The Yakovlev Yak-9 () is a single-engine, single-seat multipurpose fighter aircraft used by the Soviet Union and its allies during World War II and the early Cold War. It was a development of the robust and successful Yak-7B fighter, which was based in turn on the tandem-seat advanced trainer known as the Yak-7UTI. The Yak-9 started arriving in Soviet fighter regiments in late 1942 and played a major role in retaking air superiority from the Luftwaffe's new Focke-Wulf Fw 190 and fighters during the grand Battle of Kursk in summer 1943.
The Yak-9 had a cut down rear fuselage with an unobscured canopy. Its lighter metal structure allowed for an increased fuel load and armament over previous models built from wood. The Yak-9 was manoeuvrable at high speeds when flying at low and medium altitudes and was also easy to control, qualities that allowed it to be one of most produced Soviet fighters of World War II. It was produced in different variants including the Yak-9T with the cannon and the "large-calibre" Yak-9K with a cannon firing through the propeller hub, which was used for antitank duty and as a potent aircraft destroyer, the fighter-bomber Yak-9B with an internal bomb bay behind cockpit for up to worth of bombs, the long-range Yak-9D and the Yak-9DD with additional wing fuel tanks to escort bombers over Eastern Europe, and the Yak-9U with a more powerful engine and improved aerodynamics. The Yak-9 remained in production from 1942 to 1948, with 16,769 built (14,579 during the war).
After World War II, the Yak-9 would also be used by the North Korean Air Force during the Korean War.
Design and development
The Yak-9 represented further development of the successful Yakovlev Yak-7 fighter, a production version of the lightened Yak-7DI, taking full advantage of the combat experience with its predecessor. Greater availability of duralumin allowed for lighter construction which in turn permitted a number of modifications to the basic design. Yak-9 variants used two different wings, five different engines, six different fuel tank configurations and seven different armament setups.
Yak-9U
During December 1943, the new airframe (Yak-9U) was able to use the M-107A engine, which was more powerful than the previous VK-105PF. The engine installation was new and included individual faired exhaust pipes. The oil cooler intake was moved from beneath the nose to the port wing root as well as an enlarged radiator bath being moved further aft under the fuselage. The supercharger intake was centered on the top decking of the engine cowling. The rear antenna cable was moved inside a lengthened rear canopy which provided the pilot with a better view to the rear, while the rear fuselage was cut down and the horizontal tail surfaces were slightly reduced in size. The wings and fuselage structure were made of metal which was then skinned with Bakelite. The Yak-9U was typically armed with a ShVAK cannon firing through the hollow propeller shaft, and two Berenzin UB machine guns.
State trials were carried out from January to April 1944 and revealed that the Yak-9U had a better top speed compared to fighters in service on the Eastern front at . Unlike the I-185 the Yak-9U was stable and easy to fly. However, the M-107A engine inherited the problems of the VK-105PF and was prone to overheating, oil leaks, loss of engine pressure during climbs, spark plugs constantly burning out, and intense vibrations which would fatigue assembly bolts leading to a short engine life. These defects forced the first production batches starting during April 1944 to be powered by the more reliable M-105 PF-3 engine. Further changes were made, like increasing the fuel capacity to and in order to re-balance the aircraft, the wings were moved forward and the aircraft's Vlsh-107LO propeller being replaced with the older VISH-105S. A total production of 1,134 aircraft were constructed by December 1944.