Editing Yakovlev Yak-9 (section)

==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 {{cvt|20|mm}} ShVAK cannon firing through the hollow propeller shaft, and two {{cvt|12.7|mm}} Berezin UB machine guns.<ref name="Yak-9U WW2 Weapons">{{Cite web|url=https://ww2-weapons.com/yak-9u/|title=Yak-9U > WW2 Weapons|date=2019-02-27|website=WW2 Weapons|language=en-US|access-date=2019-10-22}}</ref>

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 {{cvt|6000|m}}. Unlike the [[Polikarpov I-185|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 {{cvt|400|L}} and in order to re-balance the aircraft, the wings were moved {{cvt|9.9|cm}} forward and the aircraft's VIsh-107LO propeller being replaced with the older VIsh-105S. A total production of 1,134 aircraft were constructed by December 1944.<ref name="Yak-9U WW2 Weapons"/>