the tail down to the floor.
Anyone care to comment on this? Keep in mind that as is probably shown by the above comments, I know NOTHING about balancing a prop.

I've balanced a number of RV-12's and countless Lycomings over the years with the basic Dynavibe. I've not seen any difference in balancing the RV-12's. One of the recent one's I did I was even able to get to 0.00 IPS (never seen that before) after attaching the final weight.

I don't think the nose gear has anything to do with it, but I have seen that if the wind is blowing it can mess with the prop disc and the readings will be slightly inconsistent.

I agree that internal engine issues would be a problem with the gearbox, but in general all of the 912's I've balanced were very smooth to start with and we were able to get them down below 0.05 using the normal process.

I do agree it's a bit of a black art at first when you start playing with the unit and different planes / engines.
 
I bought a balancer and loaned it to another RV12/Rotax owner, he has had some interesting times learning to use it. Something that has puzzled me is that while trying to balance the prop, an imbalance in the engine would be constantly changing the indications of the prop due to the odd reduction ratio.How is this compensated for?
He has come up with another thought, that the nosewheel on the floor is dampening out vibrations, and things might work better if you tied the tail down to the floor.
Anyone care to comment on this? Keep in mind that as is probably shown by the above comments, I know NOTHING about balancing a prop.

I would consider the prop to be the final component to look at when chasing a vibration. The list from the Rotax SL is a good start.

I don't know how the DynaVib or other balances filter the data but I would expect them to focus on the the prop frequency. A full spectrum analysis would give more data (described in the SL) and may not be that much of a stretch given modern technology and old test equipment.

I did observe that as the engine warms up the IPS will clock on the Rotax. Once at temp the readings were consistent.
 
Uh Scott, does that mean you don't know the answer to my questions either?:D

My point was that hundreds of RV-12's have been balanced to a high level of precision using the standard balancing procedures (loads of posts here in the forum with details)
You are correct that the prop and engine crankshaft do not remain in the same relative alignment during operation but what matters is the end result.

There are a lot of different factors that can influence being able to successfully complete the balance process.

A few are......

- Proper carb synchronization

- Propeller blades with pitch values very close to the same (preferably within .1 degrees of each other)

- Proper use of the equipment and installation of balance weight
 
I've done a fair number if 912's and have had problems with a few of them where the polar location was unstable which would make it impossible to get a good balance solution. In all cases that I can recall, balancing the carbs and/or adjusting blade pitch angle settled things down where it could be balanced.
 
I'm with Walt. You can't just go at the prop on a 912. The carbs have to be done, and the prop has to be pitched properly and equally. And make sure none of the pickup wires are anywhere near the spark plug wires. They will cause interference and weird readings.

Vic
 
I've balanced several (Lyc) engines using our EAA chapter's Dynavibe. It's not a very 'smart' balancer, and it's somewhat tricky to interpret their directions. But once you get what they're trying to tell you straight in your head, it works just fine. If you don't have it straight, you'll chase your tail for quite a while.

For Scott: I've never used any of the smarter balancers, but it sounds as if the major difference is that the DV doesn't tell you how much weight to use; it just gives you the acceleration number and the angle. But once you've added a bit of known weight & re-run the balance test, it's not that hard to zero in on the correct weight.

I've never added weight to the outside of the spinner to do my initial balance. I just go straight to longer spinner attach screws and add any weight to those, inside the backplate nutplates. Regular nyloc nutplates have worked just fine for me. I've had one or two where I had to make a weight, due to a badly out of balance situation (prop, badly cut spinner, etc). I've also split weights between two screw locations, as mentioned earlier.

On using the ignition as the trigger: Seems iffy with mags, and a nightmare if you have any kind of variable advance ignition. The optical trigger will fire at exactly the same point every time, and you can duplicate that point by simply moving the prop until the LED lights.

Nosewheel, etc: The prop does bounce the plane around quite a bit (I've done both nose & taildraggers), and it does seem to have some effect on the DV's ability to give a stable reading. I wonder if that's why DV tells us to run 3 sequences before adding weight.

Weight location: I wonder if the whole 337 thing is driven by drilling extra holes in the spinner backplate (that *is* a bit of a mod, especially on a TC'd plane). As a FWIW, I know of at least one local RV that has the balance weights from a DV balance job mounted on the flywheel, using the existing holes. Obviously takes a bit more weight because they're on a smaller radius, but it seemed to work just fine on that plane.

Charlie
 
For Scott: I've never used any of the smarter balancers, but it sounds as if the major difference is that the DV doesn't tell you how much weight to use; it just gives you the acceleration number and the angle. But once you've added a bit of known weight & re-run the balance test, it's not that hard to zero in on the correct weight.
Dynavibe has a more expensive model that does weight calcs for you.
 
I have owned a Dyna-Vibe Classic, an ACES Probalancer Sport and currently an ACES Probalancer. The Classic is accurate but does not do solutions. The other two do solutions which generally reduces the number of runs needed. As a rule a solutions balancer requires three runs. One initial run that tells where and how much weight. You can put a weight that is close to the indicated amt/location. As long as you accurately tell the balancer what was actually installed, the balancer will give an accurate amount after the second run. It will allow you to split the weights in two location and calculate the amount. The third run just verifies the final result. The biggest difference in the Probalancer over the Sport is the time of the run to get a reading. The Pro is much shorter time.
Hope this is understandable.
 
I have owned a Dyna-Vibe Classic, an ACES Probalancer Sport and currently an ACES Probalancer. The Classic is accurate but does not do solutions. The other two do solutions which generally reduces the number of runs needed. As a rule a solutions balancer requires three runs. One initial run that tells where and how much weight. You can put a weight that is close to the indicated amt/location. As long as you accurately tell the balancer what was actually installed, the balancer will give an accurate amount after the second run. It will allow you to split the weights in two location and calculate the amount. The third run just verifies the final result. The biggest difference in the Probalancer over the Sport is the time of the run to get a reading. The Pro is much shorter time.
Hope this is understandable.[/QUOT

I do agree that the Aces pro sport is probably the best information for the dollar spent. The Dynavibe just makes you do the calculations. For the initial weight used, just use the standard formula ((HP/10) +30) * ips. That is the correct weight in grams to be used if the weight is attached at the same distance from the center of the crankshaft as the piezo layer of the accelerometer is from the center of the crankshaft. If it is not the same, use an inverse ratio to modify the weight. Weight =( Calculated weight * Accelerometer dist) / attached weight distance.

You can also manually split weights using vector addition.
 
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