The Heart of a Fortress
Wright's R-1820 Cyclone engine
by Randy Wilson
Copyright © 1994 by the Confederate Air Force and Randy Wilson. All rights reserved.
The Wright Aeronautical Corporation's R-1820 nine-cylinder, air-cooled, turbo-supercharged, single-row radial was the powerplant that carried thousands of Boeing B-17s over Europe and the Pacific. Called the Cyclone 9, the R-1820 was an enlarged version of the R-1750, which produced 500 hp at 1,900 rpm in 1927. Picked to power the new Douglas DC-1 airliner, the Wright R-1820 would subsequently power most DC-2 and DC-3 civil transports although most military Douglas transports were powered by rival Pratt & Whitney R-1830 engines.
Wright engines carried a company model designation, usually a leeter and a number, to distinguish between models. The Army and Navy, however, used "dash numbers" instead of the civilian model codes. Thus the 13 pre-production Y1B-17s were powered by R-1820-39 (G2) engines, the G2 being Wright's model code. The B-17C had four R-1820-65 engines, while the B-17F and Gs had R-1820-97s, both G-200 series engines giving 1,200 hp at 2,500 rpm for takeoff.
However, what made the B-17 a great long-range, high-altitude heavy bomber was not the engines, but the General Electric B-2 and later B-22 turbo-superchargers. At higher altitudes, the air being pumped into an unsupercharged engine is thinner, producing less power. The R-1820 was fitted with a mechanically driven supercharger, like most large radial engines. But, mechanical superchargers still lose power as they climb higher.
The exhaust-driven turbo-supercharger actually increases its effectiveness in the thinner air at high altitude, up to a point. As the pressure difference increases between the hot exhaust gases in the turbo-supercharger and the outside air, the gases pass through the exhaust turbine with more force, compressing more air into the engine intake, and producing more power. Interestingly, the turbo's compressed air is fed into the mechanical supercharger, to improve the mixture and distribution of the fuel and air into the cylinders.
The next time you walk under a B-17's engine nacelle on the ramp, look up and you will see the large circular bottom of the exhaust turbine housing, through which the exhaust escapes, after giving up part of its energy to the turbo-supercharger.
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