The development of air-cooled engines for aircraft never made much progress until the war, when the British attempted to improve the performance of existing engines by a series of experiments leading eventually to the development of aluminum cylinders with steel liners and aluminum cylinder-heads with a steel cylinder screwed into the head. The advantages of these constructions and the disadvantages of other types are discussed. Results are reported of tests at McCook Field on a modern cylinder-design of this type showing good results, that lead to the belief that large air-cooled engines will be produced in the near future, equal in performance to water-cooled engines of the same power. It is claimed that, at present, there is nothing to choose in performance between water-cooled and air-cooled engines of about 25 hp. per cylinder, and that air-cooled engines of this size can be built successfully of the same compression-ratio and having the same fuel-consumption as high-compression water-cooled types.
An explanation is given of the reason for the advantages of aluminum cylinders and head constructions, and a chart is presented showing the temperature on the front and the rear of a large air-cooled cylinder of high output. The question of the cylinder-wall temperature of air-cooled and of water-cooled engines is discussed, and it is indicated that there is not much difference in temperature between the two types. The water-cooled engine is at a disadvantage on account of the number of heat transfers from one medium to another before the heat reaches the air. A statement is given of the reason air-cooled engines can perform satisfactorily with less cooling area than is required for water-cooled engines.
The subject of the resistance of airplanes having air-cooled and of those having water-cooled engines is discussed, attention being drawn to the fact that little is known of the best methods of installing and cowling these engines. Various suggestions are made for possible future development of cowling, to permit airplanes equipped with air-cooled engines to offer as lit-the resistance as those equipped with water-cooled engines.