Infallible performance and economical operation are the bases of successful commercial flying. Airplanes, having passed through the experimental and demonstration periods, must now prove their usefulness. Heretofore, because of military requirements, designers have fostered the use of power rather than refinement of design to obtain performance, but commercial operation demands efficiency, and in each of the four essentials, namely, dependability, size, total powerplant weight and cost, opportunity for decided improvement still exists.
The requirements and limiting factors of each of these essentials are discussed in turn and the conclusion is drawn that a relation exists between the amount of thrust delivered to the air and the weight put into an airplane for its propulsion. To obtain the best over-all performance, if these terms are considered as a fraction, the numerator should have the maximum and the denominator the minimum value.
The suggestion is made that a more significant measure of efficiency than the weight per horsepower of the engine alone would be the wet weight of the engine per horsepower plus the weight of the cooling-system per horsepower plus the combined weight of the fuel and oil per horsepower per hour. The figure thus obtained would have no value, however, except as an indication of the over-all efficiency of the powerplant.
The general development of airplane powerplants is outlined, the various engines in vogue at present are classified into groups according to their horsepower, and the assertion is made that the air-cooled type of engine, having already displaced the water-cooled in the 200-hp. class, is likely to do so in the 400-hp. class. Designers of water-cooled engines are said to be endeavoring to overcome the handicap of excessive weight in engines of this type, but air-cooled engines, because of their greater dependability, are considered superior for commercial purposes.
In addition to increased dependability and reduced weight, the radial type of air-cooled engine makes possible an aerodynamically superior and symmetrical fuselage and gives a high center of thrust that allows ample propeller diameter.
In motor-car work, dependability and light weight are less essential than in the aeronautic field; consequently, the urge to develop them has not been so pressing. A leaky radiator in a motor car can be easily repaired but, if it necessitates a forced landing by an airplane, may lead to disastrous results.
Detailed descriptions, accompanied by illustrations, are given of a large number of American and foreign types of aircraft engine.


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