Wright R-3350

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R-3350 Duplex-Cyclone
Wright R-3350
Type	Radial engine
National origin	United States
Manufacturer	Wright Aeronautical
First run	May 1937
Major applications	A-1 Skyraider B-29 Superfortress Canadair CL-28 P-2 Neptune
Developed from	Wright R-1820

The Wright R-3350 Duplex-Cyclone was one of the most powerful radial aircraft engines produced in the United States. It was a twin row, supercharged, air-cooled, radial engine with 18 cylinders. Power ranged from 2,200 to over 3,700 hp (1,640 to 2,760 kW), depending on the model. First developed prior to World War II, the R-3350's design required a long time to mature before finally being used to power the B-29 Superfortress. After the war, the engine had matured sufficiently to become a major civilian airliner design, notably in its Turbo-Compound forms.

Design and development

In 1927, Wright Aeronautical introduced their famous Cyclone engine, which powered a number of designs in the 1930s. After merging with Curtiss to become Curtiss-Wright in 1929, an effort was started to redesign the engine to the 1,000 hp (750 kW) class. The new Wright R-1820 Cyclone 9 first ran successfully in 1935, and would become one of the most-used aircraft engines in the 1930s and WWII.

At about the same time Pratt & Whitney had started a development of their equally famous Wasp design into a larger and much more powerful two-row design that would easily compete with this larger Cyclone. In 1935 Wright decided to follow P&W's lead, and started to develop much larger engines based on the mechanicals of the Cyclone. The result were two designs with a somewhat shorter stroke, a 14 cylinder design that would evolve into the Wright R-2600, and a much larger 18 cylinder design that became the R-3350. An even larger 2-row 22 cylinder version, the R-4090, was experimented with as a competitor to the R-4360 but was not produced.

The first R-3350 was run in May 1937, but proved to be rather temperamental. Continued development was slow, both due to the complex nature of the engine, as well as the R-2600 receiving considerably more attention. The R-3350 didn't fly until 1941, after the prototype Douglas XB-19 had been re-designed from the Allison V-3420 to the R-3350.

Things changed dramatically in 1940 with the introduction of a new contract by the USAAC to develop a long-range bomber capable of flying from the US to Germany with a 2,000 lb (900 kg) bomb load. Although smaller than the Bomber D designs that led to the B-19, the new designs required roughly the same amount of power. When preliminary designs were returned in the summer of 1940, three of the four designs were based on the R-3350. Suddenly the engine was seen as the future of Army aviation, and serious efforts to get the design into production started.

Wright R-3350 Turbo-Compound radial engine. Two exhaust-recovery turbines shown outside impellor casing area (top (silver) and lower (red blading)) that are geared to the crankshaft.

By 1943 the ultimate development of the new bomber program, the B-29, was flying. However the engines remained temperamental, and showed an alarming tendency of the rear cylinders to overheat, partially due to excessive clearance between the cylinder baffles and the cowl. A number of changes were introduced into the aircraft production line in order to provide more cooling at low speeds, and the planes were rushed to operate in the Pacific in 1944. This proved unwise, as the overheating problems were not completely solved, and the engines had a tendency to swallow their own valves. Because of a high magnesium content in the crankcase alloy, the resulting engine fires were often so intense the main spar could burn through in seconds, resulting in catastrophic wing failure.^[1]

Early versions of the R-3350 were equipped with carburetors, though it was the poorly designed elbow, or entrance to the supercharger that led to serious problems with inconsistent fuel/air distribution. Near the end of World War II, in late 1944, the system was changed to use direct injection where fuel was injected directly into the combustion chamber. This change improved engine reliability immediately. After the war the engine was redesigned, and became a favorite for large aircraft of all designs, most notably the Lockheed Constellation and Douglas DC-7.

Following the war, in order to better serve the civilian market, the Turbo-Compound^[2] system was developed in order to deliver better fuel efficiency ("gas mileage"). In these versions of the engine, three power recovery turbines (PRT) were inserted into the exhaust piping of each group of six cylinders and geared to the engine crankshaft by fluid couplings in order to deliver more power (rather than by using the exhaust to deliver additional boost as in a turbocharger). The PRTs recovered about 20 percent of the exhaust energy (around 500 HP) that would have otherwise been wasted, but unfortunately had a negative effect on engine reliability, causing many aircraft mechanics of the day to nickname them "Parts Recovery Turbines" (and worse).^{[citation needed]}

By this point reliability had improved with the mean time between overhauls at 3,500 hours and specific fuel consumption in the order of 0.4 lb/hp/hour (243 g/kWh). Engines still in use are now limited to 52 inches of Hg manifold pressure, being 2,880 HP with 100/130 octane fuel (or 100LL) instead of the 59.5 inches and 3,400 HP possible with 115/145, or better, octane fuels, which are no longer available.

Models

• R-3350-13 : 2,200 hp (1,600 kW)
• R-3350-23 : 2,200 hp (1,600 kW)
• R-3350-24W : 2,500 hp (1,900 kW)
• R-3350-26W : 2,800 hp (2,100 kW)
• R-3350-32W : 3,700 hp (2,800 kW)
• R-3350-42WA : 3,800 hp (2,830 kW)
• R-3350-53 : 2,700 hp (2,000 kW)
• R-3350-85 : 2,500 hp (1,900 kW)
• R-3350-89A : 3,500 hp (2,600 kW)
• R-3350-93W : 3,500 hp (2,600 kW)

Applications

• Beechcraft XA-38 Grizzly
• Boeing B-29 Superfortress
• Boeing C-97 Stratofreighter
• Boeing XPBB Sea Ranger
• Canadair CL-28 Argus
• Consolidated B-32 Dominator
• Curtiss XBTC-2
• Curtiss XF14C
• Curtiss XP-62
• Douglas A-1 Skyraider
• Douglas BTD Destroyer
• Douglas DC-7
• Douglas XB-31
• Fairchild C-119 Flying Boxcar
• Fairchild AC-119
• Lockheed Constellation
• Lockheed P-2 Neptune
• Lockheed XB-30
• Martin JRM Mars
• Martin XB-33 Super Marauder
• Martin P5M Marlin
• Stroukoff YC-134

Specifications (R-3350-C18-BA)

A Wright R-3350 radial engine, showing, R to L, propeller shaft, reduction gearcase, magneto (silver) with wiring, two cylinders (rear with connecting rod), impellor casing (and induction pipe outlets) and injection carburetor (black); separate accessory gearbox at extreme left

Data from Jane's^[3]

General characteristics

• Type: Twin-row 18-cylinder radial engine
• Bore: 6.125 in (155.6 mm)
• Stroke: 6.312 in (160.2 mm)
• Displacement: 3,347 in³ (54.56 L)
• Length: 76.26 in (1,930 mm)
• Diameter: 55.78 in (1,420 mm)
• Dry weight: 2,670 lb (1,212 kg)

Components

• Valvetrain: Pushrod, two valves per cylinder
• Supercharger: Two-speed single-stage
• Fuel system: Chandler-Evans downdraft carburetor
• Fuel type: 100/130
• Oil system: Dry sump
• Cooling system: Air cooled

Performance

• Power output: 2,200 hp at 2,800 rpm (takeoff power)
• Specific power: 0.66 hp/in³
• Compression ratio: 6.85:1
• Power-to-weight ratio: 0.82 hp/lb

See also

Related development

• Wright Cyclone family
• Wright R-1300 Cyclone 7
• Wright R-1820 Cyclone 9
• Wright R-2600 Cyclone 14
  

Comparable engines

• BMW 802
• Bristol Centaurus
• Pratt & Whitney R-2800
• Nakajima Homare
• Shvetsov ASh-73
  

Related lists

• List of aircraft engines
  

References

Notes

Bibliography

External links

Wikimedia Commons has media related to: Wright R-3350

• Wright R-3350-57 Cyclone Fact Sheets from the National Museum of the USAF website