It is almost humorous that man, who for centuries had nourished the secret ambition of acquiring wings, should have found his dream imperfectly realized in the twentieth century by riding in a kite. For that is all an airplane actually is. Yet a “kite” which is no longer tied to earth by a cord and which is equipped with a motor to drive it forward at a great speed has one decided advantage over the old-fashioned sort. The paper kite had to wait for a favorable breeze to catch it up and bear it aloft. We saw in the last chapter how the push of the air against the underneath side of the kite caused it to rise. If instead of the air current pushing against the kite, the kite had pushed against the air, exactly the same result would have been attained. A bird, flying in a dead calm, creates an upward pressure of air by his motion which is sufficient to support his weight. But the bird, as he flies forward against the air creates more resistance under the front portion of his body than under the rear, and this increased upward pressure would be sufficient to turn him over backward if his weight were not distributed more toward the front of his body, in order to counterbalance it.

This fact can be easily illustrated with a piece of cardboard. Take a small oblong sheet of cardboard and mark a dot at its center. If the cardboard is of even thickness this dot will be the center of its weight. Now hold the cardboard very carefully in a horizontal position and allow it to drop. It should fall without turning over, for it is pressing down evenly on the air at all points. You might say it is creating an upward air pressure beneath it, which is evenly distributed. The center of the supporting air pressure exactly coincides with the center of weight. If you have not held the cardboard in a precisely horizontal position this will not be true. The unequal air pressure will cause it to lose its balance and “upset.” This is very much the sort of experiment that Lilienthal tried when he jumped from the top of a hill in his glider, and it is easy to imagine how much skill he must have required in balancing himself in order to prevent his crude contrivance from overturning.

But now suppose that instead of dropping the piece of cardboard straight down, we give it a forward push into the air. As the cardboard moves forward it naturally creates more air resistance under the front than under the rear, and this unequal pressure will cause it to do a series of somersaults, before it reaches the floor. The same thing would happen to the bird or the airplane whose weight was evenly and equally distributed.

Now since the air pressure is greater under the front of the cardboard, add a counterbalancing weight by dropping a little sealing wax at the center front. The dot that you made in the middle of the sheet is no longer its center of weight. The center of weight has moved forward, and if it now corresponds to the center of pressure the cardboard can be made to fly out and across the room without overturning.

The whole problem of balancing a glider or an airplane is simply this one of making the center of weight coincide with the center of the supporting air pressure. Adding weight at the front of the glider is not the only way of doing this: perhaps the reader has already thought of another. Since the air pressure is caused by the weight of the cardboard and its forward motion, we could cut the sheet smaller at the front so as to lessen its air resistance there, or we could add a “tail” at the stern in order to create more air resistance at that end. Either of these plans would move the center of pressure back until it corresponded with the center of weight, and so would complete the balance of our cardboard glider.

In the bird's body all of these methods of obtaining balance are combined. His body and head taper to a point at the front in order to decrease the forward air resistance. The weight of his body is distributed more toward the front, thus counterbalancing any tendency to whirl over backward. His tail increases the stern resistance, thus helping to draw the center of pressure back to correspond to the center of weight.

We begin to see some reasons why a man equipped with wings could never be taught to fly,—as well as how perfectly the form of the bird is planned to correspond to his mode of travel. No wonder the early experimenters with wings, finding themselves so utterly helpless and awkward, attributed the bird's ease and grace of carriage to “magic.”

The modern airplane is constructed with the most painstaking attention to this principle of balance. Next to it in importance is that of wing construction: that is, the size, shape and proper curve of the supporting planes. Here again the construction of the airplane follows very closely the general form of the bird. A large bird which flew very high would be found to have his wings arched high in front, where they would have considerable thickness, and sloping down very rapidly toward the rear, while their thickness rapidly diminished. This sort of wing has great lifting power, and it is the sort that is used on an airplane which is built to “climb” rather than to develop speed.

As the arched wing cuts through the air it leaves above it a partial vacuum. Nature always tends to fill a vacuum, and so the airplane is drawn upward to fill this space. As the wings cut through the air a new vacuum is constantly created and so the airplane mounts higher and higher. The airplane is being carried upward by two forces: the air pressure beneath it and the vacuum above it which draws it up. The air pressure beneath it increases with the speed at which the airplane is traveling, and it has a tendency to press the wing into a more horizontal position, thus destroying its climbing properties. At the same time, when this happens, the thick front section of the wing presents a great “head resistance” which retards progress, and a very high speed becomes impossible.

Wings of this type can never be used on an airplane which is intended to travel at high speed. They were used on the heavy bombing and battle planes of the Great War, for they are capable of lifting a very great weight. But on the scouting planes, where speed is essential, a totally different sort of surface was employed. Here the plane is very little arched and of almost even thickness, tapering only very slightly to the rear edge. It also tapers somewhat at the front, so as to lessen its “head resistance” as it cuts through the air.

Such a surface creates little vacuum above it, and consequently has not a great lifting power. On the other hand it offers little “head resistance” and so permits a high speed. And right here it should be mentioned that a powerful motor does not in itself make a swift airplane, unless the wings are right,—for if the wings create a strong resistance in front of the airplane they destroy speed as fast as the motor generates it.

Remember that the lifting power of the airplane wing is made up of two factors. First, there is the resistance or the supporting air pressure created by the weight and speed of the wing; second, the arch of the wing creates a vacuum above it which tends to lift the airplane up. Now when for speed the arch is made very slight, the lifting power can still be increased by increasing the area of the wing, thus adding to the upward pressure. Thus for certain war duties an airplane with very large, comparatively flat wings can develop both a very good lifting power and a very high speed.

We have already mentioned the “head resistance” of the airplane wing. If the wing could strike the air in such a way as to sharply divide it into currents flowing above and below, there would be no head resistance. But the very arch of the wing in front gives it a certain amount of thickness where it strikes the air, so that instead of flowing above or below, a portion of the air is pushed along in front, retarding the progress of the airplane. This resistance is called by aviators the “drift.” The best wing is the one which has the maximum lifting power with the minimum head resistance, or, to use technical language, the greatest “lift” in proportion to its “drift.”

Of course, not only the wing but all parts of the airplane offer resistance to the air. In order to reduce this total head resistance to the minimum, every effort is made to give the body or “fuselage” of the airplane a “streamline” form,—that is, a shape, such as that of a fish or a bird, which allows the air to separate and flow past it with little disturbance. For this purpose the fuselage of the airplane is usually somewhat rounded and tapering toward the ends, often “egg shaped” at the nose.

The method of “wing warping” invented by the Wright brothers is still used on all modern airplanes to preserve lateral stability. The part of the wing which can be warped is called the aileron. There are two ailerons on every wing, one on each side at the rear, and they may be raised or drawn down by the action of a lever operated by the pilot.

If the pilot feels that the left side of his machine is falling, he draws down the aileron on that side and raises the right hand aileron. The aileron which is lowered catches the air currents flowing beneath the wing on that side. At the same time the raised aileron on the right lessens the pressure under the wing on that side and so gives it a tendency to fall. In this way, in a fraction of a minute the wings are brought level again and lateral stability is restored.

Whereas the old Wright biplane had an elevating plane in front of the main planes, most machines to-day have the elevating surfaces at the rear. By raising the “elevators” an upward motion is obtained, or by lowering them, a downward motion.

Steering to right and left is accomplished by a rudder at the rear of the airplane body or “fuselage.” This rudder may be turned to right or to left, working on a hinge.