By William Pearce

In 1925, the Soviet Air Force (Voyenno-Vozdushnye Sily or VVS) approached the TsAGI (Tsentral’nyy Aerogidrodinamicheskiy Institut, the Central Aerohydrodynamic Institute) and requested proposals for a large, heavy bomber. Under the direction of Andrei Nikolayevich Tupolev, the Tupolev OKB (Opytno-Konstruktorskoye Byuro, the Experimental Design Bureau) started design work on the aircraft in 1926, and the government finalized the aircraft’s operational requirements in 1929. The aircraft created from this program was the Tupolev ANT-6, which was given the military designation TB-3.

Model of the Tupolev TB-6 6M-44 with its six M-44 engines. Gunner stations are seen outside of the outer engines and in the wing’s trailing edge.

The large, four-engine TB-3 lifted its 137 ft 2 in (41.80 m) wingspan from earth for the first time on 22 December 1930, but plans for even larger and more ambitious aircraft were underway. In October 1929, the Scientific and Technical Committee of the Air Force (Nauchno-tekhnicheskiy komitet upravleniya Voyenno-Vozdushnye Sily or NTK UVVS) instructed Tupolev to design bombers capable of carrying a 10-tonne (22,046 lb) and a 25-tonne (55,116 lb) payload. With a 177 ft 2 in (54 m) wingspan, the 10-tonne bomber became the ANT-16, which was given the military designation TB-4. The 25-tonne bomber had a 311 ft 8 in (95 m) wingspan and became the ANT-26, which was given the military designation TB-6. However, this line of developing very large aircraft, the TB-6 in particular, quickly illustrated that there was a lack of powerful engines and that numerous smaller engines were required for the aircraft. The TB-4 required six 800 hp (597 kW) engines, and the TB-6 required twelve 830 hp (619 kW) engines. If an engine with a 2,000 hp (1,491 kW) output could be built, not only could it power these large aircraft, but it would also simplify their construction, maintenance, and control.

Back in 1928, the TsAGI had realized the need for more powerful engines and initiated work on a single-cylinder test engine to precede the design of a large, high-power bomber engine. This test engine was designated M-170; “170” was the anticipated horsepower (127 kW) output of the cylinder. The results were encouraging, and in 1930, the Institute of Aviation Motors (Institut aviatsionnogo motorostroyeniya or IAM) was tasked with the construction of a V-12 engine based on the M-170 cylinder. The 12-cylinder engine was designated M-44, and the single-cylinder test engine was renamed M-170/44.

The design of the M-44 was initiated in February 1931 under the supervision of N. P. Serdyukov. The design progressed rapidly and was completed in May. The M-44 was a four-stroke, water-cooled, 60-degree V-12. Based on a sectional drawing, the crankcase was split horizontally with main bearing caps for the crankshaft machined integral into the lower half of the case. The main bearings were secured by long bolts that passed through the lower crankcase half and screwed into the upper half. The crankshaft accommodated side-by-side connecting rods with flat-top aluminum pistons.

Sectional drawing of the IAM M-44 reveals some of the engine’s inner workings. The design was fairly conventional, just extremely large. Unfortunately, no images or other drawings of the engine have been found.

The individual steel cylinders were secured to the crankcase via hold down studs. A steel water jacket surrounded the cylinder barrel. The cylinder had a flat-roof combustion chamber, and four spark plugs were positioned horizontally at its top, just below the valves. Two spark plugs were on the outer side of the cylinder and the other two on the Vee side. Each cylinder bank was capped by a monobloc cylinder head with dual overhead camshafts. One camshaft operated the two intake valves for each cylinder, and the other camshaft operated the two exhaust valves for each cylinder. An intake manifold was attached to the Vee side of the cylinder head, and individual exhaust stacks were attached to the outer side of the cylinder head.

The normally aspirated M-44 had a compression ratio of 6 to 1 (some sources state 5 to 1). A propeller gear reduction (most likely using spur gears) was incorporated onto the front of the engine. The IAM M-44 had an 8.74 in (222 mm) bore and a 11.26 in (286 mm) stroke. Each cylinder displaced 675.6 cu in (11.07 L), and the engine’s total displacement was 8,107 cu in (132.9 L). The M-44 was the largest V-12 aircraft engine ever built. The engine produced 2,000 hp (1,491 kW) for takeoff and 1,700 hp (1,268 kW) for continuous operation. Some sources indicate that 2,400 hp (1,790 kW) was expected out of the engine after it was fully developed. The M-44 was approximately 118 in (3.00 m) long, 46 in (1.16 m) wide, and 65 in (1.66 m) tall. The engine weighed around 3,858 lb (1,750 kg).

With development of the 2,000 hp (1,491 kW) M-44 engine underway, studies were started to incorporate the engine into the ANT-16 (TB-4) and ANT-26 (TB-6) aircraft designs. Proposals to re-engine the ANT-16 with four M-44s were quickly abandoned so that work could focus on using six M-44 engines to power the ANT-26. This version of the aircraft is often cited as TB-6 6M-44. The ANT-26 design was ordered in July 1932, with construction starting soon after. Delivery of the ANT-26 prototype was expected in December 1935. Some sources state that an even larger, 30-tonne (66,139 lb) bomber with a 656 ft (200 m) wingspan and powered by eight M-44 engines was conceived, but it appears this aircraft never progressed beyond the rough design phase.

The Tupolev TB-6 6M-44 had two engines installed in each wing and two engines positioned back-to-back and mounted above the aircraft’s fuselage. The aircraft had a 311 ft 8 in (95 m) wingspan and was 127 ft 11 in (39 m) long. The TB-6 6M-44’s top speed was 155 mph (250 km/h), and it had a ceiling of 22,966 ft (7,000 m). The aircraft had a maximum bomb load of 48,502 lb (22,000 kg) and could carry a 33,069 lb (15,000 kg) bomb load 2,051 miles (3,300 km). Its maximum range was 2,983 miles (4,800 km).

This rear view of the TB-6 6M-44 illustrates the tandem engines mounted above the fuselage.

The construction of three M-44 prototypes was planned, but the first engine was delayed by continued trials of the M-170/44 test engine, which was given a higher priority. The manufacture of the first M-44 engine began in early 1933, and the engine was first run later that year. The second engine was built and run in 1934. Plans to build the third M-44 engine were suspended on account of issues with the first two engines. The M-44 test engines had trouble producing the desired power and suffered from reliability issues. It became clear that the engine was not going to be successful, and the program was cancelled in 1934.

A supercharged version of the engine, known as the M-44H, had undergone preliminary design work in 1932. However, performance specifications for this engine have not been found, and it is doubtful that detailed design work was completed. In 1935, a decision was made to build the third M-44 engine, modified for marine use. This engine was designated GM-44 and incorporated a reversing gearbox. The GM-44 produced 1,870 hp (1,394 kW), but it was no more reliable than the M-44 aircraft engine. The GM-44 engine was cancelled in 1936.

With the M-44 engine program dead, the ANT-26 design reverted back to using 12 engines (1,200 hp / 895 kW Mikulin M-34FRN). However, studies concluded that the multitude of engines created additional drag that impacted the aircraft’s performance, and the engines added so much complexity that the ANT-26 would be difficult to fly and very difficult to maintain. Simply put, the giant aircraft was impractical, and it was subsequently cancelled in July 1934. A transport/commercial version of the aircraft, designated ANT-28, was also cancelled. The ANT-26’s airframe was 75 percent complete at the time of cancellation.

With the M-44 cancelled, the 12-engine TB-6 12M-34FRN was designed to preserve the aircraft’s capabilities with reliable engines. However, one would question the practicality of such an aircraft. Note the set of tandem engines that was placed above each wing.

Sources:
– Russian Piston Aero Engines by Vladimir Kotelnikov (2005)
– Самолеты- гиганты СССР by Vladimir Kotelnikov (2009)
– Unflown Wings by Yefim Gordon and Sergey Komissarov (2013)
– OKB Tupolev by Yefim Gordon and Vladimir Rigmant (2005)

By William Pearce

In the late 1920s, the Lycoming Manufacturing Corporation of Williamsport (Lycoming County), Pennsylvania entered the aircraft engine business. At the time, Lycoming was a major supplier of automobile engines to a variety of different manufacturers. Lycoming quickly found success with a reliable nine-cylinder radial of 215 hp (160 kW), the R-680. However, the company wanted to expand into the high-power aircraft engine field.

When built, the Lycoming O-1230 was twice as large as and three times more powerful than any other aircraft engine the company had built. Lycoming essentially achieved the performance goals originally set for the O-1230, but other engine developments had made the O-1230 obsolete by the time it would have entered production.

In 1932, Lycoming became aware of the Army Air Corps’ (AAC) program to develop a high-performance (Hyper) cylinder that would produce one horsepower per cubic inch displacement and enable a complete aircraft engine to produce one horsepower per pound of weight. The AAC had contracted Continental Motors in 1932 to work with the Power Plant Branch at Wright Field, Ohio on developing an engine utilizing Hyper cylinders. The engine type was set by the AAC as a 1,200 hp (895 kW), flat, liquid-cooled, 12-cylinder engine that utilized individual-cylinder construction. The flat, or horizontally-opposed, engine configuration was selected to enable the engine’s installation buried in an aircraft’s wings.

Lycoming’s Hyper cylinder was developed into the cylinder used on the O-1230. Note the studs for attaching the camshaft housing. The intake port and coolant inlet are on the right. The exhaust port and coolant outlet are on the left.

Lycoming saw an opportunity to quickly establish itself as a high-power aircraft engine manufacturer by creating an engine that would satisfy the AAC’s requirements. On its own initiative, Lycoming began work on its own Hyper cylinder with the intent of developing a 12-cylinder engine. Lycoming chief engineer Val Cronstedt was put in charge of the project, and he was assisted by Samuel Hoffman, Clarence Wiegman, and other engineers. The AAC encouraged Lycoming’s involvement and provided developmental support, but the AAC did not initially provide financial support.

Lycoming started serious developmental work on the new engine in 1933. Various single-cylinder test engines were built and tested in 1934. Also in 1934, Hoffman and Wiegman filed for a number of patents that detailed some proposed aspects of the complete 12-cylinder engine. In 1935, the AAC became more interested in the engine and began supporting Lycoming’s efforts. Single-cylinder testing yielded positive results, with the engine passing a 50-hour test in May 1936 and producing 228.7 hp (170.5 kW) at 3,000 rpm from its 102.8 cu in (1.69 L) displacement during maximum performance tests in July 1936. That same year, the AAC contracted Lycoming to build a complete engine. Lycoming had spent $500,000 of its own money and had finalized the design of its engine, which was designated O-1230 (also as XO-1230). Construction of the first O-1230 was completed in 1937, and the engine was ready for endurance testing in December of that year.

The Lycoming O-1230 had a two-piece aluminum crankcase that was split vertically. Six individual cylinders attached to each side of the crankcase. The cylinders were made of steel and were surrounded by a steel water jacket. Each cylinder had a hemispherical combustion chamber with one intake and one sodium-cooled exhaust valve. A cam box mounted to the top of each cylinder bank, and each cam box contained a single camshaft that was shaft-driven from the front of the engine.

Top: Drawing of the O-1230 from US patent 2,119,879 for a proposed fuel injection system that can be seen in the induction manifold. The drawing shows details of the engine’s construction including the split crankcase and overhead camshaft. Bottom: Intended for installation buried in an aircraft’s wing, the O-1230’s height was kept to a minimum. The long nose case would aid in streamlined wing installations. Note that the supercharger’s diameter was slightly in excess of the engine’s height.

A downdraft carburetor fed fuel into the single-speed, single-stage supercharger mounted at the rear of the engine. The supercharger’s 10 in (254 mm) diameter impeller was driven at 6.55 times crankshaft speed. It provided air to the intake manifold that sat atop the engine. Individual runners provided air to each cylinder from the intake manifold. Lycoming had experimented with direct fuel injection on test cylinders and US patent 2,119,879 was applied for in November 1934 that detailed a fuel injection system for the O-1230. In the patent, fuel injectors would be installed in the removable cover of the intake manifold with one injector spraying into each runner and toward the cylinder. However, it is unlikely that any O-1230 ever used fuel injection.

Exhaust was expelled out the lower side of the cylinders and collected in a common manifold for each cylinder bank. An extended nose case housed beveled planetary gears for the propeller’s gear reduction. The propeller turned at .40 crankshaft speed, with options for a .50 or .333 reduction. On the top of the O-1230, just behind the gear reduction, was the engine’s sole magneto. The magneto was connected to two distributors, each driven from the front of the camshaft drive.

The O-1230 with one style of engine mount that was secured between the camshaft housing and cylinders. Note the induction manifold and individual runners atop the engine.

The O-1230 had a 5.25 in (133 mm) bore and a 4.75 in (121 mm) stroke. The engine’s total displacement was 1,234 cu in (20.2 L), and it had a 6.5 to 1 compression ratio. The O-1230 produced 1,200 hp (895 kW) at 3,400 rpm for takeoff, 1,000 hp (746 kW) at 3,100 rpm for normal operation, and 700 hp (522 kW) at 2,650 rpm for cruise operation. The engine had an overspeed limit of 3,720 rpm for diving operations. The O-1230 was 106.7 in (2.71 m) long, 44.1 in (1.12 m) wide, and 37.9 in (.96 m) tall. The engine weighed 1,325 lb (601 kg).

After completing a 50-hour type test in March 1939, the O-1230 was rated at 1,000 hp (746 kW). Continued development pushed the engine’s rating up to 1,200 hp (895 kW). The O-1230 was installed in a Vultee YA-19 attack aircraft that had been modified as an engine testbed and redesignated XA-19A (38-555). Some sources list the designation as YA-19A, but “Y” was typically used for pre-production aircraft, while “X” was for experimental aircraft. The O-1230-powered XA-19A first flew on 22 May 1940, the flight originating at Vultee Field in Downey, California. The aircraft and engine combination were transferred to Wright Field, Ohio in June 1940 and then to Lycoming on 27 March 1941. By this time, the AAC had already moved away from the buried-engine-installation concept and was interested in more powerful engines.

The XA-19A is seen with its Wright Field markings. The scoop above the cowling brought air into the engine’s carburetor. Louvered panels allowed heat generated by the exhaust manifold to escape the cowling. Note the large exhaust outlet. The radiator positioned under the engine added bulk to the O-1230’s installation. The aircraft’s tail was modified to compensate for the larger and longer nose needed to house the O-1230.

While the O-1230’s power output was on par with many of its contemporaries, such as the Allison V-1710, the O-1230 did not offer the same development potential or reliability as other engines. The O-1230 was cancelled in favor of other projects, and the engine was subsequently removed from the XA-19A airframe. The XA-19A was transferred to Pratt & Whitney on 8 August 1941, where an R-1830 was subsequently installed, and the aircraft was redesignated XA-19C.

Lycoming was still interested in developing a high-power engine and used O-1230 components to create the 24-cylinder XH-2470. In some regards, the Lycoming XH-2470 was two O-1230 engines mounted to a common crankcase. Lycoming started initial design work on the engine as early as 1938. An O-1230 is on display at the New England Air Museum in Windsor Locks, Connecticut.

The restored O-1230 on display at the New England Air Museum. The engine’s electric starter is mounted vertically just in front of the supercharger. (Daniel Berek image via Flickr.com)

Sources:
– Development of Aircraft Engines and Fuels by Robert Schlaifer and S. D. Heron (1950)
– Aircraft Engines of the World 1941 by Paul Wilkinson (1941)
– Jane’s All the World’s Aircraft 1942 by Leonard Bridgman (1942)
– “The Evolution of Reciprocating Engines at Lycoming” by A. E. Light, AIAA: Evolution of Aircraft/Aerospace Structures and Materials Symposium (24–25 April 1985)
– “Fuel Injector for Internal Combustion Engines” US patent 2,119,879 by Samuel K. Hoffman and Clarence H. Wiegman (Applied 19 November 1934)
– “Aircraft Prime Movers of the Twentieth Century” by Air Commodore F. R. Banks, Seventh Wings Club ‘Sight’ Lecture (20 May 1970)
– “Vultee Engine-Test Aircraft in World War II” by Jonathan Thompson, AAHS Journal Volume 39 Number 4 (Winter 1994)

By William Pearce

In 1930, the Bristol Aeroplane Company began to contemplate the future of aircraft engines. Their engine department was run by Roy Fedden, a prolific aircraft engine designer. At the time, Bristol was manufacturing its nine-cylinder, single-row Mercury radial engine that had an output of 510 hp (380 kW) and displaced 1,519 cu in (24.9 L). The Mercury engine was under continuous development to increase its output. However, to produce more power out of the same basic engine size, Fedden realized that a second cylinder row was needed.

The Bristol Hydra was an odd radial engine utilizing two inline rows of eight cylinders. The engine suffered from vibration issues due to a lack of crankshaft support. Note the dual overhead camshafts for each front and rear cylinder pair.

Fedden and Bristol evaluated at least 28 engine designs to determine the best path forward for a multi-row engine. At the same time, Fedden was investigating a switch to using sleeve valves, but their development at Bristol had just begun. The multi-row engine would continue to use poppet valves. At the end of 1931, a 16-cylinder, air-cooled engine design was selected for development. This engine was called the Double Octagon or Hydra.

The Bristol Hydra was designed by Frank Owner in 1932, and the project was overseen by Fedden. The radial engine was very unusual in that it had an even number of cylinders for each row. Nearly all four-stroke radial engines have an odd number of cylinders per row so that every other cylinder can fire as the crankshaft turns. In addition, the Hydra’s cylinder rows were not staggered—the first and second rows were directly in line with each other. The “Double Octagon” name represented the engine’s configuration, in which the eight cylinders on each of the engine’s two rows formed an octagon. The name “Hydra” was given to the engine because of its numerous “heads” (cylinders).

A sectional view of the Hydra created by Brian Perkins and based on a drawing found in the Bristol archives. The numbers in the drawing relate to the number of gear teeth. Note the unsupported crankshaft center section that joined the front and rear crankshaft sections. (Brain Perkins drawing via the Aircraft Engine Historical Society)

Unlike a traditional radial engine, the Hydra’s design resembled four V-4 engines mounted to a common crankcase and using a common crankshaft. In fact, a V-4 test engine was built to refine the Hydra’s cylinder and valve train design before a complete engine was built. The V-4 cylinder sections were mounted at 90-degree intervals around the crankcase, and their cylinders had a 45-degree bank angle. This configuration spaced all cylinder banks at 45-degree intervals. The V-4 cylinder sections had their exhaust ports located on the outer sides and their intake ports positioned in the Vee of each V-4 cylinder section. Two supercharger-fed intake manifolds delivered air to the Vee of each V-4 cylinder section, with each manifold servicing one front and rear cylinder. The engine’s supercharger turned at over four times crankshaft speed.

The Hydra used an aluminum cylinder that was machined all over with cooling fins. A steel barrel lined the inside of the cylinder. Each cylinder had one intake and one exhaust valve. Each front and rear cylinder formed a pair, and each cylinder pair had separate overhead camshafts that directly operated the intake and exhaust valves. At the rear of the cylinder pair, the exhaust camshaft was driven via beveled gears by a vertical shaft that was powered from the crankshaft by a gear set. A short cross shaft extended from the exhaust camshaft to power the intake camshaft. Each cylinder had two spark plugs.

Front and side view of the Hydra. Note the exhaust stacks protruding slightly above the cylinders.

The engine’s crankshaft was built-up from three pieces. The center piece joined the front and rear sections via four clamping bolts. The crankshaft only had two main bearings and no center support. Single-piece master connecting rods were used. A bevel gear reduction at the front of the engine reduced the propeller speed to .42 times that of the crankshaft. The relatively high-level of gear reduction was needed because of the engine’s high operating speed.

The Hydra had a 5.0 in (127 mm) bore and stroke. The engine’s total displacement was 1,571 cu in (25.7 L). The Hydra had a 6 to 1 compression ratio and produced 870 hp (649 kW) on 75 octane fuel. On 87 octane fuel, the engine reportedly produced 1,020 hp (761 kW). The power outputs were achieved at 3,620 rpm, a very high speed for a radial engine. The engine was 46.5 in (1.18 m) in diameter, 57 in (1.45 m) long, and weighed approximately 1,500 lb (680 kg). With its unusual cylinder configuration, the Hydra had the following cylinder firing order: 1F, 2F, 7R, 4F, 1R, 6F, 3R, 8F, 5R, 6R, 3F, 8R, 5F, 2R, 7F, and 4R.

Hydra engine installed in the sole Hawker Harrier. Note the baffling on the engine. The four-blade test club propeller was fitted for ground runs.

The Hydra V-4 test engine underwent runs in mid-1932 and eventually produced around 190 hp (142 kW) with no cooling issues. A complete 16-cylinder Hydra was first run in 1933. Later that year, the engine was installed in the sole Hawker Harrier biplane bomber prototype, J8325. The engine’s configuration made installation very easy, and the intake Vees were baffled to improve cooling airflow.

The Hydra-powered Harrier encountered some oil leaks and ignition issues, but the main trouble was with excessive engine vibration. The lack of a center main bearing on the crankshaft caused the vibration issues, which could be quite severe at certain RPMs. The short stroke of the engine combined with a short crankshaft gave the designers the false hope that the center main bearing would not be needed. A redesign of the engine was required to cure the vibration issues.

The Hydra-powered Harrier completely cowled and with its three-blade flight propeller. The aircraft was flown in this configuration during 1933, but engine vibration issues at critical RPMs limited the testing.

By 1934, the Mercury was approaching the 800 hp (597 kW) level, and the new nine-cylinder, 1,753 cu in (28.7 L) Pegasus was giving every indication that 900 hp (671 kW) was just around the corner. In addition, the sleeve valve, 1,519 cu in (24.9 L) Perseus engine had proved reliable and was producing around 700 hp (522 kW), and more ambitious sleeve valve engines were being designed. Rather than proceed with the Hydra and its double-octagon configuration, Bristol chose to develop its existing production engines and also focus on new sleeve valve engines.

The Hydra engine project was funded entirely by Bristol, although Fedden tried to get Air Ministry support. Only two Bristol Hydra engines were built; remarkably, both are reported to still exist. One is housed at the Sir Roy Fedden Heritage Centre, Bristol Branch of the Rolls-Royce Heritage Trust, in Bristol, United Kingdom. The other engine is stored at the Royal Air Force Museum London, located on the old Hendon Aerodrome.

A preserved Bristol Hydra engine held by the Bristol Branch of the Rolls-Royce Heritage Trust. Note the extensive finning on the aluminum cylinders. (Brain Perkins image via the Aircraft Engine Historical Society)

Sources:
– Fedden – the life of Sir Roy Fedden by Bill Gunston (1998)
– British Piston Aero-Engines and their Aircraft by Alec Lumsden (2003)
– An Account of Partnership – Industry, Government and the Aero Engine by George Bulman and edited by Mike Neale (2002)
– “My Wife Calls it an Obsession!!!! Part 2: Bristol Hydra” by Brian Perkins Torque Meter Volume 4, Number 2 (Spring 2005)
“The Future of the Air-Cooled Engine” Flight (25 February 1937)
– http://www.enginehistory.org/ModelEngines/Perkins/Hydra/bristol_hydra.shtml

By William Pearce

Charles Richard Fairey founded the Fairey Aviation Company (FAC) in 1915. Fairey was at Cowes, Isle of Wight, United Kingdom in September 1923 to witness a practice session for the Schneider Trophy seaplane race over the Solent. What he saw both impressed and disappointed him.

The Curtiss D-12 so impressed Richard Fairey that he went to the United States and acquired a license to produce the engine. Named the Fairey Felix, the engine was actually never produced, but 50 D-12 engines were imported.

Fairey was impressed by the Curtiss CR-3 racers, each with its compact 450 hp (336 kW) Curtiss D-12 engine turning a Curtiss-Reed metal propeller. When the race was run, the two CR-3 aircraft from the United States (US) proved to be 20 mph (32 km/h) faster than the British Supermarine Sea Lion racer. The Sea Lion was powered by a 550 hp (410 kW) Napier Lion W-12 engine that turned a wooden propeller. The two CR-3s finished the race averaging 177.266 mph (285.282 km/h) and 173.347 mph (278.975 km/h), while the Sea Lion averaged 157.065 mph (252.772 km/h). Fairey was disappointed that the British Air Ministry was not pushing its aircraft industry to make the same technological strides that were taking place in the US. Fairey was already frustrated by the constraints the Air Ministry placed on their specifications for new aircraft. With the world-beating performance of the Curtiss CR-3 aircraft fresh in his mind, Fairey resolved that if the Air Ministry would not push technology, he would.

Fairey went first to the Air Ministry seeking support for his new aircraft and was promptly turned down. Fairey then traveled to the US where, at great expense, he obtained manufacturing licenses for the Curtiss D-12 engine and Curtiss-Reed propeller. This agreement included some 50 D-12 engines to be used while FAC tooled up to manufacture their version, which was called the Felix. Fairey was so enthusiastic about the D-12, that he somehow smuggled an engine into his stateroom for his return sea voyage to Britain.

The Fairey Fox I light bomber was powered by the D-12/Felix engine. The aircraft was a private venture, and its performance surpassed other bombers and most fighters then in service. The British Air Ministry did not appreciate Fairey’s non-conformist attitude or the aircraft’s foreign power plant.

The D-12/Felix was a normally aspirated, liquid cooled, 60 degree, V-12 engine. The engine had a 4.5 in (114 mm) bore and a 6.0 in (160 mm) stroke. The D-12/Felix’s total displacement was 1,145 cu in (18.8 L), and it produced 435 hp (324 kW) at 2,300 rpm. The engine had four valves per cylinder that were operated by dual overhead camshafts.

With the engine situation under control, Fairey had his design department drew up plans for a new aircraft to be powered by the D-12/Felix. What came off the drawing board was the Fairey Fox I light bomber. Piloted by Norman Macmillan, the Fox I was flown for the first time on 3 January 1925. The Fox I had a top speed of 156 mph (251 km/h), some 50 mph (80 km/h) faster than comparable bombers then in service and also faster than most frontline fighters. Although it was built as a private venture, the Air Ministry was forced to buy a few Fox I bombers because of the aircraft’s unparalleled performance. The Air Ministry was not pleased with the situation and was downright appalled that the aircraft was powered by a US engine. Moreover, they did not want another aircraft engine manufacturer in Britain.

The Air Ministry tasked Rolls-Royce to develop an engine superior to the D-12. This new engine was developed as the Rolls-Royce Kestrel (type F) and was a stepping stone to the Merlin. The whole situation did nothing to improve the relationship between Fairey and the Air Ministry. However, had Fairey not forced the D-12 upon the Air Ministry, it is entirely possible that there may not have been a Merlin engine ready for the Battle of Britain in 1940.

British patent 402,602 outlined how passageways cast into an engine’s crankcase could bring induction air into the cylinders. The patent also states how special oil lines (h) could traverse the passageway. This would help cool the oil and heat the incoming air/fuel mixture (which is not a good idea when higher levels of supercharging are applied to the engine).

The small order of Fox aircraft meant that the Fairey Felix engine never went into production. Only 28 Fox I aircraft were built, and a number were either built with or re-engined with Kestrel engines. FAC also built the D-12-powered Firefly I fighter, which first flew on 9 November 1925 and had a 185 mph (298 km/h) top speed. No orders were placed for the Firefly I.

Failing to enter the aircraft engine business on his first attempt did not stop Fairey from trying again. In 1931, FAC had hired Captain Archibald Graham Forsyth as chief engine designer. Forsyth had previously worked with Napier and Rolls-Royce while he was with the Air Ministry. Forsyth went to work designing a new aircraft engine. During this same period, Rolls-Royce started work on their PV-12 engine, which would become the Merlin.

Forsyth developed a liquid-cooled, 60 degree, V-12 engine known as the P.12. The upper crankcase and cylinder banks of the P.12 were cast together. Each detachable cylinder head housed four valves per cylinder. Reportedly, the P.12 used a dual overhead camshaft valve train similar to that used on the D-12/Felix. Cast into the Vee of the engine was the intake manifold and the runners, which branched off from the manifold. The intake runners aligned with passages cast integral with the cylinder head that led to the cylinders. The integral intake manifolds increased the engine’s rigidity, eliminated many pipe connections, and gave the engine a much cleaner appearance.

A drawing from British patent 406,118 illustrates the induction passageways (d, e, and f) cast integral with the engine’s crankcase and head. The drawing also shows the water circulation from the crankcase to up around the cylinders and into the cylinder head. Although the valve arrangement is not specified, it is easy to see how four valves per cylinder with dual overhead camshafts could be accommodated.

The Fairey P.12 had a 5.25 in (133 mm) bore and a 6.0 in (152 mm) stroke. The engine’s total displacement was 1,559 cu in (25.5 L). Two versions of the P.12 were designed that varied in their amount of supercharging. The lightly-supercharged (some sources say unsupercharged) P.12 Prince produced 650–710 hp (485–529 kW) at 2,500 rpm. The moderately-supercharged P.12 Super Prince (or Prince II) produced 720–835 hp (537–623 kW) at 2,500 rpm. The P.12 engine weighed around 875 lb (397 kg).

The P.12 engine was first run in 1933. By 1934, three engines had been built and had run a total of 550 hours. One engine had run non-stop for 10 hours at 520 hp (388 kW) and had made three one-hour runs at 700 hp (522 kW). In late 1934, a P.12 Prince engine was installed in a Belgium-built Fox II (A.F.6022) aircraft (A.F.6022). The Prince-powered aircraft made its first flight on 7 March 1935. Ultimately, P.12 engines were run around 1,000 hours and had a final rating of 750 hp (559 kW) for normal output and 900 hp (671 kW) for maximum output.

The Fairey Fox II was used as a testbed for the P.12 Prince engine. Unfortunately, little information has been found regarding the engine or its testing. Note the two exhaust stacks for each cylinder. The arrangement was similar to that used on the D-12/Felix engine.

In 1933, the Air Ministry issued specification P27/32 for a new light bomber. Marcel Lobelle, chief designer at FAC, drew up a number of designs, including one powered by two P.12 Prince engines. However, the Air Ministry wanted a single-engine aircraft. Lobelle altered the twin-engine design into what was basically a P.12-powered early design of the Fairley Battle. The Air Ministry made it clear to FAC that it would not consider any P.12-powered aircraft, because FAC was not a recognized engine manufacturer, and the Air Ministry did not want any other firms entering the aircraft engine field. Consequently, the FAC design for the P27/33 specification was switched to A Rolls-Royce Merlin I engine in 1934. This design was contracted as the Fairey Battle. The Battle was first flown on 10 March 1936 by Christopher Staniland, but an order for 155 aircraft (under specification P.23/35) had already been placed in May 1935. The Battle was the first production aircraft powered by the Merlin engine. With no support from the Air Ministry, the P.12 Prince faded into history.

Encouraged by the early bench tests of the P.12, Forsyth designed a more powerful 16-cylinder engine in January 1935 that was designated P.16. Initially, the P.16 design was basically a P.12 with four additional cylinders to make a V-16 engine. The P.16 used the same bore and stroke as the P.12 but displaced 2,078 cu in (34.1 L). Some sources state the P.16 was guaranteed to produce 900 hp (671 kW) at 12,000 ft (3,658 m) with a weight of only 1,150 lb (522 kg). The 900 hp (671 kW) output seems low, especially when compared to the anticipated performance of the Super Prince.

FAC’s proposal to specification P27/32 included two twin-engine aircraft powered by P.12 Prince engines. The Air Ministry wanted a single-engine aircraft and would not consider anything powered by FAC engines. The specification and design eventually became the Fairey Battle.

Numerous sources suggest the P.16’s configuration was changed over concerns regarding the engine’s length combined with excessive torsional vibrations and stress of the V-16’s long crankshaft. The new, revised layout of the P.16 was an H-16 engine with two crankshafts, four banks of four cylinders, and an output of 1,540 hp (1,148 kW). This power level seems more reasonable than the 900 hp (671 kW) listed previously, but some sources give the 1,540 hp (1,148 kW) figure as an early power rating of a different engine (the P.24 Monarch). On occasion, the H-16 engine has been referred to as the P.16 Queen, but “Queen” was an early name for the P.24 Monarch. It may be that the H-16 engine never existed and has been mistaken for the P.24 over the years.

A third P.16 layout is described by other sources, which details the engine as a U-16 with two straight-eight engines mounted in parallel and geared to a common propeller shaft. FAC and Forsyth applied for a patent on 31 January 1936 (British patent 469,615) for such an engine configuration, but that date is after FAC moved away from the P.16, and the drawings depict a 12-cylinder engine. Both the H-16 and U-16 configurations would result in a much heavier engine of around 1,500 lb (680 kg).

Rather than proceed with a 16-cylinder engine, a new design had been started by October 1935. In fact, there is little evidence from primary sources that indicates a P.16 engine or an H-16 configuration were ever seriously considered. The new engine would keep the bore and stroke of the P.12 and use an H layout with four banks of six cylinders for a total of 24 cylinders. The H-24 engine design was called the Fairey P.24 Monarch.

Some sources state the P.16 engine was really two inline-eight engines coupled together as a U-16. While no drawings of a U-16 have been found, FAC and Forsyth did take out a British patent (no. 469,615) for a similar engine. This U-12 design was probably more of a stepping stone to the P.24 than a development of the P.16. Note the gun barrel (c) drawn between the cylinder banks.

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)
– Memorandum Report on Fairey P-24 (Monarch) Engine by B. Beaman, F. L. Prescott, E. A. Wolfe, and Opie Chenoweth (22 August 1941)
– “Improvements in or relating to the Induction and Lubrication Systems of an Internal Combustion Engine” British patent 402,602 by Fairey Aviation Company and Archibald Graham Forsyth (granted 7 December 1933)
– “Improvements in or relating to the Cylinder Block and Crank Case of an Internal Combustion Engine” British patent 406,118 by Fairey Aviation Company and Archibald Graham Forsyth (granted 22 February 1934)
– “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)
– “Fairey Battle Database” by W. A Harrison Aeroplane (June 2016)

By William Pearce

In 1936, the Ministère de l’Air (French Air Ministry) issued a specification for a 2,000 hp (1,491 kW) engine intended to power a flying boat for transatlantic service. The aircraft was to carry at least 40 passengers and 1,100 lb (500 kg) of cargo 3,725 miles (6,000 km) against a 37 mph (60 km/h) headwind. Hispano-Suiza already had its 12Y engine of 1,000 hp (746 kW) in production and was investigating ways to effectively double that engine. Their design efforts led to the 24-cylinder Hispano-Suiza 24Y aircraft engine.

The Hispano-Suiza 24Y Type 82 24-cylinder H engine on display in the Polish Aviation Museum in Krakow. The Type 82 was intended for use with contra-rotating propellers; however, its original propeller shaft is missing.

The idea behind the 24Y engine was to utilize as many 12Y engine components as possible. The Hispano-Suiza 12Y engine was a liquid-cooled V-12. Each bank of six cylinders was cast en bloc with an integral cylinder head. The 12Y had a 5.91 in (150 mm) bore, a 6.69 in (170 mm) stroke, and a total displacement of 2,200 cu in (36.05 L). The 12Y-50 was one of the last and most powerful versions of the engine; it produced 1,100 hp (820 kW) at 2,500 rpm.

The 24Y engine’s configuration was a vertical H-24: two cylinder banks were mounted vertically above the crankcase, and two cylinder banks were below. A crankshaft served each upper and lower cylinder bank pair. Four aluminum 12Y-50* cylinder blocks were mounted on the 24Y’s crankcase. Each cylinder block included two valves per cylinder, a single overhead camshaft, and the camshaft’s vertical drive shaft. The 7 to 1 compression pistons were connected to the hollow, one-piece crankshaft via fork-and-blade connecting rods, and all components were from the 12Y engine. Each crankshaft had six throws and was supported by seven main bearings. The two-piece, aluminum crankcase was formed by an upper and lower half and was unique to the 24Y.

Rear view of the Hispano-Suiza 24Y (Type 90) showing the engine’s four magnetos, two superchargers, four fuel pumps, and two coolant pumps.

At the rear of the engine, each crankshaft drove a single-speed supercharger at 10 times crankshaft speed. The superchargers gave the engine 2.3 psi (.16 bar) of boost. Separate intake manifolds led from each supercharger to the upper and lower cylinder banks on one side of the engine. Three carburetors were positioned along each intake manifold. Each of the engine’s 12 carburetors supplied the air/fuel mixture to a pair of cylinders.

The two spark plugs per cylinder were fired by four magnetos driven from the rear of the engine. Two magnetos were located above each supercharger. Four fuel pumps were mounted below and between the superchargers. The left and right sides of the engine had separate coolant systems, and a coolant pump was located below each supercharger.

At the front of the engine, each crankshaft had a 28-tooth gear that engaged a 55-tooth propeller gear. This combination created a .509 to 1 gear reduction for the propeller shaft. Between each crankshaft and its power gear was a Sarazin torsional vibration damper. Two versions of the 24Y were built, and they differed in their propeller drive. The 24Y Type 82 was designed to power contra-rotating propellers. In this engine, one crankshaft drove the inner propeller shaft while the other crankshaft drove the outer propeller shaft. The 24Y Type 90 was designed to power a single-rotation propeller and was available with either a normal length or extended gear reduction nose case. Some sources state the Type 90 had accommodations for a cannon to fire through the propeller shaft, but photos indicate this was unlikely.

Hispano-Suiza 24Y Type 90 engine with its extended gear reduction case for a single rotation propeller. This engine was displayed at the 1938 Salon de l’Aéronautique in Paris. Note the three carburetors for each cylinder bank.

The Hispano-Suiza 24Y had a 5.91 in (150 mm) bore and a 6.69 in (170 mm) stroke. The engine’s total displacement was 4,400 cu in (72.10 L). The 24Y produced 2,200 hp (1,641 kW) at 2,500 rpm for takeoff. Max power was 2,000 hp (1,491 kW) at 2,400 rpm at 10,827 ft (3,300 m), and cruising power was 1,500 hp (1,119 kW) at 2,250 rpm at 10,827 ft (3,300 m). The engine had a specific fuel consumption of .50 lb/hp/hr (304 g/kW/hr). The Type 82 was 6.46 ft (1.97 m) long, 3.05 ft (.93 m) wide, and 4.27 ft (1.30 m) tall. The engine weighed 2,204 lb (1,000 kg). The Type 90 had the same width and height as the Type 82 but was 3.38 ft (1.03 m) longer with the extended gear reduction case, for a total length of 9.84 ft (3.00 m). The Type 90’s weight was listed as 2,161 lb (980 kg).

Exactly when the 24Y was first run has been lost to history. The engine made its public debut in November 1938 at the Salon de l’Aéronautique (Air Show) in Paris, France. A Type 90 engine was displayed there, and it attracted a lot of attention. Unfortunately for Hispano-Suiza, that attention did not translate into sales. War in Europe was imminent by 1939, and Hispano-Suiza had turned its attention to developing the new 12Z engine. The 12Z was the next evolutionary step beyond the 12Y for Hispano-Suiza’s V-12 engines. War would interrupt the 12Z’s development, but the 12Z would later inspire another 24-cylinder engine known as the 24Z, which was configured like the 24Y. It is doubtful that the 24Y was ever flown.

The preserved 24Y Type 82 engine is missing many components. Note the vertical drive shaft for the camshaft at the end of each cylinder bank. (Polish Aviation Museum image)

Only a small number of 24Y engines were built—probably just one Type 82 and one Type 90 with an extended gear reduction case. Having disappeared during World War II, the disposition of the Type 90 is not known. The Type 82 wound up in Poland at the end of World War II. Most likely, it was part of Herman Goering’s aviation collection that was moved to Poland late in the war to keep it from being damaged during Allied bombing raids. The Hispano-Suiza 24Y Type 82 engine is currently preserved (without its original propeller shaft) and on display in the Polish Aviation Museum in Krakow.

*Some sources state that 12Y-51 cylinder blocks were used on the 24Y. The 12Y-50 and 12Y-51 were basically the same engine, the only difference being the crankshaft rotation. When viewed from the rear, the 12Y-50 rotated counter clockwise; the 12Y-51 rotated clockwise. The cylinder blocks of the 12Y-50 and 12Y-51 engines were the same.

The supercharger impellers can be seen in this view of the 24Y Type 82. Although the magnetos are gone, the fuel pumps and one coolant pump remain. (Polish Aviation Museum image)

Sources:
– Aircraft Engines of the World 1941 by Paul H. Wilkinson (1941)
– Hispano Suiza in Aeronautics by Manuel Lage (2004)
– Jane’s All the World’s Aircraft 1939 by C. G. Grey and Leonard Bridgman (1939)
– “Some Trends in Engine Design” Flight (8 December 1938)
– http://www.muzeumlotnictwa.pl/zbiory_sz.php?ido=121&w=a