It is rather difficult to make good propellers, but with a little time and patience they can be shaped and formed into good proportions. Procure two clear, straight-grained blocks of white pine, 8 in. long, 1¹⁄₂ in. wide, and ³⁄₄ in. thick. Draw a diagonal line on one block from opposite corners, as shown at S, [Fig. 3], then on the other block T, draw the line in an opposite direction. Turn the blocks over and draw opposite diagonals, as shown by the dotted lines. Draw a circle on each side exactly in the center, ¹⁄₂ in. in diameter. Drill ¹⁄₁₆-in. holes through the centers of the circles for the propeller shafts. The wood is then cut down to the lines drawn, leaving only enough material so that they will not break easily. The face of the blades should be flat and the back rounded. Leave plenty of stock near the hub. After the faces have been finished, the blades are shaped as shown at U. The propellers should be finished with sandpaper to make them perfectly smooth, as much of the success of the model will depend upon them. It will be a good plan to shellac them, and also the frame and the alighting gear. Aluminum paint costs but little, and it makes a fine finish for a model aeroplane.
The propeller shafts V, [Fig. 1], [2], and [4], are cut from bicycle spokes. An eye for the rubber band is bent in the spoke, about 2 in. from the threaded end. The end having the threads is run through the bearing block C, [Fig. 4], and the propeller fastened on with a small washer on each side of it by means of two nuts, W, cut from a bicycle nipple. These nuts may be turned up tightly with pliers.
The planes are covered with tissue paper put on tightly over the tops of the ribs, using a flour paste. The planes are movably fixed on the motor bases A by tying at the four points of contact with rubber bands. This makes it possible to adjust the fore-and-aft balance of the machine by changing the position of the planes.
The motive power, which is the most important part of the entire machine, consists of rubber bands. There are three ways of obtaining these bands. It is best, if possible, to purchase them from an aeroplane supply house. In this case, procure about 100 ft. of ¹⁄₁₆-in. square rubber, 50 ft. for each side. These are wound closely between the hooks X. This rubber can be taken from a golf ball. It will require about 40 strands of this rubber, which is removed by cutting into the ball, on each propeller. Another way of obtaining the bands is to purchase No. 19 rubber bands and loop them together, chain-fashion, to make them long enough to reach between the hooks without stretching. About 30 strands on each propeller will be sufficient. The hooks X are made in the shape of the letter “S,” to provide a way for taking out the rubber bands quickly. To prevent the hooks from cutting the rubber, slip some ¹⁄₁₆-in. rubber tubing over them. The rubber bands, or motor, when not in use, should be kept in a cool, dark place and powdered with French chalk to prevent the parts from sticking together.
Fig. 2
The Alighting Gear is Made Entirely of Bamboo and Attached to the Under Side of the Motor Frame
With the model complete, flying is the next thing in order. With a machine as large as this one, quite a field will be necessary to give it a good flight. Test the plane by gliding it, that is, holding it up by the propellers and bearing blocks on a level with your head and throwing it forward on an even keel. Shift the planes forward or back until it balances and comes to the ground lightly.
Winding up the propellers is accomplished by means of an eye inserted in the chuck of an ordinary hand drill. While an assistant grasps the propellers and motor bearings the rubber is unhooked from the front of the machine and hooked into the eye in the drill. Stretch the rubber out for about 10 ft., and as it is wound up, let it draw back gradually. Wind up the propellers in opposite directions, turning them from 400 to 800 revolutions. Be sure to wind both propellers the same number of turns, as this will assure a straight flight.
Fig. 3
The Most Difficult Part of Making the Propellers can be Overcome with a Little Patience