Nakajima B6N „Tenzan”

he Navy Carrier Torpedo Bomber “Tenzan” or Nakajima B6N, was codenamed Jill by the Allies. It was the replacement of the famous Nakajima B5N (Kate) torpedo bomber and although more B6N’s were built than B5N’s, the plane never achieved the spectacular success of its predecessor.

It entered service in the second half of 1943 but wouldn’t be used on a large scale until June, 1944 during the air and sea battles in the Philippine Sea for the Mariana Islands. Later it would be used in Taiwan, the Philippines, Iwo Jima and Okinawa and for Kamikaze attacks. The crushing air superiority of the Allies and the loss of nearly all the Japanese aircraft carriers and their trained pilots led to many Tenzans being lost, while never having had a chance to prove their worth. In spite of the fact that the Tenzan never achieved any real successes, it was undoubtedly one of the best carrier based torpedo bombers of WWII.
Design, Development, Production
In December, 1939, two years after the Nakajima B5N1 had been approved for production and a short while before the B5N2 prototype had its maiden flight, Kaigun Koku Hombu1  drew up the newest set of technical and tactical guidelines for a Navy Experimental 14-Shi Carrier Attack Aircraft, or 14-Shi Kanko2 . Their intent was to have a replacement for the B5N within two years, thus insuring a constant supply of modern torpedo bombers to the Imperial Japanese Naval Air Force (IJNAF).
The guidelines set out in the 14-Shi3  specification were for a three-man, low wing, cantilevered, all-metal plane. It size must conform to the carrier deck elevators. A top speed of 463 km/h, a cruising speed of 370 km/h and a range of 1,852 km with maximum bomb load or 3,333 km clean was required. Bomb load was to be 800 kg or one torpedo. For self defense a flexible 7.7 mm type 92 (Shiki) machine gun was to be mounted. The 14-Shi Kanko specifications were more demanding than those for the B5N2 and called for an increase in top speed of 85 km/h, in cruising speed of 111 km/h, in range with full armament of 572 km/h and in range without armament of 1,053 km/h.

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There was no contract bidding for the 14-Shi design, and the project was handed over directly to Nakajima Hikoki Kabushiki Kaisha (Nakajima Airplane Factory). It was assumed that the design team that had worked on the B5N would be most qualified to design its replacement. Engineer Kenichi Matsumura was designated head of the team which would work on the 3K (N-10) project. The 3K (N-10) was based on the airframe of its predecessor, while a radial engine with almost twice the power of the Nakajima “Sakae” 11 used on the B5N2 would deliver the increased performance. Kaigun Koku Hombu recommended a 14-cylinder “Kasei” radial engine made by Mitsubishi, but the designers insisted on using the Nakajima “Mamori” 11 radial engine which would deliver 1,870 hp. The Nakajima “Mamori” Model 11 (also know as Mamori 11-Gata or NK7A) was a completely new design and the 3K bomber would be one of the first airplanes to use it.
Work on the design went on through 1940 and the first prototype, the B6N1, was completed in March, 1941. It was a low wing, cantilevered, all-metal plane except for control surfaces, which were covered with fabric. The flat, tapered-rounded wings were in three sections; one central and two hinged for storage on an aircraft carrier, and had a dihedral of 6.5°. The wing span and area were similar to the B5N and the increase in overall weight due to the new powerplant caused a greater load on the wings. In order to compensate for this, the designers used Fowler flaps on the B6N instead of the regular flaps found on the B5N. The Fowler flaps were extended beyond the wing trailing edge on tracks and lowered 20° for take-off and 38° for landing. Although the flaps made a difference, the plane had a much higher stall speed than the B5N.
The B6N had a hydraulically retracted undercarriage with a tail wheel. The main landing gear was stowed into the underside of the wing center section towards the fuselage and the tail wheel was pulled up into the tail section. A tail hook hung in front of the tail wheel and was also retractable. The crew comprised of a pilot, navigator/bombardier and radio operator/gunner. As on the B5N the crew was under a seven part canopy. The torpedo rack was also like that of the B5N; mounted on the bottom of the fuselage 30 cm starboard from the centerline. The oil cooler was mounted on the bottom of the fuselage to port of the centerline, unlike on the B5N, where it was mounted on the centerline. A four-blade constant-speed propeller measuring 3.5 m in diameter was used on the prototype.

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