Quadplane lost control after flying in Q_LOITER or Q_STABILIZE more than 1 minute

ArduPlane VTOL Plane
1

I have VTOL quadplane with 4m wing span and weight 23kg. It has dropping system and I want to test it in Q_LOITER or Q_STABILIZE mode flying with altitude 50m and drop the payload, but something strange happened. After flying to altitude 50m and drops the payload, the altitude suddenly increasing to 100m and the plane are lost control, the pilot can’t control attitude of the plane. I did 2 tests, the first one with Q_LOITER and the second one with Q_STABILIZE. The result are the same, the plane lost control after more than 1 minute flying in multirotor mode. Can anyone tell me what happened ? I attached the log below

https://drive.google.com/drive/folders/169a7-gu4UhUxGE66xdqydS9MhpPfUB1g?usp=sharing

2

Does the CG change when you drop the payload?

3

@kd0aij thanks for the response,
I think the CG doesn’t change because the payload attached exactly on the plane’s CG.

4

The qstabilize log shows no loss of attitude control.
It does show that you commanded a pitch up of 10 to 15 degrees at an airspeed of 10 m/sec at the beginning of the climb.
How do you command the payload drop? Is it via a switch on the RC transmitter?

5

the payload dropped via RC. The pilot couldn’t fully controlled the plane, feels like it loss power but the voltage seem normal