Excuse me if this was answered already, but I can't find. The "search" removes term "tail" from my queries saying it is to common.

I mounted the normal Spirit (red) on Blade Fusion 270 instead of original AR636A. The setup is BNF Fusion with cyclic servos changed to KST, FBL to Spirit and receiver to FrSky. Rudder servo is original Spektrum H3060.

It flies, but the tail is wobbling left and right, it's around 5-8 cm amplitude at the end of it, and it's like 2 wobbles / sec, so something relatively slow. This happens regardless of rudder gain setting. It is mostly visible in hover, but also happens in flight - just less visible.

I bound rudder delay and pirouette consistency parameters to radio channels and tried adjusting in flight, but this doesn't help either. I tried rudder delay from 0-12 and pirouette consistency 130-~200.

I think everything should be OK from mechanical part, because I tried this with AR636A and it didn't have this problem.

Where should I look for the problem?

Unfortunately in the most cases (99,9%) this is caused because tail mechanics is very tight. Tighter the mechanics, the slower is the tail oscillation and in bigger amplitude.
To solve the problem, you can try to to set Rudder Delay to maybe 5. And Pirouette Consistency to 130. Then you can play with Gyro Gain.
These settings should help the most but is not correct solution. Smaller Gyro Gain + Revomix at value 4-5 can be even better.

Stock gyro could be not too precise so it is not sensing the movement and thus not making any correction which then look like everything is OK.

Correct solution is to make the mechanics as smooth as possible in all ball joints. There should be zero friction.

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Spirit System developer

Ok, that's something to check.

Do you think there's any chance of reproducing the problem indoor? I tried spooling up the heli on the floor, it becomes then very very light and moves around the floor little bit, and it is free to rotate. But tail oscillation does not happen. Would mounting the heli to some rotary plate and giving it a little but of throttle help?

The thing is it will take ages to track the problem at current weather..

Unfortunately no,
it is nearly impossible to reproduce same condition without flying in the air.

You can perform some tests indoor in a hall.

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Spirit System developer

I checked the mechanics. It wasn't tight, but had some slight friction. Reverting ball links on the tail helped, and some silicon oil helped more - it is smooth now.

However the problem is still there. I tested different combinations of gain, rudder delay and pirouette consistency. What else can be checked?

Maybe the problem is in the way how the unit is mounted.
It is important that it is holding really firmly with no room for moving.

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Spirit System developer

I had a permanent tail wobbel on my Blade 300 cfx only with testing cheap servos.
Could the servo be defect?

Once I had a to wide slot (+0,3 mm) in an aluminum tail slider (maufacturing fault).
This play lead to a short wobbel at the end of turns.

Unfortunately this is true. There are quite a lot models where pilot have to perform such small tweaks. It will reduce servo load, enlarge lifetime and improve performance very noticeably.
Usage of any oils or lubricants is not recommended. It is better there is very slight play than when it is tight in any sense.
Sadly when pilot don't know what is tight and what is smooth it is hard to distinguish.

Very likely the stock controller is built to work with this but physics can't be bypassed.

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Spirit System developer

Just to let you know, the problem solved itself over time. Not user why, maybe it needed some time... to be honest I didn't fly this heli much. But tried it recently and it now looks fine. There is slight tail wag but very small, same as it had on AR636 I think. Either it burned in or Spirit learned over time

Thank you for the great message!

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Spirit System developer