I was testing a Parallax Elev-8 V2 quadcopter that I built with a 3DR APM 2.6, external compass, GPS, etc. It also has a boom with a Tarot 2D gimbal with a GoPro camera mounted on it and a TX-5D 600 mW video transmitter.
I took the aircraft out to the field for a test mission - simple one with only 5 waypoints; the aircraft was supposed to circle around one of them and point back to a POI in transit to the next w.p. conditions were foggy and breezy.
For some reason, the aircraft did not fly the mission as I expected (I lost sight of it in the fog as it went higher than I expected, but the recovered video shows that it deviated from the planned mission. Finally, I caught sight of it overhead as it seemed to be descending for its landing. But it suddenly descended rapidly and seemed out of control as it dropped through its propwash. I immediately took over manually and tried to regain control - tried to throttle up but it did not respond; it made a hard landing.
Later that day, I decided to try flying the same mission with another Elev-8 that also has an APM 2.6 autopilot. It has a fixed mounted Drift Stealth sport camera mounted underneath and connected to a TX-5D transmitter.
The conditions were better, though still a bit breezy, but I've flown in similar conditions. After a clean liftoff, I put the aircraft in Auto mode and it flew the mission seemingly fine. It was circling around the waypoint, which is a horse's stall. Just as it was completing the orbit around the stall, it suddenly lost power and descended rapidly. Again, I tried to regain control but there wasn't enough time. Again, it did not respond to my throttle command. It landed just outside the stall in the muck, which fortunately cushioned the landing.
I'm at a loss as to what caused the power loss from both aircraft.