writes, "A Minnesota company, VeraTech, has applied for a patent on an unmanned drone that is nearly invisible to the naked eye. The Phantom Sentinel takes advantage of the phenomenon where fast moving objects appear as only a blur, so it fades out of view once it speeds up. This is achieved by rotating the entire craft. The center of gravity is in open air between two of the blade-like wings. There are some videos of a prototype in action on the VeraTech site."
The company says you could get usable video of the terrain by processing the images from a spinning camera. One version of the drone is small enough to launch by throwing it like a boomerang. And it folds for travel.