I was in a field at a local park playing with ground station. I had been flying for about 20 min and after a few successful runs I put in a new battery And was hovering at about 50ft plotting out my next flight plan and all of a sudden a helicopter passed about 50ft DIRECTLY over me! Our local police department does not have a helicopter to my knowledge but we do have a national guard training facility about 5 miles away. I assume that is who it was because it was green and rather large. It was definitely military. I have no idea why he was flying so low and what are the chances they just happened to pass by. I really can't remember ever seeing one of this type anywhere near here. Did somebody get nervous and call the cops who in turn called the National Guard? That seems a bit much. It was close enough that the wind jostled my bird quite a bit. I sure hope I don't get a knock on the door.