SD I

The SD I was a single‑seat, low‑wing monoplane designed in the early 1920s by the German engineer Walter Rethel, who had previously contributed to the Fokker D.VIII and the Hansa‑Brandenburg series. Built by Rethel’s own small workshop in Darmstadt, the aircraft first flew on 14 May 1922 at the Johannisfeld airfield. Its construction employed an all‑metal fuselage with stressed‑skin aluminium panels, a departure from the wooden structures still common at the time. Powered by a 120‑horsepower Siemens‑Halske Sh 12 rotary engine, the SD I achieved a top speed of 210 km/h and a service ceiling of 7,500 meters, giving it performance comparable to contemporary fighter prototypes. The aircraft featured a streamlined cowling, retractable ailerons and a fully enclosed cockpit with a small windshield, innovations that foreshadowed later German designs of the 1930s. Although only three prototypes were built before financial constraints halted further production, the SD I demonstrated the viability of lightweight metal airframes and influenced Rethel’s later work on the Horten‑type flying wings. Its brief existence marks an important transition point between wood‑based biplanes and the all‑metal monoplanes that would dominate pre‑World II aviation.

Walter Rethel

Classification

Production & History

Design & Classification