my 26cc gas engine plane have huge lag between desPitch and Pitch in AUTO mode
https://ardupilot.org/plane/docs/common-tuning.html
Two things that jumped out at me are the oscillations in the pitch likely a high D term value, and also the vibrations likely from the 26cc engine or possibly a prop imbalance.
My suggestion would be to first sort out the vibration levels. This could be in a soft mount for the FC, as well as the condition of the engine and prop. Verify the prop balance is good. Don’t assume the factory balance of the prop is enough. A co-worker of mine recently balanced four identical brand new props: two were perfect out of the box, and two needed significant adjustment. They were all the same part, came from the same vendor, and stored in the same location.
Once the vibrations are good, you should look back at auto-tune or consider trying manually tuning. If you are manually tuning I would work on getting the oscillations dealt with first, then try to bring up the P/I values. Review the wiki for guidance on manual tuning.
AP4.1.x has some new auto tuning functionality that seems to do a lot to reduce oscillations.
Edit: Also set SERVO_AUTO_TRIM,1
thanks allister,
yes, the vibration for the internal combustion engine plane is always too large, and I also had install the vibration isolation ( Vibration Damping — Copter documentation (ardupilot.org)) but it looks can’t slove the problem
I agree with you, I will try to change another prop to find if it can reduce the vibration level
