Before you can build successful model airplanes, you must know the principles which govern their stability, and the best way to learn these is with gliders like those shown in this chapter. When you have experimented with these, you will be so interested in the work that you will want to make motor-propelled models, and for models of this type the author refers you to his handicraft book, The Handy Boy, in which are presented instructions for building record-breaking models, for making propellers and rubber-band motors, for a motor-winder and flight-distance measuring apparatus.

For gaining an understanding of

The Matter of Balance, take a business-size envelope. Drop this flat, straight down, and it will land upon the ground without upsetting; but give it a forward movement and it will upset. Its upsetting in the one case but not in the other is due to the action of two forces, the force of gravity which pulls it groundward, and the air pressure beneath which buoys it up. In order to have it balance, the centers of these two forces must coincide.

The Center of Gravity and Center of Air Pressure. The center of gravity is always the center of weight, the balancing

Figs. 472-476. - Diagrams in Explanation of the Matter of Balance point. The center of air pressure varies. When the envelope is dropped straight down, flat (Fig. 472), this center of pressure (P) comes at the same point as the center of gravity (G),and there is perfect balance. When the envelope moves forward while dropping, however, the center of air pressure shifts forward, the front edge is forced up, and the envelope upsets (Fig 473).

To Make An Envelope Glide without upsetting it is necessary to shift the center of gravity to a point which will coincide with the center of pressure. This can be done by adding weight to the forward edge. Try a paper-clip, slipping it over the envelope as shown in Fig. 474. If, with the clip in place, the envelope darts head-on to the ground, the clip is too heavy, and has caused the center of gravity to shift beyond the center of pressure (G and P, Fig. 474). Slide the clip back and forth until the weight is so adjusted that the envelope will first dip nose-

Figs. 477 and 478. - An Envelope Glider down for a distance, then straighten up and glide horizontally, as shown in Fig. 475.

If an air current strikes the envelope sidewise, it will cause the center of pressure to shift and the envelope to upset. Bend the envelope along its center so that the

Fig. 479. - A Cardboard Glider ends will tilt up (Fig. 476), and the danger of upsetting will be lessened. The tilted surface allows the air to slip off of the ends more readily than a flat surface does. Consequently, when a side current strikes end A (Fig. 476), lifts it up, and turns end B down, as indicated by dotted lines, the envelope glider quickly rights itself because the air slips out from under tilted end A and the air pressure forces up flat end B.

Figure 477 shows

An Envelope Glider made by opening all pasted flaps of an envelope (Fig. 478), and slipping a paper clip over the folded over gummed flap.

Figure 479 shows

A Cardboard Glider. The framework, or fuselage (A), connecting the planes, is a cardboard strip of the dimensions shown in Fig. 480, folded where indicated by dotted lines, into the shape shown in Fig. 481. Main plane B and elevator plane C are pieces of cardboard of the size shown in Figs. 482 and 483. Cut a notch at the center of each of the long edges of each plane as shown. The planes must be centered on the fuselage. Cut slots in the fuselage strip, as indicated, to slip the planes through, and when you find by testing out the glider that the ends of the planes balance, fix the planes in position with pins run through from the under side of the fuselage (Fig. 481). In launching the model, turn it so that elevator plane C is in front.

The advantage in making the glider type of model air-

Figs. 480-483. - Detail of Parts of Cardboard Glider plane is that it requires neither motor nor propellers, which are the most difficult parts of model airplanes to make. The glider will not go a great distance, but if you will shoot it into the air by means of a rubber-band sling shot you can send it several hundred feet. Figure 484 shows

A Sling-Shot Glider. Cut stick A (Fig. 485) of the proportions shown, and with a saw slit one end for a distance of 2 1/2 inches to receive keel B (Figs. 486 and 488). Cut keel B out of heavy cardboard, of the dimensions given in Fig. 489, then fasten it in the slots with brads. The hook upon the bow (C, Fig. 486) is provided for the loop of the sling-shot to slip over. Make it out of a heavy hairpin. Bend the pin into a straight piece, then bend one end into a hook (Fig. 487). Make a small hole through stick A 1 inch from the bow end, slip the straight end of the wire up through the hole, and bend

Fig. 484 - A Sling-Shot Glider it down against the top of the stick as indicated by dotted lines in Fig. 487. Bind the hook to stick A by wrapping with thread (Fig. 486).

Elevator plane D and main plane E (Fig. 485) are made of heavy cardboard. Figures 490 and 491 show diagrams for marking them out. In mounting place them so stick A crosses their centers, and bind each to the stick with a strong rubber-band, passed beneath the stick and looped over the plane ends (F, Figs.485 and 486). Rest the forward edge of plane D upon hook C (Fig. 486) to give it the proper tilt.

The Sling-Shot. It is not necessary to tell how to make this of looped together rubber-bands joined to a tree-crotch, because every boy knows how.

To Launch the Sling-Shot Glider, hold the tail end of the glider with the right hand and the sling-shot with the left

Fig. 486. - Side Elevation of Sling-Shot Glider Fig. 487. - Detail of End Hook

Fig. 492. - An Aerial Top 232.

hand, as shown in Fig. 484, and release the glider in the same way that you shoot an arrow from a bow. Slide the planes backward and forward until you find the positions which give the model perfect balance while in flight.

Fig. 493. - Detail of Completed Fig. 494. - Detail showing Hub Stick

Aerial Top and Shaft Connections

Fig. 495. - Detail of Propeller Blades

The Aerial Top shown in Fig. 492 is one of the most fascinating of the simpler aerial toys. By means of its spinning cord, it can be made to rise a distance of one hundred feet or more before returning to the ground.

The top requires a stick shaft (A, Fig. 493), a hub-stick (B), for which a carpenter's dowel-stick or a flagstaff may be used, two cardboard propeller-blades (C), and a large spool (D). With a saw, slot the ends of hub B a distance of 3/4 inch, to receive the propeller-blade (Fig. 494). Cut the slots at an angle that will give the blades the pitch shown in Fig. 493, and be careful to get the angle of the slots alike. Bore a small hole through the hub-stick, and taper the shaft to fit snugly in this hole (Fig. 494). Whittle the opposite end of the shaft to fit loosely in the spool hole. Plug up the lower end of the spool hole.

Figure 495 shows the dimensions for the propeller-blades. Fasten the blades in the hub ends with brads.

To Spin the Top, wind the cord about its shaft, spinning-top fashion, then hold the spool and string as shown in Fig. 492, and pull the string. When the string has unwound, the top will rise in the air.