This famous drawing shows a screw-shaped device made of starched linen stretched over a metal frame. The linen screw was designed to rotate rapidly, powered by someone pedaling beneath it. Some consider this invention a forerunner to the modern helicopter. In 1936 the German engineer Heinrich Focke developed the first practical helicopter. Today’s helicopters use rotary propellers, rather than the screw proposed by Leonardo, to power their flight. (See also  Pedal-Powered Wing.)