Lycoming had not been successful in designing a high-power engine. They had started with an attempt to make a hyper engine that led to the 1,200 hp (890 kW) O-1230; by the time the engine was ready, however, new aircraft designs were all calling for more power. They tried again by “twinning” the engine to produce the H block H-2470, which saw some interest in the Vultee XP-54 “Swoose Goose” project. Work on the H-2470 ended when the XP-54 was cancelled.
In one final attempt, Lycoming decided to go all out and build the largest engine in the world. They put together a team under the direction of VP of Engineering Clarence Wiegman at their main Williamsport factory in the summer of 1943 and started work.
The resulting design used 9 banks of 4 cylinders arranged around a central crankshaft with each cylinder bank at a 40 degrees angle to each adjacent bank, to form a four-row radial engine. Unlike most multi-row radials, which “spiral” the cylinders to allow cooling air to reach them, the R-7755 was water-cooled, and so each of the cylinder heads in a cylinder bank were in-line within a cooling jacket. Contrast this with the 24-cylinder Junkers Jumo 222, which looked similar from the outside, as both engines used banks of four cylinders each, but ran on a V-style cycle instead of a radial and only used six banks of cylinders. The XR-7755 was 10 ft (3 m) long, 5 ft (1.5 m) in diameter, and weighed 6,050 lb (2,740 kg). At full power it was to produce 5,000 hp (3,700 kW) at 2,600 rpm, maintaining that with a turbocharger to a critical altitude that was apparently never published.
Each cylinder bank had a single overhead cam powering the poppet valves. The camshaft included two sets of cams, one for full takeoff power, and another for economical cruise. The pilot could select between the two settings, which would shift the camshaft along its axis to bring the other set of cams over the valve stems. Interestingly, the design mounted some of the accessories on the “front side” of the camshafts, namely two magnetos and four distributors. The seventh camshaft was not used in this fashion, its location on the front of the engine was used to feed oil to the propeller reduction gearing.
The original XR-7755-1 design drove a single propeller, but even on the largest aircraft the propeller needed to absorb the power would have been ridiculously large. This led to a minor redesign that produced the XR-7755-3, using a new propeller gearing system driving a set of coaxial shafts to power a set of contra-rotating propellers. The propeller reduction gearing also had two speed settings to allow for a greater range of operating power than adjustable props alone could deliver. Another minor modification resulted in the XR-7755-5, the only change being the replacement of carburetors with a new fuel injection system.
The engine first started testing at 5,000 hp (3,700 kW) in 1944 with the XR-7755-3, but demonstrated terrible reliability problems. A second example was provided, as planned, to the United States Army Air Forces at Wright Field in 1946. However, by this time the Air Force had lost interest in new piston designs due to the introduction of jet engines, and the Lycoming delivery team was instructed to simply “dump it on the ground”. This engine has since disappeared. The original test engine was later delivered to the Smithsonian Institution, where it was restored.
Specifications:
Type: 36-cylinder turbosupercharged liquid-cooled “star” (9 banks at 40 deg angles, 4 cylinders in each bank) aircraft piston engine
Bore: 6 3⁄8 in (161.9 mm)
Stroke: 6 3⁄4 in (171.4 mm)
Displacement: 7,756.3 cu.in (127.1 L)
Length: 120 in (3,050 mm)
Diameter: 60 in (1,525 mm)
Dry weight: 6,050 lb (2,745 kg)
Valvetrain: Single overhead camshaft with separate cams for takeoff and economical cruise (Variable valve timing)
Cooling system: Liquid-cooled
Power output:
5,000 hp (3,730 kW) at 2,600 rpm takeoff
4,000 hp (2,985 kW) at 2,300 rpm cruise
Specific power: 0.64 hp/cu.in (29.3 kW/L)
Specific fuel consumption:
0.70 lb/(hp·h) (0.43 kg/(kW·h)) at takeoff power
0.485 lb/(hp·h) (0.29 kg/(kW·h)) at 70% power
0.37 lb/(hp·h) (0.22 kg/(kW·h)) at minimum cruise power
Power-to-weight ratio: 0.82 hp/lb (1.36 kW/kg)
