Unmanned aircraft known as drones are here, and more are coming. The Federal Aviation Administration, which is expected to soon issue rules to allow commercial drone flights, estimates there could be 30,000 commercial and civil drones buzzing over America by 2020. The reason is simple: Robotic planes and helicopters can do many things companies pay people to do, and the drones may be able to do them at less cost and at less physical risk to workers — from inspecting rooftops of high-rises for wear and tear, to taking flyover shots for a movie.