Soviet Heavy Fighters 1926–1949

However, as was often the case in the USSR, that was not the end of changes to the technical requirements of the new aircraft. On October 26, 1927 a letter arrived at TsAGI offices, which contained a revised version of the document. The Air Force apparently went back to the idea of a four-seat, long-range reconnaissance platform carrying a payload increased to 725 kg. More changes were introduced on November 6, the scope of which rendered the existing design practically unusable. The required armament was increased from four to eight Lewis machine guns, while useful payload was once again raised – this time to 890 kg. The new draft of technical requirements called for gunner stations in engine nacelles, in addition to a pair of fuselage turrets. The crew was increased to five: pilot, co-pilot (doubling as a radio operator and rear gunner), navigator (also manning nose gun) and two engine nacelle gunners. The role of the future aircraft was described in the document as follows: “The key role of the proposed design would be to conduct reconnaissance deep behind enemy lines countering the threat of most modern and the best enemy fighters. In addition, the aircraft would have to be capable of performing escort missions on long-range bombing raids.” It was clear the new requirements would inevitably lead to increasing the overall size of the design and, what followed, degrading its performance. While the original estimates put the “cruiser’s” maximum speed at 215 – 220 km/h, that value was now described as “above 160 km/h”. Priority was given to the field of view and arc of fire, while maneuverability was way down as number six on the list. It corresponded to the requirement that the R-6 should be able to “…engage in a defensive battle with several enemy fighters at the same time”, using to its advantage spherical field of fire, rather than speed or agility.
Tupolev persevered in his lobbying tactics and eventually reached a compromise with the Air Force brass. In its final iteration the technical requirements defined the R-6 as an “air-defense fighter and army reconnaissance platform” (the “army” designation indicated “long-range” character of the mission). The aircraft once again became a four-seater with a payload of 700 – 725 kg. Gunner stations in engine nacelles were to be replaced with a retractable ventral turret. Most importantly, the overall airframe design was left largely unchanged, which was good news for TsAGI staff, who had already started, back in August 1927, manufacturing some of the components for the R-6 prototype.
The question of which powerplants would best suit the new design was still unanswered. The initial choice was a pair of Hispano-Suiza units rated at 450 and, later, 520 hp. Later the use of German BMW-VI motors was contemplated (680 – 730 hp, depending on the variant), while in January 1928 the British 480 hp Bristol Jupiter radials were eyed as a possible choice.
The work on the new design progressed at a good rate and by March 19, 1928 a full-scale wooden mockup was ready. After implementing several changes to the cockpit layout and equipment setup, the mockup was officially approved in July 1928. The Air Force had high hopes for the new aircraft, so they green-lighted full-scale production even before the prototype was ready for its first flight. The delays in prototype assembly were due mainly to problems with subcontractors and the VVS, who failed to deliver the TUR-6 gun turrets on time. Despite those hiccups, TsAGI team made up some of the lost time and finally delivered the R-6 prototype powered by a pair of BMW-VIE engines (500 – 730hp).

CiezkieSowieckie zd5

 Flight test program
The maiden flight of the R-6 prototype was a short hop flown by M.M. Gromov. Three minutes into the sortie the starboard engine began to overheat and then seized up, forcing Gromov to put the aircraft down in a hurry. He came in perpendicular to the runway, which almost cost him his life. The root cause of the engine failure was a cracked radiator, which bled engine coolant at an alarming rate. Following Gromov’s advice, the rudder was enlarged to provide better authority. During the second sortie overcompensation of elevators became apparent and it wasn’t until the third flight that all flight controls were deemed to work fine.