FIRST PRINCIPLES OF AN AIRPLANE
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."
[Illustration: DIAGRAM SHOWING THE ESSENTIAL PARTS OF AN AIRPLANE]
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.
[Illustration: WRIGHT STARTING WITH PASSENGER]
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.
[Illustration: AN EARLY FARMAN MACHINE PRIOR TO START]