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Dynamic prop balance for an LS?


Bill3558
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I get a pretty noticeable vibration at about 4200 rpm. Smooths out above or below that. My old carbonated plane didn’t do it.   I’m wondering if it could be a prop balance issue. My A&P has the equipment to do a “dynamic prop balance”, but after looking at the flimsy metal disk behind the prop he is not sure it is sturdy enough to put a counter weight on it. 
Any thoughts?  

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Your prop can be balanced and yes the backing plate is plenty strong enough. IF this is a 912ULS then the 4200 rpm range speaks more at a carb sync. Do the high end at 3300 -3500 and I bet the vibration goes away. If not then how many hours on the engine and when was the gearbox overhauled? 

If it's a 912iS then the gearbox may need to be overhauled because they had issues before the new gearbox was introduced.

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It’s an 912 Is sport. Gearbox was rebuilt at 215 hours because of the Is sport upgrade. Currently have 481 hours. Magnetic plug is clean. 
It might just be the nature of the engine in that rpm range, but I suppose it couldn’t hurt to have the prop balance checked. 
Thanks for the information . 

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Before you dynamic balance, I would certainly check the pitch of each blade. I have seen when the blades were not matched cause a vibration. I match the pitch within 1/10th of a degree when I set a prop. I have seen some out by a 1/2 a degree before.

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What year is your plane. Your 912iS engine gearbox may be the problem. Rotax had many with worn out dog hubs because of lack of oiling. They have a new gearbox now with a oiler jet that sprays oil on top of the gears inside the gearbox. I have known some 912iS owners that have had their gearbox overhauled 3-5 times. Once you get to the three times mark you can request a new gearbox.

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If it's a 2013 and the gearbox hasn't been done or it has, but it's been 150 hrs. since it was done then I would send the gearbox in. My money says gearbox. I have a 2013 CTLSi in  my hangar with about the same hours. It needed a gearbox overhaul. Send it to Leading Edge Airfoils.

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2 hours ago, Roger Lee said:

If it's a 2013 and the gearbox hasn't been done or it has, but it's been 150 hrs. since it was done then I would send the gearbox in. My money says gearbox. I have a 2013 CTLSi in  my hangar with about the same hours. It needed a gearbox overhaul. Send it to Leading Edge Airfoils.

Better than Lockwood?

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  • 2 weeks later...

Update. After doing some research I’m convinced it’s worn “dog gears” in the gear box. I have an E-Prop on order. I hope that will get me another 400 hours to next required inspection. 
This short video describes the problem. 

 

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15 hours ago, Bill3558 said:

Update. After doing some research I’m convinced it’s worn “dog gears” in the gear box. I have an E-Prop on order. I hope that will get me another 400 hours to next required inspection. 
This short video describes the problem.

I'm confused, maybe I'm misunderstanding.  You are sure you have worn gearbox components.  Are you trying to buy more time on the gearbox with a lighter prop?  If that is the plan, why not just send the gearbox in for an overhaul?  It's cheaper than a prop, and the prop won't necessarily solve the problem.

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The vibration is not that bad but is noticeable. Gearbox only has about 200 hours on it since it was rebuilt at Lockwood as part of the Rotax Is sport upgrade a few years ago. My thinking is a lighter prop and staying out of the vibration zone may allow me to fly the plane another 400 hours to the next recommended service.  I wanted to get the E-Prop anyway. It may mitigate the problem. 
If it doesn’t, I’ll send the gearbox in. Just hate the expense, hassle and down time. 
 

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Once you rebuild the gearbox 3-5 times Rotax usually will give you a new improved gearbox, but you must file a CSIR with a service center and copy of the logbook entries showing the issues and times for the other overhauls.

If it is the gearbox and a good chance it is with the 912iS engine (before the newer gearbox) then it will only get worse. The 150-200 hr. mark is about right for the box to start having issues....again.

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Good info. Thank you.
From what I can find there is little chance of catastrophic failure in flight. It’s just vibration, chattering noise and hard starting issues that get worse over time.  At the Lockwood class I recently attended,I learned a new gearbox with the oil jet is $5k. 
I’m trying to remain positive on Rotax reliability, but frankly I am getting a bit discouraged. I don’t understand why my old 912uls was smooth as silk, ( until the oil return lines came off in flight) and my  newer 912is engine has these issues eating gear boxes. 
 

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The FAA has an advisory circular out on prop balancing, and some of the symptoms.

A couple big ones are exhaust systems cracking, and things in the tail breaking. We don't really have issue with the tail thing, but exhausts sure are a pain!

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2 hours ago, Anticept said:

A couple big ones are exhaust systems cracking, and things in the tail breaking. We don't really have issue with the tail thing, but exhausts sure are a pain!

Just as an aside...I had a catastrophic exhaust failure some years back (one of the exhaust pipes completely broke in two, and the muffler had many cracks in the exhaust stack and muffler body).  I think the problem was caused by the exhaust when installed by the previous owner's mechanic being under tension and some of the components being "force fit" to others.

When adjusting or re-installing exhaust components, it's important to loosen all the cylinder flanges, then install the muffler and springs, letting everything just "hang" in place, and move it all around to your satisfaction.  Only then do you tighten at the cylinders.  If you tighten first, everything will be under tension and prone to stress cracks.  And of course, any gearbox, prop, or other FWF vibrations will only make that more likely.

Since using this method, as taught to me by Roger Lee when my failure happened on his doorstep, I have had no more issues.

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That's exactly how you're supposed to fit piping. You get everything you can roughly in place, and tighten down everything as reasonable with an emphasis on keeping stress from building. If you have to push something in place you're doing it wrong.

It annoys me to no end watching some people tighten bolts, fitting piping, and assembling parts.

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Last fall I did a pile of service on exhaust:  full removal / clean / new springs / bend spring tabs for more tension / replaced a couple exhaust studs /etc.  After everything was back in place I wrapped headers with some premium header wrap, as well as placed a clamp right over the top of joint squezing the header wrap to seal up the split line.

Pulled cowl today and gave it look over, appears just as I left it, zero signs of leaking, springs and RTV look mint.  It was a lot of work but sure was rewarding to see this upgrade paying off.

I wonder if the wrap with the clamp around the knuckle joint will tone down wear & vibration factor as a side benefit?  

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The knuckles must be unhibited from movement. They exist because stainless really likes to move when heated, and any restrictions to movement will increase stress.

Just keep it well lubricated.every 300-400 hours works with nickel antisieze, every 200 if you use copper.

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