The Yokosuka D4Y is a twin-seat carrier-based dive bomber developed by the Japanese defence manufacturer Yokosuka Naval Air Technical Arsenal. Operated by the Imperial Japanese Navy (IJN) throughout World War II, it was assigned the Allied reporting name Judy.
Development of the aircraft began in 1938 as a carrier-based dive bomber to replace the Aichi D3A, and drew considerably upon the German Heinkel He 118. A single-engine low-wing monoplane, it possessed a relatively slender fuselage along with a retractable undercarriage and wing-mounted dive brakes, permitting it to achieve high speeds and excellent maneuverability despite a relatively high wing loading. Bombs were fitted underneath the wings and within internal bomb bay in the fuselage; it was also armed with two forward-facing 7.7 mm (.303 in) Type 97 aircraft machine guns and a rear-facing 7.92 mm (.312 in) Type 1 machine gun. To achieve the stipulated long range performance, early models of the D4Y lacked both self-sealing fuel tanks and armour, akin to most other Japanese aircraft of the era.
The first D4Y1 was complete in November 1940 and made its maiden flight at Yokosuka the following month. The flight testing phase was protacted by several difficulties, including with flutter, which required structural alterations to resolve. While the aircraft was originally conceived as a dive bomber, the D4Y was used in other roles including reconnaissance, night fighter and special attack. The type made its combat debut as a reconnaissance aircraft when two pre-production D4Y1-Cs embarked aboard to take part in the Battle of Midway in 1942. It was not until March 1943 that the D4Y1 was accepted for use as a dive bomber. While the early D4Y1 and D4Y2 featured the liquid-cooled Aichi Atsuta V12 engine, a licensed version of the German Daimler-Benz DB 601, while the later D4Y3 and D4Y4 featured the air-cooled Mitsubishi MK8P Kinsei 62 radial engine, which was more reliable but possessed high fuel consumption. The D4Y1 Kai was fitted with catapult apparatus to permit operations from compact carriers. It was not until the final variant, the D4Y4, that the aircraft was fitted with bulletproof glass and armor protection for both the crew and fuel tanks.
The D4Y was one of the fastest dive bombers of the conflict, particularly the D4Y4, which aviation historian Max Gadney said was the "fastest dive-bomber of World War II... faster than the Zero". Only the delays in its development hindered the D4Y's service while its predecessor, the slower fixed-gear Aichi D3A, remained operational for much longer than intended. In October 1944, an attack by a lone D4Y resulted in the sinking of light carrier in the Battle of Leyte Gulf. Similarly, in March 1945, a single D4Y managed to hit the carrier with two bombs, nearly sinking Franklin and resulting in the loss of almost 800 of her crew. Famously, a D4Y was used in one of the final kamikaze attacks in 1945, hours after the surrender of Japan, with Vice Admiral Matome Ugaki in the rear cockpit. All remaining aircraft were withdrawn shortly after the Surrender of Japan, which ended the conflict.
Design and development Background and features The development of the Yokosuka D4Y Suisei can be traced back to early 1938 and an arrangement between the Imperial Japanese Navy (IJN) and Germany regarding the production rights for the Heinkel He 118 along with a pattern aircraft. This aircraft, designed DXHe1, underwent flight testing at Yokosuka and impressed the Naval Staff to the extent that plans to locally produce a carrier-based variant were under consideration. Instead, the Imperial Japanese Navy Aviation Bureau (Kaigun Kōkū Hombu) opted to issue requirements of a Navy Experimental 13-Shi Carrier Borne specification for an carrier-borne dive bomber to replace the Aichi D3A. According to the aviation author Rene J. Francillon, this specification was heavily influenced by the He 118. Performance stipulations included a maximum speed of 280 knots, cruising speed of 230 knots, 800 nautical mile range while carrying bombs, 1,200 nautical mile range without bombs, and the ability to operate from the IJN's smaller carriers.
In late 1938, the Yokosuka Naval Air Technical Arsenal was instructed to design an aircraft to satisfy the specification. Its design team, headed by Masao Yamana, opted for a relatively compact and aerodynamically clean monoplane with a mid-mounted wing, which possessed a similar span and area to the Mitsubishi A6M Zero fighter, thus precluding the need for a folding mechanism. Furthermore, the wing was fitted with three electrically-actuated dive brakes ahead of each landing flap. In comparison to the He 118, the airframe was both lighter and smaller as well as being more aerodynamically efficient. It was furnished with retractable undercarriage, the main wheels and legs of which retracted forward of the main spar.
The aircraft, which featured all-metal construction, was typically flown a crew of two, a pilot and a navigator/radio-operator/gunner, which were seated under a lengthy glazed canopy that provided good all-round visibility. On bomber versions of the aircraft, the pilot was provided with a telescopic bombsight. As originally designed, the aircraft was powered by a single Aichi Atsuta liquid-cooled inverted V12 engine, a licensed copy of the German Daimler-Benz DB 601, which was rated to produce up to 895 kW (1,200 hp). The radiator was located behind and below the three-blade propeller, akin to the Curtiss P-40 Warhawk.
The aircraft had a relatively slender fuselage, which enabled it to reach high speeds during horizontal flight and in dives. Furthermore, it possessed excellent maneuverability despite a relatively high wing loading for the era; the Suisei had superior performance to several contemporary dive bombers, including the US Navy's Curtiss SB2C Helldiver. In order to conform with the IJN's long distance requirement, weight was minimized by not fitting the D4Y with self-sealing fuel tanks or armour. Subsequently, the D4Y was extremely vulnerable to enemy fire and prone to catching alight when struck.
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. It was also armed with a pair of 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. This rear gun was late 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" (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.
Into flight The first (of five) prototypes was complete in November 1940 and made its maiden flight in December 1940. Early flight testing demonstrated its performance being in excess of the IJN's most optimistic expectations for the type, accompanied by largely favourable handling characteristics. However, as flight testing advanced to simulated diving bombing runs, the early prototypes revealed several shortcomings, including cracks appearing on the wing spars as well as instances of wing flutter, both of which were considered to be fatal flaws for any aircraft, but particularly one intended to routinely perform dive bombing. Accordingly, this discovery disrupted mass production plans for the type, and engineers from multiple aircraft companies were tasked with its resolution.
Instead of delaying production until the flutter issue could be fully resolved, the IJN instructed Aichi to configure early production aircraft to be use as reconnaissance aircraft, designated D4Y1-C. Modifications included the installation of a K-8 camera within the rear fuselage, and commonly being fitted with a pair of drop tanks; the reconnaissance mission profile took advantage of the D4Y's high speed and long range capabilities while not over-stressing the airframe. Accordingly, in July 1942, the D4Y1-C was ordered into production as the Navy Type 2 Carrier Reconnaissance Plane Model 11; it was only produced in small numbers due to niche requirements for specialised carrier-borne long distance reconnaissance.
Throughout 1942, development of the D4Y1 continued, leading to it being fitted with reinforced wing spars and improved dive brakes. In March 1943, by which point the aircraft's structural problems were considered to have finally been resolved and increasing losses were being incurred by the D3A, the IJN accepted the D4Y1 as a dive bomber, authorising production as the Suisei (Comet) Carrier Bomber Model 11. Thereafter, production of the type ramped quickly in response to the service's demand for dive bombers. Although the D4Y could operate from the large fleet carriers that formed the core of the Combined Fleet at the start of the conflict, it had problems operating from the smaller and slower carriers such as the , which formed a large proportion of Japan's carrier fleet after the losses of the Battle of Midway. Accordingly, catapult equipment was fitted, giving rise to the D4Y1 Kai (or improved) model.
Early versions of the D4Y proved to be difficult to keep operational largely on account of its Atsuta engines being unreliable in front-line service. From early on in development, some figures 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. Accordingly, the aircraft was re-engined with the reliable Mitsubishi MK8P Kinsei 62, a 14-cylinder radial engine, as the Yokosuka D4Y3 Model 33. Although the new engine improved ceiling and rate of climb to over and climb to in 4.5 minutes instead of 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 final version of the aircraft was the D4Y4 Special Strike Bomber, a twin-seat special attack aircraft, capable of carrying one bomb, which was put into production in February 1945. It was equipped with three rocket 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), a new four-blade metal propeller of the constant-speed type and additional armour for both the crew and fuel tanks.
Ultimately, 2,038 D4Ys across all variants were completed, the majority of which were produced by Aichi Kokuki. Operational history
The D4Y1-C reconnaissance aircraft entered service in mid-1942, when two of these aircraft were deployed aboard Sōryū at the Battle of Midway, one of which was lost when Sōryū was bombed. The other had been launched on a scouting mission and returned to ; it was then lost when Hiryū was bombed.Parshall and Tully 2005, pp. 189, 292.
The D4Y was operated from the following Japanese aircraft carriers: , , , , and .
Lacking armor and self-sealing fuel tanks, the Suiseis did not fare well against Allied fighters. They did, however, cause considerable damage to ships, including the carrier which was nearly sunk by an assumed single D4Y and the light carrier which was sunk by a single D4Y.
Marianas