Rv12iS gearbox chatter

I have about 390 hours on the engine now.. Within the past month, I've noticed a vibration in certain situations. Initially, I believed it was on unbalanced front tire which was rotating in the wind.. But after rebalancing it (only 1/4oz off), I am faced with a more expensive problem.

The vibration seems to happen when I throttle back, for instance, before I begin a descent. If I advance the throttle a bit again, the vibration seems to (immediately) go away. In stable air, at a fixed throttle setting, it does not seem to happen.

From what I've read, almost all gearboxes (especially with 100LL use) need servicing at the 600hr inspection. Ok, mine is a bit.. 30% earlier.

Assuming this is the actual problem, two questions:

1. Is the aircraft safe to operate? The vibrations aren't shaking the plane, but I can feel it in my feet. I don't want to continue operation if its going to cause damage to other parts . The last oil change 50 hours ago showed NO debris.

2. Is this a problem I want/can fix on my own? I don't mind spending some $ on specialized tools, especially if this will be happening again down the road. And if so, which gears/item #s will I want to buy before I start disassembling it (assuming I can still continue to fly it between now and the time I get parts).

Probably a good question for the Rotax Owner forum, if you’re a member. You could ask Lockwood, but they’ll likely suggest pulling the gearbox and sending it to them, but probably worth asking anyway.

Rob,

I assume your prop is balanced?

With 100LL.... perhaps check overload clutch per Rotax maintenance manual with fish-scale spring. Lead contamination is tough on the gearbox. Gears spin lead against walls of gear case and clutch plate friction surfaces get slippery.

I have early SN RV-12 and my 912ULS has >800TT operated on 93E10 with no gearbox problems. Some of the early gearboxes had improper heat-treat, but apparently I got a good one.

Also, I'm using Mobil1 Racing 4T Full Synthetic Motorcycle Oil 10W-40 exclusively....

Sounds like you use 100LL, isn’t the oil change interval more frequent than 50 hours? I thought it was like 25 with 100LL..

It used to be 25 hours but is now 50.

Henry

Gearbox

Rob, I would call Lockwood and talk to them. I had mine done at 600 hours even though there were some signs of wear we went ahead and replaced the needed parts.

Great to see your post Dave. I haven’t noted any issues but am just over 600 hours and plan to send my gearbox for service. I’ve been told by a Rotax mechanic that following the maintenance schedule on my 912 it will operate great throughout the TBO period.
I wondered how some of the higher time RV12’s followed suggested maintenance schedules.

Mine has 1280 hrs. Almost all on no lead auto gas. No gearbox issues. Mine is an ELSA, and I did not pull the gearbox for the 600 hour check. My reasoning was that this check primarily looks at the overload clutch breakaway torque on the dogs, and that is a prop strike protection. I go with plan A: no prop strikes. I do the breakaway torque check at annual that basically checks the spring pack force. It has always been in spec since first flight in 2012.

I always appreciate respectful comments that might challenge my position.

I have 1200 hours on mine also using all no lead gas. I did not do the 600 hour rebuild. I also check the specs at annual time Its always within the numbers in the manual.

Brad Stiefvater
Salem SD

I had my gearbox rebuilt at 1020 hrs. The engine has used E10 auto gas for the majority of its life, maybe only 50 gallons total of 100LL. I do not let it run below 2000 RPM. There was no indicators to overhaul the gearbox, I just wanted to see what wear was happening and get ahead of it if something was wearing. Wayne Flemmington at F70 did the work and explained the process as I helped. The biggest concern is if the spline starts to wear you need to fix the cause or it can get expensive to replace. My dogs had normal wear, nothing of concern. Since we were already inside the Bellville washers were replaced and the plastic shims. There is a step collar that was also replaced, this is an updated part with slots for better oiling of the spline.

My gearbox ran fine and could have gone a lot longer before rebuilding. The updated step collar is good to have but the original must have been doing OK.

The OP concern about a rattle when pulling off the power is a clue the splines are wearing due to the Bellville washers loosing pressure probably caused by the plastic shim washers deteriorating.

RFSchaller said:

I do the breakaway torque check at annual that basically checks the spring pack force. It has always been in spec since first flight in 2012.

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I should probably add that check to my annual (which I'm doing right now). I can report that there is still virtually no debris in the filter or on the magnets. The only abnormal thing is that I found a few red cloth fibers in the filter which is due to regularly wiping the dipstick with a red shop rag. I don't think that is enough to cause any real problems (but I will stop using shop rags!)

I also learned, just as a FYI for everyone, that gearbox S/N 74000+ has an additional oil spray nozzle. The factory/distributor upgraded my gearbox before shipping (Jan 14, 2019).. So most, if not all -12iS's should have this mod already in place.

Good info, Rob. Thanks.

When I was at the ROTAX course the instructor recommended running the engine at 2500 RPM and then shutting off the A and B ignitions in sequence to stop the engine. He claimed it resulted in less vibration and stress to the gearbox during shutdown. I’ve always followed that advice even though it conflicts with the ROTAX Operator Manual.

All rolling elements – gears, ball bearings, etc., can absorb shock and abuse much better when at speed (dynamic Vs. static). This is why car wheel bearings can withstand pot holes…. The Rotax gearbox has a hunting gear ratio for the purpose of spread wear evenly. All that said…. best practice is to keep ground ops at a reasonably high RPM. I start the engine and warm up at 2600 RPM. Cold engine runs much smoother at this speed and prevents gearbox chatter because higher prop loading sets gear lash only in one direction. Chatter is a huge no-no for gears, splines, drive cogs, and overload clutch...

I changed my prop to an E-props and it runs much smoother with no vibrations and a higher cruise speed.
It only weighs 2 kg (4.5 lb) all inclusive and low weight should be better for the gearbox.

https://aircraft.e-props.fr/product_info.php?products_id=2452

It is my aircraft in the picture on the e-props site.

Rotax Shutdown

RFSchaller said:

Good info, Rob. Thanks.

When I was at the ROTAX course the instructor recommended running the engine at 2500 RPM and then shutting off the A and B ignitions in sequence to stop the engine. He claimed it resulted in less vibration and stress to the gearbox during shutdown. I’ve always followed that advice even though it conflicts with the ROTAX Operator Manual.

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I read with interest your 2500 rpm shutdown procedure. I always cringe when the second ignition goes to off with the low rpm shutdown. The result ranges from benign to a simulated prop strike.
I used the 2500 rpm shutdown the last two flights and will never go back to minimum rpm shutdown. It seems the additional inertia allows the engine to come to a stop without the bone jarring Kerr-Whump. Thanks for sharing.

Guys, the current revision of the POH says to reduce power to 2000 rpm before shutting down. It’s the process I follow and it seems to work just fine. Earlier versions said to reduce to idle for shutting down, but it’s now 2000 rpm.

Looks like the info on 2500 RPM I got in 2012 was good. Too bad it took the thunder gods so long to figure it out.

Well, to follow up and close out this thread.. I sent (actually, the paint shop I'm using in Tennessee sent) out my gearbox to Lockwood for servicing.

Lockwood's phone support was excellent (9-10a; 2-3p) and they indicated my symptoms were very likely worn dogs. They received, repaired, and sent it back out a day after they got their hands on it. After inspection, they confirmed that it was worn dogs, but couldn't say why it had happened at only 380hrs (they had copies of my engine logbook, and didnt see anything alarming as far as maintenance or 50hr oil changes [100LL]). The problem would just get worse over time, but at no time was it in danger of failing (no power/emerg landing). Additionally, my gearbox had the optional oiler nozzle in it which was supposed to help prevent/delay this problem.

The only thing I could possibly do a better job at is to plan descents farther out, chop-n-drop less, and land with power more often.

As far as removal of the gearbox, Lockwood indicated that careful tapping the back of the prop flange with a soft mallet usually does the trick..but just be sure it doesn't get cocked while coming off, as there are dowl pins which will cause it to bind up. No absolute need for the special Rotax slide hammer for gearbox removal.

Lastly, warranty expired at 200 Hrs, but said they may cover some portion of the parts (not labor) under their goodwill program since this is problem not normally seen until much later in life.. but its at their discretion.. fingers crossed.

The costs were roughly $350 labor, $2100 in parts -- the bulk of which is a new gearset (886-548) at $1300 and dog hub (958-929) at $370. They also did the 600Hr inspection, since it was close enough to 600.

Thanks

Thanks for the info Rob.

Jcurry said:

Thanks for the info Rob.

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Ditto. I do a lot of pattern work, so I expect that I will have gearbox issues before long, so I appreciate knowing what to look for. Thx.

rcarsey said:

[100LL]).
The only thing I could possibly do a better job at is to plan descents farther out, chop-n-drop less, and land with power more often.

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Two questions.....

• Where did you get the above advice from?
  
• Did magnetic plug in gearbox have shavings?

I wonder why no answer to my two questions in above post...

"The only thing I could possibly do a better job at is to plan descents farther out, chop-n-drop less, and land with power more often"

Piper J3 said:

Two questions.....

• Where did you get the above advice from?
  
• Did magnetic plug in gearbox have shavings?

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This was my advice to myself.. since I can't think of anything else I could have been doing to upset the engine.. Based on POH (below). As far as the magnetic plug goes.. I didn't find anything abnormal this time. I'll add a photos to this post later today when I get home... but I'll describe it as just a black fine powder (but oily), on the edge of the magnet.. size or height of which wasn't enough to be measurable. However, the oil change prior to the start of any problems, I found 1 chip that was 3mmx1mm. - like a metal flake. That got my attention, but only 1 flake from an unknown source.. not much I can do at that point with that evidence.

RV12iS POH 4.13 - Descent & Approach said:

If possible, avoid windmilling the engine with the
propeller by reducing airspeed or increasing power.
When planning a descent from cruise altitude to the
airport traffic pattern, use time to destination to calculate
a realistic and comfortable rate (500 ft/minute).

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My guess is that there was nothing wrong with your gearbox components. Ramps on overload clutch drive look fully hardened and surface finish looks good – just beat to death. I would imagine a lot of ferrous (iron) particles spread around inside the engine and finally caught by oil filter or stagnated in oil tank. Surprising the magnetic plug didn’t do better job…

I have described a few times in this forum how the 912/914 series of engines are really two separate powerplants (L-R) that share a single crankshaft and how that presents special operating concerns as compared to Lycoming or Continental aircraft engines. I’ll try to convey some highlights….

Lycoming and Cont. aircraft engines have a single carburetor that feeds both sides of engine uniformly. So, either everything is Jake or it isn’t.

The 912/914 have two carburetors that need to make similar power otherwise engine is out of balance. Both sides of the engine (L-R) need to be similar in power output especially at lower RPM’s. Gears and overload clutch ramps can’t tolerate chatter and uneven power pulses at slower speeds. This will cause premature wear and ultimate component destruction. Solution is to synch carbs correctly for just-off-idle response and then always operate engine at above idle speeds. This will prolong gearbox life immensely. The gearbox has a hunting ratio to spread gear wear evenly, so most of the work is already done for us.

Cold engine start-up usually runs a bit rough. It is best to start the engine and have it run smoothly right out of the box. I start my engine and immediately advance to about 2800 RPM for warm-up. Engine runs smoothly at this speed and gearbox is very happy. Clutch ramps and gear teeth are loaded all in one direction with no chatter. Backlash is all taken out because prop is more heavily loaded at the higher engine speed.

Now to talk about reducing engine power in-flight…. When engine power is pulled back to idle, the prop is back-driven as it produces drag and all the backlash in the gearbox (gears and clutch ramps) are driven to the opposite extreme. The key here is all the backlash is removed and all components are running smooth with no chatter what-so-ever. There is no concern about reducing power to idle, while in flight.

As a secondary benefit… the 912/914 have water-cooled cylinder heads which allow reducing full-power to idle without fear of shock-cooling. Nice….

This is how I fly my airplane – reduce power to idle on downwind leg abeam runway threshold, drop both notches of flap, trim hands-off for 60 KIAS, and only add moderate power if needed to control glideslope. Airplane is in full-stabilized approach before turning base leg…

I’m approaching 850TT with a very early SN RV-12 and have no gearbox problems. My engine does not have add’l oil spray in the gearbox. A lot can gleaned about gearbox / engine cylinder health when hand-burping the engine to check oil level. When individual cylinders go over TDC as you turn the prop, the clutch ramps display the amount of free-play or backlash. Keep a good mental picture as your engine accrues time and you can use this to signal potential problems…

Sounds like?

This is my first Rotax. When it is warm it runs perfectly smooth near idle. When starting cold, a whole different story. The engine runs a little rough until it warms up and it does sound different. Like an engine does when its cold until it warms up. I'm always wondering if I'm hearing a little chatter while its warming up or if I'm hearing a cold engine. I'm not sure. Can someone describe the sound? Is it a "clacky" sound, tinny metallic sound, high speed ticking like a lifter?
Thanks

I'm very interested in learning more about what wore out your gearbox as early as it did. Maybe we'll never know, but it caused me to research the gearbox and educate myself more than I knew before.

I understand the 912iS engine has 0 degrees of "slop" between dogs whereas the ULS has 30 degrees. Hopefully I got that right. Does anyone know why Rotax made this change between ULS and iS? I understand this to mean that iS drivers can't measure the health of the springs by performing the so-called "friction test," where the torque required to move the prop through this slop with the crank locked is measured. This is basically a proxy for the axial force remaining in the springs. Learning that info kind of made me think why would you want to have ANY slop between the dogs in the first place?

Does the design of the dogs serve as a torsional damper by riding up and down the angled surface of the face of the dog ever so slightly, back and forth with the individual cylinder pulses, to "absorb" the torsional vibration in axial force via the springs?

Just trying to think through how to minimize wear on the dogs by how you treat the engine.

DanFrazer said:

Just trying to think through how to minimize wear on the dogs by how you treat the engine.

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Re-read post #25 above and adhere to good advice....

I did read your post before replying and kept thinking your discussion about carb syncing isn’t relevant for this situation since I think Rob has the iS, right? Do you also get uneven L vs R cylinder power pulses with the fuel injected version?

With regard to pattern throttle management, are you thinking your strategy is the reason you don’t have the gearbox problems Rob did? Or is that you idle yours high and he didn’t? All the things you said might be true and good things to do but I’m not finding how it explains what occurred here.

Found the photos...

The first 4 photos were at the last oil change/annual.. after I started to have problems. The amount/type of debris in these photos has been pretty typical of every oil change.

I've been using the Challenger PMA'd oil filter for ease of inspecting the filter..
which includes its own magnet. 4th photo is what the magnet looks like after its cleaned.. as reference since few use this filter.

The last two are of the only debris I've ever found.. which was the oil change BEFORE problems started. (not shown: the thickness of that flake was 0.003")

Prior comments:

I usually idle at 2200-2300 rpm (limit 2500) until I'm at 120C on the oil.. taxi is "whatever is needed", 2000 minimum..but typically I'm idling anywhere from 2300-2700.. just depends on how much I want to wear the brakes out that day. On descents, it looks like I'm at < 2000rpm.. down to 1450 at times for about 45 seconds.. from about 67kt IAS until I'm solidly on the ground and braking.. under control.. and then I'll put it up to 2000 rpm. Unfortunately, my "early" 912iS only has the factory-adjustable idle screw which is set to 1450. Newer 912iS's have an adjustable set-screw..

rcarsey said:

The last two are of the only debris I've ever found.. which was the oil change BEFORE problems started. (not shown: the thickness of that flake was 0.003")

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If nothing found on magnet plug, or oil filter, you might want to disassemble oil tank and check for metal debris. I think you said you run 100LL, and at 400 hours, might be good to clean lead sludge in bottom of tank.

DanFrazer said:

I did read your post before replying and kept thinking your discussion about carb syncing isn’t relevant for this situation since I think Rob has the iS, right? Do you also get uneven L vs R cylinder power pulses with the fuel injected version?

With regard to pattern throttle management, are you thinking your strategy is the reason you don’t have the gearbox problems Rob did? Or is that you idle yours high and he didn’t? All the things you said might be true and good things to do but I’m not finding how it explains what occurred here.

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ULS or iS... Gearbox longevity is tied to ability to withstand power pulses and this is best accomplished by keeping idle speed in higher range.

I wouldn’t get concerned over that amount on the magnetic plug for the first 100 hours. After that I would wonder what’s causing it. That’s based on what I saw with mine which now has 1200 hours.