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R-975 Whirlwind
A Continental-built R-975 from a Sherman tank

The Wright R-975 Whirlwind was a series of nine-cylinder air-cooled radial aircraft engines built by the Wright Aeronautical division of Curtiss-Wright. These engines had a displacement of about 975 in³ (16.0 L) and power ratings of 300-450 hp (225-335 kW). They were the largest members of the Wright Whirlwind engine family to be produced commercially, and they were also the most numerous.

During World War II, Continental Motors built the R-975 under license as a powerplant for Allied tanks and other armored vehicles. Tens of thousands of engines were built for this purpose, dwarfing the R-975's usage in aircraft. After the war, Continental continued to produce its own versions of the R-975 into the 1950s; some of these produced as much as 550 hp (410 kW).

Design and development

Wright introduced the J-6 Whirlwind family in 1928 to replace the nine-cylinder R-790 series. The J-6 family included varieties with five, seven, and nine cylinders. The nine-cylinder version was originally known as the J-6 Whirlwind Nine, or J-6-9 for short. The U.S. government designated it as the R-975; Wright later adopted this and dropped the J-6 nomenclature.

Like all the members of the J-6 Whirlwind family, the R-975 had larger cylinders than the R-790. The piston stroke of 5.5 in (14.0 cm) was unchanged, but the cylinder bore was expanded to 5.0 in (12.7 cm) from the R-790's bore of 4.5 in (11.4 cm). While the R-790 was naturally aspirated, the R-975, like the other J-6 engines, had a gear-driven supercharger to boost its power output.

Wright gradually developed the R-975, at first using suffix letters to indicate successive versions. The original R-975 (or J-6-9) was rated for 300 hp (224 kW),[1] while the R-975E of 1931 could do 330 hp (246 kW) thanks to an improved cylinder head design.[2][3][4] Wright later added numeric suffixes to show different power levels. The R-975E-1, introduced the same year as the R-975E, was rated at 365 hp (272 kW) thanks to higher-compression pistons and a slightly greater RPM limit.[2][5][6] An even more powerful version, the R-975E-3, was also introduced that year, with greater supercharging and a still higher RPM limit, and was progressively refined until the final model of 1935 could reach 450 hp (336 kW) for takeoff.[2][7][8]

Operational history

As the most powerful Whirlwind engine to be commercially produced, the R-975 also became the most popular. It was a powerplant for a variety of civil utility aircraft, such as the Beechcraft Staggerwing, and was also used for some early airliners, like the Ford Trimotor 4-AT-E and the Lockheed Electra 10B. In addition, it powered several U.S. military training aircraft, the North American BT-9 and Vultee BT-15 Valiant for the Army and the Curtiss-Wright SNC-1 Falcon for the Navy. It was even used in a fighter aircraft, the Curtiss F9C Sparrowhawk parasite fighter used on U.S. Navy airships.

However, the R-975 faced heavy competition from Pratt & Whitney's R-985 Wasp Junior and from their larger R-1340 Wasp. Pratt & Whitney sold many more Wasp Juniors for aircraft use than Wright sold R-975s.

Wright's production of the R-975 continued until 1945, with over 7000 engines being produced by the company.[2][9]

Production by Continental Motors

In 1939 the U.S. Army, which had already been using Continental R-670 radial engines in its light tanks, chose Continental Motors to build the R-975 under license as the engine for its M2 medium tanks. Subsequently, the same engine was selected for the M3 Lee medium tank, the M4 Sherman medium tank, the Canadian Ram tank, the M7 Priest self-propelled gun, the M18 Hellcat tank destroyer, and other Allied armored vehicles based on these. Continental versions of the R-975 for armored vehicles included the R-975E-C2, the R-975-C1, and the R-975-C4. Continental built over 53,000 R-975 engines for armored vehicles, far more than were ever built by Wright.[2]

When installed in a tank, the R-975 did not have the benefit of being cooled by an air slipstream or propeller blast, so a cooling fan was attached to the power shaft and surrounded by a shroud to provide the same effect.

After the war, Continental introduced its own R-975 version for aircraft, the R9-A. Though it was basically similar to other R-975 engines, and its compression ratio and supercharger gear ratio were unchanged from the R-975E-3, other improvements in the R9-A allowed it to achieve 525 hp (391 kW) for takeoff,[10] surpassing any Wright version. A military version, the R-975-46, could reach 550 hp (410 kW), and was used in Piasecki's HUP Retriever and H-25 Army Mule helicopters. Continental's production of R-975 engines continued into the 1950s.

Other license-built R-975s

The engine was built in Spain as the Hispano-Suiza 9Q or Hispano-Wright 9Q without modification apart from the use of Hispano's patented nitriding finishing process and, on one version only, the 9Qdr, an epicyclic output speed reducer.[11] The R-975 was also produced under licence by Fábrica Nacional de Motores in Brazil.


J-6-9 (R-975)
300 hp (224 kW) at 2,000 RPM.[1]
330 hp (246 kW) at 2,000 RPM. Higher power from improved cylinder head.[2][3][4]
365 hp (272 kW) at 2,100 RPM. Higher compression ratio.[2][5][6]
420 hp (313 kW) at 2,200 RPM up to 1,400 ft (427 m), 450 hp (336 kW) at 2,250 RPM for takeoff. Increased supercharging, slightly higher compression ratio.[2][7][8]
400 hp (298 kW) at 2,400 RPM. Built by Continental Motors under license. Designed for use in armored vehicles.[2][12]
Continental R9-A
500 hp (373 kW) at 2,300 RPM up to 4,000 ft (1,219 m), 525 hp (391 kW) at 2,300 RPM for takeoff. Continental's improved post-war version.[10]
Hispano-Suiza 9Q
Licence built R-975 J-6 Whirlwind
Hispano-Suiza 9Qa
variant of the Licence built R-975 J-6 Whirlwind
Hispano-Suiza 9Qb
variant of the Licence built R-975 J-6 Whirlwind
Hispano-Suiza 9Qd
variant of the Licence built R-975 J-6 Whirlwind
Hispano-Suiza 9Qdr
variant of the Licence built R-975 J-6 Whirlwind


Continental R-975

The engine of a Grant tank is removed for overhaul by the mechanics of a light recovery section of the Royal Army Ordnance Corps, 10 June 1942

Engines on display

Some museums which have R-975 engines on display:

Specifications (Whirlwind R-975E-3)

Data from FAA type certificate data sheet for the R-975E-3;[8] dimensions from Curtiss-Wright (1983).

General characteristics

  • Type: 9-cylinder supercharged air-cooled radial piston engine
  • Bore: 5.0 in (127 mm)
  • Stroke: 5.5 in (140 mm)
  • Displacement: 972 cu in (15.93 L)
  • Length: 43.0 in (109.2 cm)
  • Diameter: 45.0 in (114.3 cm)
  • Dry weight: 675 lb (306 kg)


  • Valvetrain: 2 valves per cylinder, pushrod-actuated
  • Supercharger: gear-driven, 10.15:1 impeller gear ratio
  • Fuel type: 80 octane
  • Reduction gear: Direct drive


  • Power output: 420 hp (313 kW) at 2,200 RPM up to 1,400 ft (427 m); 450 hp (336 kW) at 2,250 RPM for takeoff
  • Specific power: 0.43 hp/cu-in (19.7 kW/L)
  • Compression ratio: 6.3:1
  • Power-to-weight ratio: 0.62 hp/lb (1.02 kW/kg)
Specifications for different R-975 variants
Engine Power, continuous Power, takeoff Compression ratio Supercharger gear ratio Octane rating Dry weight
R-975E[4] 330 hp (246 kW) at 2,000 RPM 5.1:1 7.8:1 73 635 lb (288 kg)
R-975E-1[6] 365 hp (272 kW) at 2,100 RPM 6.1:1 7.8:1 73 660 lb (299 kg)
R-975E-3[8] 420 hp (313 kW) at 2,200 RPM 450 hp (336 kW) at 2,250 RPM 6.3:1 10.15:1 80 675 lb (306 kg)
Continental R9-A[10] 500 hp (373 kW) at 2,300 RPM 525 hp (391 kW) at 2,300 RPM 6.3:1 10.15:1 91 705 lb (320 kg)

See also



  1. 1.0 1.1 Curtiss-Wright (1940), p. 11
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Curtiss-Wright (1983), p. 2
  3. 3.0 3.1 Curtiss-Wright (1940), p. 13
  4. 4.0 4.1 4.2 FAA Type Certificate Data Sheet ATC 21
  5. 5.0 5.1 Curtiss-Wright (1940), p. 14
  6. 6.0 6.1 6.2 FAA Type Certificate Data Sheet TC 87
  7. 7.0 7.1 Curtiss-Wright (1940), pp. 14, 16, 18
  8. 8.0 8.1 8.2 8.3 FAA Type Certificate Data Sheet TC 125
  9. "Summary of Wright Engine Shipments: 1920 to 1963" (PDF). Retrieved December 15, 2009. . Transcribed from Wright Aeronautical documents by Robert J. Neal T; available from the Aircraft Engine Historical Society's reference page.
  10. 10.0 10.1 10.2 FAA Type Certificate Data Sheet E-245
  11. Lage (2004) pp152-163
  12. Curtiss-Wright (1940), p. 19
  13. "Wright R-975 Whirlwind 9". Retrieved 2009-12-25. .
  14. "Image Galleries by Gary and Janet Brossett: Pima Air and Space Museum in Tucson, Arizona". Retrieved 2009-12-15. . This page has a photo of a Wright R-975.
  15. "Aircraft Engines". Retrieved 2009-12-17. . This personal collection of museum aircraft engine photos includes a photo of the museum's R-975 under the section for the Hiller Museum.
  16. "Image Galleries by Gary and Janet Brossett: National Museum of Naval Aviation in Pensacola, Florida". Retrieved 2009-12-15. . This page has a photo of a Continental R-975-46A.
  17. "Continental R-975-46". Retrieved 2009-12-16. .
  18. "Queensland Air Museum: The Engine Collection". Retrieved 2009-12-15. .
  19. "Queensland Air Museum: Courtesy of Bruce Vander Mark". Retrieved 2009-12-15. . This page has a photo of a Continental R-975.


The following Federal Aviation Administration type certificate data sheets, all available from the FAA's Regulatory and Guidance Library:

External links

  • Engine Data Sheets: US Aero Engines — R-975 page

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