Gliding Flight

L. P. Mouillard

UNTIL within a score of years, the solution of the fascinating problem of aerial navigation has been sought for in but two directions. These are: First, the endeavor to impart sufficient speed to balloons, and, second, the imitation of the rowing birds.

Balloons were found to require enormous power for satisfactory speed, involving much more weight than they could lift; moreover, their puny envelopes, so easily deformed and so rapidly destroyed, quite preclude them from becoming a commercial success.

Flapping wings may be imitated, but only with small models; the increased strength and weight of material necessary for larger apparatus, and the great motive power required for alternative action, have proved to be obstacles not yet overcome.

It is true that a number of devices have been proposed for aeroplanes, i. e., for apparatus provided with fixed surfaces, to be driven by aerial screws etc., but every such device has involved an artificial motor of some kind, and the machine was expected to glide upon the air, as the result of some impulse originating within itself, of some motive power carried by the machine, just as in the case of the flapping birds.

Inventors, and seekers for light motive power, were therefore quite nonplussed until a new aspect of the question was presented, some years ago, by a few observers, who had been watching the larger soaring birds performing their evolutions. These observers maintained that, when once under way, soaring birds expended no muscular power whatever; that they sailed on indefinitely with rigid wings, deriving from the wind all the energy required, not only to sustain their weight, but also, paradoxical as it may seem, to advance against the wind itself. They said that the manoeuvres were simple, chiefly consisting in feats of balancing, and that there really was no good reason why, when the wind blew, man might not imitate the birds, and translate himself at will in any direction, without any other motive power.

These assertions were at first treated as rankest heresy. The larger soaring birds being very rare in those temperate regions where human activity most prevails, many able men, who had not seen the performance, simply denied the fact that any tropical birds could accomplish what was claimed: that they could sail continuously on the wind, without muscular effort; although this achievement was affirmed by such observers as Count d'Esterno, Basté, Brétonnière, Weyher, Peal, Lilienthal, and I may add myself, for I have good eyes and profess to tell the simple truth. But presently some men of the highest scientific attainments investigated the facts and believed, although they could not mathematically explain them.

Says Professor Marcy, of the Institute of France, in his lectures on the natural history of organized creatures:

"As to sailing flight, none of the oldtime falconers doubted in the least its existence. They observed it every day, and they knew that the wind was a necessary condition.

"Nobody troubled himself about an explanation in those days; but later on, when physicists attempted to explain the mechanics of flight and succeeded in conceiving the action of the wing stroke and the effects of air resistances, sailing flight appeared to them as a physical impossibility. They said that it was impossible to admit that a bird, suspended at a fixed point in the sky, should find in the action of the wind sufficient power to advance against that wind. As well, said they, might we throw an inert mass into a flowing river, and expect the current to cause the body to advance up-stream.

"And yet, modern observers have contested this verdict: M. d'Esterno and M. Mouillard demonstrated that, unless we absolutely disbelieve ocular evidence, we must accept the actual fact that sailing flight is possible, even if we have to admit that our present mechanical knowledge is insufficient to explain it. "

Count d'Esterno, now dead, dwelt in France; while I, being more fortunately located in Egypt, have continued to observe, and to ponder upon this peculiar mode of flight, and to gather additional data. I say this in all modesty, for it is my sole merit that I have been constantly impelled to study the question by the daily demonstrations which press upon the sight in that enormous aviary which we call Cairo, in Egypt. There is no mistake about it. Once the wind blows­from five to twenty-five miles an hour,­the large soaring birds cease all flapping of wings in the air, and sail for an indefinite period, making their way towards any point which they may wish to reach. They sweep in great circles, while surveying the field for a meal; they rise a mile or more, and spy the ground afar; they come floating down again, and repeat the manoeuvre twenty or thirty times a day. They flash windwards through the sky, or they advance slowly against the wind; and all this they do on rigid wings, without a single stroke, save, perhaps, to recover balance from a wind-gust, to escape an enemy, or for some bird-purpose other than that of mere translation through space.

These perpetual demonstrations of sailing flight led me to publish a book in 1881, "L'Empire de l'Air." This contained to the extent of my ability, a faithful description of what I had seen; and I have also written, in later years, another book, in which I give additional data, reaffirm that I correctly observed and described the performance, and that twelve years of additional reflection and experiment have thoroughly satisfied me that flight without flapping ("Le Vol sans :Battements," for so I mean to entitle the book,) can be imitated by man.

In this research I have endeavored to eliminate all those movements and actions of bird-life, with which man need not concern himself in endeavoring to imitate gliding flight, and this elimination has led me to believe that, when the wind blows, simple sailing, advancing, or whet I call journeying flight, is quite accessible to man, without the aid of any motor whatever (this having hitherto been the great inhibition), and that we may even dispense with the initial thrust generally obtained by birds through flapping. Having determined so much, the next question was as to what bird exerted himself least in flight, and was the best to take for a model.

Small creatures are always the most powerful in proportion to their weight: thus, it is the larger birds, which expend least effort in flight, which we must observe and then try to imitate. These are the soaring birds, which sail upon the wind with no labor save balancing, and it is among them we are to seek the model most easy to reproduce. Confining ourselves therefore to gliding flight, we find that the necessities of bird-life have developed three modes or styles: the slow soarers, the rapid soarers, and sailers of medium speed.

Slow gliding obtains chiefly among the nocturnal birds, as witness the owl tribe, whose globular body, curling feathers that catch the air, and great flat screen of moon-like face, all reduce their speed. They may be dismissed from our consideration.

Rapid gliding is absolutely indispensable to the great sea-birds. The gannets, the frigate-bird, the stormy petrels and the albatross-all birds of the wild waste of waters­must be equipped to stem the rapid gale and to keep in the air in the tempest, for they cannot alight with safety among the curling waves. They must overcome the tornado, and their aeroplane is proportioned for that very purpose. These birds have very long and excessively narrow wings, quite differing from the proportions of other flying creatures. This narrow width of their aeroplane diminishes the air resistance, and permits the perforation of the intense aerial currents which would sweep away birds of other conformations. The struggle for existence and the law of natural selection have diminished the width of the wing until, in the case of the albatross, it has become only one-twentieth of the total spread. It will probably be very difficult for man to reproduce their evolutions.

But gliding at medium speed is what most concerns us. Here the necessities are different. The medium speed soaring birds are larger and heavier than the seabirds, and must sail on calmer breezes, which furnish less supporting power, and yet they must remain aloft all day. They need therefore to be able to rise up high and there remain surveying a wide field, till they discern a meal. Their great weight, sixteen to twenty-two pounds, forbids their remaining aloft on flapping wings. No such heavy creature can row upon the air ten hours at a stretch. Then there are the ascents and descents required for the pursuit. A whole county to be surveyed, the prey to be seen and approached; perhaps the height to be regained when an enemy appears; in fine, fifteen or twenty ascensions of a mile or so each, to be performed during the bird's daily work, which he cannot hope to accomplish by sheer animal strength.

In all these exercises the vultures most excel. They are the bird that man can most easily imitate. The larger they are, the more perfect for our copy.

Observation confers knowledge. It is first important to see, and then to understand; but while I have studied many birds, I have been most fascinated by the great tawny vulture of Africa: (gyps fulvus) big as a sheep, weighing sixteen pounds, their majestic sailing on rigid wings seems the perfection of simplicity.

The great tawny vulture is not an early bird, for the wind does not quite rise with the sun, and the creature dislikes flapping. He remains on his roost, among inaccessible cliffs miles from Cairo, and, occasionally stretching his wings to limber up after disuse, he awaits the coming breeze. With half closed eyes, and head sunk between his shoulders, he remains stolid, morose and impotent. But when the morning wind begins to fan the day, the gyps wakes up, and thoughts of breakfast yearn within him. Down from the precipice he launches into space, and is at once under full sail. On rigid wings he glides along, he sweeps in circles and he rises high. He watches the smaller birds sailing at lower altitudes, and as soon as they or some brother vulture discover the dead camel or horse, down he comes to get his share.

But, here I cannot do better than to quote myself, for the "Vol sans Battements" is still unpublished, and therein I have described the scene as well as I know how,­I say:

"Yes, I will here repeat it. The peculiarity of the great vulture's flight is that he expends no force, either to sustain or to guide himself. He detests flapping; of all birds he best decomposes the forces of the wind, and utilizes them with the greatest skill. If we except those contingencies which move him to beat his wings, such as a dead calm, the defense of his person, etc., we can truly say that he never beats his wings.

"Soaring is not a chance evolution for him. It is his ordinary gait; his invariable method of searching for his food. Moreover the necessities of his hunt do not admit of any other mode of flight. The bird must not only find the dead animal, but he must be able to wait indefinitely, until the prey shall become accessible. If he were obliged to beat his wings until man, the dogs, the hyenas, etc., had taken their respective shares of the carcass, he would surely become exhausted. Instead of that, the vultures are there, up aloft, in great numbers, perhaps a hundred or more together, gliding and watching.

"At first, the observer sees nothing; but looking carefully in the zenith, little specks are observed; they are almost imperceptible, and move with a curious sluggishness; then as the ground is cleared of intruders these specks increase in size, and we see that they are vultures which have already descended a long way. (See attitudes 1, 2, 3.) Then the reel tumbling down begins; the bird avalanche falls out of the sky, and they drop from a height of two or three miles, vertically, just like a parachute.

FIGS. 1, 2, AND 3.

"The first comers are now only a hundred yards away, and it is possible to examine them closely. Their wings are partly folded and turned upwards. (See Fig. 4.) They are falling at even speed; it is the descent of a heavy body without acceleration from gravity. Their appearance is then most curious; they neither swing forward nor backward, but come down steadily and slowly without any oscillations. And above them, still the black dots continue to come into sight, until the first hundred has increased to several hundreds.

FIG. 4.

"These myriads of great birds whirl like a waterspout. The descent still continues in those enormous circlings, those mad wheelings which almost give one the vertigo merely to watch them. (See attitudes 5, 6 and 8.) Sometimes those nearest down, not quite sure of perfect safety, return upon an upward glide, and thus a broad horizontal layer of vultures serves as a base for this whirling interminable column. (See Figs. 7 and 8.) This continues until the surrounding country has been thoroughly examined and the carcass is deemed accessible; when the hungriest dart down upon it. Then follows a wonderful sight, a performance beyond description. The slow descent becomes a a frenzied fall. Each wants to be the first, and dodges past the others with incredible skill. Other birds of prey have joined the avalanche; the kites plunge down between the outstretched wings with srtident cries, the Egyptian vultures lash the air with their spear-like forms, but the gyps fulvus never hurries. He knows that there is enough for a meal, and that he can always make a place for himself, so he descends silently without haste, with the slowness of a balloon alighting. (Fig. 9)

FIG. 5

FIG. 6

FIG. 7.

The bird which is a thorough expert in sailing flight (for all are not equally gifted in this respect) and which possesses all the necessary requirements for perfect performance, that is to say considerable mass and sufficient wing surface to sustain it, that bird, I say, can on rigid wings perform the following evolutions: if the wind be feeble he can climb into the air by circling round and drifting back somewhat. If the wind be brisk enough to sustain him thoroughly he can rise perpendicularly, facing the wind, without wheeling round, and even advance into the wind while rising, this last being the ne plus ultra of gliding flight. (See Fig. 7.) Certain expert soaring birds readily perform this curious paradoxical evolution. I will not try to explain how it is done, but this is a feat which may be termed one of "aspiration", a term which has been applied by observers of soaring flight to designate the aptitude possessed by certain master soarers to advance against the sustaining current which we think ought to drift them back, and which nevertheless is performed by them with a regularity and a perfection in direct proportion to the magnitude of the mass, surface and skill of the sailing creature.

FIG. 8.

This unlimited advance against the wind current without beat of wing, without apparent effort, and almost without act of guidance, is performed every day by myriads of wing creatures, and if the reader doubts the fact he can thoroughly satisfy himself by a trip to the trade-wind latitudes. Let him specially seek the larger birds, the frigate, the sea eagle, the condor, for no bird weighing less than five pounds can demonstrate "aspiration" in all its perfection. With birds of fifteen pounds and over this faculty of penetrating the wind is wonderful; it amazes the observer who sees it for the first time, and this progression without beating the air produces an impression there is no resisting. The thought comes at once that it can be reproduced by man, that there is no effort, except a slight one for guidance, and that it is wholly a question of skill, of acquiring bird-science.

FIG. 9.

When the bird is of light weight, the force expended for mere guidance is appreciable, and may seem to exceed our muscular activity. When the small creature advances against the wind without flapping, its efforts to trim its wings to the varying breeze are so visible as to cast a doubt on our ability to reproduce these manoeuvres as fast as needed. The exercise seems so violent, requiring such skill as an acrobat, such bird-science, that we dread the experiment; but, happily for aviators, the larger birds present a different lesson. Nature undertook to produce some big birds, and for them, as for all big creatures, increase of size means decreased relative strength; and yet the bird's life was to be spent in the air. So nature resolved the difficulty by suppressing the beat of wing, by utilizing the wind, and by giving the soarer all the benefits of inertia inherent in mere weight.

FIG. 10

Having persuaded ourselves that sailing flight is well demonstrated, that it requires no motor save the wind, that its evolutions are within man's powers, and that the models for us to imitate are the largest vultures, we must study their proportions and their performances. Soaring or sailing flight­gliding flight, in a word­is based upon that property of air which enables it to resist, almost as a solid, when a large surface (in proportion to the weight) passes through it with sufficient speed. The weight should be from one to two pounds per square foot, and the speed eleven to twenty-two miles per hour.

The form required for success is an aeroplane, whether for bird or man. The bird-aeroplane varies with the genus, its mode of life and required evolutions. The man-aeroplane may be simpler, as it need perform no other functions than those of journeying flight. Observations of birds indicate that the spread across should be about six times the width of wing, and the weight about one pound to the square foot; so that for an apparatus to carry 220 pounds we need, say 216 square feet of sustaining surface, or a spread of thirty- six feet by a width of six feet.

The equipoise of such an aeroplane is governed by the position of the load. In the bird the load is to the front, quite forward of the position of stable equipoise when the bird is not under way. The equilibrium is established only when the bird attains such speed as to make the center of air pressure coincide with the center of gravity. Just so must it be for man's apparatus. Such an apparatus, suspended head downward and allowed to fall, first drops vertically, and then retrieves itself and glides upward nearly as high as the starting point; but if the load be too far to the front, it must drop too far vertically before the velocity is sufficient to produce horizontal gliding. If the load be too far back, the apparatus retrieves itself too soon, flounders about, and may even revolve upon itself, topsy-turvy. But when the position of the load is such that the centers of gravity and pressure coincide, both being to the front, and so controlled (either by the bird's or man's intelligence) that this equipoise shall not be destroyed, then the apparatus will, if the air be calm, transform its fall into forward gliding and slide over the air upon a downward course, inclined but little (say one in ten) to the horizon. In such evolution the force of gravity occasions the fall of the aeroplane. It is opposed by the resistance of air, and from these two opposing forces results the gliding forward. It is, in fact, a slow descent upon a most smooth medium, upon air, .and the more perfect the aeroplane, in shape and surface, the slower is the fall.

All this applies to still air, to a dead calm; but if an aerial current intervenes, if wind springs up, then its vivifying force, properly utilized, may produce all the evolutions of sailing flight. These are the evolutions that we need to study: the methods of utilizing the power of the wind which constitute the science of the bird, and which we must understand in order eventually to sport with the creature in the sky.

Simplifying into few words the evolutions of sailing flight, we may say that for man's use, for simple journeying, but two kinds of guidance need be produced: the directing up and down, and the steering to the right or left. The bird does this most simply, and to the practiced observer the attitude indicates, even afar off, the bird's evolution, or what he is about to do. Thus, if a vulture is gliding, as shown in Fig. 5, and turns his head, as shown in Fig. 6, he will shortly be seen to be wheeling to the left; but he has still other means of producing the same movement

Gliding upward or downward results from changes in the center of gravity to front or rear. The bird can shift his center of gravity as he will, by simply advancing or thrusting back the tips of his wings, and man will be able to utilize the same method. If the tips of the wings are advanced, then the weight of the body remains behind, the aeroplane slants backward and it ascends (see Figs. 7 and 8); but if the tips be thrust backward, the weight, being then more to the front, slants the aeroplane forward, and it descends (see Figs. 9 and 10). It glides upon the air as if on ice, and this comparison is the more accurate the larger is the surface in proportion to the weight. The yielding is very small: that is to say, the actual height lost through the air's giving way under the sailing bird is inconsiderable, and it diminishes with the speed. The greater is the bird's velocity, the more the air acts like a solid.

Steering to the right or left is effected by the bird in many ways, such as a slight bending of the body in the direction desired, a part-folding of the wing on that side, a deformation of one wing-tip, so as to impede the air at that point and to turn upon it as a pivot, etc., etc.

Such are the various means employed by the winged creatures for guidance; yet, simple as they are, all are complicated by the emergencies of life; they are also deformed to our eyes by perspective, and it will actually be more easy to reproduce them than for me to scientifically explain them in detail. Effects of perspective constantly deceive the novice. Such observers are confused as to the attitude, and cannot realize what the bird is doing. In my sketches I have shown the vultures just as they look in the sky, but it may be well to refer further to the actual action.

In Figs. 1, 2 3, the birds are journeying; they come from afar, and many miles have been glided over in this attitude, without giving a single beat of wing.

In Fig. 8, prey has been discovered. The vulture sweeps around in those enormous circles which enable him to descry from every point whether danger still lurks anywhere near the carcass.

Fig. 9.

In Fig. 7, as things did not look quite safe, the vultures simply changed their angle of incidence and glided upward again.

In Figs. 5, 6 and 10, inspection is resumed. The vultures glide along and descend slowly at an angle of about 10°, or about one foot in six. The spread-out tips of primary feathers bend upward under the weight and maintain the equipoise.

In Fig. 4, the vulture drops vertically, glides neither forward nor back.

In Fig. 9, the creature glides headlong to the feast. He has thrust his wings well forward and is falling on a steep slant.

A practical illustration may perhaps make clearer the mutual action of the bird and the wind.

The reader doubtless knows the "roller coasters" and the "montagnes russes," in which a car runs down one slope and ascends another, but never quite so high as the starting-point, because of friction and resistance of the air. Now let him fancy that, as the vehicle starts down a slope, the whole roadway moves in the contrary direction, gliding under the vehicle like the wind beneath the bird; his own mechanical instinct will at once indicate that the vehicle will then rise higher than the starting-point, (if the route admits of this), the increased rise being produced by the action of the roadway gliding past. This may be verified by constructing a little apparatus, in which the roadway shall consist of an undulating, smooth groove, either straight or circular, and the vehicle shall be a steel ball, turned perfectly true and smooth. If the groove be straight in plan, (undulating in side-view), and mounted upon wheels, then two phases of bird ascension may be simulated:

1. The ball may be started down the slope, and at the same time the grooved roadway may be briskly moved in the contrary direction. The ball will be found to rise on the upward slope higher than the point it started from. This parallels the case of the bird which, already under way, breasts the blowing wind and rises on it.

2. Or we may place the ball at rest at the bottom of one of the curves, and by simply imparting quick motion to the roadway, the ball will be seen to ascend upon the opposing slope. This parallels the case of the bird rising above his perch by simply unfolding his plumage to the breeze. This, indeed, he may do without springing up; but in this case he generally drifts back a little, while, if he gains some initial velocity, he can rise and advance simultaneously, thus exhibiting a notable case of "aspiration."

To rise in circling flight, lighter breezes suffice. This action may be simulated by making the groove circular in plan, and rotating the whole apparatus on a pivot. Its path may be made to conform to the bird's orbit by making a series of long, gentle descents, and of short and sharp ascents, the combination of the two occupying one round. Then, by rotating the pathway in one direction and starting the ball in the other, the latter will be found to ascend upon an irregular helical path; just as the bird drops a little when he is going with the wind, and rises again, and higher up, when he sweeps against the current again. This last illustration is imperfect, inasmuch as the whole pathway has been made to revolve, while the wind utilized by the bird blows on as a current. It, moreover, takes no account of the irregular wind gusts which the bird utilizes. It also exhibits much more friction than the actual performance which we have tried to simulate.

What the bird does in a wind, man can do. Our muscular strength is much too small to progress by direct action, like the flapping denizens of the air; but our brain is sufficient to supply simple gnidance when we shall have acquired the necessary skill. So, if we add life to the aeroplane and a moderate muscular power to supply the guidance, to perform in the right way and at the right time those evolutions produced by birds in gliding flight, the author believes that man may succeed in riding on the wind. To compass this, to achieve simple journeying flight in elementary form, experiment, practice, acquired skill, are doubtless requisite; but of great daring or of fresh invention there is little if any need. The principles are known, the path is pointed out by observers of birds, and now success awaits the skillful, prudent man who will thoroughly understand what he has to perform.

Mr. L. P. Mouillard was born in Lyons, France, in 1835. When fifteen years old, the chance witnessing of an evolution performed by a bird determined in him a lifelong vocation. At first, he studied art, but soon abandoned painting and purchased a farm in Algeria, where he observed the soaring birds and made several attempts to imitate them. Finding farming unprofitable, he sold out, and in 1865 went to Cairo, Egypt, where first he taught drawing in the military schools, and then engaged in trade. Then came illness; the former gymnist became crippled, and could only try his experiments by proxy; but the passion was too strong to be renounced, and he published in 1881 a book, "L'Empire de l'Air," in which he described his observations and experiments and which deservedly attracted much attention. As he intimates, he has since written another book, "Le Vol sans Battements," now nearly ready for publication.

Originally appeared in Cosmopolitan, 16, February, 1894, pp. 459-466.

Note: The word that appeared in the text was perforated. I believe this was an error in typesetting, and the phrase should have read "... nevertheless it was performed by them ..." so I changed it.