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Old 06-04-2018, 05:36 AM
dbhill916 dbhill916 is offline
Join Date: May 2013
Location: Westerville, OH
Posts: 131
Default gear reduction vs prop balance

Hi all,

I have a question about how a prop balance works on the Rotax 912.

As I understand the process, an accelerometer is temporarily mounted to the engine, and some sort of optical pickup is mounted to the prop. I don't understand how the reduction gear wouldn't interfere with the process of determining the (prop?) angle of maximum acceleration of the engine.

Apparently, that is a question that is bothering the local prop shop, too. They said they were not very familiar with doing balances on 912's, but "he'd take a look at it."

Any explanation I can relay to the prop shop (Tiffin Aire, 16G) on how to successfully balance the 912?

Thanks in advance,
David B. Hill
RV-12 #0760
Dues paid through 6-2019
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Old 06-04-2018, 06:28 AM
vic syracuse vic syracuse is offline
Super Moderator
Join Date: May 2005
Location: Locust Grove, GA
Posts: 2,374
Default N0 difference

The reduction doesn't really have anything to do with it. They are balancing the rotating mass of the propeller. I've balance a number of them without any problems.

Vic Syracuse

Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, Just Superstol, DAR, A&P/IA, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES
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Old 06-04-2018, 09:55 AM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,868

Your local prop shop is bothered because a traditional dynamic balance is dynamically balancing the propeller and crankshaft of the engine as a complete assembly.
They are correct in their thinking that that is not possible on a Rotax 912 because the relationship of the propeller to the engine crank shaft is always changing.

It can still be very beneficial though as has been proven 1000's of times.
The main reason is likely the same as comparing a simple bubble balance of a car wheel/tire to the result attained by spin balancing at high normal operating speed.

The main difference in the procedure is that they need to compensate the test settings on the equipment to reference the propeller RPM, not the engine RPM (what you see on the tach.).
I usually set 2000, because that is about the most you will see for static RPM running on the ground.
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")

Last edited by rvbuilder2002 : 06-04-2018 at 09:58 AM.
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Old 06-05-2018, 04:01 PM
dbhill916 dbhill916 is offline
Join Date: May 2013
Location: Westerville, OH
Posts: 131
Smile Thanks!

Thanks, gentlemen. I'll see if I can be the "1001'st" to get dynamically balanced!
David B. Hill
RV-12 #0760
Dues paid through 6-2019
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