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.
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 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.
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. It also included reduction gearing to allow the high-revving engine to drive a propeller at suitable speeds, hence the "-G" suffix. Aviator Jacqueline Cochran flew a special Model D-17W Beechcraft Staggerwing with this engine in 1937, setting a speed and altitude record and placing third in the Bendix transcontinental race. However, the SC-G never got past the experimental stage.

Operational history

Early versions of the Wasp Junior were used in various small civilian and military utility aircraft, but only in limited numbers. The type became more popular later in the 1930s. It was selected for the Lockheed Model 10A Electra twin-engined airliner, as well as for other small twin-engined civil transports like the Lockheed Model 12A Electra Junior, the Beechcraft Model 18, and the Grumman Goose amphibious aircraft. It was also used in single-engined civilian utility aircraft like the Beechcraft Staggerwing, the Howard DGA-15, and the Spartan Executive.
As World War II arrived, the U.S. military chose the Wasp Junior for the Vultee BT-13 Valiant and North American BT-14 basic training aircraft and for the Vought OS2U Kingfisher observation floatplane. Military versions of existing Wasp Junior-powered civilian aircraft were also produced, such as the military derivatives of the Beech 18, Beech Staggerwing, Grumman Goose, and Howard DGA-15. The Wasp Junior also powered some versions of the British Avro Anson and Airspeed Oxford twin-engined trainers. The demands of World War II led to the production of many thousands of Wasp Juniors.
Until the end of the war, the Wasp Junior's closest competitor was Wright Aeronautical's R-975 Whirlwind. However, during the war, the Wasp Junior was far more widely used in aircraft than the R-975, and Wright ceased production of the R-975 in 1945.
After World War II, many military-surplus aircraft with Wasp Junior engines entered the civilian market. New designs based on the Wasp Junior were also introduced, such as the Sikorsky H-5 helicopter, the de Havilland Beaver, and Max Holste Broussard bush airplanes, and agricultural aircraft such as the Snow S-2B and S-2C, Grumman Ag Cat, and Weatherley 201.
Pratt & Whitney ceased production of the Wasp Junior in 1953, having built 39,037 engines. Many Wasp Junior engines are still in use today in older bush planes and agricultural planes, as well as in antique aircraft. Some antique aircraft, such as the Boeing-Stearman Model 75, which originally used other engines, have had them replaced with the Wasp Junior to provide more power or for easier maintenance, since parts for the Wasp Junior are readily available.

Variants

;Wasp Junior A
;Wasp Junior TB, TB2
;Wasp Junior SB, SB2, SB3
;Wasp Junior T1B2, T1B3
;Wasp Junior B4
;Wasp Junior SC-G

Applications

Some museums which have Wasp Junior engines on display:
EnginePower, continuousCritical altitudePower, takeoffCompression ratioSupercharger gear ratioOctane ratingDry weight
Wasp Jr. A at 2,000 RPMsea levelsame5.0:17:168
Wasp Jr. TB at 2,200 RPMsea level at 2,300 RPM6.0:18:180
Wasp Jr. SB at 2,200 RPM at 2,300 RPM6.0:110:180/87
Wasp Jr. T1B2 at 2,300 RPMsame6.0:110:180/87
Wasp Jr. B4 at 2,300 RPMsame6.0:110:180/87
Wasp Jr. SC-G at 2,700 RPM at 2,850 RPM6.7:110:1100