The design appeals especially to the builder of model aeroplanes, since they can be added with very trifling weight by curving the front skids forward and stretching the cloth across their forward corners. The Valkyrie monoplane is equipped with similar rudders, in the form of half circles carried in the same position. In addition to the vertical and horizontal stability planes, many aeroplanes are now equipped with stability planes extending diagonally from the vertical axis. These are placed below the main planes, extending outward not unlike the runners of a sleigh. This box-like form tends to confine the air and affords increased support. There is even an upward tendency from this pressure of air. These planes are usually rectangular in shape, the forward or entering edge being cut away sharply. By mounting these planes on the skids, their additional weight is practically nothing. Several interesting applications of this principle are shown in the accompanying illustrations of models. CHAPTER IX MODEL AEROPLANE DESIGNS Whether one be designing the simplest paper glider, a model or a passenger-carrying aeroplane, the problem of stability is the same. To keep afloat, your air craft must be supported, as a rule, by at least two surfaces to provide longitudinal stability. To understand the principle of longitudinal stability, picture to yourself a very delicately-balanced board or "seesaw." The center of gravity naturally falls between these two planes at either end, and the wings therefore tilt up or down, or seesaw, on this invisible fulcrum. With this principle in mind, the movement of your aeroplane, which may seem so capricious, will be seen to follow definite laws. When a gust of wind forces the front plane upward, the rear plane swings down. This movement increases the angle of both planes to the horizontal; they offer much greater resistance to the air, and the speed of the machine is checked. As the aeroplane slows down, as a rule, it tries to right itself, that is, to seesaw back to balance at a horizontal position. This in turn reduces the resistance the planes offer to the wind, and the flight is continued at its original speed. [Illustration: An excellent piece of workmanship. Model by R. Mungokee] [Illustration: Model with minimum plane surface. Built by A. C. Odom] The trick, therefore, is to adjust your planes with regard to the center of gravity so that they will always seesaw back to a horizontal position; in other words, to secure automatic longitudinal stability.