Wright R-975

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), while the R-975E of 1931 could do 330 hp (246 kW) thanks to an improved cylinder head design. 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. 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.

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.

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.

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, 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. The R-975 was also produced under licence by Fábrica Nacional de Motores in Brazil.

Variants

 * J-6-9 (R-975)
 * 300 hp (224 kW) at 2,000 RPM.


 * R-975E
 * 330 hp (246 kW) at 2,000 RPM. Higher power from improved cylinder head.


 * R-975E-1
 * 365 hp (272 kW) at 2,100 RPM. Higher compression ratio.


 * R-975E-3
 * 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.


 * R-975E-C2
 * 400 hp (298 kW) at 2,400 RPM. Built by Continental Motors under license. Designed for use in armored vehicles.


 * 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.


 * 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

Applications

 * Beechcraft Staggerwing B17R, C17R, and D17R
 * Bellanca CH-300 Pacemaker
 * Cessna DC-6A Chief
 * Curtiss-Wright CW-22
 * Curtiss-Wright SNC-1 Falcon
 * Dewoitine D.31
 * Dewoitine D.35
 * Curtiss F9C Sparrowhawk
 * Fokker Standard Universal
 * Fokker T.VIII
 * Ford Trimotor 4-AT-E


 * Ireland N-2B Neptune
 * Keystone-Loening K-84 Commuter
 * McDonnell XV-1
 * North American BT-9
 * Pitcairn-Cierva PCA-2
 * Ryan B-5 Brougham
 * Stinson Detroiter SM-1F
 * Travel Air 6000B
 * VL Pyry
 * Vultee BT-15 Valiant
 * Waco JTO and JYO

Continental R-975

 * Kangaroo armoured personnel carrier
 * M3 Lee
 * M4 Sherman
 * M7 Priest
 * M18 Hellcat
 * M12 Gun Motor Carriage
 * M40 Gun Motor Carriage
 * Piasecki H-25
 * Ram tank
 * Sexton self-propelled gun
 * Sikorsky S-53
 * Skink anti-aircraft tank

Engines on display
Some museums which have R-975 engines on display:
 * Pima Air & Space Museum in Tucson, Arizona has a Wright R-975.
 * Hiller Aviation Museum in San Carlos, California has a Wright R-975.
 * National Museum of Naval Aviation near Pensacola, Florida has a Continental R-975.
 * Southern Museum of Flight in Birmingham, Alabama has a Continental R-975.
 * Queensland Air Museum in Caloundra, Queensland, Australia has a Continental R-975.