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 and power ratings of. 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, where it was overshadowed by the similar Pratt & Whitney R-985. After the war, Continental continued to produce its own versions of the R-975 into the 1950s. Some of these produced as much as.
The R-975 powered the American World War II M18 Hellcat tank destroyer which was claimed to have been the fastest tracked armored vehicle until the introduction of the turbine powered M1 Abrams in the 1980s.

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 was unchanged, but the cylinder bore was expanded to 5.0 in from the R-790's bore of 4.5 in. 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 was rated for 300 hp, while the R-975E of 1931 could do 330 hp 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 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 for takeoff.

Operational history

As the most powerful Whirlwind to be produced commercially was also the most successful. It powered a wide variety of civil utility aircraft, such as the Beechcraft Staggerwing, and various airliners, such as the Ford 4-AT-E Trimotor and the Lockheed 10B Electra. In addition, it powered U.S. military training aircraft including the North American BT-9 and Vultee BT-15 Valiant for the Army and the Curtiss-Wright SNC Falcon for the Navy. The Curtiss F9C Sparrowhawk parasite fighter operated from U.S. Navy airships was also powered by the R-975.
One notable record set by a Wright J-6 Whirlwind-powered aircraft occurred during July 28–30, 1931, when Russell Norton Boardman and John Louis Polando flew non-stop from Floyd Bennett Field, on Long Island to Istanbul, Turkey in the Cape Cod, a Bellanca Special J-300 high-wing monoplane in 49:20 hours, establishing a distance record of, the first nonstop record flight to surpasse.
However, the R-975 faced heavy competition from Pratt & Whitney's R-985 Wasp Junior and from their larger R-1340 Wasp. Pratt & Whitney R-985 outsold the Wright R-975 by a wide margin.
Wright's production of the R-975 ceased in 1945, with over 7,000 engines being produced by the company.

Production by Continental Motors

In 1939 the U.S. Army, which had 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.
In contrast to the 7,000 built by Wright, Continental built over 53,000 R-975 engines.
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 for takeoff, surpassing any Wright version. A military version, the R-975-46, could reach, 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-975-20
;R-975E
;R-975E-1
;R-975E-3
;R-975E-C2
;Continental R9-A
;Hispano-Suiza 9Q
;Hispano-Suiza 9Qa
;Hispano-Suiza 9Qb
;Hispano-Suiza 9Qc
;Hispano-Suiza 9Qd
;Hispano-Suiza 9Qdr

Applications

Wright J-6-9 and R-975

Some museums which have R-975 engines on display:
EnginePower, continuousPower, takeoffCompression ratioSupercharger gear ratioOctane ratingDry weight
R-975E330 hp at 2,000 RPM5.1:17.8:173635 lb
R-975E-1365 hp at 2,100 RPM6.1:17.8:173660 lb
R-975E-3420 hp at 2,200 RPM450 hp at 2,250 RPM6.3:110.15:180675 lb
Continental R9-A500 hp at 2,300 RPM525 hp at 2,300 RPM6.3:110.15:191705 lb