The Polikarpov Po-2 (also U-2, for its initial uchebnyy, 'training', role as a flight instruction aircraft) served as an all-weather multirole Soviet biplane, nicknamed Kukuruznik (, NATO reporting name "Mule"). The reliable, uncomplicated design of the Po-2 design made it an ideal trainer aircraft, as well as doubling as a low-cost ground attack, aerial reconnaissance, psychological warfare and liaison aircraft during war, proving to be one of the most versatile light combat types to be built in the Soviet Union. As of 1978 it remained in production for a longer period of time than any other Soviet-era aircraft. Production figures for Polikarpov U-2 and Po-2 bombers and trainers combined are between 20,000 and 30,000 with production ending as early as 1952. Precise figures are hard to obtain since low-rate production by small repair shops and air clubs likely continued until 1959. Design and development The aircraft was designed by Nikolai Polikarpov to replace the U-1 trainer (a copy of the British Avro 504), which was known as Avrushka to the Soviets. The prototype of the U-2, powered by a 74 kW (99 hp) Shvetsov M-11 air-cooled five-cylinder radial engine, first flew on 7 January 1928 piloted by M.M. Gromov. Aircraft from the preproduction series were tested at the end of 1928 and serial production started in 1929 in Factory number 23 in Leningrad. Its name was changed to Po-2 in 1944, after Polikarpov's death, according to the then-new Soviet naming system, usually using the first two letters of the designer's family name, or the Soviet government-established design bureau that created it. Production in the Soviet Union ended in 1953, but license-built CSS-13s were still produced in Poland until 1959. Operational history World War II From the beginning, the U-2 became the basic Soviet civil and military trainer aircraft, mass-produced in a "Red Flyer" factory near Moscow. It was also used for transport, and as a military liaison aircraft, due to its STOL capabilities. Also from the beginning it was produced as an agricultural aircraft variant, which earned it its nickname Kukuruznik (corn man). Although entirely outclassed by contemporary aircraft, the Kukuruznik served extensively on the Eastern Front in World War II, primarily as a liaison, medevac and general-supply aircraft. It was especially useful for supplying Soviet partisans behind the German front line. Manufacturing of the Po-2 in the USSR ceased in 1949, but until 1959 a number were assembled in