Wright R-3350 Duplex-Cyclone

The Wright R-3350 Duplex-Cyclone is an American twin-row, supercharged, air-cooled, radial aircraft engine with 18 cylinders displacing nearly 3350 in3. Power ranged from 2,200 to 3,700 hp, depending on model. Developed before World War II, the R-3350's design required a long time to mature, and was still experiencing problems with reliability when used to power the Boeing B-29 Superfortress.

After the war, the engine had matured sufficiently to be used in many civilian airliners, notably in its turbo-compound forms, and was used in the Lockheed L-1049 Super Constellation airliners into the 1950s. Its main rival was the 4360 in3, 4300 hp Pratt & Whitney R-4360 Wasp Major, which first ran some seven years after the Duplex-Cyclone. The engine is commonly used on Hawker Sea Fury and Grumman F8F Bearcat Unlimited Class Racers at the Reno Air Races.

Design and development
In 1927, Wright Aeronautical introduced its "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 design an engine in the 1000 hp class. The new Wright R-1820 Cyclone 9 first ran in 1935, and became one of the most used aircraft engines in the late 1930s and early 1940's, powering the Boeing B-17 Flying Fortress heavy bomber, General Motors FM-2 Wildcat fighter and Douglas SBD Dauntless dive bomber, among many others.

By 1931 Pratt & Whitney had started a development of their single-row Wasp nine-cylinder engine into the larger and much more powerful fourteen-cylinder, twin-row R-1830 Twin Wasp with a similar 30 L displacement that would easily compete with the single-row Cyclone. In 1935 Wright followed P&W's lead, and developed larger engines based on the Cyclone. The result was two designs, a 14-cylinder short stroke design of nearly 43 L displacement that would evolve into the Wright R-2600 Twin Cyclone, and a much larger 18-cylinder design that became the R-3350. A larger twin-row 22-cylinder version, the Wright R-4090 Cyclone 22, was experimented with as a competitor to the 4360 in3 displacement four-row, 28-cylinder Pratt & Whitney R-4360 Wasp Major, but was not produced.

With Pratt & Whitney starting development of their own 2800 in3 displacement 18-cylinder, twin-row radial as the R-2800 Double Wasp in 1937, Wright's first R-3350 prototype engines with a 3350 in3 displacement were run in May of the same year. Development was slow, due to the complexity, and the R-2600 receiving development priority. The R-3350 did not fly until 1941, after the Douglas XB-19 had been redesigned to use R-3350s instead of Allison V-3420 inlines.

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 20000 lb of bombs. Although smaller than the Bomber D designs that led to the Douglas XB-19, the new designs required just as much power. When four preliminary designs were presented in mid-1940, three of them used the R-3350. Suddenly development was a priority, and serious efforts to get it into production began. In 1942 Chrysler started building the Dodge Chicago Plant, which was ready by early 1944.



By 1943 the new Boeing B-29 Superfortress was flying with R-3350s. The engines remained temperamental, and the rear cylinders tended to overheat, partially due to inadequate clearance between the cylinder baffles and the cowl. A number of changes were introduced to improve cooling, and the aircraft was rushed into service in the Pacific in 1944. This proved unwise, as the early B-29s taking off at maximum weights, in the high temperature conditions of the B-29s tropical airfields, caused overheating that was not completely solved, and the engines also had a tendency to swallow valves. Because of a high magnesium content in the crankcase, engine fires could burn with a core temperature approaching 5600 F which could burn through the main spar in seconds, causing a catastrophic failure.

Early R-3350s used carburetors, though the poorly designed elbow entrance to the supercharger led to serious problems with fuel/air mixtures. Near the end of WWII, the system was changed to use gasoline direct injection which improved reliability. After the war the engine was redesigned and became popular for large aircraft, notably the Lockheed Constellation and Douglas DC-7.

Following the war, the Turbo-Compound system was developed to deliver better fuel efficiency. In these versions, three power-recovery turbines (PRT) were inserted into the exhaust of each group of six cylinders, and geared to the crankshaft by fluid couplings to deliver more power. The PRTs recovered about 20% of the exhaust energy (around 450 hp) that would have otherwise been lost, but reduced engine reliability. Mechanics nicknamed them Parts Recovery Turbines, since the increased exhaust heat meant a return to the engine destroying exhaust valves. The fuel burn for the PRT-equipped aircraft was nearly the same as the older Pratt and Whitney R-2800, while producing more useful power. Effective 15 October 1957 a DA-3/DA-4 engine cost $88,200.

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, giving a 34% fuel efficiency). Engines in use as of the 2020s are limited to 52 inHg manifold pressure, giving 2880 hp with 100/130 octane fuel (or 100LL) instead of the 59.5 inHg and 3400 hp possible with 115/145, a higher octane fuel, which is no longer available.

Several racers at the Reno Air Races use R-3350s. Modifications on one, Rare Bear, include a nose case designed for a slow-turning prop, taken from an R-3350 used on the Lockheed L-1649 Starliner, mated to the power section (crankcase, crank, pistons, and cylinders) taken from an R-3350 used on the Douglas DC-7. The supercharger is taken from an R-3350 used on the Lockheed EC-121 and the engine is fitted with nitrous oxide injection. Normal rated power of the original stock R-3350 was 2800 hp at 2,600 rpm and 45 inHg of manifold pressure. With these modifications, Rare Bear's engine produces 4000 hp at 3,200 rpm and 80 inHg of manifold pressure, and 4500 hp with nitrous oxide injection.

Variants



 * R-3350-13: 2,200 shp
 * R-3350-23: 2,200 shp
 * R-3350-24W: 2,500 shp
 * R-3350-26W: 2,800 shp
 * R-3350-30W:
 * R-3350-30WA:
 * R-3350-32W: 3,700 shp
 * R-3350-34: 3,400 shp
 * R-3350-35A: 2,200 shp
 * R-3350-41: Fuel injected Silverplate variant
 * R-3350-42WA: 3,800 shp
 * R-3350-53: 2,700 shp
 * R-3350-57: 2,200 shp
 * R-3350-85: 2,500 shp
 * R-3350-89A: 3,500 shp
 * R-3350-93W: 3500 shp
 * 972TC18DA1:Commercial equivalent to the -30W without water injection
 * 956C18CA1:Commercial, similar to the -26W
 * 975C18CB1:Commercial, similar to the 956C18CA1

Applications
• Beechcraft XA-38 Grizzly

• Boeing B-29 Superfortress

• Boeing XC-97 Stratofreighter

• Boeing XPBB Sea Ranger

• Canadair CP-107 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-19

• Douglas XB-31

• Fairchild C-119 Flying Boxcar

• Fairchild AC-119

• Grumman F8F Bearcat (See the Rare Bear)

• Hawker Sea Fury

• Lockheed Constellation

• Lockheed L-049 Constellation

• Lockheed C-69 Constellation

• Lockheed L-649 Constellation

• Lockheed L-749 Constellation

• Lockheed L-1049 Super Constellation

• Lockheed C-121 Constellation

• Lockheed R7V-1 Constellation

• Lockheed EC-121 Warning Star

• Lockheed L-1649A Starliner

• Lockheed P-2 Neptune

• Lockheed XB-30

• Martin JRM Mars

• Martin XB-33 Super Marauder

• Martin P5M Marlin

• Stroukoff YC-134

Engines on display

 * Wright R-3350-89 is on public display at the Aerospace Museum of California
 * Wright R-3350 is on public display at Flyhistorisk Museum, Sola, near Stavanger, Norway
 * Wright R-3350-35A is on public display at Texas Air Museum - Stinson Chapter, San Antonio, Texas
 * Wright R-3350 is on public display in the Mackenzie Engineering Building at Carleton University, Ottawa, Ontario, Canada