Category Archives: Aircraft Engines

Dutheil Chalmers Eole props rear

Dutheil-Chalmers Éole Opposed-Piston Aircraft Engine

By William Pearce

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

Dutheil Chalmers Eole patent

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

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

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

Dutheil Chalmers Eole 2 view

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

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

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

Dutheil Chalmers Eole front

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

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

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

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

Dutheil Chalmers Eole props rear

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

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

SPA-Faccioli N3 rear

SPA-Faccioli Opposed-Piston Aircraft Engines

By William Pearce

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

SPA-Faccioli N1

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

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

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

SPA-Faccioli N2

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

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

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

Faccioli N3 aircraft

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

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

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

SPA-Faccioli N3 rear

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

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

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

SPA-Faccioli N3 front

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

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

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

Faccioli N4 aircraft

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

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

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

SPA-Faccioli N4 front

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

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

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

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

SPA-Faccioli N4 rear

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

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

Isotta Fraschini Zeta rear

Isotta Fraschini Zeta X-24 Aircraft Engine

By William Pearce

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

Isotta Fraschini Zeta front

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

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

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

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

Isotta Fraschini Zeta rear

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

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

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

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

Caproni F6Z IF Zeta

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

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

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

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

Caproni Vizzola F6Z

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

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

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

Isotta Fraschini Zeta SM79

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

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

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

Isotta Fraschini Zeta SM79 cowling

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

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

Allison V-3420-A front

Allison V-3420 24-Cylinder Aircraft Engine

By William Pearce

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

Allison V-3420-A front

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

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

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

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

Allison V-3420-A rear

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

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

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

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

Allison V-3420-B NMUSAF rear

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

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

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

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

Allison V-3420-B NMUSAF

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

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

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

Allison V-3420-B right-angle drive

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

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

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

Fisher P-75A Eagle

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

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

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

Douglas XB-19A

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

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

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

Allison V-3420 XB-19A nacelle

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

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

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

Lockheed XP-58 Chain Lightning

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

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

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

Fisher XB-39

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

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


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

By William Pearce

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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


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

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

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

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


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

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

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

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


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

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


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.


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.


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).


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.


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.


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.

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),6520.0/all.html


Tips Aero Motor Rotary Aircraft Engines

By William Pearce

From a very early age, Maurice A. Tips and his younger brother Earnest Oscar were interested in aviation. By 1908, the Belgian siblings had built their first aircraft: a canard-design, pusher biplane. The first engine installed in the aircraft proved underpowered and was replaced with a Gnome rotary. The engine was geared to two shafts, each driving a two-blade pusher propeller. Although the aircraft made some flights, its handling was unsatisfactory, and the design was not developed further. The aircraft did possess unique concepts, a theme continued in Maurice’s subsequent designs.


Rear view of Maurice and Earnest Oscar Tips’ 1908 biplane pusher. The aircraft was unable to fly with its original Pipe V-8 engine, but the lighter Gnome rotary enabled the aircraft to takeoff. Note the central gearbox that provided power to the shafts that turned the propellers via right-angle drives.

After the 1908 aircraft, Maurice refocused his efforts on aircraft engines. By 1912, Maurice had designed a 25 hp (19 kW), seven-cylinder, “valveless,” rotary engine. The engine had a 2.76 in (70 mm) bore, a 4.33 in (110 mm) stroke, and a displacement of 181 cu in (3.0 L). Hollow “suction tubes” took the air/fuel mixture from the engine’s crankcase and delivered it to the cylinders. Each suction tube was geared to the engine’s fixed crankshaft. The suction tubes would spin at half the speed of the crankcase as it rotated. The top of the suction tube had two passageways. Each passageway would align with a common port near the top of the cylinder once every two revolutions of the crankcase. One passageway aligned to allow the air/fuel mixture to flow from the suction tube and into the cylinder. The second passageway aligned to allow the exhaust gases to flow from the cylinder out into the atmosphere.

Two larger versions of the seven-cylinder engine were planned. One had a 4.33 in (110 mm) bore, a 4.72 in (120 mm) stroke, a displacement of 487 cu in (8.0 L), and produced 50 hp (37 kW). The largest engine had a 4.41 in (112 mm) bore, a 5.12 in (130 mm) stroke, a displacement of 547 cu in (9.0 L), and produced 70 hp (52 kW). It does not appear that either of the larger engines were built.


Drawings of the 25 hp (19 kW) Tips engine of 1912. Air was drawn through the rotating suction tubes (5) which enable the intake port (14) and exhaust port (13) to align with the cylinder. The suction tubes were geared (9 and 10) to the stationary crankshaft (4).

Maurice worked to refine his “valveless” rotary engine throughout 1913 and 1914. The most obvious change was that the suction tube was moved to be parallel with the cylinder, rather than at an angle as seen in the 1912 engine. The newer engine design had an updated drive for the suction tubes, and the air/fuel mixture no longer passed through the crankcase; rather, it was delivered through a hollow extension of the crankshaft to a space under the suction tubes.


A 1912 advertisement for the Tips seven-cylinder engines. The ad is titled “The new rotary engines without valves,” and includes the more powerful 50 hp (37 kW) and 70 hp (52 kW) models.

A 25 hp (19 kW) Tips valveless rotary engine was installed in a monoplane built by Henri Gérard in 1914. However, it is not clear if the engine was the 1912 design or later. In addition, the results of the engine’s and aircraft’s performance have not been found. When World War I broke out, Maurice and Earnest Tips fled Belgium. Earnest made his way to Britain, where he worked with Charles Richard Fairey and helped start the Fairey Aviation Company in 1915. Earnest would return to Belgium in 1931 to start the Fairey subsidiary, Avions Fairey. He also produced the Tipsy series of light aircraft.

Maurice Tips traveled to the United States and continued to design aircraft engines. In 1917, The Tips Aero Motor Company was founded in Woonsocket, Rhode Island. That same year, Maurice applied for patents covering his new engine design, which incorporated many concepts from the earlier engines. The new Tips engine was an 18-cylinder, twin-row, rotary engine housed in a stationary frame. The new engine employed both water and air cooling. The cylinders were arranged in pairs, with one in the front row of the engine and the other in the rear row. The crankshaft had only one throw, and the pistons for both cylinders in a pair were at top dead center on their compression strokes at the same time. The engine’s compression ratio was 5.25 to 1. Each cylinder had one spark plug at the center of its combustion chamber. The spark plugs were fired by two magnetos mounted to the front of the engine and driven from the propeller shaft.


The 1913 (left) and 1914 (right) versions of the Tips rotary engine. The major changes were to the suction tube drive and rotary valves. The small tube (no. 14 on the 1913 engine and no. 40 on the 1914 engine) in the stationary crankshaft extension provided oil to the crankshaft and connecting rod.

Most rotary engines had a fixed crankshaft and a crankcase that rotated. This arrangement created much stress on the crankshaft and crankcase and also imposed severe gyroscopic effects on the aircraft. The Tips engine employed several unique characteristics to resolve the drawbacks of traditional rotary engines. The crankshaft of the Tips engine rotated and was geared to the propeller shaft. The propeller shaft was geared to the crankcase, which allowed it to rotate in the opposite direction from the crankshaft and propeller. The end result was that when the crankshaft was turning at 1,800 rpm, the propeller would turn at 1,080 rpm, and the crankcase would rotate at 60 rpm in the opposite direction. Rotary engines in which the crankshaft and crankcase rotate in opposite directions and at different speeds are often called bi-directional or differential rotary engines.

The propeller shaft of the Tips 18-cylinder engine was geared to the crankshaft at a .600 reduction; the crankshaft gear had 18 teeth, and the propeller shaft’s internal gear had 30 teeth. For crankcase rotation, the 17 teeth on the propeller shaft gear engaged 51 teeth on one side of a countershaft to give a .333 gear reduction. The other side of the countershaft had 11 teeth that meshed with a 66-tooth internal gear attached to the crankcase and resulted in a further .167 reduction. Having the propeller and crankshaft rotating in opposite directions not only eliminated the gyroscopic effect inherent to conventional rotary engines, but it also neutralized the gyroscopic effect created by the propeller attached to a fixed engine.


The 18-cylinder Tips engine of 1917 was far more complex than the earlier engines. Note the paired cylinders separated by the rotary valve (24). The propeller shaft (10) was geared to the crankshaft (7) via reduction gears (8 and 9). The crankcase was geared to the propeller shaft via a countershaft (16).

On the exterior of the cylinder castings were numerous cooling fins. In addition, internal passageways for water cooling were in the cylinder castings. Between each pair of cylinders were a series of air passageways to further augment cooling. The engine did not have a water pump; rather, thermosyphoning and the relatively slow rotation of the crankcase enabled the circulation of cooling water from the internal hot areas of the cylinders out toward the cooling fins on the exterior of the cylinders. The engine’s rotation also aided oil lubrication from the pressure-fed crankshaft to the rest of the engine. The oil pump and carburetor were located on the stationary frame at the rear of the engine.

A flange was positioned on the crankshaft, between the connecting rods of the cylinder pair. Mounted on the flange via ball bearings was an eccentric gear with 124 teeth on its outer edge. Attached (but not fixed) to the crankcase was a master valve gear that had 128 teeth on its inner edge. The gears meshing with an eccentric action resulted in the master valve gear turning four teeth per revolution of the crankshaft. On the outer edge of the master valve gear was a bevel gear with 128 teeth. These teeth engaged a 16-tooth pinion attached to a rotary valve positioned between each cylinder pair. The four teeth per revolution of the master valve gear acting on the 16-tooth rotary valve resulted in the rotary valve turning at a quarter engine speed. Each hollow rotary valve had two intake ports and two exhaust ports.

Air was drawn in through a carburetor at the rear of the engine. The air/fuel mixture flowed through a manifold bolted to the cylinder casting and into a passageway that led to a chamber around the lower part of the rotary valve. Holes in the valve allowed the air to flow up through its hollow middle and into the cylinder when the intake ports aligned. As the valve rotated, the exhaust ports would align with the cylinder, allowing the gases to escape out the top of the valve head and into the atmosphere. Passageways in the lower part of the rotary valve head brought in cooling water from the cylinder’s water jacket. Water flowed up through the rotary valve and back into the cylinder’s water jacket. The rotary valve was lubricated by graphite pads and held in place by a spiral spring and retaining cap around its upper surface.


On the left is the rotary valve shown with the intake ports aligned (Fig 3). The air/fuel mixture entered the valve through ports in its lower end (27a). On the right is the valve with the exhaust ports aligned (Fig 5). Fig 4 shows a cross section of the rotary valve with intake ports (28), exhaust ports (29), and passageways for the flow of cooling water (30). Fig 8 shows the valve gear drive. The crankshaft (7) turned an eccentric gear (44) that meshed (42 and 41) with a gear mounted to the crankcase. The result is that a bevel gear (27) engaged a gear screwed to the bottom of the rotary valve (26 on Fig 3) and turned the valve once for every four revolutions of the crankshaft.

The 18-cylinder Tips engine had a 4.5 in (114 mm) bore and a 6.0 in (152 mm) stroke. The engine displaced 1,718 cu in (28.1 L) and produced 480 hp (358 kW) at 1,800 rpm. The Tips engine weighed 850 lb (386 kg). At speed, the engine consumed 22 gallons (83 L) of fuel and 3 gallons (11 L) of oil per hour. The oil consumption was particularly high, even for a rotary engine, but the Tips engine was larger and more powerful than other rotary engines.

In 1919, the engine was mentioned in a few publications. In 1920, Leo G. Benoit, Technical Manager at Tips Aero Motors, passed away. Benoit was said to be in charge of the engine’s design and construction. No further information regarding the engine and no images of the engine have been found. This lack of information could mean that the 480 hp (358 kW) Tips engine was never built. However, given the detailed description of the engine and that it was worked on from 1917 to at least 1920, the possibility certainly exists that the engine was built and tested.

Sometime before World War II, Maurice Tips returned to Belgium. He continued to design engines and applied for a patent on a rotary piston engine in 1938. This engine was not designed for aircraft use and bore no similarities to his early aircraft engines.


Rear view of the 480 hp (358 kW) Tips engine shows the extensive fining (22) that covered the engine. The fining and air passages (23) combined to turn the whole engine into a radiator to cool the water that flowed through the engine via thermosyphoning and centrifugal force.

“Valveless Rotary Combustion Engine” US Patent 1,051,290 by Maurice Tips (granted 21 January 1913)
“Improvements in Rotary Combustion Engines” GB Patent 191307778 by Maurice Tips (application 15 April 1913)
“Improvements in or relating to Rotary Combustion Engines” GB Patent 191506821 by Maurice Tips (application 8 May 1914)
“Rotary Valve” US Patent 1,286,149 by Maurice A. Tips (granted 26 November 1918)
“Internal Combustion Engine” US Patent 1,306,035 by Maurice A. Tips (granted 10 June 1919)
“Valve-Operating Mechanism” US Patent 1,306,036 by Maurice A. Tips (granted 10 June 1919)
“Internal Combustion Engine” US Patent 2,203,449 by Maurice Tips (granted 4 June 1940)
“The Tips 480 H.P. Aero Motor” Aerial Age Weekly (17 March 1919)
Airplane Engine Encyclopedia by Glenn Angle (1921)