Realizing the utter insufficiency of man power, or of any known primary motor, some inventors have designed flying screws to be worked by new-fangled motors. Of these was the apparatus of Pomés & de la Pauze, proposed in 1871, and shown in fig. 30. The sustaining screw was inclined so as to obtain an oblique ascent, and appears to have been adjustable. The steering was to be done by a rudder, and the whole was to be worked by a gunpowder motor. The first requisite, therefore, was to perfect the gunpowder engine. It is not known how much was accomplished toward this; but the flying apparatus was never built.
FIG. 30 -- POMÈS & DE LA PAUZE -- 1871.
The next year (1872) M. Renoir, a member of the French Society, proposed an apparatus consisting of two aerial screws placed side by side in the same horizontal plane, but with shafts capable of being moved out of the vertical, in order to secure movement in both directions. They were to be driven by steam, and to rotate in opposite directions; and M. Renoir computed that the axis of rotation would have to be inclined 11° in order to obtain a horizontal course. Also, that to produce satisfactory forward speed, the additional power required would be but 10 per cent. of that required for sustaining the weight. Aside from the main question of the motor, which was left in abeyance, the important thing to ascertain was the best form of sustaining screw, in order to get the utmost support with the least expenditure of power; so the succeeding year, M. Renoir, having studied the results obtained by M. Pillet with a concave screw7 in a series of experiments beginning in 1848, tried some experiments of his own with a screw provided with a return flange or turned edge, to prevent the centritugal escape of the air, of which he gave an account in the Aéronaut for April, 1873.
He drove his screw by man power, and claimed that the results showed that a force of one horse power could sustain, by means of his screw, a weight of 165 lbs.; but Mr. Bennet, in giving an account of these experiments to the Aeronautical Society of Great Britain, in 1874, gave a somewhat different account, and said:
Two years ago M. Renoir, a member of the French Society, experimented with a screw 15 ft. in diameter, with which, by the action of his feet, he was able to lift a weight of 26 lbs. The screw was two bladed, with an increasing pitch, the angle of inclination being 3° at the front edge of the blade and increasing to 30° at the back edge. The two blades cover the entire area of the screw, and have a deep rim suspended from them to prevent the air being driven from the circumference by centrifugal force. M. Renoir estimated the power he developed was about one fifth of a horse power; but this was considered by the members of the French Society present at the experiment to be considerably below the real power exerted. As the screw was driven by the feet, after the manner of a velocipede, the body being in a good position for exerting its maximum effort, the power developed was undoubtedly nearly one horse power. A man running up a pair of stairs is able for a few seconds to exert two horse power, and mounting a ladder placed vertically, by the help of his hands, an ordinary man can do the work of 1 1/4 horse power. These facts have been determined by experiment.
While on the subject of the form of screws, it may be well to call the attention of those who may desire to study the subject further to an article upon "Propulsors," by M. Crocé Spinelli (the same gentleman who lost his life in the scientific balloon ascension of the Zenith), which will be found in the Aéronaut for April, 1870, and to another by the same author on "A Screw with Variable Pitch" in the Aéronaut for November, 1871. Also to the remarks on Screws by Mr. Wenham in the first and second reports of the Aeronautical Society of Great Britain, and to those of Mr. Thomas Moy, in the fourth report of the same society. He evidently knew what he was talking about.
In 1872 Mr. Wenham proposed a method for varying the pitch of the screw, which may be found in the report of the British Aeronautical Society of that year. The blades were to be made of some fabric, one edge being attached to a cross arm, which was made fast to the shaft of the screw. The other edge of the fabric was fastened to another cross arm, so arranged as to be placed in any position on the shaft, and firmly fixed in such position. A coiled spring was to keep the two cross arms apart, and thus maintain the fabric tightly stretched. If the adjustable arm be placed precisely in line with the fixed arm then the blade is parallel with the shaft, and by moving the adjustable arm to one side more or less, the pitch can be made anything desired.
The next experiments on screws were tried in 1877, by M. Dieuaide, formerly Secretary of the French Aeronautical Society, and the well-known Engineer and Patent Attorney, whose clever chart has furnished (by permission) almost all the illustrations contained in these articles. His apparatus is shown in fig. 31. It consisted of two pairs of square vanes set at various angles to the line of motion, so as to vary the pitch, and rotated in contrary directions by gearing. The power was furnished by a double cylinder steam-engine connected with the boiler by a flexible hose, and the lifting power of the screws could be accurately weighed by simply putting the apparatus on a scale.
FIG. 31. -- DIEUAIDE -- 1877.
The results of the experiments seemed to show "that this double screw could not, in consequence of the losses of power due to the gearing, exert a lifting force greater than that of 26.4 lbs. per horse power." This agrees closely with the results of the experiments of Giffard with a single screw he having found that 6 horse power would lift with a screw I65 lbs. at the rate of 3.28 ft. per second, or say 27.5 lbs. per horse power, from which he deduced the conclusion that the aerial screw gave out but 18 per cent. of the power exerted to drive it.
The next apparatus to be noticed was not experimented with, so far as the writer has ascertained, but was a proposal of great oddity and originality patented in 1877 by M. Mélikoff. Engineer and graduate of the school of the "Ponts-et- Chaussées." It is shown in fig. 32, and consisted in a sort of screw parachute composed of "two hyperbolic paraboloids united by their concavities into a sort of cone or pyramid with a rectangular base in projection." This was to be furnished with a series of zones, shown in section in the figure, to act upon the air and this arrangement, the one resembling a spear-head in the figure, was expected to screw itself up into the air and to act as a parachute in coming down. It was to be rotated by a gas turbine, consisting of eight curved chambers, into each of which charges of the vapor of ether mixed with air were to be successively exploded by an electric spark, and the charges allowed to expand in doing work. The surfaces were to be kept cool by melting ice and by heating the resulting water. This ice and the supply of ether were to be carried in the recipient shown just below the parachute, the turbine being shown lower down; this motor was expected to work also an ordinary screw with three arms, geared on a short axle, from which screw horizontal propulsion was expected. Below all is shown the car for the operator.
FIG. 32 -- MÉLIKOFF -- 1877.
M. Mélikoff designed his apparatus to carry up one man, and estimated its total weight at 374 lbs. Of this the apparatus proper was to absorb 1O8 lbs., the gas turbine was to weigh 92 lbs., its supplies for one hour were to amount to 40 lbs., and the operator was to be of 134 lbs. weight. The rotating surface was to measure 87 sq. ft. in area, thus giving a proportion of 4.3 lbs. to the sq. ft., which seems entirely too small, although claimed to be calculated from the tables of air pressures given by Thibault. The turbine was to be of 4 horse power, being thus estimated to weigh 23 lbs. per horse power, and it was to consume per horse power per hour 3.3 lbs. of ether and 8.7 lbs. of ice for cooling the parts, thus showing a slight discrepancy from the aggregate of 40 lbs. of supplies estimated as required for one hour.
The apparatus as a whole is scarcely worth experimenting with, and has been chiefly described because of its oddity; but the weight and power of the projected gas turbine seem to have been worked out with some care, and it might be worth while to take the subject up again, in order to ascertain whether it is practicable to construct a rotary gas motor weighing as little as 23 lbs. to the horse power.
The next experiment to be noticed was tried by M. Castel, a mechanical engineer, in 1878. He wanted to determine the amount of mechanical work required to sustain a motor in the air, and built the apparatus shown in fig. 33. It consisted of eight double screws rotated in opposite directions by a double-cylinder compressed-air engine, mounted upon wheels and fed with compressed air through a long, very light rubber hose. The weight of the whole apparatus was 49 lbs., of which 22 lbs. was in the screws and their machinery. The screws were 3 93 ft. in diameter, and weighed 132 lbs. each.
FIG. 33 -- CASTEL -- 1878.
Experiments were repeatedly tried, but they came to an early ending by the apparatus rising upon the air, taking a sheer, and smashing itself against the wall of the room. M. Castel did not publish the results accomplished in the way of lifting a measured number of pounds per horse power developed; but he stated that he "no longer had the confidence which he once possessed in screws as future instruments of aviation. Elastic surfaces with an alternating action to impart vibratory motion to the air now seem preferable to screws to solve the problem of aerial navigation with an apparatus heavier than the air." He estimated from an examination of the muscles of birds and of the amount of work which those muscles were able to give out, that the bird in full flight expended not more than 24 foot-pounds per minute for each pound of his weight, so that a bird, if he weighed 220 lbs., would only expend a maximum of 0.16 horse power.
Now, we have already seen that the average power of a man is 0.13 horse power, and that although he weighs less than 220 lbs., he cannot fly with wings by his muscular efforts, so that the estimate must be erroneous.
M. Castel proposed to build a petroleum motor to drive his proposed wing apparatus, but he probably found himself unable to keep within the necessary limits of weight.
A simpler apparatus than M. Castel's accomplished much better results, for in the same year (1878) Professor Forlanini, an Italian civil engineer, launched into the air the second steam apparatus which has flown with its contained supply of steam; the first having been that of Mr. Phillips, already described. Fig. 34 shows the flying screw arrangement experimented with by M. Forlanini.
FIG. 34 -- FORLANINI-- 1878.
It is composed of two double-bladed screws, of which the lower one is rigidly fixed to the steam-engine, while the upper one rotates; the result being that the lower screw furnishes a fulcrum upon the air, while the upper one furnishes the ascending power. The whole apparatus thus slowly rotates upon its own axis; but this feature, which would be very objectionable in a really navigable apparatus, could be eliminated by rotating both screws in inverse directions.
The upper screw was worked by a double cylinder steam engine of 1/4 horse power, supplied with steam from super heated water contained in a depending hollow globe after the manner of the well-known fireless locomotive, the initial pressure being some 120 to 160 lbs. per sq. in. It was the original design of M. Forlanini to send up his apparatus with a steam boiler attached, fired by 200 minute alcohol flames; but this proved too heavy to be lifted by the machine, and he substituted the hollow globe, tested to an internal pressure of 225 lbs. per sq. in., which, being two-thirds filled with water, is simply laid upon a fire until the desired pressure is obtained; when, on being withdrawn, the throttle-valve which admits steam to the cylinders is opened, and the apparatus rises.
It has been repeatedly tested, and its best performance seems to have been to rise to a height of 42 ft. and to remain 20 seconds in the air. M. Forlanini expressed the intention of following it up with an improved apparatus, of which he had the design, and with an engine of 2 horse power; but it is stated that he has not had the leisure to carry out this intention.
The total weight of the original apparatus was 7.7 lbs., and the aggregate area of the screws was 21.5 sq. ft., thus giving a bearing surface of about 2.8 sq. It. per pound. The weight of the steam-engine proper was 3.52 lbs. and that of the screws 1.32 lbs. The hollow globe, charged with water, weighed 2.20 lbs., and the steam-gauge and connections weighed 0.44 lbs. more, leaving 0.22 lbs. for other accessories. It will be noticed that the engine, the boiler and the gauge weigh about 80 per cent. of the whole, which proportions could not be expected to obtain in a navigable apparatus; but, on the other hand, a larger steam-engine and boiler would weigh less in proportion to its power than the minute one thus experimented with, in which steam was very wastefully used in consequence of the relatively very large proportion of radiating surfaces.
M. Forlanini designed. a self-generating steam boiler, which he expected to weigh but 13.2 lbs. per horse power; but it is not known to have been constructed.
This, then, is the best that has hitherto been done with steam. A model screw machine weighing 7.7 lbs. has risen 42 ft. into the air and flown for 20 seconds, but without taking up a self-generating steam boiler. The power developed ranged from 7,800 to 10,850 foot pounds per minute, and the total weight sustained was at the rate of 26.4 lbs. per horse power.
Some time about the year 1880 Mr. Edison -- the great Edison-at the instance of Mr. James Gordon Bennett, made some preliminary experiments to promote aerial navigation. He began very judiciously by trying to ascertain what could be done with the aerial screw as a propeller. For this purpose he is reported to have placed an electric motor of l0 (?) horse power, connected with a vertical shaft surmounted with rotating vanes upon a platform scale, and to have connected it by a wire with a source of electric power-the object being to ascertain how much the whole could be lightened by the action of the vanes upon the air.
He rigged upon the shaft first one kind of propeller, and then another, until he had tried all that he could think of; the best being a two-winged fan with long arms.
He is reported as saying that the best results obtained were to lighten the apparatus some four or five pounds of its total weight of 160 lbs., but the amount of power developed is not stated. This must have been quite small, and Mr. Edison must have been unfortunate in his selection of the screws to he tried, for we have seen, by the experiments of others, that a motor of 1O (if it was really this) horse power ought to lift 260 lbs. It is no wonder that he is reported as saying that "the thing never will be practicable until an engine of 50 horse power can be devised to weigh about 40 lbs."
It is understood that somewhat similar experiments were tried by Mr. Dudgeon:, the celebrated maker of hydraulic jacks. He tested the lifting effect of various forms of screws when rotated by steam power, and, like Mr. Edison, he stopped in disgust when he found how small was the lift in proportion to the power expended.
There may have been other experiments with lifting aerial screws in the United States, but they have not come to the knowledge of the writer. In point of fact, such aerial devices do not seem to have received much attention from inventors, and there have been but few patented proposals therefore in the United States.
In 1876 a patent was taken by Mr. Ward, of San Francisco, for an aerial vessel in which the supporting and the propelling power was to be furnished by a series of fan blowers. The fans furnishing the support were placed on horizontal shafts and the exhaust opened downward, so that the reaction would act against the force of gravity, while the fans which produced the horizontal motion were also arranged on horizontal shafts at the rear, the air being conducted to them through a duct from the front, and exhaust being to the rear, so that the reaction would force the vessel forward.
In 1877 Mr. Ward took out further patents, in which the apparatus was somewhat modified, but the general principles remained the same. It is believed that he tried some experiments; but no record of them has been met with by the writer, and a letter to the inventor has remained unanswered.
The same idea, but in a modified form, has quite lately (1892) been patented by Mr. Walker, of Texas; and perhaps experiments will be tried to test the lifting effect of air blasts under favorable circumstances, but as the efficiency of a screw, when used as a fan, is stated at only 35 per cent., while its efficiency as a propeller is stated at 70 per cent., it seems a question whether air blasts can be advantageously used in aerial navigation.
It may be pointed out here that there is a considerable difference between the fan blower and the screw propeller -a difference which should be more thoroughly understood by inventors. The most efficient fan blower is a machine which will produce the strongest current of air with any given expenditure of power . The best screw propeller is the machine which will produce the least current. If a screw propeller could be so arranged that it would not put the air in motion at all, then there would be no "slip," and the machine would be as efficient as a locomotive running on a dry rail, in which case all the power is expended upon the vehicle. In the case of a fan blower, or in the case of a steamboat moored to the wharf, and with its engines in operation all of the power is expended in moving the fluid. It is ail wasted in slip. In the case of the steamboat advancing through the surrounding fluid, or of the aerial machine, if it ever gets under way, a part of the power is expended in putting the craft in motion and another part in putting the fluid in motion, and the latter power is inefficient it is the "slip '' The best screw therefore, is the one which shall expend the greatest part of the applied force upon the craft and the least upon the fluid. It is the screw which will create as little movement as possible in the fluid in which it operates.
In 1879 Mr. Quinby patented a device consisting of two sets of screw-like sails, one set to raise the machine and the other to propel it. The drawing shows a light framework with two screws, each with two blades of fabric, one set on a vertical mast, and the other upon an inclined mast. The screws were to be driven through rope gearing by some source of power.
In the same year Mr. Greenough also patented an apparatus, which should better, perhaps, be noticed under the head of aeroplanes, but which differed from this type by having lifting screws imbedded in the surface of the aeroplane, in order to obtain a lifting action upon first getting under way, after which, by sailing at an angle, both sustaining and propelling effect could be obtained from the screws, with, however, the possible addition of a vertical screw to give increased forward motion. This inventor is understood to have tried some preliminary experiments of details, and as a result thereof to be awaiting the development of a light motor before undertaking to realize his conception upon a navigable scale.
In 1885 Mr. Foster patented an air ship consisting of two screws, four-bladed, side by side, on separate vertical shafts, which latter can be thrown at an angle by reason of a flexible portion connecting with the main driving shaft, so that the thrust may both lift and propel the apparatus. The main shaft was to be driven by the feet of an operator sitting below and half way between the two screws. These screws are apparently some 8 ft. in diameter, and the man power relied upon is evidently inadequate, so that it is quite safe to say that if the apparatus was ever tried it did not succeed in rising.
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