Aviation Engine Types
Inasmuch as numerous forms of airplane engines have been devised, it would require a volume of considerable size to describe even the most important developments of recent years. As considerable explanatory matter has been given in preceding chapters and the principles involved in internal combustion engine operation considered in detail, a relatively brief review of the features of some of the most successful airplane motors should suffice to give the reader a complete enough understanding of the art so all types of engines can be readily recognized and the advantages and disadvantages of each type understood, as well as defining the constructional features enough so the methods of locating and repairing the common engine and auxiliary system troubles will be fully grasped.
Aviation engines can be divided into three main classes. One of the earliest attempts to devise distinctive power plant designs for aircraft involved the construction of engines utilizing a radial arrangement of the cylinders or a star-wise disposition. Among the engines of this class may be mentioned the Anzani, R. E. P. and the Salmson or Canton and Unné forms. The two former are air-cooled, the latter design is water-cooled. Engines of this type have been built in cylinder numbers ranging from three to twenty. While the simple forms were popular in the early days of aviation engine development, they have been succeeded by the more conventional arrangements which now form the largest class. The reason for the adoption of a star-wise arrangement of cylinders has been previously considered. Smoothness of running can only be obtained by using a considerable number of cylinders. The fundamental reason for the adoption of the star-wise disposition is that a better distribution of stress is obtained by having all of the pistons acting on the same crank-pin so that the crank-throw and pin are continuously under maximum stress. Some difficulty has been experienced in lubricating the lower cylinders in some forms of six cylinder, rotary crank, radial engines but these have been largely overcome so they are not as serious in practice as a theoretical consideration would indicate.
Another class of engines developed to meet aviation requirements is a complete departure from the preceding class, though when the engines are at rest, it is difficult to differentiate between them. This class includes engines having a star-wise disposition of the cylinders but the cylinders themselves and the crank-case rotate and the crank-shaft remains stationary. The important rotary engines are the Gnome, the Le Rhone and the Clerget. By far the most important classification is that including engines which retain the approved design of the types of power plants that have been so widely utilized in automobiles and which have but slight modifications to increase reliability and mechanical strength and produce a reduction in weight. This class includes the vertical engines such as the Duesenberg and Hall-Scott four-cylinder; the Wisconsin, Aeromarine, Mercedes, Benz, and Hall-Scott six-cylinder vertical engines and the numerous eight- and twelve-cylinder Vee designs such as the Curtiss, Renault, Thomas-Morse, Sturtevant, Sunbeam, and others.