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  #21  
Old 01-31-2019, 10:01 AM
NinerBikes NinerBikes is offline
 
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I know it wouldn't apply to a Rotax motor, but in car engines, they put in heavier flywheel's or dual mass flywheels, to smooth the pulses out of the compression and combustion strokes, especially on turbo diesel motors. VW, Audi, Porsche do this, a lot.

Those pulses are probably what tear up the dogs.

Also when you are descending when landing and you throttle back, the propeller indeed gets feathered and sloppy, being driven by the wind, not the motor, with not much or no thrust being created by the prop. Hence the slop, if your dogs are worn.

Last edited by NinerBikes : 02-13-2019 at 10:47 PM.
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  #22  
Old 01-31-2019, 11:04 AM
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Piper J3 Piper J3 is offline
 
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I don’t agree with your assertion that prop gets “feathered” when engine is throttled back for landing.

A wind milling fixed-pitch prop creates a lot of drag. When the engine is throttled back the prop back-drives the engine, and in doing so, holds the dogs nicely against each other -so not chattering. This is why engine is running faster in glide than idle speed when stationary on the ground.

The key to gearbox longevity is engine minimum operating speed. Rolling elements – bearings, gears, etc. can take a lot of abuse under dynamic load but suffer badly if abused statically. The reason for the 1800 RPM idle speed is Rotax deems this to be the minimum speed where engine power pulses don’t hurt the gearbox.

That being said, I have my idle speed set at ~ 1700 RPM. This allows better aerodynamic drag and helps reduce float on landing. I have to manually operate minimum throttle to stay above 1800 RPM on the ground. Works very nice…
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Last edited by Piper J3 : 01-31-2019 at 11:21 AM.
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  #23  
Old 01-31-2019, 01:39 PM
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randylervold randylervold is offline
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Anyone know if the 912iS idle is even adjustable being controlled by the computer and all?
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  #24  
Old 01-31-2019, 03:15 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by randylervold View Post
Anyone know if the 912iS idle is even adjustable being controlled by the computer and all?
Yes it is.

There is an idle stop screw on the throttle body.
Van's recommends the same idle speed stop adjustment for the ULS and the iS engine (1650 RPM), with and then follow the standard operating procedure of manually keeping the engine speed above 1800 RPM at all times.
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  #25  
Old 03-01-2019, 10:16 PM
NinerBikes NinerBikes is offline
 
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Problem solved.... gearbox wear. http://www.vansairforce.com/communit...d.php?t=169246
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  #26  
Old 03-02-2019, 06:10 PM
AirHound AirHound is offline
 
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Quote:
Originally Posted by jcaplins View Post
Just out of curiosity... how would you describe the vibration?

is it just roughness only at a certain rpm?
Roughness gets more or less noticeable as you move away from that RPM?
Is it a beat freq type vibration (subtle thrumming, like a twin with props out of sync)?

note: I don't have a Rotax, nor can I say anything about your gearbox. I'm just curious.
Jeff, what are your thoughts about beat freq?
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  #27  
Old 03-03-2019, 08:14 AM
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DaleB DaleB is offline
 
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Quote:
Originally Posted by AirHound View Post
Jeff, what are your thoughts about beat freq?
You know, mine has not really a vibration so much as a rhythmic "thrumming" sound that I suspect is may be a beat between engine and gearbox. It will be interesting to see if it goes away after the gearbox service in a few hundred hours.
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  #28  
Old 03-03-2019, 08:49 AM
Azjulian Azjulian is offline
 
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Personally I find it easier and safer to follow the manufacturers recommendations. I use the oil they specify, I service at the frequency they specify, I set the engine in this case up in the detailed way described and I check the tolerances (gearbox lash etc) at the intervals they specify. Sure I could test different oil etc etc but I find that Rotax are very cautious and appear to do extensive testing and produce a quality product.

The reason I chose the RV12 as my first build was that I wanted something that was specified completely in this way, I wasn’t going to be dealing with random cooking issues because I needed to “innovate” to fix for my engine of choice etc.

Everyone gets into this for different reasons and you are always going to have a small percentage of people having issues even though they followed “the book” in the same way as I continue to do.

Before anyone questions the design of the engine or the lubricant choice or the fact it has a gearbox, consider that there are in excess of 50.0000 912 variants in production. Rotax as I said above is continually improving on that and issuing guidelines.

I’d just stress that being by “the book” is invaluable in fault resolution any issue that I have had I have resolved by doing a reset and setting up as specified by vans or Rotax.
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  #29  
Old 03-03-2019, 10:29 AM
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jcaplins jcaplins is offline
 
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Quote:
Originally Posted by AirHound View Post
Jeff, what are your thoughts about beat freq?
I do want to be clear, I don’t know anything about the Rotax or the Rotax gearbox. In my case the beat freq vibration (thrumming) was a collapsing timing chain tensioner(I have 2 chains and 2 tensioners). I believe others that had similar vibrations masked the problem by going with a different gearbox (or different engine) It only makes sense to me that to have a beat freq vibration you need at least 2 of something to be out of sync.
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