Actually, the Yak-7 started life as a trainer designated the UTI-26. It was a development of the I-26 (Yak-1) and was to be an intermediary step between the low-powered UT-2 primary trainer and the higher-powered combat aircraft.
With the chaos created by the German invasion, fighters were needed and aircraft of any type that could be used as a fighter were ordered armed and into combat. The Yakovlev Design Bureau was forced to convert the UTI-26 into a combat aircraft, redesignated as the Yak-7. The few two-seat trainers on the production line at this time were designated Yak-7V (Vyvozonyi - Familiarization).
The first two-seat fighters were designated Yak-7A, armed with a single 20mm cannon and two 12.7mm machine guns. Provisions were also made for the aircraft to carry six rockets or two 220lb bombs.
Subsequently, the instructor's seat was removed and replaced with an additional fuel tank, giving the Yak-7A increased range, about 510 miles. Because of the extra weight, some changes were made, mostly to the landing gear.
The Yak-7K (Kuryerski - Courier) was a development of the Yak-7V with two seats, but no dual controls. This was used mainly for transporting top-ranking officers and for courier flights.
There were further developments of the Yak-7; the Yak-7B, D, DI and many other sub-variants, but I don't want to write a novel as I usually do!
The decision to introduce, all at once, a large number of changes and improvements to the Yak-7 design resulted in the designation Yak-9. One of the most significant changes to occur toward the end of Yak-7 production and even moreso into production of the Yak-9 was the introduction of more metal into the structure of the aircraft. This was made possible by the increasing numbers of shipments of steel from the USA.
The Yak-9 could be considered a greatly improved Yak-7, but ultimately it's hard to draw lines because of the continual stream of changes and improvements made. The late Yak-7's were, indeed, very close to the Yak-9 in appearance. But I see the Yak-9 as being a totally different aircraft than the vast majority of Yak-7's.
Doesn't matter, though. I like 'em all!
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