X-Plane 9.70+ Yakovlev Yak-9U 1.3. The Yakovlev Yak-9 was a single-engine fighter used by the Soviet Union in World War II and after. The definitive Yak-9 variant, Yak-9U was equipped with the new Klimov VK-107A engine with 1230 kW (1650 hp). 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. The prototypes top speed of 700 km/h (435 mph) at 5600 m (18,370 ft) was faster than any other production fighter aircraft in the world at the time. Full description including changes included in the download. By Ron Norvelle.