The centre of these forces is not as might be supposed at the centre of the plane, but at a point between the centre and the front edge called the "centre of pressure." The centre of pressure approaches the front edge as the angle of the plane with the horizontal becomes less.
In order to render a better idea of how it is possible for an aeroplane to gain support in the air consider a skater moving swiftly over very thin ice which would not bear his weight, but since he is moving so rapidly that any one portion of the ice does not have time to bend to the breaking point, he is supported. In somewhat the same manner, the planes pass so rapidly on to new and undisturbed bodies of air, and stay over one body for so brief an instant that there is no time to completely overcome the inertia of the air and force it downwards.
FIG. 12. The action of the air upon a curved and a flat plane. We have seen that by the effects of the resistance of the air, an aeroplane may be sustained in the atmosphere. We must now see in what manner we can use these effects to the greatest advantage.
First of all, we have been continually speaking of a "plane" as the supporting surface, which from the definition of the word would lead one to believe that they were flat. If the wings of a bird are examined, it will soon be noticed that they are concave underneath. Since the first attempts at aviation, therefore, machines have been built with planes or wings concave on the underside. The reason for this is very apparent from Fig. 12. The first illustration shows the action of a flat surface moving through the air. The air streams, as represented by the lines do not follow the surface of the plane, but leave a considerable region of dead air. This is the reason that a flat plane is very inefficient and not capable of giving so great a lift as the curved plane in the next figure where the lines follow the outline of the plane. The less disturbance a plane causes in the surrounding air, the closer it is said to approach to "stream line form." A correctly curved plane is considerably more effectual than a flat one, giving at the same time greater "lift" and less "drift."
Built-up Planes, that is, planes having a double curve approaching true stream line form, come nearer being the ideal plane than any other from some standpoints, but do not possess any advantages when used on models of less than four feet spread.
FIG. 13. Section of a built-up plane showing how a rib is made. When made small, they offer greater "drift" or head resistance than a single curved surface plane and cannot because of the delicate structure necessary to make them light, withstand hard knocks. They have the further disadvantage of being from a constructional standpoint very hard to make smooth and rigid.
There are innumerable substances which would at first seem to recommend themselves as material for planes, but we may immediately thrust the greater portion aside. By all means avoid tracing cloth or linen, not only because its heavy weight forever precludes it from this use, but because it wrinkles and cockles so as to be absolutely useless when slightly damp or wet.
Tissue paper wrinkles easily and is not strong enough.