As long as a plane remains horizontal, or nearly so, a very narrow surface,—one, that is, with a high aspect ratio,—will exert greater lifting power than a deeper plane of the same area. An examination of the successful model aeroplanes of 1911 will show that the depth of the planes has been cut away. Planes are used with an aspect ratio as high as ten. The speed at which such a plane travels is a very important factor. As the speed increases, the efficiency of the plane surface increases, so that a model aeroplane driven rapidly may be sustained by less wing area than in the case of one which flies slowly. As the front edge of a plane is raised, the center of pressure travels backward. By the time the plane has reached an angle of about fifteen degrees, its lifting power has diminished about one-half. A very narrow plane, or one with a high aspect ratio, should, therefore, be set near the horizontal. The model should, moreover, rest upon skids at as low an angle as possible. In starting off, the planes will thus exert their maximum lift, or nearly so. If the narrow planes be elevated too much, the center of pressure will come nearer the rear than the front edge, and tend to force the aeroplane downward, or, as the phrase is, make it "sit on its tail." As long as a plane is traveling horizontally, or at low angles, its rear portion exerts very little sustaining power and may be cut away. A plane with a high aspect ratio is much more stable in flight than a surface of greater depth. The center of gravity of a flat plane would, of course, coincide with the center of the surface when the plane is in motion. When the plane tilts, the center of pressure, as we have seen, moves backward or forward. If the plane has little depth or a high aspect ratio, this center of pressure cannot move far. It must oscillate back and forth within very narrow limits. A very little tilt up or down will restore it to its normal position, so that a plane with high aspect ratio is more stable than one with a deeper surface. The efficiency of a curved surface over a flat plane was analyzed in a former volume. Such a curve, when well drawn, adds to the lifting power as well as the stability. Since a curved plane does more work than a flat surface, its size may be reduced and its aspect ratio increased. In other words, the curved plane may be narrower than a flat surface, and may be made thinner in proportion to its width. The height of the curve, or camber as it is called, has been worked out by elaborate mathematical equations, but we may take the general results without following the calculations. For a plane six inches in depth, the camber should be about one-half an inch, or one in twelve, or in this proportion. The curve should be a parabolic with the highest point well forward, one-third the way from the front edge. The front, or entering edge, of the plane should be the thickest point. It should be tapered off to a thin edge in the rear.