We have builded a quadcopter that use the flight manager kk2.1.5 with the latest firmware. When we increase the throttle, it flight. When we keep the hand on the stick we are able to maintain it but when we don't touch to the throttle, it goes up and down.
You can see an example on this video.
We have tried different values for PID but we don't know what is the best for us. We have a large quadcopter with medium propellers (may be too small).
Does the weight of the quadpcoter or the width of the propellers can be a factor? What can be the problem?