The most significant pre-Wright brothers aeronautical experimenter was the German glider pioneer, Otto Lilienthal. Lilienthal was trained in the highly regarded German technical education system and earned his living as a professional engineer. He began research in aeronautics with his brother Gustave in the late 1860s, investigating the mechanics and aerodynamics of bird flight. In the 1870s he conducted a series of experiments on wing shapes and gathered air pressure data using a whirling arm and in the natural wind. The research produced the best and most complete body of aerodynamic data of the day. Lilienthal also established definitively the widely held belief that a curved wing section, as opposed to a flat wing surface, was the optimum shape for generating lift. In 1889 he published his findings in a pathbreaking book called Der Vogelflug als Grundlage der Fliegekunst (Birdflight as the Basis of Aviation).
Lilienthal was not satisfied to restrict his work to the exploration of aerodynamic theory. Between 1891 and 1896, he put his research into practice in the form of a series of highly successful full-size glider trials. During this period Lilienthal made close to 2,000 brief flights in 16 different glider designs based on his aerodynamic investigations. Most were monoplanes with stabilizing tail surfaces mounted at the rear. He also tried a few biplane and folding wing designs, but the original monoplane glider, or Normal Segelapparat (standard sailing machine) as he called it, produced the best results. Lilienthal built at least eight gliders of this type.
The gliders had split willow frames covered with cotton twill fabric sealed with collodion to make the surface as airtight as possible. Collodion is a viscous solution of nitrated cellulose in a mixture of alcohol and ether that dries to form a tough elastic film. The wings ranged in area from 9 to 25 m2 (100 to 280 ft2), and could be folded to the rear for easier transport and storage. Control was achieved by shifting body weight, similar to modern hang glider practice. The pilot cradled himself vertically in a harness suspended below an elliptical opening between the wings. Swinging his legs from side to side and fore and aft, the pilot could adjust the center of gravity and thereby maintain equilibrium.
Lilienthal did most of his gliding from a man made hill he had constructed near his home at Gross Lichterfelde, and from the hills surrounding the small village of Rhinow, about fifty miles from Berlin. His best efforts with these gliders covered more than 300 m (985 ft) and were 12 to 15 seconds in duration.
In the summer of 1896, Lilienthal's aeronautical experiments came to an abrupt and tragic end. On August 9, while soaring in one of his standard monoplane gliders, a strong gust of wind caused the craft to nose up sharply, stall, and crash from an altitude of 15 m (50 ft). Lilienthal suffered a broken spine and died the following day in a Berlin hospital.
As successful as they were, Lilienthal's glider designs had some inherent limitations that he would have had to confront had he lived and continued his work. The principal problem was his means of controlling the craft. Lilienthal's technique of shifting body weight to maintain equilibrium did place him ahead of other experimenters in that he recognized the need for a control system and gave attention to developing one. But, as revealed in his fatal crash, the control response of his method was very limited. Even more significant, shifting body weight as a means of control placed a severe restriction on the size of the aircraft. Because control was achieved by altering the aircraft's center of gravity as a result of repositioning the pilot's body weight, the weight of the aircraft had to be kept comparatively low. This presented a great problem in the design of a powered airplane. Any aircraft capable of lifting an engine and pilot, much less any sort of a payload, would be of a size so large that shifting body weight would be totally ineffectual.
Further, the airfoil of Lilienthal's gliders, although extensively tested and documented during his earlier aerodynamic research, was very inefficient in actual practice. Lilienthal always preferred a perfect arc for the shape of his glider wings with a very deep camber of 1 in 12. His investigations demonstrated that a curved surface was the most efficient shape, but he never abandoned the perfect arc in his gliders to experiment with parabolic airfoils, which later proved to be superior. The deeply cambered perfect arcs of Lilienthal's glider wings resulted in aerodynamic efficiency and stability problems.
Despite these unresolved issues, the impact of Lilienthal's aeronautical work upon the next generation of experimenters, the generation that would finally achieve heavier-than-air powered flight, was highly influential. With his pioneering aerodynamic research and his success in the air, Lilienthal had established a new starting point for anyone entering the field. Beyond his technical contributions, he sparked aeronautical advancement from a psychological point of view as well. He demonstrated unquestionably that gliding flight was possible. Granted, he was flying for only seconds at a time, but he was truly flying. Lilienthal's tentative trips through the air made headlines everywhere. Dramatic photographs showing Lilienthal soaring gracefully over hillsides appeared in newspapers and magazines the world over. The publicity made him quite a sensation in an age when, for most, human flight still seemed a distant possibility at best. This exposure and visible proof that a human being could actually fly contributed as much to spurring other experimenters forward as did Lilienthal's ground breaking aerodynamic research. He was a great inspiration to the Wright brothers in particular. They adopted his approach of glider experimentation and used his aerodynamic data as a starting point in their own research.
The Lilienthal glider in the NASM collection was built by the German experimenter in late 1895, early 1896. It was purchased from Lilienthal by the American newspaper magnate William Randolph Hearst in the spring of 1896. Hearst sponsored test flights of the glider on a Long Island estate in April and May 1896 in an effort to create publicity and boost the circulation of his newspaper, the New York Journal. Harry Bodine, a New Jersey athlete, made most of the flights, although Journal reporters and other spectators were also allowed to test their skill. Flights as long as 115 m (375 ft) at altitudes of up to 15 m (50 ft) were made with the glider. Further flight testing, however, ceased after Lilienthal's death in August.
Hearst then gave the glider to John Brisben Walker, editor of Cosmopolitan magazine, who displayed it at a New York Aero Club show in January 1906. Alexander Graham Bell suggested to Walker that the Smithsonian Institution likely would be interested in pursuing acquisition of the Lilienthal glider. After the Smithsonian approached Walker on the matter, he presented it to the Smithsonian on February 2, 1906. Minor refurbishing was done in 1906 and 1928, and in 1967 the glider was completely restored. The horizontal tail is not original. The NASM Lilienthal glider is one of five remaining in the world.