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DYNAMIC BALANCE

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Peter H

Joined: 20 Mar 2007
Posts: 197

Posted: Tue Aug 21, 2007 1:47 pm Post subject: DYNAMIC BALANCE

As a result of finding several instances of a low frequency vibration I took the opportunity when we had a week of bad weather to remove the prop , spinner and backing plate and spent three days balancing the set at home. Very boring and tedious adding and taking 2mm screws, nuts and washers I could only do it for an hour at a time. Any draft in the house and the prop would move. When it was perfect and soon after in flight I got a very bad scary low frequency vibration when decelerating and turning left at about 140KTS.Maybe precession triggered it off. Reducing throttle did not immediately help.  

I got the set up dynamically balanced and it took a 3/4X 7/16” bolt with nut and two washers to correct the dynamic balance. I had been very satisfied with the engine running but it ran smoother than ever before when the first weight was fitted. The difference was very obvious. The prop/spinner and backing plate are now NO WAY in static balance. I have marked the parts and the prop shaft. The job took 4/4 hour and was done in the hangar.  

It means that it was my engine that was badly out of balance and is now corrected by the dynamic balancing of the prop/backing plate.  

It means that if flywheel bolts are breaking it could be due to a poorly balanced engine. My 3300 engine was factory rebuilt to zero hours in 2006.  

Based on my experience I would recommend that all users get a proper dynamic balance, making sure that the backing plate and spinner are in good sound condition at the same time.  

I understand that Pete and some others have got the equipment to do the job.  

Peter H  

        [quote][b]

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promod69camaro

Joined: 22 Aug 2007
Posts: 14

Posted: Fri Aug 24, 2007 6:07 pm Post subject: Re: DYNAMIC BALANCE

When you sent the engine in to be rebuilt, did they change out the pistons and/or connecting rods?  They may be using a different weight piston if so.  It sounds to me like each crank needs to by dynamically balanced with the pistons and rods that are going into it.  When I build a race engine, we match the pistons up according to weight.  Each one is weighed to find the lightest one.  We then remove material from all the others until they are all the same weight.  That way, the recipicating mass is the same through out the engine.  The machinest says it is easier to get a good rotating assembly balance this way and the engines seem to live longer.

I sure hope that your engine is the exception and not the rule.  These engines are to expensive for basic machining to not be performed.

Shawn

________

Zx14 vs hayabusa

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Last edited by promod69camaro on Wed Mar 09, 2011 1:23 pm; edited 1 time in total

Peter H

Joined: 20 Mar 2007
Posts: 197

Posted: Fri Aug 24, 2007 7:03 pm Post subject: DYNAMIC BALANCE

Shawn I bought the engine rebuilt from the factory and have no details how

the job was done. It was sold to me with warranty as a zero hours engine.

When an intermittent problem low frequency vibration was not solved with

careful static prop balance I got a dynamic balance and noticed an immediate

and major improvement in running the engine . It is like a different engine.

It took a 3/4X1/4" bolt,nut and washer about 4" from centre on the backing

plate to get it right.

Peter



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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Sat Aug 25, 2007 2:37 pm Post subject: Re: DYNAMIC BALANCE

Peter, 

    Having had dynamic balancing and similar weights applied to a C150 I understand your concern about the engine balance.



I do wonder though, if the prop is still the problem?  Can you be sure that the weight distribution within the prop is the same for each blade? If not, then the solution for static balance will be different to that of dynamic balance, and the reaction to yaw will also be different. 



Ralph

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Ralph - CH701 / 2200a

Peter H

Joined: 20 Mar 2007
Posts: 197

Posted: Sat Aug 25, 2007 3:07 pm Post subject: DYNAMIC BALANCE

Ralph,

I cannot be sure if the weight distribution in the prop is uniform. I can

only say that it is a wooden prop made by Ed Sterba (US) and he has a good

reputation for high performance wooden props. It would be fair to say that

the prop is an average wooden prop with average density characteristics.

Do you have any information about variability in wooden props?

In my case the correction was very large ie a 1/4"X 3/4 bolt nut and washer

at about 4" from centre. Seems like a lot of imbalance to attribute to the

light weight prop. It would be a good thing to get some more data and I

would recommend dynamic balance to all J3300 users. Provided the job is done

starting with a static balanced prop then we could see if there are any

others. It took 3/4HR and cost me AU$120.

Peter 



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promod69camaro

Joined: 22 Aug 2007
Posts: 14

Posted: Sat Aug 25, 2007 3:43 pm Post subject: Re: DYNAMIC BALANCE

Peter,

I know that what you have already done is working, but if you put weight further out on the propeller, it won't require as much to be dynamically balanced.  And the lighter weight further out will not affect the static balance as much either.  Or you could just cut of several inches of prop on the heavy side and call it fixed. 

________

Vaporizer Volcano

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Last edited by promod69camaro on Sat Feb 26, 2011 6:31 pm; edited 1 time in total

Peter H

Joined: 20 Mar 2007
Posts: 197

Posted: Sat Aug 25, 2007 5:03 pm Post subject: DYNAMIC BALANCE

Ha Ha 

But the application of weight is off axis with the prop in my case, I would

have to mount an outrigger (LOL)

Peter



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