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 carried two different wings, five different engines, six different fuel tank combinations and seven types of armament configurations. In December 1943 it became possible to install the more powerful M-107 engine on a new Yak-9U airframe: the engine mounting was new with individual faired exhaust pipes; fuselage structure and wings were made of metal and the whole aircraft was covered with a Bakelite skin . Fuel capacity was increased to 106 gallons (781 lb). In order to re-balance the model, the wing was repositioned four inches forward and in order to improve pitch control, the horizontal tail surfaces were slightly reduced. The rear part of the canopy was lengthened and the antenna cable was moved inside it. Usual armament was a ShVAK cannon with 120 rounds firing through a hollow propeller shaft and the same UBS with 170 rounds each.
The State trials took place from January to April 1944. They revealed a clear superiority in top speed over all other fighters in service on the Eastern front, up to 19,800 feet. The aircraft was simple to fly and stable. Unfortunately, the problems with the M-107A engine and moreover all the M-105 variants from which it derived, persisted: power plant overheating, oil leaks, loss of pressure in climbs, intense vibrations, burning out sparkplugs and, above all, a short engine life (25 hours). Despite these defects, the Yak.9U/VK-107 was ordered into production in April 1944 with 1,134 machines produced by December 1944.
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