Popular Science Monthly/Volume 57/September 1900/Birds as Flying Machines
|BIRDS AS FLYING MACHINES.|
FROM the day of Solomon onward the way of a bird in the air has been a subject of general interest, and the attention given to the problem of aerial navigation of late years has caused the flight of birds to be carefully studied in the hope that it might throw some light on the subject. There have been many conceptions, not to say misconceptions, regarding the flight of birds; it has been assumed that their muscles exerted a power quite beyond that of other animals, that the air sacs of some birds and the hollow bones of others gave them a degree of lightness quite unattainable by the use of ordinary materials, while some have even gone so far as to suggest the presence of some mysterious power, something like Stockton's negative gravity, whereby birds could set at naught the law of gravitation and rise at will like a balloon. The strength of a bird's muscles, of some birds' muscles at least, is not to be underrated; a hawk will plant its talons in a bird of nearly its own size and weight and bear the victim bodily away, and an osprey will carry a fish for a long distance. But a tiger has been known to fell a bullock with a single blow of the paw, to carry a man as a cat would carry a rat and to drag an Indian buffalo heavier than himself. On the other hand, some of the petrels, birds which can pass a day or so on the wing with ease, cannot rise from the water after a hearty meal, and the humming-bird, unsurpassed in aerial evolutions, may be trapped in a spider's web. This shows no great power, and long ago Marey found that the pulling force of a hawk's great breast muscle, applied through the humerus, amounted to 1,298 grams per square centimeter, something like seven pounds to the square inch; not a very heavy pull. So it seems fair to assume that while the power exerted by a bird is great, it is very far from marvelous, probably far less in proportion to size than the engine of Maxim's great aeroplane, or the naphtha motor of Professor Langley, which weighs less than ten pounds per horse power. We may get a fair idea of what this means by remembering that a bald eagle weighs from nine to fifteen pounds and that he exerts but a small fraction of a horse power.
Turning to the question of the part played by the air sacs it may be said that their value is not proved; some of the fastest birds get along without them, while birds of the most labored flight are sometimes well provided. In birds like the gannet and brown pelican the air sacs and cellular tissue about the body undoubtedly serve as buffers to break the shock of a headlong plunge into the water from a height of a hundred to a hundred and fifty feet. Or, again, they equalize the internal and external pressure when a soaring bird drops suddenly from a great height, or still more often aid in oxygenating the blood.
The hollow bones of birds are frequently cited as beautiful instances of providential mechanics in building the strongest and largest possible limb with the least expenditure of material, and this is largely true. And yet birds like ducks, which cleave the air with the speed of an express train, have the long bones filled with marrow or saturated with fat, while the lumbering hornbill that fairly hurtles over the tree tops has one of the most completely pneumatic skeletons imaginable, permeated with air to the very toe tips; and the ungainly pelican is nearly as well off. Still it is but fair to say that the frigate bird and turkey buzzards, creatures which are most at ease when on the wing, have extremely light and hollow bones, but comparing one bird with another the paramount importance of a pneumatic skeleton to a bird is not as evident as that of a pneumatic tire to a bicycle.
While it may not be easy to disprove Herr Gätke's assertion that birds sustain themselves in the air by the exercise of some power beyond our own, it is pretty safe to assume that they do not, and it would seem that the burden of proof should lie with those who take the affirmative side of the question.
If we have nothing to learn from birds in the way of building an engine that shall exert great power for its size and weight we may still have something to gain in the matter of speed, although here the popular idea is apt to be exaggerated. We often read that ducks fly at the rate of a mile a minute, or that the swallow has a speed of two hundred miles an hour, but it is very difficult to lay hands upon any facts that will sustain these assertions.
So, too, homing pigeons are frequently stated to have travelled for long distances at the rate of sixty miles an hour, but some of the published records show that one hundred and twenty miles in two hours and a quarter is unusually fast traveling, and this is at the rate of only nine-tenths of a mile per minute, a speed not unusual for express trains. However, it may be said that actual observations show that ducks do travel from forty to fifty miles an hour, and any sportsman will readily believe that under some conditions they attain a velocity of a mile and a quarter a minute, although a confession of faith is not a demonstration of an assured fact.
So far the lesson taught by the bird is that a machine of low power may attain a very considerable speed and it remains to be seen if there is anything to be learned concerning methods of flight. Broadly speaking, there are two, possibly three, distinct modes of flight, by repeated strokes of the wings and by soaring or sailing, although we find every intermediate stage between the two, or combinations of flapping and sailing, and as a matter of fact no bird can entirely dispense with strokes of the wing.
The humming-bird represents the perfection of one method, the frigate bird of the other, and in his own line each is unrivaled. These two modes of flight are associated with equally distinct modifications of structure, and just as we have every intermediate state of flight between flapping and soaring so the two structural extremes are merged into one another. The humming-bird flies as the Irishman played the fiddle, by main strength, the frigate bird relies on his skill in taking advantage of every varying current of air, and the skeleton of the one indicates great muscular power while that of the other shows its absence. No other bird has such proportionately great muscles as the humming-bird, the keel of the sternum or breast bone from which these muscles arise runs from one end of the body to the other while at the same time it projects downward like the keel of a modern racing yacht. These muscles drive at the rate of several hundred strokes a minute a pair of small, rigid wings, the outermost bones of which are very long while the innermost are very short, a feature calculated to give the greatest amount of motion at the tip of the wing with the least movement of the bones of the upper arm, to which the driving muscles are attached. Another peculiar feature is that the outermost feathers, the flight feathers or primaries, are long and strong, while the innermost, those attached to the forearm, are few and weak; so far as flight is concerned the bird could dispense with these secondaries and not feel their loss. Finally the heart, which we may look upon as the boiler that supplies steam for this machinery, is large and powerful, as is necessary for such a high-pressure engine as the little humming-bird. It is hardly to him that we would look for aid in constructing a flying machine, the expenditure of force is too great for the results attained, the space required for boiler and engine leaves no room for carrying freight.
As just intimated the frigate bird is exactly the reverse of his tiny relative; the body is a mere appendage to a pair of wings, while the breast muscles are so small as to show at a glance that of all flying creatures the frigate bird is the one which has most successfully solved the problem of the conservation of energy and can obtain the greatest amount of power with the least expenditure of muscle.
There is also a great difference between the hummer and the frigate bird, or between flapping and sailing birds generally, in the complexity of what may be termed the muscles of adjustment, the little muscles that run from the shoulder to the elbow and forearm and, among other duties, are concerned in keeping free from wrinkles that portion of the wing which lies between the shoulder and the wrist, forming a triangular flap with the base forming the front edge of the wing and the apex lying in the elbow joint.
The wing of the frigate bird, too, is quite the opposite of that of the hummer, for it is the inner portion of the wing, the upper arm and forearm, which is elongated, and instead of the six feeble secondaries of the humming-bird there are no less than twenty-four; instead of a short, stiff, rounded wing we have one that is long, flexible and pointed. Instead of a wing driven at the rate of several hundred strokes a minute there is a wing that may be held outstretched and apparently motionless for minutes at a time, the muscles of the frigate bird being almost as constantly in repose as those of the other are perpetually in motion.
If the frigate bird represents the highest type of soaring flight two more familiar birds, the turkey buzzard and albatross, are not far behind, and these represent two methods of sailing flight and two distinct modifications in the type of wings. The albatross is continually on the move, ever quartering the water as a well-trained setter does the ground, and yet with all this movement rarely mounting higher than fifty feet above the water and never wheeling in great circles in mid-air. This bird has that type of wing which best fulfills the conditions necessary for an aeroplane, being long and narrow, so that while a fully grown albatross may spread from ten to twelve feet from tip to tip, this wing is not more than nine inches wide. This spread of wing, like that of the frigate bird, is gained by the elongation of the inner bones of the wing and by increasing the number of secondaries, there being about forty of these feathers in the wing of the albatross.
The turkey buzzard is emphatically a high flyer, wheeling slowly about, half a mile or a mile above the earth, while his cousin, the condor, so Humboldt tells us, has been seen above the summit of Chimborazo. If any bird knows how to utilize every breath of wind to the utmost that bird is the albatross, and it is equally a delight and a marvel to see this bird apparently setting at naught all natural laws as he sails with outstretched pinions almost into the eye of the wind or hangs just off the lee quarter of a ship reeling off ten or twelve knots an hour. In this last trick, however, the gull is almost equally expert, evidently making use of the draft from the sails as well as of the eddies caused by the passage of the vessel.
It has long been evident that if man is to navigate the air it must be done after the method of the albatross rather than that of the humming-bird, by the aeroplane and not by any device to imitate the strokes of a bird's wings, for not only do the largest birds and those of the longest flight for the most part sail or soar, but it is apparent that the limit of size in a vibrating wing must soon be reached, since in a strong wind with its varying eddies it would be quite out of the question to manipulate such a piece of mechanism.
But in spite of the fact that sailing flight calls for the exercise of comparatively little muscular power, the structure of the skeleton suggests that the wing of a soaring or sailing bird needs a particularly strong point of support, for birds which sail or soar have the bones which sustain the direct pull of the wing strengthened or braced as other birds do not. The shoulder joint of a bird is formed by the shoulder blade and coracoid, this last being the bone which is attached to the breast bone and on which comes the direct pull of the wing, and in front of the coracoids, running downwards towards the sternum, is the wishbone or furcula, corresponding to oar collar bones or clavicles. It is evident that the greater the length of the coracoid the less able would it be to resist the strain brought upon it, and it is also evident that the simultaneous downward stroke of the wings must have a tendency to force the coracoids inwards, or towards one another. Obviously the greater the strain the greater the need of strengthening or bracing the coracoid to resist it, and there are in the shoulder girdle of a bird various devices looking towards this end. In some birds, the albatross, for example, the coracoid is short and stout, while in others extra bracing is obtained from the wishbone.
In the humming-bird the wishbone is light and weak and so short that it does not come near the sternum; the pigeon, a bird of powerful flight, is little better off, for the wishbone is so long and slender that it does little or nothing towards strengthening the shoulder joint, and in both these birds which fly by rapid wing strokes the entire pull of the wing is taken by the coracoid. In the frigate bird, on the contrary, the wishbone is not only strong, but it rests upon and is firmly soldered to the breastbone, while at its upper end it fuses with the coracoid, thus making the firmest possible support to the wing. The cranes, which soar well, also have the wishbone united with sternum, and in the albatrosses and petrels the wishbone touches the breastbone and is so curved forward as to gain strength in this way while, as previously noted, the strength of the coracoid is increased by its shortness. The turkey buzzard and birds of prey, some of which both soar and flap, have the wishbone strengthened by having more material added to make the furcula thick and strong while at the same time it is shaped like a wide U instead of a V.
Either there is more force exerted in sailing than is at first sight apparent or else extra strength is called for in making sudden turns, or when it becomes necessary, as it does more or less frequently, to take a sudden wing stroke. As wings are levers of the third order the longer the wing the more force is required to move it and more strength is needed at the fulcrum or shoulder joint, and since sailing birds have long wings the need of strength is evident.
Neither birds nor any creatures that live or have lived afford us any criterion as to the limit of size that must be placed on an aeroplane. The largest of whales is weak and insignificant beside an ocean liner, and the condor and albatross, with their spread of ten or twelve feet and weight of ten to twenty pounds, tell us nothing of what may be the possibilities of size and weight.
Among the various problems confronting the would-be navigator of the air is that of at times making headway against a medium moving at the rate of ten, twenty, or thirty miles an hour, sometimes even more, a difficulty that neither locomotive nor steamer is called upon to meet. True, an aeroplane would, to use a technical term, probably lie within two and one-half points of the wind and could thus advantageously beat to windward, but any deviation from a straight course means loss of time, and nowadays time is everything.
The mode of propulsion may be, undoubtedly will be, as entirely different from a wing as the propeller is unlike the tail of a fish, and as the study of fish has thrown little or no light on the problems of the proper form or best motor for a ship, it is doubtful if the study of birds will do more for the aerodrome. Nor does it seem likely that a study of the bird will suggest any new devices in the way of joints, braces, or rudders, for what must be discouraging to those engaged in solving the problems of flight is the utter inadequacy of the bird's wing, from a mechanical standpoint, for the work it is called upon to do, for in all its articulations there is a freedom of movement, an amount of play that would be inadmissible in any machine. The shoulder, elbow and wrist joints are but loose affairs, depending for their efficiency on the pull of the muscles; subtract the element of life from the wing of a bird and it becomes at once limp and useless. And herein is the key to the bird's success as a flying machine; it has life, and while the wing may reveal certain principles of balancing, it cannot teach us all the art, for it is done instinctively. The bird has back of it untold ages of experience and its actions during flight demand no thought; the muscles respond instinctively to each change in the pressure and direction of the wind, and the bird need take no thought as to how it shall fly.
Mr. Chanute has taken the greatest step yet made towards overcoming the difficulty of responding to changes in the velocity of the fickle air, but whether or not it will be possible to construct apparatus that will not only adjust itself to changes in the force of the wind, but to eddies and changes in direction as well, remains to be seen, the more that it must act not on planes six feet in length, but on surfaces infinitely larger. The proper method of constructing the wings of an aeroplane so as to insure stability and utilize the power of the wind to the best advantage, and some hints as to balancing and steering are the main assistance that we seem likely to gain from a study of the structure and flight of birds.