Tupolev SB

The Tupolev ANT-40, also known by its service name Tupolev SB and development co-name TsAGI-40, was a high speed twin-engined three-seat monoplane bomber, first flown in 1934. The Tupolev design was advanced but lacked refinement, much to the dismay of crews, maintenance personnel, and Stalin, who pointed out that "there are no trivialities in aviation".
Numerically the most important bomber in the world in the late 1930s, the SB was the first modern stressed skin aircraft produced in quantity in the Soviet Union and probably the most formidable bomber of the mid-1930s. It was produced in the Soviet Union and was also built under license in Czechoslovakia. Many versions saw extensive action in Spain, the Republic of China, Mongolia, Finland and at the beginning of World War II against Germany in 1941. It was also used in various duties in civil variants, as trainers and in many secondary roles. Successful in the Spanish Civil War because it outpaced most fighters present, the aircraft was obsolete by 1941 as faster fighters had by then been introduced. By June 1941, 94 percent of bombers in the Red Army air force Red Army were SBs.

Development

In 1933 the Soviet Air Force ministry issued an outline requirement for a high-speed bomber. Work on this proposal at TsAGI began in January 1934. The SB was designed and developed in the Tupolev KB by a team led by A. A. Arkhangelski. Two versions were planned, one with Wright Cyclone radial engines, and one with the Hispano-Suiza 12Y liquid-cooled V12 engines. The skills gained in the design of the MI-3 and DI-8 aircraft were widely used. The first two prototypes were called ANT-40.1 and ANT-40.2. The Cyclone powered prototype flew first, on 7 October 1934, while the first Hispano-Suiza powered prototype, which featured a larger wing, flew on 30 December 1934, demonstrating superior performance.
The second Hispano-Suiza powered aircraft, the ANT-40₂ was considered a production prototype, and its performance was impressive but it was plagued by teething problems, leading unhappy test personnel to cover the ANT-40₂ with placards listing the aircraft's defects prior to a visit by Sergo Ordzhonikidze, the Commissar for Heavy Industry. On seeing these placards, Ordzhonikidze summoned Tupolev to a meeting at the Kremlin to discuss these shortfalls. When Tupolev stated that most of the defects were trivial, Joseph Stalin said,
The first production aircraft, designated SB, rolled off the production line before the end of 1935, and before ANT-40₂ had completed its flight test programme. The aircraft entered full production in 1936, and was produced in two plants, State Aircraft Factory No 22 at Moscow and No 125 at Irkutsk until 1941.
Despite the fact that the assembly lines were plagued with a constant string of modifications, some 400 SBs were delivered by the end of 1936—a number of these being diverted to Spain—and 24 Soviet Air Forces squadrons were in the process of working up with the new bomber. Giving excellent performance in the Spanish Civil War, it acquired the popular name "Katyusha".
In 1937, negotiations were concluded between the Soviet and Czechoslovak governments, for the supply of SB bombers and a licence for local production, in exchange for the right to produce the Škoda 75 mm Model 1936 mountain gun. The version of the SB to be supplied to, and subsequently license-built as the Avia B-71 was the SB 2M-100A but fitted with the Avia-built Hispano-Suiza 12-Ydrs engine. A 7.92 mm vz. 30 machine gun supplanted the twin ShKAS machine guns in the nose and similar weapons were provided for the dorsal and ventral stations. Sixty aircraft were to be flown to Czechoslovakia by mid-1938. The planned licensed production program took a decidedly leisurely course, despite the increasingly dangerous political situation. By 15 March 1939, when the German Wehrmacht occupied Bohemia and Moravia, not one Czech-built aircraft had been delivered.
Development of the SB continued with revisions being made to reflect the lessons of early operations in Spain. As problems were encountered in converting pilots to fly the SB, a trainer version, the USB was built in September 1937, with a modified nose with an open cockpit for an instructor and dual controls. Problems were also encountered with the armament, the nose guns having limited traverse and so being little use against head-on attacks and later aircraft were modified with a better field of fire. From 1940, the dorsal gun position was replaced by an enclosed turret, while the ventral gun position, which was difficult to use, was also modified.
The aircraft was also progressively fitted with improved engines. At first it was equipped with the Klimov M-100, a license-built version of the Hispano-Suiza 12Ybrs engine, but this was soon replaced by the more powerful M-100A, and from 1938 by the yet more powerful M-103. While the engine installation of the SB 2-M103 initially retained the drag inducing frontal radiators of the M-100 powered aircraft, an improved engine installation was developed with the radiators slung under the engines. On 2 September 1937 M.Yu. Alexeev set an official altitude record of with load of in an M-103 powered SB. He had earlier set an unofficial record of.
In an attempt to further improve the performance of the SB, which by 1939 was becoming obsolete, the development of two second-generation versions were authorised, a direct replacement for the SB and a specialised dive bomber. The level bomber, known as the SN-MN or MMN, had a new wing of reduced wing area and was powered by more powerful Klimov M-105 engines. Performance was little better than the standard aircraft and it was abandoned. The dive-bomber, SB-RK was similar to the MMN but was fitted with dive brakes and it was ordered into production.
Even though the SB was no longer a state of the art aircraft, production continued to increase through 1939 and 1940, as the Soviet Union tried to build up the strength of their air forces to compete with the growing threat of Nazi Germany, with almost 4,000 being built in these two years. The SB was phased out of production in early 1941, being replaced by the Petlyakov Pe-2. A total of 5,695 were built at Factory No 22 at Moscow before it was evacuated to Kazan, while Factory No 125 built a further 1,136 at Irkutsk. Three prototypes were built at the Tupolev design bureau, while Aero Vodochody and Avia in Czechoslovakia built 45 and 66 respectively, giving a total of 6,945 built.

Design

The SB was an all-metal monoplane powered by two Klimov M-100 12-cylinder water-cooled engines which drove fixed-pitch two-bladed metal propellers. The engines were provided with honeycomb-type frontal radiators enclosed by vertical thermostat-controlled cooling shutters. At an early production stage, the M-100 engine gave place to an improved M-100A engine, driving ground-adjustable three-pitch propellers, with speed being boosted to at. Because of its broad, high aspect ratio wing, that gave it a good altitude performance – Soviet crews nicknamed the SB the "Pterodactyl".

Operational history

There were a number of foreign customers for the SB. They were mostly satisfied with the aircraft's performance. There were some complaints about the noise, cramped crew compartments, hard undercarriage suspension and in particular about the front gunner's position, which could be reached only through a hatch under the fuselage, preventing the gunner from escaping in the event of a ditching or belly landing. Czechoslovakia signed an agreement to produce the ANT-40 as Avia B-71.

Spanish Civil War

While only 54 SBs had been delivered to the Soviet Air Forces by 1 July 1936, this did not stop the new Tupolev bomber being amongst the first shipments of military equipment sent by the Soviet Union to support the Spanish Republicans when the Spanish Civil War broke out on 17 July 1936. An initial batch of 31 SBs arrived in Cartagena aboard the Soviet Freighter Komsomol in October 1936, flying their first mission, a bombing raid by four SBs against Tablada airfield, Seville on 28 October. The SBs were used to equip Grupo 12 of the Spanish Republican Air Force, which at first was mainly manned by Soviet volunteers and under Soviet control.
The SB could outpace the Fiat CR.32 and Heinkel He 51 biplane fighters of the nationalist forces, and was therefore difficult to intercept, with dives from high altitude being the only way to intercept the SB. On 29 May 1937 two SBs attacked the German pocket battleship Deutschland, mistaking it for the Nationalist cruiser Canarias, killing 31 and injuring an additional 83 German sailors. In June–July, a second consignment of 31 SBs were received, allowing Grupo 12 to return to full strength, and a new unit, Grupo 24, to be established. The delivery of Messerschmitt Bf 109s to re-equip the German Condor Legion meant that the SB could no longer evade Nationalist fighters by sheer speed, and losses rose.
A third and final batch of 31 SBs arrived in June 1938, allowing operations to continue, although losses continued to be high. By the time the Civil War ended in April 1939, 73 SBs had been lost, 40 of them to enemy action. Nineteen SBs were taken over by the Nationalists, and used to form a bomber squadron. Although some were re-engined with French Hispano-Suiza 12Ybrs engines to aid maintenance, they were still subject to spares shortages, and in April 1943 only three were airworthy. When Junkers Ju 88s were received in December 1943, the remaining SBs were used for occasional training flights until withdrawn and scrapped in 1948.

China

In July 1937, the Second Sino-Japanese War broke out. The Soviet Union signed the Sino-Soviet Non-Aggression Pact on 21 August 1937, and as part of this agreement, supplied large amounts of military equipment to the Chinese Nationalists, as well as deploying complete air force units, nominally manned by Soviet volunteers. An initial delivery of 62 SBs was made in September–October 1937, with combat operations by Soviet forces starting in December with attacks on Japanese ships on the Yangtze River. On 23 February 1938, to celebrate Soviet Army Day, Soviet SBs carried out a long range attack on Japanese airfields on Taiwan, claiming 40 Japanese aircraft destroyed on the ground.
A further 60 SBs were delivered to China in early 1938, these being heavily used to attack Japanese forces during the Battle of Wuhan. Losses were heavy, forcing the Chinese SB units to be temporarily withdrawn from combat. The Soviet units operating the SB over China re-equipped with the Ilyushin DB-3 in 1939, allowing their SBs to be transferred to Chinese units, but the Chinese made limited use of these reinforcements.
The Soviet Union supplied a further 100 SBs in 1941, just before it signed the Soviet–Japanese Neutrality Pact. The SB was gradually phased out of front-line operations against the Japanese with the delivery of more modern American bombers from 1942, being partly replaced by Lockheed Hudsons and B-25 Mitchells. Limited numbers of SBs continued in non-combat use, including operations against opium plantations, before being used against the Communists when the Chinese Civil War flared up in 1945, being finally withdrawn in 1946.