ArduCopter 3.21 on APM 2.6 Hexa: strange yaw "pumping" and u

Hi !

Thats a long title for my problem :slight_smile:

I have 2 Problems:

When i switch to Alt Hold in a relatively windstill environment the copter yaws CW or CCW and then corrects itself back. E.G. in 10 seconds it can do this 2 to 3 times. It also drifts a bit back/forth left/right but thats probably related and due to NOT beeing 100% windstill and the prop backwash i am nut sure that that is a problem…

The frame is very cheap yet very stiff as it is heavy built, its called “Argonaut” and its made of Carbon, see an image attached. There is no noticable arm twist.

Its a small & cheap Platform i wanted for testing the WiDV thingy (blue box, switched off right now, not related to the problem).(I have larger Hexas with 2.6 that fly just fine, did not want those for WiDV)

The frames COG is balanced okay, its using cheap 1806 motors with 5x3 Props.

ACC & Gyro & Compass seem fine to me, GPS is okay but for the tests i just used Stabilize & Alt Hold.

The APM is a bit forward from the COG (5cm), but pretty much in the middle in the other two axis. This did not pose a problem to me before even if the APM was off-COG worse but i better mention that :slight_smile: .

ESC are calibrated, Vibrations are okay to my taste.

If you look at the screenshot from MP log view you can see 2 strange things:

The 6 motor outputs are drastically different (something between 1400 <—> 1700 !

And also in the log: the desired yaw stay flat (as i did not yaw on the RC) but the “real” yaw drifts up and down in a smooth curve, exactly what i am experiencing.

I also included the full log, so you can see where i am flying :slight_smile:

For the life of me i cannot see how this comes.

As the motor outputs are so drastically different i did a primitive test of the motors.

I fixed the Hexa to a table and did motor test in APM.

(Motor #1 was front right and then the numbers went around the Hexa in CW motion, is that correct, i think so, but i better ask).

I measured RPM at 25% / 50% and 75%.

While they were NOT a perfect match i hope that the variations are in the grren zone and also my RPM-measurung tool is not too precise so allow ±100 RPM for it.

The results i got was:

Motor # (MP) RPM @ 25% RPM @ 50% RPM @ 75%
1 10230 14000 16440 CCW front right
2 10700 14760 16900 CW right
3 10200 14200 16600 CCW back right
4 10600 14600 16900 CW back left
5 10400 14300 16750 CCW left
6 10350 14400 16700 CW front left

Do you guys have any idea whats going on ?

Thanks a lot for helping me out…