Wind tunnel and flight testing of Otto Lilienthal's Experimental Monoplane from 1895

Abstract

In 1895, Otto Lilienthal patented, built and repeatadly flew a research aircraft equipped with a set of control surfaces actuated by the pilot. It had rubber-band activated leading edge flaps for automatic pitch control, and spoilerons, wing warping and rudder for yaw and roll control, which were actuated by a hip cradle. A full-scale replica and a 1:5 model were built for this investigation. All structural materials relevant to the flying qualities, were selected with great care. The 1:5 model was tested in two different wind tunnels at the "Institute of Aerodynamics and Flow Technology" of the German Aerospace Research Center (DLR) in Göttingen, Germany. The full-scale replica was used for a set of tethered flight tests on the Outer Banks (NC) in cooperation with Kitty Hawk Kites. The wind tunnel tests gave new insights into the performance, trim state, flight stability and controllability. Based on the commonly used classification of Lilienthal's gliders the "Experimentiergerät" or in the remainder of this text "Experimental Monoplane" has a wingspan of 8.8m and a wing surface area of 23m2 (appr. 250ft2). Lilienthal wrote that those larger dimensions lead to better performance at low wind speeds compared to his previous designs, but required additional means of control, as his weight shift method provided only very limited control authority for wing spans of more than 7m. Wind tunnel measurements and a limited set of flight tests were performed in order to investigate, whether the patented automatic pitch control and the control surfaces were sufficient to control the glider.