SAILING FLIGHT.

BY 0. CHANUTE

THE soaring, or as the French term it more properly, the sailing flight (vol-ô-voile) of certain species of birds, that is to say their power of progressing through the air and of translating themselves at will without any flapping action whatever, has always seemed such a mechanical paradox that its very existence has been questioned by those who have not carefully observed the performance of these birds.1

That a bird should float on outstretched wings high in air for hours, with no muscular exertion whatever save the passive one of keeping his wings rigidly extended, seems so preposterous, so much against all our mechanical instincts and experience of the law that expenditure of energy is necessary to produce locomotion, that even when the feat of soaring is first witnessed, the mind doubts the evidence of the eyes and seeks for some undetected movement to account for the forward advance.

Yet there is nothing more certain than that the soaring birds are supported and propelled without flap of wing. It is generally conceded by observers that they extract from the wind the energy necessary to the performance, but the exact way in which this is done is not at all agreed upon.

One principal reason why this extraordinary mode of flight has not attracted more attention, is that the performance is comparatively rare in northern latitudes. It requires a combination of favorable circumstances to entice to the locality the birds which practise it, such as regular diurnal breezes, neither too weak nor too strong, abundant food, and a mild climate. These circumstances chiefly obtain in subtropical latitudes, and in the vicinity of the sea, and in such regions the soaring birds are plentiful, while in the colder regions, where man's activities are greatest, only the eagles, the hawks, and an occasional visiting vulture are to be seen.

Buzzards and some other soaring birds are, however, abundant in the Southern United States, and much the most convincing evidence would be obtained by personal observation; but for the benefit of those readers who have never seen the performance under favorable circumstances, the following list of authors may be given, who have either described sailing-flight, or advanced theories for its explanation.

LIST OF AUTHORS.

Singularly enough, none of the above authors save the last, and perhaps Mr. Goupil, have dwelt upon the particular wing shape of the soaring birds, or explained in their theories why certain species of birds can sail and certain others can only glide 2 (that is to say, slide over the air on fixed wings by utilizing previously acquired momentum or the force of gravity), while the sailing birds can extract from the wind all the energy needed for support or propulsion.

This seems to require a certain mass, say, from a pound's weight upward, to equate the irregularities of the wind, and a particular conformation of bird to extract energy from the wind to the best advantage, so that the main conditions of sailing flight seem to be:

1. There must be wind, although it may be light.
2. No flapping whatever is needed when under way.
3. The bird must have a peculiar conformation.
4. The bird needs a certain mass or weight.

Few of the above-named authors give any precise measurements, so that it is impossible to test their theories by numerical examples and computations. The writer was thereby induced to gather such data for himself. He now proposes, first, to give the results of some of his observations, and then to attempt to reduce the data to the sway of mathematical laws.

PART I.--THE OBSERVATIONS.

In the Southern United States the sailing bird most frequently seen is the turkey- buzzard (Cathartes aura). He is an accomplished soarer, but is somewhat inaccessible, so that it is difficult to watch his manoeuvres at close range. The frigate-bird and the pelican are generally still higher up in air or more distant, so that while it is easy to become entirely satisfied that while sailing they obtain all their support and motion without flapping their wings, it is more difficult to observe just how they do it.

After watching many of these birds in Texas, Louisiana, and Florida, the writer found that his desire for exact observations was best gratified by the gulls of California; and omitting herein therefore all the other observations made of other species of birds at longer range, he proposes to confine his relation to the acts of birds which he could watch currently at fifteen to twenty feet distance.

The gulls of San Diego (California) are extraordinarily tame. They are not suffered to be molested, because they act as scavengers of floating refuse food about the harbor, and while the greater number of their kind live out in the open sea, upon fish and the slop of passing vessels, the knowing ones dwell in the bay and feed upon the broken victuals thrown from ships, and upon the garbage of the city. They wait for "Steamer day" with all the eagerness of old Californians before the days of railways, and when a craft comes in they congregate about it on the wing, awaiting the refuse from the kitchen.

They are absolutely fearless of man, and will float or fly within 15 to 30 feet of him while performing their evolutions, so that more may be learned of the minute acts of soaring from them in an hour than from months of observations of other birds which soar comparatively high in the air. Indeed, it is possible to detect their every motion, however slight, to see how they get started underway, how they glide or sail, how they balance themselves in a variable wind, and how they stop their headway to alight. When they sail, there can be no question of there being undetected movements, for the wings are absolutely rigid, the operation being often performed in such close proximity to the observer that the fluttering of a stray bit of down, protruding from below the secondary feathers, can be plainly seen.

By either standing on the lower, or on the middle, or on the upper deck of a steamer, the gulls can be watched either from below, or on a level with the eye, or from above, as may be preferred, so that one observation shall check another concerning the same act of manoeuvring.

These acts, for the present purpose (which does not concern itself with rowing flight), may be divided under four heads:

• 1st. Starting, or getting under way.
• 2d. Sailing, or soaring on the wind.
• 3d. Balancing, or maintaining the equilibrium.
• 4th. Alighting, including stopping the motion.

It may be well to state here, for the benefit of those whose observations have been confined to rowing birds, that while during the first few vigorous strokes, the weight is all sustained by the reaction of the downward blow of the wing on the air, as t soon as forward motion is obtained the bird acts as an aeroplane, and the support is mainly or all derived from the air pressure due to the forward speed; that is to say, that the under surface of the wings, whether flapping or extended rigidly, and when progressing as an inclined plane, receives a pressure from the impinging air which acts at right angles to the surface. This pressure is resolved into two components, one vertical which sustains the weight, and the other horizontal which either opposes the motion, if the angle of incidence be above the horizon, or acts as a propelling force if the angle be inclined below the horizon. Thus the flappings of the wings (which the investigations of Professor Marey show to be delivered at a varying angle) chiefly serve as a propelling force when the bird is under full headway, and the bird may also be sustained when gliding or sailing upon rigidly extended wings, provided he have headway enough. In still-air gliding this headway has to be furnished by gravity or from previous momentum, but in sailing the same effect is produced by the passing wind, provided it has sufficient motion with respect to the bird.

Sailing flight depends upon this latter condition, and as the wind constantly varies in intensity, as was well shown by Professor Langley, a certain amount of momentum or mass is required to perform sailing flight properly, and hence it is best exhibited by the larger and heavier birds.

It thus appears that the first important thing for the bird to accomplish in order that he may soar, is to acquire and to retain initial velocity, so that he may at all times, against the wind or with the wind, obtain support from the air by glancing over it at a sufficient speed and angle to produce a sustaining pressure. The first manoeuvre to observe, therefore, is how he obtains this initial velocity, and the following comprise the results of the writer's observations of gulls at San Diego.

STARTING.

The gulls take their start either from the surface of the water or from a perch. If from the former, they first face the wind, paddle a few strokes, spring from the water into the air and flap vigorously for a time, flying directly against the wind and nearly on a level course, some 4 to 6 feet above the water. The flaps at first average 3 per second, and are of great amplitude, or nearly half of a circle, but as headway is gained, say in a distance of 20 to 30 feet, the amplitude is reduced to about 45°, each side of the wing pivot, and the number of strokes gradually diminishes to 2 per second, and eventually to 1 stroke per second, the bird rising meanwhile at an angle rather less than 45°, to a height of 30 or 40 feet above the water, this altitude being gained in a course of 150 to 200 feet, at the end of which the velocity of the bird (measured in still air) is from 26 to 30 feet per second, when, if the wind be sufficient, he is in good condition for soaring flight.

If the gull be on a perch (and the bird seems much to prefer this position), say on the ridge of a building, on the edge of a wharf, or on a pile-head, his exertions to start are much less. He faces the wind as before, launches himself forward and downward, gaining impetus from gravity, and with a very few flaps-- sometimes without any--he acquires a velocity of 26 feet per second, then he rises and is prepared to soar upon the wind, which is evidently more rapid at a height of 30 or 40 feet above the water than it is at the surface. The bird may alternately flap and glide for a few seconds, but as soon as he feels himself in a sufficiently strong current, say of 18 to 20 feet per second, he ceases all active exertions and abandons himself wholly to the pleasures of sailing.

If the breeze be steady and strong enough, say 19 to 22 measured feet per second at the surface of the wharf, the gulls occasionally exhibit a third and very remarkable method of getting under way.

This is sometimes performed from the ridge of a building, or from a ship's spar, but the most satisfactory observations are obtained when the bird is on a pile-head about 10 feet above the water, and therefore on a level with the eye of the observer on the wharf, and some 15 or 20 feet distant. The writer has seen the feat performed many times and will describe the best instances.

The bird stands on the pile-head, and faces the wind. He opens his wing wide, but keeps the front edges depressed, so that the wind glances over them and presses him downward: then, when the breeze freshens, he changes the plane of his wings by a slight twist, until they present an angle of incidence of about 20° above the horizon. The wind, pressing under the wings, lifts the bird up 2 or 3 feet. The first part of this ascent is quite vertical, but presently the gull, still rising, drifts back about 5 feet, when he comes to a poise for an instant as he changes his angle of incidence, so that it becomes negative and points below the horizon, when his course is reversed. He plunges downward and against the wind until he is within about 4 feet of the water, when, having acquired an initial speed of his own, of about 26 feet per second, he again changes his angle of incidence to some 12° or 15° above the horizon, and he rises again, and is henceforth master of his movements, continuing to sail without flapping, apparently in any direction he desires.

The wind, on the occasions when this performance was exhibited, blew with comparative steadiness at a speed of 14 to 23 feet per second, with an average of 21.12 ft. per second, or 14.4 miles per hour. The sea breeze at San Diego sprang up every morning, being caused by the rarefaction of the air in the sun over the California deserts, and during the writer's visit the wind blew every day from the same quarter and with nearly the same intensity, making this location an ideal one for the performance of sailing flight.

Thus it is seen, that by this last starting manoeuvre, it is in the power of a soaring bird, if the wind serves, to rise from his perch without flapping, to remain aloft indefinitely, as will be more particularly described under the head of Sailing, and to return to a perch again, without furnishing a single stroke of wing. In point of fact the birds not infrequently give a flap or two, but this seems to be done more as a matter of convenience, to trim the course, or to maintain the velocity.

SAILING.

Once well underway, with their own speed of 26 to 30 feet per second (17.7 to 20.4 miles per hour), the gulls seem to be master of their movements, and, if the breeze blows from 14 to 23 ft. per second (12.92 to 15.64 miles per hour), to be able to translate themselves to any point they choose.

The manoeuvre which can be observed at closest range consists in patrolling at lunch time along the water side of a steamer tied to the wharf. The birds then sail back and forth along the side, at a distance of 20 to 30 ft. therefrom and at a height of about 30 ft. above the water. They are attending upon the cook, in hope of garbage.

On the occasion at which the most satisfactory measurements were made the gulls sailed along back and forth a distance of about 100 ft. parallel with the steamer, and some 20 ft. therefrom, so that from the middle deck they could be looked in the eye and the least movement detected. They floated on a horizontal course, turning upon their heel at the end of each lap, and returning over the same path. This was done entirely without beat of wing, or tremor of a feather, save an occasional fluttering of a bit of down projecting from beneath ill-matched feathers, and it entirely set at rest in the mind of the writer any assumption as to support being gained from any minute movements of the feathers or tail.

Whenever a bucket of slops was emptied from the kitchen, the gulls wheeled at once, poised themselves for an instant, and swooped down towards the food. When within 5 ft. of the water they began a curve, throwing their wings at an angle into the attitude of an obtuse angle, and describing nearly a semicircle, they snapped up a mouthful from the water. Then they remounted with a few vigorous flaps, to a soaring attitude on a level with the upper deck of the steamer; thence, a fresh poise having been obtained, and the food swallowed, the operation would be repeated, and when all the slop was eaten up, the sailing along the side would be resumed until the next bucketful.

The wind was blowing at an observed mean velocity (6 measurements) of 18.83 ft. per second, or 12.78 miles per hour, and at an angle of 30° (as near as could be judged) with the line of the ship. The gulls progressed against this side wind at a measured speed of 10 ft. per second, with reference to the ship, and their speed with reference to the wind was therefore equal to its velocity, multiplied by the cosine of the angle of incidence, plus their own apparent motion. Hence the relative speed was 18.83 X 0.866 + 10 = 26.3 ft. per second, or 17.88 miles per hour. At this speed the birds were perfectly sustained, and floated back and forth on extended wings slightly arched, wheeling with a short turn at the end of each course, their heads turning from side to side, and their little eyes eagerly watching. Their angle of incidence above the horizon was 5° to 7°, as near as it could be measured when going against the wind, but this angle of incidence is very difficult of measurement, and moreover it constantly varies with the speed, or when the bird rises or falls, or goes with or against the wind. It was estimated by holding a pencil on the line of the water horizon.

During all the time, the birds were constantly balancing themselves, as will be more fully explained hereafter, so that it must not be understood that when they are spoken of as floating on the breeze, they made no movements whatever to regulate their poise or angle of incidence, or to counteract the variations in the direction and intensity of the air currents. Such movements were, however, very slight, about as active as those made by a man to balance himself in walking, and clearly they did not furnish any motive-power for support or propulsion. This power all came from the wind, as will be more fully discussed when the attempt is made to account mathematically for the phenomenon.

There being a question as to the horizontality of the wind, this was tested by liberating bits of tissue paper (such as that placed between visiting cards), from the edge of the steamer. The wind blew them upward at various angles, generally from l0° to 20°; so that it was concluded that the side of the ship produced an ascending trend in the wind; what that trend was at the birds, 20 feet away, there was no mode of ascertaining, but it is believed that it materially assisted them in their evolutions.

Meantime, in other parts of the harbor, other gulls were observed patrolling the sky. They generally soared higher in the air, at a height of 100 to 200 feet, probably to avail of the stronger breeze prevailing at that height, and presumably blowing with exact horizontality. With these latter birds the favorite course seemed to be quartering with the wind, back and forth, for 500 to 800 feet. Their relative speed, after allowing for that of the wind, seemed, by measurement, to be from 22 to 24 feet per second, but the writer believes it to have been somewhat more, as the wind was measured at the surface of a dock, while the birds were aloft. On some mornings, before the sea breeze had fully set in, and when the wind was consequently light, a favorite course seemed to lie just above and to the leeward of a set of " coal pockets " on a dock, which forms a wind-break 650 feet long and about 25 feet high, thus lending additional countenance to the theory that a soaring bird finds assistance from ascending currents of air, deflected from their horizontal course by some obstacle or by rising ground; but when the wind grew stronger, the gulls soared indifferently all over the harbor.

At other times the gulls circled in the air, gradually drifting to leeward. These circlings were seldom long continued, and lacked the majesty of the great sweeps of the buzzard when surveying a township from aloft. The gulls circle rather irregularly, but observation indicated that they dropped earthward while sailing with the wind, thus gaining speed from gravity, and rose more than they had dropped when they reached the quarter circle performed against the wind, thus utilizing the increased velocity to regain the lost altitude. At times, they seemed poised, absolutely fixed in one spot, their own initial velocity at that time being of course exactly equal to that of the wind, and at other times they rose upwards but drifted backward, a little gust of wind having apparently furnished a surplus of sustaining power, but at the expense of forward motion.

In one or two rare instances, the birds were seen both to rise and to advance in a straight line against the wind simultaneously, and this is the hardest manoeuvre to explain mathematically. It has, however, been well described and figured by M. Baste, and his diagrams will be used when an attempt is made to account for this paradox.

The birds having no particular motive to demonstrate that they can sail indefinitely without flapping, occasionally resort to the latter in their manoeuvres. When gliding or sailing upon the breeze, they perform an occasional flap, a mere kick, as it were, either to limber up the outstretched wings, or to maintain the speed they require to obtain a sustaining reaction, or to overcome the head resistance without changing their angle of incidence when the wind chances to weaken. Sometimes, also, they glide down some distance and gain speed at the expense of height, to be recovered when the breeze freshens; but there is nothing more certain in the writer's mind than the fact that wind is required for sailing flight, and that the few observers who claim to have seen soaring performed in a dead calm, must in some way have been mistaken.

By a calm, or by a very light breeze of 2 to 4 miles per hour, such as generally prevails in the early morning hours at San Diego, sailing flight does not seem to be performable by the gulls, and if they want to go somewhere they flap exclusively. Their speed when flapping in a calm is from 30 to 33 ft. per second (20.4 to 22.44 miles per hour), this being the result of many measurements along distances of 350 to 500 ft. The harbor gulls are lazy birds, however, and evidently dislike the exertion, for, during a dead calm, most of their time is spent on firm support, the favorite places being the outer ends of piers with little traffic, where there is a chance for the birds to walk about and squawk, apparently in gossip with each other.

When the sea-breeze springs up, generally before noon, the more active or hungry gulls start out upon a cruise, and by rowing flight, continued until they are high in the air, find a wind sufficiently strong for them to sail in, and when this has freshened to 11 or 15 miles an hour at the dock surface, all the gulls leave their perch and float upon the breeze, sailing then seeming to be preferred by them to perching, as involving no more exertion and being a pleasanter mode of passing the time, even when a meal is no longer a desideratum.

If, however, the wind increases to over 30 miles an hour (a rare occurrence), the gulls seem to find sailing unpleasant and generally seek some quiet spot, more or less sheltered, to allow the storm to blow over.

The greatest exertion performed by the gulls is, apparently, during the act of hovering, when beating their wings rapidly so as to remain at a fixed point just above the water, to inspect closely a suspicious morsel. The body is then held at an angle of about 40° with the water, some 18 or 24 inches above it, and the wings are vibrated fast, though without a great amplitude, while the bird remains several seconds in one spot, and cranes his neck forward. If the morsel be deemed acceptable, he sweeps down, snatches it in his beak, and with much flutter, balancing, and effort, he rises again with vigorous flaps to resume his soaring flight, generally preferring, however, to do this at an angle of 30° to 45° to the wind, back and forth, so as to convey to the observer the idea that he is making a series of tacks against the wind.

Almost all observers are agreed (D'Esterno, Mouillard, Penaud, De Louvrie, Baste, Bretonniere, Proctor, Peal, Langley, etc.) that wind is absolutely necessary for the performance of sailing flight. All of the observations of the writer confirm this, although he has seen the feat performed by buzzards and by hawks, when the breeze measured only 5 or 6 miles an hour at the surface. It may have been more rapid higher up, where the bird was. The gulls do not seem to soar well unless the wind blows at about 10 miles per hour. The difference is probably accounted for by the difference in proportion of carrying surface to the weight, this being, according to the writer's measurements, very nearly 1 lb. to the square foot for the gull, while it is 0.88 lb. per square foot for the buzzard and 0.55 lb. per square foot for the chicken-hawk.

Before making the observations above described a "herring gull" had been shot, weighed, and measured. Its weight was 2.188 lbs., its entire surface, wings, tail, and body projected, measured 2.015 square feet, its cross-section of body at the maximum point was 0.126 square feet, and its cross-section of wings, projected at the point of maximum anterior thickness, was 0.098 square feet. These figures will hereafter be used in computing the support and resistances.

BALANCING.

All sailing birds are accomplished acrobats. It is difficult to detect the more minute movements in the hawks, the buzzards, the frigate-bird, etc., which generally sail at a considerable altitude, but they can be closely studied in the fearless gulls of San Diego, which perform their evolutions right under the observer's nose.

As the wind varies in intensity, or as the birds wish to rise or to fall, they are constantly changing their angle of incidence and their poise. This is done by advancing or moving to the rear the tips of the wings, which are stretched out in a soaring attitude. The movement is slight, and, as the writer suspects, is also automatic. It alters the poise at once fore and aft, and the bird either rises or falls, or he restores the adjustment between his own speed and that of the wind.

If he wants to wheel to one side, a manoeuvre which is done very gracefully, he apparently increases the flexion of the wing on the side to which he wants to turn, the body tilts to that side in consequence of its disturbed balance, and the bird wheels to that side. The same result is also thought to follow the advancing of one wing more than the other, but the writer does not feel that his observations are quite conclusive on that point.

But most of the continual balancing is effected with the head and the feet, which, when the wind is at all gusty, are almost constantly in action. The fore and aft balancing is sometimes effected with the head alone, the neck being stretched out or drawn in, or it may be swung from side to side to preserve the transverse equilibrium. Often, however, the legs also come into action. When in full sailing activity in a steady breeze, they are rigidly extended out back under the tail, but when a gust of wind compromises the balance, the legs drop downward, making an angle at the knee, and the feet are adjusted as required to preserve the fore and aft balance, by altering the leverage due to their weight, making thus an adjustable pendule of great efficiency.

As has been said, these movements are slight, and hence difficult to detect except at very close range; nor are they continual, as they are only made as occasion requires, without apparently taking more thought than a man does while walking. They are doubtless due to reflex action as guided by acquired instinct, but they point out the enormous difficulties to be encountered by man if he seeks to imitate the bird, and to sail upon the gusty wind, before he has acquired the science of balancing, or produced an automatic apparatus of his own.

It is difficult to determine accurately the angle of incidence which the bird makes with the horizon when sailing. The general plane of the wings does not seem to be parallel with the lower edge of the body, and the eye becomes confused in estimating the angle. This angle, moreover, constantly varies with the speed within small limits, but the writer deems it to range between 3° and 15° above the horizon when the bird is on a level course. As he rises or falls the angle of incidence is materially altered, both above and below the horizon, but from the best projections made by the writer against the sky line, he deems that the most usual angle of the wings, when sailing upon a level course, is from 5° to 7° above the horizon, and the latter figure will be used in the mathematical computations.

One peculiarity of the sailing gull consists in his attitude,--the wings are arched downward like a bow. Land-sailing birds generally hold their wings extended either horizontally, or so as to make with each other a slight diedral angle above the horizon, while the gulls and many other sea birds hold their wings when sailing in the singular position described. Why this should be so the writer has been unable to surmise.

ALIGHTING.

While the manoeuvres performed in alighting exhibit as many varieties as those for starting, they can be described briefly. The object of the bird is evidently to stop his forward motion so as to avoid a shock upon alighting, and this is very neatly performed in a variety of ways. The method preferred is to arrive at the point selected at a lower level and to rise to the perch, thus destroying speed by the action of gravity. The bird generally rises some 18 inches above the selected perch, this rise being performed at an angle of about 45 or more, and then he poises for an instant and gently drops feet downward on the perch. Sometimes the arriving course is on a level with the point selected; in that case the bird tilts himself to an angle of some 40° to the line of motion and without beat of wing, if he has calculated just right, he finds his headway stopped just above the perch and drops down to it as previously described.

Or the motion may be stopped by backward beat of wing, or by hovering with vibrating wings just above the spot selected, so that the pendant legs soon touch the perch, or the gull's body settles upon the water, but in every case the bird contrives to arrive from such a direction that he faces the wind, if any, when at the perch, and utilizes this as a retarding force.

These are the observations. They were made in March, 1892, and written out in full at the time, substantially as now i published, but it took me three years to arrive at a satisfactory explanation of what I had seen, and to compute the forces in action. The chief trouble was that the calculations of weights sustained, at the observed speeds and angles of incidence, were based upon the known pressures of air upon plane surfaces, adding, however, a coefficient obtained, with a pigeon's wing, a non-soaring bird. It was only when Herr Lilienthal's table of air pressures was obtained, in the "Handbook for Aeronauts and Aviators"3 that I was enabled to figure up satisfactory reactions, with the coefficients obtained by actual experiments with surfaces shaped like the wings of sailing birds.

3"Taschenbuch für Flugtechniker und Luftschiffer." 1895. W. H. Kuhl, 73 Jägerstrasse, Berlin W.

Webmaster's note:This article was copied from the 1896 edition of James Mean's The Aeronautical Annual. Two tables in this document were typeset a little differently in Chanute's article. I took the liberty of employing the table format because it is a reasonable approximation of the original "look and feel". HTML isn't always sufficiently powerful to do the things one might like. Scan by Cory Kotowsky. HTML by Gary Bradshaw.