In 1863 M. de Louvrié, a French engineer and mathematician, proposed the apparatus shown in fig. 43, which he called an "aeroscaphe." It consisted in a kite-like surface, stiffened by light cords to a mast above and to the car below, and capable of acting as a parachute, as well as of being folded like the wings of a bird. It was intended to progress through the air either through the agency of a screw driven by a motor, or more directly by the reaction upon the air of some explosive substance, such as gunpowder. It was submitted to the French Academy of Sciences, but no experiments were made on a practical scale.
Fig. 43. -- DE LOUVRIÈ-- 1863.
This proposal was the result of a mathematical investigation of the action of air upon aeroplanes and under the wings of birds, which was published by M. de Louvrié first in Les Mondes and then in the Aéronaute,17 wherein he showed that Navier and other French mathematicians had grossly overestimated the power required for flight. He also contended that the formulas then in current use for calculating the reactions of air upon oblique surfaces were erroneous, and he advanced the empirical formula of Duchemin, herein before given, as agreeing much more closely with the observed facts. These writings of M. de Louvrié were sharply attacked by other aviators, 18 who had been promoting the imitation of flapping wings, and who denied altogether the possibility of soaring or sailing flight of birds, so that a lively controversy ensued, which has continued to this day, for M. de Louvrié published in 189019 a new formula on a rational basis, of fluid action upon oblique surfaces, which agrees very closely in its results with those of the Duchemin formula, and he has also written an article on the ''Theory of Sailing Flight," which is to appear shortly.
The "aeroscaphe" was practically a kite without string or tail, and its stability would probably have been found deficient, but M. de Louvrié tried in 1866 some experiments with a model weighing some 9 lbs. and exposing a surface of about 11 sq. ft. This ran upon a car on an inclined plane, and the "lift" and the "drift" were carefully measured by means of dynamometers. The results of these experiments demonstrated the fact that at all velocities and for all angles the "drift" or resistance of a plane to motion is to the "lift" or supporting pressure as the sine of the angle of incidence to its cosine; as indeed was to be expected from theoretical considerations, and as had been laid down in 1809 by Sir George Cayley, in the article which has already been herein quoted.
As the sine is to the cosine as the tangent of the angle M. de Louvrié deduced from his experiments the fundamental formula for the resistance of aeroplanes to be
R = W tang. @,
in which R is the resistance or "drift,'' W the weight, and @ the angle of incidence; and calling L the "lift" and P' the normal pressure, at that angle of incidence, we may write further:
R = W tang. @ = P' sin. a
L = W= P' cos. a,
which formulas furnish at once the "drift" and the "lift" when either the normal pressure or the weight is known. In 1868 M. de Louvrie took out a second French patent for an aeroplane, in which he chiefly described the method of stretching the kite sail by adjustable radiating arms. It was to have a flat diedral angle to provide stability sideways, and was to be driven by a hot air engine, but no experiments seem to have been made. Since then he is understood to have abandoned the promotion of aeroplanes, and to expect more favorable results from his "anthropornis," which has already been noticed under the head of "wings and parachutes."
It may be incidentally here mentioned that a somewhat similar proposal for a circular kite or flat parachute was patented in the United States by Mr. Wooton in 1866. It must have been found, if experimented with, quite deficient in stable equilibrium; any square, round, or polygonal surface being quite inferior in stability to the comparatively long and narrow wings which form the sustaining aeroplanes of the birds
While it seems certain, from the shape and arrangement of the apparatus of Carlongford, Du Temple and De Louvrié, that they had in mind the possibility of soaring upon the wind like a bird, they all proposed some kind of an artificial motor, and none of them was bold enough, in the face of existing prejudice, to propose to trust to the wind alone as a motive power. This was reserved for Count D' Esterno, who published in 1864 a very remarkable pamphlet20 upon the evolutions and flight of birds, in which he gave the results of his many years of close observation, and formulated what he considered to be "the seven laws of flapping flight and the eight laws of soaring flight." He held that the act of flight involved the providing for three distinct and indispensable requirements -- i.e., equilibrium, guidance, and impulsion and that the latter could be obtained from the wind whenever it blew strongly enough. He says in his pamphlet:
"Sailing flight labors under the disadvantage that it cannot take place without wind; but, on the other hand, we can derive from the wind, when it blows, an unlimited power, and thus dispense with any artificial motor. In sailing (or soaring flight, a man can handle an apparatus to carry lo tons. just as well as one only carrying his own weight. Whoever has seen large birds of prey sailing upon the wind, knows that without one flap of their wings they direct themselves as they choose, save when they want to go dead with the wind or dead against it, on which occasions they must either tack or sweep in circles."
He patented in 1864 the apparatus shown in fig. 44, consisting of two wings hinged to a frame at the side of a central car, so that they could be set at any diedral angle, or even flapped should a motor be applied. The front end of the frame was provided with trunnions fitting in sockets inserted in the car, so that the rear end of the wings could be raised or lowered, thus altering the angle of incidence. and incidentally moving the wings forward or backward with respect to the car. The wings themselves were to be rigid within the triangles next to the car, and made flexible in the rearward portion, where the curved ribs are shown, which latter might be made of whalebone. It was claimed that these wings would thus be capable of three movements, corresponding to the first three "laws" laid down by DEsterno: 1. An up-and-down or flapping action; 2. A forward or backward inclination, and 3. A torsional or twisting motion. The tail was connected with the car with a universal joint, and had also three motions corresponding to the next three laws--viz.: 4. An up-and-down oscillation; 5. A lateral displacement sideways, and 6. Torsion. The car was provided with a movable seat, and the operator could either sit thereon, and shift his weight forward and back or sideways, or he might stand up and effect the same object by swaying his body or moving about, this action displacing the center of gravity of the apparatus, and corresponding to the seventh law of flapping flight, or means for the maintenance of equilibrium; to which for sailing flight D'Esterno added still an eighth requirement or "law" in affirming the necessity for an initial headway.
FIG. 44. -- D'ESTERNO -- 1864.
He indicates in his book that an apparatus for one man would weigh, with the operator, approximately 330 lbs., and spread an equal number of square feet of horizontal surface, 215 sq. ft. of which would be in the two wings, each being approximately 15 1/2 ft. long by 7 ft. wide, and he describes in his patent mechanism somewhat crude, chiefly consisting of ropes and drums, for producing the various motions described.
The proposed mode of actual operation is not described, but it must have been nearly identical with the evolutions of the sparrow-hawk in the excursion which has been described. The apparatus would first descend in order to obtain an initial velocity, after which, having a speed of its own, it might utilize that of the wind. During this descent the fore-and-aft plane of the wings would have to point below the horizon, and it the reader will refer to the various attitudes of the sparrow-hawk on fig. 36, he will note that between points A and B his wings were scarcely open, in order to diminish the ''drift." Once the speed gained, the apparatus would needs alter its center of gravity in order to change upward the angle of incidence and to come on a nearly level keel. If it went dead with the wind, its relative speed would have to be great enough to furnish support; if going dead against the wind, it would lose headway but gain elevation; and it might tack on a quartering wind or describe a series of helical orbits, like those of the birds. If the latter were chosen, the apparatus would, when it had the wind in its quarter, be sailing into the "eye of the wind" and faster than it blew, just as in the case of the ice boat; while it would probably need to descend a little on the part of the circle when it was going with the wind, and would be enabled to rise materially when, upon facing the wind, the force of the latter was added to its own initial velocity. Thus, at every turn height would be gained, this being acquired when going against the wind; and height once obtained, the apparatus would be able to sail in any direction by descending.
It will be noted, however, how many delicate manoeuvres are requisite to accomplish these evolutions: to alter the angle of incidence, the direction, the speed, and to maintain the equilibrium at all times. The bird does all this by instinct; he performs the exact manoeuver required accurately, at exactly the right time, and he is always master of his apparatus; but the man would be at a terrible disadvantage, his perceptions and his operations would be too slow, and a single false movement might be fatal.
There probably would have been many mishaps at first with Count D'Esterno's apparatus had it been experimented, and being aware of this difficulty, he proposed that the experiments should be conducted over water sufficiently deep to break the fall, the apparatus being raised, like a kite, by a cord fastened ashore, which the operator could hold fast or abandon at will, while he was acquiring the science of the birds.
At that time (1864) aviation was not in public favor, and the very existence of soaring or sailing flight of birds was strenuously denied. It was held that there must be some small movements of the wings, which sustained the bird in the air, and which the observers had failed to detect, and it was not till the subsequent observations of Pénaud, Wenham, Baste, Peal, Darwin, Mouillard and others that it was admitted that a bird might sail by the sole force of the wind without flapping.
Count D'Esterno's proposal was generally laughed at as an evidence of mild lunacy, and whether because of this reason or because he distrusted the efficacy of his own mechanism, he did not build his apparatus. This is the more to be regretted because, being in possession of an ample fortune, he might have tried a number of valuable experiments which, if they did not result in success (as they probably would not), might yet have greatly advanced the fund of knowledge upon this intricate subject. Later on, when aviation grew in favor, and he was urged by members of the French Aeronautical Society, he conferred with various ingenious mechanics, and in 1883 he made an arrangement with M. Jobert to build a soaring apparatus. This was well under way when, in that same year, Count D'Esterno died at the age of 77. The apparatus was never completed, and, such as it is, has been deposited in the Exposition Aéronautique.
A singular proposal was that of M. Claudel, who patented in 1864 the apparatus shown in fig. 45. This consisted of two aeroplanes, one at the front and one hack propelled by two wings, lozenge-shaped, and rotated upon pivots at each apex. If they were made flexible the resistance of the air would bend these wings into an elongated screw, and some propelling effect might be produced. They were to be driven by bevel gears set in motion by a steam engine in the car; but it is not known whether any practical experiments were ever tried.
FIG. 45. -- CLAUDEL -- 1864.
In 1866 Mr. F. H. Wenham patented the meritorious proposal of superposing planes or surfaces above each other, so as to increase the supporting area without increasing the leverage. These were to be "kept in parallel planes by means of cords, or rods, or webs of woven fabric.... The long edges of the surface," made of silk or other light material, to be placed "foremost in the direction of motion." This system of surfaces being arranged above a "suitable structure for containing the motive power." If manual power was employed, the body of the operator was to be placed in a horizontal direction, and "the arms or legs to work a slide or treadle from which the connecting cords convey a reciprocating motion to oars or propellers, which are hinged above the back of the person working them."
In a very able paper, which has become classical, read at the first meeting of the Aeronautical Society of Great Britain, in 1866 Mr. Wenham gave an account of his observations, concluding with a very valuable discussion of the problem of flight, and with the following description of his experiments:
Having remarked how thin a stratum of air is displaced beneath the wings of a bird in rapid flight, it follows that in order to obtain the necessary length of plane for supporting heavy weights, the surfaces may be superposed or placed in parallel rows, with an interval between them. A dozen pelicans may fly, one above the other, without mutual impediment, as if framed together; and it is thus shown how two hundred weight may be supported in a transverse distance of only 10 ft.
In order to test this idea, six bands of stiff paper 3 ft. long and 3 in. wide were stretched at a slight upward angle in a light rectangular frame, with an interval of 3 in. between them, the arrangement resembling an open Venetian blind. When this was held against a breeze, the lifting power was very great; and even by running with it in a calm it required much force to keep it down. The success of this model led to the construction of one of a sufficient size to carry the weight of a man. Fig. 46 represents the arrangement, being an end elevation; a a is a thin plank tapered at the outer ends, and attached at the base to a triangle, b, made of similar plank for the insertion of the body. The boards a a were trussed with thin bands of iron c c, and at the ends were vertical rods d d. Between these were stretched five bands of holland 15 in. broad and 16 ft. long. the total length of the web being 80 ft. (100 sq. ft. Of surface). This was taken out after dark into a wet piece of meadowland one November evening, during a strong breeze, wherein it became quite unmanageable. The wind acting upon the already tightly stretched webs, their united pull caused the central boards to bend considerably, with a twisting, vibratory motion. During a lull, the head and shoulders were inserted in the triangle, with the chest resting on the base board. A sudden gust caught up the experimenter, who was carried some distance from the ground, and the affair, falling over sideways, broke up the right-hand set of webs.
FIG. 46. -- WENHAM -- 1866.
In all new machines we gain experience by repeated failures, which frequently form the stepping-stones to ultimate success. The rude contrivance just described (which was but the work of a few hours) had taught, first, that the webs or aeroplanes must not be distended in a frame, as this must of necessity be strong and heavy to withstand their combined tension; second, that the planes must be made so as either to furl or fold up for the sake of portability.
In order to meet these conditions, the following arrangement was afterward tried: a a, fig. 47, is the main spar, 16 ft. long, 1/2 in. thick at the base, and tapered, both in breadth and thickness, to the end; to this spar was fastened the panels b b, having a base board for the support of the body. Under this, and fastened to the end of the main spar, is a thin steel tie band, e e, with struts starting from the spar. This served as the foundation of the superposed aeroplanes, and, though very light, was found to be exceedingly strong; for when the ends of the spar were placed upon supports, the middle bore the weight of the body without any strain or deflection; and further, by a separation at the base-board, the spars could be folded back with a hinge to half their length. Above this were arranged the aeroplanes, consisting of six webs of thin holland 15 in. broad (giving 120 sq. ft. of supporting surface); these were kept in parallel planes by vertical divisions 2 ft. wide of the same fabric, so that when distended by a current of air, each two feet of web pulled in opposition to its neighbor; and finally, at the ends (which were sewn over laths), a pull due to only 2 ft. had to be counteracted, instead of the strain arising from the entire length, as in the former experiment. The end pull was sustained by vertical rods, sliding through loops on the transverse ones at the ends of the webs, the whole of which could fall flat on the spar till raised and distended by a breeze. The top was stretched by a lath, f, and the system kept vertical by stay-cords taken from a bowsprit carried out in front. All the front edges of the aeroplanes were stiffened by bands of crinoline steel. This series was for the supporting arrangement, being equivalent to a length of wing of 96 ft. Exterior to this two propellers were to be attached, turning on spindles just above the back. They are kept drawn up by a light spring, and pulled down by cords or chains running over pulleys in the panels b b, and fastened to the end of a swiveling cross-yoke sliding on the base-board. By working this cross piece with the feet, motion will be communicated to the propellers, and by giving a longer stroke with one foot than the other, a greater extent of motion will be given to the corresponding propeller, thus enabling the machine to turn, just as oars are worked in a rowing boat. The propellers act on the same principle as the winds of a bird or bat; their ends being made of fabric stretched by elastic ribs, a simple waving motion up and down will give a strong forward impulse. In order to start, the legs are lowered beneath the base-board, and the experimenter must run against the wind.
FIG. 47. -- WENHAM -- 1866.
An experiment recently made with this apparatus developed a cause of failure. The angle required for producing the requisite supporting power was found to be so small that the crinoline steel would not keep the front edges in tension. Some of them were borne downward, and more on one side than the other, by the operation of the wind, and this also produced a strong fluttering motion in the webs, destroying the integrity of their plane surfaces, and fatal to their proper action.
Another arrangement has since been constructed having laths sewn in both edges of the webs which are kept permanently distended by cross stretchers. All these planes are hinged to a vertical central board, so as to fold back when the bottom ties are released, but the system is much heavier than the former one, and no experiments of any consequence have yet been tried with it.
It may be remarked that although a principle is here defined, yet considerable difficulty is experienced in carrying the theory into practice. When the wind approaches to 15 or 20 miles per hour, the lifting power of these arrangements is all that is requisite, and, by additional planes, can be increased to any extent; but the capricious nature of the ground- currents is a perpetual source of trouble.
If Mr. Wenham tried any further experiments with his apparatus, he has not, to the writer's knowledge, published an account of the results. They would be nearly certain to be unsatisfactory for want of stable equilibrium. The Wenham aeroplane was even more unstable than that of the bird and the latter is constantly in need of adjustment to counteract the "ground currents" and the variations in speed and in the angle of incidence. Moreover, the horizontal position selected by Mr. Wenham was most unfavorable because unnatural to man, in directing the movements of an apparatus; so that as often as he might rise upon the wind, just so often he was sure to lose his balance and to come down with more or less violence. The two propellers described by him would of course have proved quite ineffective in sustaining the weight, because man's muscular power is quite insufficient to have worked them with a speed adequate to that purpose, but they might have served to direct the course, had the equilibrium of the apparatus been stable.
Indeed, the writer believes that the first care of the aviator who seeks to solve the problem of flight, must be to seek for some form of apparatus which shall be, if possible, more stable in equilibrium than the bird. The latter is instinct with life; he meets an emergency instantly. Man's apparatus will be inanimate, and should possess automatic stability. Safety is the first requisite--safety in starting, in sailing, and in alighting, and the latter operation must be feasible almost everywhere without special preparation or appliances before the problem can be said to be fairly solved. It will probably prove the most difficult detail to accomplish, but it does not seem impossible when we see the feat performed by the birds so many times every day.
Mr. Wenham's proposal to superpose planes to each other in order to obtain large supporting surfaces without increasing the leverage, and consequent weight of frame, will probably be found hereafter to be of great value. The French experimenter Thibaut found that when two equal surfaces were placed one behind the other, in the direction of fluid motion, the resistance more nearly equaled that of the two separate surfaces than might be supposed. Thus for two square planes, placed at a distance apart equal to their parallel sides so as to cover each other exactly, M. Thibaut found the resistance equal to 1.7 times that of one single surface. When the hinder plane projected by 0.4 of its surface beyond the front plane, the resistance was 1.95 times that of the single surface. This diminished to 1.84 times, when it became 0.9. Beyond this it reached nearly twice the resistance. 21
Professor Langley found in his experiments with superposed planes, 15 X 4 in., soaring at horizontal speed, that "when the double pairs of planes are placed 4 in. apart or more, they do not interfere with each other, and the sustaining power is, therefore, sensibly double that of the single pair of planes; but when placed 2 in. apart, there is a very perceptible diminution of sustaining power shown in the higher velocity required for support and in the greater rapidity of fall."22
We may hence conclude that there will result a material, indeed a great advantage in superposing planes, provided they are so spaced as not materially to interfere with each other, and provided also that they are arranged so as to afford a good equilibrium.
September, October and November, 1868
18L'Aéronaute, December 1868; January and February, 1869.
19Revue de l'Aeronautique, Vol. 3, page 102.
20 "Du Vol des Oiseaux," D'Esterno, 1864. 21 Derval, "Navigation aerienne," 1889 p. 185.
22Langley, " Experiments in Aerodynamics," p. 40.
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