The automobile experiment naturally suggested the aeroplane, and after much reading of magazines and animated discussions as to the relative advantages of biplanes, monoplanes, gliders, etc., the boys decided to try their skill on a biplane of their own design, a combination of the features and proportions of the Curtiss and Wright machines.

The automobile was child's play compared with the problems confronting the young aviators in designing and working out a flying machine, and, as in the former case, the question of motive power was the most difficult. We might add it has not yet been satisfactorily solved.

Fig. 48 shows the general appearance of the boys' model, which was eighteen inches long from front to back, and the planes, made of light card-board, were 14 inches long and 3¹/₂ inches wide. The frame, braces, rudder, and tilting plane were made of ¹/₈-inch basswood, put together with ¹/₂-inch brads clinched wherever the points came through.

The parts composing the frame were made first, and all small details, such as rudder, propeller, tilting plane, etc., cut out later.

The separate parts are shown in the drawing. Four straight pieces like a were required to support the tilting plane in front, and two pieces each b and c for the rudder in the rear. Two pieces a, one of b and c were fastened together by means of two uprights d, forming one complete side of the machine. This was completed, and the second side made identical with it.

These two sides were then fastened parallel with each other, rigidly, by means of the two rudder posts e e and the cross pieces f f, by brads. The rudder posts bound the two sides rigidly at the rear, the cross pieces at the centre, and at the forward end the tilting plane was held in position by the brads, which also acted as pivots.

This made a remarkably light and yet strong framework. The card-board planes were not placed in position until everything else was finished, as they could be attached easily and quickly, but were very much in the way when experiments were being made on the propelling apparatus.Of course there had to be a propeller, and the problem of making it required some practice.

Ralph introduced the subject by showing Harry how to make an old-fashioned toy, shown in the detail drawing, of two pieces, one the propeller, the other a balancing stick.

The propeller was made of a piece of ³/₈-inch basswood, 4 inches long and ¹/₂ inch wide. A ³/₁₆-inch hole was first drilled at the exact centre. The two ends were then whittled down to the shape shown at k. The balancing stick was next whittled down until one end fitted tightly into the hole drilled in the propeller, and the rest of the stick then rounded until it was of uniform diameter. This stick was glued into the hole, and allowed to dry.

There was plenty of work to do while the glue was hardening, as the cross pieces g g had to be fastened to the frame to prepare for the installation of the power plant.

When the glue was dry, Ralph took the balancing stick between the palms of his hands, drew his right hand toward him with a quick motion, at the same time releasing the stick. To Harry's amazement, the whole thing flew up and struck the ceiling, and for a few minutes aeroplanes were forgotten while the two played with this interesting but ancient toy.

Ralph explained that the propeller was simply part of a screw thread, and had actually worked its way through the air just as a screw works its way into a piece of wood. Its lifting power had been shown by the way it carried the balancing stick with it up to the ceiling.

"Now," he continued, "when we place a propeller horizontal it will worm its way forward through the air in the same way and carry the aeroplane with it, for the simple reason that it is so placed in the frame it can't get out. As the free space it has to revolve in is only 3 inches, we shall have to cut the blades down to about 2³/₄ inches to give it clearance."

They whittled out a shaft 1¹/₂ inches long and fastened the two notched pieces h h to it after placing the propeller in position between the two cross pieces g g which had been previously drilled with ¹/₄-inch holes to act as bearings.

New rubber bands were then passed over the notches, stretched out to the front and rear of the frame, and tied to cross pieces.

By winding up the propeller, these bands were twisted tightly, and when the propeller was released, the bands unwound, causing it to revolve rapidly.

The rudder was now pivoted in position by brads, and the two planes fastened by the same method.

The power derived from the bands was not sufficient to propel the aeroplane fast enough to support it in the air, so it was necessary to experiment with strong thread until the centre of gravity was found. It proved to be near the centre of the planes. Small holes were made with an awl at this point, the thread passed through them and tied. By suspending the aeroplane from a chandelier it took up a horizontal position.

Then the forward tilting plane was elevated slightly and the propeller wound up. On being released the aeroplane slowly and majestically sailed through the air in a great circle, limited by the length of the suspending thread.

The boys never tired of this toy and all it lacked was the ability to fly in the open air, which would require a more powerful motor. This would more than double the weight of the machine, and therefore call for larger planes to support it. There you have the great problem of the aviator.Ralph wisely suggested that as they had not yet reached the stage of designing gasolene motors they had better leave the aeroplane as it was, or it would be necessary to abandon their woodwork, which neither of them had any intention of doing.