By William Pearce

Mitsubishi Heavy Industries was Japan’s largest aircraft engine producer and had developed a number of reliable and powerful engines. During 1942, Mitsubishi investigated a 3,000 hp (2,237 kW) engine design. Given the designation A19, the radial engine design had four rows of seven cylinders. The A19 had a 5.51 in (140 mm) bore and a 6.30 in (160 mm) stroke. This gave the 28-cylinder engine a displacement of 4,208 cu in (69.0 L). However, in the spring of 1943, Mitsubishi engineers concluded after extensive testing that the rear rows of the engine would not have enough airflow for sufficient cooling. The A19 was never built.

Although in a sorry state, the Mitsubishi A21 / Ha-50 preserved at the Museum of Aviation Science in Narita, Japan gives valuable insight into a lost generation of Japanese aircraft engines and 22-cylinder aircraft engines. Nearly all of the non-steel components have rotted away. (campns.jp image)

To solve the cooling issues, Mitsubishi turned to a two-row radial engine design with 11-cylinders per row. The new engine carried the Mitsubishi designation A21. The Imperial Japanese Army (IJA) approved of the engine design and instructed Mitsubishi to proceed with construction. The A21 was given the IJA designation Ha-50. Many sources state the engine was later assigned the joint Japanese Army and Navy designation [Ha-50]. However, [Ha-52] would have been more fitting for the engine’s configuration, and the [Ha-50] designation may be the result of confusion with the IJA’s Ha-50 designation. The Imperial Japanese Navy (IJN) was not involved with the engine’s development.

At the time, Mitsubishi was already developing an 18-cylinder radial based on their 14-cylinder [Ha-32] Kasei engine. To speed development of the Ha-50, Mitsubishi decided to continue the practice of adding additional Kasei-type cylinders to a new crankcase. The resulting air-cooled, 22-cylinder, two-row, radial configuration was common with only two other engines: the Hitachi/Nakajima [Ha-51] and the Wright R-4090. Using two rows of 11 cylinders kept the engine short and relatively simple compared to a four-row configuration. The two-row configuration also enabled a rather straightforward engine cooling operation without resorting to complex baffles. However, the large number of cylinders in each row increased the engine’s frontal area and caused greater stresses on the crankshaft’s crankpins.

The Ha-50 had a substantial amount of space between the first and second cylinder rows. Note the pistons frozen in their cylinders. (Rob Mawhinney image via the Aircraft Engine Historical Society)

The Ha-50 used a three-piece, steel crankcase that was split vertically along the cylinder center line and secured via internal fasteners. Aluminum alloy housings were used for the gear reduction and the supercharger. Each cylinder was secured to the crankcase by 16 studs. The cylinders were formed with a cast aluminum head screwed and shrunk onto a steel barrel. Relatively thin fins were cut into the steel cylinder barrels to aid cooling. Each cylinder had one intake valve and one exhaust valve. The intake and exhaust ports for each cylinder faced toward the rear of the engine. The cylinders had a compression ratio of 6.7. Following the typical two-row radial configuration, the second row of cylinders was staggered behind the first row. Ample space existed between the cylinders in the front row for cooling air to reach the cylinders in the rear row. A fairly large space existed between the front and rear cylinder rows, perhaps signifying a rather robust center crankshaft support.

Two-stage supercharging was used in the form of a remote turbosupercharger for the first stage and a gear-driven, two-speed supercharger for the second stage. However, the test engines had only the gear-driven supercharger, which turned at 7.36 times crankshaft speed in low gear and 10.22 times crankshaft speed in high gear. The Ha-50 used fuel injection, and water-injection was available to further boost power. At the front of the engine was a planetary gear reduction that turned the propeller at .412 times crankshaft speed. Some sources state that contra-rotating propellers were to be used, but only a single propeller shaft was provided on the initial engines. A cooling fan was driven from the front of the gear reduction.

Left—An Ha-50 aluminum cylinder head still attached to the cylinder barrel. Note the valve in the intake port. Right—Detailed view of a cylinder barrel illustrates the cooling fins cut into its middle and the threaded portion at the top for cylinder head attachment. (Rob Mawhinney images via the Aircraft Engine Historical Society)

The Ha-50 had a 5.91 in (150 mm) bore and a 6.69 in (170 mm) stroke. Its total displacement was 4,033 cu in (66.1 L). The engine had a takeoff rating of 3,100 hp (2,312 kW) at 2,600 rpm and 8.7 psi (.60 bar) of boost. Normal ratings for the engine were 2,700 hp (2,013 kW) at 4,921 ft (1,500 m) and 2,240 hp (1,670 kW) at 32,808 ft (10,000 m). The normal ratings were achieved at an engine speed of 2,500 rpm and with 5.8 psi (.40 bar) of boost. The Ha-50 was 56.9 in (1.45 m) in diameter, 94.5 in (2.40 m) long, and weighed 3,395 lb (1,540 kg).

Front view of the Ha-50 illustrates the ample space between the front-row cylinders, enabling air to reach the rear-row cylinders. Note the single rotation propeller shaft. (Rob Mawhinney image via the Aircraft Engine Historical Society)

Construction of the Ha-50 started in April 1943, and the first engine was completed in 1944. Engine testing began immediately, and severe vibrations were encountered that reportedly shook one engine apart on the test stand. Some sources indicate the Ha-50 was an optional power plant for the Kawanishi TB, a four-engine transoceanic bomber ordered by the IJA. The Kawanishi TB was a smaller and lighter competitor to the Nakajima Fugaku, which had become exclusively an IJN project. Six Ha-50 engines were ordered for the Kawanishi TB, but the bomber project was cancelled before any aircraft were built. Three of the Ha-50 engines were finished, but their operational issues and the cancelling of the Kawanishi TB resulted in the Ha-50 engine project being abandoned. Two of the engines were damaged in a bombing raid, but the surviving Ha-50 reportedly achieved 3,200 hp (2,386 kW) in July 1945.

The three Ha-50 engines were thought to have been destroyed at the end of World War II and before the arrival of US forces. However, one Ha-50 engine was discovered in November 1984 during expansion work at the Haneda Airport (Tokyo International Airport). Some sources indicate the surviving engine was found by US forces shortly after the war and delivered to Haneda Airport for later shipment to the United States. Apparently, plans changed, and the engine was subsequently bulldozed into a pit and covered with dirt. The discovered Ha-50 was in an advanced state of decay, but it was recovered, and efforts were made to preserve the engine and prevent its continued deterioration. The engine’s condition was stabilized, and it was put on display at the Museum of Aviation Science in Narita, Japan. The surviving Ha-50 is the sole example of any 22-cylinder aircraft engine.

The supercharger and accessory case completely rotted off the Ha-50 during its near 40-year interment. Note the threads cut into the top of the steel cylinder barrels. (Rob Mawhinney image via the Aircraft Engine Historical Society)

Sources:
– Japanese Aero-Engines 1910–1945 by Mike Goodwin and Peter Starkings (2017)
– The History of Mitsubishi Aero-Engines 1915–1945 by Hisamitsu Matsuoka (2005)
– http://www.arawasi.jp/on%20location/narita1.html
– https://ja.wikipedia.org/wiki/ハ50_(エンジン)