I had modified this aircraft extensively over time - the latest mods were the addition of MN3420 T-Motors a voltage/current sensor and use of a 6S lipo. After completing the software configurations in Mission Planner, I took it out for a test flight - initial flight went well. Elated at the results, I created a waypoint mission, uploaded the wapoints into the aircraft and attempted to fly it. The autopilot didn't seem to start the aircraft off to the first waypoint, so I took over manually in Stabilize mode, later switching to RTL. During it's descent, I manually yawed the aircraft and it suddenly wobbled uncontrollably and crashed, completely destroying the aircraft. I have not determined the cause of the instability. I have a second, similar quadcopter - a Taroto 650 Sport, which is grounded (I don't have a lipo for it as I was using the one I had between the two aircraft and it is badly damaged).