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Rotax 912 ULS Engine Vibration Solved

romaja

Well Known Member
I built a Rans S-20 Raven in 2016 and installed a new 912 ULS engine. 302 hours on the Hobbs. Early this year I started experiencing an engine vibration at cruise power settings. Very Frustrating. Troubleshooting went as follows:

1. Tested with new carbs
2. Tested with new ignition boxes
3. New Prop spinner
4. Dynamically balanced prop.
5..New spark plugs, several times
6. Balanced carbs, several times
7. Repitched prop before dynamic balance
8. Compression check, compression leakdown check
9. New floats

Apparently there have been problems with some gearboxes. I read a post on the yahoo S7 forum that sounded hauntingly familiar. This person even replaced pistons and cylinders troubleshooting this vibration at a considerable expense. His problem went away after he replaced his gearbox. I pulled my gearbox and sent it away to have new gears pressed in. The vibration is now gone! After considerable expense and a great deal of trouble shooting, the problem has been solved. I wanted to share my story in hopes that it might help someone else with similar symptoms.

Best,
Jim
 
I thought this was on gearboxes on the 912 iS models, circa 2014 or 2015 builds? I've seen mentions made of keeping the engine speed up to 2500 rpm to keep wear to a minimum on the dogs inside that gear box. What rpm's do you normally run and warm up at?
 
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I thought this was on gearboxes on the 912 iS models, circa 2014 or 2015 builds? I've seen mentions made of keeping the engine speed up to 2500 rpm to keep wear to a minimum on the dogs inside that gear box. What rpm's do you normally run and warm up at?


When starting cold, I idle at 2000 rpm for 2 minutes, then to 2500 until warm. Yes, they had problems with the 912is but no mention of the 912ULS. No unusual wear on the dogs upon inspection.
 
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When starting cold, I idle at 2000 rpm for 2 minutes, then to 2500 until warm. Yes, they had problems with the 912is but no mention of the 912ULS. No unusual wear on the dogs upon inspection.

Fuel you run, and brand of motor oil that you run? Thinking out loud, if the motor oil also needs some gearbox lubricant supplement, much like what Mobil 1 does for motorcycle gearboxes, or with their racing motor oils. See Rotax list of approved motor oils, with certain fuels.
 
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Fuel you run, and brand of motor oil that you run? Thinking out loud, if the motor oil also needs some gearbox lubricant supplement, much like what Mobil 1 does for motorcycle gearboxes, or with their racing motor oils. See Rotax list of approved motor oils, with certain fuels.

I use Aeroshell Oil Sport Plus 4. Also, Rotax oil filters.
 
I run Aeroshell Oil Sport Plus 4 and use Rotax oil filters because . . .
that?s what Rotax recommends, regardless of the fuel used.
 
My second engine had significant vibration issues especially compared to the first one. I chased down all the stuff mentioned in the post but also would add:

Loosen the exhaust and re-center it. Make sure it's not breaking the springs.
Go through the engine mount bolts (all of them, not just at the engine).
Make sure the bowman cables have nice clean runs especially at the carb.
Try Vans new carb springs.

Honestly, I think the new carb springs did the trick. It's a much more conventional design that doesn't rely on hooks. My exhaust has run through quite a few springs but none since repositioning it. It's a known problem.

For me, it was easy to use my ADAHARS to troubleshoot the problem. At 5200 RPM the engine would set up a resonance that would send the ADHARS offline. When the autopilot was engaged the plane would bank and dive.

Rotax SL912-010 speaks to the known vibration issues on the 912.

You should also be measuring blade track and testing the friction torque for the gearbox. Don't let the gearbox run at minimum torque.
 
Honestly, I think the new carb springs did the trick. It's a much more conventional design that doesn't rely on hooks.

This sounds like a more plausible explanation for a mid-range vibration issue. The 912 is really two separate engine powerplants that share a common crankshaft and prop reduction gearbox. Each half of the engine is independent of the other and controlled separately by its own carburetor. Carbs can be synchronized easily at idle especially with the crossover vacuum tube. Likewise, full throttle is well synched with each throttle plate nearly wide open and main fuel jet likewise nearly wide open.

Mid-range is where asynchronization is most likely to occur. Synchronization in mid-range is largely dependent on how well the throttle plate follows the displacement of the throttle cable via the return spring. Cable stiffness, bend radius, weak return springs, missing return spring ? all have effect on mid-range smoothness, or lack thereof?
 
Bingo! Thank you Driftdown!
Page 4 and Page 5.

They recommend full synthetic motor oil, if running Mogas or unleaded. You might also take notice that if your oil temperature is high, and you run unleaded gas/ Mogas, they recommend a FULL synthetic motor oil, made by Mobil Oil.

Personally, I stopped running Shell synthetic motor oils since 2002, they do poorly on NOACK tests, have high volatility, even in their rated "synthetic" motor oils, lose volume, and thicken in viscosity. Heat burns off volatiles in Shell motor oi, and you have to add some make up oil. This is quite common in diesel motors, which run at much lower temperatures, even turbocharged, than a Rotax does.

BTW, I am not trying to make this an "oil war". What I am suggesting is that rather than follow sheep blindly, do your own research on oil results. I've had numerous diesel vehicles that consumed Shell Rotella T6 motor oil in 5w-40, their "full synthetic", 3/4 to a full qt every 10,000 miles between oil changes, and as soon as I switched to Mobil Delvac-1, my oil consumption stopped, completely, for the next 170,000 miles. This happened on 4 different VW TDI's that I owned. YMMV, but I stopped running Shell products, AND sold all my stock in Shell / Royal Dutch, as a result.

Do your own research, a Rotax motor is 7 to 8x as expensive to R&R, my observations are that there are a lot better lubricant products out there than what Shell Oil offers. If you have to add "make up" oil in a Rotax, with Shell motor oils, I'd suggest you find something to replace it, the next oil change, that doesn't require make up oil. This isn't a Lycoming or Conti motor. The tolerances are very, very tight, such that the motor shouldn't consume any motor oil.


https://www.daec.de/uploads/media/ROTAX_SI-912-016.pdf
 
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Page 4 and Page 5.

They recommend full synthetic motor oil, if running Mogas or unleaded. You might also take notice that if your oil temperature is high, and you run unleaded gas/ Mogas, they recommend a FULL synthetic motor oil, made by Mobil Oil.

Personally, I stopped running Shell synthetic motor oils since 2002, they do poorly on NOACK tests, have high volatility, even in their rated "synthetic" motor oils, lose volume, and thicken in viscosity. Heat burns off volatiles in Shell motor oi, and you have to add some make up oil. This is quite common in diesel motors, which run at much lower temperatures, even turbocharged, than a Rotax does.

BTW, I am not trying to make this an "oil war". What I am suggesting is that rather than follow sheep blindly, do your own research on oil results. I've had numerous diesel vehicles that consumed Shell Rotella T6 motor oil in 5w-40, their "full synthetic", 3/4 to a full qt every 10,000 miles between oil changes, and as soon as I switched to Mobil Delvac-1, my oil consumption stopped, completely, for the next 170,000 miles. This happened on 4 different VW TDI's that I owned. YMMV, but I stopped running Shell products, AND sold all my stock in Shell / Royal Dutch, as a result.

Do your own research, a Rotax motor is 7 to 8x as expensive to R&R, my observations are that there are a lot better lubricant products out there than what Shell Oil offers. If you have to add "make up" oil in a Rotax, with Shell motor oils, I'd suggest you find something to replace it, the next oil change, that doesn't require make up oil. This isn't a Lycoming or Conti motor. The tolerances are very, very tight, such that the motor shouldn't consume any motor oil.


https://www.daec.de/uploads/media/ROTAX_SI-912-016.pdf


Your writings "smack of condescension". First off to suggest that I and others on the forum should " rather than follow sheep blindly, do your own research on oil results" is nonsensical.

So, if we use the oil suggested in a Rotax Service Bulletin in a link that you provided above, then we are "blindly following sheep" ?? Huh? please explain

You contend that we should be running full synthetic (#1below) if running unleaded or Mogas. Using the service bulletin you have provided. Here is an excerpt of your link:

1 Full-synthetic motorcycle oil with gear additives. Highly recommended for occasionally high oil temperature
operation (higher than 120 ?C / 250 ?F) using only unleaded fuels.
2 Semi-synthetic aviation oils with gear additives. Highly recommended for normal (lower than 120 ?C /
250 ?F) and occasionally high oil temperature (higher than 120 ?C / 248 ?F) operation using leaded or unleaded
fuels.


I choose #2. I do not run Full synthetic oil as don't run my engine over 120C/250F ever! Not even close. In addition those temps are running very close to max oil temperature for the Rotax 912 ULS. I never see those temps or even close. If you are running oil temps in excess of 120F/250F, this deserves it's very own thread.

Your suggestion to do your own research is also a bit over the top. Plenty of research has been done and the verdict is Aeroshell Oil Sport Plus 4 as suggested by the Rotax bulletin you have provided. You also contend that you do not want to start an "oil war". You seem to have a disdain for Shell products and have an axe to grind.

You mention that you have had 4 VW TDI's consume shell oil until you switched brands and the consumption stopped after switching to Mobil for the next 170000 miles. In addition, you sold all of your stock in Shell/Royal Dutch. Not sure of the relevancy of this info you have provided?.

Maybe you are a troll that hates shell products.
I suggest you stop condemning people for using recommended oil.

PS. Any other forum members out there that "blindly follow sheep" as suggested by "Ninerbikes" use Aeroshell Oil Sport Plus 4 in their 912ULS's? Please let me know or chime in.

Best,
Jim
 
Your writings "smack of condescension". First off to suggest that I and others on the forum should " rather than follow sheep blindly, do your own research on oil results" is nonsensical.

So, if we use the oil suggested in a Rotax Service Bulletin in a link that you provided above, then we are "blindly following sheep" ?? Huh? please explain

You contend that we should be running full synthetic (#1below) if running unleaded or Mogas. Using the service bulletin you have provided. Here is an excerpt of your link:

1 Full-synthetic motorcycle oil with gear additives. Highly recommended for occasionally high oil temperature
operation (higher than 120 °C / 250 °F) using only unleaded fuels.
2 Semi-synthetic aviation oils with gear additives. Highly recommended for normal (lower than 120 °C /
250 °F) and occasionally high oil temperature (higher than 120 °C / 248 °F) operation using leaded or unleaded
fuels.


I choose #2. I do not run Full synthetic oil as don't run my engine over 120C/250F ever! Not even close. In addition those temps are running very close to max oil temperature for the Rotax 912 ULS. I never see those temps or even close. If you are running oil temps in excess of 120F/250F, this deserves it's very own thread.

Your suggestion to do your own research is also a bit over the top. Plenty of research has been done and the verdict is Aeroshell Oil Sport Plus 4 as suggested by the Rotax bulletin you have provided. You also contend that you do not want to start an "oil war". You seem to have a disdain for Shell products and have an axe to grind.

You mention that you have had 4 VW TDI's consume shell oil until you switched brands and the consumption stopped after switching to Mobil for the next 170000 miles. In addition, you sold all of your stock in Shell/Royal Dutch. Not sure of the relevancy of this info you have provided?.

Maybe you are a troll that hates shell products.
I suggest you stop condemning people for using recommended oil.

PS. Any other forum members out there that "blindly follow sheep" as suggested by "Ninerbikes" use Aeroshell Oil Sport Plus 4 in their 912ULS's? Please let me know or chime in.

Best,
Jim

Would you debate that a full synthetic motor oil provides the same or better wear results in an internal combustion engine than a semi synthetic motor oil?

Does full synthetic motor oil flow better or worse than semi synthetic motor oil when cold, at start up?

Is there anything that a Full synthetic motor oil does worse than a semi synthetic motor oil?

I look at aircraft engines with pistons having a gearbox in them running at 5200 to 5800 rpm, at full throttle for 5 minutes as having extreme demands being placed upon the motor oil, as the motor oil in an air cooled engine acts both as lubricant and as a coolant, carrying heat away from motor parts. I look at a gearbox up front having straight cut teeth in it as having the ability to shear motor oil and break it down quite quickly, which is something full synthetic motor oils do not suffer from unlike motor oils with petroleum based stock in them.

Then let's throw in that the average flight around the patch is 20 to 30 minutes of run time on the motor. How many full throttle applications are you doing on a 50 hour semi synthetic oil change, versus using the plane at average 1 to 2 to 3 hour flights?

I'd be really interested in finding out what motor oil brand the people with gearbox failures are running, to see if there is a correlation, between semi synthetic motor oils and full synthetic motor oils. Also their oil service intervals.

There is no doubt that fuel injected engines with precise air fuel mixtures is one of the leading advances to extended oil changes in modern combustion engine motors, with lower pollution values, both to the air, and contaminants that end up trapped in the motor oil, leading to extended intervals between oil changes.

Rotax is plenty happy to sell you new carb floats and new dogs for your gearbox. They'll take your money, gladly.

Until Rotax 912 users start using something besides Shell Aero 4, which your gearbox failed on, and, since so many of you run Shell Aero 4 in your 912's, we'll never know if the failures are lack of lubrication induced, based on semi synthetic oil usage, as once the oil film barrier is lost, the galling starts with metal to metal rubbing and grinding, or is it metallurgy failure of the design, lack of hardness of the metal parts, or the higher compression of the 100 hp ULS motor over the 80 hp UL motor that's causing it? That Rotax suggests idling closer to 2500 rpms on warm up suggests what as being the problem for the 912 ULS or 912 IS? Something is breaking the lubrication barrier down in that gearbox, such that galling is occurring, there is no buffer or cushion of lubricant between the metal parts.
 
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NinerBikes -

82 posts VAF

Mtn Biking, Fly fishing, Ham Radio

Wants to get a sport pilot's license

No direct Rotax experience, yet resident expert

Hmmmm?
 
Dude... no. You're linking to an OLD version of a Rotax SI that has been superseded several times. The most recent one can ALWAYS be found on the Rotax web site. The latest is from 2017 and specifies, as have the last few revisions, precisely ONE oil for the Rotax 912. That oil is AeroShell Sport Plus 4, which was developed specifically for the Rotax 912 (or so I have been told, but I'm just some guy on the internet). You can't trust some random document that Google finds to be correct. Go to flyrotax.com and get the straight story. I won't even post a link to the most recent here, because I don't want someone finding this thread five years from now and getting a way out of date version and thinking it's current.
 
NinerBikes -

82 posts VAF

Mtn Biking, Fly fishing, Ham Radio

Wants to get a sport pilot's license

No direct Rotax experience, yet resident expert

Hmmmm?

All owner/operators have heard, and I dare say subscribe to, ?oil is cheap?. Suggest, if financially feasible, he gain experience with oil change intervals based on personal usage variables for his class research project.
 
Page 4 and Page 5.

They recommend full synthetic motor oil, if running Mogas or unleaded.

Do your own research, a Rotax motor is 7 to 8x as expensive to R&R, my observations are that there are a lot better lubricant products out there than what Shell Oil offers. If you have to add "make up" oil in a Rotax, with Shell motor oils, I'd suggest you find something to replace it, the next oil change, that doesn't require make up oil. This isn't a Lycoming or Conti motor. The tolerances are very, very tight, such that the motor shouldn't consume any motor oil.


https://www.daec.de/uploads/media/ROTAX_SI-912-016.pdf

Dude... no. You're linking to an OLD version of a Rotax SI that has been superseded several times.

As Dale pointed out, your are basing you position off of a document that was superseded 2+ years ago. The only oil that Rotax now officially recommends is Aeroshell Sport Plus 4 (still on page 4 and 5 of the document).
The most current Rev document Rev. 10 in this case) can always be found on the Rotax Owner .com web site. This is the only official source for factory documentation. The current Rev level of SI-912-016 can be accessed HERE
From my understanding, the reason for the change was that Rotax had no control over the formulation of any of the oils that were previously approved. More than once, a manufacturer made a formulation change to an oil that had been the same for many years and it resulted in different performance results in a geared Rotax engine. So Rotax collaborated with Shell to revise their Rotax specific oil to meet Rotax engineerings desires, I imagine with the agreement that it will not be changed with further consultation. It is a semi synthetic oil that full-fills all of the previous requirements depending on what fuel you were predominately using. It does not require adding any additional modifiers. It meets all of Rotax's requirements as is.
 
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As Dale pointed out, your are basing you position off of a document that was superseded 2+ years ago. The only oil that Rotax now officially recommends is Aeroshell Sport Plus 4 (still on page 4 and 5 of the document).
The most current Rev document Rev. 10 in this case) can always be found on the Rotax Owner .com web site. This is the only official source for factory documentation. The current Rev level of SI-912-016 can be accessed HERE
From my understanding, the reason for the change was that Rotax had no control over the formulation of any of the oils that were previously approved. More than once, a manufacturer made a formulation change to an oil that had been the same for many years and it resulted in different performance results in a geared Rotax engine. So Rotax collaborated with Shell to revise their Rotax specific oil to meet Rotax engineerings desires, I imagine with the agreement that it will not be changed with further consultation. It is a semi synthetic oil that full-fills all of the previous requirements depending on what fuel you were predominately using. It does not require adding any additional modifiers. It meets all of Rotax's requirements as is.

And yet galling still occurs in the gearbox, with metal on metal contact, with use of the specified oil, well before 2000 hours, in plenty of observed cases.

Should we point the finger at operator error?

Design failure?

Vendor failure for materials used and specified on the part?

Or oil failure? Rotax spec'd the oil... and gearboxes are failing, early.

Rotax has Qualitiy Control over all specifications except the end user, and operator error. Just like their Carb floats. Which they say is fine with Mogas up to E10, and they still keep failing. They've got a new part number out for the carb floats. Makes me wonder if it will ever happen for motor oil, too.

I'd love to see a failure analysis report as to the cause, because once that oil boundary layer is gone between the gearbox parts, the damage starts getting done. And there are fundamental differences in additive packages for motor oils, and in gear oils.

As to my Qualifications, let's just say I worked for the manufacturer of a bottle brush hone company that puts the final sealing finish on every single kind of sealing bore that you can imagine, both in the new and remanufactured engine industry. I've seen plenty of failures of surfaces that need lubrication, the metal finish our tools provided needs to work in conjunction with the tribologists concoctions. Failure isn't an option on hydraulic seals in bores when your John Deere is broken down and it's harvest time for the crops. We also worked closely with, back then, Parker Hannifin, with sealing surfaces compatible with their O ring products. So, I am aware of what causes seizures, and rebuilding and remanufacturing the surfaces, instead of just throwing all new parts at the failure. I also worked at a Fortune 50 aerospace company that made gyro and ring laser gyro guidance systems for the F-14, E2C and AWACS, before GPS, back when Fuel Injected motors were first coming into vogue for pollution control standards. Back then, for me, it was a profession, not a hobby.

So I apologize if I come across as condescending, this sounds like a project that could use some failure analysis as to the root cause. RV-12's are great planes, it's a shame they can get marred by Rotax part failures, long before 2000 hours on the power plant and drive train for the prop. Or it sounds like a great opportunity for an aftermarket parts manufacturer to offer replacement parts that don't fail so soon. Or an oil that works in it's intended application.

Because, something is under engineered, that the failures keep occurring. It could be something as simple as oil jets need to direct oil directly at the parts in question that keep failing, both for lubrication and boundary layer protection.

The good part is there are so many RV-12's already built and flying, with Rotax 912 engines in them, so there's a very large sample of Rotax engines to gather data from in one location.

Don't get mad, just because I think outside of the box. My lack of pilot experience has nothing to do with my experience in engines, and gearboxes, and lubrication issues. It's about how to prevent the failures... unless you enjoy more pulling gearboxes and sending them off to get them fixed and getting handed a bill, than you do flying your RV-12.
 
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I was going to sit on the sidelines, but here goes…

I don’t think any fingers need pointing. Most 912 operators (me included) are enjoying very good service life with very little maintenance. I have 400TT on my 12 with no gearbox problems. Very little metal fillings on the gearbox magnetic plug. If any metal fillings, I would attribute to the surface finish from the hobbing process when gears are manufactured. Virtually no oil consumption in 75-hour oil change. In my opinion the 912 is a perfect match for this airframe.

How many Lycoming’s make it to TBO? A quick look at sales ads for used aircraft show an awful lot of MOH before 1000TT. Cam/lifter spalling among other known problems.
 
Vladimir has almost 5000 hours on his o320 still runs great. Truth of the matter is Lycoming 4 and 6 cylinder engines very routinely exceed tbo.
 
And yet galling still occurs in the gearbox, with metal on metal contact, with use of the specified oil, well before 2000 hours, in plenty of observed cases.

Should we point the finger at operator error?

Design failure?

Vendor failure for materials used and specified on the part?

Or oil failure? Rotax spec'd the oil... and gearboxes are failing, early.

Rotax has Qualitiy Control over all specifications except the end user, and operator error. Just like their Carb floats. Which they say is fine with Mogas up to E10, and they still keep failing. They've got a new part number out for the carb floats. Makes me wonder if it will ever happen for motor oil, too.

I'd love to see a failure analysis report as to the cause, because once that oil boundary layer is gone between the gearbox parts, the damage starts getting done. And there are fundamental differences in additive packages for motor oils, and in gear oils.

As to my Qualifications, let's just say I worked for the manufacturer of a bottle brush hone company that puts the final sealing finish on every single kind of sealing bore that you can imagine, both in the new and remanufactured engine industry. I've seen plenty of failures of surfaces that need lubrication, the metal finish our tools provided needs to work in conjunction with the tribologists concoctions. Failure isn't an option on hydraulic seals in bores when your John Deere is broken down and it's harvest time for the crops. We also worked closely with, back then, Parker Hannifin, with sealing surfaces compatible with their O ring products. So, I am aware of what causes seizures, and rebuilding and remanufacturing the surfaces, instead of just throwing all new parts at the failure. I also worked at a Fortune 50 aerospace company that made gyro and ring laser gyro guidance systems for the F-14, E2C and AWACS, before GPS, back when Fuel Injected motors were first coming into vogue for pollution control standards. Back then, for me, it was a profession, not a hobby.

So I apologize if I come across as condescending, this sounds like a project that could use some failure analysis as to the root cause. RV-12's are great planes, it's a shame they can get marred by Rotax part failures, long before 2000 hours on the power plant and drive train for the prop. Or it sounds like a great opportunity for an aftermarket parts manufacturer to offer replacement parts that don't fail so soon. Or an oil that works in it's intended application.

Because, something is under engineered, that the failures keep occurring. It could be something as simple as oil jets need to direct oil directly at the parts in question that keep failing, both for lubrication and boundary layer protection.

The good part is there are so many RV-12's already built and flying, with Rotax 912 engines in them, so there's a very large sample of Rotax engines to gather data from in one location.

Don't get mad, just because I think outside of the box. My lack of pilot experience has nothing to do with my experience in engines, and gearboxes, and lubrication issues. It's about how to prevent the failures... unless you enjoy more pulling gearboxes and sending them off to get them fixed and getting handed a bill, than you do flying your RV-12.

No, no owner/operator would be mad, more likely we are confused by your slightly one off dissertations. Have you considered conferring with the manufacturer?
 
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