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Vibrations at higher RPMs


neoplx

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I am quite new CTSW owner (Rotax 912 UL). A week ago I noticed vibrations during engine run-up at higher RPM. First I thought it may be because that day the wind was blowing from the back during the run-up. When I turned and tried higher RPM again, it seemed much less noticable but still a bit noticeable. I though it may be normal to hear vibrations when wind is blowing from the back so I just tried to avoid that for the next time.

 

Next day when I was taxiing and wind was from the back during taxi it was again noticeable a bit (less then on run-up though). What made me start worrying is that it vibrates also when flying straight and setting RPM > ~4600. The feel/sound and it's frequency is almost like a percussion drill and intensity varies - 2 seconds almost not hearable then 2 seconds hearable. On straight flight with 4300 RPM it can't be heard, I think it also can't be heard during take off.

 

What could cause it? I guess it is not normal... Thanks for help!

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These vibrations can be caused by a couple of things. At that RPM it can be carb sync is out. Then it could be one or more prop blades not the same pitch as the others. You may need to do a dynamic prop balance. Your gearbox may need to be re-shimmed.

 

These are the most common causes I would do a carb sync and check the prop blades first.

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If all of the above fails, check the bolts securing the engine to the engine mount.  They are supposed to be either safety-wired or torqued every annual.  Mine were not checked over the first two years I owned my airplane and two were loose and one backed out completely.

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If the balance tube came off the engine would probably shut down. I have never heard of this happening. What I believe he means about the tube coming off is the carb  vent tube or in the CTSW the tube can also come too far out the side of the airbox and cause the same roughness.

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. . . "What I believe he means about the tube coming off is the carb vent tube or in the CTSW the tube can also come too far out the side of the airbox and cause the same roughness.

 

Yeah Roger, that's the one.  My airplane is an '06 CTSW.

I think it became disconnected somewhere during my cross country ferry from San Jose to Tampa.

I did not notice any extreme symptoms, but discovered it on a subsequent pre-flight inspection, after I finally got the airplane home in Tampa.

After conferring with Jeremy, I just reconnected it and all has been well since then.  It has not become disconnected since.

 

Thanks for the prompt answer. . . . . :)

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I have the opposite issue.  My engine is very smooth, but gets a vibration at *low* rpm.  When I pull back to idle in the pattern I can notice a little bit of shake.  Nothing alarming, and it smooths out if I just add a tiny bit of throttle (like 100-200rpm, just enough to get it out of idle).  This is somewhere in the 2000-2200rpm range.  I only notice this in the air, not when idling on the ground or taxiing. 

 

Any ideas on that?

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Well, I don't know, but my RPM at idle on approach is a couple of hundred higher than in idle sitting stationary or taxiing.

 

Yes, that is normal because of airflow through the propeller at (even slow) flying speed has less resistance than still air on the ground.  My idle is usually 1650-1800rpm on the ground, and 2000-2200rpm in the air.

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I have the opposite issue.  My engine is very smooth, but gets a vibration at *low* rpm.  When I pull back to idle in the pattern I can notice a little bit of shake.  Nothing alarming, and it smooths out if I just add a tiny bit of throttle (like 100-200rpm, just enough to get it out of idle).  This is somewhere in the 2000-2200rpm range.  I only notice this in the air, not when idling on the ground or taxiing. 

 

Any ideas on that?

 

Andy,

Same here.

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Each engine setup including the prop has its own slightly different personality.

Sync the carbs at idle and around 3500 rpm. Check the prop blades to make sure they are all the same pitch, check the prop tracking and lastly if these fail get a dynamic prop balance.

 

All this said each engine I have seen has a unique vibration at different and reduced power setting in flight.

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I have had vibrations in all the modes mentioned here, including the mode which started this thread which was a vibration during run nup when I experienced a quartering tailwind. I originally checked and corrected uneven pitch on my prop blades and rebalanced my carbs. This helped vibrations during low speed flying but didn't help the run up vibration. All engine mount bolts on the engine and firewall were checked and found to be correctly torqued. Prop track was checked and found to be to spec. I then had the prop dynamically balanced. This improved my cruise vibrations but didn't help my ramp run up vibration. I then checked the "torque to turn" of the prop. This value can be determined by locking the crankshaft and then applying a fish scale at a known distance out from the center of the prop. The prop is turned approximately 30 degrees and the lb./ft. result is recorded. I found this to be on the absolute low side of the spec. Discussion with Roger Lee and others resulted in the conclusion that my gearbox needed to be inspected and probably re-shimmed. The box was inspected and re-shimmed and reinstalled. This reduced my vibration during run up to a level that I considered to be highly acceptable and compared to other CT's which my friends own. It also improved vibrations in other modes of operation, including steady cruise.

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