Popular Science Monthly/Volume 64/March 1904/Aerial Navigation
THERE are now dawnings of two possible solutions of the problem of aerial navigation; a problem which has impassioned men for perhaps 4,000 or 5,000 years. Navigable balloons have recently been developed to what is believed to be nearly the limit of their efficiency, and after three intelligent but unfortunate attempts by others, a successful dynamic flying machine seems to have been produced by the Messrs. Wright.
It is therefore interesting to review the present status of the question, the prospects of its solution and the probable uses of the hoped-for air-ships.
As to balloons, we may pass over the early gropings and failures to make them navigable. It was recognized very soon that the spherical balloon was the sport of the wind, that it was necessary to elongate it in order to evade the resistance of the air, and that, inasmuch as aerial currents are much more rapid than aqueous currents, it was necessary to obtain considerable speeds in order to have a useful air-ship. This means that there must be great driving power, and that this power shall weigh as little as possible; for in any case the balloon itself with its adjuncts and passengers will absorb the greater part of its lifting power.
Giffard was the first to apply in 1852 an artificial motor to an elongated balloon. This motor consisted in a steam-engine of three horse power, which weighed with its appurtenances 462 pounds, and Giffard obtained only 6.71 miles per hour, although his balloon was 144 feet long and 39 feet in diameter, or about the size of a tramp steamer.
Dupuy de Lome in 1872 went up with a balloon 118 feet long and 49 feet in diameter, but, having a wholesome dread of the contiguity of fire and inflammable gas, he employed man power (weighing about 2,000 pounds to the horse power) to drive his screws, and he obtained less speed than Giffard. The accidents to Wölfert and to DeBradsky have since shown the soundness of his fears.
Next came Tissandier in 1884, who employed an electric motor of 1 1/2 horse power, weighing some 616 pounds, with which he attained 7.82 miles per hour.
Meanwhile the French war department took up the problem. It availed itself of the labors of the previous experimenters and made careful and costly investigations of the best modes of construction, of the best shapes to cleave the air and of the weight and efficiency of motors. This culminated in 1885 when Messrs. Renard and Krebs, of the Aeronautical Section, brought out the war balloon 'La France' which attained about 14 miles an hour (or half the speed of a trotting horse) and returned to its shed five times out of the seven occasions on which it was publicly taken out.
This air ship was 165 feet long, 27 1/2 feet in diameter and was provided with an electric motor of 9 horse power, weighing with its apurtenances some 1,174 pounds. The longitudinal section was parabolic, somewhat like a cigar rolled to a sharp point at both ends, the largest cross-section being one fourth of the distance from the front, and it was driven, blunt end foremost, by a screw attached at the front of the car. No better shape and arrangement have yet been devised and subsequent experimenters who have wandered away therefrom have achieved inferior results, so far as the coefficient of resistance is concerned.
In 1893 the French War Department built the 'General Meusnier,' named after an aeronautical officer of extraordinary merit of the first French Republic. This war balloon is said to be 230 feet long, 30 feet in diameter, 120,000 cubic feet in capacity and to have been originally provided with a gasoline motor of 45 horse power. It is said by all the writers on the subject that it was never taken out. Possibly the French were waiting for a war which fortunately never came ; but, be this as it may, it is probable that with the reduction which has since taken place in gasoline motors this balloon could carry an engine of some 70 horse power, and attain a speed of about 30 miles an hour, which is greater than that of transatlantic steamers.
Some unsuccessful experiments were carried on in Germany in 1897. First by Dr. Wölfert, whose balloon was set on fire by his gasoline motor and exploded in the air, killing both himself and his engineer, and later by Schwarz, whose aluminum balloon proved unmanageable and was smashed in landing. The most ambitious attempt, however, was that of Count Zeppelin, who built in 1900 a monster air-ship 420 feet long and 39 feet in diameter. It was a cylinder with paraboloid ends, but the shape was inferior and almost all the lifting power was frittered away on a internal frame of aluminum, so that the gasoline motor could be of only 32 horse power, and the speed attained has variously been stated at 8 to 18 miles per hour. Nevertheless the design of Count Zeppelin contained many excellent features, and a movement is now on foot in Germany to enable him to try again, through means of a popular subscription. The mere size, if he builds again as large, is a great element of success, for as the cubic contents and lift increase as the cube of the dimensions, while the weights increase in a far smaller ratio, a balloon of this great size ought to be able to lift a very powerful motor, and to attain a speed of 30 or more miles per hour. He has shown that the size is not beyond the possibility of control.
Meanwhile gasoline motors had been increasing in efficiency and diminishing in weight. The French war department gave no sign and it was reserved for a Brazilian, Mr. Santos Dumont, to show to the Parisians what could be accomplished by equipping an air-ship with a gasoline motor. The history of his triumphs is so present to all minds that it need only be alluded to, but it may be interesting to give some details of the sizes and arrangements of his various balloons. His first idea seems to have been that, in order to make it manageable, a balloon should be made as small as possible, and that it was practicable to disencumber it of many adjuncts hitherto considered indispensable. Neglecting to study carefully what had been found out by his predecessors, he had to learn by experience, and he built five balloons, all navigables, before he produced in 1901 his No 6, with which he won the Deutsch Prize, by sailing 3 1/2 miles and return in half an hour. This balloon was 108 feet long, 20 feet in diameter and was provided with a gasoline motor of 16 horse power which might be driven up to 18 or 20 horse power. While the speed over the ground was 14 miles an hour, retarded as it was by a light wind, the speed through the air was about 19 miles an hour, a small but marked advance over any previous performance; but the result would have been still better if the shape had been that of Colonel Renard's balloon.
Since then Mr. Santos Dumont has built four new navigable balloons. His No. 7, with which he expects to compete at St. Louis in 1904, is 160 feet long and 23 feet in diameter and is to be provided with a motor of 60 horse power. His No. 8, which was sold to parties in New York last year. His No. 9, which is his visiting balloon, being only 50 feet long and 18 feet in diameter and provided with a 3 horse power motor. Its speed is only 10 miles an hour, but it is handy to ride around to breakfast or afternoon teas. He is now finishing his No. 10, the omnibus, which is 157 feet long and 28 feet in diameter, with a motor of 46 horse power. Fares are to be charged for by the pound of passenger when it comes out next spring.
Emulators of Santos Dumont there have been that have come to grief. Mr. Roze built in 1901 a catamaran consisting of two twin balloons, which, although 148 feet long, failed to raise their own weight serviceably. Mr. Severo built in 1902 a navigable balloon which was so injudiciously constructed that the car broke away in the air, and the inventor was killed as well as his engineer. Later in the same year DeBradsky built a navigable balloon equipped with a gasoline motor located so near the vent for the gas that the latter took fire, exploded the balloon, and the inventor and his engineer were killed, thus for the second time verifying the fears of the experts who discountenanced this combination.
Some meritorious projects have been published but not yet carried out. Among these may be mentioned that of Mr. Yon, now deceased, and that of Mr. Louis Godard. The latter project was for a balloon 180 feet long and 36 feet in diameter, with two steam motors of 50 horse power each. It was expected to attain a speed of 30 miles per hour.
One navigable balloon which was built this year, that of the Lebaudy brothers, has achieved a great success. It is 185 feet long, 32 feet in diameter, and is equipped with a gasoline motor of 40 horse power. It has beaten the speed of Santos Dumont, having on many occasions, it is said, attained 24 miles an hour.
There is also a navigable balloon being built in Paris by Mr. Tatin for Mr. Deutsch, the donor of the famous prize. This is 183 feet long, 27 feet in diameter and is equipped with a gasoline motor of 60 horse power.
Besides these there are said to be a number of navigable balloons either being built or proposed in France. They are those of the Marquis de Dion, of Pillet & Robert, of Girardot, of Boisset and of Bourgoin, but there is no telling how many of them will materialize.
These are all French balloons, while there are in England the balloon of Mr. Spencer, 93 feet by 24 feet with nominally 24 horse power; of Mr. Beedle, 93 feet by 24 feet with 12 horse power, and that of Dr. Barton, now in construction, with dimensions of 170 feet in length, 40 feet in diameter, and equipped with a number of aeroplanes and three gasoline motors of 50 horse power each. It is a question whether the weight of the aeroplanes will leave sufficient margin to lift 150 horse power.
The ultimate practicable size for balloons is not yet known, but the mathematics of the subject are now tolerably well understood. The larger the balloon the more speed it can attain, and it is possible to design it so that the results shall not be disappointing. Those inventors who expect to attain 70 to 100 miles an hour by some happy combination do not know what they are talking about.
It is interesting to speculate which of the above-mentioned navigable balloons would, if competing, stand a chance of winning the $100,000 prize which has been offered by the St. Louis Exposition of 1904. So far as can now be discerned, the only vessels which are likely to develop the required minimum speed of 20 miles an hour over the ground, which speed really requires about 25 miles an hour through the air as there will almost invariably be some wind, will be the Santos Dumont No. 7, the Lebaudy and the Deutsch air-ships, all of them French. The English vessels of Spencer and of Beedle are too small to lift sufficient power to drive them at 25 miles an hour. The balloon of Dr. Barton might gain this speed if it were not 40 feet in diameter, besides being loaded down with aeroplanes, and it remains to be seen what will be the effect of this combination. The American air-ships all seem to be too small to lift enough power to give them the required speed save the Stanley air-ship, 228 feet by 56 feet in diameter, begun in San Francisco. Should this be completed in time, and should the weights be kept approximately near those stated in the circulars, it might have a chance to obtain 25 miles an hour, but it would need more than three times the 50 horse power contemplated in order to do so, and the weight of the aluminum shell and framing would probably absorb much of the lifting power.
If the aeronautical contest at St. Louis were scheduled to take place a few years later, thus giving time to consummate recent success, it is not improbable that the main prize would be carried off by a flying machine. This yet lacks the safe flotation in the air which appertains to balloons, but it promises to be eventually very much faster.
The writer found, somewhat to his surprise, when on a visit to Paris last April, that a decided reaction has set in among the French against balloons. It seemed to be realized that the limit of speed had been nearly reached for the present, and that but small utility was to be expected from navigable balloons. They must be large, costly and require expensive housing, while they are slow and frail and carry very small loads. As commercial carriers they are not to be thought of, but they may be useful in war and in exploration.
Hence the French are turning their thoughts towards aviation and propose to repeat some of the experiments with gliding machines which have taken place in America. Even Colonel Renard, the celebrated pioneer of the modern navigable balloon, is now said to have become a convert to aviation and to say that the time has come to try the system of combined aeroplanes and lifting screws for flying apparatus.
A good deal of experimenting has been done with power-driven flying models. The more recent types have been actuated by twisted rubber threads, by compressed air and by steam, and the most notable experiments in order of date are those of Penaud, Tatin, Hargrave, Phillips, Langley and Tatin and Richet. The data for these (except the first) will be found by searchers in such matters in the London Times edition of the 'Encyclopaedia Britannica,' in the article on aeronautics. The most successful experiment was that of Professor Langley, who obtained in 1896 three flights of about three fourths of a mile each with steam-driven models, the apparatus alighting safely each time and being in condition to be flown again.
The one great fact which appears from all these various model experiments is that it requires a relatively enormous power to obtain support on the air. Omitting the cases in which the power was probably overestimated, the weights sustained were but 30 to 55 pounds to the horse power expended, thus comparing most unfavorably with the weights transported by land or by water; for a locomotive can haul about 4,000 pounds to the horse power upon a level track, and a steamer can propel a displacement of 4,000 pounds per horse power on the water at a speed of 14 miles an hour.
But models are, to a certain extent, misleading. They seldom fly twice alike and they do not unfold the vicissitudes of their flight. Moreover, the design for a small model is sometimes quite unsuited for a large machine, just as the design for a bridge of ten feet opening is unsuited for a span of one hundred feet.
After experimenting with models three celebrated inventors have passed on to full-sized machines, to carry a man. They are Maxim, Ader and Langley, and all three have been unsuccessful, simply because their apparatus did not possess the required stability. They might have flown had the required equilibrium and strength been duly provided.
At a cost of about $100,000, Sir Hiram Maxim built and tested in 1894 an enormous flying machine, to carry three men. It consisted in a combination of superposed aeroplanes, portions of which bagged under air pressure, and it was driven by two screws 17 feet 10 inches in diameter, actuated by a steam engine of 363 horse power with steam at 275 pounds pressure. The supporting surface was about 4,000 square feet, and the weight 8,000 pounds. The machine ran on a track of 8-feet gauge, and was prevented from unduly rising by a track above it of 30-feet gauge. At a speed of 36 miles per hour all the weight was sustained by the air, and on the last test the lifting effect became so great that the rear axle trees were doubled up and finally one of the front wheels tore up about 100 feet of the upper track ; when steam was shut off and the machine dropped to the ground and was broken. Its short flight disclosed that its stability was imperfect and Sir Hiram Maxim has not yet undertaken the construction of the improved machine which he is understood to have had under contemplation.
Having already built in 1872 and 1891 two full-sized flying machines with doubtful results, Mr. Ader, a French electrical inventor, built in 1897 a third machine at a cost of about $100,000 furnished by the French War Department. It was like a great bird, with 270 feet supporting surface and 1,100 pounds weight, being driven by a pair of screws actuated by a steam engine of 40 horse power which weighed about 7 pounds per horse power. Upon being tested under the supervision of the French army officers, the equilibrium was found so defective that further advance of funds was refused. The amount lifted per horse power was 27 pounds.
The data for the full-sized flying machine of Professor Langley, tested October 7 and December 8, 1903, have not yet been published. From newspaper photographs it appears to be an amplification of the models which flew successfully in 1896, and this, necessarily, would make it very frail. The failures, however, seem to have been caused by the launching gear and do not prove that this machine is worthless. Like the failures of Maxim and of Ader, it does indicate that a better design must be sought for, and that the first requisites are that the machine shall be stable in the air, shall be quite under the control of its operator, and that he, paradoxical as it may appear, shall have acquired thorough experience in managing it before he attempts to fly with it.
This was the kind of practical efficiency acquired by the Wright Brothers, whose flying machine was successfully tested on the seventeenth of December. For three years they experimented with gliding machines, as will be described farther on, and it was only after they had obtained thorough command of their movements in the air that they ventured to add a motor. How they accomplished this must be reserved for them to explain, as they are not yet ready to make known the construction of their machine nor its mode of operation. Too much praise can not be awarded to these gentlemen. Being accomplished mechanics, they designed and built the apparatus, applying thereto a new and effective mode of control of their own. They learned its use at considerable personal risk of accident. They planned and built the motor, having found none in the market deemed suitable. They evolved a novel and superior form of propeller; and all this was done with their own hands, without financial help from anybody.
Meantime it is interesting to trace the evolution which has led to this result and the successive steps which have been taken by others.
It is not enough to design and build an adequate flying machine; one must know how to use it. There is a bit of tuition which most of us have seen, that of the parent birds teaching their young to fly, which demonstrates this proposition. Even with thousands of years' evolution and heredity, with adequate flying organs, the birdlings need instruction and experience.
Safety is the all-important requisite. It is indispensable to have a flying machine which shall be stable in the air, and to learn to master its management. Nothing but practise, practise, practise, will gain the latter, and upon this the school of Lilienthal and his followers is founded.
Otto Lilienthal was a German engineer of great originality and talent, who after making very valuable researches, assisted by his brother, published a book in 1889, 'Der Vogelflug als Grundlage der Fliegekunst,' which it is very desirable to have translated and published for the benefit of English investigators. Then, putting his theories to the test of practise, he built from 1891 to 1896 a number of aeroplane machines with which he diligently trained himself in gliding flight, using gravity for a motive power, by starting from hillsides. He grew exceedingly expert, and made, it is said, more than 2,000 flights, until one rueful day (August 9, 1896) he was upset and killed by a wind gust, probably in consequence of having allowed his apparatus to get out of order.
He was followed by Mr. Pilcher, an English marine engineer, who slightly improved the apparatus, but who, after making many hundred glides, was also upset and killed in October, 1899, through structural weakness of his machine.
The basis for the equilibrium of an apparatus gliding upon the air being that the center of gravity shall be on the same vertical line as the center of air pressure, both Lilienthal and Pilcher reestablished this condition by moving their bodily weight to the same extent that the center of pressure varied through the turmoils of the wind. The writer ventured to think this method erroneous, and proposed to reverse it by causing the surfaces themselves to alter their position, so as to bring the center of pressure back vertically over the center of gravity. He began experimentally with man-carrying gliding machines in June, 1896, and has since built six machines of five different types, with three of which several thousand glides have been effected without any accidents. The first was a Lilienthal machine, in order to test the known before passing to the unknown, and this was discarded some six weeks before Lilienthal's sad accident.
With three of the other machines favorable results were obtained. The best were with the 'two-surface' machine, equipped with an elastic rudder attachment designed by Mr. Herring, and this was described and figured in the 'Aeronautical Annual' for 1897.
Three years later Messrs. Wilbur and Orville Wright took up the problem afresh and have worked independently. These gentlemen have placed the rudder in front, where it proves more effective than in the rear, and have placed the operator horizontally on the machine, thus diminishing by four fifths the resistance of the man's body from that which obtained with their predecessors. In 1900, 1901, 1902 and 1903 they made thousands of glides without accidents and even succeeded in hovering in the air for a minute and more at a time. They had obtained almost complete mastery over their apparatus before they ventured to add the motor and propeller. This, in the judgment of the present writer, is the only course of training by which others may hope to accomplish success. It is a mistake to undertake too much at once and to design and build a full-sized flying machine ab initio, for the motor and propeller introduce complications which had best be avoided until in the vicissitudes of the winds bird-craft has been learned with gravity as a motive power.
Now that an initial success has been achieved with a flying machine, we can discern some of the uses of such apparatus, and also some of its limitations. It doubtless will require some time and a good deal of experimenting, not devoid of danger, to develop the machine to practical utility. Its first application will probably be military. We can conceive how useful it might be in surveying a field of battle, or in patrolling mountains and jungles over which ordinary means of conveyance are difficult. In reaching otherwise inaccessible places such as cliffs, in conveying messages, perhaps in carrying life lines to wrecked vessels, the flying machine may prove preferable to existing methods, and it may even carry mails in special cases, but the useful loads carried will be very small. The machines will eventually be fast, they will be used in sport, but they are not to be thought of as commercial carriers. To say nothing of the danger, the sizes must remain small and the passengers few, because the weight will, for the same design, increase as the cube of the dimensions, while the supporting surfaces will only increase as the square. It is true that when higher speeds become safe it will require fewer square feet of surface to carry a man, and that dimensions will actually decrease, but this will not be enough to carry much greater extraneous loads, such as a store of explosives or big guns to shoot them. The power required will always be great, say something like one horse power to every hundred pounds of weight, and hence fuel can not be carried for long single journeys. The north pole and the interior of Sahara may preserve their secrets a while longer.
Upon the whole, navigable balloons and flying machines will constitute a great mechanical triumph for man, but they will not materially upset existing conditions as has sometimes been predicted. Their design and performance will doubtless be improved from time to time, and they will probably develop new uses of their own which have not yet been thought of.
- Paper read before Section D, American Association for the Advancement of Science, December 30, 1903.