Category Archives: World War II

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 flickr.com)

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.

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

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.

Sources:
https://it.wikipedia.org/wiki/Isotta_Fraschini_Zeta
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.

Sources:
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)

fiat-a38-rc15-45-v-16-engine

FIAT A.38, A.40, and A.44 Aircraft Engines

By William Pearce

In the early 1930s, Italy was a world leader in aviation and had developed both liquid-cooled and air-cooled engines. In 1933, the Italian Air Ministry decided to focus on air-cooled radial engines, and the development of liquid-cooled inline engines was essentially abandoned. By 1939, the shortsightedness of this decision became clear as most premiere frontline fighters from Britain, France, Germany, the Soviet Union, and the United States were powered by liquid-cooled engines. As a result, the Ministero dell’Aeronautica (Italian Air Ministry) began to encourage the development of liquid-cooled engines.

fiat-a38-rc15-45-v-16-engine

The FIAT A.38 RC15-45 was a 2,118 cu in (34.7 L) inverted V-16. The supercharger was mounted between the cylinder banks to decrease the engine’s length. Note the magnetos and contra-rotating propeller shafts.

In 1939, the Italian Air Ministry asked FIAT to design a new aircraft engine to power the next generation of Italian fighter aircraft. FIAT engineers Antonio Fressa and Carlo Bona began designing the new engine, designated A.38. The A.38 was initially an upright V-16 engine closely based on the FIAT AS.8, which was originally designed to set speed records. While the AS.8 had individual cylinders, the A.38 used two cast cylinder blocks.

After the initial upright engine design, the Italian Air Ministry was inspired by the German Daimler-Benz 600 series of inverted V-12s and requested the A.38’s configuration be changed to an inverted engine. Fressa completely redesigned the A.38, leaving very little in common with the AS.8. The AS.8 engine was a 45 degree V-16 with a 5.51 in (140 mm) bore and stroke, and by 1940, the A.38 had become an inverted, 90 degree V-16 with a 5.43 in (138 mm) bore and a 5.71 in (145 mm) stroke.

The A.38’s 16-cylinder arrangement was selected to maximize the engine’s power output while keeping its cylinder size and supercharger boost within known and reliable limits. However, a V-16 engine is very long, and its crankshaft is subject to torsional vibrations. To keep the engine’s length as short as possible, Fressa used a 90 degree cylinder bank arrangement and positioned the supercharger horizontally between the cylinder banks. This resulted in a rather complex supercharger drive.

fiat-a38-test-cell

The AC.38 in a test cell. The supercharger arrangement greatly increased the engine’s otherwise small frontal area. The 1,200 hp (895 kW) engine could have sufficed with a single-rotation propeller, but the contra-rotating unit would eliminate asymmetrical torque.

The A.38 was of all-aluminum construction with two detachable monobloc cylinder blocks. Each cylinder bank had eight cylinders, and each cylinder had two inlet and two exhaust valves. The valves were actuated by dual overhead (underhead in this case) camshafts that were driven by a single vertical shaft from the front of the engine. Two spark plugs were installed in each cylinder, and the spark plugs for each cylinder bank were fired by two magnetos driven at the front of the engine. The A.38 had a compression ratio of 7 to 1.

The engine had contra-rotating propeller shafts that were driven at .514 engine speed. Between the cylinder banks were the carburetor, supercharger, intake manifolds, and water pump. There were plans to use fuel injection, but this was never completed. The single-stage supercharger had two-speeds that gave critical altitudes of 4,931 ft (1,500 m) and 14,764 ft (4,500 m). The supercharger was powered by a shaft driven from the front of the engine and situated in the Vee between the cylinders. This shaft also drove the oil and water pumps. The supercharger’s outlet was at the center of the engine, and the air was fed into four manifolds, each serving four cylinders.

The engine was officially designated A.38 RC15-45: “RC” for Riduttore de giri (gear reduction) and Compressore (supercharged), and 15/45 for the altitudes (in hectometers) at which maximum power was obtained. The A.38 had a 5.43 in (138 mm) bore, a 5.71 in (145 mm) stroke, and a displacement of 2,118 cu in (34.7 L). The engine produced 1,200 hp (895 kW) at 2,800 rpm at 4,931 ft (1,500 m) and 14,764 ft (4,500 m). The 1,200 hp (895 kW) output was not normally enough to justify the use of contra-rotating propellers, but a photo of the engine in a test cell and a drawing of the FIAT G.55 fighter powered by the A.38 show propellers with just two-blades. It would appear that contra-rotating propellers were used more to eliminate asymmetrical torque than to compensate for exceeding the capabilities of a single-rotation propeller. The engine weighed 1,698 lb (770 kg).

fiat-a38-powered-g55

The FIAT G.55 fighter was originally designed to use the A.38 engine with contra-rotating propellers (top), but the aircraft was redesigned once the switch to a single-rotation propeller (bottom) was made. Delays with the A.38 led to the Daimler-Benz DB 605 being installed in the G.55.

Three A.38 engines were ordered, but it is not clear if all were built. The A.38 underwent tests in 1941 and was able to achieved 1,300 hp (969 kW), but even more power was desired. Some developmental changes to the engine included switching to a single-rotation propeller shaft. Trouble was experienced with the engine’s crankshaft and supercharger drive, and despite multiple attempts, the engine failed to pass airworthy certification tests. Fressa continued to work on the engine into 1942, but the Italian Air Ministry had already obtained licenses to produce Daimler-Benz engines and was no longer interested in the A.38—FIAT would build the DB 605 as the RA 1050 Tifone (Typhoon). It is interesting to note that the AS.8 had proven itself reliable and probably would have been a faster and better starting point for Fressa than an all-new engine design.

A number of aircraft designs were made to accommodate the A.38 engine. The only design that was actually built was the G.55. The G.55 was originally planned to be powered by the A.38 turning contra-rotating propellers, but the design was later altered for a single-rotation, three-blade propeller. In late 1941, it became obvious that the G.55 airframe would be completed before the A.38 engine was cleared for flight tests. As a result, a change to the DB 605 engine was initiated. First flown on 30 April 1942, the G.55 arguably became the best Italian fighter of World War II. Due to the state of the Italian aircraft industry in wartime, the G.55 was never made in sufficient numbers to have any impact on the conflict.

fiat-a40-x-24-engine

The FIAT A.40 was a 2,000 hp (1,491 kW) X-24 that had the same bore and stroke as the A.38. Although two A.40 engines were built, they were never tested because of shifting priorities during World War II. Note the cannon installed in the upper Vee on the side view drawing.

In 1940, Fressa tasked Dante Giacosa to create a new engine to compete with the A.38 and produce 2,000 hp (1,491 kW) at 8,202 ft (2,500 m). Instead of the V-16 layout, Giacosa turned to an X-24 configuration with four six-cylinder banks positioned 90 degrees from each other. The X-24 engine was designated A.40 RC20-60, and it used the same 5.43 in (138 mm) bore and 5.71 in (145 mm) stroke as the A.38. The A.40 engine had a single crankshaft and used one master connecting rod with three articulated connecting rods for each row of cylinders. The induction manifold was installed in the Vee between the lower cylinder banks and fed the two-speed supercharger mounted at the rear of the engine. The A.40 used a fuel injection system that Giacosa and his team had designed. The gear reduction unit raised the single-rotation propeller shaft, which enabled a 20 mm or 37 mm cannon to be fitted in the Vee between the upper cylinder banks and to fire through the propeller hub. The A.40 displaced 3,176 cu in (52.1 L), and an output of 2,000 hp (1,491 kW) was expected at 6,562 ft (2,000 m) and 26,247 ft (6,000 m). Reportedly, two A.40 engines were built in 1943, but Italy’s surrender prevented the engines from ever being tested. No information has been found on the disposition of any A.38 or A.40 engines.

While Fressa was working on the A.38, he also designed a more powerful engine. There is some evidence that suggests the engine was originally designated A.42 and used four A.38 cylinder blocks in an H-32 configuration. However, the engine was redesigned and redesignated A.44 RC15-45. The FIAT A.44 was comprised of two V-16 engines stacked together to form an X configuration. The V-16 engine sections were independent of each other, and each section powered half of the A.44’s contra-rotating propeller at a .429 reduction. A.38 cylinder blocks, pistons, and crankshafts were used, but the V-16 engine sections had a wider bank angle of 135 degrees. The X-32 engine displaced 4,235 cu in (69.4 L) and was forecasted to produce 2,400 hp (1,790 kW) at 2,800 rpm and a maximum of 2,800 hp (2,088 kW) at 2,950 rpm. The engine was estimated to weigh 3,307 lb (1,500 kg), and the design progressed through 1942. While FIAT designed a few aircraft to be powered by the A.44, like the CR.44 fighter/bomber and the BR.44 torpedo bomber, the engine failed to gain the support of the Italian Air Ministry and was never built.

fiat-cr44-f-b-a44

The FIAT CR.44 fighter/bomber was planned around the 2,400 hp (1,790 kW) FIAT A.44 engine. The A.44 X-32 engine was essentially two V-16 engines mounted together. The A.44 engine would have shared most parts with the A.38, except the crankcase. Neither the A.44 nor the CR.44 were built.

Sources:
Aeronuatica Militare Museo Storico Catalogo Motori by Oscar Marchi (1980)
Ali D’Italia Fiat G 55 by Piero Vergnano and Gregory Alegi (1998)
Forty Years of Design with Fiat by Dante Giacosa (1979)
“Fantasmi di aerie e motori Fiat dal 1935 al 1945 (prime parte)” by Giovanni Masino; Ali Antiche 106 (2011)
“Fantasmi di aerie e motori Fiat dal 1935 al 1945 (seconda parte)” by Giovanni Masino; Ali Antiche 108 (2012)
http://www.secretprojects.co.uk/forum/index.php?topic=14405.0
http://www.secretprojects.co.uk/forum/index.php/topic,6520.0/all.html

FKFS Gruppen-Flugmotor A mockup copy

FKFS Gruppen-Flugmotor A, C, and D

By William Pearce

In 1930, German engineer Wunibald Kamm founded the FKFS (Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart or Research Institute of Automotive Engineering and Vehicle Engines Stuttgart). The FKFS was an organization that tried and tested new, inventive ideas in the field of automotive technology. However, it was not long before Kamm’s thoughts and some of the FKFS’s resources were directed toward aircraft engines.

FKFS Gruppen-Flugmotor A mockup copy

Crankcase mockup of the FKFS Gruppen-Flugmotor A with Hirth HM 512 cylinders. Visible on the side of the engine is a mockup of the axial supercharger. (Kevin Kemmerer image)

In mid-1938, Kamm was able to persuade Willy Krautter of Hirth Motoren to join the FKFS as head of the FKFS’s Special Engine Group. Krautter’s first project at FKFS was building FKFS’s first aircraft engine. The result was a flat, air-cooled, two-stroke, four-cylinder engine that displaced 31 cu in (.51 L) and produced 25 hp (18 kW). The engine was used in the Hirth Hi-20 MoSe motor glider. Next, Krautter designed an improved and updated version of the four-cylinder engine, but priorities had shifted with the outbreak of World War II. Inspired by an open request from the RLM (Reichsluftfahrtministerium or German Ministry of Aviation), the FKFS focused on designing much larger engines.

The RLM was interested in large, powerful engines for bombers being designed to reach targets in North America. Both Kamm and Krautter believed that air-cooled engines were overall superior for aircraft use, and they designed a 32-cylinder engine intended for the RLM. This radial engine had eight cylinder banks evenly spaced at 45 degree intervals around the crankcase. Each cylinder bank consisted of four inline, air-cooled cylinders and a single overhead camshaft. The cylinder proposed for the engine was designed by Krautter and was undergoing tests at the FKFS.

FKFS Gruppen-Flugmotor A crankcase copy

Two views of the Gruppen-Flugmotor A’s crankcase. In the left image, note the rear accessory drive housing with provisions to power the axial supercharger. Also note the large roller bearing in the nose case. In the right image, note the crankshaft and camshaft position for each engine section. Crankcase finning is also visible in both images. (Kevin Kemmerer images)

Building a new aircraft engine from scratch is a massive undertaking, so Krautter suggested grouping together existing, proven engines to quickly create a larger, more powerful unit. Kamm supported Krautter’s idea of a Gruppen-Flugmotor (Group Aircraft Engine), and the detailed design of such a power unit commenced in 1939. The first engine was known as the Gruppen-Flugmotor A (or just Motor A), and it utilized almost all of the components from four Hirth HM 512 engines (excluding their crankcases) to create a new 48-cylinder engine. Undoubtedly, Krautter’s experience at Hirth Motoren influenced his decision to use HM 512 parts.

The Hirth HM 512 was an inverted, air-cooled, V-12 engine. Its individual cylinders were arranged in two rows spaced at 60 degrees and attached under an elektron (magnesium alloy) crankcase. Four long studs held each cylinder to the crankcase, and the cylinders were staggered to allow the use of side-by-side connecting rods. Each cylinder was made of cast iron and had an aluminum cylinder head. In the Vee of the engine, one intake and one exhaust valve per cylinder were actuated by individual pushrods driven by a single camshaft. Each cylinder had two spark plugs—one on each side of the cylinder. The intake and exhaust manifolds were mounted to the outer sides of the cylinder banks. The HM 512 had a 4.13 in (105 mm) bore, a 4.53 in (115 mm) stroke, and a displacement of 729 cu in (11.9 L). The engine produced 450 hp (336 kW) at 3,100 rpm.

FKFS Gruppen-Flugmotor A complete copy

The complete 48-cylinder Gruppen-Flugmotor A. Note the intake manifolds leading from the axial supercharger to the two adjacent V-12 engine sections. The four Bosch magnetos are visible at the rear of the engine. (Kevin Kemmerer image)

For the Gruppen-Flugmotor A, an HM 512 crankshaft occupied each corner of the engine’s large, square-shaped, aluminum crankcase, and a camshaft was located at the apex of each corner. The cylinder banks for each engine section were on adjacent sides of the Gruppen-Flugmotor A’s crankcase. The two-piece aluminum crankcase was split horizontally and incorporated cooling fins on its exterior.

At the front of the Gruppen-Flugmotor A’s crankcase was a central combining gear that took power from each of the four crankshafts and transmitted it to a single propeller shaft. The crankshaft of each engine section could be decoupled from the combining gear if the engine section were damaged or to conserve fuel and increase an aircraft’s range. It was believed that the Gruppen-Flugmotor A’s economy could be increased up to 56% by decoupling two of the engine sections while cruising during a long-range flight.

FKFS Gruppen-Flugmotor A axial supercharger housing copy

The housing for an axial supercharger used on the Gruppen-Flugmotor A. Visible are the four stator rows. Each blade was inserted into a dovetail groove and held in place by a screw, visible on the outside of the housing. (Kevin Kemmerer image)

Another unique feature of the Gruppen-Flugmotor A was its use of two axial superchargers that provided 11.6 psi (0.8 bar) of boost. The left and right sides of the engine each had one supercharger located between the cylinder banks. The axial superchargers had four stages (although photos appear to show three compressor stages and four stator rows) and were driven from the accessory section at the rear of the engine. Fuel was injected ahead of the superchargers and subsequently mixed with air. The air/fuel mixture was then fed to the cylinders via long induction manifolds. The engine’s four Bosch dual magnetos and other accessories were mounted to the rear of the engine.

The Gruppen-Flugmotor A had 48 cylinders with a 4.13 in (105 mm) bore and a 4.53 in (115 mm) stroke. The engine’s total displacement was 2,917 cu in (47.8 L). Unfortunately, most of the engine’s specifications have been lost, but it was around 6.07 ft (1.85 m) long and 4.27 ft (1.30 m) in diameter. The engine produced 1,970 hp (1,470 kW) at 3,200 rpm with manifold fuel injection. A switch to direct fuel injection was made by changing to Hirth HM 512 D cylinders that had an unused port in the cylinder head. With direct fuel injection, the Gruppen-Flugmotor A’s output was increased by 200 hp (150 kW) to 2,170 hp (1,620 kW). The engine was tested during 1941 and 1942, but test information has not been found. Photos indicate some trouble was encountered with the axial superchargers failing in dramatic ways. After the war, Krautter stated that the engine was capable of 2,400 hp (1,790 kW).

FKFS Gruppen-Flugmotor C

This drawing of the Gruppen-Flugmotor C illustrates how the engine design was a link between the Gruppen-Flugmotor A and D engines. Note the cooling fan and side-by-side connecting rods. (“Wunibald I. E. Kamm – Wegbereiter der modernen Kraftfahrtechnik” image)

As the Gruppen-Flugmotor A was proving the concept of a grouped-engine power unit, Kamm, Krautter, and the FKFS had already designed a larger, more powerful engine in 1941. Called the Gruppen-Flugmotor C (or Motor C), the 48-cylinder engine had the same basic layout as the earlier engine but used new components. An engine-driven cooling fan was employed to help minimize cooling drag, as both Kamm and Krautter felt the Gruppen-Flugmotor A’s cooling drag was excessive. The engine also had contra-rotating propeller shafts and two five-stage axial superchargers that provided 11.6 psi (0.8 bar) of boost.

FKFS Gruppen-Flugmotor D Cylinder copy

The 122 cu in (2.0 L) cylinder used on the Gruppen-Flugmotor D. The triangular cover conceals the camshaft drive for the valves. The baffle around the cylinder helped direct air to maximize cooling efficiency. (Kevin Kemmerer image)

The individual cylinders of the Gruppen-Flugmotor C were of Krautter’s maturing design, each having a capacity of 67 cu in (1.1 L). The cylinder had a hemispherical combustion chamber with two spark plugs and a port for direct fuel injection. The cylinder barrel and head were cast from aluminum as one piece, and the cylinder bore was chrome plated. A flange at the base of the cylinder attached it to the crankcase. Atop the cylinder was a housing for the intake and exhaust valves. The two valves were actuated via roller rockers by a single overhead camshaft, which served all the cylinders of one bank. Each camshaft was driven by a vertical shaft at the front of the engine.

The Gruppen-Flugmotor C had a 4.33 in (110 mm) bore, a 4.53 in (115 mm) stroke, and a total displacement of 3,201 cu in (52.5 L). The engine was 7.17 ft (2.185 m) long and 4.43 ft (1.35 m) in diameter. The Gruppen-Flugmotor C was forecasted to produce 3,500 hp (2,610 kW) at 4,000 rpm with the original 67 cu in (1.1 L) cylinders, but studies of larger 92 cu in (1.5 L) and 122 cu in (2.0 L) cylinders indicated outputs of 4,290 hp (3,200 kW) and 5,920 hp (4,415 kW), respectively. While some components of the Gruppen-Flugmotor C were built for testing, a complete engine was never built.

Seeing the potential of the Gruppen-Flugmotor C with 122 cu in (2.0 L) cylinders inspired Kamm and Krautter to create the Gruppen-Flugmotor D. Designed in 1943, the engine was very similar to the Gruppen-Flugmotor C, with 48 cylinders, a cooling fan, and contra-rotating propellers, but it used fork-and-blade connecting rods. The 122 cu in (2.0 L) cylinder was basically an enlargement of the 67 cu in (1.1 L) cylinder design. The Gruppen-Flugmotor D had four (one on each side of the engine) five-stage axial superchargers that provided 13.8 psi (0.95 bar) of boost.

FKFS Gruppen-Flugmotor D copy

Drawing of the 48-cylinder Gruppen-Flugmotor D. Note the cooling fan, contra-rotating propeller drive, and fork-and-blade connecting rods. One five-stage axial supercharger can be seen on the right side of the drawing. The engine was estimated to produce 5,920 hp (6,000 ps / 4,415 kW). (Kevin Kemmerer image)

The Gruppen-Flugmotor D had a 5.31 in (135 mm) bore and a 5.51 in (140 mm) stroke. The engine’s total displacement was 5,870 cu in (96.2 L), and it was forecasted to produce 5,920 hp (4,415 kW) at 4,000 rpm. Reportedly, a complete engine was ready for tests in April 1944, but the state of the war and the progress of jet engines rendered the Gruppen-Flugmotor D and its further development irrelevant. At the time, Germany was in need of interceptor fighters, not long-range bombers.

At war’s end, Kamm and Krautter were brought to the United States under Operation Paperclip, a program to extradite the best German scientists, engineers, and technicians and apply their skills and knowledge to further industries in the United States. The two men worked at Wright Field in Dayton, Ohio until they were released from their service. In the 1950s, Krautter founded his own engineering firm, the Wilkra Company, where he designed everything from engines for boats and motorcycles to lawn tractors and ski bikes.

Kamm 60-cylinder compound-diesel

The Kamm-designed 60-cylinder compound-diesel engine incorporating five V-12 engine sections around a central turbine. The engine’s concept was similar to that of the Napier Nomad. (“Wunibald I. E. Kamm – Wegbereiter der modernen Kraftfahrtechnik” image)

For a time, Kamm worked with Krautter at Wilkra but returned to Germany in 1955. Kamm revisited the Gruppen-Flugmotor concept when he designed a 60-cylinder compound diesel-turbine engine. This engine consisted of five V-12 engine sections mounted around a central turbine. The V-12 engine sections were based on an extremely-high-output diesel engine Kamm had helped design while at the Stevens Institute of Technology in Hoboken, New Jersey in the early 1950s. The V-12s were air-cooled, two-stroke, loop-scavenged engines with side-by-side connecting rods. The turbine had a nine-stage axial compressor section, a combustion section, and a five-stage exhaust turbine section. High-pressure air from the compressor section would provide the incoming charge for the diesel engine. The diesel’s exhaust would be expelled into the exhaust section of the turbine. The turbine’s combustion section could run independently of the piston engine sections to increase the compound engine’s overall output. The engine’s bore and stroke were around 2.75 in (70 mm) and 4.5 in (114 mm), respectively, giving a total displacement of approximately 1,604 cu in (26.3 L). The 60-cylinder compound engine was designed to produce 2,950 hp (2,200 kW) without additional power from the turbine’s combustion section and 4,025 hp (3,000 kW) with the additional power. The engine would have had a low specific fuel consumption of .296 lb/hp/h (180 g/kW/h) and was forecasted to be 6.56 ft (2.00 m) long and 4.10 ft (1.25 m) in diameter. The 60-cylinder engine was never built.

Note: Kamm and Krautter’s Gruppen-Flugmotoren were not the first time that multiple engine sections were combined to create a large, powerful engine. In the 1920s, the French firm Bréguet created the Bréguet-Bugatti 32-cylinder Quadimoteurs in a similar but less complex fashion.

FKFS Gruppenmotor 48-Zyl copy

This drawing dated October 1943 depicts a 48-cylinder engine and lists its displacement as 37.6 L (2,294 cu in). The engine’s bore and stroke appear to be the same but are not listed on the drawing. A 100 mm (3.94 in) bore and stroke would give a displacement of 37.70 L (2,300 cu in). It is not clear how this engine fits into the overall history of the Gruppen-Flugmotoren, but its design is similar to the C and D engines. (Kevin Kemmerer image)

Sources:
Correspondence with Kevin Kemmerer, grandson of Willy Krautter
Wunibald I. E. Kamm – Wegbereiter der modernen Kraftfahrtechnik by Jurgen Potthoff and Ingobert C. Schmid (2012)
“Why Multicylinder Motorcycle Engines?” by W. Krautter, Design of Racing and High Performance Engines edited by Joseph Harralson (1995)
Aircraft Engines of the World 1944 by Paul H. Wilkinson (1944)
Engine-Transmission Power Plants for Tactical Vehicles by Emil M. Szten et. al. (1967)

Hispano-Suiza 24Z left

Hispano-Suiza 24Z (Type 95) Aircraft Engine

By William Pearce

Starting around 1938, Hispano-Suiza began to halt development of its other aircraft engines to focus on its latest V-12, the 12Z (Type 89). The 12Z drew heavily from the Hispano-Suiza 12Y but had many improvements, including two-speed supercharging and four-valves per cylinder actuated by dual-overhead camshafts. France was in desperate need of high-powered aircraft engines to keep its air force comparable to those of other European nations during the build-up to World War II. The 12Z was developing 1,400 hp (1,044 kW) at 2,600 rpm when France surrendered in June 1940.

Hispano-Suiza 24Z right

The Hispano-Suiza 24Z incorporated the improved features of the 12Z engine but was very similar to the 24Y. Note the high position of the propeller shafts to accommodate a cannon mounted between the upper cylinder banks and firing through the propeller hub. The magnetos for the right side of the engine can be seen mounted to the propeller gear reduction housing.

One of the engine programs that was suspended because of the 12Z was the Hispano-Suiza 24Y: a 2,200 hp (1,641 kW), 24-cylinder H engine utilizing many 12Y components. Not long into the 12Z program, engineers started to wonder what level of performance could be achieved by a 24-cylinder engine using 12Z components. In 1943 and under German occupation, development began on the Hispano-Suiza 24Z (Type 95) engine.

The 24Z had the same vertical H-24 configuration as the earlier 24Y with two cylinder banks situated above crankcase and another two cylinder banks below. The two-piece crankcase was made from aluminum and split horizontally. A crankshaft served each upper and lower cylinder bank pair. Each one-piece crankshaft had six-throws, was counterbalanced, and was supported by seven main bearings. The pistons were connected to the crankshafts via fork-and-blade connecting rods. The crankshafts, connecting rods, and pistons were the same as those used in the 12Z engine.

Each of the 24Z’s four cylinder banks was made up of a six-cylinder block with an integral crossflow cylinder head. The two intake valves and two exhaust valves for each cylinder were controlled by separate overhead camshafts. The camshafts were driven from a vertical shaft at the rear of each cylinder bank. The cylinder banks and their valve train were from the 12Z engine.

Hispano-Suiza 24Z left

Each left and right half of the 3,600 hp (2,685 kW) 24Z engine could operate independently of the other half. Note the intake manifolds routing air from the supercharger to the cylinder banks and the drive shaft for the fuel injection pump extending from the rear of the engine.

Two superchargers were mounted at the rear of the engine with their impellers parallel to the engine’s crankshafts. Originally, single-speed supercharges were used, but these were later replaced with two-speed units. At low speed, the supercharger’s impeller spun at 6.72 times crankshaft speed. At high speed, the impeller spun at 9.52 times crankshaft speed. Supercharger speed change and boost control were automatic. Separate intake manifolds led from each supercharger to the inner side of the upper and lower cylinder banks. Mounted on each side of the crankcase was a fuel pump that injected fuel directly into each cylinder. Each fuel pump was driven via a shaft from the rear of the engine. The two spark plugs per cylinder were positioned below the intake valves. The spark plugs for each upper and lower cylinder bank pair were fired by two magnetos mounted to the propeller gear reduction case.

The front of each crankshaft engaged a separate propeller shaft at a .44 to 1 gear reduction. The two propeller shafts made up a contra-rotating unit. The 24Z was not built with a single-rotation propeller shaft. The engine had provisions for a cannon to be mounted between the upper cylinder banks and fire through the hollow propeller shaft. Each upper and lower cylinder bank pair on the 24Z constituted a 12-cylinder engine section, and each engine section could operate independently of the other.

Sud-Est 580 HS 24Z

The Sud-Est SE 580 under construction with the 24Z engine installed. Note the supercharger intake on both sides of the cowling. Also note the upper and lower row of exhaust stacks. The scoop behind the cockpit was for the radiators.

The Hispano-Suiza 24Z had a 5.91 in (150 mm) bore and a 6.69 in (170 mm) stroke. The engine’s total displacement was 4,400 cu in (72.10 L). The 24Z produced 3,600 hp (2,685 kW) at 2,800 rpm for takeoff. This power was achieved at an over-boosted condition of 7.7 psi (.53 bar); normal boost was 7.0 psi (.49 bar). Max power with low-speed supercharging was 3,200 hp (2,386 kW) at 2,800 rpm at 8,202 ft (2,500 m). Max power with high-speed supercharging was 2,640 hp (1,969 kW) at 2,800 rpm at 26,247 ft (8,000 m). The engine’s normal rating was 3,000 hp (2,237 kW) at 2,600 rpm at 8,202 ft (2,500 m) with low-speed supercharging and 24,606 ft (7,500 m) with high-speed supercharging. The 24Z’s cruising power was 1,500 hp (1,119 kW) at 2,100 rpm at 9,843 ft (3,000 m) with low-speed supercharging and 18,373 ft (5,600 m) with high-speed supercharging. The engine had a specific fuel consumption of .48 lb/hp/hr (292 g/kW/hr). The 24Z was 10.72 ft (3.27 m) long, 4.27 ft (1.30 m) wide, 4.54 ft (1.39 m) tall, and weighed 3,197 lb (1,450 kg).

World War II hindered the 24Z’s construction. The engine was first run in 1946, and it was exhibited at the Salon de l’Aéronautique (Air Show) in Paris in November 1946. Bench tests of the 24Z revealed some serious issues, the extent of which have not been found. In September 1947, the engine’s compression was lowered to 6.75 to 1 from its original value of 7.0 to 1. Perhaps this change was an attempt to cure issues with detonation. Later, the gear reduction failed while a 24Z was under test, destroying the engine.

The 24Z’s prime application was the Sud-Est* SE 580/582 fighter that was designed during the war. However, issues with the 24Z resulted in the substitution of an Arsenal 24H engine for the SE 580. The SE 580 itself was later scrapped before the aircraft was completed. Many other projects, mostly flying boats and transports, were proposed with 24Zs as their power plant. None of these projects made it off the drawing board.

Hispano-Suiza 48Z Late 133

Drawing of two Hispano-Suiza 48Z engines installed in the wing of a Latécorère 133 flying boat. (image relabeled, but originally from “Latécorère: Les avions et hydravions” by Jean Cuny)

A further Hispano-Suiza proposal consisted of coupling two 24Z engines together to create the 48Z (Type 96) engine. In this configuration, the propeller shaft of the rear 24Z engine section passed between the upper cylinder banks of the front 24Z engine section and extend though the propeller shaft of the front engine. The rear 24Z powered the front propeller of a coaxial contra-rotating unit, while the front 24Z powered the rear propeller. The 48Z would have used four turbosuperchargers—two mounted near the front of the engine and two mounted at the rear. The 48-cylinder 48Z engine displaced 8,800 cu in (144.20 L), had a takeoff rating of 7,200 hp (5,369 kW) at 2,800 rpm, and produced 5,200 hp (3,878 kW) at 13,123 ft (4,000 m). Like many large engine projects at the dawn of the jet age, the 48Z existed only on paper.

With the 24Z’s developmental issues and no tangible prospects for installation in an aircraft, the engine program was stopped in 1948. At least two 24Z engines were built, but probably not many more. One engine survives and is preserved in the Musée de l’Air et de l’Espace in le Bourget (near Paris), France.

Hispano-Suiza 24Z

The Hispano-Suiza 24Z preserved in the Musée de l’Air et de l’Espace in le Bourget, France. Most likely, this is the engine that was displayed at the 1946 Salon de l’Aéronautique and was installed in the SE 580. (image via Le Rêve d’Icare)

*The SE 580/582’s development began at what was Dewoitine, which had been nationalized into SNCAM (Société nationale des constructions aéronautiques du Midi or National Society of Aircraft Constructors South). SNCAM was absorbed into SNCASE (Société nationale des constructions aéronautiques du Sud-Est or National Society of Aircraft Constructors Southeast), which is often shortened to just Sud-Est.

Sources:
Hispano Suiza in Aeronautics by Manuel Lage (2004)
Jane’s All the World’s Aircraft 1947 by Leonard Bridgman (1947)
Aircraft Engines of the World 1947 by Paul H. Wilkinson (1947)
Latécorère: Les avions et hydravions by Jean Cuny (1992)
“Engines at the Paris Show” Flight (21 November 1946)

Armstrong Siddeley Deerhound III

Armstrong Siddeley ‘Dog’ Aircraft Engines

By William Pearce

The British firm Armstrong Siddeley Motors (ASM) was formed in 1919 when Armstrong Whitworth (founded in 1847) purchased Siddeley-Deasy (founded in 1912). Prior to the merger, both Armstrong and Siddeley were active in the automotive and aeronautical fields. Siddeley first began manufacturing aircraft engines in 1915 under a contract to build the Royal Aircraft Factory’s RAF 1A engine. In 1916, Siddeley had built its first aircraft engine—the Puma—which was developed from the B.H.P. (Beardmore-Halford-Pullinger) six-cylinder engine. The Puma was the first in a long line of engines that were produced by ASM into the 1940s and named after cats (felines)—the last being the Cougar of 1945.

Armstrong Siddeley Hyena AW16

Two versions of the cowling used to cover the 15-cylinder Armstrong Siddeley Hyena installed in an Armstrong Whitworth A.W.XVI (A.W.16). The cowling on the right is illustrative of the Hyena’s inline radial cylinder arrangement.

In 1932, ASM worked to develop a new line of air-cooled, radial engines. These engines would be a design departure from their existing cat-engines, so they decided to name the engines after dogs (canines). Unfortunately, none of these engines were successful, and information about them is frustratingly hard to find and occasionally contradictory. To add to the confusion, some of the engine names were used more than once, and the engines possess many of the same characteristics.

The first engine of the new dog-series was the Mastiff (this name was used again later). This engine was built in 1932 and was a large radial engine with two rows of seven cylinders. The specifications of the 14-cylinder engine are currently not known. In reviewing a photo (that is unfortunately not publishable) of the Mastiff, the engine closely resembles a larger version of the 1,996 cu in (32.7 L) ASM Tiger in appearance and construction. The Mastiff was supercharged and had a one-piece crankcase and gear reduction. A cam ring at the front of the engine acted on pushrods that actuated each cylinder’s two valves. One Mastiff was built for Italy, but it is not known if the engine was ever tested.

Armstrong Siddeley Deerhound I side

This photo gives a good view of the 21-cylinder Armstrong Siddeley Deerhound I’s configuration. Note the engine’s inline radial layout and the vertical shaft in front of each cylinder bank to drive the overhead camshaft.

The second dog-engine was the Hyena. The Hyena’s 15 air-cooled cylinders were arranged in three rows of five. Even more unique than the three-row arrangement was the fact that the cylinder rows were inline rather than staggered. Between each of the engine’s five cylinder banks was a camshaft that acted upon short pushrods to actuate the two valves per cylinder. The camshafts were geared to the crankshaft.

The Hyena had a 5.39 in (137 mm) bore and a 4.92 (125 mm) stroke. The engine’s total displacement was 1,687 cu in (27.64 L), and it produced 620 hp (462 kW) at 2,250 rpm. The Hyena was first run in 1933 and was installed in an Armstrong Whitworth A.W.XVI (A.W.16) biplane fighter later that year. The Hyena-powered A.W.XVI (registered as G-ABKF) first flew on 25 October 1933. The basic engine proved itself to be mechanically sound but rather heavy for its power. Issues were encountered with cooling the rear cylinders. This led to a number of different engine cowlings being tried, but the overheating issues persisted. Eventually, further development of the Hyena was abandoned, and only one or two engines were built. The Hyena was proposed to power the A.W.21 and A.W.28 fighters, but these projects did not proceed past the design stage.

Armstrong Siddeley Deerhound I rear

This rear view of the Deerhound I shows the supercharger housing with intake manifolds leading to each bank of cylinders.

Lessons learned from the Hyena were applied to the next dog-engine: the Deerhound. Led by Lt. Col. F. L. R. Fell, the design of the Deerhound was underway by late 1935, and it retained the inline radial cylinder configuration of the Hyena. However, the Deerhound had two addition cylinders for each row, making three rows of seven cylinders. Each cylinder bank of the Deerhound used a single overhead camshaft to actuate each cylinder’s two valves. The camshafts were driven from the crankshaft by a vertical shaft at the front of the engine. The Deerhound had a single-stage, two-speed supercharger and a propeller reduction gear of .432.

The 21-cylinder Deerhound had a 5.31 in (135 mm) bore and a 5.00 in (127 mm) stroke. The engine displaced 2,330 cu in (38.18 L) and had a forecasted output of 1,500 hp (1,119 kW). The Deerhound was seen as insurance against the potential failure of the Bristol Hercules engine then under development. The designers of the Deerhound would have preferred to create a liquid-cooled engine in the 1,500 hp (1,119 kW) class, but ASM management (John Siddeley) insisted on air-cooling. One of the engine’s designers, W. H. “Pat” Lindsey, stated the engine was “old-fashioned” and did not possess many then-modern refinements.

Armstrong Siddeley Deerhound construction

This photo shows five Deerhound engines in various stages of assembly. Most likely, all of the engines are Deerhound Is, but it is possible one is a Deerhound II. From left to right, the engines appear to be numbered D1, D5, D3, D4, and D2. The engine marked D5 is definitely a Deerhound I.

The Deerhound was first run in 1936, and it was not long before cooling and other problems were encountered. Most likely, five engines were built, and the last achieved 1,370 hp before it failed. In 1937, the ASM board tasked Fell to redesign the engine to cure its ills. The redesign resulted in the Deerhound II, which will be described later. The Deerhound was proposed for the Armstrong Whitworth A.W.42 heavy bomber.

Another engine from 1935 was the Terrier. The Terrier was a two-row radial engine in which each row had seven cylinders. Again, specifics of the engine’s configuration are not available, but most likely the Terrier was effectively a two-row, 14-cylinder Deerhound. Retaining the Deerhound’s 5.31 in (135 mm) bore and a 5.00 in (127 mm) stroke, the engine would have a displacement of 1,553 cu in (24.45 L). The Terrier had a 6.6 to 1 compression ratio.

Armstrong Siddeley Deerhound II side

An Armstrong Siddeley Deerhound II partially assembled. The front of the engine, gear case, and valve covers have all be redesigned from that used on the Deerhound I. Note the overhead camshaft and valve arrangement visible on the upper cylinder bank.

Like the Deerhound, the Terrier had a single-stage, two-speed supercharger. The engine produced a maximum of 550 hp (410 kW) at 2,700 rpm for takeoff, 510 hp (380 kW) at 2,100 rpm at 6,500 ft (1,981 m), and 476 hp (355 kW) at 3,100 rpm at 14,700 ft (4,481 m). Normal outputs were 470 hp (350 kW) at 2,700 rpm at 5,000 ft (1,524 m), and 450 hp (336 kW) at 2,700 rpm at 12,000 ft (3,658 m). The Terrier was proposed for a number of projects including the Armstrong Whitworth F.9/35 turret fighter proposal, the Blackburn M.15/35 torpedo bomber proposal, and the Avro 672 and 675 multi-role aircraft designs. None of those projects were built, and work on the Deerhound prevented the Terrier from being constructed.

Also in 1935, the Whippet was designed. Specifics of the Whippet are not available, but the 250 hp (186 kW) engine may have had two rows of seven cylinders with a bore and stroke of around 4.02 in (102 mm). That cylinder size would give the engine a total displacement of around 712 cu in (11.67 L). The Whippet did not proceed beyond the design phase.

The next engine design was initiated around 1936 and was for the Wolfhound (this name was used again later). The inline radial Wolfhound was an outgrowth of the Deerhound and had four rows of seven cylinders. Specifics of the engine are not available. However, 28 cylinders with the Deerhound’s 5.31 in (135 mm) bore and 5.00 in (127 mm) stroke would displace 3,106 cu in (50.90 L) and produce around 1,800 hp (1,342 kW). The Wolfhound did not make it off the drawing board.

Deerhound II engine Whitley

A Deerhound II engine installed in an Armstrong Whitworth A.W.38 Whitley bomber. Note the relatively small diameter of the engine compared to that of the firewall. The 44 in (1.12 m) diameter Deerhound replaced the 51 in (1.29m) diameter Armstrong Siddeley Tiger that was originally installed in the Whitley II.

In 1937, the Deerhound was redesigned and became the Deerhound II. The engine’s configuration changed little. However, refinements were made, and the cylinder bore was increased from 5.31 in (135 mm) to 5.51 in (140 mm). The stroke was unchanged, but the larger bore increased the engine’s displacement by 175 cu in (2.88 L), bringing the total displacement to 2,505 cu in (41.06 L). The engine’s forecasted output was still 1,500 hp (1,119 kW). The Deerhound II had a 6.75 to 1 compression ratio and was 44 in (1.12 m) in diameter. Extensive baffles were installed around the cylinders to help direct air flow and cool the rear cylinders.

The Deerhound II was first run in 1938, but more issues were encountered, including a broken crankshaft during a type test in October 1938. Fell and his team were under immense pressure from the ASM board to fix the engine’s issues. Two Deerhound II engines were installed in an Armstrong Whitworth A.W.38 Whitley bomber (serial no. K7243) for flight tests, and the aircraft first flew in January 1940. Fell’s contract with ASM was not renewed when it expired on 9 February 1939. Lindsey temporarily took over Deerhound development until Stewart S. Tresilian was brought on staff in mid-1939.

Deerhound engine cowling Whitley

Any aerodynamic advantages achieved by the close-fitting cowling covering the Deerhound engine installed on the Whitley must have been undermined by the bulbous cooling-air intake under and the large induction scoop above the engine. This Whitley was eventually lost, but through no fault of the engines.

Although the date is not recorded, the engine did achieved and output of 1,500 hp (1,119 kW) at 2,975 rpm with 5 psi (.34 bar) of boost. On the Whitley bomber, the engines were housed in special nacelles. Cooling air was taken in under the spinner and directed from the rear of the engine forward through the cylinders. However, engine cooling issues persisted, and the designers believed increasing the cooling fin area of the cylinders would resolve the problem.

Flight testing continued until 6 March 1940 when the Deerhound II-powered Whitley bomber was lost on takeoff, killing all three people on board. The crash was attributed to an improperly set trim tab and had nothing to do with the engines. With the testbed destroyed, ASM decided to curtain development of the Deerhound II and focus on an improved version that would cure the overheating issues. The new engine, the Deerhound III, will be described later. Five Deerhound II engines were built.

Armstrong Siddeley 36-cyl Mastiff

This drawing illustrates a coupled arrangement for two 36-cylinder Armstrong Siddeley Mastiff engines. The engine’s nine banks of four cylinders can be deduced from the drawing. A similar arrangement was purposed with two Deerhound engines.

In 1937, the Boarhound was designed. This engine possessed the larger 5.51 in (140 mm) bore of the Deerhound II and had a longer 6.30 in (160 mm) stroke. The real design change was the engine’s layout—the Boarhound had three inline rows of nine cylinders. With its 27 cylinders, the Boarhound displaced 4,058 cu in (66.50 L). Its initial and rather conservative estimated output was 2,300 hp (1,715 kW) at 2,700 rpm. The Boarhound had a diameter of 51 in (1.30 m). With all resources focused on the Deerhound, the Boarhound was never built.

Around 1938, the Mastiff name was resurrected and given to a further development of the Boarhound. The new Mastiff had four inline rows of nine cylinders. While the cylinder’s bore was still 5.51 in (140 mm), the stroke was lengthened to 6.69 in (170 mm). The 36 cylinders of the Mastiff engine displaced an impressive 5,749 cu in (94.21 L), and output was estimated at 4,000 hp (2,983 kW). Like the Boarhound, the Mastiff was not developed.

In 1940, Tresilian went to work on the Deerhound III to create an engine free from the issues experienced with the original Deerhound and the Deerhound II. The Deerhound III possessed the same bore, stroke, displacement, and 44 in (1.12 m) diameter as the Deerhound II. However, the engine was essentially redesigned, and its output was increased to 1,800 hp (1,342 kW). The engine was first run in late 1940. High-power tests revealed detonation issues with the first row of cylinders, but some sources state the engine did achieve 1,800 hp (1,342 kW) on the dyno. An updated design, the Deerhound IV, was proposed but never built. Only one Deerhound III was built.

Armstrong Siddeley Deerhound III

This picture shows the sole Armstrong Siddeley Deerhound III engine. Again, revisions to the front of the engine, gear reduction, and valve covers can easily be seen. Reportedly, this engine survived into the 1970s.

In mid-1941, the Wolfhound name was reused for a new Tresilian-designed engine. The new Wolfhound had four inline rows of six cylinders. The 24-cylinder engine had a 5.91 in (150 mm) bore and a 5.51 in (140 mm) stroke. Total displacement was 3,623 cu in (59.38 L), and the engine was to produce 2,600–2,800 hp (1,939–2,088 kW) at 2,800 rpm. The engine had a two-stage supercharger and was designed to power contra-rotating propellers.

In October 1940, a bombing raid severely damaged the Armstrong Siddeley Aero Development shop and destroyed several Deerhound engines. Another raid on 8 April 1941 further damaged the shop and set engine development back even more. ASM dog-engine development continued at a slow pace until October 1941, when the British Ministry of Aircraft Production (MAP) cancelled further work. The ASM dog-engines would not be ready in time to be of any use in the war, and the MAP wanted the company to focus on turbine engines (ASM named theirs after snakes). The sole Deerhound III engine was thought to have survived into the 1970s, but there are no known ASM dog-engines currently preserved.

Armstrong Siddeley Deerhound IV

Drawing of the Armstrong Siddeley Deerhound IV engine that was never built. Even if this design from 1941 proved successful, it would not have been developed in time to see much use during World War II.

Sources:
Armstrong Siddeley — the Parkside Story 1896–1939 by Ray Cook (1988)
Parkside: Armstrong Siddeley to Rolls-Royce 1939–1994 by Roy Lawton (2008)**
Sectioned Drawings of Piston Aero Engines by Lyndon Jones (1995)**
Armstrong Whitworth Aircraft since 1913 by Oliver Tapper (1973)
British Secret Projects: Fighters & Bombers 1935–1950 by Tony Buttler (2004)
British Piston Aero-Engines and Their Aircraft by Alec Lumsden (1994/2003)
http://www.designchambers.com/wolfhound/index.htm#Articles (and pages therein)
http://www.secretprojects.co.uk/forum/index.php?topic=5580.0

**Rolls-Royce Heritage Trust books that are currently in print and available from the Rolls-Royce Heritage Trust site.