FIG. 73.--LILIENTHAL--1891.
At the annual meeting of the American Association for the Advancement of Science, held in Buffalo, N. Y., in 1886, a paper on The Soaring Birds was read by Mr. Lancaster, then of Chicago, which paper attracted great interest and attention.
Mr. Lancaster, in the hope of surprising the secret of the birds, had the pluck, in 1876, to exile himself to the wilderness of Southwestern Florida, on the Gulf coast, near the Everglades, and there to remain for five consecutive years watching the sailing of the master soarers. He published some of his observations and deductions in the London Engineer, in the reports of the Aeronautical Society of Great Britain, and in the American Naturalist but the subject was by no means exhausted, and his description of the phenomena observed was so interesting to the members of the association, few of whom had ever seen a soaring bird at close range, that they demanded to hear more upon a subsequent day.
Unfortunately for Mr. Lancaster, upon the latter occasion he attempted to give a mechanical and mathematical explanation of the performances which he had previously so well described, and his theory was so plainly erroneous that he was subjected to harsh ridicule and criticism. He had witnessed some remarkable feats of "Aspiration," he had attempted to reproduce them artificially, but he was clearly wrong in his expounding of the mystery, and his critics did not properly discriminate between the statements of observed facts and the attempted explanation.
The principal issue, however, was made concerning some attempts to imitate soaring action, which Mr. Lancaster claimed to have successfully made in Florida, and which he unwisely declined to exhibit at Buffalo. He had described them in his paper as follows:
I constructed floating planes which, for lack of a better name, I have termed "effigies," and which are an example in point. I have made scores of them. They would draw into the breeze from the hand and simulate the soaring birds perfectly, moving on horizontal lines or on an inclination to a vertical. They would float in the best winds with neither ledge, rough front surface nor rear curve if very nicely adjusted; but one of this construction I never induced to pass beyond the limits of vision, as the equilibrium was so very delicate that a little inequality in the wind current would capsize it.
There is every probability that such an experiment would invariably fail if tried in any but a perfectly steady sea breeze, an inflowing curent of air with peculiar conditions; but it does not follow that the action described is impossible, for if we presume that current to have an upward trend (and the writer knows, of his own knowledge, that such upward trends are not rare in sea breezes), we can readily see that an aeroplane, tipped forward so as to point below the horizon, may be both sustained and "aspirated," or possess a forward component of pressure, so that it may advance against the wind, and rise at the same time, like the kite of M. Myers.
The equilibrium is, of course, excessively delicate, and hence the requirement for a sea breeze, for a local homogeneous mass of air flowing from the water over the land to replace the rising quantity heated by the sun; but it is erroneous to suppose that the experiment described by Mr. Lancaster involves a mechanical impossibility. It is, doubtless, difficult to repeat it successfully, because it requires a combination of peculiar conditions; but the soaring birds are daily performing the feat, and apparently in horizontal winds.
In order that those who are favorably circumstanced may test the matter experimentally, the following description of the device is copied from a paper of Mr. Lancaster, published in 1882.
Take a stick of wood 1 in. square and 18 in. long, and point one end. Slit the other end 3 or 4 in., and insert a piece of stiff cardboard 6 in. wide and I ft. long. This will represent the body and tail of the bird. Fasten on both sides near the pointed end a tapering stick 2 ft. long, with the outer ends slightly elevated, and fasten to these and the body a piece of cardboard 10 in. wide and 2 ft. Iong. Have the tail vertical instead of horizontal, as in the bird. Round off the outer rear corners of the wings for 3 or 4 in. The imitation of the natural bird is now complete. There is no need of exactness, as the air you are to try it in will be an unknown quantity, and it may just suit the shape you make. An indispensable part is now to be added, which is to preserve the equilibrium and is not used by the natural bird. A tapering stick, say Ii in. wide, 3/8 in. thick at the top, 3/8 in. square at the other end, and 18 in. long is used. This piece is to be securely fastened by a small bolt through the upper end of the body piece, about 5 in. from the from end. It must be capable of adjustment by allowing the lower end to swing front and back through say 4o . To the lower end is fastened a muslin bag which will hold 2 lbs. of shot. Expose the effigy to a breeze of from 3 to 20 miles an hour, from as high a situation as it is possible for you to obtain, by holding it by the pendant stick near the body. Adjust the weighted stick forward or back, and add or subtract shot until the effigy has a tendency to spring from your hand against the current of air, when it may be released at a moment of greatest steadiness of breeze.I have made hundreds of these toys, with all kinds of success, but have never yet succeeded in getting one to travel beyond the limits of vision. They have proceeded directly against the breeze for 500 yards, and obliquely, up or down, or to right or left, within those limits, when they would dose their balance and come down Sometimes almost any kind of one that was presented to the air would float creditably, while at others none would succeed. The pendant weight for maintaining equilibrium, though the best I have ever devised, is far too sluggish for perfect work. The momentum of this weight prevents the best results, for, if a succession of puffs of wind upon the same wing should occur quickly together, the weight would swing far enough, in obedience to the impulse given, to capsize the effigy. Such a succession of puffs is sure to occur, sooner or later, at each trial. These toys operate long enough, however, to prove the purely mechanical character of flight, and serve to materially strengthen the theory.
The writer may confess that he has tried this experiment several times under special instructions furnished by Mr. Lancaster, but that he has never succeeded in floating one of these "effigies" so that they would advance against the wind. Others have, to his knowledge, tested the matter, and had no better success, yet it is not rational to say that the feat is impossible, for it is very clear that if the wind have an ascending trend, and the "effigy" be slightly tipped toward the front, the horizontal component of air pressure will drag it forward, while the vertical component will sustain or elevate it, as already explained. It is probably because of the uncertain prevalence of ascending trends that Mr. Lancaster complains that sometimes almost all these toys would succeed in simulating soaring, and sometimes none at all.
In 1888 Mr. Lancaster moved to Colorado, where he has been experimenting with a view to the solution of the problem of soaring flight; and in the American Naturalist for September, 1891, he gave an account of some of his experiments, with the conclusions which he deduced therefrom. The following extract contains an account of a remarkable occurrence. He says:
I can produce true soaring flight in natural wind, with a plane exceeding 2 lbs. to a square foot of surface, whenever I wish to do so and can obtain wind strong enough for the purpose. During the past three years I have made about 50 planes [aeroplanes ?] of various shapes and sizes, and from 25 lbs. to 400 lbs. in weight. These planes are not set free in wind, but used in the experimental cases above described, but with rigid rods in place of the parallel wires. These rods run in large rings and have a cross-head at their outer ends allowing the plane to run to the front until its edge rests against the rings. In the best trial the parallel [with the plane ?]] component is neutralized at 10 from horizontal, far exceeding my expectations derived from observations of the birds, their angle of obliquity being rarely over 5°.On a few occasions these planes accidentally escaped me in time of highest wind, and were ruined at once for all purposes excepting firewood, in each case being a loss of two or three months' work, and playing havoc with my finances. One that I valued particularly plunged to the front in a violent blast of wind with force sufficient to tear out the rings. It rose into the air. gradually higher and higher, until an elevation of at least 3,000 ft. was attained, when some part of the device giving away, it lost equilibrium and plunged through the air, striking the earth about 21 miles from the 2 1/2 and 1,000 ft. higher than that locality. Another mile would have carried it to the summit of the Flat Top Mountains. It was in the air about three hours, and I walked beneath it during its flight. Its course was directly against the highest wind I have experienced during my residence here. At times it did not progress, bat went higher. It weighed 110 lbs., and had been well balanced for experimenting on surface manipulation. There was no lesson taught in this flight, the birds having been doing the same thing for a long time. It was an interesting spectacle to look at: so is a large bird in the same act. I presume Mr. Darwin's provisional solution would apply to this plane as well as to the condors; but I am trying to explain the actual mechanical activity of both.
The best effects produced were with a plane of 400 lbs. weight and 80 sq. ft. of surface. In a wind that would be rightly termed a gale, arising about midnight, this plane was thrown about 7° from horizontal. It ran to the front against the rings at 10°, where the entire parallel component was neutralized, and at 7° it hugged the rings with a force that required a backward pull of 15 lbs. to detach it.
This plane would make a splendid navigator, and I would have no hesitation in trusting myself to it, when steering. equilibrium and alighting or stopping items had been worked out. I mean to say that it would navigate wind. I am now just entering on a course of experiments in calm air.
This very interesting case of "aspiration" may have been produced by the same cause as in the case of M. Myers's kite--i.e., an ascending trend of wind; but certainty concerning this depends upon the shape of the surface. Mr. Lancaster writes of it as a "plane ;" but as he mentions also the "front ledge" and the "rear curve," the surface operated upon by the wind was probably a more or less compound surface, for which there is no specific name, but which may be described as an aeroplane. If it was shaped like those of the soaring birds, then "aspiration" might occur with a horizontal wind, but the equilibrium would be very unstable, and, as Mr. Lancaster points out, the steering, alighting, and stopping would be the important points to work out.
Among the most systematic and carefully conducted series of experiments that have ever been made in the direction of artificial flight are those of Herr Otto Lilienthal, of Berlin, Germany, a mechanical engineer and constructor, and a prominent member of the German Society for the Advancement of Aerial Navigation.
The general position that he maintains, and in pursuance of which he has made his more recent experiments. is that bird flight should be made the basis of artificial flight. Dexterity alone, as he maintains, invests with superiority the native denizens of the air, and, therefore, man, if he possessed sufficient skill, might participate in flight. He evidently believes, like M. Mouillard, that for the soaring birds ascension is the result of the skillful use of the power of the wind, and that no other force is required; and, therefore, that to imitate them no engines or other external sources of power are needed, but that all the necessary apparatus consists of properly constructed sustaining surfaces skillfully operated.
Herr Lilienthal, instead of first flying at conclusions, began by a systematic analysis of the problem, verified by experiments, which latter were carried on by himself and his brother, G. Lilienthal, during a period covering nearly 25 years, and he published in 1889 a book on "Bird-Flight as the Basis of the Flying Art,"34 in which he gave the result of his investigations.
From a review of this remarkable book in the Aéronaute for January, 1892 the following account of its contents has been prepared.
Herr Lilienthal seems to have begun by observing the sailing of various sea birds following vessels at sea, and of the stork, an expert soarer, which inhabits Germany; he drew the conclusion that plane surfaces present undue resistance, and that success in artificial flight is only to be expected from concavo-convex sustaining surfaces; a belief which, as we have already seen, was also entertained by Le Bris, Beeson, Goupil Phillips , and others.
He declares that the laws of air resistances and reactions which, unfortunately, are as yet but imperfectly known, form the whole basis for the "technique" or actual performance of flight, and that the shapes and methods of birds so completely utilize these laws and offer such appropriate mechanical movements that failure must follow if they be discarded.
Herr Lilienthal's experiments were in great part directed toward an investigation of the resistances and reactions of air, and the power necessary for flight. One of these consisted in suspending himself from a spar projecting from a house and operating a set of six wings opening and closing like concave Venetian blinds, through which he measured the lifting effects of wing strokes performed with the muscles of the legs, so that the step of each foot would produce a double stroke of the wing. The weight of the operator and wings combined was 176 lbs., and they were counterweighted with 88 lbs. suspended to a rope passing over two pulleys. With some practice he was enabled, by operating the wings with the pedals, to lift himself 30 ft. from the earth, thus proving that he obtained, through his mechanism, wing power sufficient to lift the remaining 88 lbs.--a very excellent performance, and much in excess of most of those hitherto described in this review of Progress in Flying Machines.
This and other experiments, together with a consideration of the power to be obtained from the wind, convinced him that artificial flight was accessible to man, aided by considerably weaker motors than have generally been thought indispensable, and, indeed, under favorable circumstances of wind, with no motor at all.
Herr Lilienthal therefore, carefully analyzed the shapes and methods of the living birds and the exact proportions of their concavo-convex surfaces. He went into this in detail, and finally formulated in his book the following conclusions:
Such are the principal considerations which must be observed in the application of the theories herein expounded.
Governed by these considerations, equipped with much preliminary experiment and analysis, Herr Lilienthal put his theories and conclusions to practical test, in the Lilienthal of 1891 by undertaking a series of experiments with a pair of curved wings designed for soaring alone--that is, to serve as sustaining surfaces and not for flapping or propulsion.
The following account of these experiments has been furnished by Mr. George E. Curtis, of Washington, D. C., who has also obtained from Dr. C. Kassner, of the Meteorological Institute at Berlin, the very graphic photographs from which the engravings have been made.
The Lilienthal apparatus is shown in fig. 73, and consists of a pair of extended bird-like wings, incurvated from front to back on parabolic lines, and sinuous in the direction of their lengths. The area of sustaining surface, as at first constructed, was 107 sq. ft., but it was diminished in the course of numerous changes and remodellings to 86 sq. ft. There was, as will be observed, a horizontal tail and a vertical rudder or keel. The framework was made of willow, and covered with sheeting fabric. The weight of the whole apparatus, without the operator, was 39.6 lbs.
FIG. 73.--LILIENTHAL--1891.
In order to become accustomed to the management of these artificial wings, Herr Lilienthal first practiced in his garden. Here he had a spring-board, toward which he ran for a distance of about 26 ft.; and with the velocity thus acquired, together with the reaction of the springboard, he launched himself into the air, where he could learn to operate and to manage the wings.
After these preliminary experiments had given him dexterity and facility in the management of the apparatus, he betook himself to a hilly region in the suburbs of Berlin, and there practiced soaring flight in natural winds of moderate velocities. The plan, of course, consisted in first running against the wind, and thus deriving there from the necessary sustaining air pressure.
Having selected a hill whose downward inclination faced the prevailing wind, he ran along the summit straight toward the wind, until a sufficient velocity was attained at the brow, where he was carried into the air and landed safely at the foot of the hill, having sailed a distance of 65 to 82 ft.
When the wind velocity became greater than 11 to 13 miles per hour. the management of the apparatus became exceedingly difficult, and Herr Lilienthal advises an experimenter not to venture to leave the ground under such circumstances, unless he has attained, through long practice, a considerable degree of dexterity in manoeuvre
The results attained in the practice of the season of 1891 were sufficiently encouraging to warrant the further prosecution of these experiments in the following year; but they disclosed a number of points to which additional attention needed to be given in order to overcome the practical difficulties in imitating the birds. These points related to a better adjustment of the center of gravity, to methods for obtaining greater stability, and to the mode of management of the apparatus when the wind blew more rapidly than 11 to 13 miles per hour.
In the issue of the Zeitschrift für Luftschiffahrt for November, 1892 Herr Lilienthal published an article on "Soaring and its Imitation, "in which he gives a brief account of his experiments in the summer of 1892 from which the following abstract has been prepared:
Many theories have been proposed to explain soaring. My own explanation is based upon the advantageous relations of air resistances incident to the use of slightly curved wing surfaces (as I have demonstrated) and upon the gently rising trend of alr current which I have found to prevail.A flying apparatus which has the same proportions as those of a good soaring bird and is of sufficient size to carry a man, can scarcely be held fast by three or four men together when exposed to a brisk wind. When we look at the safe and quiet sailing of the birds, it almost seems as if some undiscovered mechanical principle were at work, some feature in the elastic
properties of air or in the elastic curvature of the feathers which accounts for the mystery of sailing flight; but my experiments have taught me that there is no mystery, and that the same mechanical science which has explained the theory of the steam engine and followed the orbits of the planets is adequate to explaining the operations of soaring flight.
Dexterity alone, in my opinion, invests the native inhabitants of the air with superiority over man in that element.... Inasmuch as continuous soaring with large wings in high winds can terminate in scarcely anything but the destruction of the foolhardy fellow who may first attempt the experiment without previous practice, I first undertook last year to gain some expertness with a smaller apparatus and in moderate winds. In spite of my caution the wind several times played the mischief with me. Even with only 86 sq. ft. of sustaining surface, I was several times tossed up into the air by unexpected gusts of wind, and but for the circumstance that I was able to release myself quickly from my apparatus, I might have had a broken neck instead of the sprains in feet or arms which always healed in a few weeks.
Almost every Sunday, and sometimes on week days, I went out to practice on the hill between Grosshreutz and Werder. A mechanic, Herr Hugo Eulitz, the maker of my apparatus, went with me, and each practiced alternately while the other rested. Thus we obtained dexterity in gliding down on the air and in landing at the foot of the hill without mishap.
Herr Kassner, of the Meteorological Institute, was so kind as to photograph me in the air, and has thus enabled me to exhibit to the members of the society how I sailed right over the head of the miller of Derwitz (in whose barn I stored my apparatus) and of his esteemed poodle dog.
Equipped with the experience gained in 1891, I this year attempted to soar with wings measuring 172 sq. ft. in surface. My apparatus weighed 53 lbs., and my own weight is 176 lbs., so that the whole was 229 lbs. Each Square foot of surface, therefore, sustained 229/172 = 1.33 lbs.
The up-thrust of the wind (the lift) upon the wing surface is perhaps half as much as the pressure of the same wind upon the same surface if turned perpendicular thereto.35 Now, as the apparatus therefore needs to sustain it a wind producing a pressure of 1.33 X 2 = 2.66 lbs. per square foot we see that (by ordinary tables of wind pressures) it must blow at a velocity of about 23 miles per hour.
I have, however, been very cautious about exposing myself to such a wind with this large apparatus; and in such high winds have used smaller surfaces for my sailing practice.
This year I selected a locality between Stegiitz and Südende. It had, however, the disadvantage that only westerly starts were possible. Herr Kassner has again taken instantaneous photographs of my apparatus, which have been laid before the society (fig. 74) The strongest winds in which I practiced had a velocity which I estimated at between 15 and 16 miles per hour. By running I obtained an additional velocity of 7 miles an hour, making the total relative velocity 23 miles an hour, which was required for soaring. Under these circumstances the first part of my flight was almost horizontal, and the alighting was always a gentle one.... Each apparatus had a vertical and horizontal tall, without which it is impracticable to practice in the wind. In conclusion, I will remark that sailing flight near the earth's surface must be much more difficult than at greater heights, where the wind blows more regularly, while every irregularity of the ground at lower levels starts whirls in the air.
FIG. 74. -- LILIENTHAL --1892
34"Der Vogelflug
als Grundlage der Fliegekunst," Von Otto Lilienthal, Berlin, 1889.
35Lilienthal, "Der
Vogelflug als Grundlage der Fliegekunst," Tafel VII.
| List of Illustrations | Table of Contents | Index |