In theory, it is possible to model a plane so delicately that it will fly against the wind by the pressure of the wind itself. It is extremely important that both sides of the plane be brought to this curve as accurately as possible. An efficient plane must, therefore, be covered smoothly on both sides. Such a plane again offers very little skin friction to the wind. It is difficult to lay down any hard and fast rules for the relation of weight to wing surface, since the types of aeroplanes differ so widely. It has been found, however, that in large models one square foot of surface will support about one-half a pound of weight, when traveling at a high rate of speed. You will find that your model, if its wings have a spread of thirty inches os thereabouts, will approach one pound in weight. The figuring will show you that two wings, whose combined area is less than 150 square inches, will be comparatively small and certainly well under those generally employed a year ago. The planes used on this season’s models are marvels of lightness and strength. Much has been learned by studying the methods employed by the builders of man-carrying aeroplanes. It is a simple matter to build a three-foot plane which weighs complete less than one ounce, and is strong enough to withstand many a violent shock. [Illustration: A geared model built by Leslie V. Robinson] [Illustration: An ingenious biplane] It will be found a good plan first to lay out the exact form of your plane on a smooth board. Make the depth of the plane one-fifth of its length. It will be noticed that this plane is much more slender than those used last year. Next draw a line at the center the entire length of the board, and mark off one-tenth of the length of the plane from either end. From this center describe a quarter circle at either end, on the same side of the line. This will form your main or entering wedge. The rear corners should be cut sharply away and only slightly rounded. There is no better material for the main frame than a thin reed, cane or bamboo. The longer ribs may be made of any light lath and the cross ribs of a thin flat strip of the same material. Soak the reed overnight to make it as pliable as possible, or heat it over a flame. Now lay the reed over the outline of the plane, and hold it in this position by driving thin brads on both sides and bending them over the cane. When the outer edge is complete, mortise the ends slightly and tie and glue firmly together. With the outer frame held rigidly in position, it will be found a much easier matter to introduce the ribs. If you are building a flexed plane first, insert a stick of wood from end to end before placing your cross ribs in position. The thickness of this temporary stick will, of course, determine the curve of your plane. For a three-foot plane, a height of one-half an inch will answer.