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.

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

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.

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.

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