Engine-Base Construction
One of the important parts of the power plant is the substantial casing or bed member, which is employed to support the cylinders and crank-shaft and which is attached directly to the fuselage engine supporting members. This will vary widely in form, but as a general thing it is an approximately cylindrical member which may be divided either vertically or horizontally in two or more parts. Airplane engine crank-cases are usually made of aluminum, a material which has about the same strength as cast iron, but which only weighs a third as much. In rare cases cast iron is employed, but is not favored by most engineers because of its brittle nature, great weight and low resistance to tensile stresses. Where exceptional strength is needed alloys of bronze may be used, and in some cases where engines are produced in large quantities a portion of the crank-case may be a sheet steel or aluminum stamping.
Crank-cases are always large enough to permit the crank-shaft and parts attached to it to turn inside and obviously its length is determined by the number of cylinders and their disposition. The crank-case of the radial cylinder or double-opposed cylinder engine would be substantially the same in length. That of a four-cylinder will vary in length with the method of casting the cylinder. When the four-cylinders are cast in one unit and a two-bearing crank-shaft is used, the crank-case is a very compact and short member. When a three-bearing crank-shaft is utilized and the cylinders are cast in pairs, the engine base is longer than it would be to support a block casting, but is shorter than one designed to sustain individual cylinder castings and a five-bearing crank-shaft. It is now common construction to cast an oil container integral with the bottom of the engine base and to draw the lubricating oil from it by means of a pump, as shown at Fig. 140. The arms by which the motor is supported in the fuselage are substantial-ribbed members cast integrally with the upper half.
The approved method of crank-case construction favored by the majority of engineers is shown at the top of Fig. 141, bottom side up. The upper half not only forms a bed for the cylinder but is used to hold the crank-shaft as well. In the illustration, the three-bearing boxes form part of the case, while the lower brasses are in the form of separately cast caps retained by suitable bolts. In the construction outlined the bottom part of the case serves merely as an oil container and a protection for the interior mechanism of the motor. The cylinders are held down by means of studs screwed into the crank-case top, as shown at Fig. 141, lower view. If the aluminum cylinder motor has any future, the method of construction outlined at Fig. 142, which has been used in cast iron for an automobile motor, might be used for an eight-cylinder Vee engine for airplane use. The simplicity of the crank-case needed for a revolving cylinder motor and its small weight can be well understood by examination of the illustration at Fig. 143, which shows the engine crank-case for the nine-cylinder “Monosoupape” Gnome engine. This consists of two accurately machined forgings held together by bolts as clearly indicated.