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.
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).
Shortly after the announcement of the 9 Shi’s design objectives, Nakajima’s engineers Takao Yoshoda and Yasuo Fukuda projected the new aircraft. It was given the factory designation “Q”, and later the military designation “9 Shi Experimental Torpedo-Bomber” (B4N1). It was a classic fixed-undercarriage biplane with a mixed construction.
The work was delayed, and two prototypes were completed only in 1936. The first one was powered by the Nakajima Hikari 1 engine, and the second used the Nakajima Kotobuki 3 power unit. However, none of the above-mentioned planes met the assumed requirements and was not approved by the Imperial Navy.
The situation of the deck aviation, as in the case of the 7 Shi requirements, forced the mass production of the B4Y1 (developed in Kugisho). It was considered the best option, although it did not meet all the expectations.
At the Aviation Arsenal in Yokosuka, a team of designers working on the development of a new torpedo-bomber was headed by Eng. Sanae Kawasaki. The design of the machine, which was given the designation “9 Shi Experimental Torpedo-Bomber” (B4Y1), made it possible to install various types of engines. In order to make the construction pace as fast as possible, generally available materials and components were used during the construction. For this purpose, a new fuselage structure was developed, and its tail and wings were adapted from the Kawanishi E7K1 reconnaissance seaplane. The plane had a mixed structure and a fixed landing gear. The pilot’s cabin was uncovered, and the remaining cabins were covered with a common, richly glazed canopy. The first prototype, which was completed and flown in 1935, was powered by a 620 hp Hiro Type 91-I V-engine. Four more prototypes were built over the course of the following year. They had an increased wingspan and new power units. The second and third prototypes were powered by Nakajima Kotobuki 3 engine with a take-off power of 640 hp. Fourth and fifth prototype received Nakajima Hikari 2 radial engine with a take-off power of 840 hp. The tests showed that the last two prototypes had the best performance, and they were just selected for the comparative tests with the B4M1 and B4N1. In November 1936, Kaigun Koku Hombu officially accepted the B4Y1 prototype into series production, giving it the designation “Type 96 Torpedo-Bomber”. As with the B3Y1, the production of the new machine was split between Nakajima, Mitsubishi, and the Hiro Naval Aviation Arsenal (Kaigun Kokusho). Production was discontinued in 1938. A total of 205 copies were built.
Until 1940, the B4Y1 aircraft operated from the “Akagi”, “Kaga”, “Sōryū” and “Ryūjō” carriers, actively participating in the second Japanese Chinese conflict. They were not very modern and significantly differed in performance from the Mitsubishi A5M deck fighters. That is why the cooperation of the individual airborne units was very difficult. After the outbreak of the Pacific War, the B4Y1 planes were already moved to second line and training units. Only the oldest Japanese carrier, “Hōshō”, still had eight machines of this type, but after commencing combat operations, it was rearmed with Nakajima B5N2 torpedo-bombers. In the Allied code, B4Y1 received the designation Jean.
Birth of B5N
In 1936, the obsolete and “emergency” torpedo-bombers were replaced with newer B4Y1s, but this was a temporary solution. The new plane had a top speed of 277 km/h and a range of 1,574 km, which was enough at that time, but it could not be enough in future. In 1935, the monoplane fighter, later known as the Mitsubishi A5M Claude, made its first flights, reaching a speed of 449 km/h at an altitude of 3,000 m and proving that the era of biplanes came to an end.
In 1935, Kaigun Koku Hombu released the technical order of the 10 Shi containing radically new requirements resulting from a completely new approach to the subject of a deck torpedo-bomber. The given characteristics were much higher than any aircraft of this type developed so far. The set task was very ambitious, but both Mitsubishi and Nakajima decided to try their hand at this competition.
The 10 Shi requirements expected that the new airplane should reach a maximum speed of not less than 330 km/h and have an overall performance impossible to achieve by a biplane. During the tests of the A5M Claude there were problems with deviations from the course in the last stages of the landing approach, which could be very dangerous for the aircraft landing on the carrier, but it was found that the problem could be solved by the use of flaps, which at that time were considered as novelty, although in their early form they were already used during the World War I.
A great help for the constructors was getting acquainted with the Northrop Gamma 5A plane. Its only prototype (registration number X 14997) was purchased in order to learn about new technologies. On October 29, 1935, it was sent from USA to Japan. The machine was handed over to the Navy, where it was given the designation BXN1. During the tests, the plane was crashed and completely destroyed, but before it happened, a lot of valuable information was collected.
The Gamma 5A, for its times, had many innovative solutions. It used, among other novelties, the results of the flap work carried out at NACA in 1933. In the USA, its design was modernized by adding a retractable landing gear and a more powerful engine, thus creating the Northrop A-17 light bomber, some of which were used by the RAF during World War II under the designation Nomad. The Gamma 5A design solutions have also been used as a starting point by Douglas in their TBD Devastator and SBD Dauntless projects. Eng. Jiro Horikoshi (the creator of the famous A6M Zero) believed that this plane was the most important for the Japanese from all those bought abroad in the 1930s. The influence of the data gathered during the analysis of its construction and capabilities was enormous on further Japanese designs.
The 10 Shi specification stated that the wingspan of the new aircraft should not exceed 16 m, and 7.5 m when folded to be hangared. These limitations were due to the size of the lift platforms on Japanese carriers. The plane should be capable of carrying 800 kg of bombs or torpedoes, and the defence armament was to consist of one 7.7 mm machine gun. At an altitude of 2,000 m, the maximum speed was to be 330 km/h. Normal flight duration should be 4 hours, while on economic speed of 250 km/h, the maximum flight duration should last 7 hours. The new machine was to be powered by a Nakajima Hikari or Mitsubishi Kinsei radial engine, and the crew was to be three people.
Earlier failures in the development of a modern torpedo-bomber under the 7 and 9 Shi programs made the 10 Shi program practically the last chance to catch up with the world leaders in this field and Kaigun Koku Hombu had high hopes for it. The Naval Aviation Command was well aware that in the event of a war with any enemy except China, there would be a catastrophe, as the biplanes are not only outdated, but also unable to fulfil their tasks. A critical situation arose. Time was running and the Japanese lagged further and further behind their likely opponent..
Two companies took part in the competition: Nakajima Hikoki Kabushiki Kaisha and Mitsubishi Jukogyo Kabushiki Kaisha. Shortly after the competition was announced, the Nakajima team of designers presented a preliminary design of the machine, which was given the factory designation Type K. The aircraft was to be an all-metal, cantilever low-wing, with a working sheathing and a hydraulically retractable main landing gear. The landing gear mechanism of the future B5N was based on the solutions used in the Northrop A-17, and the B5N itself was one of the first aircraft of this type in Japan. The long, covered cabin housed three crew members: a pilot, a bombardier-navigator, and a radioman-machine gunner. The wing with a trapezoidal outline was divided into a centre wing and two hydraulically folding consoles. When folded, they overlap over the richly glazed canopy. Thanks to this solution, the plane needed much less space in the carrier’s hangar. Such a complicated hydraulic system was used in Japan for the first time, therefore the problems related to its functioning that arose during the tests were not surprising and were successfully solved, while the experience gained was used in later designs.
In the first draft of the preliminary design, the fuselage of the plane was slim, but long, so the plane would barely fit on the lift platform. Therefore, in the second draft of the project, the fuselage was shortened to 10.3 m and Fowler flaps and a three-bladed metal constant speed propeller with variable pitch were used. The plane was to be powered by a Nakajima Hikari 2 9-cylinder radial engine with a take-off power of 840 hp, equipped with a NACA cowling, characterized by a low aerodynamic drag.
The design of the new aircraft in this form was submitted to the competition. Soon it was approved, and the construction of the prototype began. The assembly was completed in December 1936. It was given the military designation “10 Shi Experimental Torpedo-Bomber” (B5N1). The machine first took off in January 1937. Despite the use of a relatively low-power engine, the aircraft developed a high, at that time, top speed of 370 km/h, far exceeding the specifications. The designers from Nakamura’s team, however, were far from euphoric, because during the tests, both on the ground and in the air, many defects were revealed. The hydraulic system for folding the main landing gear and wingtips was the most problematic issue, but there were many more minor problems, some of which were very difficult to fix. One of the reasons was the fact of using many new solutions in the construction of the aircraft, which were not fully refined and, as a result, caused problems. However, very good performance and flight properties spoke in favour of the project.
The Naval Aviation Command was very incredulous about Nakajima’s innovative approach, fearing that the complex structure would make the aircraft more difficult to operate and increase its failure rate. Problems with the hydraulic system seemed to confirm these concerns. Considering the above, the designers were instructed to simplify the construction of the second prototype. Instead of a hydraulic one, a somewhat crude but reliable manual wing folding mechanism was used, and Fowler flaps, which also caused problems, were replaced by classic slotted flaps. Integral fuel tanks with a capacity of 1,150 l were placed in the centre wing, and the engine was switched to a Hikari 3 with a take-off power of 720 hp. It was not planned to use any protection for the cockpit and fuel tanks. In order to enable the aircraft to carry bombs or torpedoes (depending on the need), various suspension nodes were developed, which the technical staff could quickly replace or remove.
The good forward visibility from the cockpit was essential for the deck plane pilot. Unfortunately, with the tail down, it was very poor on B5N1, that’s why during take-off and landing the pilot’s seat could be raised so his head was levelled with the upper edge of the windscreen. The navigator-bombardier/observer sat behind the pilot, facing forward, and had two small windows in the sides of his cabin to allow observation of fuel consumption indicators located on the upper surface of the centre wing. In order to target the bomb drop, he opened a small door in the floor. The radioman-machine gunner sat with his back to the direction of flight, most often with a retracted machine gun and a closed cockpit cover, which improved the aerodynamics of the aircraft. Early radio stations operated at low frequencies and were equipped with a long, droppable antenna. Communication between crew members was via a voice tube, and oxygen equipment was usually not fitted.
In this form, the second prototype began comparative tests with its rival—the Mitsubishi B5M1, and then went into the mass production.
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