sentences:  A glider – the Colditz Cock – was even built secretly by POWs as a potential escape method at Oflag IV-C near the end of the war in 1944.Foot-launched aircraft had been flown by Lilienthal and at the meetings at Wasserkuppe in the 1920s.However the innovation that led to modern hang gliders was in 1951 when Francis Rogallo and Gertrude Rogallo applied for a patent for a fully flexible wing with a stiffening structure.