Yak-9U is a version of Yak-9 with improved aerodynamic characteristics and a new engine M-107A. On February 23, 1942 at the time of testing the aircraft developed the speed of 680 kph at a height of 5,800 m. Hardship with tweaking the propeller delayed the beginning of serial production for more than a year, and first serial aircrafts Yak-9U appeared at the front only in October, 1944. They showed absolute superiority over the German fighters. By August, 1945 over 3,900 aircrafts Yak-9U were built.