Yokosuka D4Y - Design and Development

Design and Development

Development of the aircraft began in 1938 at the Yokosuka Naval Air Technical Arsenal as a carrier-based dive bomber to replace the Aichi D3A.

The aircraft was a single-engine, all-metal low-wing monoplane, with a wide-track retractable undercarriage and wing-mounted dive brakes. It had a crew of two: a pilot and a navigator/radio-operator/gunner, seated under a long, glazed canopy which provided good all-round visibility. The pilot of bomber versions was provided with a telescopic bombsight. The aircraft was powered by an Aichi Atsuta liquid-cooled inverted V-12 in-line engine, a licenced copy of the German DB 601, rated at 895 kW (1,200 hp). The radiator was behind and below the three-blade propeller, as in the P-40.

It had a slim, elegant fuselage that enabled it to reach high speeds in horizontal flight and in dives, while low wing loading gave excellent maneuverability, with the Suisei having superior performance than contemporary dive bombers such as the SB2C Helldiver. In order to conform with the Japanese Navy's doctrine of ensuring that its aircraft could outrange potential enemies, weight had to be minimized with the result that the D4Y was not fitted with self-sealing fuel tanks or armour. In consequence, the D4Y was extremely vulnerable and tended to catch fire when hit.

Bombs were fitted under the wings and in an internal fuselage bomb bay. It carried one 500 kg (1,100 lb) bomb, but there were reports that the D4Y sometimes carried two 250 kg (550 lb) bombs, for example during the attack on the light aircraft carrier USS Princeton. Only 30 kg (70 lb) bombs were carried externally. The aircraft was armed with two 7.7 mm (.303 in) machine guns in the nose, and one 7.92 mm (.312 inm) Type 1 machine gun selected for its high rate of fire in the rear of the cockpit. Later, the rear gun was replaced by a 13 mm (.51 in) Type 2 machine gun. This light gun armament was typical for a Japanese carrier-based bomber. The forward machine guns were retained in the kamikaze version.

The first D4Y1 prototype made its maiden flight in December 1940. After the prototype's successful trials, development continued, and the first problems appeared. During dive-bombing trials, the wings of the D4Y started to flutter, a fatal flaw for an airframe subject to the stresses of dive bombing. Because of this, initial production aircraft were used as reconnaissance aircraft, as the D4Y1-C, which took advantage of its high speed and long range, while not over-stressing the airframe. Production of the D4Y1-C continued in small numbers until March 1943, when the increasing losses incurred by the D3A resulted in production switching to the D4Y1 dive-bomber, the aircraft's structural problems finally being solved. Although the D4Y could operate successfully from the large and fast fleet carriers that formed the core of the Combined Fleet at the start of the war, it had problems operating from the smaller and slower carriers such as the Hiyō class which formed a large proportion of Japan's carrier fleet after the losses received in the Battle of Midway. Catapult equipment was therefore fitted, giving rise to the D4Y-1 Kai (or improved) model.

These early versions of the D4Y were difficult to keep in service because the Atsuta engines were unreliable and difficult to maintain in front-line service. From the beginning, some had argued that the D4Y should be powered by an air-cooled radial engine, a type Japanese engineers - and maintenance crew - had experience with and trusted. The aircraft was therefore fitted with the reliable Mitsubishi MK8P Kinsei 62, a 14-cylinder two-row radial engine. This version was the Yokosuka D4Y3 Model 33.

Although the new engine improved ceiling and rate of climb (over 10,000 m/32,800 ft, and climb to 3,000 m/9,800 ft in 4.5 minutes, instead of 9,400 m/30,800 ft and 5 minutes), the higher fuel consumption resulted in shorter range and a slower cruising speed, while the bulky engine obstructed the forward and downward view of the pilot, hampering carrier operations. These problems were tolerated because of the increased availability of the new variant.

The last version was the D4Y4 Special Strike Bomber. This one-seat kamikaze aircraft, capable of carrying one 800 kg (1,760 lb) bomb, was put into production in February 1945. It was equipped with three RATO boosters for terminal dive acceleration. This aircraft was an almost ideal kamikaze model: it had a combination of speed (560 km/h/350 mph), range (2,500 km/1,550 mi) and payload (800 kg/1,760 lb) probably not matched by any other Japanese aircraft.

The D4Y5 Model 54 was a planned version designed in 1945. It was to be powered by the Nakajima NK9C Homare 12 radial engine rated at 1,361 kW (1,825 hp), would have a new four-blade metal propeller of the constant-speed type, and would have more armour protection for the crew and fuel tanks.

Ultimately, 2,038 of all variants were produced, mostly by Aichi.