Wright R-975 Whirlwind / J-6 Whirlwind Nine / J-6-9
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.
Model designations included Whirlwind 330, 365, and 420.
The R-975 cylinders are of a steel barrel over which an aluminium alloy head is screwed and shrunk. Intake ports are at the rear with exhaust ports on the forward side of cylinder.
The crankcase assembly is composed of four major castings of aluminium alloy. The cam follower housing carrying the tappet guides is cast integral with the main section of the crankcase.
Two-piece single-throw crankshaft with one-piece master rod and “H” section articulated rods.
Aluminium alloy pistons, cross ribbed on under side of head, and fitted with full floating hollow pins held in place by expanding spring wire locks.
Tulip shaped valves, solid stem inlet valves and hollow stem exhaust valves.
Rotary induction system of Wright design, with “pre-heating” device on carburettor. Provision is made for obtaining, at all times, a clean supply of air to the carburettor.
Lubrication system is designed to eliminate all external oil pipes to the engine.
Equipment supplied was air cleaner and heater, nose cowling, complete exhaust manifold, priming pump, ignition switch, tool kit, external oil filter, and instruction book.
Accessories available at extra cost were Ecllipse hand inertia starter, Eclipse generator, Eclipse generator control box, standard steel propeller hub, fuel pump, Eclipse combination hand and electric inertia starter, Eclipse hand starter with booster magneto, Eclipse electric inertia starter, propeller hub for wooden propeller, metal propeller, and hubs for two or three blade props.
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.
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.
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 until 1945.
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.
Type: 9 cylinder, air cooled fixed radial
Military Rating: 300 hp at 2000 rpm
Commercial Rating: 300 hp at 2000 rpm
Displacement: 972 cu.in
Compression ratio: 5.1-1
Bore: 5 in
Stroke: 5 1/2 in
Length: 41 7/16 in
Diameter: 45 in
Weight: 520 lb
Fuel consumption: not more than .55 lb/hp/hr
Oil consumption: not more than .035 lb/hp/hr
Lubrication: Pressure pump
Ignition: Scintilla, dual
Carburation: Stromberg, single bbl
Spark plugs: 2 per cylinser