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At the start of the Pacific War, the Japanese air force mainly used small seaplanes launched from heavy warships for maritime reconnaissance. The service also operated shore-based bombers and multi-engine heavy flying boats on long-range reconnaissance missions. In 1942, the Imperial Navy became interested in a high-speed, long-range reconnaissance plane. The service wanted the plane to be uninterceptable by enemy fighters. Naval leaders aimed to match the capabilities of the Mitsubishi Ki-46 reconnaissance aircraft used by the Imperial Army. In 1942, the Navy ordered the development of the plane, requiring it to reach a maximum speed of 355 mph at an altitude of 20,000 feet. The Yokosuka Naval Arsenal was assigned the project, and the original design was formally designated R1Y Seiyun or “blue cloud.” It was supposed to be powered by new 24-cylinder, liquid-cooled engines that Mitsubishi was developing. However, since those engines would not be ready soon, the engineers decided to use two Mitsubishi MK10A air-cooled engines instead. This R1Y variant resembled the Ginga bomber, but its expected performance did not meet the requirements.
Design of Yokosuka R2Y Keiun
As a result, R1Y was canceled. Later, the Yokosuka Naval Arsenal developed a better version of the R1Y reconnaissance aircraft, designated the Yokosuka R2Y Keiun. It was designed to reach a maximum speed of 435 mph and a cruising range of over 2,000 miles. The Yokosuka R2Y Keiun was powered by the Aichi Ha-70 engine, which consisted of two Aichi Atsuta 12-cylinder engines, each producing 1,700 horsepower. The design leader for the new Yokosuka R2Y Keiun aircraft was Captain Shiro Otsuki. But the focus on reconnaissance planes changed after Japan was defeated at the Mariana Islands, as fighters and bombers were needed. The threat of war near Japan made long-range maritime reconnaissance aircraft unnecessary. The Keiyun faced the same challenges as its carrier-based version, the Seiyun. But the idea of turning it into a bomber gave the project new hope. In late 1944, plans were proposed to develop a jet bomber based on the Yokosuka R2Y Keiun. To do this, the twin engines in the body would be replaced by a fuel tank. Then, two Mitsubishi Ne-330 turbojet engines, each producing 2,800 pounds of thrust, would be installed under the wings. The bomber would carry one 1,700-pound bomb under the body. It would also have four 30 mm cannons mounted in the front. The expected maximum speed would be 430 mph at 33,500 ft, compared to 435 mph for the piston-engine version. The rate of climb was expected to improve, but the combat radius was likely to decrease.
The Cancellation
The Navy liked the Yokosuka R2Y Keiun design, so to speed up prototype construction, it decided to reuse as many parts as possible from the earlier version. The service chose to build the piston-engined R2Y1 as an aerodynamic prototype before the jet-powered R2Y2 was complete. Meanwhile, the designers started working on new versions of the aircraft, called the R2Y2-kai. Unlike the R2Y2, the R2Y2-kai would have jet engines placed in the fuselage, with air intakes positioned in front of the wing. By making the front section smaller to reduce drag, they expected it to reach a top speed of 450 mph. The R2Y2-kai was expected to be Japan's fastest aircraft during WWII. After production finished, the R2Y1 prototype was sent to Kisarazu on April 27, 1945, for flight testing. During testing, problems with the landing gear (nose-wheel shimmy) and engine overheating were identified. The first flight happened on May 8, led by Lieutenant Commander Kitaima. However, this flight was cut short because the oil temperature increased sharply. A few days later, it became clear that the engine needed to be replaced. Before this could happen, the aircraft was bombed by the Americans. The second R2Y1 and the R2Y2 prototypes were still being built at the Yokosuka arsenal and were not finished before the war ended. After Japan’s defeat, work on the aircraft project stopped. In the Grounded Dreams series, the Yokosuka R2Y Keiun was a good aircraft but could not prove itself due to the war’s end. Read more Grounded Dreams articles HERE.
