The Arado SD I was a fighterbiplane 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. The wooden sesquiplane wings were braced with N-type interplane struts without any wires - typical Fokker.
In flight, performance and handling proved disappointing at anything but very low speeds. Questions also arose as to the structural soundness of the design, and development was terminated very soon thereafter.