The Arado SD I was a fighter biplane, developed in Germany in the 1920s. It was intended to equip the clandestine air force that Germany was assembling at Lipetsk. The layout owed something to designer Walter Rethel's time with Fokker. Of conventional configuration, the SD I featured a welded steel tube frame, metal-covered ahead of the cockpit, and fabric-covered aft of it. The wooden sesquiplane wings were braced with N-type interplane struts, without any wires - a typical Fokker feature.