Category Archives: World War II

Mitsubishi Ha-50 campns

Mitsubishi A21 / Ha-50 22-Cylinder Aircraft Engine

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.

Mitsubishi Ha-50 campns

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.

Mitsubishi Ha-50 side

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.

Mitsubishi Ha-50 cylinders

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).

Mitsubishi Ha-50 front

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.

Mitsubishi Ha-50 rear

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)

Daimler-Benz DB 604

Daimler-Benz DB 604 X-24 Aircraft Engine

By William Pearce

In July 1939, the RLM (Reichsluftfahrtministerium, or Germany Air Ministry) issued specifications for a new medium bomber capable of high-speeds. Originally known as Kampfflugzeug B (Warplane B), the aircraft proposal was eventually renamed Bomber B. The Bomber B specification requested an aircraft that could carry a 2,000 kg (4,410 lb) bomb load 3,600 km (2,237 mi) and have a top speed of 600 km/h (373 mph). To power the Bomber B aircraft, the RLM requested engine designs from BMW, Junkers, and Daimler-Benz. The respective companies responded with the BMW 802, the Junkers Jumo 222, and the Daimler-Benz DB 604.

Daimler-Benz DB 604

The Daimler-Benz DB 604 was designed in 1939 to power the next generation of German fast bombers under the Bomber B program. However, the engine was not selected for production.

The DB 604 was an all-new, liquid-cooled, 24-cylinder engine. Four banks of six cylinders were arranged in an “X” configuration with each cylinder bank spaced at 90 degrees. The X-24 engine consisted of a two-piece aluminum alloy crankcase split horizontally at its center. The engine’s single crankshaft had six crankpins that were spaced at 0 degrees, 120 degrees, 240 degrees, 240 degrees, 120 degrees, and 0 degrees. This arrangement resulted in cylinders firing evenly at every 30 degrees of crankshaft rotation. Attached to each crankpin was a master connecting rod that accommodated three articulated connecting rods. A gear reduction at the front of the engine turned the propeller at .334 crankshaft speed. A supercharger mounted to the rear of the engine had an upper and a lower outlet. Each outlet was connected to two intake manifolds that ran along the inner Vee side of the cylinder banks.

The DB 604’s fuel system was located in the upper and lower Vees of the engine and consisted of fuel injection pumps and individual fuel injectors for each cylinder. Each cylinder had two intake and two exhaust valves, all of which were actuated by a single overhead camshaft. The camshaft for each cylinder bank was driven via a vertical shaft from the rear of the engine. The exhaust ports were positioned in the left and right Vees, as were the two spark plugs per cylinder. The spark plugs were fired by two magnetos positioned in the left and right Vees and mounted to the propeller gear reduction housing.

Daimler-Benz DB 604 side

The DB 604 was a rather compact design. A magneto can be seen at the front of the engine between the exhaust ports of the upper and lower cylinder banks. Note the supercharger at the rear of the engine. (Evžen Všetečka image via www.aircraftengine.cz)

The DB 604 had a 5.31 in (135 mm) bore and stroke and displaced 2,830 cu in (46.4 L). The engine had a 7.0 to 1 compression ratio and weighed 2,381 lb (1,080 kg). The DB 604 prototype was first run in late 1939. The first engine produced 2,313 hp (1,725 kW) at 3,200 rpm. This engine may have had a single-speed supercharger. The DB 604 A and DB 604 B engines were produced quickly after the first prototype. These engines had a two-stage supercharger that provided 6.17 psi (.43 bar) of boost. The difference between A and B versions was the rotation of the engine’s crankshaft. The DB 604 A/B had a maximum output at 3,200 rpm of 2,660 hp (1,984 kW) at sea level and 2,410 hp (1,797 kW) at 20,600 ft (6,279 m). The engine’s maximum continuous output was 2,270 hp (1,693 kW) at sea level and 2,120 hp (1,581 kW) at 21,000 ft (6,401 m), both figures at 3,000 rpm. Maximum cruise power was at 2,800 rpm, with the engine producing 1,830 hp (1,365 kW) at sea level and 1,860 hp (1,387 kW) at 20,000 ft (6,096 m). The DB 604 was flight tested in a Junkers Ju 52 trimotor transport, but it is not clear which version of the engine was tested. At least five DB 604 engines were made.

The Bomber B proposals that moved forward as prototypes were the Dornier Do 317, Focke-Wulf Fw 191, and Junkers Ju 288. Despite the DB 604 showing some promise, the RLM chose the Jumo 222, and work on the DB 604 was stopped in September 1942. No records have been found that detail the DB 604’s reliability, and many other X-24 aircraft engine designs were prone to failure. The sole surviving Daimler-Benz DB 604 engine is on display at the Flugausstellung L.+ P. Junior museum in Hermeskeil, Germany.

Daimler-Benz DB 604 right

Some of the fuel injection equipment is just visible in the engine’s upper Vee. The sole surviving DB 604 engine is on display at the Flugausstellung L.+ P. Junior museum in Hermeskeil, Germany. (Evžen Všetečka image via www.aircraftengine.cz)

Ultimately, the Ju 288 was selected as the winner of the Bomber B program. Delays with the 2,500 hp (1,964 kW) Jumo 222 led to it being substituted with the 2,700 hp (2,013 kW) Daimler-Benz DB 606, and that engine was later replaced by the 2,950 hp (2,200 kW) DB 610. The DB 606 consisted of two DB 601 inverted V-12 engines coupled side-by-side, while the DB 610 was the same arrangement but with two DB 605 engines. The Ju 288 aircraft and the Jumo 222 engine never entered large-scale production.

An enlarged version of the DB 604 was contemplated, with the engine’s bore increased .2 in (5 mm) to 5.51 in (140 mm). This gave the engine a displacement of 3,044 cu in (49.9 L). The larger 90-degree, X-24 engine was very similar to the DB 604 but incorporated a three-speed, three-stage supercharger. The engine was forecasted to produce 3,450 hp (2,575 kW) at 36,089 ft (11,000 m). Development of the larger engine did not progress beyond the initial design phase.

Daimler-Benz DB 604 left


Despite a number of X-24 aircraft engines being made, none truly were produced beyond the prototype phase, and the DB 604 was no exception. Note that the two intake manifolds between the upper (and lower) cylinder banks were connected at the front of the engine to equalize pressure. (Evžen Všetečka image via www.aircraftengine.cz)

Sources:
Flugmotoren und Strahltriebwerke by Kyrill von Gersdorff, et. al. (2007)
German Aero-Engine Development A.I.2.(g) Report No. 2360 by G. E. R. Proctor (22 June 1945)
Luftwaffe: Secret Bombers of the Third Reich by Dan Sharp (2016)
Jane’s All the World’s Aircraft 1945–46 by Leonard Bridgman (1946)
https://de.wikipedia.org/wiki/Daimler-Benz_DB_604
http://www.aircraftengine.cz/Hermeskeil/

Lycoming XR-7755-3 side

Lycoming XR-7755 36-Cylinder Aircraft Engine

By William Pearce

Since 1933, the Lycoming Division of the Aviation Manufacturing Corporation had worked to create a high-power engine for the United States Armed Forces. Its first attempt was the 1,200 hp (895 kW), 12-cylinder O-1230, which was outclassed by the time it first flew in 1940. Lycoming’s second attempt was the 2,300 hp (1,715 kW), 24-cylinder XH-2470. The engine had shown some promise, but its performance was eclipsed by other engines when the XH-2470 was first flown in 1943. Lycoming set out to design an engine that was more powerful than any other and that would meet the power needs of future large aircraft.

Lycoming XR-7755-3 side

The Lycoming XR-7755 was the most powerful aircraft engine in the world when it was built. The XR-7755 was the culmination of Lycoming’s experience with radial and liquid-cooled engines. Conceived in 1943, such a large engine was not needed by the time it first ran in 1946.

In mid-1943, Lycoming engaged in talks with personnel from the US Army Air Force (AAF) at Wright Field, Ohio. Different sources list the involvement of the Air Materiel Command, Air Technical Service Command, and the Power Plant Lab. By December 1943, the engine concept had been solidified as a very large displacement, high-compression, liquid-cooled engine designed for optimum fuel economy and intended to power the next generation of very large aircraft. Lycoming’s experimental engine was designated XR-7755 and given the “Materiel, Experimental” code MX-434.

Clarence Wiegman headed the Lycoming XR-7755 design team. The engine consisted of nine banks of four inline cylinders positioned radially with 40-degrees of separation around a forged steel crankcase. This formed a 36-cylinder inline radial engine. The crankcase was made up of five sections, each split vertically through the cylinders. The crankcase sections were secured together by nine bolts that extended the length of the case. The individual steel cylinders each had their own water jacket. Each bank of four cylinders shared a common cast aluminum cylinder head. Each four-cylinder bank was secured to the crankcase by 16 long studs that passed through the cylinder head.

Lycoming XR-7755-3 stand

The worker gives some perspective to the XR-7755’s large size. However, the engine’s three-ton (2.74 t) weight is hard to imagine. The engine’s two magnetos and four distributors are visible on the front of the cylinder banks.

Each cylinder had one intake and one exhaust valve. Both valves were sodium cooled, with a hollow stem for the intake valve and a hollow stem and head for the exhaust valve. The valves for each bank of cylinders were actuated by a single overhead camshaft, driven via a vertical shaft at the front of the engine. Each camshaft had two sets of lobes for different valve timing—one lobe set was optimized for power and the other set for economic cruise. The camshafts shifted axially to engage the desired set of lobes. When the camshaft was shifted, the spark plug timing was automatically changed. Ignition was provided by two magnetos and four distributors. Each unit was camshaft-driven and mounted to the front of a separate cylinder bank. The spark plug leads passed through the valve covers and to the spark plugs, which were positioned in opposite corners of each cylinder.

Lycoming XR-7755-1 test stand

The XR-7755-1 on the test stand with its single propeller shaft. With each of the 36-cylinders displacing 215 cu in (3.5 L), witnessing the XR-7755 run was most likely a very memorable event. Note the robust upper engine support.

The crankshaft had four crankpins, each spaced at 180 degrees. The crankshaft was made up of five sections and assembled through the four one-piece master connecting rods. The crankshaft sections were joined at the rear of the crankpin via face splines and secured by four bolts. Five roller bearings supported the crankshaft in the crankcase.

At the rear of the engine was a single-stage, single-speed supercharger. The supercharger’s impeller was 14.4 in (366 mm) in diameter and spun at six times crankshaft speed. The supercharger fed air to nine intake manifolds, each mating with the right side of a cylinder bank. Fuel was provided to the cylinder via either a carburetor or fuel injection. Individual exhaust stacks were attached to the left side of each cylinder. Provisions were also made to incorporate two turbosuperchargers.

Although a single rotation engine was tested, the engine accommodated contra-rotating propellers using SAE #60L-80 spline shafts. The inner shaft rotated counterclockwise, and the outer shaft rotated clockwise. A two-speed propeller gear reduction was hydraulically shifted and used planetary gears. A .2460 reduction was available for high engine speeds, and a .3536 reduction was used for cruise operations with low engine rpm.

Lycoming XR-7755-1 Maxwell and Cervinsky

Lycoming workers Red Maxwell (left) and Paul Cervinsky (right) pose next to the completed XR-7755-1. It appears Maxwell is ready for the big engine to be stuffed in an airframe to see what it will do. Note the ring on the nose case and around the propeller shaft. No other image found has that ring.

The XR-7755 had a 6.375 in (162 mm) bore and a 6.75 in (171 mm) stroke. The engine displaced 7,756 cu in (127.1 L) and had a compression ratio of 8.5 to 1. The XR-7755 produced 5,000 hp (3,728 kW) at 2,600 rpm (.2460 propeller gear) for takeoff, 4,000 hp (2,983 kW) at 2,300 rpm (.2460 propeller gear) for normal operation, and 3,000 hp (2,237 kW) at 2,100 rpm (.3536 propeller gear) for cruise power. Specific fuel consumption at normal cruise power was .43 lb/hp/hr (262 g/kW/hr), but the rate dropped to around .38 lb/hp/hr (231 g/kW/hr) at low cruise power of around 1,500 hp (1,119 kW). The engine was 61.0 in (1.55 m) in diameter, 66.25 in (1.68 m) tall, and 121.35 in (3.08 m) long. The XR-7755 weighed 6,050 lb (2,744 kg).

Lycoming XR-7755 ad Dec 1946

Lycoming ad from December 1946 featuring the XR-7755. If the engine was not going to go into production, Lycoming might as well get some press out of it. One can only wonder how those responsible for marketing imagined the huge, liquid-cooled engine would factor into the decision-making process of a person buying a small, air-cooled engine.

The XR-7755 was first run in July 1946. At the time, some 10,000 hours of single-cylinder testing had been completed. The Lycoming factory was located near a residential area. Reportedly, a nearby grocery store’s canned goods would vibrate off the shelves as the XR-7755 underwent high-power tests. A good neighbor, Lycoming went to the store and installed strips on the shelf edges to keep the cans from falling. At takeoff power, the XR-7755’s fuel consumption was 580 gallons (2,196 L) per hour, or 20.62 fl oz (.61 L) per second. The engine’s coolant pump flowed 750 gpm (2,839 l/m) to dissipate 95,600 BTUs (23,956 kcal) per minute. The flow rate was enough to fill a 55 gallon (208 L) drum every 4.4 seconds. The oil pump circulated 71 gpm (269 l/m) at 100 psi (6.89 bar). Lycoming had an optimistic opinion of the engine and believed that an output of 7,000 hp (5,220 kW) was possible.

Most sources indicate that two XR-7755 engines were built: an XR-7755-1 with a single rotation propeller shaft and an XR-7755-3 with a contra-rotating propeller shaft. Both of these engines used carburetors. There is some indication, including the recollections of those who had family members involved with the project, that a third engine was built: an XR-7755-5 with fuel injection. Reportedly, the -1 underwent a 50-hour test run, but the results are not known. The -3 was delivered to the AAF in 1946, but it is unlikely this engine underwent much testing. It is not clear what happened to the -5, if it was completed. By the time the XR-7755 had run, the concept of an aircraft larger than the Convair B-36 Peacemaker had fallen out of favor, as had the idea of modifying the B-36 with larger piston engines. Rather, jets would be used to improve performance of the aircraft. There was no application for the XR-7755 in a post-war world with the performance of jet aircraft quickly being realized. The XR-7755 never flew.

Lycoming XR-7755 AAF Fair Oct 1945

The XR-7755 on display at the Army Air Forces Fair held at Wright Field, Ohio in October 1945. Note what appears to be a mockup of the contra-rotating propeller shafts.

One curious anomaly in the XR-7755’s story is an appearance of the engine at the Army Air Forces Fair held at Wright Field, Ohio in October 1945. This predates the engine’s run date and its supposed delivery to the AAF. However, the engine appears to have a mockup of its contra-rotating propeller shafts installed. It would seem that the engine is not complete and was shipped the 460 miles (740 km) from Lycoming’s factory in Williamsport, Pennsylvania to Dayton, Ohio to be displayed with other unusual treasures from the war. Presumably, the engine was returned to Lycoming after the show and was subsequently completed and tested in 1946.

The sole XR-7755-3 has been preserved by the Smithsonian National Air and Space Museum and is on display in the Steven F. Udvar-Hazy Center in Chantilly, Virginia. Many consider the XR-7755 the largest aircraft engine ever built. However, the Soviet IAM M-44 (8,107 cu in / 132.8 L) of 1933 and Yakovlev M-501 (8,760 cu in / 143.6 L) of 1952 were larger engines. At the time it first ran, the XR-7755 was the world’s most powerful reciprocating aircraft engine.

Lycoming XR-7755-3 NASM

The restored XR-7755-3 on display in the Steven F. Udvar-Hazy Center of the Smithsonian National Air and Space Museum. The bottom of the engine is on the left, marked by the drain tube from the gear reduction housing and the sump built into the valve cover. Note the two spark plug leads for each cylinder passing through opposite sides of the valve covers. (Sanjay Acharya image via Wikimedia Commons)

Sources:
– “5,000-Hp. Lycoming Revealed” by J. H. Carpenter, Aviation (December 1946)
– “The Evolution of Reciprocating Engines at Lycoming” by A. E. Light, AIAA: Evolution of Aircraft/Aerospace Structures and Materials Symposium (24–25 April 1985)
Aircraft Engines of the World 1948 by Paul Wilkinson (1948)
The History of North American Small Gas Turbine Aircraft Engines by Richard A. Leyes II and William A. Fleming (1999)
Studebaker’s XH-9350 and Their Other Aircraft Engines by William Pearce (2018)
https://generalaviationnews.com/2007/04/20/the-xr-7755-the-whole-story/
https://airandspace.si.edu/collection-objects/lycoming-xr-7755-3-radial-36-engine

Lycoming XH-2470 side

Lycoming XH-2470 24-Cylinder Aircraft Engine

By William Pearce

The Lycoming Division of the Aviation Manufacturing Corporation was located in Williamsport (Lycoming County), Pennsylvania. The company had started producing aircraft engines in the late 1920s. In 1937, Lycoming became aware that its most powerful engine to date, the 12-cylinder O-1230, would not produce the power needed for future frontline military aircraft. Development of the O-1230 engine started in 1933, but the anticipated power needs of state-of-the-art aircraft were beyond what the 1,200 hp (895 kW) engine could provide. Lycoming moved quickly to apply knowledge gained from the O-1230 to a new aircraft engine.

Lycoming XH-2470 side

The design of the Lycoming XH-2470 started with the concept of mounting two O-1230 engines to a common crankcase. Note that the propeller shaft is raised above the centerline of the engine.

Lycoming started the design of its new engine in 1938, and detailed design work commenced in mid-1939. The 24-cylinder engine had an H-configuration and consisted of components from two O-1230 engines combined with a new crankcase. The new two-piece aluminum crankcase was split horizontally and accommodated a left and right crankshaft. Each crankshaft served two banks of six cylinders, with one bank above the engine and the other bank below. Fork-and-blade connecting rods were used, with the forked rods serving the lower cylinders. The H-24 engine was designated XH-2470 and given the “Materiel, Experimental” code MX-211. The US Army Air Corps (AAC) initially felt that the engine was too small, but the US Navy supported the design. The Navy ordered a single prototype engine on 11 December 1939, and the AAC started to show some interest in the engine in 1940.

The Lycoming XH-2470 utilized individual cylinders that consisted of a steel barrel screwed and shrunk into an aluminum head. The liquid-cooled cylinder was surrounded by a steel water jacket. The aluminum head had a hemispherical combustion chamber with one intake valve and one sodium-cooled exhaust valve. A cam box was mounted to the top of each cylinder bank, and each cam box contained a single camshaft that was shaft-driven from the rear of the engine.

Lycoming XH-2740 top

Top view of the XH-2470 shows the intake manifold positioned between the cylinder banks. The narrow engine could be installed horizontally (on its side) in an aircraft’s wing. Bell and Northrop pursued this installation for project aircraft, but the designs were not built.

A downdraft carburetor fed fuel into the supercharger’s 12 in (305 mm) diameter impeller mounted to the rear of the engine. Lycoming had experimented with direct fuel injection on test cylinders and planned to have fuel injection available for the XH-2470, but it is unlikely that any complete engines ever used fuel injection. The XH-2470-1, -3, and -7 engines had a single-speed, single-stage supercharger that was driven at 6.142 times crankshaft speed. The XH-2470-5 had a two-speed supercharger that was driven at 6.06 and 7.88 times crankshaft speed. Intake manifolds ran between the upper and lower cylinder banks. Depending on the installation, exhaust gases were either expelled from the outer side of the cylinders via individual stacks or collected in a manifold common to each cylinder bank. Provisions were made for the engine to accommodate a turbosupercharger.

The XH-2470-1, -3, and -5 were available with a single-rotation propeller shaft using a SAE #60 spline shaft. The -1 and -3 had a .38 gear reduction. The -5 was listed as having a two-speed reduction, but the ratios have not been found. The XH-2470-7 had contra-rotating propeller shafts and a two-speed gear reduction with speeds of .433 and .321. The contra-rotating shafts were SAE #40-60 splines, with the inner shaft rotating counterclockwise and the outer shaft rotating clockwise. The gear reduction for all engines was achieved through spur gears, and the propeller shaft was positioned above the engine’s centerline. The engine could be installed in either a vertical or horizontal position.

Lycoming XH-2470-2 drawing

The XH-2470-2 and -4 were engines intended for the Navy. The -2 was similar to the AAF’s engine with a single propeller shaft. The -4 had contra-rotating propeller shafts and was similar to the -7.

The H-2470 had a 5.25 in (133 mm) bore and a 4.75 in (121 mm) stroke. The engine’s total displacement was 2,468 cu in (40.4 L), and it had a 6.5 to 1 compression ratio. The H-2470 produced 2,300 hp (1,715 kW) at 3,300 rpm at 1,500 ft (457 m) for takeoff, 2,000 hp (1,491 kW) at 3,100 rpm at 3,500 ft (1,067 kW) for normal operation, and 1,300 hp (969 kW) at 2,400 rpm at 15,000 ft (4,572 m) for cruise operation. In addition, the two-speed supercharged engine could achieve 1,900 hp (1,417 kW) at 3,300 rpm at 15,000 ft (4,572 m) for emergency power and 1,750 hp (1,305 kW) at 3,100 rpm at 15,000 ft (4,572 m) for normal operation. The XH-2470 had a 3,720-rpm overspeed limit for diving operations. The single-rotation engines were 89.9 in (2.28 m) long, 30.5 in (.77 m) wide, 50.3 in (1.28 m) tall, and weighed 2,430 lb (1,102 kg). The contra-rotating XH-2470-7 was approximately 114 in (2.90 m) long and weighed 2,600 lb (1,179 kg).

Before the XH-2470 had even run, Lycoming proposed a variant of the engine to satisfy the AAC’s Request for Data R40-D, which was issued on 6 March 1940. R40-D sought the design of a 4,000 to 5,500 hp (2,983 to 4,101 kW) aircraft engine for use in long-range bombers. Lycoming proposed coupling two H-2470 engines together, creating a 48-cylinder XH-4940. The XH-4940 would produce 4,800 hp (3,579 kW) at 3,100 rpm up to 8,500 ft (2,591 m) with the aid of a single-speed, single-stage supercharger. The engine had a projected maximum speed of 3,400 rpm and would weigh 6,200 lb (2,812 kg). The AAC’s R40-D ended up going nowhere, and the request was cancelled in mid-1940.

Lycoming XH-2470 test stand

An XH-2470 mounted on a test stand with a tractor propeller. Installed in the XP-54 as a pusher; the blades on the XH-2470 had their angle reversed. Note the individual exhaust stacks.

The XH-2470 was first run in July 1940. The engine was proposed for the Curtiss XF14C Naval fighter and the Vultee XP-54 Swoose Goose AAC fighter. The XP-54’s original power plant was the Pratt & Whitney X-1800 (XH-2240 / XH-2600), but development of this engine stopped in October 1940. It was the cancellation of the X-1800 that led to the AAC’s interest in the XH-2470, and the AAC ordered 25 (later increased to 50) engines in October 1940. The AAC was also interested in potentially using the XH-2470 to power the Lockheed XP-58 Chain Lightning. Bell and Northrop also expressed interest in the engine for future projects.

The XH-2470 completed a Navy acceptance test in April 1941. At the time, the XF14C and XP-54 prototypes were in the detailed design stage. However, the Army Air Force (AAF—the AAC had changed its name in June 1941) continued to alter the XP-54 requirements throughout 1941. Added to the Vultee project were Turbosuperchargers, a pressurized cockpit, and the option of the Wright R-2160 Tornado engine. It was not until 1942 when R-2160 development was seriously behind schedule that the engine was dropped from the XP-54 and a more focused installation of the XH-2470 was presented. An XH-2470-7 engine with contra-rotating propellers was intended for the XP-54, but a single rotation engine was substituted because of delays with the contra-rotating gearbox. The AAF specified two Wright TSBB turbosuperchargers for the first XP-54 prototype and a single, experimental General Electric (GE) XCM turbosupercharger for the second aircraft. Reportedly, the Navy ordered 100 XH-2470 engines in May 1942. However, this may have been a total of 100 engines on order with 50 going to the AAF and 50 to the Navy.

The first XP-54 (41-1210) made its first flight on 15 January 1943, taking off from Muroc (now Edwards) Air Force Base, California. With the exception of the first flight, the XH-2470 engine installed in the XP-54 turned a 12 ft 2 in (3.71 m) Hamilton Standard propeller. Although the aircraft handled well, its development had suffered through constant changes in design and intended role. The aircraft underperformed, and the XH-2470 engine had some issues, such as oil foaming at high RPM or at altitudes above 20,000 ft (6,096 m).

Lycoming XH-2470 Vultee XP-54

The Vultee XP-54 was a very large aircraft. Even so, the installation of the XH-2470 appears to be quite cramped. Note the large exhaust manifold linking the engine to the turbosupercharger, which was positioned behind the cockpit.

The first XP-54 was flown to Wright Field, Ohio on 28 October 1943. After the next flight, a close inspection of the XH-2470 revealed some minor issues as well as damage to the supercharger impeller. The engine was removed and sent to Lycoming for repairs. The cost to fix the engine was more than the AAF was willing to pay, which showed the AAF’s lack of interest in the XH-2470 program. The first XP-54 was removed from flight status and used as a source of spare parts for the second XP-54 aircraft. The first XP-54 had completed 86 flights and accumulated 63.2 hours of flight time.

To make matters worse, the Navy cancelled its XH-2470 order in December 1943, deciding to power the XF14C with a turbosupercharged Wright R-3350 instead. Factors that influenced this change were the Navy’s long-standing preference for air-cooled engines, a shift of the XF14C’s role to that of a high-altitude fighter, issues with the XH-2470’s developmental progress, and doubts that the engine would be ready in time to see combat during World War II. At the same time, Lycoming had moved on to another aircraft engine project, the 36-cylinder XR-7755. Lycoming had invested over $1,000,000 of its own money into the XH-2470 engine.

Lycoming XH-2470 Vultee XP-54 top

The two exhaust outlets from the turbosupercharger protrude quite visibly behind the cockpit. The panel behind the exhaust was stainless steel, and hot exhaust burned the paint off the cowling on early flights. The upper cowling was later replaced with an unpainted stainless steel unit, and the rudders were painted around the same time. (Aerospace Legacy Foundation Archive image)

The second XP-54 prototype (42-108994, but incorrectly painted as 41-1211) made its first flight on 24 May 1944, taking off from Vultee Field. After at least three flights, the GE XCM turbosupercharger and the XH-2470 were removed from the aircraft. Some incompatibility between the turbosupercharger and engine had caused damage to both units. A new engine and turbosupercharger were installed, and the XP-54 flew again in December 1944. The second XP-54 made at least 10 flights, the last ending with an engine failure on 2 April 1945. The airframe had accumulated 10.7 hours of flight time.

At least one XH-2470 engine has been preserved. An XH-2470-7 is in storage at the Smithsonian National Air and Space Museum. The engine, which was never installed in any aircraft, has contra-rotating propellers and a two-speed gear reduction. The Smithsonian also lists an XH-2470-1 engine from the XP-54 in its inventory. However, no further evidence of this engine’s existence has been found.

The preserved XH-2470-7 is in storage at the Smithsonian National Air and Space Museum. Although the engine was never installed in any aircraft, at least it may be displayed one day. (NASM image)

The preserved XH-2470-7 is in storage at the Smithsonian National Air and Space Museum. Although the engine was never installed in any aircraft, at least it may be displayed one day. (NASM image)

Sources:
Aircraft Engines of the World 1947 by Paul Wilkinson (1947)
American Secret Pusher Fighters of World War II by Gerald H. Balzer (2008)
Development of Aircraft Engines and Fuels by Robert Schlaifer and S. D. Heron (1950)
U.S. Experimental & Prototype Aircraft Projects Fighters 1939–1945 by Bill Norton (2008)
Experimental & Prototype U.S. Air Force Jet Fighters by Dennis R. Jenkins and Tony R. Landis (2008)
The History of North American Small Gas Turbine Aircraft Engines by Richard A. Leyes II and William A. Fleming (1999)
Studebaker’s XH-9350 and Their Other Aircraft Engines by William Pearce (2018)
Preliminary Model Specification for Engine Aircraft Model XH-2470-4 for Opposite Rotating Propellers by Aviation Manufacturing Corporation Lycoming Division (18 April 1940)
– “The Evolution of Reciprocating Engines at Lycoming” by A. E. Light, AIAA: Evolution of Aircraft/Aerospace Structures and Materials Symposium (24–25 April 1985)
https://airandspace.si.edu/collection-objects/lycoming-xh-2470-7-h-24-engine
https://airandspace.si.edu/collection-objects/lycoming-xh-2470-1-h-24-h-type-engine

Nakajima HA-54 engine front

Nakajima [Ha-54] (Ha-505) 36-Cylinder Aircraft Engine

By William Pearce

On 6 December 1917, Chikuhei Nakajima founded one of the first aircraft manufacturing companies in Japan. Originally known as the Japanese Aeroplane Research Institute (Nihon Hikoki Kenkyūsho), the company went through a few name changes and was reorganized as the Nakajima Aircraft Company, Ltd (Nakajima Hikōki KK) in December 1931, with Chikuhei Nakajima as its chairman. At the onset of World War II, Nakajima felt that the United States had vast industrial resources and that the attack on Pearl Harbor would have dire consequences for Japan. Nakajima believed that the only way to ensure a Japanese victory was for Japan to have a reliable method to consistently attack the US mainland. Such attacks would require the US military to divert offensive forces to defend and protect the US.

Nakajima HA-54 engine front

The 36-cylinder, 5,000 hp (3,728 kW) Nakajima [Ha-54] was an ambitious engine program. The engine was needed long before it would be ready. The project was cancelled to reallocate resources to more urgent needs.

In 1942, Nakajima made his thoughts known to the Imperial Japanese Army (IJA) and Imperial Japanese Navy (IJN), but neither service gave Nakajima much consideration or any support. Frustrated, Nakajima used his company’s own resources to design a large bomber capable of striking the US mainland from Japan. Nakajima designated his bomber design Project Z (or Z Airplane). After months of research, Nakajima again approached the IJA and IJN in April 1943 with his Project Z proposals. The IJA and IJN were more receptive, but there was no consensus between the services on the aircraft’s design, specifications, or mission. The IJA and IJN submitted an updated proposal to Nakajima in June 1943, and through the remainder of 1943, Nakajima worked to solidify the Project Z aircraft design.

In August 1943, Nakajima distributed his thesis, “Strategy for Ultimate Victory,” to IJA and IJN officials and to prominent politicians. The thesis outlined strategic use of Project Z bombers to attack industrial targets in the US. The bombers could fly from Japan to Germany and strike targets in the western US as well as industrial areas in the mid-western states. Once in Germany, the bombers would be rearmed and refueled to make the return trip to Japan, again striking the US. The attacks, combined with their effect on production, was predicted to halt the US advance against Japan and aid in the defense of Germany.

Nakajima HA-54 section

Sectional drawing of what is believed to be the final configuration of the [Ha-54] engine. A single-rotation propeller is depicted. Note the cooling fan at the front of the engine, the annular intake manifold at the middle of the engine, and the supercharger impeller at the rear of the engine. Induction manifolds are shown at the top of the drawing, and exhaust manifolds are at the bottom.

Although several different designs and configurations were investigated, the Project Z aircraft that entered development was a six-engine, long-range bomber of all metal construction. The proposed aircraft had a wingspan of 213.2 ft (64.98 m), a length of 147.6 ft (44.98 m), and a height of 28.8 ft (8.77 m). Project Z had an empty weight of 143,300 lb (65,000 kg) and a maximum weight of 352,739 lb (160,000 kg). The bomber’s top speed was 422 mph (679 km/h) at 32,808 ft (10,000 m). Project Z had a maximum range of 11,185 miles (18,000 km) and a ceiling of 49,213 ft (15,000 m).

Such a massive aircraft required very powerful engines; however, no engine in Japan had the power that Project Z required. To solve this issue, Nakajima decided to make a power plant that could support Project Z by coupling together two existing [Ha-44] engines. The [Ha-44] was the most powerful engine that Nakajima was developing, and the new combined engine for Project Z was designated [Ha-54].

The heritage of Nakajima’s radial engines lies in licenses the company acquired to produce the Bristol Jupiter, Pratt & Whitney R-1340 Wasp, Wright R-1820 Cyclone, and Gnome-Rhône 14M. Nakajima obtained the Jupiter license in 1925, the Wasp license in 1929, the Cyclone license in 1933, and the 14M license in 1937. While Jupiter engines were produced, the Wasp and Cyclone licenses were obtained to gain knowledge, and Nakajima did not produce those engines. Many Nakajima radial engines used a 146 mm bore, which directly corresponded to the Jupiter’s 5.75 in bore, and several Nakajima personnel spent a few months in the US being instructed by Wright as part of the license agreement. The general construction of Nakajima radials resembled a combination of Bristol, Pratt & Whitney, and Wright engines, and Nakajima engineers consistently incorporated their own thoughts and ideas into their engines.

Nakajima HA-54 engine mount

This model of the [Ha-54] illustrates the significant structure proposed to mount the engine. Such a mounting system would make engine maintenance very difficult, with the entire engine needing to be pulled to change a cylinder on the rear engine section. Note the exhaust collector ring and its outlet, which would lead to a turbosupercharger.

When Nakajima started to develop their own engines in the late 1920s, their designation system consisted of their name and the engine type followed by the engine model letter. As an example, the designation “NAL” represented Nakajima Air-cooled L model. When Nakajima started to develop a new class of high-power engines in the early 1940s, the focus was on air-cooled engines. Subsequently, the letter “N” and the engine type were dropped from the engine designation system. A new letter was used to designate the next generation of engines, followed by the engine model letter. As an example, the designation “BH” stood for B generation H model.

Japanese companies, the IJA, and the IJN all had their own designations for engines. To eliminate confusion (or perhaps add more), a joint designation system was introduced in May 1943. Nakajima [Ha-44] was the joint designation for the engine that would be the basis for Project Z’s engines. The [Ha-44] carried the Nakajima designation BH, the IJA designation Ha-219, and the IJN designation NK11. The [Ha-44] was an experimental, 2,942 cu in (48.2 L), 18-cylinder, air-cooled, radial engine. Development of the [Ha-44] began around 1941, and the engine was based on the 14-cylinder Nakajima [Ha-34] (Nakajima NAL, IJA Ha-41/Ha-109, and IJN NK5). By mid-1943, the [Ha-44] was producing 2,400 hp (1,790 kW) at 2,700 rpm. Two [Ha-44] power sections would be connected to make up the [Ha-54] engine.

Nakajima HA-54 engine cooling flow

This drawing depicts the cooling air flow of the [Ha-54] engine. This was the proposed configuration, but sufficient cooling for the rear cylinders was never achieved. Note the cooling fan at the front of the engine and the intake manifold ring at the center of the engine.

The [Ha-54] engine carried the Nakajima designation D-BH, for Double-BH. It was also designated Nakajima Ha-505 by the IJA but had no known IJN designation. Developed under chief designer Kiyoshi Tanaka, the engine was a 36-cylinder, air-cooled radial with two-stage supercharging. The [Ha-54] was comprised of two [Ha-44] engine sections bolted together via an intermediate section. The two engine sections formed a single four-row radial engine with nine-cylinders in each row. The crankshafts of the two engine sections were directly joined, and neither engine section could operate without the other.

The [Ha-54] used cylinders constructed of an aluminum head that was screwed onto a steel barrel. Each cylinder had two valves that were actuated by pushrods driven by cam rings. Exhaust gases flowed into a collector ring positioned behind the engine. Two ducts led from the collector ring, with each duct delivering the exhaust to a turbosupercharger located farther aft in the engine nacelle. The pressurized air from the turbosupercharger was fed through an intercooler and into a three-speed, mechanically-driven supercharger at the rear of the engine. The supercharger’s impeller was 19.69 in (500 mm) in diameter, and it was driven at 1.00, 4.77, or 5.88 times crankshaft speed. The fully-charged air from the supercharger was directed to an annular manifold at the center of the engine. Induction air for the front and rear engine sections was distributed from this central manifold. Because of this configuration, cylinders on the front engine section had rear-facing intake and exhaust ports, while the cylinders on the rear engine section had front facing intake ports and rear facing exhaust ports. Other methods to distribute air into the cylinders were investigated, including the use of two supercharger impellers at the rear of the engine, with each impeller dedicated to one engine section. In addition, some versions of the engine did not employ a turbosupercharger.

Nakajima HA-54 with baffles

Nakajima tests indicated that cooling the [Ha-54] engine would be difficult. This model shows one of the final cooling configurations and matches the above drawing. Air flowed through the cylinders of the front engine section and exited at the center of the engine. Air that flowed over the front cylinders was directed down to flow through the cylinders of the rear engine section.

Some sources state a low pressure fuel injection system was planned for the [Ha-54] engine. Other Nakajima engines injected the fuel directly before the impeller at 14 psi (.97 bar). However, a different fuel injection point may have been selected to avoid the constant presence of a volatile air/fuel mixture in the annular induction ring at the center of the engine. Anti-detonation injection was also available for takeoff or emergency power.

Cooling the huge engine posed a serious issue, and much research was devoted to finding an adequate solution. Investigations were conducted to employ fan-assisted, forced-air cooling with an engine-driven fan positioned at either the front or rear of the engine. Another plan involved ram-air cooling for the front engine section and reverse cooling for the rear engine section. In this configuration, baffling separated the air flow from the front and rear engine sections. Air to cool the rear engine section was brought in via the wing’s leading edge, made a 180-degree turn, flowed through the cylinders of the rear engine section, and exited cowl flaps positioned at the middle of the engine.

Nakajima HA-54 no turbo

A fully-cowled model of the [Ha-54] with ram-air (no fan) cooling for the front engine section and reverse cooling for the rear engine section. Note how the cooling air for both engine sections exited the cowl flaps at the center of the nacelle. Also, the engine depicted did not use a turbosupercharger and had nine exhaust stacks protruding from the cowling.

The final cooling configuration involved an engine-driven fan at the front of the cowling. Air was ducted through the cooling fins of the front engine section’s cylinders. The air then exited cowl flaps at the center of the engine cowling. Separately, air was ducted over the cylinders of the front engine section and through the cooling fins of the rear engine section’s cylinders. This air then exited via cowl flaps at the rear of the engine cowling.

At the front of the [Ha-54] engine, the crankshaft drove a propeller gear reduction and the cooling fan. The planetary gear reduction turned the propeller at .414 times crankshaft speed. Both single-rotation and contra-rotating propellers were considered, with the final decision in favor of six-blade (or possibly eight-blade) contra-rotating propellers that had a diameter of 14.8 ft (4.5 m). The [Ha-54] engine would be supported by a series of mounting rings that connected to the engine at its center and rear.

The Nakajima [Ha-54] had a 5.75 in bore (146 mm) and a 6.30 in (160 mm) stroke. The 36-cylinder engine had a 7.5 to 1 compression ratio and a total displacement of 5,885 cu in (96.4 L). The [Ha-54] had a maximum output of 5,000 hp (3,728 kW) at 2,800 rpm with 11.6 psi (.80 bar) of boost for takeoff and 4,600 hp (3,430 kW) at 2,800 rpm with 7.73 psi (.53 bar) of boost at 29,528 ft (9,000 m). The engine had a diameter of 61 in (1.55 m) and was 141 in (3.58 m) long. The [Ha-54] weighed approximately 5,400 lb (2,450 kg).

Nakajima HA-54 two impellers

This drawing of the [Ha-54] shows some of the engine mount positions. Note that there are two supercharger impellers depicted. In this version of the engine, each impeller provided induction air to one engine section.

The question of cooling the [Ha-54] was never entirely resolved, and development of the engine was forecasted to take too long. By January 1944, work on the [Ha-54] had stopped, and it is unlikely that a complete engine was ever built. With the the cancellation of the 5,000 hp (3,728 kW) [Ha-54], a scaled-down version of the Project Z aircraft was designed and powered by [Ha-44] engines, which had been developed to 2,500 hp (1,864 kW). The new aircraft, designated Nakajima G10N Fugaku (Mount Fiji) was slightly smaller but still possessed the same, six-engine configuration as Project Z. By the summer of 1944, all of Japan’s resources were needed to protect the homeland, and the prospect of a long-range bomber attacking the US mainland was out of reach. The G10N Fugaku was cancelled, and all work stopped on its engines.

After World War II, the Nakajima Aircraft Company became the Fuji Industrial Company, Ltd. (Fuji Sangyo KK). In July 1953, the company merged with four others to form Fuji Heavy Industries, Ltd. (Fuji Jūkōgyō Kabushiki-gaisha). Fuji Heavy Industries manufactured Subaru automobiles and supplied equipment for the aerospace and transportation industries. In April 2017, Fuji Heavy Industries was renamed the Subaru Corporation.

Nakajima Project Z

The final version of the original Nakajima Project Z bomber with its six [Na-54] engines. Development of the [Na-54] was forecasted to take too long, so [Na-44] engines were substituted, and the aircraft was scaled-down as the Nakajima G10N Fugaku. As Japan’s prospects for an offensive victory faded, the project was cancelled, and resources were reallocated to defense.

Sources:
Japanese Aero-Engines 1910–1945 by Mike Goodwin and Peter Starkings (2017)
Japanese Secret Projects 1 by Edwin M. Dyer III (2009)
– “Nakajima Engine Design and Development” Air Technical Intelligence Group Report No. 45 by J. H. Morse (5 November 1945)
– “Nakajima Aircraft Company, LTD” The United States Strategic Bombing Survey, Corporation Report No. II (June 1947)
https://www.secretprojects.co.uk/forum/index.php?topic=24084.0
http://www.enginehistory.org/Piston/Japanese/japanese.shtml
http://forum.valka.cz/topic/view/199974/Ha-505-Ha-54-01
https://ja.wikipedia.org/wiki/%E5%AF%8C%E5%B6%BD