Weighing only 16 grams, the Black Hornet looks like a tiny toy helicopter. But it’s really a nano-size piece of military hardware unlike anything on the battlefield today — experimental robot flies and hummingbirds not withstanding. The PD-100 Black Hornet Personal Reconnaissance System, unveiled to the American public for the first time last week at the Association of the United States Army Expo in Washington, D.C., is a drone (actually, a pair of them) that a soldier can carry and operate as easily as he or she would a radio.