When you have completed cutting the propellers, place them at their centers across the edge of a knife-blade, and if they do not balance perfectly, locate the trouble and correct it. Finish the work with fine emery-paper, and then shellac it. Some boys glue silk over the ends of their propeller blades, for a distance of ½ inch or so, to reinforce them and make them less likely to split.
The Propeller-Shafts are made of heavy piano-wire, bent into a hook at one end (Fig. 38) to receive the rubber strands of the motor, and cut of the right length to extend through the hole in the bearing, through a glass bead, through the propeller, and then to bend over the side of the hub (Figs. 37 and 38). By bending over the end of the shaft against the hub, it is held securely in place.
The Motors consist of twelve strands of ⅛-inch flat rubber, each, and as these are 1 yard in length, exactly 24 yards of rubber are required. The rubber is not connected direct to the hooks on the bow and propeller-shafts, as the wire would quickly cut through the strands. Instead, small rings are bent out of wire, with pieces of small rubber-tubing slipped over the wire, and the ends of the rubber strands are looped through these rings and bound in place with thread (Fig. 39). The wire rings are then slipped on and off the hooks quickly. As light and heat cause rubber to deteriorate, you must remove the motors from the machine after use, pack away in a covered box, and keep in a cool place, in order to get the longest life possible out of the rubber.
