Where to look for cause of excessive tail vibration?

[PaoloFCantoni]

I have an MSH Prôtos 700 Leggero/Evolzione Franken heli. It's been flying quite well, but since the new year, I've found that the tail "drifts" during and after some manoeuvres. I've noticed that there is a large tail vibration and mistakenly assumed that that was affecting the Brain2 FBL. Rik has since "put me straight" and I now understand what his admonition about clearing up vibrations BEFORE looking at the FBL is about.

So that leaves me with a problem. Below are 3 screen snips from the vibrations log file of the Brain2. The first shows the heli on the bench with NO blades attached (neither mains nor tails). As you can see, all seems fine.
The second shows the heli on the bench with NO mains, but tails attached. A bit worse but still quite reasonable.
The last shows the heli in flight (obviously with both mains and tails). As you can see the tail vibration (and the overall Global vibration) level has "gone through the roof". I've checked everything I know might contribute to tail vibration and all seems fine, but my knowledge is now exhausted.

Any ideas on what might be causing this issue in flight? It seems to be related to the forces at play in flight, but where?

NO blades:


Tails ONLY:


In Flight:


TIA,
Paolo

[jrman83]

Have you tried changing to a different blade or checking the balance of the blades? Is this vibration level always present or is this just the peak? Does your tail rod have a slight bend in it or is it straight? Have you pulled the grips off and checked bearings? Tail drift sounds like a tuning issue.

[blkdout]

Maybe check the I.D. of your thrust bearing races. I have read getting those mixed up can cause some nasty vibes.

[JWeber]

Tail blades is most likely. Also, how tight do you run them? Too loose blades can cause vibes in air.

I agree with what was said before
Check that the tail blades weight the same and are balanced.
Make sure that your thrust bearings are in the correct direction per the manual.
Make sure all bearings in the tail are smooth
Make sure all tail mechanics are smooth

[PaoloFCantoni]

Originally Posted by jrman83

Have you tried changing to a different blade or checking the balance of the blades? Is this vibration level always present or is this just the peak? Does your tail rod have a slight bend in it or is it straight? Have you pulled the grips off and checked bearings? Tail drift sounds like a tuning issue.

Done all that, Ben, thanks.

[PaoloFCantoni]

Originally Posted by blkdout

Maybe check the I.D. of your thrust bearing races. I have read getting those mixed up can cause some nasty vibes.

I haven't actually cheek them since I installed them, but the vibes weren't there earlier and the tail assembly hasn't changed.

Paolo

[PaoloFCantoni]

Originally Posted by JWeber

Tail blades are most likely. Also, how tight do you run them? Too loose blades can cause vibes in the air.

I agree with what was said before
Check that the tail blades weight the same and are balanced.
Make sure that your thrust bearings are in the correct direction per the manual.
Make sure all bearings in the tail are smooth
Make sure all tail mechanics are smooth

As mentioned, done all that! Loose blades were my first thought!

Interestingly, after Rik put me right about the FBL, I checked my XLP 700 and it has almost as much of a tail vibe (50-60%) and it DOESN'T have the drift! Weird!

Paolo

[EDIT: Also, I forgot to mention that if I hover the heli and go "hands off the sticks" there is NO drift. It only occurs during some manoeuvres (not even very smack ones- I'm a "big air" flyer). Some new mains have arrived today so I'll try switching mains to see if that affects things when I get home.]

[OnTheSnap]

The battery trays generally allow the packs to "jiggle" which causes funny things to happen. I always flew with extra straps between the chassis and packs to mitigate this.

[PaoloFCantoni]

Originally Posted by OnTheSnap

The battery trays generally allow the packs to "jiggle" which causes funny things to happen. I always flew with extra straps between the chassis and packs to mitigate this.

Mine are both strapped in and 3M Dual Locked in! "Dey aynt movin'", but thanks for the thoughts.

I'm home now and waiting for the packs to charge before I go fly and see if the new mains help.

[Edit: New mains didn't help. If anything, it seems slightly worse. I can visibly see the tail fin vibrating

The large amount of "grass" seems to suggest a more general malaise.]

[PaoloFCantoni]

Random thought...

Could tail belt tension be an issue?

Paolo

[flint723]

No I don't believe belt tension would cause such a severe vib.

After everything you checked there isn't a whole lot to do now but throw parts at it. Starting with a different tail hub (or hub/grip assembly), then tail shaft if hub doesn't help. If you don't want to open new parts, you can swap the hub/grip assembly from your other Max, assuming it's known good.

I did have a new hub that had vibs from day one so it is possible to have a bad part.

[jrman83]

I would definitely try a new tailshaft. Never been crashed?

[JWeber]

bent tail hub or tail shaft might do it.

I learned a trick to see if you have a bent tail hub. Remove the hub from the tail shaft. Put some pressure on the tail blades back and forth (not rotating them) and see if the tail hub rotates on it's own. If it's bent the pressure through the grips will make it move on it's own. If it's not bent then it will not.

Also, do you fly with anyone else who can double check everything with you? I've missed obvious things before that a second set of eyes caught.

[PaoloFCantoni]

Hi All,

Life intervened, but last week I managed to make some changes. I changed the mains and tail blades. Having observed that there was quite a bit of "grass", I decided to tighten everything up. I also observed that the skids weren't as rigid as they perhaps could be and I secured them to the lower frame more rigidly.

Anyway, the attached flight log shows that the vibration at the tail blades has reduced to manageable levels and the "grass" has reduced. However, the Global vibration level is still too high. The tail drift is STILL there.

I've attached the flight log and the current Brain2 Configuration.

I'm stumped as to where to go next. I currently plan to reset the FBL to the factory settings and reconfigure it manually from scratch.

Any other ideas? Haven't had a chance to try Jim's test yet, but my suspicion is that it won't show up anything (given the changes) in vibrations so far.

TIA,
Paolo

[Andy from Sandy]

On the bench with the control rod disconnected can you move the tail blades and see them self centre?

If you are absolutely sure the grips are not binding in any way put a small piece of electrical tape on one tail blade and see what it does for the vibration.

Play with the tape from blade to blade moving it along the blade to see the effect on the vibration level.

[jrman83]

I can't see a drift from these vibrations. Maybe your servo has slipped its original position or your rod is too long/short.

When you remove the tail grips how do the vibes look? How much up/down play is in your main shaft? FWIW, my vibes are high-ish in the tail area also. When you remove the grips it goes nearly flat. Not too worried about it right now.

[PilotPin]

Try locking the inner race to the tail shaft. I had bad vibrations on a tail once. There was a little slop between the inner race and tail shaft.

[ss1-2013]

Please check the head dampers, main shaft, main gear, spindle screws, tail hub & tail shaft.
Make sure all screws are nice & snug.

Any recent crashes?


Robin.

[blkdout]

Are your storage conditions room climate? Like not a garage or barn or shed, etc.? I'm wondering if those huge main gears could be warped perhaps?

Sounds like you've checked a bunch so just trying to think elsewhere.

[PaoloFCantoni]

Thanks, guys, I'll try some of the suggestions over the next week or so.