Leonardo wrote of the building of a model plane out of parchment, and of testing some of his early ornithopter, an aircraft that flies by flapping wings, and parachute designs using paper models. Thereafter, Sir George Cayley explored the performance of paper gliders in the late 19th century. Other pioneers, such as Clément Ader, Prof. Charles Langley, and Alberto Santos-Dumont often tested ideas with paper as well as balsa models to confirm (in scale) their theories before putting them into practice.[citation needed]