In connecting up your propeller with the motor it is very important that the shaft should turn freely and that the bearings offer the least possible resistance. If you have built your aëroplane from the drawing (see [Plate A]), now drill a hole exactly in the center of the stick which crosses the triangle at the rear of the frame. This hole will come on a line with the apex of the prism, or exactly in the center of the triangle. When the turning of the motor pulls the ends of the frame together, the strain will therefore be exactly distributed among the three sides or braces.
The propeller must be kept clear of the frame and must never touch or scrape against it. First a thin strip of metal, drilled to take the axle or hat-pin, should be nailed over the hole in the crosspiece. A sheet of aluminium such as is used for name-plates is just the thing. Now on the propeller-wire or axis string a smooth, symmetrical glass bead, and pass the axle through the metal strip and the crosspiece. This will give you an excellent substitute for ball-bearings. The end of the wire should then be turned into a hook well inside the frame. The propeller should be mounted so carefully that it will turn freely without friction and without wabbling from side to side.
The simplest and most effective motor is formed by connecting the two hooks with many turns of a long, thin strand of rubber, which can be bought by the yard or pound. The thinner strands of rubber will exert more force than the heavy bands, and red rubber is more durable than any other. The bands should be looped loosely between the two hooks, just as you would wind a skein of zephyr—over the hook on the propeller-“shaft,” then around the hook at the other end, then down over the propeller-shaft hook, and so on. If the hooks be three feet apart the combined strands should form a band one inch or more in diameter. If you cannot buy the rubber in this form, a number of two-inch rubber bands, such as you buy by the box at the stationer’s, may be lopped chain fashion together to form a continuous rope from hook to hook.
To store up energy for the flight, simply turn your propeller round and round until the rope of rubber bands is tightly knotted. You can readily tell when it is sufficiently wound and the danger-point is reached, which comes when the pull of the rubber grows too strong for your frame. The average motor should be turned about one hundred and fifty times. When the propeller is released the rubber bands in unwinding will give you back almost exactly the same number of revolutions, less perhaps one or two, which represents the loss through friction.
Model Constructed from Diagram, Plate A.
If the propeller simply buzzes around, coming to rest in a few seconds, without raising your aëroplane, it is probably too small for the weight of the aëroplane. When fully wound up the propeller should run for about ten seconds. On the other hand, if the propeller be too large, it will quickly twist the aëroplane out of its course and drive it to earth. It is well to try out your motor thoroughly to make sure of its running smoothly before attempting any actual flights.
Do not yield to the temptation of trying your wings, however, until the skids have been attached. Most of the regular full-size aëroplanes run on ordinary bicycle wheels, although the Wrights use runners like a sleigh. These skids or runners enable the machine to run along the ground with the least possible friction and greatly assist in rising. In the models of aëroplanes the skids serve a double purpose in protecting the machine when it alights.
A serviceable skid may be made by building a triangle of thin strips and attaching it to the frame with the broad side downward, as shown in the accompanying drawing. Skids made of reed curving down from the main body of the aëroplane will also serve to take up the shock. There are many ways of constructing these skids, and a study of the models here illustrated will give many suggestions. If you intend to have your aëroplane start from the ground, the front skids should be somewhat longer than those in the rear to give it the proper lift.
The friction of the skids is greatly reduced by mounting them on wheels. Small metal wheels may be borrowed from toy automobiles, or small disks of wood or cork will answer for the purpose. A very simple axis may be formed by running a long hat-pin through the uprights of the skids. The photographs of the best models will be found full of suggestions. You will need at least three skids to form a tripod for your aëroplane. It makes little difference if you use one leg in front, or two.