Development and description On 9 April 1945, the
Council of People's Commissars ordered the Yakovlev OKB to develop a single-seat jet fighter to be equipped with a single German Jumo 004 engine. To save time, Yakovlev based the new design (known as the Yak-3-Jumo or Yak-Jumo) on the latest version of his successful
Yakovlev Yak-3 piston-engined fighter. The piston engine was removed and the jet engine was mounted underneath the forward
fuselage so that its exhaust exited underneath the middle of the fuselage. To protect the fuselage, a steel
heatshield was added to its bottom. The deeper forward part of the fuselage caused the configuration of the aircraft to resemble a "pod-and-boom". Very few changes were made to the metal fuselage other than at the aircraft's nose. This was recontoured to accommodate the armament of two
Nudelman-Suranov NS-23 autocannon, an additional fuel tank above the engine and the engine itself. No changes were made to the wings other than the elimination of the air intakes for the oil cooler and the bending of the front wing
spar into an inverted U-shape to clear the engine. The
vertical stabilizer was slightly enlarged, but the
tailplane was unmodified. The
conventional landing gear was also unmodified other than the tailwheel which now used several steel
leaf springs as shock absorbers. The Yak-Jumo carried a total of of fuel.
Taxi tests began in October 1945, but the heatshield proved to be too short and the heat from the engine exhaust melted the
duralumin skin of the rear fuselage as well as the rubber
tire of the tailwheel. Modifications to rectify the problems took until late December. By this time a second prototype had been completed with a solid steel tailwheel and an enlarged tailplane. After a few taxiing tests, it was transferred to the Central Aerohydrodynamic Institute (
TsAGI) for full-scale
windtunnel testing that lasted until February 1946. On the 26th of that month, the Council of People's Commissars issued requirements that the aircraft should have a maximum speed of at
sea level and a speed of at an altitude of . It should be able to climb to that altitude in minutes or less and it should have a range of at 90% of maximum speed. Two prototypes were to be ready for flight testing on 1 September. The
Mikoyan-Gurevich OKB was developing the
MiG-9 at the same time. According to aviation historians
Bill Gunston and
Yefim Gordon, representatives from Yakovlev and Mikoyan-Gurevich tossed a coin on 24 April 1946 to determine which aircraft would be the first Soviet jet to fly. Yakovlev lost and the Yak-Jumo made one
circuit of the airfield before landing. The manufacturer's flight testing of the aircraft was completed on 22 June, but its early success caused the
Council of Ministers to issue a new requirement on 29 April for two aircraft powered by the Soviet-built RD-10 engine (known as the Yak-15, Yak-15RD10 or Yak-RD). Aside from the new engine, the requirement differed from the previous one only in a range of at optimum cruise speed and a reduction of the maximum
ceiling to . Two prototypes were ordered to be available for flight testing on 1 September 1946. The tests revealed a number of problems in that the thick wing inherited from the Yak-3 limited the top speed of the aircraft, the engine exhaust 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. ==Survivors==