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.

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.

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.

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.

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/

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.

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.

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.

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 planetary propeller gear reduction was hydraulically shifted by engine oil boosted to 300 psi (20.68 bar) by a pump in the nose case. A .2460 reduction was available for high engine speeds, and a .3536 reduction was used for cruise operations with low engine rpm. Due to its size, the XR-7755 required two starters. Both starters were mounted vertically on the crankcase in front of the cylinder banks; one was at the 2:30 position, and the other was at the 9:30 position.

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).

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 (24,107 kcal) per minute. That is 2,504 hp (1,681 kW) of heat being rejected into the coolant, and the system’s 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). The oil system absorbed 25,500 BTUs (6,430 kcal) per minute, which is 601 hp (448 kW). 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.

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.

Sources:
– “5,000-Hp. Lycoming Revealed” by J. H. Carpenter, Aviation (December 1946)
– Lycoming XR-7755 Aircraft Engine and Engineering Laboratories by Lycoming Division, The Aviation Corporation (31 October 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

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 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.

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.

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.

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).

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.

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.

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

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.

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.

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.

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.

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.

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.

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).

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.

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

By William Pearce

The first original engine made by the Fairey Aviation Company (FAC) was the P.12 Prince. Designed by Captain Archibald Graham Forsyth, the Prince was a 1,559 cu in (25.5 L) V-12 that was ultimately rated at 750 hp (559 kW) for normal output and 900 hp (671 kW) for maximum output.

Seeking more power, Forsyth investigated a 16-cylinder engine designated P.16 that displaced 2,078 cu in (34.1 L). A V-16 design was initially considered, but the configuration was changed over concerns regarding the engine’s length combined with excessive torsional vibrations and stress of the long crankshaft. The P.16 configuration was switched to an H-16 with four banks of four cylinders. However, by switching to an H-24 configuration with four banks of six cylinders, a more powerful engine could be developed that would possess the same frontal area as the H-16. In fact, there is little evidence from primary sources that indicates a P.16 engine or an H-16 configuration were ever seriously considered. Design work on the H-24 engine had begun by October 1935. The H-24 engine was designated P.24 and may have been initially named “Prince” (like the P.12). For a time, the P.24 was called “Queen,” but the name was later changed to “Monarch.”

The Fairey P.24 Monarch was a vertical H engine in which the left and right 12-cylinder engine sections could be operated independently of each other. The aluminum crankcase was made of an upper and lower half. Each bank of six cylinders was mounted to the crankcase. An aluminum cylinder head attached to the cylinder bank. The P.24 retained the bore and stroke of the P.12 (and P.16) engine. Each cylinder had two intake valves and one exhaust valve, all actuated by a single overhead camshaft driven from the rear of the engine. The 1.8125 in (46 mm) diameter intake valves were operated by a T-type tappet, and the 2.25 in (57 mm) diameter exhaust valve was operated directly by the camshaft. It is interesting to note that various British patents (no. 463,501 and 465,540) filed by FAC and Forsyth show four valves per cylinder. No information has been found of a four-valve head being fitted to a P.24 engine.

Mounted to each side of the engine was a single-stage, two-speed supercharger with two carburetors mounted on its inlet. The superchargers were driven from the rear of the engine at 8.3475 times crankshaft speed through three step-up gears. Via a short manifold, each supercharger delivered air into passageways cast integral with the crankcase, cylinder bank, and cylinder head. A single passageway brought air into two cylinders. It was noted in British patent 463,501 that having intake passageways cast integral with the crankcase made the engine cleaner, more rigid, and opened the vertical space between the cylinder banks for either an oil cooler or gun. However, if a gun were to be considered, the crankcase construction did not allow it to fire through the propeller hub. Rather, the gun would need to be synchronized to fire through the propellers.

The air/fuel mixture was ignited by two spark plugs—one on each side of the cylinder. The spark plugs were fired by four magnetos mounted to the rear of the engine. The upper two magnetos fired the spark plugs located on the inner side of the cylinders, and the lower two magnetos fired the spark plugs located on the outer side of the cylinders. The left and right sides of the P.24 had the same firing order: 1U, 5L, 4U, 1L, 2U, 4L, 6U, 2L, 3U, 6L, 5U, and 3L. The first and last cylinders had their own exhaust stack, but the middle four cylinders were paired off with a shared exhaust stack. This arrangement gave the P.24 16 exhaust stacks, which often caused the engine to be mistaken for an H-16.

At the front of the engine, each crankshaft was geared to a separate propeller shaft at a .5435 reduction. The two propeller shafts made up a coaxial, contra-rotating unit. The crankshafts and accessories for each engine section rotated in opposite directions. When viewed from the rear, the left crankshaft rotated clockwise and powered the front propeller. The right crankshaft rotated counterclockwise and powered the rear propeller. The P.24’s fully-adjustable, contra-rotating propeller was developed by FAC. Each crankshaft had six throws and was supported by eight main bearings. The pistons were connected to the crankshaft by fork-and-blade connecting rods, with the fork rod servicing the lower cylinders and the blade rod servicing the upper cylinders. Since they rotated in opposite directions, the crankshafts and engine accessories were not interchangeable.

When either the left or right side of the engine was shut down, the other side of the engine would continue to operate and power its propeller. This gave any aircraft powered by a P.24 engine the reliability of two engines without the drawback of asymmetrical thrust in an engine-out situation. This configuration also allowed half of the engine to be shut down to extend the aircraft’s range or loiter time. Although the cooling and oil systems of the engine sections were separate, they used common tanks and coolers during engine tests. It was noted that completely separate coolers and tanks could be installed in an aircraft to make the engine sections truly independent of one another.

The Fairey P.24 Monarch had a 5.25 in (133 mm) bore, a 6.0 in (152 mm) stroke, and a compression ratio of 6 to 1. The engine displaced 3,117 cu in (51.1 L) and was originally rated for 2,000 hp (1,491 kW) at 2,600 rpm. However, Forsyth felt the engine could be developed to 2,600 hp (1,939 kW) at 3,000 rpm. The two-speed superchargers gave critical altitudes of 6,000 and 13,000 ft (1,829 and 3,962 m). Fuel consumption was .63 lb/hp/hr (383 g/kW/h) at takeoff, .55 lb/hp/hr (335 g/kW/h) at full power, and .45 lb/hp/hr (274 g/kW/h) at 60% throttle. The P.24 engine was 86.25 in (2.19 m) long, 43.0 in (1.09 m) wide, and 52.5 in (1.33 m) tall. The engine weighed around 2,330 lb (1,057 kg), although 2,180 lb (989 kg) is given by many sources. Perhaps the discrepancy in reported weight is a result of the engine’s weight changing with the installation of various accessories.

The P.24 was first run in 1938. The test cell at FAC was not built for the full power of the P.24, so just half of the engine was run at a time. By 6 October 1938, the engine was successfully completing two-hour runs without issues. Arrangements were made to install the P.24 in a Fairey Battle light bomber, and the engine was being considered for use in the Hawker Tornado fighter. The P.24 started its 50-hour civil type test on 13 May 1939 and successfully completed the test on 14 June 1939. For the test, the engine ratings were a normal output of 1,200 hp (895 kW) at 10,500 ft (3,200 m) and 2,400 rpm, and a maximum output of 1,450 hp (1,081 kW) at 10,500 ft (3,200 m) and 2,750 rpm. It appears that a single-speed supercharger delivering just .5 psi (.04 bar) of boost was used for the 50-hour test.

The P.24-powered Battle (K9370) first flew on 30 June 1939, piloted by Christopher S. Staniland. A short time later, a P.24 test engine achieved 1,540 hp (1,148 kW) at 2,600 rpm with 3 psi (.21 bar) of boost for takeoff. With P.24 tests moving forward, FAC looked to apply the engine to new aircraft designs. On 19 October 1939, FAC proposed using the P.24 engine in aircraft designed to specifications N8/39 and N9/39. Designed by FAC chief designer Marcel Lobelle, the same basic single-engine, two-seat fighter design was used to satisfy both specifications, with the addition of a turret for N9/39. In Lobelle’s drawings, the FAC engine was labeled as the “Queen,” but it was visually identical to the P.24 as built. In addition, the late-1939 time frame was after FAC had moved away from 16-cylinder engine designs. In the late-1939 proposals, the engine was rated at 1,320 hp (984 kW) for takeoff and 1,170 hp (872 kW) at 15,000 ft (4,572 m).

During 1940, a number of short runs were made up to 1,750 hp (1,305 kW), but there were no long periods of running at these high outputs. Trouble was experienced with the superchargers and main bearings. A maximum of 2,200 hp (1,641 kW) was achieved, but not with both halves of the engine running at the same time. Each individual engine half achieved an output of 1,100 hp (820 kW), adding up to the 2,200 hp (1,641 kW) figure. In fact, each engine half made two runs at 1,100 hp (820 kW), but the runs were only a short duration of two minutes each.

In October 1940, the Air Ministry made it clear that no production orders for the P.24 engine would be placed. Instead, the focus was on other engines already further developed and being designed by established engine manufacturers with greater facilities for production. In April 1941, the Fleet Air Arm (FAA) expressed some interest in the P.24. Since the P.24 had twin-engine reliability, and its contra-rotating propellers eliminated torque reaction, the engine was ideally suited for carrier operations. However, the Air Ministry again asserted that there would be no production of the engine. This did not stop FAC from continuing to push the P.24 engine, even proposing in mid-1942 that the Fairey Firefly carrier fighter should be re-engined with a P.24 that would produce 2,150 hp (1,603 kW) at 3,000 rpm. The Air Ministry was not interested, and the plan went no further. The P.24-powered Battle was turned over to Royal Aircraft Establishment Farnborough on 12 July 1941. By this time, P.24 engines on test had been run 20 hours at 2,000–2,100 hp (1,491–1,566 kW) and five hours at 2,100–2,300 hp (1,566–1,715 kW).

Fairey had discussed the P.24 engine with United States Army Air Force (AAF) officials in June 1941. The following month, Forsyth visited the US to give more details about the engine. Fairey and Forsyth felt that the Air Ministry had made a mistake in exclusively backing the Napier Sabre and not providing any support for the P.24. They wanted P.24 development and production to continue in the US; production in the US by an established engine manufacturer would be easier than FAC undertaking the task themselves. By this time, the name “Monarch” was applied to the P.24 engine. In August 1941, the AAF stated that they were not interested in the development or production of the P.24 engine, but they were interested the in the contra-rotating propellers developed for the engine. However, Fairey stated in a letter dated November 1941 that Wright Field in Dayton, Ohio had prepared three-view drawings of the P.24 installed in the Republic P-47 Thunderbolt fighter and the Curtiss A-25 Shrike (SB2C Helldiver) dive bomber. Fairey also stated that the Ford Motor Company was engaged in discussions about producing the engine.

The apparent reversal of the AAF’s interest in P.24 production seems odd, and it may have been more optimism on Fairey’s part than what was really expressed by the AAF. Fairey did want to get the engine to the US, and claiming that the AAF was interested was the quickest way to get the cooperation of the Air Ministry, who had been battling Fairey for quite some time. Regardless of the AAF’s level of interest in the engine, they were certainly interested in the contra-rotating propellers. The P.24-powered Battle was shipped to the US on 5 December 1941. Another P.24 engine was delivered to Farnborough for further testing, and a third Monarch engine was prepared for shipping to the US. With the Japanese attack on Pearl Harbor, the second P.24 engine was never sent to the US.

Many sources state that the P.24 engine was considered to replace the Pratt & Whitney R-2800 in the P-47. It should be noted that an order for 773 P-47Bs (602 were finished as P-47Cs) was placed in September 1940; the XP-47B made its first flight on 6 May 1941, and an order for 805 P-47Ds was placed in October 1941. All of this P-47 activity occurred before the AAF touched the P.24 engine and before the US entered World War II. An additional 1,050 P-47Ds and 354 P-47Gs were ordered in January 1942, before the AAF had much, if any, time to evaluate the P.24 engine. It would seem that the AAF was quite content with the R-2800 engine, since full-scale P-47 production was underway, and 2,982 aircraft were on order. A request from Fairey dated January 1943 sought a new set of propellers for a P.24 engine installed in a P-47. At the time, the P-47 was two months away from entering combat, with hundreds of aircraft produced and thousands on order. It seems highly unlikely that any engine change would have been seriously considered by the AAF.

When the AAF received the P.24-powered Battle at Wright Field in Dayton, Ohio, it had around 87 hours of flight time. The P.24 engine had a takeoff rating of 2,100 hp (1,566 kW) and military ratings of 2,100 hp (1,566 kW) at 6,000 ft (1,829 m) and 1,850 hp (1,380 kW) at 13,000 ft (3,962 m); all ratings were at 3,000 rpm with 9 psi (.62 bar) of boost and using 87 octane fuel. Forsyth believed the engine was capable of 2,600 hp (1,939 kW) with 100 octane fuel. The AAF found the coaxial propellers and the “double engine” concept novel, but found the rest of the engine conventional. The AAF felt the design of the P.24 limited its further development. Since the intake manifolds were cast into the engine’s crankcase, a complete redesign would need to be undertaken to improve their flow. As it was, the intake manifolds fed the air/fuel mixture clumsily into the cylinders through five turns, some of which were fairly sharp 90-degree bends. The integrally cast manifold also caused the air/fuel mixture to be heated as it flowed to the cylinders. Another issue was that the gear reduction housing was cast integral with the crankcase. If a failure caused damage to the gear reduction housing, the entire crankcase would need to be scrapped. With all of the integral components, the two crankcase castings were large and complex. The AAF also noted the non-interchangeability of the crankshafts and other components. The AAF remarked that BMEP (brake mean effective pressure) achieved by the P.24 at 3,000 rpm with 9 psi (.62 bar) of boost was matched by the Allison V-3420 with 3.75 psi (.26 bar) of boost at the same rpm. In addition, the relatively low critical altitude of the P.24 meant the Rolls-Royce Merlin 60 series had a 160 hp (119 kW) advantage at 30,000 ft (9,144 m).

Of course, enhancements to the P.24 could be made, and Forsyth suggested that further development of the engine be conducted in the US. This included increasing the bore of the P.24 to 5.5 in (140 mm), which would give a displacement of 3,421 cu in (56.1 L) and an output of 3,000 hp (2,237 kW). In addition, a P.32 could be built with four banks of eight cylinders for a total displacement of 4,156 cu in (68.1 L). The bore of the 32-cylinder P.32 could also be increased to 5.5 in (140 mm) for a total displacement of 4,562 cu in (74.8 L). A 24-cylinder engine with a 6.0 in (152 mm) bore and stroke could be developed that would displace 4,072 cu in (66.7 L) and produce 3,400 hp (2,535 kW).

Additions to the basic P.24 included developing a two-stage supercharger that would enable the engine to produce around 1,800 hp (1,342 kW) at 36,000 ft (10,973 m). The AAF mentioned that the stages would be separate, with one stage driven from the propeller gear reduction at the front of the engine. In May 1942, Forsyth applied for a US patent (no. 2,470,155) that addressed some of the AAF’s concerns regarding the engine’s gear reduction and supercharger. The patent outlines the basic P.24 engine but with a detachable propeller gear reduction. The engine could be used in a variety of aircraft, and different gear reductions would be fitted to provide the desired propeller rpm based on the aircraft’s role. An extension could be installed between the engine and propeller gear reduction that had a mounting to drive an additional supercharger on each side of the engine. In addition, different superchargers with their own gearing could be installed on the engine to provide different levels of boost based on the needs of the aircraft. In some configurations, superchargers could be engaged as the aircraft gained altitude. In addition, US patent 2,470,155 described how P.24 engines could be coupled to create an H-48 engine. A 48-cylinder engine would displace 6,234 cu in (102 L) and produce over 4,400 hp (3,281 kW).

In June 1942, FAC and Forsyth applied for another US patent (no. 2,395,262) that detailed another supercharger configuration, but with a turbosupercharger mounted to the front of the engine. This configuration was briefly mentioned in US patent 2,470,155. In US patent 2,395,262, the turbosupercharger was the first stage and was powered by the exhaust gases from the top row of cylinders. Exhaust from the lower cylinders was combined with the exhaust from the turbosupercharger. The pressurized air flowed from the turbosupercharger to the gear-driven supercharger at the rear of the engine. The air was pressurized further and directed into the engine via manifolds as seen on the P.24.

Forsyth had been considering the combined turbo and supercharger arrangement since before the P.24 first ran. This concept was outlined in a British patent (no. 463,984) applied for in 1935 and granted in 1937. In this patent, the turbosuperchargers would be mounted on the sides of the engine, outside of the induction manifolds. The gear-driven supercharger would be run alone for low altitude operation, and the turbosupercharger would provide additional boost at higher altitudes. At a certain altitude, valves would open, allowing engine exhaust into the turbosupercharger to start its operation. At the same time, valves on the gear-driven supercharger’s inlet pipes would close. The turbosupercharger would become the first stage of charging and feed air into the gear-driven supercharger, in which fuel would be added and the mixture further pressurized before being fed into the cylinders.

Forsyth also suggested developing a jet engine section to be added to the P.24. British patent 591,048 described the P.24 employed in a semi-motorjet configuration. Behind the piston engine was a supercharger, and behind the supercharger was a large, engine-driven compressor. Fuel was injected and ignited in the compressor to generate thrust. Three different power options were available: the engine could drive the propellers only; the engine could drive both the propellers and the compressor; or the propellers could be disengaged, and the engine would drive the compressor alone, the compressor generating the thrust needed to maintain flight. A series of clutches connected or disconnected the propellers, piston engine, and compressor.

British patent 591,189 described a very similar engine concept as the semi-motorjet listed above; however, two compressors were used. Each engine section had its own compressor section, and the piston engine’s superchargers were again located on the side of the engine. In addition to the power options listed in the previously mentioned patent, one engine section could drive one propeller, while the other engine section could drive one compressor. Both of the patents that incorporated compressors with the P.24 engine were applied for in 1944 and granted in 1947.

Forsyth even thought about using P.24 components to create a marine engine. US patent 2,389,663 outlines P.24 cylinder banks being used in a U-12 configuration. Two U-12 engine sections would be combined to create a U-24 engine. The drive for the contra-rotating marine propellers would come from between the two U-12 engine sections. The patent notes that the six-cylinder engine sections could be run independently and that the superchargers could be engaged or disengaged. Low-speed operation would consist of running one six-cylinder engine section without supercharging. High-speed operation would employ all 24 cylinders and four superchargers.

All of these inventive propulsive ideas came to naught. As previously mentioned, the British supported the Sabre and not the P.24. The AAF felt that only 2,460 hp (1,834 kW) at 3,000 rpm would be obtained from the P.24 (as built) with 100 octane fuel and that no part of the engine was so remarkable that it warranted production in the US. This opinion did not stop the AAF from adding 250 hours of flight time to the P.24 before the Battle was returned to Britain in 1943. The aircraft logged around 340 hours powered by the P.24 engine.

Some suggest that if FAC had received the resources given to Napier for development of the Sabre, the P.24 would have been a phenomenal engine. The P.24 was a good engine, but its performance does not appear to be exceedingly remarkable for the era in which it was developed. While it is true that the P.24 performed reliably, the 3,117 cu in (51.1 L) engine was producing under 2,000 hp (1,491 kW) for most of its developmental life. A P.24 Monarch engine and its contra-rotating propellers survived and are currently on display in the Royal Navy Fleet Air Arm Museum at Yeovilton. The engine and propellers were most likely those used on the Battle.

Sources:
– Fairey Aircraft since 1915 by H. A. Taylor (1988)
– British Piston Aero-Engines and their Aircraft by Alec Lumsden (2003)
– World Encyclopedia of Aero Engines by Bill Gunston (2006)
– Fairey Firefly by W. Harrison (1992)
– Report on 50 Hours Civil Category Type Test Fairey P.24 – Series I (October 1939)
– Memorandum Report on Fairey P-24 (Monarch) Engine by B. Beaman, F. L. Prescott, E. A. Wolfe, and Opie Chenoweth (22 August 1941)
– Memorandum Report on Evaluation of P-24 Engine and Coaxial Rotating Propellers by Air Cops Material Division (27 August 1941)
– “Improvements in or relating to Gaseous Fuel Induction Pipes for Internal Combustion Engine” British patent 463,501 by Fairey Aviation Company and Archibald Graham Forsyth (granted 1 April 1937)
– “Improvements in or relating to Supercharging Internal Combustion Engines” British patent 463,984 by Fairey Aviation Company and Archibald Graham Forsyth (granted 9 April 1937)
– “Improvements in or relating to Valve Mechanism for Internal Combustion Engines” British patent 465,540 by Fairey Aviation Company and Archibald Graham Forsyth (granted 10 May 1937)
– “Improvements in or relating to Power Plants for Aircraft” British patent 469,615 by Fairey Aviation Company and Archibald Graham Forsyth (granted 29 July 1937)
– “Marine Power Unit” US patent 2,389,663 by Archibald Graham Forsyth (granted 27 November 1945)
– “Supercharging Arrangement” US patent 2,395,262 by Archibald Graham Forsyth (granted 19 February 1946)
– “A Power Unit for Aircraft and the like” British patent 591,048 by Fairey Aviation Company and Archibald Graham Forsyth (granted 5 August 1947)
– “Improvements in or relating to Power Units” British patent 591,189 by Fairey Aviation Company and Archibald Graham Forsyth (granted 11 August 1947)
– “Supercharged Multiple Motor Internal-Combustion Unit for Aircraft” US patent 2,448,789 by Archibald Graham Forsyth (granted 7 September 1948)
– “Power Plant Assembly” US patent 2,470,155 by Archibald Graham Forsyth (granted 17 May 1949)
– http://ww2aircraft.net/forum/threads/fairey-aero-engines-any-good-info.30710/