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Pratt & Whitney R-985 Wasp Junior

Pratt & Whitney R-985 AN-1


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 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 civil 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.

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 525 hp (391 kW) at an altitude of 9,500 ft (2,900 m) and could produce 600 hp (447 kW) 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.

Early versions of the Wasp Junior were used in various small civil 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 amphibian. It was also used in single-engined civil 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 civil 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-engine trainers. The demands of World War II led to the production of many thousands of Wasp Juniors.

Up 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 civil market. Production of the Beech 18 with the Wasp Junior engine continued until 1970. 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 like 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.


Air Tractor AT-300
Avro Anson (Mk V)
Barkley-Grow T8P-1
Beechcraft Model 18 and military derivatives
Beechcraft Staggerwing D17S, D17W, G17S
Bell XV-3
Bellanca 300-W
Berliner-Joyce OJ
Boeing-Stearman Model 75 (in aftermarket conversions)
Bratukhin G-3
CAC Winjeel
de Havilland Canada DHC-2 Beaver and L-20/U-6 military versions
Douglas C-26 Dolphin
Fleetwings BT-12
Grumman G-164 Ag Cat (some models)
Grumman G-21 Goose
Howard DGA-11
Howard DGA-15P
 Koolhoven F.K.51 (some models)
Lockheed Model 10-A Electra
Lockheed Model 12-A Electra Junior
Max Holste MH.1521 Broussard
McDonnell XHJH Whirlaway
North American BT-14
Seversky BT-8
Sikorsky H-5 helicopter (and S-51 civil version)
Sikorsky S-39 amphibian
Snow S-2B and S-2C
Spartan Executive 7W
Stinson Reliant SR-9F and SR-10F
Vought OS2U Kingfisher
Vultee BT-13 Valiant
Waco S3HD
Waco SRE Aristocrat
Weatherly 201 series
Weatherly 620

Wasp Jr. A
R-985-1 U.S. military version
First production version.
Power, continuous: 300 hp (224 kW) at 2,000 RPM sea level
Power, takeoff: 300 hp (224 kW) at 2,000 RPM
Compression ratio: 5.0:1
Supercharger gear ratio: 7:1
Octane rating: 68
Dry weight: 565 lb (256 kg)


Wasp Jr. TB, TB2
R-985-9, -11, -11A, -21, -46 U.S. military versions
Early B-series versions rated for sea-level performance.
Power, continuous: 420 hp (313 kW) at 2,200 RPM sea level
Power, takeoff: 440 hp (328 kW) at 2,300 RPM
Compression ratio: 6.0:1
Supercharger gear ratio: 8:1
Octane rating: 80
Dry weight: 640 lb (290 kg)


Wasp Jr. SB, SB2, SB3
R-985-13, -17, -23, -33, -48, -50; R-985-AN-2, -4, -6, -6B, -8, -10, -12, -12B, -14B U.S. military versions
Common B-series versions rated for performance at altitude.
Type: 9-cylinder supercharged air-cooled radial piston engine
Power, continuous: 400 hp (298 kW) at 2,200 RPM 5,000 ft (1,500 m)
Power, takeoff: 450 hp (336 kW) at 2,300 RPM
Bore: 5 3⁄16 in (132 mm)
Stroke: 5 3⁄16 in (132 mm)
Displacement: 985 cu in (16.14 L)
Length: 41.59 in (1,056 mm)
Diameter: 45.75 in (1,162 mm)
Compression ratio: 6.0:1
Supercharger: Single-speed gear-driven General Electric centrifugal supercharger, with impeller driven at 10 times crankshaft speed
Octane rating: 80/87 aviation gasoline
Dry weight: 640 lb (290 kg)
ComponentsValvetrain: Two overhead valves per cylinder, pushrod-actuated
Reduction gear: Direct drive
Specific power: 0.406 hp/ (18.5 kW/lt)
Power-to-weight ratio: 0.625 hp/lb (1.03 kW/kg)


Wasp Jr. T1B2, T1B3
R-985-25, -27, -39, -39A; R-985-AN-1, -1A, -3, -3A U.S. military versions
Common B-series versions with improved sea-level performance.
Power, continuous: 450 hp (336 kW) at 2,300 RPM 1,500 ft (460 m)
Power, takeoff: 450 hp (336 kW) at 2,300 RPM
Compression ratio: 6.0:1
Supercharger gear ratio: 10:1
Octane rating: 80/87
Dry weight: 653 lb (296 kg)


Wasp Jr. B4
R-985-AN-5, -7 U.S. military versions
Vertically mounted development of T1B3, for helicopters.
Power, continuous: 450 hp (336 kW) at 2,300 RPM 2,300 ft (700 m)
Power, takeoff: 450 hp (336 kW) at 2,300 RPM
Compression ratio: 6.0:1
Supercharger gear ratio: 10:1
Octane rating: 80/87
Dry weight: 684 lb (310 kg)


Wasp Jr. SC-G
Experimental high-powered version with propeller reduction gearing.
Power, continuous: 525 hp (391 kW) at 2,700 RPM 9,500 ft (2,900 m)
Power, takeoff: 600 hp (447 kW) at 2,850 RPM
Compression ratio: 6.7:1
Supercharger gear ratio: 10:1
Octane rating: 100
Dry weight: 864 lb (392 kg)






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