Blériot-SPAD S.51

The Blériot-SPAD S.51 was a French fighter aircraft developed in 1924 in response to a French Air Force requirement for an aircraft to replace the obsolete Nieuport-Delage NiD.29. Designed by André Herbemont, it shared its basic configuration with his other aircraft of the period, being a biplane with a swept upper wing and an unswept lower wing joined by I-shaped interplane struts, and unlike earlier designs it used metal construction for the wings. The prototype was rejected by the French authorities, but revised versions found export customers in the Polish Air Force, which bought 50, and the Turkish and Soviet air forces, which each bought a single example. The S.51/3 was fitted experimentally with the first controllable-pitch propeller developed in France.