History of aicraft piston engines

By 1920 most large european engines were g eared in

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Unformatted text preview: he Renault), the 8-cylinder-in-line M ercedes, and the 2 20-hp Hispano-Suiza. These were soon followed by the Rolls-Royce Eagle, w ith planetary gears. T he need for propeller gearing results from the fact that the propeller speed for optimum propeller efficiency is usually lower than the speed at which the engine gives its best performance. Without gearing, the speed for the engine-propeller combination is chosen as a c ompromise—too h igh for best propeller efficiency and too low for maximum engine power. As i mproved engine design called for higher engine speeds, this compromise b ecame more unsatisfactory. By 1920 most large European engines were g eared. In the United States, however, the general use of reduction gears c ame much later. For that matter, in 1924 gearing was actually eliminated from the Curtiss D -12 engine in order to save 25 pounds of weight! Nevertheless, by 1930 it was evident that large engines should be geared to allow of optimum performance. P ratt & W hitney used an internal gear in 1931, a nd both Wright Aeronautical and Pratt & Whitney adopted the Farman p lanetary gear for use in the DC-3 in 1933 (fig. 69). From that time on, 78 Figure 69.—Bevel planetary reduction gear system as used on Farman engines, after World War I. T his type was also used by some American engine b uilders under license from Farman. Below, Spur-type planetary reduction gear system which superseded the bevel t ype. >///////;//M////w/»/}, p ropeller reduction gears became an integral part of all large airplane engines, s pur type planetary gears being standard for most radials and plain 2 -element s pur gears for V-type engines (fig. 70). The plain spur gears used by the Rolls-Royce Merlin of 1945 c arried 2,200 takeoff horsepower satisfactorily on a face 2-in. wide, a remarkable achievement in gear design. Other Developments O nly brief mention can be made here of the numerous secondary, though often very important, problems encountered and solved in the development of reciprocating aircraft engines. Among these, however, should be m entioned the following: I GNITION SYSTEMS. As mentioned previously, the Wright Brothers' engine of 1903 used " make-and-break" i gnition. This system involved a pair of c ontacts within the cylinder, one insulated and connected to a battery 79 Figure 70.—Two-element spur reduction gear as used on Renault Type 12-Kh liquid-cooled engine. This type of gear has been generally used in V-type and in-line engines since about 1930. (From Aerosphere 1939, p. 649) a nd coil system and one operated by a shaft protruding through the cylinder wall. This shaft was operated from the camshaft so as to "break" the contact p oints apart at the moment of ignition. The low voltage arc so formed was an effective igniter. However, the mechanical complication involved, a nd the difficulty of cooling the contact levers within the cylinder made i t impractical for any but very low-output engines. This system was soon...
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