Storing energy for a long distance flight
CHAPTER V THEORY AND PRACTICE OF PLANE CONSTRUCTION
THE planes of your model aeroplane need no longer be a blind experiment whose merits or defects may only be learned by actual test. The science of wing-building is much better understood to-day than a year ago. Without going into the complicated equations dealing with aspect ratio, pressure, and gravity, it is important that one bear in mind a few definite rules in designing even the simplest planes. A great many useless experiments may be avoided.
In a previous volume, it was pointed out that a narrow plane, or one with a high aspect ratio, driven with its broader side forward, would yield greater support than a square surface of the same area. (The aspect ratio, it may be well to repeat, is the relation between the width and depth of the plane. A wing, for instance, whose width is five times its depth, is said to have an aspect ratio of five.)
It has been found that on small planes the center of pressure is situated about one-third the distance back from its front or entering edge. The center of pressure in flexed planes occupies about the same position.
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.