Wind Tunnel Testing of Otto Lilienthal's Production Aircraft from 1893

Abstract

A full-scale wind tunnel test of a replica of Otto Lilienthal's Normalsegelapparat ("Normal soaring apparatus") from 1893 was conducted in a closed, low-speed wind tunnel. The goal was to determine the flight performance and characteristics of this glider for the first time. Resolving the open question about the static stability of his design is of particular interest here. In addition, historical sources are ambiguous on how Lilienthal treated the fabric covering the wings. Therefore, tests were performed both with an impermeable, sealed fabric as well as untreated, permeable wings. Forces and moments were measured for free stream velocities between 5 m/s and 13 m/s over an angle of attack range covering the linear lift increase up to fully stalled conditions. The recorded data is used to investigate the flight performance and characteristics of the glider with an emphasis on its static stability. The impact of the permeability of the fabric in comparison to the impermeable configuration is discussed.