page_title: Yokosuka D4Y
section_title: Design and development
hierarchical_section_title: Yokosuka D4Y / Design and development
caption_attribution_description: Yokosuka D4Y2 "Suisei" before taking off. www.ijnafpics.com
context_page_description: The Yokosuka D4Y Suisei is a two-seat carrier-based dive bomber developed by the Yokosuka Naval Air Technical Arsenal and operated by the Imperial Japanese Navy from 1942 to 1945 during World War II. Development of the aircraft began in 1938. The first D4Y1 was complete in November 1940 and made its maiden flight at Yokosuka the following month.
  While the aircraft was originally conceived as a dive bomber, the D4Y was used in other roles including reconnaissance, night fighter and special attack. It made its combat debut as a reconnaissance aircraft when two pre-production D4Y1-Cs embarked aboard the Sōryū to take part in the Battle of Midway in 1942. It was not until March 1943 that it was accepted for use as a dive bomber. The early D4Y1 and D4Y2 featured the liquid-cooled Aichi Atsuta engine, a licensed version of the German Daimler-Benz DB 601, while the later D4Y3 and D4Y4 featured the Mitsubishi MK8P Kinsei radial engine.
  Like many other Japanese aircraft of the time, the D4Y lacked armor and self-sealing fuel tanks and it was not until the final variant, the D4Y4, that the aircraft was given bulletproof glass and armor protection for the crew and fuel tanks.
context_section_description: Development of the aircraft began in 1938 at the Yokosuka Naval Air Technical Arsenal  when the Kaigun Kōkū Hombu issued requirements of a Navy Experimental 13-Shi Carrier Borne specification for an carrier-based dive bomber to replace the Aichi D3A. Its design was inspired by the Heinkel He 118 which the Japanese Navy had acquired from Germany in early 1938. 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 V12 engine, a licensed 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.
  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 Curtiss 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. Subsequently, 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.
  The first (of five) prototypes complete in November 1940 and 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.
caption: Yokosuka D4Y1 before takeoff
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