Its lighter metal structure allowed for an increased fuel load and armament over previous models built from wood.

[3] The Yak-9 was manoeuvrable at high speeds when flying at low and medium altitudes and was also easy to control, qualities that allowed it to be one of the most produced Soviet fighters of World War II.

[citation needed] It was produced in different variants including the Yak-9T with the 37 mm (1.5 in) cannon and the "large-calibre" Yak-9K with a 45 mm (1.77 in) cannon firing through the propeller hub, which were used for antitank duty and as potent aircraft destroyers, the fighter-bomber Yak-9B with an internal bomb bay behind the cockpit for up to 400 kg (880 lb) worth of bombs, the long-range Yak-9D and the Yak-9DD with additional wing fuel tanks to escort bombers over Eastern Europe, and the Yak-9U with a more powerful engine and improved aerodynamics.

[5] 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.

[6] 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 6,000 m (20,000 ft).

[8] After the Battle of Smolensk, in the second half of 1943, the famed Free French Normandie-Niémen unit became a Groupe and was equipped with the Yak-9.

Nevertheless, in the course of 398 sorties, the unit claimed 27 Focke-Wulf Fw 190As and one Bf 109G-2, for the loss of two Yaks in dogfights, one to flak and four in accidents.

The Yak-9U contributed greatly toward the Soviets gaining air superiority, and the Germans learned to avoid the Yaks “without antenna mast”.

Flying a type-T (equipped with a 37mm NS-37 cannon in the nose) he achieved 19 air victories, plus nine shared and was awarded the Gold Star Medal of the Hero of the Soviet Union in June 1945.

[14] At the beginning of the Cold War, Yak-9 fighters began buzzing American, British, and French flights in the air corridors to West Berlin.

A section of the aircraft's operating manual was accidentally omitted from the translation from Russian into some languages: before starting the Yak-9, it was necessary to hand-crank a small cockpit-mounted oil pump 25 times to provide initial lubrication to the Klimov V12 engine, unlike World War II German and Western fighters equipped with forced closed-cycle lubrication systems.

Skipping this unusual but vital step resulted in frequent engine seizures during the takeoff roll and initial climb, causing several fatalities during 1950.

A problem corrected during prototype tests was poor quality control that led to multiple oil and coolant leaks from cannon recoil.

Poor handling with a full bomb and fuel load and lack of special aiming equipment limited its combat usefulness.

A Yak-9D with the cockpit moved 0.4 m (1 ft 4 in) to the rear like the Yak-9T, as well as numerous fixes and improvements based on experience with previous versions.

A Yak-9M with slightly reduced fuel capacity, the Klimov VK-105PF2 engine with 970 kW (1,300 hp), and radio and navigational equipment for night and adverse weather flying for PVO Strany.

Poor performance due to the unreliable engine dramatically improved with adoption of the Klimov M-106PV with water injection, with the aircraft reaching 12,500 m (41,000 ft) during testing.

The main visual difference from the Yak-9T was in the oil coolers in the wing roots, like on the Yak-3, and in that plywood covered the fuselage, instead of fabric.

It did not enter production because the VYa was considered unsatisfactory and because the one cannon, one machine gun armament seen on previous models offered a significant increase in range.

The definitive Yak-9 variant, the Yak-9U (VK-105) was equipped with the new 1,230 kW (1,650 hp) Klimov VK-107A engine, and with the 20 mm (0.79 in) ShVAK with 120 rounds replacing the VYa cannon.

Early test flights in 1943 indicated that the only comparable Soviet fighter was the Polikarpov I-185 prototype which was more difficult to fly and less agile due to higher weight.

In the early 1990s, Yakovlev started limited production for the warbird market of Yak-9 and Yak-3 replica aircraft using original World War II equipment and Allison V-1710 engines.