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  #1  
Old 12-21-2017, 08:01 PM
waterboy2110 waterboy2110 is offline
 
Join Date: Jul 2010
Location: SF Bay Area
Posts: 241
Default Prop balancing using the DynaVib

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.
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  #2  
Old 12-22-2017, 09:01 PM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
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What did that toy set you back? I think they run about a grand, donít they?
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  #3  
Old 12-22-2017, 09:22 PM
waterboy2110 waterboy2110 is offline
 
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$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.
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  #4  
Old 12-23-2017, 08:21 AM
Azjulian Azjulian is offline
 
Join Date: Dec 2015
Location: Scottsdale, Arizona
Posts: 131
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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.
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  #5  
Old 12-23-2017, 09:13 AM
waterboy2110 waterboy2110 is offline
 
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Location: SF Bay Area
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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.
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  #6  
Old 12-23-2017, 09:30 AM
banjo johnny banjo johnny is offline
 
Join Date: Jul 2013
Location: Millville, NJ
Posts: 3
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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
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  #7  
Old 12-23-2017, 12:00 PM
Aluminum Aluminum is offline
 
Join Date: Jul 2017
Location: San Jose, CA
Posts: 124
Default

Quote:
Originally Posted by Azjulian View Post
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.
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  #8  
Old 12-23-2017, 01:21 PM
waterboy2110 waterboy2110 is offline
 
Join Date: Jul 2010
Location: SF Bay Area
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Quote:
Originally Posted by Aluminum View Post
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.
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Instrument Rating
A&P / Rotax iRMT 9 Series Maintenance
RV12 Flying.
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  #9  
Old 12-24-2017, 08:26 AM
Aluminum Aluminum is offline
 
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Location: San Jose, CA
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Quote:
Originally Posted by waterboy2110 View Post
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.
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  #10  
Old 12-27-2017, 08:26 AM
jeterjim jeterjim is offline
 
Join Date: Dec 2009
Location: Orange Beach, AL
Posts: 24
Default Dynavibe

Santa gave me a Dynavibe this year. How did you attach weights to prop and did you balance with spinner on or off?
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