Ascending trends of wind are by no means rare, as abundantly proved by published observations since M. Pénaud called attention to the many causes which must produce such trends. This was shown in a very able paper on "Sailing Flight," which was published in part in the Aéronaute for March and April, 1875 but which, unfortunately, was left unfinished. M. Pénaud demonstrated that such winds must necessarily result from even moderate undulations of the ground (and therefore a fortiori from mountains or deep valleys), from natural or artificial objects acting as wind breaks, from the meeting of air currents flowing in different directions, or even from the heating effect of the sun. He doubtless expected to show, in the portion of the paper remaining unpublished, that an upward trend of 1/9 to 1/6 (from 6° to 10° in the wind was quite sufficient to enable a sailing bird to progress against the breeze by inclining his aeroplane so that the horizontal component of the pressure would have a forward direction, while the wind still acted on the under side; for we have already seen in computing the foot-pounds expended by a 1-lb. pigeon in gliding, that with a speed of 40 miles per hour and an angle of incidence of 3 the "drift" will be 0.05647 lbs., while the body resistance and that of the edges of the wings together will be 0.05555 lbs., and that at 5 (30 miles per hour) the "drift" will be 0.08892 lbs., and the resistance of the body and edge of wings will be 0.03124 lbs., so that in both these cases the "drift" (calculated even with the coefficients which have been obtained with planes, and which are known to be inferior to those to be expected from concavo-convex surfaces) is sufficient, if directed forward, to overcome the resistances and to give to the sailing bird a forward impulse; this reversal in direction of the "drift," as previously explained, occurring when the plane becomes inclined so as to point forward below the horizon.
Since Pénaud's day a great many observations have confirmed the frequent prevalence of both ascending and descending currents. Aeronauts, more particularly, have noted that the atmospheric currents follow the undulations of the ground, causing their balloons to subside upon approaching a valley, or to rise when nearing a cliff or a mountain. They have also inferred, from the fact that they have found butterflies a mile or more above the earth while sailing over table lands, that these trends are frequent in such regions, although their effect upon the balloon is less immediately noticeable than in mountainous countries, where the angle of ascent often is 45 or more. In such broken countries very curious observations have been made as to the invariable prevalence of steeply ascending winds in certain well-defined localities when the wind blows from a particular quarter; such, for instance, as the observations of M. Mouillard in the Lybian chain near Cairo, and those of M. Bretonniere in the vicinity of Constantine, Algeria, where certain zones or gaps of ascending winds seem to exist, which the sailing birds utilize to gain elevation by circling. There they congregate in crowds, forsaking the rest of the sky, and spirally mount on rigid wings, until they have gained sufficient altitude to carry them toward any point which they may want to reach in descending.
It is probably in sub-tropical regions that such phenomena are most numerous and permanent; but the reader, who is accustomed to thinking of the wind as blowing horizontally, may be quickly edified by watching the smoke issuing from a tall chimney even in northerly climates. This smoke will be seen at various hours, or on various days, to trend either upward or downward or with exact horizontality, as may depend upon the undulations of the great atmospheric waves which are produced by the impinging upon each other of the currents flowing and crossing at various altitudes; or if the observer have the good fortune to be in the regions inhabited by the sailing birds, he may satisfy himself as to the similar atmospheric undulations which are constantly taking place, even in a perfectly flat country, such as the plains of Texas or the sea beaches of Florida, by liberating bits of down or threads of smoke fro rn the same spot at various times or days. He will also-observe the local ascending currents permanently produced by a mere wind break, such as a belt of trees facing the in flowing sea breeze. He may satisfy himself (by attaching light strips of bunting or bright-colored threads to the tops of those trees) that the breeze is deflected upward just over their upper branches, and he will then understand why these spots constitute the favorite haunts of the sailing birds when the breeze is light. He will see the soarers for hours gliding back and forth and back and forth on pulseless wings just above the top of the wind break formed by these belts of trees, evidently utilizing the ascending current to patrol the adjoining beach while awaiting, with no labor, whatever food may be brought by the incoming tide, or an opportunity of eating it undisturbed.
It is not intended here to convey the impression that ascending trends of wind are absolutely necessary for sailing flight. The writer has seen the feat performed many times, when every test seemed to prove that the current was absolutely horizontal; but it then seemed to him that on such occasions the equilibrium was more difficult to maintain, and that the bird had to bestow greater attention upon the nice adjustments required to preserve his balance and to produce "aspiration" when the wind varied in intensity and direction; just as an acrobat experiences greater fatigue in walking a tight rope, through the attention and care expended to avoid falling, than in walking many times the same distance on the ground, where no particular care is required to preserve the balance. It is probably because of such relief from all cerebral strain that the soaring birds seem to sail with less care and with far greater steadiness whenever they are utilizing an ascending current. They are then easily and safely sustained, and so mechanical does the performance seem that some observers have expressed the opinion that they then sleep on the wing. There is no doubt, moreover, that ascending trends of wind enable the creatures to soar in lighter breezes than would otherwise be possible, and when the faint morning wind first begins to blow, many of the sailing birds will be seen congregated just above wind breaks, while the other parts of the sky are vacant,
But to return from this digression, occasioned by the feat of "aspiration" performed by M. Myers kites, it will be discerned that the principle of flexibility alluded to confers stability upon the well-known Japanese kite specimens of which are now to be found in almost all toy shops. This kite flies without a tail, the frame being so light and elastic that the surface adjusts itself constantly to the irregularities of the breeze. The side pockets catch the wind, and by springing back of the medial line form a diedral angle which confers lateral balance, while the flexibility up and down confers longitudinal equilibrium. The same principle is exemplified in the upward bending of the extremities of the feathers of birds in flight, which doubtless adds much to their stability, and, indeed' so universally is this principle illustrated by all creatures which navigate fluids, that Dr. Amans, in a work upon the locomotive organs of fishes,32 lays it down as an axiom derived from physiological considerations, that an aeroplane of rigid form is contre nature, or in direct antagonism with all the inferences to be drawn from the observation of creation.
The Japanese are expert kite-flyers, and have produced many shapes besides that which has been above alluded to. They are said to use kites as weather vanes, and to have hitching posts in their gardens to which the device is almost permanently affixed. Indeed, it is said that these kites sometimes remain 8 or 10 consecutive days up in the air--an astonishing achievement to European and American kite fanciers, who seldom succeed in keeping their apparatus up more than a few hours. The explanation is probably to be found in the greater regularity and permanence of the air currents in the regions of trade winds, and these too are the regions where the soaring birds are most numerously found, probably because they are there sure of a sustaining breeze every day, through the use of which they may evade the fatigue of flapping flight.
The various forms of the Chinese kites are even more numerous than those of the Japanese, and most of the tailless kind are said to depend upon the same principle of flexibility for their equilibrium. It would not at all be surprising to find, should a stable aeroplane be hereafter produced, that it has its prototype in a Chinese kite; but the writer has discovered very little information in print upon the subject; the following article, translated from La Nature by the Scientific American and published in its issue of March 24th 1888, being perhaps the best available:
One of our correspondents in China, Mr. Huchet, at present in Paris, has had the kindness to have made for our purposes, by a skillful Chinese manufacturer, a series of models representing the different types of kites used everywhere in China, Annam, and Tonkin, and which the same gentleman has been obliging enough to bring to us in person.
Fig. 69 represents the simplest form of these kites. Its frame is formed solely of a stiff bamboo stick, A B, and two slightly curved side rods, C D and E F. To this frame is pasted a sheet of paper, which is somewhat loose at the extremities C E and D F where, under the action of the wind, pockets are formed that keep the affair bellied and in an excellent position of equilibrium. Our engraving shows the mode of attaching the strings that serve to hold it. Kites of this kind are usually about 3 ft. in width.
FIG. 69 -- SIMPLEST CHINESE KITE.
Fig. 70 shows the appearance of the musical kite, so called because it is provided with a bamboo resonator, R containing three apertures, one in the center, and one at each extremity. When the kite is flying the air, in rushing into the resonator, produces a somewhat intense and plaintive sound, which can be heard at a great distance. This kite is somewhat like the preceding, but the transverse rods of its frame are connected at the extremities and give the kite the aspect of two birds' wings affixed to a central axis. This kite sometimes reaches large dimensions--say TO ft. in width. There are often three or four resonators placed one above another over the kite, and in this case a very pronounced grave sound is produced. Mr. Huchet informs us that the musical kite is very common in China and Tonkin. Hundreds of them are sometimes seen hovering in the air in the vicinity of Hanoi. This kite is the object of certain superstitious beliefs, and is thought to charm evil spirits away. To this effect it is often, during the prevalence of winds, tied to the roofs of houses, where, during the whole night, it emits plaintive murmurs after the manner of Eolian harps.
FIG. 70 -- CHINESE MUSICAL KITE.
Among ingenious fancies of the Chinese is their bird kite, fig. 71, the frame of which is made elastic. The thin paper attached to the wings moves under the action of the wind and simulates the flapping of the wings. This kite is sometimes 3 ft. in length.
FIG. 71 -- CHINESE BIRD KITE.
The most curious style of Chinese kite is the dragon kite, fig. 72. It consists of a series of small elliptic, very light disks formed of a bamboo frame covered with India paper. These disks are connected by two cords which keep them equidistant. A transverse bamboo rod is fixed in the long axis of the ellipse, and extends a little beyond each disk. To each extremity of this is fixed a sprig of grass which forms a balancing plume on each side. The surface of the foremost disk is slightly convex, and a fantastic face is drawn upon it, having two eyes made of small mirrors. The disks gradually decrease in size from head to tail, and are inclined about 45 in the wind. As a whole, they assume an undulatory form, and give the kite the appearance of a crawling serpent. The rear disk is provided with two little streamers that form the tail of the kite. It requires great skill to raise this device.
FIG. 72 -- CHINESE DRAGON KITE.
This last device resembles in arrangement the multiple disk kites for life saving of the Rev. Mr. Cordner, already described, and suggests that the superposition of kites affords a good field for experiment, There is a limit in size beyond which the increasing leverage will so add to the required strength and weight of the frame as to make a kite unduly heavy as well as unwieldy,33 and superposition naturally suggests itself for experiments intended to test the efficacy and equilibrium of kite aeroplanes. There will be many practical details to work out in devising the best mode of attachment of such aeroplanes with each other, so that all surfaces may pull together and yet counteract the effects of wind gusts, so that experiments with kites seem to offer the readiest, quickest, and least expensive method of working out this part of the problem.
The attention of experimenters is specially called to the form of kite shown in fig. 70. It resembles in shape and attitude those of the soaring birds, which, as already remarked, perform their manoeuvres with peculiarly curved and warped surfaces, and it will be seen hereafter that the nearest success in compassing gliding flight hitherto obtained--that of M. Lilienthal--has been achieved with just such surfaces.
Inventors seem to have bestowed but little attention upon kites, less than a score of such devices having thus far been patented in the United States. These patents chiefly cover various methods of making the frames to fold, so that the kite may be more portable, while but few inventors seem to have considered how the stability may be increased. Among these latter may be mentioned Mr. Clarke (No. 96,550), who proposes the insertion of a spring on one of the three cords which compose the bridle. By the yielding of this spring the angle of incidence of the kite may vary somewhat with the varying velocities of the wind, and thus diminish the perturbations.
Mr. Maddans (No. 121,056) proposes a kite with a convex surface, this being obtained by providing a stick across the top, which stick is sprung into a bow by attaching its ends to each other; but this bowing seems to have been chiefly devised to attach a flapping tongue, rotating on the bowstring, and so making a drumming noise, while there is no doubt that the convexity of the kite must add to its stability.
Mr. Thompson (No. 225,306) patents a reversible convex or concave kite, with a frame like that of an umbrella; but nothing is said of the equilibrium or of dispensing with a tail, the object being, apparently, to provide for convenience in carrying.
Mr. Coldly (No. 354,098)) provides for the stability by inserting in the middle surface of a kite a wind bag rearwardly projecting, which is distended by the breeze and prevents the kite from darting. This is virtually the same device as that of Mr. Copie, already mentioned, which was found to require a central opening to allow the escape of the air when experimented in large dimensions. It is evident that such a device, if applied to a navigable aeroplane, would largely increase the resistance to forward motion; but this might be minimized by making such wind pockets very shallow, and inserting a large number in the aeroplane. The experiment may be worth trying by kite fanciers.
While several forms of folding frames for kites have been patented by inventors, few seem to have been designed to act as parachutes also. This has been accomplished recently by Mr. Moy in a very simple way (British patent No. 1.916, A.D. 1892) by providing the folding frame with a central hub, to which a trapeze bar may be suspended when such a kite is used for conveying passengers or for exploration. By using two lines, the angle of incidence may be controlled, and the kite be made either to raise a weight or to descend slowly to the ground as a parachute.
As already intimated, the writer has found singularly little on record concerning kites, and that little bears but slightly upon the important question of the stability of aeroplanes. It may be for lack of more thorough search that only fragmentary information has been gathered. Kites are supposed to have been invented 400 years before the Christian era by Archytas, a resident of Smyrna (where the flying of kites remains a national sport to this day), and the Asiatics have always been and are now the great kite experts of the world. It is, therefore, not improbable that search in books of travel or inquiries addressed to Orientals might elicit information bearing directly upon the flying machine problem; and it is much to be desired that some competent person shall undertake to write a critical account of kite experiments as well as of the kites of all nations, and of the influence of form as to stability and sustaining power. There is a large collection of Chinese kites in the National Museum at Washington, and it would certainly be interesting to have an account of the various principles exemplified and of the behavior of the various shapes in the air.
des organes de la locomotion aquatique, P. C. Amans.
33The largest kite on record is said to belong to a Japanese gentleman, and is 50 ft. X 45 ft., weighing 1,700 lbs. Its frame is composed of 350 pieces of wood.
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