try this thing
Disconnect tail servo linkage and move with your fingers.
The slider must move without impediments.
After (remove rotor blades and tail blades) and turn on the helicopter, while the tail rotor is turning the slider must always move without impediments.
I have had this problem in the past.
When helicopter was stopped the slider is perfect and when I turned on the engine the slider it got hard.
And in flight I had a swing nervous and fast similar to yours

Thanks I will test this. What did you do to make it smooth while running?

Every helicopter is different.
you try if the slider it gets hard or no, when it starts to turn helicopter and let me know the results.

Hello Chris,
if you see that there is a problem the only way to go to disassemlbe the stert rotor to pull out sliding sleeve - Now you can polish either the tail rotor spindle, what is a bit simpler to do or you can polish the sleeve inside with wet sandpaper - if you want to polish the tail rotor spindle you can run the motor in a low rpm and then sand the shaft carefully with a little bit of oil maybe until the stiffness has disappeared -
So I have done that - at least you have to clean carefully the shaft that there is no more dust on it - then assemble it back together and test it - that has to be done maybe several times that it works without sticking -

Best regards

I fix the helicopters of my friends.
In my friend's helicopter they had assembled thrust bearings incorrectly and in the black levers instead of mounting the brass bushings someone entered the spacers which are used in the servo.
If the slider it gets hard when the helicopter turn it means that the problem is mechanical.
If it's always the same the problem is found somewhere else.
He must take the test and must let us know the result

Okay I tried the tip franz88 gave me. But there was no harshness when rotating. Only the self centering due to the centrifugal force.

It means it's ok.
from the first posts I read that in spirit you did everything right so let's think about something else...
but you How do you set the tail?
with the servo at 90 degrees you have the degrees of tail to hold the nose helicopter still ?
In Hovering in normal mode (no AVCS) the nose of the helicopter It must not rotate clockwise or counterclockwise.
I think you have put servo 90 degrees and slider centered and it's not good.

Hi, when I deactivate the heading hold by reducing the gyro gain the heli starts rotating around the z axis, not only drifting but quite fast.

When my servo arm is at 90° to the pushrod (pic2), the tail rotor has a bit pitch (picture 1). I thought that would be right because you need some compensation for the main blade momentum.
Am I wrong there?

the radio values they are relative
in diagnostic (rectangle red image) use a switch position to have +50% head lock
and one position switch to have +50% normal mode
you push on with helicopter in avcs mode and after switch in normal mode
during hovering in normal mode without pressing the stick the nose the helicopter must remain still (move linkage tail if it doesn't stay still).

But first of all
you write model servo tail
and what bec do you use? (model name)
and you can attach the spirit configuration file?

I have to check some things
you remember the tail of the spirit is the best tail that I have ever tried.
The tail is it's really perfect with spirit, you have a problem but we need to understand which one.
If you have rpm logs it would be a good thing understand if you have constant rpm otherwise the tail it will never be okay, only if rpm is ok the tail he behaves well

Hello Franz,

the ESC with BEC is a OmPhobby 85A.
All other information you need is written in my first post.

When you say „move linkage tail“ do you mean change the servo midpoint electrically or di you mean I should change the mechanical length of the pushrod?

I ask because while I have a spare pushrod I have no option to calibrate the correct lenght. I can only shorten it and hope it’s okay. The mechanics of the OMP M4max has no mechanical option to change the linkage or the midpoint of the tail blades. I only can adjust the servo midpoint.

Attached you find the rpm log.

Thanks,
Chris