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R-985 Wasp Junior
Pratt & Whitney R-985 at thePima Air & Space Museum
Type	Air-cooled 9-cylinderradial 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
Manufactured	1929-1953
Number built	39,037
Developed from	Pratt & Whitney R-1340 Wasp
Developed into	Pratt & Whitney R-1535 Twin Wasp Junior

ThePratt & Whitney R-985 Wasp Junior is a series of nine-cylinder, air-cooled,radialaircraft engines built by thePratt & Whitney Aircraft Company from the 1930s to the 1950s. These engines have adisplacement 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 engines were built, and many are still in service today.

Pratt & Whitney developed the R-985 Wasp Junior as a smaller version of theR-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-speedcentrifugal type supercharger. Its cylinders were smaller, however, with abore andstroke of5+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.^[1] The first run of the Wasp Junior was in 1929,^[2] and sales began in 1930. The initial version, theWasp Junior A, produced 300 hp (224 kW).^[3][4]

The U.S. military designated the Wasp Junior as theR-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 highercompression ratios, greater RPM limits, and more effective supercharging, and they led to the "B series". The first B series model was theWasp Junior TB, which could maintain 420 hp (310 kW) at sea level and could reach 440 hp (330 kW) for takeoff.^[3][5] The TB was tuned for best performance at sea level; it was soon joined by theWasp Junior SB, which was tuned for best performance at altitude and could sustain 400 hp (300 kW) at altitudes up to 5,000 ft (1,500 m), with 450 hp (340 kW) available for takeoff.^[3][6] A still later model, theWasp Junior T1B2, had improved performance at low level, being able to sustain 450 hp (340 kW) up to 1,500 ft (460 m)^[3][6] while still matching the SB's power at high altitudes.^[7] 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, theWasp Junior B4, was especially designed for vertical mounting in helicopters.^[6][8]

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, theWasp Junior SC-G, could sustain 525 hp (391 kW) at an altitude of 9,500 ft (2,900 m) and could produce 600 hp (450 kW) for takeoff.^[8] It also included reduction gearing to allow the high-revving engine to drive a propeller at suitable speeds, hence the "-G" suffix. AviatorJacqueline Cochran flew a special Model D-17WBeechcraft Staggerwing with this engine in 1937, setting a speed and altitude record and placing third in theBendix transcontinental race. However, the SC-G never got past the experimental stage.

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 theLockheed Model 10A Electra twin-engined airliner, as well as for other small twin-engined civil transports like theLockheed Model 12A Electra Junior, theBeechcraft Model 18, and theGrumman Gooseamphibious aircraft. It was also used in single-engined civilian utility aircraft like theBeechcraft Staggerwing, theHoward DGA-15, and theSpartan Executive.

As World War II arrived, the U.S. military chose the Wasp Junior for theVultee BT-13 Valiant andNorth American BT-14basic training aircraft and for theVought OS2U Kingfisher observationfloatplane. 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 BritishAvro Anson andAirspeed 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 wasWright Aeronautical'sR-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 theSikorsky H-5 helicopter, thede Havilland Canada DHC-2 Beaver, andMax Holste Broussardbush airplanes, andagricultural aircraft such as theSnow S-2B and S-2C,Grumman Ag Cat, andWeatherley 201.

Pratt & Whitney ceased production of the Wasp Junior in 1953, having built 39,037 engines.^[2] 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 theBoeing-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.

Wasp Junior A

U.S. military version: R-985-1^[9]

300 hp (220 kW) at 2,000 RPM at sea level and for takeoff.^[3][4] First production version.

Wasp Junior S2A

Wasp Junior TB, TB2

U.S. military versions: R-985-9, -11, -11A, -21, -46^[9]

420 hp (310 kW) at 2,200 RPM at sea level, 440 hp (330 kW) at 2,300 RPM for takeoff.^[3][5] Early B-series versions rated for sea-level performance.

Wasp Junior SB, SB2, SB3

U.S. military versions: R-985-13, -17, -23, -33, -48, -50; R-985-AN-2, -4, -6, -6B, -8, -10, -12, -12B, -14B^[9][10]

400 hp (300 kW) at 2,200 RPM up to 5,000 ft (1,500 m), 450 hp (340 kW) at 2,300 RPM for takeoff.^[6][11] 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^[9][10]

450 hp (340 kW) at 2,300 RPM up to 1,500 ft (460 m) and for takeoff.^[3][6] Common B-series versions with improved sea-level performance

Wasp Junior B4

U.S. military versions: R-985-AN-5, -7.^[9][10]

450 hp (340 kW) at 2,300 RPM up to 2,300 ft (700 m) and for takeoff^[6][8] Vertically mounted development of T1B3, for helicopters

Wasp Junior SC-G

525 hp (391 kW) at 2,700 RPM up to 9,500 ft (2,900 m), 600 hp (450 kW) at 2,850 RPM for takeoff^[8] Experimental high-powered version with propeller reduction gearing.

Some museums which have Wasp Junior engines on display:

• TheNational Air and Space Museum'sSteven F. Udvar-Hazy Center nearWashington Dulles International Airport inVirginia: This engine has been sectioned and motorized for display.^[12]
• EAA AirVenture Museum inOshkosh, Wisconsin^[13]
• Hill Aerospace Museum nearOgden, Utah.^[14][15]
• Museum of Flight inSeattle, Washington^[16]
• National Museum of Naval Aviation nearPensacola, Florida^[17]
• National Museum of the United States Air Force nearDayton, Ohio^[18][19]
• New England Air Museum inWindsor Locks, Connecticut^[20]
• Pima Air & Space Museum inTucson, Arizona^[21][22]
• Southern Museum of Flight inBirmingham, Alabama.^[23]
• Strategic Air and Space Museum (formerly the Strategic Air Command Museum) nearAshland, Nebraska^[16]
• Canada Aviation and Space Museum inOttawa, Ontario^[16]
• Royal Air Force Museum Cosford, nearWolverhampton, United Kingdom^[24][25]
• Queensland Air Museum inCaloundra, Queensland.^[26]

Data fromFAAtype certificate data sheet for the Wasp Junior SB;^[6] dimensions from Pratt & Whitney (1956), p. A2.

• Type: 9-cylinder supercharged air-cooled radial piston engine
• Bore:5+3⁄16 in (132 mm)
• Stroke:5+3⁄16 in (132 mm)
• Displacement: 985 in³ (16.14 L)
• Length: 41.59 in (1,056 mm)
• Diameter: 45.75 in (1,162 mm)
• Dry weight: 640 lb (290 kg)

• Valvetrain: Twooverhead valves per cylinder, pushrod-actuated
• Supercharger: Single-speed gear-drivenGeneral Electriccentrifugal supercharger, withimpeller driven at 10 times crankshaft speed
• Fuel type: 80/87 gradeaviation gasoline
• Reduction gear: Direct drive

• Power output: 400 hp (300 kW) at 2,200 RPM up to 5,000 ft (1,500 m); 450 hp (340 kW) at 2,300 RPM for takeoff
• Specific power: 0.406 hp/in³ (18.5 kW/L)
• Compression ratio: 6.0:1
• Power-to-weight ratio: 0.625 hp/lb (1.03 kW/kg)

• Pratt & Whitney Wasp series

Related development

• Pratt & Whitney R-1340 Wasp
• Pratt & Whitney R-1535 Twin Wasp Junior

Comparable engines

• Alvis Leonides
• Bristol Jupiter
• Wright R-975 Whirlwind

Related lists

• List of aircraft engines

• Gunston, Bill.World Encyclopedia of Aero Engines: From the Pioneers to the Present Day. 5th edition, Stroud, UK: Sutton, 2006.ISBN 0-7509-4479-X
• White, Graham.Allied Aircraft Piston Engines of World War II: History and Development of Frontline Aircraft Piston Engines Produced by Great Britain and the United States During World War II. Warrendale, Pennsylvania: SAE International, 1995.ISBN 1-56091-655-9
• Pratt & Whitney (1956),Index of Wasp Jr. & R-985 Designated Engines(PDF), archived fromthe original(PDF) on October 31, 2010, retrievedDecember 21, 2009. Available from the Aircraft Engine Historical Society'sreference page.
• Tsygulev (1939).Aviacionnye motory voennykh vozdushnykh sil inostrannykh gosudarstv (Авиационные моторы военных воздушных сил иностранных государств) (in Russian).Moscow: Gosudarstvennoe voennoe izdatelstvo Narkomata Oborony Soyuza SSR. Archived fromthe original on 2009-03-24.

The followingFederal Aviation Administrationtype certificate data sheets, all available from the FAA'sRegulatory and Guidance LibraryArchived 2016-11-12 at theWayback Machine:

• Wasp Jr. A:Type Certificate Data Sheet ATC 39, archived fromthe original on February 4, 2017, retrievedDecember 21, 2009.
• Wasp Jr. SB, T1B2, B4:Type Certificate Data Sheet E-123, retrievedDecember 21, 2009.
• Wasp Jr. TB:Type Certificate Data Sheet TC 85, retrievedJanuary 7, 2010.
• Wasp Jr. military models:Type Certificate Data Sheet TC 5E-1, archived fromthe original on February 4, 2017, retrievedJanuary 7, 2010.

Wikimedia Commons has media related toPratt & Whitney R-985.

• "Radial Engines". Covington Aircraft. RetrievedOctober 26, 2019.

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• Aircraft air-cooled radial piston engines
• Pratt & Whitney aircraft engines

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