Jetrader - July/August 2008 - 29

Left. The Pratt & Whitney, 3,500HP R-4360 Wasp Major was one of the last piston-powered engines to dominate the skies before the jet and turboprop engines took its place. Right. The Wright 3,250HP R-3350 Turbo Compound engine, which was one of the most powerful radial aircraft engines of its day. a service ceiling of 33,000 feet—when an upper cylinder enclosing a passenger compartment and ﬂight deck was added to the B50 to convert it into the Stratocruiser, it could still make 32,000 feet. (The DC7C’s ceiling was 21,700 ft and the L1649A’s, 23,700 ft.) The Wright Flyer’s engine was fabricated in the bicycle workshop by the brothers’ mechanic, Charlie Taylor, in six weeks. They had calculated, using their aerodynamic drag coefﬁcients from their wind tunnel and the parasitic drag of the frontal area, including the pilot, that they needed at least eight horsepower to convert propeller thrust of ninety pounds to achieve the required airspeed of 23 mph. In actual fact their invention of today’s propeller gave 132 pounds of thrust with an efﬁciency of 75 percent. The four cylinder 175-lb engine could run for only a short time before overheating but gave out twelve horsepower, giving a weight-to-power ratio of 15:1—more than enough for the job the Flyer was designed to do on December 17, 1903. The Other Side In contrast the Langley Aerodrome—which made a vertical ﬂight from the roof of a boathouse into the Potomac River on December 8, 1903—had a ﬁve-cylinder radial engine, weighing 208 lbs, which developed 52.4 hp (4:1 ratio). The builder was graduate mechanical engineer Charles Manley. He was also the unfortunate pilot on that short ﬂight into the frigid water. WWI was the catalyst for the ﬁrst stage of development of the radial engine: the problem was the biplanes were too slow to provide sufﬁcient cooling air over the cylinders, so the rotary engine was adopted by both sides of the conﬂict for their aircraft. By 1917 they powered 80 percent of the aircraft employed and were obsolete by 1918. Originally developed in the U.S. and Australia in the 1890s for cars and boats, it was the French Seguin Brothers’ Societe des Moteurs Gnome who produced a 50-hp, seven-cylinder rotary engine in 1908. By the end of 1917 when they were supplanted by liquid-cooled designs, they developed 100 HP with nine-cylinders. The Sopwith Camel of Snoopy fame was powered by a 130 HP 9-cylinder Clerget (2.9:1 wt/power ratio). Although a French engine, it was also manufactured in England. The rotary design has the crankshaft bolted to the airframe, and the rest of the engine rotates around it. The crankshaft axis is off center from that of the rotating engine, so the pistons reciprocate giving the suck, squeeze, bang and blow of a normal four-stroke engine. There is no carburetor, so to control the engine when landing the pilot had a blip switch to momentarily cut the ignition and hope the engine would start again. Fuel, air and the castor oil lubricant mixed together in the crankcase, and being a total-loss oil system, both burnt and pure castor oil from the exhaust sprayed the front of the fuselage and pilot; it is reported that no pilot suffered from constipation. On the contrary, there were many unscheduled landings to satisfy an urgent call of nature. The tremendous gyroscopic force generated by the whirling of such a heavy mass, resulted in the turn rate to the left of the Gnome engine four times that of a turn to the right. The Clerget rotated in the opposite direction, so it was the right-hand turn that a skilled pilot could exploit in a dogﬁght if an opponent got on his tail. The German pilots in their Fokker Dr.1. with the Oberusel rotary engine employed the same tactics. Unlike a reciprocating engine in those early airplanes—which had no heavy ﬂywheel—the rotary’s ﬂywheel effect, due to the rotating mass, generated very low vibration levels without the need of counterweights. Its time between overhauls was around 15-20 hours, and the high fuel and oil consumption limited endurance. By August 1916, the increase in power output of liquid-cooled reciprocating engines—particularly the Spanish 1,700 rpm Hispano-Suiza V-8 Abadal ﬁtted to the French SPAD VII, with its synchronized machine guns ﬁring through the propeller disc—gave the allied air forces superiority over the German aircraft. At long last, its 180 hp overcame the weight of the water jacket, pump, plumbing and radiator of the cooling system. However, more powerful air-cooled radial engines were not practical unless a solution was found to cool the cylinders. Shown here, the 100 hp Gnome Monosoupape rotary engine. Photo courtesy of The Science Museum, London. REFERENCES: • John D. Anderson Jr., The Airplane, A History of its Technology, American Institute of Aeronautics and Astronautics, 2002 • Peter L. Jakob, Visions of a Flying Machine, Smithsonian Institution, 1990 • Manuel Large, Hispano Suiza in Aeronautics, SAE International, 2004 • Kimble McCrutcheon: www.enginehistory.org/ gnome • My own ﬂight log book Jetrader 29

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Jetrader - July/August 2008

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