The Soviets and the Germans had very different ways of determining the effectiveness of a fighter in combat. The Germans, just like we in the United States, view the success of a fighter by the performance and the kill/loss ratio. For the Soviets in the Great Patriotic War, the effectiveness of a fighter was determined by its ability to assist the advance of the forces on the ground, and its ability to protect attack aircraft from enemy air attempts to disrupt Soviet air support effectiveness.
The Yak-1 entered service shortly before the German attack on the Soviet Union: Operation Barbarossa. It remained in service throughout the war. Even as late as December 1943 Yak-1s were still provided as replacement aircraft to units in active combat against German forces. Improvements of the aircraft continued throughout the production run, taking place on the assembly line, or in the field. Modifications of the aircraft tended to increase the weight of the aircraft, negatively affecting its performance so work continued to find ways to increase the power of the engine, improve the efficiency of the propeller, reduce the aircraft weight, and aerodynamic and parasitic drag.
Rocket tubes under the wings, 37 mm cannon, increased rate of fire in the 20 mm cannon were all tried. Self-sealing fuel tanks, inert gas pressurization systems for the fuel tanks, armored glass for the windscreen, dust filters to protect the engine, modified exhaust stubs to provide additional thrust, increased the safety and usefulness of the aircraft.
During the initial battles against German forces the majority of Yak-1s in the Western and Southwestern fronts were destroyed, primarily on the ground. The aircraft was new to the crews and the pilots had not yet learned how to use them. In spite of this, one unit in three days in June 1941, destroyed 26 enemy aircraft on the approaches to Kiev.
Yak-1s were used in the early days, primarily in the air defense of Moscow. Unusually, they were also used to strafe advancing German ground troops. Their numbers increased as the year advanced. By the end of 1941 the Yak-1s were being joined by Yak-7s. Production of the Yak-1 ended in July 1944.
During the battle for the Kursk Salient 659 Yak-1s and Yak-7Bs served with the Second and the Sixteen Air Armies. Yak-1s took part in the 1944 Belorussian Strategic Offensive Operation, which destroyed the German Army Group Center, by protecting ground attack aircraft supporting Soviet troops. By this time the Yak-9 began to replace the Yak-1s and Yak-7s, although these were still providing reconnaissance services.
Some of the greatest Russian aces began their careers flying Yak-1s, including Aleksandr I. Pokryshkin with 53 personal kills and six shared kills.
Source: Yakovlev Fighters of World War Two, Yefim Gordon, Sergey Komissarov, and Dmitriy Komissarov, Hikoki Publications Ltd, Manchester, England, 2015