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Prop/Engine Vibrations

pilotyoung

Well Known Member
I purchased my RV-13 last February with 338 hours on it. It now has 432 hours on it. This summer I started noticing that when I reduce power from cruise, around 4400 down to about 3800 I get a vibration. I think it started after I complied with the Service Bulletin on the cracks in the front of the propeller hub. I hear the vibration more that I feel it. Two other pilots who have ridden with me also think it is from the propeller or gear box. The engine runs smoothly at cruise and at idle.

About a month ago I rechecked the front hub for cracks, and found none, and reset the blade angles at 71.4 and 71.3. The vibrations continued. This last week I replaced the soft engine isolators with the new ones from Van's and replaced all the springs on the muffler.

When I flew the aircraft, it still has the vibrations. Today I rechecked the blade angles and they are still 71.4 and 71.3. I checked the torque on the bolts and they are fine. I looked all over the engine for anything loose that could be vibrating and didn't find anything.

The propeller was dynamically balanced at 105 hours.

So I am asking for ideas, suggestions on what might be causing these vibrations. Has anyone else experienced vibrations like this?

Thanks.
 
Dynamic prop balance? Also, I had a similar problem and I found one of my throttle cables was sticking slightly in mid-throttle range. I rerouted it slightly near the carb and secured it better so it wouldn't move. That worked for me.
 
Hi John --

Which prop hub front did you install - D1 or F ? I understand that you can install the blades 'in' a little from where they should be with the 'F' hub. I think it has been discussed that after the new hub, assuming the blades were installed correctly, there still can be a balance difference. Be sure the blades are in there most outward position, and rebalance it. I did mine, and it helps a bunch.
 
Vibrations

I just reinstalled the original hub after inspecting it. I am planning on ordering the new style hub and installing it.
 
John,
Did you really set the blades at different angles (71.4 and 71.3), or is that a typo in your post?
Alex
 
About a month ago I rechecked the front hub for cracks, and found none, and reset the blade angles at 71.4 and 71.3.
Why tie difference? Was that just the closest you were able to get them to being the same?

Ha... Alex was faster on the keyboard than I was, I see.
 
Vibrations

I set them both to 71.4 and after I got all the bolts finally torqued one was at 71.3 and one at 71.4. Since the tolerance is .2 I left them alone.
 
Seeing you’ve already balanced the engine, and assuming you haven’t changed the spinner or prop pitch, it could be things like a mis-firing spark plug(s) or carburetor balance.

If you’ve taken the spinner off to inspect - be sure you put it back on in the same orientation. It’s only gram weights that make a difference and the spinner is a very common cause. I’d also look for things like missing washers.

Lastly, do another engine balance and look for 2nd or 3rd level harmonics. It could be something like the alternator - but the most likely is something more innocuous like the spinner or a washer on a spinner screw.
 
The carbs on the ULS tend to run the engine a bit on the rich side in the mid throttle range. If you have a float that is just beginning to get a bit heavy and floating a bit lower, it could be causing that carb to be even richer but possibly only show up at mid range throttle settings.
 
I purchased my RV-13 last February with 338 hours on it. It now has 432 hours on it. This summer I started noticing that when I reduce power from cruise, around 4400 down to about 3800 I get a vibration. I think it started after I complied with the Service Bulletin on the cracks in the front of the propeller hub. I hear the vibration more that I feel it. Two other pilots who have ridden with me also think it is from the propeller or gear box. The engine runs smoothly at cruise and at idle.

About a month ago I rechecked the front hub for cracks, and found none, and reset the blade angles at 71.4 and 71.3. The vibrations continued. This last week I replaced the soft engine isolators with the new ones from Van's and replaced all the springs on the muffler.

When I flew the aircraft, it still has the vibrations. Today I rechecked the blade angles and they are still 71.4 and 71.3. I checked the torque on the bolts and they are fine. I looked all over the engine for anything loose that could be vibrating and didn't find anything.

The propeller was dynamically balanced at 105 hours.

So I am asking for ideas, suggestions on what might be causing these vibrations. Has anyone else experienced vibrations like this?

Thanks.

I second what Scott has suggested; also, carb sync...it is amazing the difference it makes when done right in overall smoothness of engine performance. Took me numerous attempts before getting the hang of it....and not wanting to start a war on the forum (I disagree with Van’s sync instructions) PM me for phone#; and i’ll share a slick method that has worked on my -12 and many other 912 Rotax equipped a/c. The typical comment i’ve recieved after tuning their engines...i’ve never seen it run so smooth...
 
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Vibration

Has anyone with an RV-12 had to have the gear box repaired? I have about 450 hours now and after doing extensive trouble shooting the vibration still exists. A couple of people have said the problem is in the bear box. So I am wondering if anyone else has had gear box problems on the original RV-12 with about 450 hours on it.

Thanks.
 
The gearbox, it's shims, Bellvue washers and the dogs can and do wear on the Rotax 912. Rotax has recommended higher speeds of 2500 rpm for warmup procedure, to lessen the chance of the prop slapping the dogs back and forth when there is near zero thrust on the prop. ie, you should be taxiing at that speed also, and be on the brakes to check your taxi speed. Brake pads are cheap.

I believe the guy that teaches with Sling 2's in the Carson City, NV area has had the same problem with his Rotax gearboxes, his name is Paul... Maybe check on the Rotax forum for this problem?
 
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The gearbox, it's shims, and the dogs can and do wear on the Rotax 912. Rotax has recommended higher speeds of 2500 rpm for warmup procedure, to lessen the chance of the prop slapping the dogs back and forth when there is near zero thrust on the prop?

Along with NinetBikes comments....you bought your a/c with 300+ hours. If run @ lower idle speeds for ??? Times....this could account for wear.
 
Engine Vibration

I have run the idle above 2,000 towards the top of the green arch on the tach when the engine is cold every since I bought it. The prior owner or his mechanic told me about the slow idle problem so I have avoided it.
 
I have run the idle above 2,000 towards the top of the green arch on the tach when the engine is cold every since I bought it. The prior owner or his mechanic told me about the slow idle problem so I have avoided it.

Have it pulled and inspected, before you rule it out. Learn how to check end play and side to side play on it. There are some type of thrust washers on it that it's common for them to go out in 100 to 600 or 800 hrs and the lash values get too large..
 
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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.
 
vibration

When I am at cruise speed, 5300-5400 and I reduce power to slow down, about 4600 it starts vibrating and I hear it and it continues to about 3000 rpm.

Both hear and feel it.
 
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.
 
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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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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.
 
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?
 
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.
 
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.
 
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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