damaged the surface of the airfield, the cockpit often filled with smoke from kerosene and oil that had dripped onto the engine, and the aircraft was very short-ranged. Despite these problems, the Yak-15 proved to be very easy to fly, even for pilots accustomed to piston-engined fighters, and caused the VVS to accept the fighter as a conversion trainer. Even before the State acceptance trials were completed, the Council of Ministers ordered the aircraft into production in December 1946. 50 aircraft were to be built between January and April 1947, equally split between single-seat aircraft and two-seat trainers, armed with only a single cannon. The trainer ran into serious development difficulties and all the aircraft of the first batch were single-seaters. Fifty of these participated in the May Day flypast in Moscow in 1947. A total of 280 Yak-15s were produced through the end of the year, exclusive of prototypes. The aircraft were distributed in small numbers to fighter aviation regiments based in the USSR, Poland, Romania, Hungary, and Manchuria for use as conversion trainers. The aircraft's manoeuvrability led it to be used by a number of informal acrobatic display teams throughout the late 1940s. A single prototype of the two-seat trainer was the first aircraft of the first production batch built by Factory No. 31 in the fall of 1946. The prototype did not begin manufacturer's flight testing until 5 April 1947, even though the primary differences from the single-seat version were limited to a redesigned forward fuselage that accommodated an additional cockpit for the trainee where the armament used to be and a sideways-opening, canopy. The trainer was initially designated as the Yak-Jumo vyvoznoy, but it was eventually designated as the Yak-21 although some documents refer to it as the Yak-15V, Yak-15UT or Yak-21V. Further work on the trainer was cancelled with the success of the trainer version of the Yak-17 with its tricycle undercarriage. One Yak-15 was used to test a prototype aerial refueling system in 1949, although the installation on the fighter and the Tupolev Tu-2 bomber used as the tanker were both dummies to test procedures and fit. Yak-17-RD10 On 29 April 1946, five days after the Mikoyan-Gurevich I-300 (MiG-9 Prototype) and the Yak-Jumo made their first flights, the Council of Ministers ordered that the Yakovlev OKB begin design of a new aircraft similar to the Yak-Jumo, using the RD-10 engine with improved aerodynamics. This