Pratt & Whitney Wasp Junior
| R-985 Wasp Junior | |
|---|---|
 |
|
| Pratt & Whitney R-985 AN-1 mounted in a Boeing Stearman | |
| Type | Air-cooled 9-cylinder radial piston engine |
| National origin | United States |
| Manufacturer | Pratt & Whitney |
| First run | 1929 |
| Major applications |
Beechcraft Model 17 Beechcraft Model 18 de Havilland Canada DHC-2 Beaver Grumman G-21 Goose Sikorsky H-5 Vought OS2U Kingfisher Vultee BT-13 Valiant |
| Produced | 1929-1953 |
| Number built | 39,037 |
| Developed from | Pratt & Whitney R-1340 Wasp |
| Developed into | Pratt & Whitney R-1535 Twin Wasp Junior |
The Pratt & Whitney R-985 Wasp Junior is a series of nine-cylinder, air-cooled, radial aircraft engines built by the Pratt & Whitney Aircraft Company from the 1930s to the 1950s. These engines have a displacement of 985 in3 (16 l); initial versions produced 300 hp (220 kW), while the most widely used versions produce 450 hp (340 kW).
Wasp Juniors have powered numerous smaller civil and military aircraft, including small transports, utility aircraft, trainers, agricultural aircraft, and helicopters. Over 39,000 of these engines were built, and many are still in service today.
Pratt & Whitney developed the R-985 Wasp Junior as a smaller version of the R-1340 Wasp to compete in the market for medium-sized aircraft engines. Like its larger brother, the Wasp Junior was an air-cooled, nine-cylinder radial, with its power boosted by a gear-driven single-speed centrifugal type supercharger. Its cylinders were smaller, however, with a bore and stroke of 5 3⁄16 in (132 mm), giving a 27% lesser total displacement. The Wasp Junior used many parts from the Wasp and even had the same mounting dimensions, allowing an aircraft to easily use either the smaller or the larger engine. The first run of the Wasp Junior was in 1929, and sales began in 1930. The initial version, the Wasp Junior A, produced 300 hp (224 kW).
The U.S. military designated the Wasp Junior as the R-985, with various suffixes denoting different military engine models. However, Pratt & Whitney never adopted the R-985 designation scheme for its civilian Wasp Juniors, identifying them simply by name and model (e.g. "Wasp Junior A").
Pratt & Whitney followed the Wasp Junior A with more powerful models in the "A series". These had higher compression ratios, greater RPM limits, and more effective supercharging, and they led to the "B series". The first B series model was the Wasp Junior TB, which could maintain 420 hp (313 kW) at sea level and could reach 440 hp (328 kW) for takeoff. The TB was tuned for best performance at sea level; it was soon joined by the Wasp Junior SB, which was tuned for best performance at altitude and could sustain 400 hp (298 kW) at altitudes up to 5,000 ft (1,500 m), with 450 hp (336 kW) available for takeoff. A still later model, the Wasp Junior T1B2, had improved performance at low level, being able to sustain 450 hp (336 kW) up to 1,500 ft (460 m) while still matching the SB's power at high altitudes. The SB and T1B2, and later versions of these with similar performance, were the most popular Wasp Junior models. One later development of the T1B2, the Wasp Junior B4, was especially designed for vertical mounting in helicopters.
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