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  #21  
Old 12-29-2017, 06:49 PM
Jimzim Jimzim is offline
 
Join Date: Mar 2009
Location: Arvada, CO
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Default 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! Id be happy to pay a reasonable fee to get my 7 smoothed out a bit; o360, Sensenich FP prop..
Thanks, Jim
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  #22  
Old 01-03-2018, 02:54 PM
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DonFromTX DonFromTX is offline
 
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Location: La Feria Texas
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Angry

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.
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  #23  
Old 01-03-2018, 03:18 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by DonFromTX View Post
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......
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  #24  
Old 01-03-2018, 03:25 PM
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DonFromTX DonFromTX is offline
 
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Uh Scott, does that mean you don't know the answer to my questions either?

Quote:
Originally Posted by rvbuilder2002 View Post
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......
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  #25  
Old 01-03-2018, 03:56 PM
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DaleB DaleB is offline
 
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Location: Omaha, NE (KMLE)
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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.
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  #26  
Old 01-03-2018, 04:09 PM
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akarmy akarmy is offline
 
Join Date: Jan 2005
Location: Auburn, WA
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Default

Quote:
Originally Posted by DonFromTX View Post
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.
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  #27  
Old 01-03-2018, 04:24 PM
waterboy2110 waterboy2110 is offline
 
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Location: SF Bay Area
Posts: 241
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Quote:
Originally Posted by DonFromTX View Post
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.
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  #28  
Old 01-03-2018, 04:27 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by DonFromTX View Post
Uh Scott, does that mean you don't know the answer to my questions either?
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
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  #29  
Old 01-03-2018, 04:34 PM
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Walt Walt is offline
 
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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.
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  #30  
Old 01-03-2018, 05:18 PM
vic syracuse vic syracuse is offline
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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
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