Vibrations at higher RPMs

[neoplx]

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!

[Bobby CAU]

hello there,

for me is quite normal that cases.

 

what is your WOT during take off?.

you feel the vibrate as well?

[Jim Meade]

What prop do you have?

[neoplx]

There is Woodcomp 3-blade in-flight adjustable propeller installed.

Ok, thanks for help. I will fly it to the authorised centre then - better to be safe. 

I will let you know what it was if they fix it.

[Andy]

I've had vibration above 4000RPM when the little tube between carbs popped off one of 'em (which, I believe, will give you the same symptoms as carbs out of synch.) 

[WmInce]

I've had vibration above 4000RPM when the little tube between carbs popped off one of 'em (which, I believe, will give you the same symptoms as carbs out of synch.) 

Roger,

Can you chime in on this?  What is that tube for?

[Doug G.]

It is a balance tube, and you are correct it can make the carbs go you of synch if it is detached. Not easy for this to happen in a CT, it is a pipe with short rubber tubes at the ends.

[WmInce]

It is a balance tube, and you are correct it can make the carbs go you of synch if it is detached. Not easy for this to happen in a CT, it is a pipe with short rubber tubes at the ends.

Thanks Doug.

[Jim]

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.

[WmInce]

. . . "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. . . . .

[Doug G.]

Just when you think you know what you are talking about...

[Tom Baker]

Just when you think you know what you are talking about...

Everyone seems to think in terms of their aircraft. They don't consider that someone might be flying something else, and things might be different than what they are used to.

[Jim Meade]

Everyone seems to think in terms of their aircraft. They don't consider that someone might be flying something else, and things might be different than what they are used to.

Like the old Indian said, "what you see depends on where you stand".

[FlyingMonkey]

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?

[Jim Meade]

Backlash in the gearbox? The prop is pushing the engine more than the engine is pushing the prop.

[FlyingMonkey]

Backlash in the gearbox? The prop is pushing the engine more than the engine is pushing the prop.

 

Hmm, that's an interesting idea, but wouldn't that cause an RPM drop?  If this *is* the case, the next question would then be:  "is it harmful"?

[Jim Meade]

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.

[FlyingMonkey]

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.

[WmInce]

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.

[FlyingMonkey]

Andy,

Same here.

 

Maybe it's just normal then when the idle is set low enough.

[Ian]

Andy,

Same here.

 Me too, but balancing the carbs sorted it out as Roger suggested.

[Runtoeat]

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.

[Anticept]

Also, quartering tailwind causes funny effects on props, and many planes vibrate when it gets like that.

[IrishAl]

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.

Dick,

How many hours were on your engine when you reshimmed the box?

[FlyingMonkey]

 Me too, but balancing the carbs sorted it out as Roger suggested.

 

My carbs have been recently balanced, and I had the prop dynamically balanced (to within 0.54) at last annual in May.  Before the prop balanced I had noticeable vibration at all RPM, enough that you could see the mushroom shaking a little.  As I said, everything is buttery smooth now except at that tiny RPM range, and only in flight.  I'm just going to chalk it up to my plane's personality I think.