Pratt & Whitney R-4360 Wasp Major
Starting in 1940, and production begining in 1945, just too late for the Second World War, the engineers at P&W were tasked with developing a 3,000 h.p.-plus engine. At the beginning of the R-4360's development, state-of-the-art engines were struggling to achieve 2,000 h.p. P&W decided on air cooling and, after a number of variations and permutations of cylinder arrangement had been investigated, the final concept, which went into production, was four rows of seven cylinders, giving a total of 28.
Cooling high-performance air-cooled engines was always a challenge, for the R-4360. Each row of pistons was slightly offset from the previous, forming a semi-helical arrangement to facilitate efficient airflow cooling of the successive rows of cylinders, with the spiraled cylinder setup inspiring the engine's "corncob" nickname. Seven plenums, one between each cylinder bank, created the necessary cooling air path. A complex tight baffling system ensured that cooling air was forced through the cylinders in a quasi-cross-flow pattern. To ensure an unobstructed path, intake manifolds were routed over the top of the cylinders, terminating in a downdraught flow into the hemispherical combustion chamber.
A mechanical supercharger geared at 6.374:1 ratio to engine speed provided forced induction, while the propeller was geared at 0.375:1 so that the tips did not reach inefficient supersonic speeds. General Electric (GE) had designed most previous P&W superchargers, but this time P&W decided to do it in-house. According to former R-4360 engineers, P&W's supercharger was more efficient than GE's. Most superchargers were single-stage, with variable speed or single speed. Additionally, most R-4360 applications were augmented by GE turbosuperchargers with intercooling, A two-stage gear-driven supercharger was also developed.
Inevitable teething problems arose when the R-4360 entered service, such as frying the ignition system, intake manifold problems and, perhaps most seriously, poor oil scavenging owing to aeration of the oil. The disarmingly simple solution for the last of these problems was to incorporate perforated sheet-metal plates in the rear housing, which took out much of the entrapped air.
Although reliable in flight, the Wasp Major was maintenance-intensive. Improper starting technique could foul all 56 spark plugs, which would require hours to clean or replace. As with most piston aircraft engines of the era, the time between overhauls of the Wasp Major was about 600 hours when used in commercial service.
Engine displacement was 4,362.50 cu.in (71.5 lt), hence the model designation. Initial models developed 3,000 hp (2,240 kW), and later models 3,500 hp, but one model delivered 4,300 hp (3200 kW) using two large turbochargers in addition to the supercharger. Engines weighed 3,482 to 3,870 lb (1,579 to 1,755 kg), giving a power-to-weight ratio of 1.11 hp/lb (1.83 kW/kg), which was matched or exceeded by very few contemporary engines.
Designed for military use, the R-4360 also saw commercial use as the "Wasp Major”. The R-4360 was used for a number of applications, including commercial aviation, military aircraft and air racing. The R-4360-8 powered the Douglas XTB2D-1 Skypirate, the contra-rotating propeller shafts each drove a Hamilton Standard four-bladed propeller. The Skypirate was cancelled after one prototype.
Wasp Majors were produced between 1944 and 1955; 18,697 were built.
A derivative engine, the Pratt & Whitney R-2180-E Twin Wasp E, was essentially the R-4360 "cut in half". It had two rows of seven cylinders each, and was used on the postwar Saab 90 Scandia airliner.