Wright R-975 Whirlwind

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.

R-975 Whirlwind
A Continental-built R-975 from a Sherman tank
Type Air-cooled 9-cylinder radial piston engine
National origin United States
Manufacturer Wright Aeronautical
Continental Motors
Major applications Beechcraft Staggerwing
North American BT-9
Vultee BT-15 Valiant
M4 Sherman tank
Piasecki HUP Retriever
M18 Hellcat
Produced 1929-1950s
Number built over 7,000 by Wright
over 53,000 by Continental

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).

The R-975 is most famous for being used as the power plant for the M18 Hellcat tank destroyer, the fastest and most efficient American tank killer of World War II. The R-975 powered M18 remained the fastest tracked armored vehicle in the world until the introduction of the turbine powered M1 Abrams in the 1980s.[1]

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),[2] while the R-975E of 1931 could do 330 hp (246 kW) thanks to an improved cylinder head design.[3][4][5] 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.[3][6][7] 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.[3][8][9]

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.

One notable record set by a Wright J-6 Whirlwind-powered aircraft occurred on July 28–30, 1931, when Russell Norton Boardman (age 33) and John Louis Polando (age 29) flew from Floyd Bennett Field, usually abbreviated as "FBF" — a famous New York City-area early airport on western Long Island from which many record flights originated — to Istanbul, Turkey aboard a Bellanca Special J-300 high-wing monoplane named Cape Cod, registration NR761W, making it safely to Istanbul nonstop in 49:20 hours, establishing a distance record of 5,011.8 miles (8,065.7 km), the first known nonstop record flight in aviation history whose distance surpassed either English (5,000 mi) or metric (8,000 km) mark. As the runway at FBF was just 5,000 feet (1,500 m) long, Boardman and Polando needed to have a fence removed and clear a parking lot to add another thousand feet to meet their required takeoff distance. The phone and electric utilities even took down poles along Flatbush Avenue.[10]

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.[3][11]

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.[3]

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,[12] 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.[13] The R-975 was also produced under licence by Fábrica Nacional de Motores in Brazil.[14][15]


J-6-9 (R-975)
300 hp (220 kW) at 2,000 RPM.[2]
300 hp (220 kW) for airship use
330 hp (250 kW) at 2,000 RPM. Higher power from improved cylinder head.[3][4][5]
365 hp (272 kW) at 2,100 RPM. Higher compression ratio.[3][6][7]
420 hp (310 kW) at 2,200 RPM up to 1,400 ft (430 m), 450 hp (340 kW) at 2,250 RPM for takeoff. Increased supercharging, slightly higher compression ratio.[3][8][9]
400 hp (300 kW) at 2,400 RPM. Built by Continental Motors under license. Designed for use in armored vehicles.[3][16]
Continental R9-A
500 hp (370 kW) at 2,300 RPM up to 4,000 ft (1,200 m), 525 hp (391 kW) at 2,300 RPM for takeoff. Continental's improved post-war version.[12]
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 9Qc
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

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;[9] 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)



Specifications for different R-975 variants
EnginePower, continuousPower, takeoffCompression ratioSupercharger gear ratioOctane ratingDry weight
R-975E[5] 330 hp (246 kW) at 2,000 RPM5.1:17.8:173635 lb (288 kg)
R-975E-1[7] 365 hp (272 kW) at 2,100 RPM6.1:17.8:173660 lb (299 kg)
R-975E-3[9] 420 hp (313 kW) at 2,200 RPM450 hp (336 kW) at 2,250 RPM6.3:110.15:180675 lb (306 kg)
Continental R9-A[12] 500 hp (373 kW) at 2,300 RPM525 hp (391 kW) at 2,300 RPM6.3:110.15:191705 lb (320 kg)

See also

Related development

Comparable engines

Related lists



  1. Patton's Vanguard: The United States Army Fourth Armored Division, By Don M. Fox. P.25
  2. Curtiss-Wright (1940), p. 11
  3. Curtiss-Wright (1983), p. 2
  4. Curtiss-Wright (1940), p. 13
  5. FAA Type Certificate Data Sheet ATC 21
  6. Curtiss-Wright (1940), p. 14
  7. FAA Type Certificate Data Sheet TC 87
  8. Curtiss-Wright (1940), pp. 14, 16, 18
  9. FAA Type Certificate Data Sheet TC 125
  10. "'Cape Cod's' Success Climaxes 5 Years [of] Bellanca Records". The Sunday Morning Star, Wilmington, DE. August 2, 1931. Retrieved September 4, 2013.
  11. 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 Archived 2010-02-04 at the Wayback Machine.
  12. FAA Type Certificate Data Sheet E-245
  13. Lage (2004) pp152-163
  14. "O bom caminhão a casa torna". Jornal O Globo (in Portuguese). March 21, 2013. Retrieved October 1, 2018.
  15. "Opinião Leandro Sauerbronn: A história da Fábrica Nacional de Motores". pleno.news (in Portuguese). Retrieved October 1, 2018.
  16. Curtiss-Wright (1940), p. 19
  17. Wright R-975 Whirlwind 9, retrieved 2009-12-25.
  18. Image Galleries by Gary and Janet Brossett: Pima Air and Space Museum in Tucson, Arizona, archived from the original on 2009-08-27, retrieved 2009-12-15. This page has a photo of a Wright R-975.
  19. 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.
  20. Image Galleries by Gary and Janet Brossett: National Museum of Naval Aviation in Pensacola, Florida, archived from the original on 2009-08-27, retrieved 2009-12-15. This page has a photo of a Continental R-975-46A.
  21. Continental R-975-46, retrieved 2009-12-16.
  22. Queensland Air Museum: The Engine Collection, archived from the original on 2009-10-04, retrieved 2009-12-15.
  23. Queensland Air Museum: Courtesy of Bruce Vander Mark, retrieved 2009-12-15. This page has a photo of a Continental R-975.
  24. Patrick Dillon has a number of fully restored engines and vehicles currently on display at the Lake Boga Catalina Museum.


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

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