I prepared for the flight like the several dozen previous ones. Put the props on the correct motors and gave them a quick spin. Watched them make a few revolutions before bottoming out and making the motor turn a little. Also checked visually that they were bottomed out. Went out and launched, hovered about 10' up for a few seconds - all looked normal. I then gave it full throttle up to about 80' to get above some trees, and let the left stick return to center. At that time, a prop came off to the side. The PV turned upside down and plummeted. It rubbed against a bushy evergreen tree, and landed on its feet in the grass with the battery popped out. The prop came fluttering down and landed about 20' from the PV. I looked everything over and saw no damage. I reinstalled the battery and prop, and flew it up to 600' for some pictures, and it flew as normal. Got my second battery, and flew at 1000' with no problems. My theory is that with the sudden motor deceleration when I backed off the throttle, the angular momentum in the prop plus the rotational thrust placed on the prop from the upward momentum (acting like a turbine) was enough to loosen it. These are the original self-tightening props, and it was one of the black ones. From now on, I plan to snug them up with the little wrench they provide for removal.