For the most part, American aerospace testing was done on American aircraft. However, beginning with WWI, whenever the United States obtained examples of foreign aircraft - either from friendly countries through cooperative arrangements or from enemies via capture or defection- they were likely to wind up at McCook Field or Wright Field for a thorough evaluation which included flight testing if possible.
"During WWII evaluations at Wright Field included allied aircraft like the Russian Yak-9 and the British Spitfire and Mosquito, and enemy aircraft including the German JU-88, ME-109, FW-190, ME-262, and the Japanese Zero. The end of the war brought large numbers of captured aircraft for evaluation. As with other test flight activities, much of the foreign aircraft evaluation moved to Muroc Air Base (later Edwards AFB) after the war, but even then the occasional foreign aircraft came to the Miami Valley for testing, as a MiG-15 (courtesy of a North Korean defector) at Patterson Field attests." (Source: Against the Wind)
In June 1941, the German Army swept into Russia. Caught by surprise, Russia's Air Force was decimated on the ground and in the air. Alexander Yakovlev moved his design and manufacturing facilities east of the Ural Mountains and began production of the YAK-9 in 1942. Eventually 16,769 YAK-9 models were built, more than any other aircraft in the Russian Air Force.
The YAK-9 was designed for mass production and durability. Due to shortages in Russia, it incorporated a minimum of scarce strategic materials. They were designed to outnumber the enemy, not for technical superiority. While not good at one-on-one fighting, it was a good fighter in numbers. It was also a durable fighter, capable of absorbing a lot of battle damage and still make it home. Some models were also used for ground attack
The YAK-9 is one of the Allied aircraft that was study by the WADC in the 1950's.
Type - Single-seat Fighter
Wings - Low-wing cantilever monoplane. Two outer wings attached to a centre-section which forms part of the fuselage and floor of cockpit. Structure consists of two spars of extruded metal, wood ribs, and skin of fabric over plywood, varnished and lacquered. Fabric-covered ailerons with trim tabs at trailing-edge.
Fuselage - Framework of welded steel tubing, covered by two half-shells of wood attached to tubular framework by webbing bands. This also fabric-covered and lacquered.
Tail Unit - Cantilever monoplane type. All-wood tailplane and fin. Fabric-covered elevators and rudder. Trimming-tabs in elevators.
Landing Gear - Tail-wheel type. Main wheels retract inwards into wing roots. Tail wheel retracts into rear of fuselage. Oleo-pneumatic shock-absorbers. Messier type brakes on main wheels.
Power Plant - One 1,210 h.p. VK-105PF twelve-cylinder Vee liquid-cooled engine, driving three-blade metal variable pitch airscrew. Coolant radiator under fuselage just aft of cockpit, and additional air-intake under nose, on cowling.
Accommodation - Pilot in all-round vision cockpit. Sliding hood.
Armament - One ShVAK 20 mm. motor cannon in nose. Two Beresin 12.7 mm. machine-guns mounted in cowling.
Dimensions - Span 10 m. (32 ft. 10 in). Length 8.48 m. (27 ft. 10 in.).