B-29D (XB-44)

Underpowered for its weight, the Wright powered B-29 could handle substantial power upgrades if available. The B-29D program, redesignated the XB-44, was installed with the new four-row 28-cylinder Pratt & Whitney R-4360 Wasp Major air-cooled radial engine producing 3500hp. The aircraft was easily distinguishable from the B-29 with these new engines. In 1945, the first orders for the aircraft were placed. Shortly thereafter the aircraft was redesignated the B-50A in an attempt to win appropriations for a new aircraft. It was shown that the changes made created essentially a new aircraft which spawed the B-50 program.

The XB-44 flew in May 1945 before the end of the Pacific War. The remarkable R-4360 Wasp Major was developed by Pratt & Whitney beginning in 1942, but culminated twenty years of piston engine development at the company. Designers in the mid-1930s developed Twin Wasp R-1830 by Pratt & Whitney (World’s most produced radial engine that propelled the Douglas DC-3/ C-47, the PBY Catalina, certain Helicopters and even Sherman Tank types) at max 1,200 hp. But 10 years later, there came this R-4360 with a max output of 3,500-4,300 hp. The cc/hp ratio went from 35-40 hp per liter of displacement to over 50-60 hp per liter. The R-4360 is a 28-cylinder, air-cooled radial engine that produces a maximum of 3,500 hp and weighs approximately 3,500 pounds (1,575 kg).

The new engines had 4 rows of 7 cylinders each, a total of 28 cylinders in the “corn cob” configuration. With 56 spark plugs per engine, a nightmare for a mechanic in case the plugs fouled. The huge 4360 cu in or 71,500 cc displacement per engine reflected also the very end of the development line for the ever bigger Piston aero engines.

For all its mass, this 28-cylinder, 4,360 cubic inch powerhouse was the lightest engine for its horsepower of any aircraft powerplant of its time. It weighed 3,405 pounds but produced 3,650 horsepower, a ratio of .93 pounds per horsepower. It was fuel-injected, gear-supercharged, and featured one crankshaft with four throws, each throw connecting seven cylinders to the crankshaft. One of each throw's pistons was a "master" piston, having the connecting rod form the crank bearing, maintaining alignment of the bearing with the crankcase. The other six piston connecting rods were connected to the master rod's bearing with pinned hinge connections. Each cylinder had two spark plugs and was offset to maximize cooling as air moved across the engine.

The R-4360-4 engine is an air-cooled engine having 28 cylinders arranged in four radial rows of seven cylinders each The rows are located about the crankcase in such a way as to form a right hand helix consisting of seven banks of four cylinders. The front row (propeller end) is designated as the 'T" row and the rear row (anti-propeller end) is designated the "A" row. The banks are numbered from 1 to 7, inclusive, counting clockwise from the rear of the engine, Cylinder number A-l is the top rear cylinder. Master rod locations are A—1, B-4, C-4, and C—1. Each bank of four cylinders is fired by an individual magneto firing both left and right plugs. Each magneto is mounted on the nose section directly in front of the bank that it fires.

The propeller shaft and reduction gearing are of the single rotation, single speed type, having an integral torqu-smeter for determining propeller shaft torrue. The supercharger is a single stage, variable speed type driven through hydraulic couplings which provide sufficient boost to maintain take-off power from sea level to 1500 feet altitude in maximum slip. The impeller has an inducer on the entrance side of the main impeller wheel for the purpose of pre—boosting and improving entrance flow characteristics. The variable speed hydraulic drive is of the dual drive split coupling type.

Engine oil is supplied to the couplings through a valve which meters the oil flow and reduces the pressure. The oil is then metered out centrifugally through holes in the periphery of the coupling housing until the outflow is equal to the input of oil. The amount of relative slip between the "driver" and "runner" halves of the coupling is dependent upon the oil level maintained in the coupling during rotation, which is, in turn, dependent upon the flow of oil regulated by the metering valve.

The supercharger revolved six times for each revolution of the crankshaft and the propeller shaft turned once for every two revolutions of the crankshaft. The revolutions of the propeller had to be slowed to avoid exceeding the speed of sound at the tips, which would have drastically reduced propeller efficiency. The R-4360 was a very reliable engine, but each flight usually required that aircraft mechanics re-tighten all engine mounting bolts!

R-4360s have been used to power various post-WWII USAF bombers, cargo/transports and aerial tankers, including the B-36 bomber, the B-35 Flying Wing, the C-74 Globemaster, the C-97 Stratofreighter, the Consolidated XC-99, the C-119 Flying Boxcar and the C-124 Globemaster II aircraft. It represents the most technically advanced and complex reciprocating aircraft engine produced in large numbers in the United States.