Rarely has human flight achieved the gracefulness of the birds. Manned airplanes, no matter how sleek, are still largely tubes with wings, pulled through the sky by powerful engines. Helicopters are little better, their spinning blades creating a frenzy of noise and dust wherever they go. Even simple drones, like the U.S. military's hand-tossed RQ-11 Raven, land with all the grace of a dropped LEGO tower. At the University of Maryland, researchers working on a series of drones also named Raven have created that rare, beautiful machine: a drone that flaps its wings to fly, a robotic heir to the sky.