Nakajima B5N Kate

When Pacific War broke out, the Japanese naval aviation had the world’s most modern torpedo bomber—the Nakajima B5N2.

At that time, it was much better than the American Douglas TBD-1 Devastator and put the British Fairey Swordfish biplane one generation behind. The Japanese planes were faster, had better manoeuvrability, and could drop torpedoes from a greater height. After a successful debut at Pearl Harbour, B5Ns took part in all major naval battles up to 1944, permanently entering the history of aviation. Only at the turn of 1943/1944, this excellent plane began to be replaced by the modern Nakajima B6N Tenzan. But the road to B5N was not easy and was full of failures.

The Japanese Imperial Navy very early saw the need to have a modern torpedo-bomber, but subsequent development programs did not meet the expectations of Kaigun Koku Hombu. In the early 1930s, the Imperial Navy aviation had huge problems with equipping units with the appropriate class of aircraft of this type. The first Mitsubishi torpedo-bomber, the 1MT1 triplane, turned out to be unsuccessful and inconvenient to use due to its size. Further constructions improved the situation only temporarily.
The deck torpedo-bombers constituted an important factor in the Japanese doctrine of sea aviation, therefore the efforts to develop a world-class torpedo-bomber were not ceased. However, the competitions for a new machine announced every two years gave very poor results. In April 1932, Nakajima and Mitsubishi companies received the 7 Shi technical requirements for a new torpedo-bomber, but this time Kaigun Koku Hombu ordered the commencement of design work also at the Naval Aviation Arsenal in Yokosuka (Kaigun Kokusho) to protect against another failure. Soon there was a design for a fixed-undercarriage biplane with a mixed structure, powered by a Hiro Type 91 Model 2 V-engine with power of 750 hp. The assembly of the first prototype was completed the same year. Although it was a completely new design, the machine was given the designation “Modernized Type 13 Deck Torpedo-Bomber”, implying that it was a development of the Mitsubishi B2M.

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As feared, the results of the 7 Shi competition were tragic. Both Mitsubishi (3MT10) and Nakajima (B3N1) prototypes were lost in crashes, and Aichi’s prototype—AB-8—which entered into the competition as a private venture, performed only slightly better than machines of that time. In this situation, despite the fact that the aircraft developed in Yokosuka proved difficult to fly during the tests and had great problems with maintaining air stability, it was decided to develop it further. In August 1933, the prototype was accepted for serial production. It was designated as “Type 92 Model 1 Deck Torpedo-Bomber” (B3Y1).
In total, about 130 aircraft of this type had been made by 1936, when production was halted. Type 92 Model 1 took an active part in the second Japanese Chinese conflict, taking off from the decks of aircraft carriers and land bases. B3Y1 was plagued by frequent engine failures, and it quickly turned out that they were ineffective. It was clear that the successor should be seriously considered.
In 1934, the command of the Imperial Navy announced an urgent need for a new torpedo-bomber that could replace the “not-so-successful” B3Y1. In this situation, Kaigun Koku Hombu developed the specification 9 Shi. In February 1934, Mitsubishi and Nakajima were traditionally invited to participate in the competition. In addition to them, the Yokosuka Naval Aviation Arsenal was also involved to develop its own project.
At Mitsubishi construction bureau, the development of the new aircraft was progressing fast. A team of designers led by Eng. Hajime Matsuhara, the creator of 3MT10, based on the earlier design, which accelerated the works enormously. The analysis of the causes of the 3MT10 failure showed that the aircraft had too much weight in relation to engine power. At the same time flight tests showed that the radial engines are more reliable, and one of the main criticisms of earlier designs was the failure rate of the in-line engines. Taking these observations into account, Eng. Matsuhara decided to use a radial engine on the new Ka-12 torpedo-bomber (Ka-12 was a factory designation given to this machine), because, in addition to greater reliability, this solution allowed to reduce the weight of the aircraft. On the assumption that a proprietary product is better, the Mitsubishi A-9 engine, which still was developed, was chosen instead of a refined Nakajima Hikari 1 engine with similar power. The latter turned out to be a big failure. As early as August 1934, a prototype of the new aircraft was ready. It was a classic mixed-structure biplane with a fixed landing gear. The machine took off for the first time from the Kagamigahara airfield on August 25, 1934. After handing it over to the Navy for further trials, the aircraft was given the designation “9 Shi Experimental Torpedo-Bomber” (B4M1).


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