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 and played a major role in retaking air superiority from the Luftwaffe 's new Focke-Wulf Fw and Messerschmitt Bf G fighters during the grand Battle of Kursk in summer 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 remained in production from to , with 16, built 14, during the war. 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. 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 20 mm 0. State trials were carried out from January to April and revealed that the Yak-9U had a better top speed compared to fighters in service on the Eastern front at 6, m 20, ft. Unlike the I the Yak-9U was stable and easy to fly. However, the MA engine inherited the problems of the VKPF 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.
