use of small flapping wings providing the thrust for a larger fixed wing; this innovation eliminated the need for gear reduction, thereby simplifying the construction. E. P. Frost made ornithopters starting in the 1870s; first models were powered by steam engines, then in the 1900s, an internal-combustion craft large enough for a person was built, though it did not fly. In the 1930s, Alexander Lippisch and the National Socialist Flyers Corps of Nazi Germany constructed and successfully flew a series of internal combustion-powered ornithopters, using Hargrave's concept of small flapping wings, but with aerodynamic improvements resulting from the methodical study. Erich von Holst, also working in the 1930s, achieved great efficiency and realism in his work with ornithopters powered by rubber bands. He achieved perhaps the first success of an ornithopter with a bending wing, intended to imitate more closely the folding wing action of birds, although it was not a true variable-span wing-like those of birds. Around 1960, Percival Spencer successfully flew a series of uncrewed ornithopters using internal combustion engines ranging from displacement, and having wingspans up to . In 1961, Percival Spencer and Jack Stephenson flew the first successful engine-powered, remotely piloted ornithopter, known as the Spencer Orniplane. The Orniplane had a wingspan, weighed , and was powered by a -displacement two-stroke engine. It had a biplane configuration, to reduce oscillation of the fuselage. Crewed flight Crewed ornithopters fall into two general categories: Those powered by the muscular effort of the pilot (human-powered ornithopters), and those powered by an engine. Around 1894, Otto Lilienthal, an aviation pioneer, became famous in Germany for his widely publicized and successful glider flights. Lilienthal also studied bird flight and conducted some related experiments. He constructed an ornithopter, although its complete development was prevented by his untimely death on 9 August 1896 in a glider accident. In 1929, a man-powered ornithopter designed by Alexander Lippisch (designer of the Messerschmitt Me 163 Komet) flew a distance of after tow launch. Since a tow launch was used, some have questioned whether the aircraft was capable of flying on its own. Lippisch asserted that the aircraft was actually flying, not making an extended glide. (Precise measurement of altitude and velocity over time would be necessary to resolve this question.) Most of the subsequent human-powered ornithopters likewise used a tow launch, and flights were brief simply because human muscle power diminishes rapidly over time. In 1942,