Why General Electric put an airplane engine on a truck and drove it to the top of Pikes Peak.
- By Donald Sherman
- Air & Space magazine, May 2001
(Page 3 of 4)
The British government’s Royal Aircraft Establishment experimented with reciprocating-piston air compressors and Roots blowers, which have intermeshing vanes, but the trials bore no fruit. So the RAE concentrated instead on high-speed centrifugal blowers. A BE2C biplane powered by a 537-cubic-inch air-cooled eight-cylinder engine took 35 minutes to climb to 8,500 feet. With an experimental gear-driven supercharger, it climbed 3,000 feet higher. But engine maestro Sam Heron at the Royal Aircraft Factory had doubts, which he expressed in muted terms: “The observer sat forward with his feet under the fuel tank and over the supercharger’s gear drive. The gears were quite inadequate and the pinion failed in flight, producing showers of sparks and a feeling of distinct concern.”
August Rateau, an enterprising inventor, engineer, and industrialist in France, dusted off a 1909 idea of Alfred Buchi’s for a turbocharger and fitted it to SPAD, Breguet, and ALD types with some success. One turbocharged Renault engine improved the rate of climb at 14,000 feet by 15 percent and boosted top speed from 104 to 120 mph. The British evaluated Rateau’s equipment, noting a 23 percent improvement in the rate of climb, but suspended research after a catastrophic turbine failure at 13,500 feet.
Rateau’s turbocharger caught the eye of the U.S. Army Air Service’s technical experts stationed in Paris, and soon investigations were under way at McCook Field in Dayton, Ohio, with an experimental unit running on a Liberty engine. Excessive heat caused persistent failures. A parallel effort initiated in November 1917 by William Durand, chairman of the National Advisory Committee for Aeronautics, was more fruitful. Earlier, Durand had been at Cornell, where he first learned of Moss and his research. He was also well aware of GE’s prominence in steam turbines and centrifugal compressors. Durand promptly petitioned GE’s president for Moss’ assistance.
Engineering drawings of the Rateau device were available, but Moss’ GE team had its own ideas. By June 1918, GE’s Lynn Steam Turbine Department in East Lynn, Massachusetts, had shipped a prototype to the War Department’s Airplane Engineering Division at McCook Field for adaptation to a Liberty 12 aircraft engine. It was a teenage marriage: an untried turbocharger wed to an engine that one year earlier had been just a glimmer in the War Production Board’s eye.
Moss’ first turbo consisted of two 10-inch-diameter wheels mounted on a steel shaft turning at 20,000 rpm and supported by bearings at each end that were lubricated by engine oil. One wheel had surfaces that absorbed the blast of the engine’s exhaust gases and thereby spun the steel shaft. At the other end was another wheel that drew air in from the atmosphere and compressed it so it could be routed to the engine intake. Engine coolant circulated through a jacket surrounding the bearing on the hot side, where the engine exhaust gas flowed, in order to carry away the heat. Large welded-steel manifolds gathered the exhaust and delivered it to a nozzle box that directed the exhaust at the turbine wheel.
Moss mounted the turbo at the front of the engine so the propeller’s slipstream would cool the compressor housing, the nozzle box, and the exhaust manifolds. Valves called waste gates, located at the rear of the exhaust manifolds, could be opened to modulate the flow of exhaust gas to the turbine and thereby control the amount of boost generated.
But in the low-altitude flatlands of Ohio, McCook Field sat at the very bottom of the atmosphere. To test the turbocharger at McCook would mean compressing air that was already thick. The engine wasn’t built to tolerate loads imposed by air at high pressure, and every test risked destruction. Then Moss got his idea about climbing a mountain with the test engine mounted on a truck. And he found a mountain with a road all the way to the top—Pikes Peak, in Colorado.
Together with McCook Field engineer C. P. Grimes and a small crew of technicians, Moss assembled a mobile laboratory: the turbocharged Liberty engine with a huge propeller to absorb the power, a dynamometer to gauge the torque produced, and various support systems. The whole thing was mounted on a Packard motor truck and looked like a carnival contraption.