Learning to fly in the U.S. Army

by E. N. Fales

The second way to get good efficiency is to choose the shape of the wings properly. For instance, early experimenters tried to get results with flat wings, and failed completely, for the flat wing proved to be very inefficient. When it was observed that birds had curved wings, this principle was applied to early experiments and then for the first time man was able to obtain support in a flying machine. The fundamental principle of efficiency in wings is that they must be curved, or cambered, as it is sometimes called. This is because as the wing rushes onward it wants to sweep the air downward smoothly and without shock, as can be done only when the wing is curved (see air flow, Fig. [21]).

Fig. 21.—Efficiency of curved and flat wing.

(a) Air flow past curved wing is smooth without much eddying; (b) air flow past flat wing produces eddies above it.

The question of wing curvature is exceedingly important then; we find that the curvature of its upper surface is particularly so. We notice that airplane wings all have a certain thickness in order to enclose the spars and ribs; it is not necessarily a disadvantage for them to be thick, due to the fact that the upper curve of the wing does most of the lifting anyway, and the lower side is relatively unimportant. You can make the lower surface almost flat, without much hurting the effect of the wing, so long as the upper surface remains properly curved. However, the upper surface must be accurately shaped, and is so important that in some machines we find cloth is not relied on to maintain this delicate shape, but thin wood veneer is used (I refer to the front upper part of the wing). In general, then, wings are thick toward the front and taper down to a thin trailing edge.

Fig. 22.—Diagram of vacuum and pressure on airplane wings.

Note in biplane reduced vacuum on bottom wing.

You may wonder how it was found that the upper surface of the wing was the most important; and I will say that this was one of the interesting discoveries of the early history of aerodynamics. People at first thought that a wing sweeping through the air derived its support entirely from the air which struck the bottom of the wing, and they assumed that if the bottom of the wing were properly shaped, the top did not matter; that is, all the pressure in the air was delivered up against the bottom surface. But a French experimenter conceived the idea of inserting little pressure gages at various points around the wing. He found, it is true, that there was considerable pressure exerted in the air against the bottom of the wing; but he found a more surprising fact when he measured the condition above the wing. When he applied his gage to the upper surface of the wing, it read backward, that is, showed a vacuum, and a very pronounced one. He found that there was a vacuum sucking the top part of the wing upward twice as hard as the pressure underneath was pushing; so that two-thirds of the total lift on this wing was due to vacuum above it (see Fig. [22]).

In the diagram the shaded area on top of the wing represents vacuum above, that below the wing represents pressure beneath.