Category Archives: Aircraft Engines

Fairey P24 Monarch engine

Fairey P.24 Monarch Aircraft Engine

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.

Fairey P24 Monarch engine

The Fairey P.24 Monarch was a final attempt by Fairey Aviation to produce a piston aircraft engine. The engine proved to be reliable and had some unique features, but nothing about it was revolutionary. Both Britain and the United States passed at the opportunity to produce the engine.

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.

Fairey P24 integral passageways

Drawings taken from British patent 463,501 illustrate the integral passageways of the P.24’s induction system. The drawn configuration was nearly identical to that used on the actual engine. The sectional view on the right illustrates the numerous sharp turns the air/fuel mixture made on its way into the cylinders.

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.

Fairey P24 valves

Another drawing from British patent 463,501 details the induction, valves, and exhaust. Although the drawing has two exhaust valves per cylinder, the P.24 as built had only one exhaust valve. Note that each bank of six cylinders only has four exhaust stacks. This gave a total of 16 stacks for the P.24 engine, which is why it is occasionally 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.

Fairey P24 Monarch installation drawing

The P.24 installation drawing illustrates the H-24 engine as built. British patent 463,501 mentioned the possibility of installing a gun between the upper cylinder banks, but no provisions for such equipment were incorporated into the actual engine.

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.

Fairey P24 Monarch Battle GB side

This side view of the P.24 engine installed in Fairey Battle K9370 illustrates the very large radiator installed under the aircraft. It is easy to understand why the engine, with its 16 prominent exhaust stacks, is often thought to be an H-16. Note that the propellers do not have a complete spinner, which was installed later.

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.

Fairey P24 Monarch Battle GB front

A well circulated image of the P.24-powered Battle in Britain. Taken at the same time is another image that shows the front propeller turning (left side of the engine running). A doctored set of these images had the exhaust stacks removed to keep the engine’s configuration a secret. Note the complete spinner.

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.

Fairey P24 Monarch Battle US side

The P.24 was flown extensively in the US, but the AAF was mostly interested in the contra-rotating propeller. The Battle retained its serial number, but the British roundels were painted over, and US markings were applied. Note the star under the wing and the stripes on the tail. Some contend the Battle was designed to be powered by the P.24. However, the Battle’s origin can be traced to 1933, and the P.24’s design was initiated over two years later, in 1935. An early design of the Battle was powered by the Fairey P.12 Prince.

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. 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.

Fairey P24 Monarch FAA side

The sole surviving P.24 Monarch engine and its propellers are preserved and housed in the Fleet Air Arm Museum at Yeovilton. Only around three P.24 engines were built. (Rory57 image via

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).

Fairey P24 Turbosuperchargers

Forsyth envisioned adding turbosuperchargers to the P.24 as another stage of charging. This concept is outlined in British patent 463,984 (top, granted in 1937) and US patent 2,395,262 (bottom, granted 1946). In addition, a front mounted supercharger driven from the propeller gear reduction was contemplated. All of these arrangements were to give the P.24 better high-altitude performance, but none were built.

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.

Fairey P24 with compressors

As the jet age dawned, Forsyth looked to incorporate the new technology into the P.24. The top drawing is from British patent 591,048 and describes a single compressor (H) mounted behind the engine (A) and supercharger (F). Induction pipes (34) lead from the supercharger to the engine. The bottom drawing is from British patent 591,189 and describes a compressor (M) mounted behind each engine section. Both configurations allow the engine to drive the propellers, the compressor, or both. In addition, fuel could be injected and ignited into combustion chambers (I top and S bottom) for additional thrust. The patents were applied for in 1944 and granted in 1947.

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.

Fairey U-24 marine engine

For marine use, Forsyth mounted the four banks of the P.24 on a new crankcase to create a U-24 engine. This drawing from US patent 2,389,663 shows the engine and how it would drive a contra-rotating propeller. The bevel drive to the supercharger is shown by number 10.

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.

Fairey P24 engine configurations

A variety of P.24 engine configurations were illustrated in US patent 2,470,155. All of the different configurations are reminiscent of what Allison envisioned for the V-1710 and V-3420. Note the bevel gear drives for the power shafts.

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)

Fairey Fox II P12 engine run

Fairey P.12 Prince Aircraft Engine

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.

Curtiss D-12 Fairey Felix

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.

Fairey Fox bomber D-12 Felix

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.

Fairey P12 induction side

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.

Fairey P12 engine section

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.

Fairey Fox II P12 engine run

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.

Fairey P27-32

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.

Fairey U engine

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 barrel (c) drawn between the cylinder banks.

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)

Dutheil Chalmers Eole props rear

Dutheil-Chalmers Éole Opposed-Piston Aircraft Engine

By William Pearce

In 1906, the French company Société L. Dutheil, R. Chalmers et Cie (Dutheil-Chalmers) began developing aircraft engines for early aviation pioneers. The company was headquartered in Seine, France and was founded by Louis Dutheil and Robert-Arthur Chalmers. Although most of their engines were water cooled, the Dutheil-Chalmers’ horizontal aviation engines may have been the first successful versions of the horizontal type that is now used ubiquitously in light aircraft. Continuing to innovate for the new field of aviation, Dutheil-Chalmers soon developed a line of horizontal, opposed-piston engines.

Dutheil Chalmers Eole patent

Taken from the Dutheil-Chalmers British patent of 1909, this drawing shows the layout of the horizontal, opposed-piston engine. The dashed lines represent the bevel-gear cross shaft that synchronized the two crankshafts.

On 23 November 1908, Dutheil-Chalmers applied for a French patent (number not found) that outlined their concept of an opposed-piston engine. The French patent is referenced in British patent 26,549, which was applied for on 16 November 1909 and granted on 21 July 1910. In the British patent, Dutheil-Chalmers stated that the engine would have two crankshafts. The output shaft would not be a power shaft that connected the two crankshafts. Rather, the crankshafts would rotate in opposite directions (counter-rotating), and a propeller would mount directly to each crankshaft. This is the same power transfer method used in the SPA-Faccioli opposed-piston aircraft engines. While the Dutheil-Chalmers and SPA-Faccioli engines shared a similar concept and were built and developed at the same time, there is no indication that either company copied the other.

The Dutheil-Chalmers opposed-piston engines are sometimes referred to as Éole engines. It is not clear if Dutheil-Chalmers marketed the engines for a time under a different name or if Éole was just the name they gave to their line of opposed-piston engines. Éole is the French name for Aeolus, the ruler of the winds in Greek mythology. The engines were primarily intended to power airships. The two counter-rotating propellers would cancel out the torque associated with a single propeller on a standard engine. In addition, the opposed-piston engine’s two-propeller design did not require the heavy and cumbersome shafting and gears necessary for a conventional single-crankshaft engine to power two propellers.

Dutheil Chalmers Eole 2 view

Top and side view drawings of the four-cylinder, opposed-piston engine. The drawings show no valve train and differ slightly from photos of the actual engine, but they give an idea of the engine’s general layout.

Four different horizontal, opposed-piston engine sizes were announced, all of which were water-cooled. Three of the engines had the same bore and stroke but differed in the number of cylinders used. These engines had two, three, and four cylinders. Each had a 4.33 in (110 mm) bore and a 5.91 in (150 mm) stroke, which was an 11.81 in (300 mm) stroke equivalent with the two pistons per cylinder. The two-cylinder engine displaced 348 cu in (5.7 L) and produced 38 hp (28 kW) at 1,000 rpm. The engine weighed 220 lb (100 kg). The three-cylinder engine displaced 522 cu in (8.6 L) and produced 56 hp (42 kW) at 1,000 rpm. The engine weighed 397 lb (180 kg). The four-cylinder engine displaced 696 cu in (11.4 L) and produced 75 hp (56 kW) at 1,000 rpm. The engine weighed 529 lb (240 kg). It is not clear if any of these engines were built.

The fourth engine was built, and it was the largest opposed-piston engine in the Dutheil-Chalmers line. The bore was enlarged to 4.92 in (125 mm), and the stroke remained the same at 5.91 in (150 mm)—an 11.81 in (300 mm) equivalent with the two pistons per cylinder. The four-cylinder engine displaced 899 cu in (14.7 L) and produced 97 hp (72 kW) at 1,000 rpm. Often, the engine is listed as producing 100 hp (75 kW). The four-cylinder engine weighed 794 lb (360 kg).

Dutheil Chalmers Eole front

This Drawing illustrates the front of the Dutheil-Chalmers opposed-piston engine. Note the cross shaft that synchronized the two crankshafts. The gear on the cross shaft drove the engine’s camshaft. The pushrods, rockers, and valves are visible.

Only the 97 hp (72 kW) engine was exhibited, but it was not seen until 1910. The engine was displayed at the Paris Flight Salon, which occurred in October 1910. The engine consisted of four individual cylinders made from cast iron. The horizontal cylinders were attached to crankcases on the left and right. Threaded rods secured the crankcases together and squeezed the cylinders between the crankcases. Each crankcase housed a crankshaft, and the two crankshafts were synchronized by a bevel-gear cross shaft positioned at the front of the engine. A two-blade propeller was attached to each crankshaft. The propellers were phased so that when one was in the horizontal position, the other was in the vertical position.

Near the center of the cross shaft was a gear that drove the camshaft, which was positioned under the engine. The camshaft actuated pushrods for the intake valves on the lower side of the engine and the exhaust valves on the upper side of the engine. The pushrods of the intake valves travel between the cylinders. All of the pushrods acted on rocker arms that actuated the valves positioned in the middle of the cylinder. Each cylinder had one intake and one exhaust valve.

No information has been found that indicates any Dutheil-Chalmers Éole opposed-piston engines were used in any airship or aircraft. Still, it is a unique engine conceived and built at a time of great innovation, not just in aviation, but in all technical fields.

Dutheil Chalmers Eole props rear

The 97 hp (72 kW), four-cylinder, eight-piston engine on display at the Paris Flight Salon in 1910. The engine has appeared in various publications as both a Dutheil-Chalmers and an Éole. Note the rods that secured the crankcases together. What appears to be the camshaft can be seen under the engine.

Les Moteurs a Pistons Aeronautiques Francais Tome II by Alfred Bodemer and Robert Laugier (1987)
“Improvements in or connected with Motors especially applicable to Aviation and Aerostation Purposes” GB patent 26,549 by L. Dutheil, R. Chalmers and Company (granted 21 July 1910)
“Motors for Aerial Navigation—V” by J. S. Critchley, The Horseless Age (26 October 1910)
“Aerial Motors at the Salon” by Oiseau, Flight (5 November 1910)

SPA-Faccioli N3 rear

SPA-Faccioli Opposed-Piston Aircraft Engines

By William Pearce

Aristide Faccioli was an Italian engineer. In the late 1800s, he became fascinated with aviation and worked to unravel the mysteries of powered flight. With little progress in aviation, Aristide had turned to automobile development by 1898. He worked for Ceirano GB & C and designed Italy’s first automobile, the Welleyes. Ceirano GB & C did not have the finances to produce the automobile, so a new company was established for automobile production. This company was called Fabbrica Italiana Automobili Torino or FIAT, and it bought the rights, plans, and patents for the Welleyes. The Welleyes became FIAT’s first production automobile, the 3 ½ CV.

SPA-Faccioli N1

The SPA-Faccioli N.1 engine with its four cylinders, each housing two opposed pistons. At the rear of the engine (bottom of image) is the cross shaft linking the two crankshafts. Note the gear on the cross shaft that drove the camshaft.

Aristide became FIAT’s first technical director, but he left in 1901 to start his own automobile company. In 1905, Aristide moved from automobile production to engine design. However, Aristide’s focus returned to aviation once he learned of the successful flights of the Wright Brothers and other early pioneers. In 1907, Aristide shut down his companies and worked on aircraft and aircraft engine designs. In 1908, Aristide visited a close friend, Matteo Ceirano, seeking financial support. Matteo was one of Ceirano GB & C’s founders and was a co-founder of SPA (Società Ligure Piemontese Automobili). Matteo and SPA backed Aristide and encouraged him to continue his aeronautical work.

Aristide’s first engine was the SPA-Faccioli N.1. The N.1 was a water-cooled, horizontal, opposed-piston engine. Each side of the engine had a crankshaft that drove pistons in the engine’s four, individual cylinders. Attached to each crankshaft was a propeller. The crankshafts and their propellers turned in opposite directions (counter-rotating). When viewed from the rear of the engine, the right propeller turned clockwise, and the left propeller turned counterclockwise. The two-blade, wooden propellers were phased so that when one was horizontal the other was vertical. The dual, counter-rotating propeller design was an effort to eliminate engine vibrations and cancel out propeller torque.

SPA-Faccioli N2

This rear view of the SPA-Faccioli N.2 illustrates that the engine was much more refined than the N.1. Note the magneto driven above the cross shaft and the gear train driven below.

The two crankshafts were synchronized by a bevel-gear cross shaft that ran along the rear of the engine. Geared to the cross shaft was a camshaft that ran under the engine. The camshaft actuated the intake and exhaust valves that were located in the middle of each cylinder. As the two pistons in each cylinder came together, the air/fuel mixture was compressed. Once the mixture was ignited by the spark plug in the middle of the cylinder, the expanding gasses pushed the pistons back, operating like any other four-stroke engine. The N.1 had a 4.41 in (112 mm) bore and a 5.91 in (150 mm) stroke. The two pistons per cylinder effectively gave the N.1 an 11.81 in (300 mm) stroke. The engine displaced 721 cu in (11.82 L) and produced 80 hp (60 kW) at 1,200 rpm. The N.1 weighed 529 lb (240 kg).

The N.1 engine was installed in the Faccioli N.1 aircraft, which was a triplane pusher design. Flown by Mario Faccioli, Aristide’s son, the engine, aircraft, and pilot all made their first flight on 13 January 1909. The aircraft quickly got away from Mario, and the subsequent crash injured Mario and destroyed the aircraft. Although brief, the flight marked the first time an Italian-designed and built aircraft was flown with an Italian-designed and built engine. With all parties undeterred, the N.1 engine was installed in the Faccioli N.2 aircraft (a biplane pusher with a front-mounted elevator) and flown by Mario in June 1909. After a few flights, Mario and the N.2 aircraft were involved in an accident that again injured Mario and destroyed the aircraft.

Faccioli N3 aircraft

Mario Faccioli sits on the Faccioli N.3 aircraft in 1910. Note the covers over the N.2 engine’s cross shaft bevel gears. Since the propellers rotated in opposite directions, when one was vertical, the other was horizontal.

After these setbacks, Aristide designed a new engine, the SPA-Faccioli N.2. The N.2 had many features in common with the N.1: water-cooling, opposed-pistons, dual crankshafts, a bevel-gear cross shaft, and counter-rotating propellers. However, the N.2 was a single cylinder engine. The engine’s magneto was driven from the cross shaft. The N.2’s intake was positioned on the bottom side of the engine, and exhaust was expelled from the top side. The N.2 had a 3.94 in (100 mm) bore and a 5.12 in (130 mm) stroke—a 10.24 in (260 mm) equivalent for the two pistons per cylinder. The engine displaced 249 cu in (4.08 L) and produced 20 hp (15 kW) at 1,200 rpm and 25 hp (19 kW) at 1,500 rpm. The N.2 weighed 106 lb (48 kg).

The N.2 engine was installed in the Faccioli N.3 aircraft. With a very similar layout to the N.2 aircraft, the N.3 pusher biplane was smaller and did not have the front-mounted elevator. Mario was again the test pilot, and he first flew the aircraft on 12 February 1910. Many flights were made throughout February and March. On 26 March 1910, one propeller came off the engine and damaged the aircraft while it was in flight. Mario was injured in the subsequent crash, and the N.3 aircraft was damaged. Aircraft and pilot flew again in the summer, but Aristide was already working on a new aircraft design.

SPA-Faccioli N3 rear

This rear view of the SPA-Faccioli N.3 shows many features common with the N.2 engine. However, note the 20 degree cylinder angle extending from the crankshafts. The camshaft was driven from the cross shaft and extended through the engine. Two pushrods extend from both the top and bottom of the camshaft. The black plugs in the center of the cylinders cover ports for spark plugs. (W. R. Pearce image)

The N.2 engine was installed in the Faccioli N.4 aircraft, a further refinement of the Faccioli line. The aircraft was first flown by Mario in July 1910. On 15 October 1910, Mario used the N.4 aircraft to get his Italian pilot’s license (No. 21). This was the first time an Italian-designed and built aircraft was used to obtain a pilot’s license.

For his next aircraft, the Faccioli N.5, Aristide needed more power. The new SPA-Faccioli N.3 engine was built upon knowledge gained from the previous engines. Again, the engine was water-cooled with opposed-pistons and had dual crankshafts (synched by a bevel-gear cross shaft) that drove counter-rotating propellers. However, the cylinder arrangement of the N.3 was unique. In essence, the N.3 was made up of two V-4 engines mounted horizontally and attached together via their combustion chambers. The cylinders of the complete engine formed a diamond shape, with the cylinders angled at 20 degrees relative to the crankshaft. This gave the cylinders a 160 degree bend at their middle. Technically, the pistons no longer shared a common cylinder, but the cylinders did still share a combustion chamber. Some sources define the N.3 as a four-cylinder opposed-piston engine, and other sources define it as an eight-cylinder engine in which opposed pairs of cylinders shared a common combustion chamber.

SPA-Faccioli N3 front

The N.3 engine’s intake manifold can be seen on the left side of the image; the exhaust ports are also visible to the right of the valves. Note the camshaft extending through the engine, and the pushrods that actuated the valves. The front side of the engine still has its two spark plugs.

Two magnetos were driven from the cross shaft at the rear of the N.3 engine. The magnetos fired one spark plug per cylinder pair. The spark plugs were positioned either on the front of the engine or on the back, depending on the cylinder. The cross shaft also drove a short camshaft that extended through the diamond between the cylinders. Via pushrods and rocker arms, the camshaft actuated the one intake and one exhaust valve for each cylinder pair. An intake manifold mounted to the front of the engine brought air and fuel into the right side of the engine, and the exhaust was expelled from the left side of the engine. The N.3 had a 2.95 in (75 mm) bore and a 5.91 in (150 mm) stroke. The engine displaced 324 cu in (5.30 L) and produced 40 hp (30 kW) at 1,200 rpm and 50 hp (37 kW) at 1,600 rpm. The N.3 weighed 198 lb (90 kg).

The N.3 engine was finished in early 1911, but the Faccioli N.5 aircraft was not. The N.3 engine was installed in the N.4 aircraft, and Mario continued his role as chief pilot. The N.3-powered N.4 aircraft was entered in various competitions during the Settimana Aerea Torinese (Turinese Air Week) held in June 1911. On 25 June 1911, the last day of the competition, a mechanical failure on the aircraft caused Mario and the N.4 to crash. As with previous crashes, Mario was injured, and the aircraft was destroyed.

Faccioli N4 aircraft

The Faccioli N.4 aircraft was originally powered by the SPA-Faccioli N.2 engine. In 1911, the eight-cylinder SPA-Faccioli N.3 engine was installed. This image was taken in June 1911, with the N.3 engine installed and Mario in the aircraft.

It is not clear if the Faccioli N.5 aircraft was ever completed. Aristide’s involvement in aviation seemed to wane after the crash of the N.4 aircraft. In fact, the last SPA-Faccioli engine may have been a development of the N.3 undertaken exclusively by SPA without much involvement from Faccioli.

Built in late 1911 or early 1912, the SPA-Faccioli N.4 engine was an enlarged and refined N.3. With the N.4, eight cylinders were again positioned in a diamond configuration, angled at 20 degrees at the crankshafts and 160 degrees at the combustion chambers. Each opposed cylinder pair shared a common combustion chamber. Each cylinder pair now had two spark plugs, and they were fired by two magnetos, one driven directly from the rear of each crankshaft. The cross shaft synchronizing the crankshafts also served as the camshaft. At the rear of the engine, the cross shaft drove pushrods that acted on rocker arms mounted to the top and bottom of the engine. The rocker arms actuated the one intake and one exhaust valve per cylinder pair, positioned at the center of the cylinders. The intake manifold was positioned behind the engine, to the left of center. The manifold fed the air/fuel mixture to a passageway in the cylinder casting that ran on the left side of the valves. The exhaust was expelled to the right of the valves.

SPA-Faccioli N4 front

The SPA-Faccioli N.4 was the final refinement of the Faccioli engine line. The magnetos can be seen behind the engine; each was driven from the rear of a crankshaft. Note the two spark plugs per cylinder pair. (W. R. Pearce image)

The N.4 engine had a 3.74 in (95 mm) bore and a 5.91 (150 mm) stroke. The engine displaced 519 cu in (8.51 L) and produced 80 hp (60 kW) at 1,200 rpm and 90 hp (67 kW) at 1,600 rpm. The N.4 was 54 in (1.38 m) wide, 32 in (.82 m) long, 22 in (.57 m) tall, and weighed 441 lb (200 kg). No information has been found to indicate that the engine was installed in any aircraft.

After surviving so many close calls, Mario Faccioli was sadly killed in a plane crash in March 1915. The type of aircraft involved in the crash is not known. Aristide Faccioli never achieved the success he strived for and never recovered from his son’s death. He took his own life on 28 January 1920.

SPA-Faccioli N.3 and N.4 engines are preserved and on display in the Museo Storico dell’Aeronautica Militare in Vigna di Valle, Italy. An N.4 engine is displayed in the Museum of Applied Arts & Sciences, Museums Discovery Centre in Castle Hill, Australia. The museum lists the engine as a “300 hp, model 2-A,” undoubtedly confusing the eight-cylinder SPA-Faccioli engine with a SPA Type 2-A straight-eight engine. Also, the N.4 is positioned upside-down in its display stand.

SPA-Faccioli N4 rear

This rear view of the N.4 engine shows how the cross shaft also acted as the camshaft and directly drove the pushrods. The valves in the foreground are for the intake. The port for the intake manifold can just be seen at the center of the engine. Note the mounts for the magnetos and that the engine is upside-down in its display stand. (Museum of Applied Arts & Sciences image)

Origin of Aviation in Italy by Piero Vergnano (1964)
Aeronuatica Militare Museo Storico Catalogo Motori by Oscar Marchi (1980)
Jane’s All the World’s Aircraft 1912 by Fred T. Jane (1912/1968)

Isotta Fraschini Zeta rear

Isotta Fraschini Zeta X-24 Aircraft Engine

By William Pearce

In 1900, Cesare Isotta and Vincenzo Fraschini formed Isotta Fraschini (IF) in Milan, Italy. The firm originally imported automobiles, but began manufacturing its own vehicles by 1904. In 1908, IF started experimenting with aircraft engines and began producing them by 1911. The company went on to build successful lines of air-cooled and water-cooled engines. In the early 1930s, IF experienced financial issues caused in part by the great depression. In 1932, the Italian aircraft manufacturer Caproni purchased IF and continued production of automobiles and engines (both aircraft and marine).

Isotta Fraschini Zeta front

The Isotta Fraschini Zeta used many components from the Gamma V-12 engine. The air-cooled, X-24 Zeta had its cylinder banks at 90 degrees, and cooling the rear cylinders proved to be a problem. (Kevin Kemmerer image)

In the late 1930s, IF developed a pair of inverted, 60 degree, V-12, air-cooled engines. The first of the engines was the Gamma. The Gamma had a 4.92 in (125 mm) bore and a 5.12 in (130 mm) stroke. The engine displaced 1,168 cu in (19.1 L) and produced 542 hp (404 kW) at 2,600 rpm. The second engine was the Delta; it had the same architecture as the Gamma but had a larger bore and stroke of 5.20 in (132 mm) and 6.30 in (160 mm) respectively. The Delta displaced 1,603 cu in (26.3 L) and produced 790 hp (589 kW) at 2,500 rpm.

In 1939, the Ministero dell’Aeronautica (Italian Air Ministry) worked to import Daimler-Benz aircraft engines from Germany and obtain licenses for their production. IF decided to design an engine powerful enough to compete with the Daimler-Benz engines or replace them if sufficient quantities could not be imported.

To speed engine development, IF created the new engine using as much existing technology as possible. Essentially, two Gamma engines were mounted on a common crankcase in an X configuration to create the new engine, which was called the Zeta. The use of air-cooling and a single crankshaft simplified the design of the 24-cylinder Zeta engine.

Isotta Fraschini Zeta rear

All of the Zeta’s accessories were driven at the rear of the engine. A camshaft housing spanned all of the cylinders for one cylinder bank. Note the two spark plug leads for each cylinder extending from the top of the camshaft housing. The pipes for the air starter can been seen on the upper cylinder bank. (Kevin Kemmerer image)

The Isotta Fraschini Zeta was made up of an aluminum crankcase with four cylinder banks, each with six individual cylinders. All cylinder banks were positioned 90 degrees from one another. Each air-cooled cylinder was secured to the crankcase by ten bolts, and the cylinder’s steel liner extended into the crankcase. Each cylinder had two spark plugs that were fired by magnetos positioned at the rear of the cylinder bank.

Each cylinder had one intake and one exhaust valve. Mounted to the top of each bank of cylinders was a camshaft housing that contained dual overhead camshafts. A vertical shaft at the rear of the cylinder bank directly drove the exhaust camshaft. A short cross shaft drove the intake camshaft from the exhaust camshaft. The crankshaft was supported by seven plain bearings, and each connecting rod served four cylinders via a master rod and three articulating rods.

An accessory section at the rear of the engine drove the magnetos, vertical drives for the camshafts, and a single-stage supercharger. The supercharger forced air through intake manifolds between the upper and lower cylinder Vees. The exhaust gases were expelled from the cylinders via individual stacks between the left and right cylinder Vees. A pressurized air starting system was used, and the engine had a compression ratio of 6.5 to 1. The Zeta maintained the 4.92 in (125 mm) bore and 5.12 in (130 mm) stroke of the Gamma. The Zeta displaced 2,336 cu in (38.3 L) and produced 1,233 hp (919 kW) at 2,700 rpm. The engine was around 68 in (1.73 m) long, and 39 in (1.00 m) wide and tall. The Zeta weighed approximately 1,675 lb (760 kg).

Caproni F6Z IF Zeta

The Caproni Vizzola F.6MZ was the only aircraft to fly with a Zeta engine. The close-fitting cowl can be seen bulging around the engine’s cylinder banks, and the removed panels show just how tight of a fit the cowling was. Note the gap around the propeller for cooling air.

The Zeta RC45 was first run on 28 February 1941, and development was slowed due to various design issues. The engine was also having trouble making the forecasted output, with only around 1,085 hp (809 kW) being achieved. As development progressed, many of the issues were resolved, but the engine still lacked power. In May 1943, the Zeta RC24/60 with a two-speed supercharger was run, but the engine was not able to pass its type test. A number of aircraft were considered for conversion from their initial engines to the Zeta, but serious progress was made on only two aircraft.

The Caproni Vizzola F.6M was an all-metal aircraft based on the Caproni Vizzola F.5 but powered by a 1,475 hp (1,100 kW), liquid-cooled, Daimler-Benz DB 605 engine. While the F.6M was being developed, the design of a second version of the aircraft powered by a Zeta RC45 engine was initiated on 7 October 1941. The new design was called F.6MZ (or just F.6Z). The Zeta-powered aircraft was ordered on 16 June 1942, and it was assigned serial number (Matricola Militare) MM.498. The engine change came about because reliable deliveries of the DB 605 and its license-built contemporary, the FIAT RA 1050, could not be assured.

Progress on the Caproni Vizzola F.6MZ was delayed because of the engine. While the F.6M first flew in September 1941, it was not until 14 August 1943 that the F.6MZ took flight. The F.6MZ had a tight-fitting cowling that bulged around the engine’s four valve covers, and four rows of short exhaust stacks protruded from the cowling. Cooling air was taken in from around the spinner, and the air was expelled via an annular slot at the rear of the cowling. An oil cooler was housed in a chin radiator below the cowling.

Caproni Vizzola F6Z

The F.6MZ was first flown on 14 August 1943. The two rows of exhaust stacks can be seen near the cylinder bank bulges. The cooling air exit flaps can just be seen at the rear of the cowling.

First flown by Antonio Moda, the F.6MZ had an estimated top speed of 391 mph (630 km/h), some 37 mph (60 km/h) faster than the F.6M. This speed seems optimistic, considering the Zeta had an output of at least 225 hp (168 kW) less than the DB 605 and that the F.6MZ could not have produced significantly less drag or have been much lighter than the F.6M. The Zeta engine experienced overheating issues throughout the flight test program—the rear cylinders did not have sufficient airflow for proper cooling. Some modifications were made, but further flight tests were halted with Italy’s surrender on 8 September 1943. Two F.6MZ aircraft were ordered, but only the first prototype was built.

In October 1941, Regia Aeronautica (Italian Royal Air Force) requested that Reggiane (Officine Meccaniche Reggiane) replace the DB 605 / FIAT RA 1050 in its RE 2005 Sagittario fighter with the IF Zeta RC24/60. Reggiane was another company owned by Caproni. The Zeta-powered aircraft, developed after the RE 2005, was the Reggiane RE 2004, and seven examples were ordered. Although Reggiane was less enthusiastic about the Zeta than Caproni Vizzola, they did work on designing a firewall-forward engine package.

Isotta Fraschini Zeta SM79

These four images show the Zeta RC24/60 engine installed in the nose of a SM.79. Once tested, this installation would be applied to the Reggiane RE 2004. Note how the exhaust stack arrangement was completely different from that used on the F.6MZ.

A Zeta engine was not delivered to Reggiane until 1943. At the time, Reggiane was building Savoia-Marchetti SM.79 Sparviero three-engined bombers. One SM.79 was modified to have the Zeta engine installed in the nose position. This would enable the engine to be flight tested, and the cooling characteristics of the cowling configuration could be evaluated before the engine was used in the RE 2004. Compared to the F.6Z cowling, the Reggiane cowling had a larger diameter but was a cleaner design. Again, cooling air was brought in from around the spinner and exited through an annular slot at the rear of the cowling, and an oil cooler was positioned below the cowling. The Reggiane installation used exhaust stacks that ended with two close rows along the sides of the cowling. It appears that the Italian surrender occurred before the Zeta engine was ever flown in the SM.79. In fact, the Zeta RC24/60 was never cleared for flight, and the engine used in the SM.79 was most likely a mockup without all of its internal components. Although never built, the RE 2004 had an estimated top speed of 385 mph (620 km/h), 36 mph (58 km/h) slower than the RE 2005. At 7,117 lb (3,228 kg), the RE 2004 was 842 lb (382 kg) lighter than the RE 2005.

IF also designed the Sigma, a larger X-24 engine using cylinders and other components from the inverted, V-12, air-cooled Delta. The Sigma had a 5.20 in (132 mm) bore and 6.30 in (160 mm) stroke. The engine displaced 3,207 cu in (52.5 L) and had an estimated output of 1,578 hp (1,178 kW) at 2,400 rpm. The Sigma was never built, but its approximate dimensions were 82 in (2.08 m) long, and 45 in (1.15 m) wide and tall. The engine weighed around 2,160 lb (980 kg).

Isotta Fraschini Zeta SM79 cowling

The Zeta installation for the RE 2004 (as seen on the SM.79) was fairly clean but somewhat spoiled by the large oil cooler under the cowling. Note the cooling air exit gap at the rear of the cowling.

Tutti gli aerie del Re by Max Vinerba (2011)
Italian Civil and Military Aircraft 1930-1945 by Jonathan W. Thompson (1963)
I Reggiane dall’ A alla Z by Sergio Govi (1985)
The Caproni-Reggiane Fighters 1938-1945 by Piero Prato (1969)
Ali E Motori D’Italia by Emilio Bestetti (1939)
Isotta Fraschini: The Noble Pride of Italy by Tim Nichols (1971)

Allison V-3420-A front

Allison V-3420 24-Cylinder Aircraft Engine

By William Pearce

In the mid-1930s, the United States Army Air Corps (AAC) was interested in a long-range bomber. Boeing won a contract to build the aircraft, which was originally designated XBLR-1 (eXperimental Bomber Long Range-1), but ultimately became the XB-15. By 1935, the AAC realized that current engines, and those under development, lacked the power needed for such a large aircraft. At the time, the AAC was pursuing its next experimental long-range bomber, the Douglas XBLR-2. The AAC requested the Allison Engineering Company build a 1,600 hp (1,193 kW) engine for the XBLR-2, which later became the XB-19.

Allison V-3420-A front

The Allison V-3420 was much more than two V-1710 engines coupled together. However, as many V-1710 components were used as possible, resulting in only 340 new parts. This is a V-3420-A engine with an attached single-rotation gear reduction.

In 1935, Allison was in the middle of developing its 1,000 hp (746 kW) V-1710 engine. The AAC requested that the new 1,600 hp (1,193 kW) engine have a single crankshaft and use as many V-1710 components as possible to keep development time to a minimum. After evaluating a few different configurations, Allison decided to double the V-1710 to create a 24-cylinder engine in an X configuration. This engine became the X-3420.

The X-3420 would have an entirely new crankcase, crankshaft, gear reduction, supercharger, and accessory section, but it would keep the basic V-1710 cylinder and head. The X-3420 had a flattened X arrangement with a left and right cylinder bank angle of 60 degrees, an upper cylinder bank angle of 90 degrees, and a lower cylinder bank angle of 150 degrees. The fuel-injected engine would produce 1,600 hp (1,193 kW) at 2,400 rpm for takeoff and 1,000 hp (746 kW) at 1,800 rpm for economical cruise. The engine would have an 8.5 to 1 compression ratio and weigh 2,160 lb (980 kg).

While using as many V-1710 components as possible made Allison’s job easier, the X-3420’s single crankshaft and its master and articulating rods required much design work, as did its fuel-injection system. Very quickly, Allison realized it did not have the resources to develop the X-3420 and needed to focus on the V-1710, which was encountering technical issues. Development of the X-3420 was effectively abandoned in 1936. As an alternative, Ron Hazen, Allison’s Chief Engineer, proposed a new 2,000 hp (1,491 kW) engine that had two crankshafts and was more closely based on the V-1710. The engine would produce more power than the X-3420 and be developed in less time. The AAC approved of Hazen’s proposed engine, which became the V-3420. The engine was often referred to as a W-24 or double Vee (DV) and was occasionally called the DV-3420.

Allison V-3420-A rear

Rear view of the V-3420-A shows the supercharger mounted behind the right engine section and various accessories mounted behind the left engine section. The V-3420’s design enabled the engine to produce more power than its X-3420 progenitor.

The Allison V-3420 design was more complex than just coupling two V-1710 engines together. As with the proposed X-3420, a new crankcase, gear reduction, supercharger, and accessory section were at the center of the engine, but the V-3420 would utilize many V-1710 components. The use of two V-1710 crankshafts along with their connecting rods made the V-3420’s design and development much more manageable for Allison. The engine consisted of two 60 degree V-12 engine sections mounted on a common crankcase and separated by 90 degrees, which gave the inner cylinder banks 30 degrees of separation.

As V-1710 development progressed, Allison was able to offer the V-3420 with 2,300 hp (1,715 kW) for takeoff. At 2,300 lb (1,043 kg), the engine would only weigh 140 lb (64 kg) more than the single crankshaft X-3420, but it would produce an additional 700 hp (522 kW). In May 1937, the AAC contracted Allison to build the V-3420 engine prototype.

A large aluminum crankcase sat at the center of the 24-cylinder V-3420 engine. Attached to the crankcase were four cylinder banks. Each cylinder bank consisted of six steel cylinder barrels shrink fitted to a one-piece aluminum cylinder head. Each cylinder barrel was surrounded by an aluminum water jacket. A single overhead camshaft actuated two intake and two exhaust valves for each cylinder. Each cylinder had a 5.5 in (140 mm) bore and a 6.0 in (152 mm) stroke. The engine displaced 3,421 cu in (56.1 L) and had a compression ratio of 6.65 to 1. At the rear of the engine was a supercharger driven by the right crankshaft, and all accessories were driven by the left crankshaft. The engine was also intended to be used with a General Electric turbosupercharger.

Allison V-3420-B NMUSAF rear

This V-3420-B was the type installed in the Fisher XP-75. About 15 ft (4.6 m) of shafting separated the engine from the gear reduction. Note the much larger supercharger compared to the image of the V-3420-A engine. The V-3420-B used a two-stage supercharger and no turbosupercharger. (Gary Brossett image via the Aircraft Engine Historical Society)

There were only 340 parts unique to the V-3420 engine, and those accounted for 930 pieces of the 11,630 that made up the engine. Initially, the V-3420 had a takeoff rating of 2,300 hp (1,715 kW) at 3,000 rpm, a maximum rating of 2,000 hp (1,491 kW) at 2,600 rpm, and a cruise rating of 1,500 hp (1,119 kW) at 2,280 rpm. The basic 24-cylinder engine was 97.7 in (2.48 m) long, 60.0 in (1.52 m) wide, and 38.7 in (.98 m) tall. The engine weighed 2,665 lb (1,209 kg)—365 lb (166 kg) more than the original estimate.

In January 1938, Allison was authorized to release V-3420 engine specifications to aircraft manufacturers and airlines. This resulted in a number of aircraft designs incorporating the engine; however, only four V-3420-powered aircraft types were actually flown. The V-3420 engine was first run in April 1938, followed by an AAC order for six engines in June 1938. An engine was also displayed in the 1939 World’s Fair in New York.

The US Navy was aware of the V-3420 engine and asked Allison if it could be converted for marine use. Allison responded with the appropriate designs. In December 1939, the Navy ordered two V-3420 marine engines for installation in a new, aluminum-hulled Patrol Torpedo boat designated PT-8. The two V-3420 marine engines were delivered to the Navy, and the PT-8 boat started trials in November 1940. The PT-8 was tested through 1941, but no further boats or V-3420 marine engines were ordered. The sole PT-8 was later re-engined and still exists as of 2017.

Allison V-3420-B NMUSAF

On the V-3420-B engine, an idler gear kept the crankshafts in sync. The engine’s large crankcase can be seen in this image. The large aluminum casting had front and rear covers and a magnesium oil pan. (Gary Brossett image via the Aircraft Engine Historical Society)

For aircraft use, the V-3420 required further development, which was slow due to Allison’s ongoing commitments to the V-1710 engine as well as the AAC’s preoccupation with vastly expanding its resources for the coming war. In late 1940, Allison focused on two major models of the V-3420 engine: -A and -B. The V-3420-A had crankshafts that rotated the same direction—either clockwise or counterclockwise, depending on the desired rotation of the propeller. The -A engine used a single-rotation propeller with either an attached or remote gear reduction, but most commonly with an attached gear reduction. The V-3420-B had crankshafts that rotated in opposite directions and was used with contra-rotating propellers. Different versions of the -B engine could accommodate either an attached or remote gear reduction, which allowed a number of propeller shaft configurations, including right-angle drives. The -B engine almost always had a remote gear reduction. The two crankshafts of the V-3420-B were kept in sync by idler gears at the front of the engine. The idler gears also balanced power loads from the crankshafts to the contra-rotating propeller shafts.

In September 1940, Allison’s V-1710 commitments became overwhelming, and development of the V-3420 engine was put on hold. As a result, the XB-19 had four 2,000 hp (1,491 kW) Wright R-3350 18-cylinder radial engines installed in place of the V-3420s. However, the R-3350 was encountering its own extensive developmental issues that put its use in the Boeing B-29 Superfortress in question. In February 1941, the AAC requested that Allison restart development of the V-3420-A with an output of 3,000 hp (2,237 kW) as a possible replacement for the Wright R-3350. The B-29 bomber was too important for its fate to be tied to one engine.

Allison V-3420-B right-angle drive

One V-3420-B engine was built to be mounted in an aircraft’s fuselage with extension shafts leading through the wings to right angle drives that would connect to the propellers. This type of engine configuration would have been used in the McDonnell Model 1. Only one engine was built with this configuration.

A V-3420 engine was delivered to Wright Field in October 1941, but with the bombing of Pearl Harbor in December, the V-3420 program was again put on hold so that Allison could focus on the V-1710 engine. History repeated itself in mid-1942 when the suitability of the R-3350 engine was again in question. Allison was instructed by the Army Air Force (AAF—the AAC was renamed in June 1941) to prepare the V-3420 for installation in a B-29, which was redesignated XB-39. Nine engines were built and delivered by October 1942. On 1 October 1942, the AAF ordered two Fisher XP-75 Eagle fighter prototypes that were powered by the V-3420-B engine. This was followed by an order placed on 28 October for 500 V-3420-A engines for installation in 100 production B-39 aircraft.

As the aircraft projects were underway, continued development of the V-3420 engine increased its output to a takeoff rating of 2,600 hp (1,939 kW) at 3,000 rpm with 8 psi (.55 bar) of boost, a normal rating of 2,100 hp (1,566 kW) at 2,600 rpm at 25,000 ft (7,620 m), and a cruise rating of 1,575 hp (1,175 kW) at 2,300 rpm at 25,000 ft (7,620 m). However, the engine could be overboosted in emergency situations to 3,000 hp (2,237 kW) at 3,000 rpm with 10.2 psi of boost (.70 bar).

Fisher P-75A Eagle

The Fisher P-75A was the end of a very tumultuous fighter program. The original design consisted of various parts from other aircraft that, when combined, would somehow make an aircraft superior to all others. The reality was that the combined parts created an aircraft that was downright dangerous and needed to be redesigned. A partial redesign did not completely cure the problems, and problems still existed after a subsequent complete redesigned. Still, 2,500 aircraft were ordered before better judgment prevailed and the program was cancelled. The P-75 was the only aircraft flown with V-3420-B engines.

The first aircraft to fly with the V-3420 was the Fisher XP-75. Developed by the Fisher Body Division of General Motors, the XP-75 was a long-range escort fighter. Through 1943, the AAF felt a desperate need for such an aircraft and ordered six additional XP-75 prototypes, bringing the total to eight. In addition, the AAF expressed its intent to purchase 2,500 P-75s if the prototypes met their performance estimates. The V-3420-B engine for the P-75 had a two-stage, variable speed supercharger (and no turbosupercharger) that was hydraulically coupled to the right crankshaft. The engine alone weighed 2,750 lb (1,247 kg), and its weight increased to 3,275 lb (1,486 kg) with its 3.5 in (89 mm) diameter extension shafts and remote gear reduction.

The XP-75 first flew on 17 November 1943, and the aircraft almost immediately ran into issues. Its V-3420-B engine was not entirely trouble free either; unequal fuel distribution was a continuing problem for the V-3420. The issue was mostly solved by having each alternate engine section fire every 30 degrees of rotation, rather than both engine sections firing every 60 degrees of rotation. The aircraft was redesigned to correct its deficiencies and was given the new designation of P-75A. The AAF ordered 2,500 P-75As on 7 June 1944, and production started immediately. However, the entire P-75 program was cancelled four months later, in October 1944. The P-75A did not live up to expectations, it was outmatched by aircraft already in service, and the end of the war was in sight. Eight XP-75 and six P-75A aircraft were built, but three of the aircraft crashed during testing. One P-75A was preserved and is on display in the National Museum of the US Air Force. The rest of the surviving aircraft were scrapped.

Douglas XB-19A

With V-3420-A engines installed, the Douglass XB-19A realized a boost in its performance. While the engines proved reliable, it was very time-consuming for Fisher to design and fabricate the new nacelles to house the V-3420. The same basic nacelle was also used on the XB-39.

Actual work to install V-3420-A engines in the XB-19 started in November 1942 at Fisher. The aircraft was redesignated XB-19A and flew for the first time with its V-3420 engines in January 1944. The V-3420 installation served as a test for the engine’s use in the XB-39. With the exception of range, the XB-19A’s performance increased across the board: maximum speed increased by 40 mph (64 km/h); cruising speed increased by 50 mph (80 km/h); service ceiling increased by 16,000 ft (4,877 m), but normal range decreased by 1,000 miles (1,609 km). The XB-19A was strictly an experimental aircraft and was never intended to enter production.

In February 1943, V-3420-A engines were selected to power the Lockheed XP-58 Chain Lightning. The V-3420 was not Lockheed’s first choice, or second, or third. The XP-58 heavy fighter program was initiated in 1940 but was beset with constant design and role changes, which were made worse by developmental issues of the aircraft’s previously selected engines. By the time it was completed, the XP-58 was oversized, overweight, underpowered, and not needed. First flown on 6 June 1944, the aircraft’s lackluster performance matched Lockheed and the AAF’s enthusiasm for the project. Only one prototype was built, and the XP-58 program was cancelled in May 1945.

Allison V-3420 XB-19A nacelle

The men working on the V-3420 installed in the XB-19A give some perspective as to the engine’s size and the size of the aircraft. The V-3420’s radiator, oil cooler, turbosupercharger, and intercooler were all mounted in the nacelle, under the engine. This configuration prevented the need for heavily modifying the aircraft.

Even though it helped spur the V-3420 engine program, the V-3420-powered B-29 was the last aircraft to take flight with the engine. A B-29 (actually a YB-29, the first pre-production aircraft) was delivered to Fisher for conversion to an XB-39 with V-3420-A engines. Work on the XB-39 was slow because Fisher’s main focus was the XP-75. The XB-39 finally flew on 9 December 1944. Performance of the XB-39 was superior to that of the B-29: its top speed was 50 mph (80 km/h) faster, and it had a 3,000 ft (914 m) higher service ceiling. However, standard B-29s were proving to be more than adequate, and it was not worth the time or trouble to convert any other airframes to V-3420-power.

To meet the power needs for extremely large aircraft designs during World War II, Allison proposed the DV-6840. The DV-6840 consisted of two V-3420s driving a common remote gearbox for contra-rotating propellers. A gearbox for the DV-6840 was completed in 1946, but no information has been found regarding it being tested. Allison had also planned a further development of the V-3420. This fuel-injected V-3420-C engine had a forecasted emergency output of 4,800 hp (3,579 kW) and a takeoff/military rating of 4,000 hp (2,983 kW)—both ratings at 3,200 rpm with water injection. However, the V-3420-C was never built.

Lockheed XP-58 Chain Lightning

The Lockheed XP-58 was another program than inexplicably pressed on despite the many signs that it was heading nowhere. Somewhere between three to seven engines were selected before the V-3420-A was finally chosen to power the aircraft. It was not Lockheed’s fault; they had no control over which experimental engines would actually be produced. Lockheed also had no control over the constantly changing roles the AAF asked the XP-58 to fulfill.

The Allison V-3420 was not a trouble-free engine, but it did work well in its few applications once initial issues were resolved. The engine held a lot of potential, but that potential faded as its development languished. At the start of 1944, only 33 V-3420 engines had been delivered, and two of those were marine engines. Had the AAC committed to the engine in 1936 and provided Allison with the resources needed to develop the engine, the V-3420 very well could have powered the B-29 and various post-war aircraft. The four aircraft projects that used the V-3420 did not fail because of the engine. By the time the V-3420 program was in order in 1944, other engines were adequately fulfilling the 3,000 hp (2,237 kW) role.

Allison built a total of 157 V-3420 engines: 37 -A engines (including the two marine engines) and 120 -B engines. A number of V-3420s were sold as surplus after the war. Some eventually made their way into museums, while other engines were used in a hydroplane (Henry J. Kaiser’s Scooter Too driven by Jack Regas) and a tractor puller (E. J. Potter’s Double Ugly). However, none of the V-3420 engines took flight again.

Fisher XB-39

The Boeing / Fisher XB-39 program is what put the V-3420 engine back on track to production. It was the most promising aircraft out of the four powered by the V-3420. Delayed by Fisher’s work on the XP-75, there was little point to the aircraft when it took to the air in December 1944. The image above shows the V-3420 engines being installed at the Fisher plant in Cleveland, Ohio. Fisher was producing various subassemblies for the B-29, which can be seen in the background. On the right side of the image, just behind the XB-39’s wing, is the fuselage of a P-75A.

Vees For Victory!: The Story of the Allison V-1710 Aircraft Engine 1929-1948 by Dan Whitney (1998)
The Allison Engine Catalog 1915-2007 by John M. Leonard (2008)
Jim Allison’s Machine Shop: The First 30 Years by John M. Leonard (2016)
Aircraft Engines of the World 1946 by Paul H. Wilkinson (1946)
Allied Aircraft Piston Engines of World War II by Graham White (1995)
US Army Air Force Fighters Part 2 by William Green and Gordon Swanborough (1978)
McDonnell Douglas Aircraft since 1920: Volume I by Rene J. Francillon (1988)
Lockheed Aircraft since 1913 by Rene J. Francillon (1982/1987)
Boeing Aircraft since 1916 by Peter M. Bowers (1966/1989)


Daimler-Benz DB 602 (LOF-6) V-16 Diesel Airship Engine

By William Pearce

Around 1930, Daimler-Benz* developed the F-2 engine, initially intended for aviation use. The F-2 was a 60 degree, supercharged, V-12 engine with individual cylinders and overhead camshafts. The engine had a 6.50 in (165 mm) bore and an 8.27 in (210 mm) stroke. The F-2’s total displacement was 3,288 cu in (53.88 L), and it had a compression ratio of 6.0 to 1. The engine produced 800 hp (597 kW) at 1,500 rpm and 1,000 hp (746 kW) at 1,700 rpm. The engine was available with either direct drive or a .51 gear reduction, and weighed around 1,725 lb (782 kg). It is unlikely that the Daimler-Benz F-2 powered any aircraft, but it was used in a few speed boats.

The Daimler-Benz OF-2 diesel engine was very similar to the spark ignition F-2. Note the dual overhead camshafts in the Elektron housing above the individual cylinders. This was one of the OF-2’s features that was not incorporated into the LOF-6.

The Daimler-Benz OF-2 diesel engine was very similar to the spark ignition F-2. Note the dual overhead camshafts in the Elektron housing above the individual cylinders. This was one of the OF-2’s features that was not incorporated into the LOF-6.

In the early 1930s, Daimler-Benz used the F-2 to develop a diesel engine for airships. This diesel engine was designated OF-2, and it maintained the same basic V-12 configuration as the F-2. The individual cylinders were mounted on an Elektron (magnesium alloy) crankcase. Each cylinder had four valves that were actuated by dual overhead camshafts. The OF-2 had the same bore, stroke, and displacement as the F-2, but the OF-2’s compression ratio was increased to 15 to 1.

Fuel was injected into the cylinders at 1,330 psi (91.7 bar) via two, six-plunger injection pumps built by Bosch. The fuel was injected into a pre-combustion chamber located between the four valves in the cylinder head. This design had been used in automotive diesels built by Mercedes-Benz. Sources disagree on the gear reduction ratio, and it is possible that more than one ratio was offered. Listed ratios include .83, .67, and .58.

The Daimler-Benz OF-2 engine had a normal output of 700 hp (522 kW) at 1,675 rpm, a maximum output of 750 hp (559 kW) at 1,720 rpm, and it was capable of 800 hp (597 kW) at 1,790 rpm for very short periods of time. Fuel consumption at normal power was .392 lb/hp/hr (238 g/kW/hr). The engine was 74.0 in (1.88 m) long, 38.6 in (.98 m) wide, and 42.5 in (1.08 m) tall. The OF-2 weighed 2,061 lb (935 kg).


This view of a display-quality DB 602 engine shows the four Bosch fuel injection pumps at the rear of the engine. The individual valve covers for each cylinder can also be seen.

The OF-2 passed its type test in 1932. At the time, Germany was developing its latest line of airships, the LZ 129 Hindenburg and LZ 130 Graf Zeppelin II. These airships were larger than any previously built, and four OF-2 engines would not be able to provide sufficient power for either airship. As a result, Daimler-Benz began developing a new engine to power the airships in 1933. Daimler-Benz designated the new diesel engine LOF-6, but it was soon given the RLM (Reichsluftfahrtministerium or Germany Air Ministry) designation DB 602.

Designed by Arthur Berger, the Daimler-Benz DB 602 was built upon lessons learned from the OF-2, but it was a completely new engine. The simplest way to build a more powerful engine based on the OF-2 design was by adding two additional cylinders to each cylinder bank, which made the DB 602 a V-16 engine. The two banks of eight cylinders were positioned at 50 degrees. The 50 degree angle was selected over the 45 degree angle typically used for a V-16 engine. This gave the DB 602 an uneven firing order which helped avoid periodic vibrations.

The individual steel cylinders were mounted to the aluminum alloy crankcase. About a third of the cylinder was above the crankcase, and the remaining two-thirds protruded into the crankcase. This arrangement helped eliminate lateral movement of the cylinders and decreased vibrations. The crankcase was made of two pieces and split horizontally through the crankshaft plane. The lower part of the crankcase was finned to increase its rigidity and help cool the engine oil.

Daimler-Benz LOF-6 DB602 V-16 diesel engine

Originally called the LOF-6, the Daimler-Benz DB 602 was a large 16-cylinder diesel engine built to power the largest German airships. Note the three-pointed star emblems on the front valve covers. Propeller gear reduction was achieved through bevel planetary gears.

A single camshaft was located in the Vee of the engine. The camshaft had two sets of intake and exhaust lobes per cylinder. One set was for normal operation, and the other set was for running the engine in reverse. The fore and aft movement of the camshaft to engage and disengage reverse operation was pneumatically controlled. Separate pushrods for the intake and exhaust valves rode on the camshaft and acted on duplex rocker arms that actuated the valves. Each cylinder had two intake and two exhaust valves. Four Bosch fuel injection pumps were located at the rear of the engine and were geared to the camshaft. Each injection pump provided fuel at 1,600 psi (110.3 bar) to four cylinders. Fuel was injected into the center of the pre-combustion chamber, which was situated between the four valves. For slow idle (as low as 300 rpm), fuel was cut from one cylinder bank.

The DB 602 engine was not supercharged and had a .50 propeller gear reduction that used bevel planetary gears. The engine used fork-and-blade connecting rods that rode on roller bearings fitted to the crankshaft. The camshaft also used roller bearings, but the crankshaft was supported by plain bearings. Two water pumps were driven by a cross shaft at the rear of the engine. Each pump provided cooling water to one cylinder bank. The engine’s compression ratio was 16.0 to 1, and it was started with compressed air.

The DB 602 had a 6.89 in (175 mm) bore and a 9.06 in (230 mm) stroke, both larger than those of the OF-2. The engine displaced 5,401 cu in (88.51 L). Its maximum continuous output was 900 hp (671 kW) at 1,480 rpm, and it could produce 1,320 hp (984 kW) at 1,650 rpm for 5 minutes. The DB 602 was 105.9 in (2.69 m) long, 40.0 in (1.02 m) wide, and 53.0 in (1.35 m) tall. The engine weighed 4,409 lb (2,000 kg). Fuel consumption at cruising power was 0.37 lb/hp/hr (225 g/kW/hr).


The ill-fated LZ 129 Hindenburg on a flight in 1936. The airship used four DB 602 engines housed in separate cars in a pusher configuration. Note the Olympic rings painted on the airship to celebrate the summer games that were held in Berlin.

Development of the DB 602 progressed well, and it completed two non-stop 150-hour endurance test runs. The runs proved the engine could operate for long periods at 900 hp (671 kW). Four engines were installed in both the LZ 129 Hindenburg and the LZ 130 Graf Zeppelin II. Each engine powered a two-stage compressor. Each compressor filled a 3,051 cu in (50 L) air tank to 850 psi (59 bar) that was used to start the engine and to manipulate the camshaft for engine reversing.

Plans for a water vapor recovery system that used the engines’ exhaust were never implemented, because the airships used hydrogen instead of the more expensive helium. The recovery system would have condensed vapor into water, and the collected water would have been used as ballast to help maintain the airship’s weight and enable the retention of helium. Without the system in place, expensive helium would have been vented to compensate for the airship steadily getting lighter as diesel fuel was consumed. With the United States unwilling to provide helium because of Germany’s aggression, the airships used inexpensive and volatile hydrogen, as it was readily available. The Hindenburg was launched on 4 March 1936, and the Graf Zeppelin II was launched on 14 September 1938.

Engines for the Hindenburg were mounted in a pusher configuration. In April 1936, the Hindenburg’s DB 602 engines experienced some mechanical issues on its first commercial passenger flight, which was to Rio de Janeiro, Brazil. The engines were rebuilt following the airship’s return to Germany, and no further issues were encountered. The Hindenburg tragically and famously burst into flames on 6 May 1937 while landing at Lakehurst, New Jersey.


Front view of the DB 602 engine in the Musée de l’Air et de l’Espace, in Le Bourget, France. Above the engine are the cooling water outlet pipes. In the Vee of the engine is the induction manifold, and the pushrod tubes for the front cylinders can be seen. Note the finning on the bottom half of the crankcase. (Stephen Shakland image via

The Graf Zeppelin II was still being built when the Hindenburg disaster occurred. Design changes were made to the Graf Zeppelin II that included mounting the DB 602 engines in a tractor configuration. The inability of Germany to obtain helium, the start of World War II, and the end of the airship era meant the Graf Zeppelin II would not be used for commercial travel. The airship was broken up in April 1940.

The DB 602 engine proved to be an outstanding and reliable power plant. However, its capabilities will forever be overshadowed by the Hindenburg disaster. Two DB 602 engines still exist and are on display; one is in the Zeppelin Museum in Friedrichshafen, Germany, and the other is in the Musée de l’Air et de l’Espace, in Le Bourget, France. Although the DB 602 was not used on a wide scale, it did serve as the basis for the Mercedes-Benz 500 series marine engines that powered a variety of fast attack boats (Schnellboot) during World War II.

*Daimler-Benz was formed in 1926 with the merger of Daimler Motoren Gesellschaft and Benz & Cie. Prior to their merger, both companies produced aircraft engines under the respective names Mercedes and Benz. After the merger, the Daimler-Benz name was used mostly for aircraft engines, and the Mercedes-Benz name was used mostly for automobiles. However, both names were occasionally applied to aircraft engines in the 1930s.


Rear view of the DB 602 engine on display in the Zeppelin Museum in Friedrichshafen, Germany. A water pump on each side of the engine provided cooling water to a bank of cylinders. (Stahlkocher image via Wikimedia Commons)

Aircraft Diesels by Paul H Wilkinson (1940)
Aerosphere 1939 by Glenn D. Angle (1940)
Diesel Engines by B. J. von Bongart (1938)
High Speed Diesel Engines by Arthur W. Judge (1941)
Diesel Aviation Engines by Paul H Wilkinson (1942)
“The Hindenburg’s New Diesels” Flight (26 March 1936)
“The L.Z.129’s Power Units” Flight (2 January 1936)