Before even connecting (via telemetry) to your plane, you should set up a rectangular mission (as shown in the following screenshot). It's important to have long straightaways so that you can see the actual ability of the plane to adhere to a straight line:
The first thing you'll notice is that this rectangle is actually 5 points with 6 entries in the Waypoints window (bottom). I'm sure there's a logic to it, but mission planner does not allow you to simply add a repeat behavior to the end of a mission. Instead, you need to create an extra waypoint at the end as a dummy waypoint, and insert a DO_JUMP behavior just ...
