As promised, my final installment on Yak-4 derivatives ...
In 1940, Yakovlev produced a fighter variant of the Yak-2 recce-bomber. Designated BB-22IS (aka I-29), this 'least-mod' fighter derivative failed to impress during testing and was quickly abandoned. A more thorough re-design resulted in a heavy interceptor, internally designated BB-22TI (for Heavy Destroyer BB-22). This variant would have the crew compartments moved to the centre-of-gravity with a new nosecone containing a heavy cannon armament. Construction of a prototype was approved for evaluation.
The Yakovlev 'BB-22TI' was assigned an official designation of DIS-3 (for Dvukmotorny Istrebitel Soprovozhdeniya or Twin-Engine Escort Fighter). This fit better with VVS policy goals but, in fact, Alexandr Yakovlev had not intended for his design be an escort. Although ordered as the DIS-3 escort fighter, officialdom was quite aware that Yakovlev was building a heavy interceptor. [1] Based as it was on the BB-22 airframe, the prototype DIS-3 was completed quite quickly. It was flown without armour or armament but performance was not exceptional. The DIS-3 inherited the BB-22 series' instability.
There were also growing concerns about the DIS-3's intended armament installation. Could Soviet industry produce sufficient ShKAS cannons to satisfy general VVS needs let alone the 6-gun armament proposed for the DIS-3? In the end, the DIS-3 prototype never flew with its offensive armament. The entire programme was ultimately cancelled and the prototype was turned over to the Letno-Issledovatel'skii Institut - the VVS' Flight Research Institute - for purely experimental purposes.
Top LII-VVS Yakovlev DIS-3 fitted with four Western 'Gispano' 20 mm cannons for comparative testing with Soviet ShKAS guns, 1942.
A much more radical fighter development of the BB-22 design was the I-36 high-altitude interceptor. Devised in cooperation with the Moscow-based GAZ-1 factory, the Yakovlev I-36 combined a revised BB-22 airframe with the huge Mikulin AM-35 engine from the MiG-3 fighter. This combination was quite apropos. After building 81 BB-22s, GAZ-1 had switched to producing MiG-3 fighters. As such, the Moscow plant was ideally suited to building the I-36 airframe. However, the I-36 was no straightforward adaptation of the BB-22 to a single powerplant.
For its high-altitude role, the I-36 was to be fitted with twin turbosuperchargers - one mounted on each side of the rear fuselage. A large belly fairing contained a pair of engine coolant radiators flanking a large intercooler for the turbosuperchargers. In appearance, the I-36 resembled Yakovlev's I-26 frontal aviation fighter. But the I-36 was a much larger aircraft. It was ordered off the drawing board into limited production as a target defence interceptor for the PVO (Protivovozdushnoi Oborony) as the TVI-4 (Tyazhelyi Vysotny Istrebitel or Heavy High-Altitude Destroyer). [2]
As with the DIS-3, prototype construction was quickly completed but delivery of its turbosuperchargers and other specialized equipment was another matter. Once the German invasion was underway, doubts emerged about the wisdom of expending resources on defending targets which then might need to be immediately abandoned to the rapidly advancing enemy. When the TVI-4 prototype suffered a turbo fire on its fourth flight, a final nail was driven into the coffin of Yakovlev's high-altitude interceptor. GAZ-1 would continue with MiG-3 fighter production. The PVO would not return to a high-altitude fighter requirement until the middle of 1943. [3]
________________________
[1] Including 'escort' in the official designation may have been an attempt to dodge accusations of duplicating the heavy interceptor efforts of Vsevolod Tairov's design bureau.
[2] This designation was a strange one. The '4' in TVI-4 obviously related to 'Yak-4' but, under the Soviet system, fighter aircraft were usually given 'odd' number designations.
[3] This requirement would be satisfied by the I-220 (aka MiG-11) - a much smaller aircraft than Yakovlev's TVI-4 - but no production was undertaken. By the time that the MiG was under test, the threat of Luftwaffe bombers beyond the Urals had largely disappeared.