The marked advance that has been made in the last 10 years in constructional details and in performance of airplane engines and in airplane performance is reviewed, beginning with the year 1916 when the Curtiss OX-5 eight-cylinder water-cooled engine was brought to its final stage of development. The author describes briefly each type of engine produced successively by the company he represents and tells of the changes that were made to improve the performance.
From the 8-cylinder V-type the constructors changed to the 6 and 12-cylinder water-cooled type and are now developing a 9-cylinder air-cooled radial engine that was built in 1925. An important field of usefulness is foreseen for the air-cooled engine. Up to 500 hp. the air-cooled engine weighs less per horsepower than the water-cooled engine when installed; from 500 to 1000 hp. the weight of the two types, installed, is about the same; and at powers of more than 1000 hp. the water-cooled engine begins to have the advantage in installed weight. In larger powers than 500 hp. the indications point to the V and X types of air-cooled engine as preferable to the radial type. An air-cooled Liberty-12 engine has been built successfully and tested by the engineering division of the Air Service at McCook Field, which has also developed the cylinder of the 400-hp. air-cooled Curtiss engine, in which the exhaust-valves are as well cooled as those in any water-cooled engine.
The problem in designing aircraft engines is to obtain the best relation of (a) weight, (b) frontal area, (c) cost, (d) bulk, (e) high performance, and (f) reliability; and all of these factors are involved with the proper arrangement of parts and the proper selection of materials. More than 40 different kinds of material are used in the modern engine, of which the more interesting are the light and unusual alloys, such as magnesium, aluminum-bronze, duralumin, and Y alloy. The characteristics of each of these that make it especially suitable for different parts of the engine are given. Fourteen different materials are used in one cylinder-block of the Curtiss 500-hp. engine, as follows: duralumin, magnesium, aluminum-bronze, nickel-chromium steel in two analyses, carbon-steel in two analyses, cast iron, brass, copper, cold-rolled steel, drill rod, chrome-vanadium steel, and silchrome steel.
Future development of the water-cooled aircraft engine probably will be the combination of reduction gearing from the crankshaft to the propeller with high engine-speed and supercharging. The gear-reduction problem has been solved and gears can now be made as reliable as the engine. Availability of the lighter alloys at reasonable cost, together with supercharging and gear reduction, should make possible the reduction of the engine weight to close to 1 lb. per hp. During the last 10 years the weight has been reduced from more than 4.00 lb. to less than 1.33 lb. per hp. The rotary inductor on the Curtiss 400-hp. air-cooled engine has made possible a fuel economy of less than 0.50 lb. of gasoline per hp-hr. at one-quarter throttle opening.


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