Category Archives: Between the Wars

Continental-O-1430-engine

Continental Hyper Cylinder and the O-1430 Aircraft Engine

By William Pearce

In the late 1920s, British engine expert Harry R. Ricardo hypothesized that the spark-ignition internal combustion engine with poppet valves had reached its specific power-producing zenith. The foundation for this belief was rooted in the fuel quality and technology employed at the time. Ricardo recommended that a single sleeve valve should replace the cylinder’s poppet valves and would enable the continued increase of an engine’s specific power output.

Continental-Hyper-Cylinder-No-2-sectional

Sectional drawing of the Continental Hyper No. 2 cylinder from August 1933. The domed exhaust valve is on the left. The domed piston had recesses to provide clearance for the valves.

British expatriate turned American citizen Sam D. Heron was also an engine expert and was employed at the time by the Army Air Corps (AAC) at Wright Field in Dayton, Ohio. Heron was involved in engine research, and with the approval of the AAC, he began to explore the power limits of the spark-ignition internal combustion cylinder with poppet valves. However, Heron had access to one thing that Ricardo did not consider: sodium-cooled exhaust valves.

Around 1923, Heron had developed an air-cooled cylinder for use on the Liberty V-12 engine. This cylinder had a 4.625 in (117 mm) bore, a 7.0 in (178 mm) stroke, and displaced 117.6 cu in (1.93 L). Around 1925, Heron developed the sodium-cooled exhaust valve. These valves had a hollow stem that was partially (approximately 2/3) filled with sodium. Once the valve reached 208° F (98° C), the sodium melted. The up-and-down movement of the valve sloshed the sodium in the valve. The sodium absorbed heat from the valve head, cooling it, and transferred the heat to the valve stem. The valve stem extended out of the cylinder and transferred the heat to the valve guide boss and subsequently to the cooling fins (if air cooled) or the water jacket (if water-cooled). The exhaust valve was a hot spot inside the cylinder that could cause detonation. Detonation is the spontaneous combustion of the remaining air and fuel mixture inside the cylinder prior to the flame front propagating from the spark plug, after it has fired, reaches that part of the cylinder. The sodium-cooled valve reduced the valve’s temperature, helping to prevent the possibility of detonation, and enabled the cylinder to produce more power.

Around 1930, Heron took the air-cooled Liberty cylinder with a sodium-cooled exhaust valve and converted it to water-cooling by adding a water jacket around the cylinder barrel. The cylinder was used on a single-cylinder test engine and quickly produced more power than the poppet valve limits described by Ricardo. At the time, an average aircraft engine cylinder produced a mean effective pressure (mep) of around 150 psi (10.3 bar). Using a single sleeve valve engine, Ricardo was able to achieve an mep of 450 psi (31.0 bar). Heron’s test cylinder was able to achieve an mep of 360 psi (24.8 bar) on its first run. Heron’s test cylinder was reworked, and an mep of 500 psi (34.5 bar) was ultimately recorded.

Continental-Hyper-Cylinder-No-2-side-bottom

Two views of the same Hyper No. 2 cylinder after its 49-hour test run in August 1933. The exhaust port is on the same side as the coolant pipe.

Encouraged by Heron’s test results, the AAC sought to develop a high-performance (Hyper) cylinder to be used on an aircraft engine. The cylinder kept the 4.625 in (117 mm) bore, but the stroke was reduced to 5.0 in (127 mm) to permit an engine speed of up to 3,400 rpm. With the change, the cylinder displaced 84.0 cu in (1.38 L). A proposed V-12 engine would incorporate 12 Hyper cylinders for a total displacement of 1,008 cu in (16.5 L) and a goal of producing 1,000 hp (746 kW). The AAC also desired a pressurized cooling system that ran straight ethylene glycol at 300° F (149° C). The then-current practice was to use normal water as the coolant, which limited the temperature to around 180° F (82° C). The high temperature was selected in an effort to decrease the size of the radiator needed in the aircraft. For proper cooling of a complete engine with the desired 300° F (149° C) coolant temperature, the AAC believed that individual cylinder construction would be required rather than six-cylinders together in a monobloc. However, an engine constructed with individual cylinders is less rigid than using monobloc construction, making the crankcase and cylinders prone to cracking when the engine is highly stressed. Individual cylinder construction also makes the engine heavier and longer, which increases torsional stresses on the crankshaft.

On 5 October 1932, a contract to develop the Hyper cylinder and design a complete 12-cylinder engine was issued to the Continental Motors Company. At the time, Continental built engines for a number of different automotive manufacturers and built medium-size air-cooled radial engines under their own name. Continental had also been contracted for experimental work on single sleeve valve engines by both the AAC and the US Navy.

Continental set up an office in Dayton, Ohio to work with Heron and the AAC regarding the design of the first test cylinder, Hyper No. 1. Continental built Hyper No. 1 to the AAC’s specifications at their main facility in Detroit, Michigan. Hyper No. 1 was constructed of a forged steel cylinder barrel screwed and shrunk into a cast aluminum head. A separate steel water-jacket was shrunk over the barrel and a shoulder of the head. The cylinder had a hemispherical combustion chamber with a single intake and a single sodium-cooled exhaust valve. The valves were actuated by an overhead camshaft via rockers. The rockers had a roller that rode on the camshaft and a pad that contacted the valve stem. Hyper No. 1 was first tested in early 1933 and soon produced 84 hp (63 kW) at 3,000 rpm, achieving the goal of producing 1 hp per cu in (.7 kW per 16 cc). However, there was some concern that a 1,008 cu in (16.5 L) engine producing 1,000 hp (746 kW) would be highly stressed, resulting in decreased reliability.

Continental-O-1430-drawing-1933

A drawing of the O-1430 included in U.S. patent 2,016,693 from October 1933 shows the engine’s basic layout. The cylinder appears to be nearly identical to that of Hyper No. 2, and the engine’s configuration matches what was ultimately built in 1938.

The AAC allowed Continental to develop a larger cylinder bore, resulting in Hyper No. 2. Hyper No. 2 had the bore increased by .875 in (22 mm) to 5.5 in (140 mm). This change increased the cylinder’s displacement by 34.8 cu in (.57 L) to 118.8 cu in (1.95 L). An engine with 12 Hyper No. 2 cylinders would displace 1,425 cu in (23.4 L), an increase of 417 cu in (6.8 L) over using Hyper No. 1 cylinders. Other AAC requirements, such as 300° F (149° C) coolant, individual cylinders, and a 1,000 hp (746 kW) output remained unchanged.

An endurance test report of Hyper No. 2 dated 3 August 1933 states that two cylinders were used for the test. The first cylinder failed due to cracks after 11 hours at 3,000 rpm and 9.8 psi (.68 bar) of boost. The second cylinder was run for 49 hours and produced 83 hp (62 kW) at 3,000 rpm with 6.9 psi (.48 bar) of boost. This gave an indicated mep of 211 psi (14.5 bar) and would enable a 12-cylinder engine to produce 1,000 hp (746 kW). However, the second cylinder also exhibited cracks at the end of the run, and numerous parts of both cylinders failed during or were worn out after the test. The report also states that the cylinder had a compression ratio of 5.9 to 1 and that the intake and exhaust valves were both sodium-cooled, but it is not clear if this was also the case with Hyper No. 1. The report includes a drawing of a piston listed as having a 5.75 to 1 compression ratio.

As testing of Hyper No. 2 was underway, serious discussions commenced regarding the design of a 12-cylinder engine. The AAC now wanted a flat (horizontally opposed cylinder) engine that could be installed in an aircraft’s wing and tasked Continental to build such an engine. The result was the O-1430, which utilized Hyper No. 2 cylinders. Sometimes the engine is referred to as OL-1430, for Opposed Liquid-cooled. It was assumed that a complete O-1430 engine would be built quickly and that the engine could be rapidly placed into service, with only a few years elapsing from design to production.

Continental-O-1430-mockup

Wooden mockup of the Continental O-1430 engine. The model was very detailed and closely matched the actual engine. The model survived and is in a private collection. Note the intake manifold and its individual runners atop the engine.

The Continental O-1430 was a horizontally opposed (flat-12 or 180° V-12) aircraft engine. The two-piece aluminum crankcase was split vertically at its center. Six individual steel cylinders were attached via studs to each side of the crankcase. As installed on the engine, the air and fuel mixture entered the cylinder via a port on the top side, and the exhaust gases were expelled via a port on the bottom side of the cylinder. A camshaft housing was attached atop all of the cylinders on each side of the engine. The single overhead camshaft for each cylinder bank was driven from the front of the engine via a shaft and bevel gears. A magneto was mounted to the rear of each camshaft. One magneto fired one spark plug in each cylinder, and the other magneto fired the other spark plug. The spark plugs were both positioned on the intake side of the cylinder and flanked the intake port. The pistons were connected to the crankshaft via fork-and-blade connecting rods.

At the front of the engine was an accessory drive and propeller gear reduction. A double set of spur gears enabled the reduction and kept the propeller shaft on the same axis as the crankshaft. A gear reduction of .455 or .556 could be fitted without any modification to the reduction housings. Additionally, the accessory drive was designed so that swapping two gears would reverse the rotation of the accessory drive shaft relative to the crankshaft. In other words, the setup enabled the accessories to be driven in the same direction whether the crankshaft rotated clockwise or counterclockwise. There was no need for special accessories or gearsets when the engine was installed in handed operation. Reversing the relative positions of the starter and generator mounted to the sides of the front accessory drive and flipping their common drive shaft enabled those units to operate regardless of the clockwise or counterclockwise rotation of the crankshaft.

Continental-O-1430-engine-top

Top view of the complete O-1430 engine shows the accessory section at the front of the engine with the starter and generator. Note the camshaft drives and the leads from the magnetos to the spark plugs.

A downdraft carburetor was positioned at the extreme rear of the O-1430 engine. It fed air and fuel into the single-speed, single-stage supercharger, which was mounted to the rear of the engine. The supercharger impeller was 10.5 in (267 mm) in diameter and turned at 6.45 times crankshaft speed. An intake manifold led from the supercharger and sat atop the engine. Individual runners branched off the manifold and supplied the air and fuel mixture to each cylinder. A water pump with two outlets, one for each cylinder bank, was driven from the bottom of the supercharger drive housing.

The O-1430 had a 5.5 in (140 mm) bore and a 5.0 in (127 mm) stroke. The engine displaced 1,425 cu in (23.4 L) and had compression ratio of 6.1 to 1. Takeoff power was 1,150 hp (858 kW) at 3,150 rpm, and continuous power was 1,000 hp (746 kW) at 3,000 rpm up to 25,000 ft (7,620 m). The O-1430 was 104.5 in (2.65 m) long, 44.3 in (1.13 m) wide, and 24.2 in (.61 m) tall. The engine weighed 1,300 lb (590 kg).

Construction of the O-1430 was delayed by the development of the Hyper No. 2 cylinder. Almost all of the time from 1932 to 1938 was spent on refining the cylinder’s design. The AAC wanted the cylinder to be fully developed before the complete engine was built, and it took Continental years to fully satisfy the AAC’s requirements. Cracks in the cylinder were a constant issue as Hyper No. 2 was developed. Additionally, Continental seemingly did not want to spend any of its own money on the cylinder or engine, even though the company would eventually be reimbursed by the AAC. Rather, Continental sent each change and every purchase through the AAC for contractual approval. While this funding bottleneck severely slowed work, Continental was struggling financially in the Depression era. In addition, Continental believed that the engine would not be suitable for commercial use and that it would only power fighter aircraft. They felt that a fighter engine would not offer a significant return on any money that they invested into the project. At the same time, the AAC had very limited funds available for the experimental engine project.

Continental-O-1430-engine

Although the O-1430 achieved its desired output of 1,000 hp (746 kW), its protracted development rendered the engine obsolete. Had it been completed in 1935, the O-1430 may have found an application and been put into production.

The O-1430 was finally completed and run in 1938. This was about two years past the AAC’s originally envisioned timeline for the engine to be in production and powering various aircraft. The engine passed a 50-hour development test at 1,000 hp (746 kW) in April 1939. By this time, the concept of installing a flat engine in the wing of a fighter had fallen out of favor, as a fighter’s wings were too thin to house such an engine. In addition, a 1,000 hp (746 kW) engine was not powerful enough for fighters under development. The Allison V-1710 and the Rolls-Royce Merlin had both passed more stringent tests and produced more power years prior. In addition, Allison had convinced the AAC that 250° F (121° C) coolant was just as, if not more, efficient as 300° F (149° C) coolant. At 300° F (149° C), a lot of heat is transferred into the oil, necessitating a larger oil cooler. A larger radiator is needed at 250° F (121° C), but the oil cooler can be smaller, resulting in the same overall drag of the comparative cooling systems. Furthermore, the engine and all surrounding components and accessories lasted longer at the lower temperature. It was also found that pure ethylene glycol did not transfer heat as well as a 50/50 mixture of water and ethylene glycol.

A redesign of the O-1430 was offered in which the engine would be altered into a compact Vee configuration. With recent advancements, such as increased supercharging and better fuels, it was believed that the redesigned engine could be made to produce 1,600 hp (1,193 kW) and would be well suited for fighter aircraft. The engine was subsequently redesigned as an inverted V-12. It was officially designated as the Continental XIV-1430 and later became the XI-1430. Work on the O-1430 was halted.

On 11 September 1939, the AAC issued Request for Data R40-A seeking an 1,800–2,400 hp (1,342–1790 kW) engine for installation in a bomber’s thick wing. Continental proposed doubling the O-1430 to create the 24-cylinder XH-2860. This was the same thing Lycoming had done with its O-1230 when creating the XH-2470. However, the Continental XH-2860 did not find favor with the AAC, and the engine never proceeded beyond the preliminary design phase. The decision against the XH-2860 was based in part to allow Continental to focus on developing the XI-1430.

Continental-XI-1430-left-right

The XI-1430 was the final development of the O-1430 and Hyper cylinder program. Although the engine exhibited impressive performance, achieving 2,100 hp (1,566 kW) in August 1944, it had reliability issues and came too late to have any impact in World War II.

Sources:
Development of Aircraft Engines and Aviation Fuels by Robert Schlaifer and S. D. Heron (1950)
Report of 49-Hour Endurance Test of Continental “Hyper” Engine No. 2 by R. N. DuBois (3 August 1933)
Continental! Its Motors and its People by William Wagner (1983)
Tornado: Wright Aero’s Last Liquid-Cooled Piston Engine by Kimble D. McCutcheon (2001)
– “Engine Support” U.S. patent 2,016,693 by Norman N. Tilley (filed 2 October 1933)
– “Reversible Accessory Driving Mechanism for Engines” U.S. patent 2,051,568 by Harold E. Morehouse (filed 7 June 1935)
– “Reversible Starter and Generator Drive for Engines” U.S. patent 2,053,354 by Norman N. Tilley (filed 7 June 1935)
http://www.enginehistory.org/Piston/HOAE/Continental2.html

Lorraine 12Fa

Lorraine-Dietrich ‘W’ Aircraft Engines

By William Pearce

In the early 1900s, Lorraine-Dietrich was a French manufacturer of wagons, rail equipment, and automobiles. During World War I, the company’s factory in Argenteuil, France started manufacturing aircraft engines under the name “Lorraine.” The Argenteuil factory was led by Marius Barbarou, the engineer that designed the aircraft engines.

Lorraine 12F

The Lorraine 12F of 1919 was the first of the company’s W-12 engines and followed the design outlined in the 1918 patent. Note the exposed pushrods and enclosed valves.

By 1918, Lorraine had developed aircraft engines in the form of an inline-six, a V-8, and a V-12. However, Barbarou began to experiment with engines of a W configuration. The W (or broad arrow) engine configuration had the benefit of being more rigid and slightly lighter than a comparable V-12, with the drawback of being slightly taller and wider. On 5 June 1918, Lorraine (under Barbarou) applied for a patent in which the benefits of a W engine with either four, six, or eight cylinders per bank was described. While the British Napier Lion W-12 was being developed at the same time, the patent illustrates that the Lorraine W engines were a parallel development and not a copy of the Lion. French patent 504,772 was awarded on 22 April 1920 for the W engine design.

The first generation of Lorraine’s W engines was designed around 1918 and known as the 12F (many sources do not give a designation for this engine, and “12F” was used again). The liquid-cooled, 12-cylinder engine consisted of a two-piece aluminum crankcase that was split horizontally along the crankshaft’s axis. Three banks of cylinders were mounted atop the crankcase, and the left and right banks were angled 60 degrees from the center, vertical bank. Each cylinder bank had two pairs of two cylinders. Each pair of steel cylinders was surrounded by a welded steel water jacket. Atop each cylinder was a single intake valve and a single exhaust valve. The enclosed valves were each actuated by a partially exposed rocker and a fully exposed pushrod. All of the pushrods were controlled by two camshafts—one positioned in each Vee between the cylinder banks. The push rods that controlled the exhaust valves for the left and right cylinder banks had a lower roller rocker that followed the camshaft.

A single-barrel updraft carburetor was positioned on the outer side of the right cylinder bank. An intake pipe led from the carburetor, passed between the two cylinder pairs of the right bank, and joined a manifold. The manifold split into four branches that fed each of the cylinders on the right bank. Employing a similar configuration, a two-barrel carburetor on the left side of the engine fed both the left and center cylinder banks. Each cylinder had two spark plugs that were fired by two magnetos located at the rear of the engine. The left magneto fired the spark plugs on the intake side of the cylinders, and the right magneto fired the exhaust-side spark plugs.

Lorraine 24G

With a new crankcase, crankshaft, and camshafts, the 24-cylinder 24G of 1919 was more than just two 12F engines coupled together. Note the ignition system driven from the propeller shaft.

The flat-plane crankshaft had four throws and was supported by three main bearings. A master connecting rod was attached to each crankpin. The master rods were connected to the aluminum pistons in the vertical cylinder bank. Articulated rods connected the pistons in the left and right cylinder banks to the master connecting rods. The engine had a compression ratio of 5.2 to 1. The propeller was attached directly to the crankshaft without any gear reduction. The Lorraine 12F had a 4.96 in (126 mm) bore and a 7.09 in (180 mm) stroke. The W-12 engine displaced 1,826 cu in (29.9 L) and produced 500 hp (372 kW) at 1,600 rpm. The 12F weighed 960 lb (435 kg).

While work on the 12F was underway, a 24-cylinder engine was designed that was basically two 12Fs. The W-24 engine was designated 24G (many sources do not give a designation for this engine, and a different G-series emerged later). Other than having twice the number of cylinders, the main change from the 12F was that the ignition system was driven at the front of the engine. The 12G’s eight throw crankshaft was supported by five main bearings. The W-24 engine displaced 3,652 (59.9 L) and produced 1,000 hp (746 kW) at 1,600 rpm. The direct drive engine weighed 1,874 lb (850 kg), and it was estimated that a 16 ft 5 in (5 m) propeller would be needed to harness its power.

The 12F and 24G engines were built during 1919 and displayed at the Salon de Paris in December of that year. There is some indication that the valve arrangement was problematic at high engine speeds, but the engines were displayed at the next two Salons in November 1921 and December 1922. No applications are known for the 12F or the 24G, which were too large for almost all aircraft. It is unlikely that more than a few of these engines were built.

Lorraine 12Eb no mags

A direct-drive 12E-series engine with exposed valves and overhead camshafts. Unseen are the magnetos positioned at the rear of the engine.

While enduring the rough start with the first generation of W engines, Barbarou had already designed the second generation—starting with the 12E-series. The first engine in this series was the 12Ew, which was derived from the 370 hp (276 kW) Lorraine 12D (V-12) and conceived to fill the power gap between that engine and the 500 hp (373 kW) 12F. The 12Ew was similar in layout to the 12F, but had a completely different valve arrangement. The exposed valves for each cylinder bank were actuated via rockers by a single overhead camshaft. The camshaft was driven by the crankshaft via bevel gears and a vertical shaft at the rear of the engine. It appears that the two magnetos were initially located at the front of the engine but later relocated to the rear of the engine. The engine had a compression ratio of 5.5 to 1. The propeller was attached directly to the crankshaft without any gear reduction.

The Lorraine 12Ew had a 4.72 in (120 mm) bore and a 7.09 in (180 mm) stroke. The engine displaced 1,491 cu in (24.4 L) and produced 420 hp (313 kW) at 1,800 rpm. The 12Ew was 54.1 in (1.37 m) long, 47.6 in (1.21 m) wide, and 44.8 in (1.14 m) tall. The engine weighed around 860 lb (390 kg). The 12Ew was first run around late 1919, but development was slowed due to work on other engines and other projects. The 12Ew was used in a few aircraft, and the engine was developed into the 12Eb.

The Lorraine 12Eb was dimensionally the same as the 12Ew, but it had a compression ratio of 6.0 to 1 and produced 450 hp (336 kW) at 1,850 rpm. The engine weighed 822 lb (373 kg). The 12Eb was first run in late 1922 or early 1923, and 30 test engines were built in 1923. The 12Eb quickly proved itself to be a successful engine. In March 1924, the 12Eb was the most economic engine at an endurance competition (Concours de Moteurs de Grande Endurance) held at Chalais-Meudon, near Paris. The engine operated for a total of 410 hours at 1,850 rpm. During that time, three cylinders were replaced due to water leaks.

Lorraine 12Eb museaum

A 12Eb engine with the magnetos driven from the front of the engine. Power from the magnetos was taken to the distributors, which were driven by the back of the left and right cylinder bank camshafts. (Pline image via Wikimedia Commons)

12Eb production started in late 1924, and approximately 150 engines were built in 1925. From 1924 to 1927, a number of licenses were purchased by other countries to manufacture the 12Eb: CASA and Elizalde in Spain; SCAT in Italy; FMA in Argentina; Hiro, Nakajima, and Aichi in Japan; PZL in Poland; Škoda and ČKD in Czechoslovakia; and IAR in Romania. The Blériot-SPAD S.61 fighter, the Breguet 19 light bomber, and the Potez 25TOE reconnaissance bomber were the 12Eb’s primary applications.

In 1925, a geared version of the 12Eb was developed, and it was designated 12Ed (sometimes referred to as 12Ebr). The planetary gear reduction turned the propeller at .647 times crankshaft speed. At 59.9 in (1.52 m), the 12Ed was 5.8 in (.15 m) longer than the direct-drive engine. Engine weight also increased 86 lb (39 kg) to 908 lb (412 kg). The 12Ed produced the same 450 hp (336 kW), but this was achieved at 1,900 engine rpm and 1,226 propeller rpm. The main application for the 12Ed was the CAMS 37 reconnaissance flying boat.

Lorraine 12Ed

The 12Ed engine with a propeller gear reduction was the same basic engine as the 12Eb. The early engines had a smooth gear reduction housing, but ribs were added later for extra strength.

The 12Ee debuted in 1926. This engine was basically a 12Eb with its compression ratio increased to 6.5 to 1. The 12Ee produced 480 hp (358 kW) at 2,000 rpm and had a maximum output of 510 hp (380 kW). The engine weighed 846 lb (383 kg). The 12E-series engines were used in the FBA-21 flying boat and Villiers IV seaplane to set numerous seaplane payload and distance records. Lorraine built around 5,500 E-series W-12 engines, and licensed production added another 1,775, for a total of approximately 7,275 engines. In all, the 12E-series engines were used in around 24 countries.

In December 1926, a Lorraine W-18 engine was displayed at the salon de l’Aviation in Paris. The 18-cylinder engine was designated 18K, and it was based on the E-series. The engine had been under development by Barbarou since at least 1923. The 18K had individual cylinders, rather than the paired units used on the E-series. The cylinder banks had an included angle of 40 degrees. Each of the cylinder banks had two carburetors, with each carburetor feeding three cylinders. Otherwise, the induction system was similar to that used on the 12E, including the two barrel carburetors on the left side of the engine for the left and center cylinder banks. The 18K had a compression ratio of 6.0 to 1, and its crankshaft was supported by seven main bearings.

The Lorraine 18K had the same 4.72 in (120 mm) bore and a 7.09 in (180 mm) stroke as the 12E-series engines. The W-18 engine displaced 2,236 cu in (36.6 L) and weighed around 1,287 lb (584 kg). The 18Kb was the direct drive variant that produced 650 hp (485 kW) at 2,000 rpm. The engine was 79.2 in (2.01 m) long, 36.2 in (.92 m) wide, and 43.3 in (1.10 m) tall.

Lorraine 18K

The 18K engine had the same construction as the 12E engines but used individual cylinders. Note that each carburetor fed two inductions pipes—one supplied the left cylinder bank and the other the center bank. The two one-piece magneto/distributor units are driven from the camshaft drive.

A version with a propeller gear reduction was designated 18Kd. The 18Kd turned the propeller at .647 times crankshaft speed and produced up to 785 hp (585 kW) at 2,500 rpm, but its continuous rating was the same as the 18Kb. With a total length of 83.5 in (2.12 m), the 18Kd was 4.3 in (109 mm) longer than the direct drive variant. The 18Kd weighed 1,365 lb (619 kg).

The 18Kd underwent official trials in mid-February 1927, and it was selected for the single-engine Amiot 122 bomber. The 18K may have been installed in other prototype aircraft, but the Amiot 122 was its only production application. A total of approximately 100 18Kb and 18Kd engines were made, and it was not considered a commercial success.

In 1928, Barbarou and Lorraine developed the third generation of W-12 engines, known as 12Fa Courlis. This was a reuse of the “12F” designation that was first applied in 1918. The F-series Courlis engines had a crankcase similar to that of the E-series, but the cylinder bank was a monobloc aluminum casting with enclosed valves. The steel cylinder liners were screwed into the cylinder banks, and the engine’s compression ratio was 6.0 to 1. Compared to the 12E, the cylinder bore diameter was increased, and the stroke length was decreased. Each cylinder had two intake and two exhaust valves, all actuated by a single overhead camshaft. The intake and exhaust ports were on the same side of the cylinder bank, and the carburetors mounted directly to the cylinder bank. The crankshaft was supported by five main bearings.

The Lorraine 12Fa Courlis had a 5.71 in (145 mm) bore and a 6.30 in (160 mm) stroke. The engine displaced 1,944 cu in (31.7 L) and produced 600 hp (447 kW) at 2,000 rpm. Sources indicate that the engine was capable of 765 hp (570 kW) at 2,400 rpm. Without gear reduction, the 12Fa Courlis was 62.2 in (1.66 m) long, 44.9 in (1.14 m) wide, 41.7 in (1.06 m) tall, and weighed 933 lb (423 kg). While the .647 propeller gear reduction did not increase the engine’s length by any noteworthy value, it did add 59 lb (27 kg), resulting in a weight of 992 lb (450 kg).

Lorraine 12Fa

With its enclosed valves and monobloc cylinder banks, the 12Fa Courlis was a modern engine design when it appeared in 1929. The gear reduction mounted to the crankcase in place of the direct-drive propeller shaft housing. The rest of the engine, including the crankshaft, was the same between the direct drive and geared variants.

The 12Fa Courlis was first run around 1928 and was tested by the Ministére de l’Air (French Air Ministry) from 10 to 17 June 1929. During the test, 52 hours were run at 2,000 rpm. In July 1929, the 12Fa made its public debut at the Olympia Aero Show in London. The French authorities officially approved the engine for service on 21 August 1929. The 12Fa was installed in a Potez 25 for engine development tests, which were conducted in 1930.

Developed in 1930, the 12Fb Courlis had a simplified induction system compared to the 12Fa. The 12Fb Courlis had a single, three-barrel carburetor mounted at the rear of the engine. Three separate intake manifolds extended from the carburetor, with one manifold connecting to each cylinder bank. The engine had cross-flow cylinder heads, with the exhaust ports on the side opposite of the intake ports. The 12Fb had the same basic specifications as the 12Fa, but fuel delivery issues initially reduced its rating to 500 hp (372 kW) at 1,900 rpm. However, continued development of the 12Fb soon brought its power up to 600 hp (447 kW) at 2,000 rpm, the same as the 12 Fa. Although installed in a few prototypes, the 12Fb did not power any production aircraft. By the early 1930s, air-cooled radial engines were increasing in popularity for transports and liquid-cooled V-12 engines for fighters. The Lorraine F-series Courlis did not find the success of the E-series. Around 30 F-series Courlis engines were built.

Lorraine 12Fb

The 12Fb had a simplified induction system with one carburetor and three intake manifolds. However, unequal fuel distribution was an issue.

Around 1932, an updated 12Eb was designed that incorporated some features from the 12F-series. Designated 12E Hibis, the engine used aluminum four-valve heads similar to those employed on the 12F engines. The Hibis had a 4.80 in (122 mm) bore and a 7.09 in (180 mm) stroke. The engine’s total displacement was 1,541 cu in (25.3 L), and it produced 500 hp (373 kW) at 2,000 rpm. While the engine was proposed around 1932, it is not clear if any were actually produced. The Hibis had disappeared by 1934.

In 1930, Barbarou created the 18-cylinder Lorraine 18Ga Orion. This W-18 engine combined the configuration of the 18K and the improved construction techniques of the F-series Courlis engines. The 18Ga had three monobloc cylinder banks set at 40 degrees. Each bank had six cylinders with a single overhead camshaft that operated the four valves per cylinder. The left and right cylinder banks had their intake and exhaust ports on their outer side. The carburetors were also mounted directly to the outer side of the cylinder bank. The center cylinder banks had a crossflow head with the carburetor and intake ports on the left side and the exhaust port on the right side. The crankshaft was supported by seven main bearings, and the engine had a .647 planetary gear reduction. It does not appear that there was a direct-drive variant.

Lorraine 18Ga

The 18Ga Orion combined the 18-cylinder 18K engine with the modern construction of the 12F-series. Note that the outer cylinder banks have intake and exhaust ports on the same side, while the center cylinder bank has intake and exhaust ports on opposite sides.

The 18Ga Orion had a 4.92 in (125 mm) bore and a 7.09 in (180 mm) stroke. The engine displaced 2,426 cu in (39.8 L) and produced 700 hp (522 kW) at 2,100 rpm and 870 hp (649 kW) at 2,500 rpm. The W-18 engine was 83.1 in (2.11 m) long, 36.6 in (.93 m) wide, and 43.7 in (1.11 m) tall. The engine weighed 1,252 lb (568 kg). The 18Ga completed a 50-hour type test prior to its public debut at the salon de l’Aviation in Paris in November 1930. The engine was used in at least one prototype aircraft, the Amiot 126 bomber. The 18Ga did not enter production, and only around 10 engines were built.

In November 1934, a supercharged version of the 18G Orion was displayed at the salon de l’Aviation in Paris. An updraft carburetor fed the gear-driven, centrifugal supercharger that was mounted to the rear of the engine. Three intake manifolds delivered the air and fuel mixture to the cylinder banks, just like the 12Fb engine. The revised cylinder banks included four valves per cylinder that were actuated by dual overhead camshafts. Each camshaft pair was driven by a vertical shaft at the rear of the engine. The supercharged 18G produced 1,050 hp (783 kW) at 2,150 rpm, but no additional specifications have been found.

A few 12E-series engines are preserved in various museum. No Lorraine F-series, 18-cylinder, or 24-cylinder engines are known to exist.

Lorraine 18G supercharged

The supercharged 18G Orion that was debuted in November 1934. Note the appearance of the new cylinder banks, which included four valves per cylinder.

Sources:
Lorraine-Dietrich by Sébastien Faurès Fustel de Coulanges (2017)
Aerosphere 1939 by Glenn D. Angle (1940)
Les Moteurs a Pistons Aeronautiques Francais Tome I by Alfred – Bodemer and Robert Laugier (1987)
Le moteur Lorraine 12 Eb de 450 ch by Gérard Hartmann (undated)
Moteur “Lorraine” 450 C.V. 12 Cylinders en W by Société Lorraine (circa 1925)
Les Moteurs Lorraine by Société Générale Aéronautique (circa 1932)
Moteur “Lorraine” 600 CV (Type 12 Fa.) by Société Lorraine (10 November 1929)

Pratt Whitney R-2060 Yellow Jacket

Pratt & Whitney R-2060 ‘Yellow Jacket’ 20-Cylinder Engine

By William Pearce

Around 1930, the United States Army Air Corps (AAC) was interested in a 1,000 hp (746 kW), liquid-cooled aircraft engine. Somehow, the AAC persuaded Pratt & Whitney (P&W) to develop an experimental engine at its own expense to meet this goal. The engine was the R-2060 Yellow Jacket, and it carried the P&W experimental engine designation X-31. The “Yellow Jacket” name followed the “Wasp” and “Hornet” engine lines from P&W.

Pratt Whitney R-2060 Yellow Jacket

The Pratt & Whitney R-2060 Yellow Jacket was an experimental liquid-cooled engine. Note the annular coolant manifold around the front of the engine that delivered water to the water pumps.

While the R-2060 would be P&W’s first liquid-cooled engine, the company had experimented with liquid-cooled cylinders as early as 1928. In addition, many of P&W’s engineers had experience with liquid-cooled engines while working for other organizations—in particular, those workers who had helped develop liquid-cooled engines at Wright Aeronautical.

The R-2060 had a one-piece, cast aluminum, barrel-type crankcase. Attached radially around the crankcase at 72-degree intervals were five cylinder banks. The lowest (No. 3) cylinder bank was inverted and hung straight down from the crankcase. Each cylinder bank consisted of four individual cylinders arranged in a line. This configuration created a 20-cylinder inline-radial engine. Attached to the front of the crankcase was a propeller gear housing that contained a planetary bevel reduction gear. Mounted to the rear of the crankcase was the supercharger and accessory section.

The crankshaft had four throws and was supported by five main bearings. Mounted to each crankpin was a master connecting rod with four articulated connecting rods—a typical arrangement found in radial engines. Each individual cylinder was surrounded by a steel water jacket. Mounted atop each bank of cylinders was a housing that concealed a single overhead camshaft. The camshaft actuated the one intake valve and one exhaust valve in each cylinder. Each camshaft was driven from the front of the engine by a vertical shaft and bevel gears. Driven from the rear of each camshafts was a magneto that fired the two spark plugs in each cylinder for that cylinder bank. The spark plugs were installed horizontally into the combustion chamber and placed on each exposed side of the cylinder. The camshaft housing on the lower cylinder bank was deeper and served as an oil sump.

Pratt Whitney R-2060 Yellow Jacket right

The 20-cylinder R-2060 was a fairly compact and light engine. Note the camshaft housings atop each cylinder bank and that the housing of the lower bank was deeper to serve as an oil sump. (Tom Fey image via the Aircraft Engine Historical Society)

Air was drawn into the downdraft carburetor mounted at the rear of the engine. Fuel was added, and the mixture then passed into the supercharger, which was primarily used to mix the air and fuel rather than provide boost. The air and fuel flowed from the supercharger through five outlets—one between each cylinder bank. The outlets were cast integral with the crankcase. Attached to each outlet was an intake manifold that branched into two sections, with each section branching further into two additional sections. The four pipes were then connected to the four cylinders of the cylinder bank. The exhaust ports were on the opposite side of the cylinder bank.

Cooling water flowed from the radiator into two inlets on an annular manifold mounted around the rear of the engine. The manifold had five outlets, one for each cylinder bank. Water flowed from the annular manifold into a pipe that ran along each cylinder bank. Branching off from the pipe were connections for each cylinder, with the mounting point near the exhaust port. The water passed by the exhaust port and through the water jacket, exiting near the intake port. The water from each cylinder was collected in another pipe that led to a smaller annular manifold mounted around the front of the engine. Two water pumps driven at the front of the engine took water from the front manifold and returned it to the radiator.

Pratt Whitney R-2060 Yellow Jacket left close

For each cylinder bank, the inlet for the intake manifold was cast into the crankcase. Note the water manifolds attached to the cylinders. The generator can be seen mounted on the left. (Tom Fey image via the Aircraft Engine Historical Society)

The Pratt & Whitney R-2060 Yellow Jacket had a 5.1875 in (132 mm) bore and a 4.875 in (124 mm) stroke. Creating an oversquare (bore larger than the stroke) engine was not typical for P&W and was repeated only with the R-2000, which was derived from the R-1830 with minimal changes. However, the comparatively short stroke helped decrease the engine’s diameter. The R-2060 displaced 2,061 cu in (33.8 L) and was projected to produce 1,500 hp (1,119 kW) at 3,300 rpm. The Yellow Jacket was 68 in (1.73 m) long and 47 in (1.19 m) in diameter. The engine weighed 1,400 lb (635 kg).

Serious design work on the R-2060 was started in March 1931, and single-cylinder testing began in August of the same year. The engine was first run in July 1932, and issues were soon encountered with oil circulation and coolant leaks. Throughout the rest of 1932, P&W worked to solve the oiling issues, control excessive oil consumption, prevent hot spots in various cylinder banks, and eliminate cracks in the cylinder water jackets. On one of its last tests, the R-2060 achieved 1,116 hp (820 kW) at 2,500 rpm, but reaching 1,500 hp (1,119 kW) at 3,300 rpm was beyond what the engine could handle. A major redesign of the engine was needed, and the Yellow Jacket project was subsequently cancelled in early 1933 after accumulating just 46 hours of test running. Only one R-2060 engine was built.

Cancellation of the R-2060 allowed P&W to focus on the development of the air-cooled, two-row, 14-cylinder R-1830 Twin Wasp radial engine. The R-1830 became the most produced aircraft engine of all time, with 173,618 examples built. The sole R-2060 Yellow Jacket was preserved and is part of Pratt & Whitney’s Hangar Museum in East Hartford, Connecticut.

Pratt Whitney R-2060 Yellow Jacket rear

Rear view of the R-2060 illustrates the engine’s carburetor and supercharger housing. The annular manifold around the rear of the engine supplied cooling water to the five cylinder banks. (Kimble D. McCutcheon image via the Aircraft Engine Historical Society)

Sources:
– The Liquid-Cooled Engines of Pratt & Whitney by Kimble D. McCutcheon (presentation at the 2006 Aircraft Engine Historical Society Convention)
Development of Aircraft Engines and Fuels by Robert Schlaifer and S. D. Heron (1950)
The Engines of Pratt & Whitney: A Technical History by Jack Connors (2009)

Farman 18T engine

Farman 18T 18-Cylinder Aircraft Engine

By William Pearce

The rules of the Schneider Trophy Contest stated that any country that won the contest three consecutive times would retain permanent possession of the trophy. By 1930, Britain had two consecutive victories and were favored to win the next contest scheduled for September 1931. Frenchman Jacques P. Schneider had started the contest, and France won the first competition held in 1913. The possibility of losing the contest forever spurred France to action, and the STIAé (service technique et industriel de l’aéronautique, or the Technical and Industrial Service of Aeronautics) ordered at least five aircraft types and three different engines for the 1931 contest. One of the engines ordered was the Farman 18T.

Farman 18T engine

The Farman 18T was specifically designed for installation in the Bernard flying boat. The unusual 18-cylinder engine had no other known applications.

Avions Farman (Farman) was founded in 1908 by brothers Richard, Henri, and Maurice. In October 1917, the company moved to produce engines built under license to support the war effort. The first of these engines was built in mid-1918, and production stopped after World War I. In 1922, Farman started to design their own line of engines under the direction of Charles-Raymond Waseige.

The Farman 18T was designed by Waseige and had an unusual layout. The water-cooled engine had three cylinder banks, each with six cylinders. The left and right cylinder banks were horizontally opposed, with a 180-degree flat angle across the engine’s top side. The lower cylinder extended below the crankcase and was perpendicular to the other cylinder banks. This configuration gave the 18-cylinder engine a T shape.

The engine used a two-piece cast aluminum crankcase that was split vertically. Steel cylinder liners were installed in the cast aluminum, monobloc cylinder banks that were bolted to the crankcase. The four valves of each cylinder were actuated via pairs of rockers by a single overhead camshaft. Each camshaft was driven by a vertical shaft at the rear of the engine.

The 18T used aluminum pistons and had a compression ratio of 6.0 to 1, although some sources say 8.5 to 1. The connecting rods consisted of a master rod for the lower cylinder bank and two articulated rods for the left and right cylinder banks. Each cylinder had two spark plugs, one installed in each side of the cylinder bank. The spark plugs were fired by magnetos driven from the rear of the engine. A nose case at the front of the engine contained the Farman-style bevel propeller reduction gear that turned the propeller at .384 crankshaft speed.

Farman 18T Paris Air Show 1932

The 18T (lower left) was proudly displayed as part of the Farman exhibit at the Salon de l’Aéronautique in November 1932. The other Farman engines are a 350 hp (261 kW) 12G (middle) and a 420 hp (313 kW) 12B (right).

For induction, air passed through carburetors at the rear of the engine and into a centrifugal supercharger that provided approximately 4.4 lb (.3 bar) of boost. The air/fuel mixture flowed from the supercharger into an intake manifold for each cylinder bank. The intake manifolds ran along the bottom of the cylinder bank for the left and right banks and along the right side (when viewed from the non-propeller end) of the lower cylinder bank. The exhaust ports were on the opposite side of the cylinder head from the intake.

The 18T had a 4.72 in (120 mm) bore and stroke. The engine displaced 1,491 cu in (24.4 L) and produced a maximum of 1,480 hp (1,104 kW) at 3,700 rpm. The 18T was rated at 1,200 hp (895 kW) at 3,400 rpm for continuous output. The engine was 65.98 in (1.68 m) long, 44.65 in (1.13 m) wide, 32.56 (.83 m) tall, and weighed 1,069 lb (485 kg).

Two Farman 18T engines were ordered under Contract (Marché) 289/0 (some sources state Marché 269/0) issued in 1930 and valued at 3,583,000 Ғ. The two engines were to power a flying boat built by the Société des avions Bernard (Bernard Aircraft Company). An official designation for the flying boat has not been found, and it was not among the known aircraft ordered for the 1931 Schneider Contest. There is some speculation that a lack of funds prevented the aircraft from being ordered for the 1931 race, but it would be ordered in time for the 1933 race.

Farman 18T Paris Air Show 1932 display

The display at the air show in Paris announced the 18T’s 1,200 hp (895 kW) continuous rating. Note that the supercharger housing extended above the crankcase, which was otherwise the engine’s highest point.

The design of the Bernard flying boat was led by Roger Robert and developed in coordination with the 18T engine. The all-metal aircraft had a low, two-step hull with sponsons protruding from the sides, just behind the cockpit. A long pylon above the cockpit extended along the aircraft’s spine, and the pylon supported the engine nacelle and wings. The engines were installed back-to-back in the middle of the nacelle. The engines’ lower cylinder banks extended into the pylon, and the left and right cylinder banks extended into the cantilever wings, which were mounted to the sides of the nacelle. Surface radiators for engine cooling covered the sides of the pylon, and extension shafts connected the propellers to the engines. The aircraft had a 36 ft 1 in (11.0 m) wingspan and was 35 ft 5 in (10.8 m) long. The engine nacelle was 17 ft 1 in (5.21 m) long. A 12.5 to 1 scale model of the flying boat was tested at the Laboratoire Aérodynamique Eiffel (Eiffel Aerodynamics Laboratory) in Auteuil (near Paris), France.

The 18T engines were bench tested in 1931, but the most power achieved was only 1,350 hp (1,007 kW). While further development was possible, at the time, the chance of France fielding a contestant in the 1931 Schneider Contest was virtually non-existent. The chances of the Bernard flying-boat being built were even worse. Although the aircraft had an estimated top speed of over 435 mph (700 km/h), and a detailed study was submitted to the Service Technique (Technical Service), the flying boat was seen as too radical and was never ordered. The limited funds were needed for the more conventional racers.

The Supermarine S.6B went on to win the 1931 Schneider Contest, giving the British permanent possession of the trophy. The 18T was marketed in 1932 and displayed at the Paris Salon de l’Aéronautique (Air Show) in November. However, there was little commercial interest in the 18T, and the project was brought to a close without the engine ever being flown; most likely, full testing was never completed.

Bernard - Farman 18T Schneider 3-view

Powered by two 18T engines, the Bernard flying boat racer had an estimated top speed of over 435 mph (700 km/h). This speed was substantially faster than the Supermarine S.6B that won the 1931 Schneider race at 340.08 mph (547.31 km/h) and went on to set an absolute speed record at 407.5 mph (655.8 km/h). However, the estimated specifications of unconventional aircraft often fall short of what is actually achieved.

Sources:
Aerosphere 1939 by Glenn D. Angle (1940)
Les Moteurs a Pistons Aeronautiques Francais Tome 1 by Alfred Bodemer and Robert Laugier (1987)
Schneider Trophy Seaplanes and Flying Boats by Ralph Pegram (2012)
Les Avions Bernard by Jean Liron (1990)
Les Avions Farman by Jean Liron (1984)

IAM M-44 sectional view

IAM M-44 V-12 Aircraft Engine

By William Pearce

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

Tupolev TB-6 6M-44 top

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

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

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

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

IAM M-44 sectional view

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

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

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

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

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

Tupolev TB-6 6M-44 side

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

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

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

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

Tupolev TB-6 12M-34FRN

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

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