The Propeller Blank. Draw a line around the four faces of this block at the exact center of the length. Then on faces C and D, lay off a distance of ½ inch on the center-line, measuring from the edge of face B, for the thickness of the propeller-hub, and draw diagonal lines from the upper and lower left-hand corners of faces C and D to the end of the hub center-line (Step B). Then cut away the portions outside of these lines, as shown in Step C. Lay out the hub upon faces A and B of the block, with a ½-inch diameter, and bore a small hole through the center to receive the propeller-shaft (Step C). Draw diagonals from the corners to the center-line of the hub (Step D); then cut away the wood outside of these lines (Step E).
The next step (F) consists in laying out the form of the propeller blade upon all four sides and ends of the block, and Step G is the final one of cutting out the propeller, scooping out its blades concave on one side, and carving them convex on the opposite side. A very sharp knife must be used for cutting; and the work must be done slowly and carefully, because the least slip is likely to ruin the propeller. The entering-edge of each blade is the almost straight edge, and should be cut very thin. The ends of the blades should also be cut thin, while the hub should be cut away as much as can safely be done without weakening the propeller.
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 1/8-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.
It has been found that rubber motors can be wound much farther by lubricating them with glycerine. It is only necessary to put a few drops of the glycerine upon a clean cloth, and rub it over the outside strands; then wind the motors, and it will work over the surface of the inner strands until all parts are covered.
