Tail Fin Vibration

Hello,
I am running a Logo 550SE (50 flights) flying sport aerobatic with 1750rpm on the head.
I have an intermittent but persistent vibration on the vertical tail fin 3 or 4 fins in amplitude
After a 7 minute flight the Vbar log shows only 3 or 4 raised vibration warnings.
The Vibration analyser (No main blades) shows the head, tail, and motor RPMs all below 40.
I have rebuilt the tail but things are no better.
I would be grateful for any suggestions of where to start looking next.
Thanks in advance, John

Hi John,

I guess you do the vibration analysis without blades? Then, the blades probably cause vibration or oscillation, which shows on the part farthest away and long enough to oscillate, which is the tail fin.

Usually the tail fin shows tail rotor vibration, so I would weigh the tail blades (and if by try and error, put some scotch tape to one, in the cog, see if it gets better or worse). Could also be the tail shaft running slightly untrue, which does not show enough without blades, but with (more weight, pull), it does.

I'm not sure what to make from

Quote:

johnpssa
vibration on the vertical tail fin 3 or 4 fins in amplitude

though, maybe a brief video would help understanding better?

Best,

Eddi

Born to fly ...
forced to work.

Hi Eddi, thanks for the response.
For the Vibration analysis tail blades are fitted, I only remove the main blades and disconnected the tail from the servo so it can centre.

Last Saturday I replaced the shaft, hub and flange bearings, refitted the grips but there was no improvement.

As I said it might be OK for a few outings and then it returns.

The aluminium tail clamp in my last post was purchased as part of my investigations

John

Hi John,

OK, maybe you can get somebody film the situation, upload the vid somewhere (youtube, private), and let me see?

I guess something is resonating, but it's of course difficult to pinpoint it, from a distance.

Kind regards

Eddi

Born to fly ...
forced to work.

Hi Eddi, thanks again.

It does seem like a resonance issue, from what you have said it is most likely to coming from the tail are there any other areas worth looking at?

The Logo flies beautifully, there are no signs of any issues apart from the fin.

I will see what I can do regarding the vid although it will not be until the weekend.

Regards
John

Hi John,

depends on the kind of 'blur' you see on the tail fin. The main rotor usually only creates a wobble, skids and chassis (and everything) shaking.
The motor and gears running at high speed imo would create a fine 'blur', with not much of an amplitude, but high frequency.
And the tail and parts running at their speed a 'blur' with a slightly higher amplitude.
The tail boom supports would probably also resonate ...

Kind regards

Eddi

Born to fly ...
forced to work.

Hi Eddi

Thanks for the information, given all my efforts on the tail, with little or no change, I was just wondered if I was looking in the wrong place.

Amplitude at the bottom of the fin is 3 or 4 fins in width although it is a blur I do not think it is very high frequency, you can almost see the 3 fins. Only other thing is that it seems to start a few seconds after the helicopter lifts off.

Regards,
John

Hi John,

please also check the belt, tension and that it's twisted only 90°, and check, if you run the mechanics by hand, if you can feel the belt 'stepping' into the pulleys, or if it runs smoothly.

Kind regards

Eddi

Born to fly ...
forced to work.

Hi Eddi, latset update.

There has always been a 0.5mm radial movement in the tail blade grips, so I thought I would check to see if I had got something wrong. I did not find any issues, no missing bits and all correctly fitted, I had some spare blade grips so I gave them a try and the radial movement was much reduced, do not know why as I used the same hardware that I had removed.

No sign of the tail fin vibrations during today’s flights, Not sure if changes made any difference or if it is just one of the non vibrating days.

Frustrating! As/when it happens again I will update the post.

All appears smooth turned by hand, could you clarify the statement “belt stepping into the pulleys”

Kind Regards
John

Hi John,

usually, under tension, there is no more axial or radial play in the blade grips, everything settles firmly on the outer ring of the thrust bearing, held by the washer and screw. But maybe, for whatever reason, there was too much residual play/movement.

The belt's teeth, like gears, usually mesh perfectly into their counterparts, the teeth of the pulleys. Of course the tight diameter adds some friction, but that's (usually) it.

If the belt was not perfect, or if it has been overstressed at some point, it might not mesh perfectly any more, so the teeth might kind of click or step into the pulley's teeth, not just run smoothly around the pulleys.

Fingers crossed that you sorted it out

Best,

Eddi

Born to fly ...
forced to work.



Edited 1 time(s). Last edit at 10/22/2018 08:51AM by Eddi E. aus G..

Hi all.
Just saw this post, and i want to upload videos with my problem of tail fin vibrations.
All parts in tail are brand new(exept tail fin and some small parts) and everything is doublechecked and adjusted,balanced but...it was a huge vibrations on certain headspeed and i was trying to chase them almost 2 days.
Finally found that tail fin causes resonance vibrations.
On second video you can see my custom tail fin from 2 cf tubes and old broken tail fin and there are almost no vibrations at all.
I'm assuming,the form,thikness and material(fiberglass) of original tail fin needs to be slight modified.
I haven't tried the new design tail fins(they are carbon fiber) but i think they will work better due to cf does not resonance that much.
Any comments on that?

PS
I know that those rpm's are kinda too much for logo 690 but i run 600mm blades sometimes and raise my HS to 2200 for hard 3d.
Besides it will resonate anyways depending on length of tail boom,weight of tail blades etc...

https://www.youtube.com/watch?v=GVTwIVZjiaU
https://www.youtube.com/watch?v=bvZQklGqJBs
My regards.
George



Edited 3 time(s). Last edit at 11/29/2018 03:48AM by racer-g.

Hi George,

well, something must cause the tail fin to resonate. Certainly, different materials will react at different rpm.

I would be curious what the actual vibration analysis shows, the small gauge on the start screen is more a GOM, but if you'd click the button with the green alps right above it, you'd see the real magic.

And then, I'd start with the tail shaft only, no tail rotor assembly, no blades, run the model through it's rpm, check the display, feel the shaft if it runs absolutely true or if you can feel anything already.

Then, tail assembly but no tail blades, check the difference on the display, and check/feel the tail boom and fin.

Last, the blades, because they will amplify everything which is not running true by a large scale.

Of course you can use a different fin, but this just remedies the symptoms, not the cause.

Kind regards

Eddi

Born to fly ...
forced to work.