The ducted-fan, tilt-rotor helicopters of “Avatar” fame have yet to find their way into the U.S. military, perhaps to the chagrin of sci-fi fans everywhere. [caption id="attachment_23571" align="alignleft" width="300" caption="Andrew Grandin, mechanical design engineer with American Unmanned Systems, takes pictures of the Guardbot while swimming in the water beside it during testing."][/caption] But the rol ...