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Yokosuka D4Y Suisei
Design & Delivery

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 Warhawk.


Yokosuka D4Y.
[Source: Unknown]

The aircraft had a slim fuselage that enabled it to reach high speeds in horizontal flight and in dives, while it had excellent maneuverability despite high wing loading, with the Suisei having superior performance to contemporary dive bombers such as the SB2C Helldiver. In order to conform with the Japanese Navy's requirement for long range, weight was minimized by not fitting the D4Y 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 usually 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) Type 97 aircraft machine guns in the nose and a 7.92 mm (.312 in) Type 1 machine gun selected for its high rate of fire, in the rear of the cockpit. The rear gun was replaced by a 13 mm (.51 in) Type 2 machine gun. This armament was typical for Japanese carrier-based dive-bombers, unlike "carrier attack bombers" (i.e. torpedo bombers) like the Nakajima B5N and B6N which were not given forward-firing armament until the late-war Aichi B7A, which was expected to serve as both a dive-bomber and torpedo-bomber, and was given a pair of 20mm Type 99-2 cannon. The forward machine guns were retained in the kamikaze version.


Yokosuka D4Y.
[Source: Unknown]

The first (of five) prototypes made its maiden flight in December 1940. After the prototype trials, problems with flutter were encountered, a fatal flaw for an airframe subject to the stresses of dive bombing. Until this could be resolved, early 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 from the large 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 of the Battle of Midway. Catapult equipment was fitted, giving rise to the D4Y1 Kai (or improved) model.

Early versions of the D4Y were difficult to keep operational because the Atsuta engines were unreliable in front-line service. From the beginning, some had argued that the D4Y should be powered by an air-cooled radial engine which Japanese engineers and maintenance crew had experience with, and trusted. The aircraft was re-engined with the reliable Mitsubishi MK8P Kinsei 62, a 14-cylinder two-row radial engine as 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 reduced range and cruising speed and the 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, a single-seat kamikaze aircraft, capable of carrying one 800 kg (1,760 lb) bomb, which 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.


Instrument panel of a Yokosuka D4Y4 "Suisei".
[Source: Unknown]

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), a new four-blade metal propeller of the constant-speed type and more armour for the crew and fuel tanks. Ultimately, 2,038 of all variants were produced, mostly by Aichi.

Sources:
The Complete Encyclopedia of World Aircraft, 1997, Barnes & Nobles Books, ISBN: 0 7607 0592 5
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