Nakajima B5N Kate

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that in the event of a war with any enemy except China, there would be a catastrophe, as the biplanes are not only outdated, but also unable to fulfil their tasks.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..

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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.

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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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