SIMPLE paper airplanes that you can build in a few minutes can serve as valuable models in illustrating many fundamental principles of aeronautics. With them you can see in action characteristic features of such designs as the airplane, glider, flying wing, helicopter, and autogiro. The drawings on this and the following pages show the steps in making three basic models-the "helicopter," "glider," and "flying wing." With models like these, you can study the effect of wing loading, the reason for dihedral, the operation of ailerons, rudders, and elevators, the importance of correctly locating the center of gravity, the use of wing flaps, the effect of increasing wing camber, and other aerodynamic principles that are necessary for a sound understanding of the nature of flight.

You may drop, for instance, the "helicopter" from a height and see the rotating blades retard its descent. The blades of a true helicopter are power-driven, and though your model might be more accurately called a "gyroplane," of which the auto-giro is the most familiar example, its downward movement is that of a helicopter, power off, windmilling to a glide landing.

helicopterglider

These are the steps in making a "glider" from a single sheet of paper. After the model has been folded, a staple or paper clip will hold it firmly. At right, the nail is being tried to determine proper location for a model's center of gravity.

A plane's speed is governed in part by wing loading, which is its weight divided by the surface area of its wings. The greater this wing loading is, the greater the speed must be to keep the plane in the air. Examples are the light weight and large wings (low wing loading) of a slow primary trainer and the relatively small wings and greater weight (high wing loading) of a fighter with its oversize, high-powered engine and burden of guns, ammunition, armor, and fuel.

Make several glider models of the same size and shape, one of oinionskin or tissue paper and one each using one, two, and three thicknesses of typewriting paper, for a study of the effect of changing weight for the same wing area. Then make two more models of the same kind of paper, one with wings as in the drawing and the other with the last fold shifted to form a wider wing. The model with the larger wing surface (lower wing loading) will travel considerably more slowly.

Dihedral-mounting wings on a fuselage so that they tilt slightly upward-helps to produce lateral stability. Fold the wings of a glider downward, and it will very likely roll over and over in flight, but if the wings are folded up and away from the fuselage, they will counteract this tendency and keep the model flying level.

The location of the center of gravity contributes to longitudinal stability. Mount a small nail or brad between the wings of a glider, keeping it in place with a staple, and watch how it affects the flight when shifted back and forth. You will find that the model flies well only when its center of gravity remains within certain limits in relation to the wings.

Directional stability-ability to fly in a straight line-depends on the fuselage and fin. Comparison of the paths of flights of the glider model, which has a relatively long fuselage, and the flying wing, which has none, offers a good example.

For ailerons, bend the rear edge of one glider wing up and the other down, and the model will fly in a circle. If the bends are reversed, the rotation will be in the opposite direction. Bend both of the rear edges up or down to show the effect of elevators. In the former case, the wind against the upturned surfaces will cause the tail to go down and the nose to rise, while the glider will dive if the edges are bent down.

aileronsletters in these drawings

By carefully following the letters in these drawings, you can fold a sheet of paper into a "flying wing" without much difficulty. For some demonstrations a fuselage will be needed, while for some others ailerons may be made, as below.

To study the effect of a rudder

To study the effect of a rudder, bend the tail end of the fuselage of another glider to the left. Your model will go into a spin toward that side. This can be counteracted by aileron control-bending the rear of the right wing up a,nd that of the left wing down. A few trials may be necessary to get the proper correction to make the model turn evenly toward the left. In this test, the aileron controls are used opposite to the way they would be used on a plane because your rudder is under the wings.

The directional stability of your flying wing can be improved by adding a boom, or fuselage, as shown in the drawings. Bending the end of this boom upward will help still more. Make several flying wings as nearly alike as possible and provide each with the inserted fuselage. Cement together the upper and lower surfaces of one model's wing so that it lies perfectly flat, and separate the wing of another by pushing in at the ends. The wing with the curved surfaces (camber) should stay in the air longer and fly more slowly than the flat-wing model. Cut flaps in the lower surface of the wing of a third flying-wing model and bend them down. This should make the model sail more smoothly than one which has not been treated in this way.

With a little experimenting, you can add to these ideas and demonstrate many other aeronautical principles, including such spectacular feats as loops, rolls, and other aerobatics.