Nakajima B6N - Design and Development

Design and Development

The B5N carrier torpedo-bomber's weaknesses had shown themselves early in the Second Sino-Japanese War and, as well as updating that aircraft, the Imperial Japanese Navy began seeking a faster longer-ranged replacement. In December 1939 it issued a specification to Nakajima for a Navy Experimental 14-Shi Carrier Attack Aircraft capable of carrying the same external weapons load as the B5N. The new plane was to carry a crew of three (pilot, navigator/bombardier and radio operator/gunner) and be of low wing, cantilevered, all-metal construction (though control surfaces were fabric-covered). Further requirements included a top speed of 250 knots (290 mph; 460 km/h), a cruising speed of 200 knots (230 mph; 370 km/h) and a range of 1,000 nmi (1,900 km; 1,200 mi) with an 800 kg (1,800 lb) bomb load or 2,072 nmi (3,837 km; 2,384 mi) without external armament.

The Navy had requested installation of the proven Mitsubishi Kasei engine as the B6N's powerplant but Engineer Kenichi Matsumara insisted on using Nakajima's new 1,870 hp (1,390 kW) Mamori 11 14-cylinder air-cooled radial due to its lower fuel consumption and greater adaptability. This became an unfortunate choice as the Mamori engine was plagued with mechanical defects and never achieved its expected power rating.

Constrained by the standard-sized aircraft elevators then in use on most Japanese carriers, designer Matsumara was obliged to use a wing similar in span and area as that of the B5N and to limit the aircraft's overall length to 11 m (36 ft). This latter restriction accounted for the B6N's distinctive swept-forward tail fin and rudder. The outer wing panels folded upward hydraulically, reducing the B6N's overall span from 14.9 m (49 ft) to approximately 6.3 m (21 ft) for minimal carrier stowage. In order to lessen increased wingloading due to the heavier powerplant, Fowler flaps were installed which could be extended beyond the wing's trailing edge. These were normally lowered to an angle of 20 degrees during take-off and 38 degrees when landing. Despite the use of these flaps, however, the B6N had a much higher stall speed than its predecessor.

The prototype B6N1 made its maiden flight on 14 March 1941. Following continued testing, however, several problems became evident. In particular, the aircraft exhibited an alarming tendency to roll while in flight, the cause of which was traced to the extreme torque developed by the four-bladed propeller. To compensate, the aircraft's tail fin was thinned down and moved 2 degrees ten minutes to port. This modification greatly improved the plane's handling characteristics.

The B6N1's Mamori 11 engine was found prone to severe vibrations and overheating at certain speeds and was at first judged too unreliable (an important consideration given that the plane was expected to fly long distances over open water). Following a series of modifications, though, the engine's performance was finally deemed promising enough that carrier acceptance trials were begun at the end of 1942. Subsequent test flights conducted aboard the carriers Ryuho and Zuikaku indicated the need to strengthen the tail hook mounting on the plane's fuselage. Some attempts were also made to use RATOG (rocket-assisted take-off gear) units on several B6N1s in order to qualify the aircraft for use on smaller carriers but the results were unsatisfactory.

The B6N1 was officially approved for production status in early 1943 and given the designation Navy Carrier Attack Aircraft Tenzan Model 11. Modifications based on testing of the initial prototypes included: the addition of a flexible Type 92 machine-gun in a ventral tunnel at the rear of the cockpit and a 7.7mm Type 97 machine-gun to the port wing (the latter was eventually deleted after the seventieth production aircraft); angling the torpedo mounting rack 2 degrees downward and adding torpedo stabilization plates to prevent the torpedo from bouncing during low-altitude release; strengthening of the main landing gear. A proposal by the designers to replace the B6N1's unprotected fuel tanks with self-sealing ones would have resulted in a 30% drop in fuel capacity, a loss in range the Navy decided was unacceptable.

After only 133 B6N1s had been produced by July 1943, the Japanese Ministry of Munitions ordered Nakajima to halt manufacture of the Mamori 11 engine in order that the Navy reduce the number of different engines then in use. Pending availability of the 18-cylinder Nakajima Homare engine, Nakajima was asked to substitute the 1,850 hp (1,380 kW) Mitsubishi MK4T Kasei 25 engine on the B6N1 airframe, the very engine the Navy had originally requested them to use. As the Mamori 11 and Kasei 25 were similar in size, installation was relatively straightforward, requiring only that the nose be extended to maintain the aircraft's center of gravity and minor alterations to the oil cooler and air intakes on the engine cowling. A smaller 3.4 m (11 ft) diameter four-bladed propeller and shorter spinner were also installed at this time and a fixed tail wheel replaced the original retractable one, resulting in a small weight-savings. Finally, the single exhaust stacks on either side of the engine cowling were replaced with multiple smaller stubs to reduce glare at night. The resulting modification was designated Navy Carrier Attack Aircraft Tenzan Model 12 or B6N2.

Starting in the fall of 1943, one of every three B6N2s manufactured was equipped with 3-Shiki Type 3 air-to-surface radar for detecting enemy ships. Yagi antennas were installed along the wing leading edges and also protruded from the sides of the rear fuselage.

A final version of the aircraft, designated B6N3 Model 13, was planned for land-based use as, by this point in the war, all of Japan's large carriers had been sunk and those few smaller ones remaining lacked catapults for launching heavier carrier-borne aircraft like the B6N. Changes included installation of a Kasei Model 25c engine, a more streamlined engine cowling and crew canopy, strengthening of the main landing gear, a retractable tail wheel and removal of the tail hook. Two B6N3 prototypes were completed but Japan surrendered before this variant could be put into production.

By war's end in August 1945, Nakajima had completed a total of 1,268 B6Ns (almost all of them B6N2s) at its plants in Okawa in the Gumma district and at Handa in the Aichi district. Production never exceeded more than 90 planes per month.