Like Cayley, Lilienthal remained convinced that the first powered aircraft would be an ornithopter. He had demonstrated that birds produce thrust by the propeller action of their outer feathers, and just before his death had completed a glider with ornithopter wing-tips operated by a small carbonic-acid gas motor. As with Cayley's 'flappers', they did not work well in ground trials and it is unlikely that they would have worked in flight, but maybe...