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.
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