Pratt & Whitney R-985 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 ; initial versions produced , while the most widely used versions produce .

Wasp Juniors have powered numerous smaller civil and military aircraft, including small transports, utility aircraft, trainers, agricultural aircraft, and helicopters. Over 39,000 engines were built, and many are still in service today.

Design and development

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 , 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 .

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 at sea level and could reach 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 at altitudes up to , with available for takeoff. A still later model, the Wasp Junior T1B2, had improved performance at low level, being able to sustain up to 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.

During the mid-1930s, Pratt & Whitney developed a still greater improvement of the Wasp Junior, the "C series", with an even higher compression ratio and RPM limit. The only type produced in this series, the Wasp Junior SC-G, could sustain at an altitude of and could produce for takeoff.

: at 2,000 RPM at sea level and for takeoff.

: at 2,200 RPM up to , at 2,300 RPM for takeoff. Common B-series versions were rated for performance at altitude.

;Wasp Junior T1B2, T1B3

:U.S. military versions: R-985-25, -27, -39, -39A; R-985-AN-1, -1A, -3, -3A

EAA AirVenture Museum in Oshkosh, Wisconsin
Hill Aerospace Museum near Ogden, Utah.
Museum of Flight in Seattle, Washington
National Museum of Naval Aviation near Pensacola, Florida
National Museum of the United States Air Force near Dayton, Ohio
New England Air Museum in Windsor Locks, Connecticut
Pima Air & Space Museum in Tucson, Arizona
Southern Museum of Flight in Birmingham, Alabama.
Strategic Air and Space Museum (formerly the Strategic Air Command Museum) near Ashland, Nebraska
Queensland Air Museum in Caloundra, Queensland.