Nakajima B6N „Tenzan”

The maiden flight of the B6N took place on March 14, 1941. Soon after, test flights would be carried out by pilots from the Navy Arsenal in Yokosuka4  Hiko Jikkenbu and Yokosuka Kokutai squadron which exposed a number of serious problems with the design. The first problem to be fixed was the tendency of the plane to roll during flight due to the powerful torque of the four-bladed propeller. In order to correct this the tail fin was thinned and keyed 2° 6’ to port. Test flights made at the end of 1942 on the carriers “Ryuho” and “Zuikaku” revealed a weakness in the tail hook assembly, which was quickly fixed. The toughest problems for the engineers were those with the “Mamori” engine, which turned out to be highly undependable. At certain speeds it caused major vibrations, overheated and never achieved its planned power ratings. The engine wasn’t accepted for use until the end of 1942 and after many modifications had been made. After almost two years since the first prototype had been built, the B6N1 was officially accepted by Kaigun Koku Hombu for series production as the Kanko Tenzan Ichi-Ichi-Gata, or Navy Carrier Attack Aircraft Tenzan Model 11. The word Tenzan literally means “Heavenly Mountain” and was the Japanese name for the Tien Shan mountain range in China.
The B6N1 Model 11 differed from the prototypes as a result of further modifications introduced during production. It was fitted with a second flexible machine gun (Type 92) mounted in a ventral tunnel at the rear of the cockpit. A 7.7 mm Type 97 (97-Shiki) with a 400-round magazine was mounted in the center wing section on the port side. The effectiveness of this armament was questionable (small caliber arms were useless against either surface targets or enemy fighters) and from the seventy-first plane to roll off the assembly line, this type of armament was removed. The main landing gear was strengthened, the torpedo rack, positioned on the starboard side of the belly, was angled down 2° and thinner, longer exhaust stacks were used. Tests were conducted using rocket assisted take-off on a few B6N1’s for use on small carriers, but the concept was considered too complicated and offered no solution to the problem of landing such a heavy plane on a short deck. The designers proposed replacing the unprotected fuel tanks with self-sealing ones, but this resulted in a 30% reduction in the B6N1’s fuel capacity, and the Navy insisted on using the original fuel tanks to maintain range. A total of 133 B6N1 Model 11’s were built in the Nakajima plant in Koizumi between February and July, 1943

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The planned date for replacing the B5N with the B6N was sometime in 1941, but the delays during development forced the Navy to renew production of the B5N2 in 1942. Meanwhile, problems with the new bomber continued. Soon after production of the B6N1 Model 11 had begun, Kaigun Koku Hombu ordered a halt to the production of the “Mamori” engine. It had been decided for logistical reasons to unify the engines being used by the Navy as much as possible. The Mitsubishi “Kasei” was dependable and widely used, and the new Nakajima “Homare” engine looked very promising and used the same pistons as the mass produced “Sakae” engine. Kaigun Koku Hombu ordered the B6N1 to be adapted to use the “Homare.” This engine would be used on many of the planes designed at the time, including the Nakajima C6N “Saiun” observation plane, the Kawanishi N1K-J “Shiden” fighter plane and the Kugisho P1Y “Ginga” bomber.

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With the “Homare” just entering production and the need for bombers at a critical level, the Nakajima engineers first adapted the B6N to use the Mitsubishi “Kasei” engine, which was readily available. The 1,850 hp fourteen cylinder Mitsubishi “Kasei” Model 25 (Kasei 25-Gata or MK4T) was picked for its size, which was close enough to the “Mamori” to make installation trouble-free. It’s lighter weight forced the need to extend the nose to maintain the correct center of gravity. Following this were changes made to the oil cooler intake, the engine cowling and air intake. A new four-blade propeller with a smaller (3.4 m) diameter, wider blades and shorter spinner was fitted. The adaptations for the “Kasei” engine made this version of the plane somewhat longer than the B6N1.
Additionally, the exhaust stacks were changed from the original two to individual ones for each cylinder. This was to eliminate flames which were blinding the pilots during night maneuvers, and also helped by adding some drag. The oil reserve was reduced from 90 liters to 70 liters and the fuel capacity was increased by 10 liters. The tail wheel was mounted in a permanent position, as its ­retraction was considered too much of a luxury. The overall weight of the plane dropped by 140 kg, and even with the weaker engine, the plane reached a top speed of 482 km/h, and it had a better climb rate. The armament was not modified.

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Kaigun Koku Hombu named the new plane Kanko Tenzan Ichi-Ni-Gata, or Navy Carrier Attack Aircraft Tenzan Model 12 (B6N2 Model 12), and production was begun in June, 1943. In November, 1944, changes were made to the armament based on suggestions from front line combat groups. The dorsal Type 92 machine gun was replaced with a 2‑Shiki (Type-2) 13mm machine gun, and the ventral machine gun was replaced with a 1-Shiki (Type-1) 7.92 mm machine gun. The radio was upgraded to the newer model 96-Gata Ku 3-Go 4-Gata. This model was named the Kanko Tenzan Ichi-Ni-Ko-Gata, or Navy Carrier Attack Aircraft Tenzan Model 12A (B6N2a Model 12A) and was produced till the war ended in August, 1945. Nakajima manufactured a total of 1,133 B6N2’s and B6N2a’s at their plants in Okawa in the Gumma prefecture and at Handa in the Aichi prefecture. According to factory records a total of 1,268 B6N’s including prototypes, B6N1’s, B6N2’s and B6N2a’s were built: the first 298 at Koziumi and 970 at Handa. According to Navy records only 1,262 planes were built. Production was slow, with only 18 B6N2’s built by October, 1943 and a maximum rate of only 90 planes per month was ever achieved. From the fall of 1943 about 1/3 of the planes leaving the factory were equipped with 3-Shiki Type 3 ship-seeking radar with Yaga antennas mounted on the wing leading edges and on the sides of the rear fuselage. The planes with radar were not marked in any special way.

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