INVASION OF THE SKY
FROM the day when man began to assert his superiority over other
animals he began to cast longing eyes at the sky. Fired with ambition
and stirred with a spirit of mastery he chafed at the bonds that bound
him to earth; but the heavens remained a forbidden kingdom to him. Many
a bold adventurer who dared to emulate the birds paid the penalty of
his temerity with a broken limb and even with his life. It seemed as if
man were destined forever to grovel on the bed of the atmospheric ocean
with never a chance to rise except in dreams and fancy.
It was not until near the close of the eighteenth century that a means
of rising off the earth was discovered. Two brothers, Stephen and
Joseph Montgolfier, of Annonay, France, were sitting before a fire,
watching the smoke curl up the chimney, when it occurred to one of them
that smoke might serve as a vehicle to carry them up into the air. They
belonged to a prominent paper-manufacturing family and naturally turned
to that material as the most suitable for trapping and harnessing the
smoke. They began their experiments with a large bag of thin paper
which they filled with smoke and floated up to the ceiling. The next
step was to fasten a dish filled with burning embers to the bag, so
that the balloon carried its own smoke generator. The experiment was
tried in the open air and the balloon arose to a great height. Larger
bags were made of linen and paper and on the fifth clay of June, 1782,
a public exhibition was given. A pit was dug in the ground in which a
fire was lighted and over this was placed a huge balloon which weighed
300 pounds. Eight men were required to hold it down while it was
filling with heated air and, when released, it shot up to an elevation
of about 6,000 feet and came to earth about a mile and a half away.
THE FIRST HYDROGEN BALLOON
When the news of this event reached Paris a professor of physics named
Charles suggested that hydrogen, being much lighter than air, would
raise the balloon without the use of fire. By a popular subscription
funds were raised to defray the expenses of securing enough iron
filings and sulphuric acid to generate the hydrogen necessary to fill a
balloon of 22,000 cubic-foot capacity. On the 27th of August, 1783, a
flight was attempted. The big bag arose without mishap and disappeared
in the clouds. Three quarters of an hour later it landed in a field
fifteen miles away much to the astonishment of the villagers thereabout
who gathered around the strange bobbing monster with mingled fear and
curiosity. One of the number, more daring than the rest, advanced and
shot the balloon, whereupon the crowd closed in and tore it to pieces
with their pitchforks.
In November of the same year the Montgolfier brothers built a balloon
48 feet in diameter and 74 feet high, and Jean François Pilatre de
Rozier, a professor of natural history, made several ascents with the
balloon held captive. Then, in company with the Marquis d’Arlandes, the
balloon was cut loose and the balloon voyage was undertaken. Below the
car of the balloon was an iron vessel in which a fire was maintained to
furnish the heated air. The aeronauts each carried a bundle of fuel to
feed the fire and a wet sponge to extinguish sparks that might ignite
the bag. Despite this precaution a number of holes were burnt in the
envelope, but nevertheless the flight was successful and the daring
voyagers came to earth without mishap after a short journey.
M. de Rozier’s career as an aeronaut was a short one. The first man
to be carried up in a balloon, he was also the first balloon victim.
He undertook to combine the Montgolfier and the Charles systems by
building a balloon that employed both the hot air and the hydrogen
principles, but the balloon took fire and De Rozier with his companion,
the Marquis de Maisonfort, were both killed. Two months after De
Rozier’s first balloon ascension a flight was made at Lyons in a huge
fire balloon which carried seven passengers. This big bag was 100 feet
in diameter and 130 feet high, with a capacity of 590,000 cubic feet.
The invasion of the air was now well under way, although for a time
it made little real progress. Owing to the danger of fire, hot air
eventually gave way to hydrogen and later to coal gas, which, although
it did not have half the lifting power of hydrogen, possessed the
advantage of being much cheaper.
THE WEIGHT OF AIR
It seems hardly necessary in this day and generation to explain that a
balloon rises because it is lighter than the air it displaces, but it
does seem astonishing that a balloon weighing several tons may yet be
lighter than an equal volume of air. We do not ordinarily think of air
as having any weight. We know that the ocean of air bears on the earth
with a pressure of about 14½ pounds per square inch, or about a ton per
square foot at sea level. This amounts to 28 millions tons per square
mile and, when we multiply this figure by the number of square miles of
surface in the total area of the earth, we find that the whole ocean
of air has a weight of 5,500,000,000,000,000 tons--a figure which is
far beyond our conception, but it does impress us with the fact that
air is a ponderable substance. Of course, the atmosphere that exerts
a pressure of 14½ pounds per square inch is scores of miles deep, but
even a small quantity of air has appreciable weight. If weighed in a
vacuum a cubic foot of air would tip the scales at 1¼ ounces, and 100
cubic feet would weigh close to 8 pounds. The air in an empty room 10
feet square and with a 10-foot ceiling weighs nearly 80 pounds at a
temperature of 32 degrees Fahrenheit. In other words, if we had a box
measuring 10 feet on each side and weighing less than 80 pounds in a
vacuum it would float in the atmosphere when the thermometer was down
to the freezing point, provided there was no air in the box to weigh it
down.
Vacuum balloons were proposed long before the time of the Montgolfier,
but the problem was to construct a vessel strong enough to resist the
crushing pressure of the atmosphere. The best bracing for the walls
of the vessel is some gas dense enough to exert a pressure equal to
that of the atmosphere but whose weight is less than that of the air.
The weight of the vessel must then not exceed the difference between
the weight of the gas and an equal volume of air. The Montgolfier
brothers used heated air to keep their box or envelope distended. Air
expanded by heat is lighter than cold air. However, it does not make a
very good filler because 1,000 cubic feet of air heated to 212 degrees
Fahrenheit weighs 59 pounds, leaving only 21 pounds on a freezing cold
day for the absolute weight of our 1,000-cubic-foot box. Coal gas,
on the other hand, weighs about 40 pounds per 1,000 cubic feet at 32
degrees Fahrenheit, while the same volume of hydrogen weighs only 5½
pounds. If coal gas were used to brace our box against the pressure
of the air we should have a surplus of about 40 pounds for the weight
of the box, while hydrogen would allow us 74½ pounds. Of course, the
weight of a volume of gas depends upon its density. No matter how small
a weight of gas we placed in the box it would fill the box completely,
but when we speak of a 1,000 cubic feet of gas or hydrogen we mean a
volume sufficiently dense to bear against the container with the same
pressure that the atmosphere presses outside; _i. e._, 14½ pounds per
square inch. Temperature also affects the pressure. In the case of
fire balloons the hot air inside the envelope is less dense and hence
lighter than cool air outside, but the pressure is equal because the
former is expanded by heat. The volume of gas in a hydrogen or coal-gas
balloon varies greatly with the temperature to which it is subjected.
For instance, if on rising through a cloud or a blanket of fog, the
balloon should encounter the rays of the sun, the heat would burst open
the envelope were no vent provided. The mouth of the bag is kept open,
however, for just such emergencies so that the surplus gas may escape.
On the other hand, sudden chilling of the gas will contract it and send
it down to earth, or the balloon might encounter a downward current
of air, when the only salvation of the aeronaut is to throw out the
ballast.
BALLOON NAVIGATION
Balloons seem like very helpless craft, and yet they are capable of
skillful navigation at the hands of an experienced pilot. Although the
balloonist has no means of self-propulsion and must drift with the
winds, he is capable of controlling motion in the vertical direction
and can choose the particular air currents on which he desires to
ride. By throwing out sand ballast the bag may be made to rise and
by letting out the gas it may be made to descend, and a pilot who
is familiar with prevailing currents of the atmosphere or able to
interpret meteorological indications, may locate the air stream that
will carry him to his destination. Sand is the balloonist’s fuel; when
that is gone the balloon may as well come to earth at once. Its course
can no longer be directed and there is nothing to prevent it from being
suddenly dashed to earth should it run into an “air hole,” which is
another name for a downward air current. When a balloon comes to earth
it is liable to be dragged by the wind and many accidents from dragging
occurred in the early days of aeronautics until John Wise, an American,
invented the rip panel by which the envelope may be ripped open by
pulling a cord, thus freeing the gas and permitting the bag to collapse
instantly.
[Illustration: A LOOM PROVIDED WITH A JACQUARD ATTACHMENT
Note the belt of cards that determine the pattern that is to be woven]
[Illustration: A BATTERY OF MULE-SPINNING FRAMES]
[Illustration: JACQUARD ATTACHMENT FOR LOOMS]
SEVEN MILES ABOVE THE EARTH
The greatest altitude ever attained by a balloon has been claimed
for Glaisher and Coxwell, who in 1862 went up to a height of seven
miles. Both men were overcome by the extreme rarity of the atmosphere.
Coxwell, however, although nearly paralyzed and unable to move his
arms, succeeded in seizing the safety valve rope in his teeth and
pulled it before he lost consciousness. The balloon was then at an
altitude of 29,000 feet and rising at the rate of 1,000 feet per
minute. Thirteen minutes elapsed before he regained his senses, and
then the balloon was falling at the rate of 2,000 feet per minute.
From the data furnished, it has been estimated that an altitude of
37,000 feet was reached. While there has been much dispute as to the
authenticity of this record, the altitude of 35,100 feet established in
1901 by Professor Berson and Dr. Suring in the German balloon Preussen,
probably represents a higher ascent than that of Glaisher’s balloon.
The new system of aerial transportation proved very useful in the
nineteenth century. During the siege of Paris, in 1870-1871, sixty-six
balloons arose out of the beleaguered city, and all but seven made
their escape in safety. In our own Civil War captive balloons were
first used to direct the fire of artillery. It was in one of these
balloons that Friedrich von Zeppelin, who was a German military
attaché with the Union forces, made his first aerial ascent. He was
so impressed with the advantages of military observation from such a
lofty aerie that he became from that moment an aeronautical enthusiast,
and on his return to Germany urged upon the military authorities the
importance of the balloon in war. It was then that he began his
aeronautic studies and experiments which culminated in the construction
of the gigantic ships of the air with which Germany undertook to carry
the terror of war into England.
KITE BALLOONS
In the World War captive balloons were extensively used by both sides,
an important development being the kite balloon. The common spherical
captive balloon is very unsteady except in perfectly still air. It bobs
around and swings and turns, making an unfavorable base for careful
observation. In heavy winds it is liable to be dashed to earth by
sudden gusts. The kite balloon is held up not only by its own buoyancy,
but also by the wind in the same way that a kite is supported. The
bag is elongated (sausage-shaped) and is fastened to the anchor line
in such a way that it is tilted like a kite. To assist in holding it
in this position the balloon is ballasted at the after end and the
ballast, if you please, is air. A big air-filled bag hangs from the
stern of the balloon and serves as a rudder while two other horizontal
bags serve as stabilizing fins.
SELF-PROPELLED BALLOONS
No sooner had the Montgolfier brothers proved that it was possible to
rise off the earth upon a bubble of hot air, than inventors began to
devise schemes of propelling the bubble and controlling its course.
The first dirigible balloon was built in 1784 and, strangely enough,
oars were used to propel it. The balloon had a fish-shaped body, just
like that of modern airships, and the bag was kept inflated by means
of air as it is to-day. It was realized even at that early date that
the envelope must retain its shape if it is to plow through the air
with a minimum of friction. As the gas would gradually leak out of the
envelope the bag would become flabby and present a crumpled bow to the
ocean of the air and, to overcome this, a double envelope was provided
and air was introduced between the inner and outer skin. Means were
provided for pumping air in as the gas slowly escaped, thus keeping
the envelope fully inflated. As the air was pumped in sand ballast was
thrown overboard so that the buoyancy of the airship was not impaired.
To-day air is used to keep gas envelopes inflated, but instead of
placing the air in an outside envelope it is introduced into a small
bag inside called a “ballonet.”
From time to time other means of propulsion were proposed and tried. In
1852 Henri Giffard, who is known as the Fulton of aerial navigation,
built and operated a dirigible driven by a steam engine, which he hung
at a considerable distance below the gas bag, so that the fire box
would not be liable to ignite the highly inflammable gas. A draft was
maintained in the fire box as it is in a locomotive by letting the
steam discharge into the smoke stack, but in this case the stack was
inverted and the smoke and gases were blown downward and away from the
coal gas with which the balloon was inflated. The danger of fire and
the difficulty of building a power plant light enough to be carried
aloft and powerful enough to give the craft any material headway were
the chief obstacles that had to be overcome. At the close of the
nineteenth century, Santos Dumont, a Brazilian inventor, built a
balloon that was driven by a gasoline engine. Despite the apprehension
of other aeronauts this machine proved an unqualified success. Two
years later, in 1900, Count Zeppelin introduced the rigid dirigible,
which to-day is the standard for large airships.
THREE TYPES OF DIRIGIBLES
There are three types of dirigible airships; the flexible, the
semirigid, and the rigid. In the flexible type, as we have observed,
the envelope must be kept tightly filled in order to hold its shape
when driven against the air. The car is suspended from the gas bag. In
the semirigid type a rigid frame or backbone serves as a keel for the
gasbag and helps to prevent it from crumpling up. In the rigid type a
casing incloses the gas or rather bags; for the gas Is contained in a
number of separate bags fitted into separate compartments. The casing
is composed of a framework of duralumin, which is an alloy of aluminum,
with a percentage of copper and nickel. It weighs but little more than
aluminum, but is five times as strong. Over the duralumin framework is
stretched a sheathing of rubberized fabric. Because of the weight of
this casing rigid dirigibles must be made in large size.
ACROSS THE ATLANTIC IN A DIRIGIBLE
Before the war large Zeppelins were built that were fitted with
luxurious cabins and dining rooms and made regular scheduled voyages.
The big British dirigibles are of the Zeppelin type. The R-34, was
672 feet long and 79 feet in diameter. It was fitted with nineteen
gas bags and had a total capacity of over two million cubic feet of
hydrogen. It was driven by five engines, each developing from 250 to
275 horsepower, and was capable of making from 50 to 75 miles per
hour, depending upon whether or not the engines were pushed. The big
dirigible left the Royal Naval Air Station, near Edinburgh, on the
2d of July, 1919, and landed at the Roosevelt Field near Mineola on
the 6th, having made the trip in four days and two hours. The course
covered about 3,100 sea miles, but the actual air mileage was about
6,300 miles because head winds were encountered. In aeronautic voyages
it is the distance through the air that must be reckoned rather than
the distance over the ground or sea. An airship may be traveling at
the rate of 50 miles per hour through the air, but if there is a wind
of 30 miles per hour blowing against the course of the dirigible, the
latter will be making only 20 miles per hour over the ground, or if the
wind is blowing with the airship it will be making 80 miles per hour
over the ground. Because of the head winds the transatlantic flight of
the R-34 was so much longer than had been anticipated that its stores
of fuel were almost completely exhausted. And yet, when the airship
started out from Edinburgh, it carried 81 tanks of gasoline, each
containing nearly 70 gallons, or a total of 4,900 gallons. This fuel
weighed nearly 16 tons. Almost a ton of oil and 3 tons of water added
to the load and the baggage and crew amounted to 4 tons more. The total
weight carried was over 24 tons and the dirigible fully loaded weighed
altogether about 60 tons. When the dirigible started out it had to fly
low, but as the fuel was consumed it grew lighter and rose higher. The
surplus hydrogen had to be pumped into steel tanks where, owing to its
compression, it was heavy and served as ballast which could at any time
be fed back into the gas bags to increase the buoyancy of the airship.
We can no longer think of air as having no weight when we consider that
all this tonnage was supported by air.
The R-34 was by no means the largest dirigible built, but we dare not
boast of the size of the airships of to-day when the aeronautics is in
its in-fancy, because our present dirigibles may seem puny alongside
the big aircraft that may be built to-morrow. The dimensions of the
R-34 have been given because of the historic interest in this first
dirigible to span the Atlantic Ocean.
HEAVIER-THAN-AIR MACHINES
Marvelous as was the achievement of the Montgolfiers and wonderful
as were the aeronautic developments that followed the invention of
the balloon, the dominion of the birds was not really conquered until
man had learned how to fly in a machine heavier than the air. Captive
aeroplanes date back to the remote ages of ancient history. Kites are
really “heavier-than-air machines.” They maintain themselves in the air
because they travel through the air at a considerable velocity. True, a
kite may be stationary, or practically so with respect to the ground,
but if we detach ourselves from the ground and view the situation from
a drifting balloon, the earth will appear to be moving under us and the
kite will rush past us as it is dragged by the earth to which it is
tied. The idea of propelling a kite through the air, not by tying it to
the earth, but by furnishing it with its own propeller and power plant,
was conceived long ago, but the problem was to find a power plant
light enough. The honor of being the first man to rise off the ground
in an aeroplane belongs to C. Ader, who made several short flights
between 1890 and 1896 in a machine driven by a twenty-horsepower steam
engine. Our own S. P. Langley did some most important pioneer work in
flying and built a man-lifting, steam-driven machine in 1903 which
would have flown had it not been for an accident to its launching gear.
In fact, this very model was flown successfully a number of years
later. However, it was not until the gasoline engine was developed that
the power plant problem was solved. The internal-combustion motor was
made more and more powerful in proportion to its weight until now there
are several types that weigh less than two pounds per horsepower.
But the power plant was only one obstacle to be overcome. The real
problem was to learn how to control the machine after it rose into
the air. Otto Lilienthal attempted to learn how to fly in a motorless
flying machine. He provided himself with wings and, jumping off a
height or running down a slope, depended upon gravity to furnish him
with the necessary propulsion through the air. Unfortunately after five
years of gliding experiments a fatal accident terminated his aeronautic
research.
THE WRIGHT BROTHERS
In this country the Wright brothers, Orville and Wilbur, took up the
work of Lilienthal and they, too, undertook to learn to fly before they
built a flying machine. In all his years of experiment, Lilienthal
did not have more than five hours of experience in the air. The
Wright brothers determined to spend more time in the air and less in
theoretical speculations at home, so they built a gliding machine that
would sustain a man at a speed of eighteen miles per hour and picked
out a spot on the Atlantic coast where they were assured of fairly
constant winds of sixteen to twenty-five miles per hour. At first
the machine was used as a kite and various experiments were made in
balancing it. Then short gliding flights were made from the tops of the
sand dunes. Not until the art of balancing the glider and controlling
it in unsteady air currents was any attempt made at building a
motor-driven flying machine. It was by these experiments that the
Wright brothers discovered the system of warping the wings so as to
preserve the lateral balance of the machine. After several seasons of
experimental gliding, and not until they felt that they had learned
how to fly, was a power machine built. This made its first flight on
December 17, 1903. The first flight lasted only twelve seconds, while
the fourth flight lasted fifty-nine seconds. Many months were spent in
perfecting the machine and in solving the various problems of flight,
and not until September, 1905, did the Wright brothers feel that they
had mastered the art of flying. After that three years elapsed before
the world was actually convinced of the reality of airplane flight and
recognized the work of the pioneers.
TRANSATLANTIC AEROPLANE FLIGHTS
The development of the heavier-than-air machine is so recent and is
still advancing so rapidly that we dare not give more than a brief
outline of its progress here. The more important advances are familiar
to most of us and a record of achievements to-day will be hopelessly
out of date to-morrow. The war gave a tremendous impetus to flying.
Pilots were trained by the thousand. Machines grew in speed up to 150
miles per hour. Huge bombing machines were built, with a wing spread
of over 125 feet, and weighing ten to fifteen tons. These were capable
of carrying a load of four to five tons. The first flight across the
Atlantic was made in June, 1919, by the United States navy flying boat
NC-4, which flew to Newfoundland, then to the Azores, and from there
to Lisbon, Portugal. The trip was finally completed by a flight to
Plymouth, England.
The first nonstop flight was made in the same month by a Vickers Vimy
bomber which, with a favoring wind of thirty miles per hour, made the
trip in less than eighteen hours at a rate of 120 miles per hour.
To-day all-metal aeroplanes are being flown successfully. Plans are
under way to build aeroplanes for service at extremely high altitudes,
where greater speeds are possible owing to the tenuity of the air and
the consequent lowering of head resistance. These machines are to have
inclosed bodies in which air at normal pressure will be maintained by
means of blowers. The blowers would also furnish the engines with air
necessary for proper combustion of the fuel.
We are not going to give a history of the progress of aviation since
the invention of the Wright biplane, but instead we shall look briefly
and in a very elementary way into the principles underlying the flight
of heavier-than-air machines.
WHY A KITE STAYS UP
What is it that makes a plane or a kite stay in the air? The answer is
inertia. The balloon shows us that air possesses weight; the aeroplane
shows us that air possesses inertia. This is a natural consequence.
Every body possesses inertia and the heavier the body the greater its
inertia. By inertia we mean resistance to change of motion or rest. The
pressure of air against the face of a fan is due to its resistance to
a change from state of rest to a state of motion, while the pressure
of wind against a surface represents the resistance of air in motion
to being brought to a state of rest. The more sudden the change the
higher is the resistance or pressure developed. If an open newspaper
be laid over one half of a ruler, while the other half extends beyond
the edge of the table, the ruler may be broken by a sharp blow on the
overhanging end, not because the other end is held down by the weight
of the newspaper, but because the inertia of the air bearing on the
broad area of the paper prevents the ruler under the paper from rising
in response to the sudden blow at the overhanging end. It is the
inertia of the air, _i. e._, its resistance to rapid displacement that
keeps a parachute from falling like a solid shot to earth.
Figure 62 shows how a kite is maintained in the air. The line _AB_
represents the plane of the kite, the line _CO_ at right angles to this
plane is the pressure against the center of the kite surface. The wind
pressure _DO_ is resisted by the pull of the kite string and exerts
a lift _EO_, which resists the vertical pull of gravity. The sum of
the forces _DO_ and _EO_ must be equal to the force _CO_. If _EO_ is
greater than the force of gravity the kite will rise, and if it is less
the kite will fall. The magnitude of the force _EO_ depends upon the
velocity of the wind _DO_ and the angle of the kite _AB_ to the wind.
If the plane of the kite were parallel to the direction of the wind
the angle would be zero and the lift would also be zero.
FORCES THAT SUPPORT AN AEROPLANE
[Illustration: FIG. 62.--FORCES WHICH HOLD UP A KITE]
In an aeroplane conditions are just the same as in a kite, except
that a propeller drives the plane through the air with a force equal
to _CO_. The plane is slightly curved, so that air which is deflected
or forced down at the forward edge will continue to press against the
plane all the way to the rear edge. The shape of the top of the plane
is also important. As the plane is driven through the air a partial
vacuum is formed above the plane, so that the difference in pressure
between the two sides is increased and there is a greater lifting
effect. This partial vacuum is known as “cavitation.” In the early days
of the aeroplane it was not realized how important was the effect of
cavitation on rapidly moving objects. Every spar and member of the
airplane as it plows through the air builds up a head resistance in
front and is followed by a wake of rarefied air unless it is given
a stream-line construction. This is illustrated in Figure 63, which
shows at the left a rectangular body traveling through the air and at
the right a stream-lined body. The flow of the air is indicated by the
lines and it is evident that head resistance and cavitation are reduced
by forming the body with a bulging bow and tapering stern. To-day all
the exposed parts of aeroplanes are stream-lined as far as possible.
In the early Wright machines wire braces were extensively used. It was
not supposed that a wire would offer much resistance to the air, but,
upon investigation, it was found that the wire braces would vibrate
laterally and present virtually a broad surface which materially
increased the head resistance.
[Illustration: FIG. 63.--FLOW OF AIR AROUND A RECTANGULAR BODY AND A
STREAM-LINED BODY]
The angle of the plane determines to a large extent the head resistance
of the plane. If the plane is horizontal the head resistance is at a
minimum, but the lift is also very slight. If the wing planes were
perfectly flat, there would be no lift at all, but because of the
curve of the wings there is a certain amount of lift when they are
horizontal, and even when they are tipped slightly downward, provided
the machine is traveling at high speed. The angle of an aeroplane’s
wings is therefore much flatter than that of a kite.
MAINTAINING THE EQUILIBRIUM OF AN AEROPLANE
In order to maintain itself in equilibrium the center of gravity
must coincide with the center of pressure, otherwise there will be a
turning action about the center of gravity and the machine will upset.
The location of the center of pressure depends upon the angle of the
plane. The greater the angle the farther it is from the forward edge
of the plane and the pilot can maintain fore-and-aft equilibrium by
tilting the horizontal planes of his rudder so as to change the angle
of the machine, and hence of the main or forward planes. Unfortunately
the air is a turbulent ocean filled with invisible air currents and
the aeroplane must be capable of adjustment to meet the variations
of pressure due to sudden gusts of wind. This is particularly true
of lateral balance. A gust coming from the side will put a greater
pressure on one side of the aeroplane than on the other. To meet this
the angle of the plane at one side must be less than that at the
other. This was discovered by the Wright brothers and their method of
overcoming the lateral variations of pressure was the key to their
early success. They warped their wings or twisted them so that the
angle of the wing was reduced on the side from which the gust came
and was increased on the other, and thus the center of pressure over
the whole wing was kept on the line of the center of gravity. At the
same time there was a variation in head resistance which had to be
corrected by moving the rudder, and the Wright brothers used a single
lever to warp the wings and at the same time to operate the rudder so
as to keep the aeroplane on its course.
Before the Wright brothers made public their invention other aeronauts
had great difficulty in making turns. When turning, one side naturally
has to move through the air faster than the other. This produces
an increase of pressure on one side over the other which may be
counterbalanced by warping the wings. The same effect is produced by
the use of small wing planes, called “ailerons,” at each side of the
main planes. An automatic means of stabilizing an aeroplane has also
been devised, a description of which will be found in Chapter XXI.