The Airplane - Jay Spenser [101]
Another clever individual, a Canadian named W. R. Turnbull, independently invented a variable-pitch propeller in the late 1920s that was electrically rather than hydraulically actuated. Impressed by tests of Turnbull’s technology, the Curtiss-Wright Corporation negotiated an exclusive license but did not do anything with it until 1935, when the U.S. Navy ordered Curtiss electric propellers to improve the performance of its flying-boat patrol planes.
Hamilton Standard and Curtiss-Wright kept trying to outdo each other in the constant-speed propeller field. This beneficial competition spurred further innovation and provided a vital technology during World War II.
Remember Army pilot John Macready’s 1921 high-altitude flight over Ohio in the supercharger-equipped LePere biplane? Even as the Wright brothers were inventing the airplane, Sanford Moss was at Cornell University building and evaluating what amounted to an embryonic jet engine. This fascination with high-speed gas turbines won him a doctorate from Cornell and a research position with the General Electric Company in its industrial gas turbine division.
During World War I, NACA approached GE and Dr. Moss to develop a turbo-supercharger (today turbocharger) for aviation. By compressing air before feeding it to the engine, a supercharger prevents decreasing atmospheric pressure from robbing an airplane’s engine of power as it climbs. Superchargers can either be directly driven by the engine (mechanical supercharging) or—as studied by Moss—indirectly by passing the engine exhaust through a turbine whose extended shaft drives a parallel turbine that pressurizes intake air.
Other countries were also pursuing this idea. In the lead was France, where a clever engineer introduced an aviation turbo-supercharger in 1917. Auguste Rateau’s design built on decades of experience creating industrial turbines for use in steel mills, mine shafts, and electrical-generation plants. His aviation efforts were not destined to be as influential as those of Sanford Moss only because France did not share America’s vision of high-altitude flight.
In 1918, a Liberty engine equipped with Moss’ supercharger was carted by cog railway to the top of Pike’s Peak in Colorado. It performed flawlessly at an elevation of 14,109 feet (4,300 meters), putting out full rated power despite the thin air. This success paved the way for the flight experiments by Macready and others.
Despite scant interest between the world wars, Moss and GE continued to refine this technology, which finally came into its own in the late 1930s. The GE turbo-supercharger made it possible for the Boeing 307 Stratoliner, history’s first pressurized airliner, to cruise above the weather rather than slog through it. It also allowed the Army’s B-17 Flying Fortress bombers to operate in the substratosphere, where reduced air resistance yielded true airspeeds above 300 mph (482 km/h).
Air heats up when pressurized, of course, but introducing hot air into an engine’s cylinders is not desirable because it robs the engine of power and risks a premature explosion of the fuel-air mixture. Consequently, GE supercharger installations included an intercooler, which is a heat exchanger that cools the supercharged airflow before feeding it to the engines.
GE’s turbo-superchargers were a critical technology in World War II. All U.S. heavy bombers were equipped with them, and so were the P-38 Lightning and P-47 Thunderbolt fighters. Also vital to the war effort were engine-driven superchargers, which likewise boosted the power and altitude performance of many Allied and Axis warplanes. All airplanes equipped with Merlin engines (among them the Spitfire, Mosquito, Lancaster, and P-51 Mustang) had engine-driven superchargers. So did most German, Japanese, and Italian high-performance warplanes.
After the war, superchargers contributed to the performance of the final generation of piston-engine airliners. Particularly complex was the