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Prop Problems ?

dinowal

Active Member
After a flight recent flight, I returned to join the circuit (pattern), and as I reduced power to 4000 - 4500 rpm I noticed some unusual & significant new vibration. All had seemed normal on cruise at 5200 -5500 ish rpm while the prop was pulling. On inspection of the prop I found a 5/8 by 5/8 inch flake of paint had departed from the back of the metal strip on the leading edge, approx. 4 inches from the tip. I realise that our props are hollow & made from Carbon Fibre & are very light, but I am surprised that such a small amount of missing weight could affect things so much. Has anyone had a similar situation,or have experience & knowledge to share? Thanks, DEAN
 
Your vibration may not be prop related. Small flake of paint missing, even at tip, shouldn't be noticeable. Sounds to me more like carburetor. Could be heavy float causing rich mixture and imbalance L-R. Floats work great until they don't...
 
I remember reading a thread last year pertaining to vibrations at reduced throttle settings. If memory serves me correctly, I believe the vibrations were determined to be coming from the gear reduction unit ... so keep that in mind as a possibility after the carbs are eliminated as a problem.
 
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Rotax has made some recommendations to decrease clutch and dog wear, as well as sometimes needing the belleville washers which provide tension, being replaced in the gearbox. at low hours

Rotax now recommends warm up at 2500 rpms, not 2000, to save wear and tear on the gearbox dogs. There have been failures at 300 to 400 hours on the dogs on RV-12 SLSA planes.

Speculation about brands of oil is another issue, as those that run only Mogas, no 100LL, have gone to a full synthetic Mobil 1 Full Synthetic 10w-40 T4 motorcycle oil with additives for shared gearbox/engine lubrication, to reduce gearbox wear. The jury is still out on that one, but if almost all you run is Mogas, it might be a better solution.


See Cammie Patch posts on this in a search. They have replaced dogs, and belleville washers to keep their school rental plane up and running on the line, until parts arrived. The Rotax gearbox parts are a pretty penny, best to address it shortly it that is the cause.
 
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Thanks to you 3 gentlemen for your replies.. I have contacted Sensenich & the more I read & think, my Prop is probably not the problem.- I can assure you that I will be seriously p'd off if it's sinking carby floats, - I am on my 4th set, with only 52 hours on the Hobbs meter. I will of course be even more unhappy if gearbox is involved, - my friend, (who services Rotax engines for a living) & I will continue to investigate & report back in a few weeks.. Cheers, DEAN
 
Just a thought and may not be helpful in this case, but is your engine in the serial number range that needs to have the crankshaft journal checked for cracking? If so might want to do the check since it is quick and easy. As I said, not sure the symptoms match the failure mode, but at least you could eliminate one more thing.
 
Aaah, yes it was!!

Yes, thanks Jim ! On restarting the engine with top cowl off, the RH carby was spewing fuel from the little overflow tube!. Guess what ? The floats were way overweight, & the ones in the LH carby were right at the upper limit. Now, I know you folks are really polite & you don't swear much, so I am sparing you a lot of what's in my head! However I will ask: what the bloody **** is going on? This is really unacceptable - this is my 4th set after 52 hours of flying , - I'm not sure if the problem lies with Mr. Bing, Mr Rotax, or even Mr Van's, but would love to hear from someone with a permanent fix. My dream of having an RV12 ready to fly anytime after a quick preflight seems to be disappearing..
The only good thing about this saga is that the Australian Rotax agent has paid for all these floats, -thanks to them, - hopefully they can give Bing/Rotax a poke? Cheers, (not happy), DEAN
 
One thing I always do at the same time as I change my engine oil is clean the pistons on the carbs and put a small amount of 3in1 oil on them. I do this because I have noticed that the pistons can start to feel rough in their bores possibly due to combustion / fuel etc deposits.

I find that doing this creates very smooth transitions in power.
 
Thanks

Thanks Azjulian, - yes, we found the piston in the RH carby was a little sticky & had some black deposits on & around it.. Possibly was another part of the problem, but floats were definitely overweight. Cheers, Dean
 
What it really was!

Finally I have sorted my vibration problems! To recap, I had a small ding from a stone on the metal leading edge of my propeller. It was an allowable sized dent according to Sensenich and caused no changes to performance of my aircraft for quite some time. But, as previously described I came back from a flight in very turbulent conditions to find lots of vibration in the airframe, and some paint split off the back of the metal leading edge. I repaired that area under advice from Sensenich, but the vibration never went away. After trying everything on the engine, a friend said some of the very light carbon fibre props can develop a tip flutter or flexing of the blade. I contemplated sending my blades back to Sensenich for refurbishment, but the cost including shipping was excessive, so I have fitted an Australian made BOLLY 3 blade prop, and Wow! no more vibration when running up the engine! I still have to organise the pitot tube through the spinner before a test flight, but at last I'm expecting a SMOOTH, trouble free experience... Hope this story may help others, Cheers, DEAN
 
Both of our 12's had a mid rpm 'grumble zone'.

We concluded it was the gearbox coming on and going off drive slightly so ignored it and tried to avoid those mid rpms.
 
A lot of 912’s have a mid-range RPM vibration that is inherent to the design. The most likely cause is carb imbalance. As discussed many times in this forum, the 912 is really two separate and distinct power sources that share a common crankshaft. We adjust synchronization at idle, and just off-idle, so engine is smooth at slow speeds. This is very necessary to prevent uneven power pulses seen by the gearbox. Uneven power pulses are very destructive for the gearing at low RPM. This is why Rotax recommends >2500 RPM warmup speed. Throttle plate position is probably not uniformly linear between the two carbs with springs pulling against cable. This shows up as disproportionate throttle positions in the mid-range. The carbs pretty much auto-synch at high RPM’s with throttle plates nearly wide open. It’s the mid-range that we can’t adjust or compensate for. All this can be seen by monitoring R-L EGT’s.

With all that said, we pretty much have to live with the mid-range problem. The best practice is to not operate continuously in the mid-range. Transition through the mid-range and only operate in mid-range if necessary, to extend glide, etc. Treat the mid-range as a harmonic imbalance that should be avoided.
 
Don’t know if I would call it mid-RPM rumble, but every time I pull it back to idle for landing the 912 seems to grumble at me. Used to alarm me a little, but it has never been a problem.
 
Yeah, something like that, no exact RPM range... Again, very important to synchronize at idle and just off-idle. The vacuum cross-over tube helps at low engine speeds by fooling the engine into believing there is a single manifold with both carbs acting as a single unit. As engine speed increases the small cross-over tube loses its effect and then the carbs act independently. Wide open, or nearly wide-open throttle will be in very close synch just by nature of high-volume air flow even if throttle plates are slightly different position. It’s the mid-range that shows asynchronization. This can be from a variety of causes – mismatched cable pull, wear on main jet and needle from vibration, wear on main needle circlip, etc., etc. Difficult to make two carbs behave alike through entire power range.

I saw in an earlier thread (sorry, I don’t know author) where a second set of springs were added to the carbs to make cable pull more uniform and repeatable. The author kept the new torsion springs on the throttle shafts but added back the old extension springs that work on the throttle arms. This affectively adds more spring pull to the cables and makes the throttle plates track position more accurately. I believe the author claimed improvement in mid-range vibration. I intend to try this and will report soonest…
 
Yeah, something like that, no exact RPM range... Again, very important to synchronize at idle and just off-idle. The vacuum cross-over tube helps at low engine speeds by fooling the engine into believing there is a single manifold with both carbs acting as a single unit. As engine speed increases the small cross-over tube loses its effect and then the carbs act independently. Wide open, or nearly wide-open throttle will be in very close synch just by nature of high-volume air flow even if throttle plates are slightly different position. It’s the mid-range that shows asynchronization. This can be from a variety of causes – mismatched cable pull, wear on main jet and needle from vibration, wear on main needle circlip, etc., etc. Difficult to make two carbs behave alike through entire power range.

I saw in an earlier thread (sorry, I don’t know author) where a second set of springs were added to the carbs to make cable pull more uniform and repeatable. The author kept the new torsion springs on the throttle shafts but added back the old extension springs that work on the throttle arms. This affectively adds more spring pull to the cables and makes the throttle plates track position more accurately. I believe the author claimed improvement in mid-range vibration. I intend to try this and will report soonest…
Seagull made that post. He was having problems balancing the carbs on his RV-12, one of the throttle cables was a bit sticky. https://vansairforce.com/community/showthread.php?t=187659
 
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