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Lost position when apply rudder

Hi everyone,
when apply rudder at LOITER mode, my helicopter my does not rotate on the axis.
Attached video and logs.

Thanks in advance.

Video:
https://drive.google.com/file/d/1CdqeCWjBLKtjx5JJ0u-FoVtwvCkMCeqH/view?usp=sharing

Log:
https://drive.google.com/file/d/1Qn2M8w8CEcgDHxt8I3nxLEAgTlOLxmV_/view?usp=sharing

@Loren_DS I had looked at your data from the previous post regarding yaw oscillation but ran out of time to post. One thing I noticed was that you were holding a lot of pedal in a hover. I think it was like 50% right pedal to hover. I haven’t looked at the data from this flight but I am wondering if you lost tail rotor effectiveness. I only say that because of the high pedal for a hover and the fact that it spun to the left which is what would happen if you lost the tail rotor. Just my initial thoughts.

Sorry @bnsgeyer but what is PEDAL?

Sorry. I come from manned aviation. It is the same as rudder.

Yes i input full rudder with the stick y normally any helicopter rotates centered on the axis of the main rotor, but in this case it is not. Why?

@Loren_DS First, the left yawing motion was due to you applying full left rudder?

In Loiter, it is a position controller. So I could imagine that if you applied full left rudder, then the aircraft could have been drifting because of the tail rotor thrust. The aircraft sensed it was getting away from the hover position and was applying elevator and aileron to move the aircraft back into posiiton. Depending on how well your position controller is tuned, I could see that the aircraft could respond like it did. That is all my opinion based on what you’ve told me.

I have one other effect in mind, that might have caused your problem. It seems your GPS is installed on the tail of the helicopter, correct? I did that too and then noticed that the helicopter is trying to rotate around the GPS-receiver instead of the rotor shaft when I was yawing. Your helicopter is too heavy to do that for the fast rotation rate you’re commanding, so the position controller is constantly hunting the changing position.

Imagine the helicopter yawed for 180 degrees around its CG. Now the GPS-module is in a different position than it was when the helicopter started the maneuver. To bring it back to the position where it was in the beginning, the helicopter has to fly forward (which it is trying to do in your video). It almost reaches that position, but until then you have yawed the airframe for another 180 degrees. That means, that the helicopter is pointing in the same direction as when it started the maneuver, but it is positioned as if the GPS module was in its nose. So it has to fly forward again to compensate for the deviation and bring the GPS back to its original position. And that is exactly what you can see in the video.

To get rid of the behavior, try setting the position offsets of the GPS and also the flight controller itself from the center of gravity. The parameters for that are:

GPS_POS1_X, GPS_POS1_Y, GPS_POS1_Z

INS_POS1_X, INS_POS1_Y, INS_POS1_Z
INS_POS2_X, INS_POS2_Y, INS_POS2_Z
INS_POS3_X, INS_POS3_Y, INS_POS3_Z

You can also read this manual for more information like the directions of the offset parameters:
https://ardupilot.org/copter/docs/common-sensor-offset-compensation.html
I hope, this will help to solve your problem.

Yes, full left rudder

ok @Felix, thanks for your information.

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