Thanks for the XML.
Personally I don't think anything is wrong. You programmed the module to fly a circle with a radius of about 100 meters. Of course your plane is small and you probably feel uncomfortable to make it fly further away (ask Florian about this, he had the same problem
).
Your average flying height is 20(!) meters. You need to fly at least at a height of 80 meters (this is 4 times as high). The furthest distance the plane flew from you is 100 meters.
Even though it's a flying wing, you need to fly it as if it was a big glider. When activating the autopilot mode, try to fly in a clockwise direction around the take-off position at a distance a lot further than it is now.
To make you feel more comfortable: Change the maximum roll angle to 40 degrees. Change the radius of you circle to CIRCLE_REL (0, 0, 60, 60) or something similar.
But even so: try to fly higher and make yourself feel comfortable with the plane being around 100 meters away from the take-off position.