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In describing the various proposals and experiments which have been made to compass artificial flight by means of rotating screws, the latter will chiefly be considered as instruments from which to obtain support of a given weight in the air. There is no question that they can serve as propellers if the support be otherwise obtained, nor that if a screw can lift and sustain its own prime motor, it can also be made to progress horizontally, either by inclining it at the proper angle or by adding a vertical screw.

It was to be expected that when inventors found how difficult it is to obtain a lifting effect from flapping wings, they should turn to aerial screws to sustain them in the air. Man has succeeded in out-traveling both land and marine animals by substituting rotary motion for the reciprocating action of their limbs: the locomotive far outstrips the horse, and the paddle-wheel and screw have, for large vessels, superseded the oar, so that it seems natural to expect that some rotating device shall be found the preferable propeller, should aerial navigation ever be accomplished.

It will be seen, from the accounts which follow, that the chief obstacle has hitherto been the lack of a sufficiently light motor in proportion to its energy; but there has recently been such marked advance in this respect, that a partial success with screws is even now almost in sight.

Curiously enough, the Aerial Screw considerably antedates the marine screw, although, unlike the latter, it has not been brought into practical use. We have already seen that Leonardo Da Vinci experimented with paper screws, which mounted into the air, as early as A.D. 1500, and we may add that a sketch has been found in his note books for a proposed aerial screw machine 96 ft. in diameter to be built of iron and bamboo framework, covered with linen cloth thoroughly starched. He probably abandoned all idea of constructing it when his experiments with models showed the power that would be required.

A similar proposal was made by Paucton, a learned mathematician, in 1768. when, in a treatise upon the Archimedean screw, he described an apparatus which he called a "Pterophore," consisting of two aerial screws, one to sustain and the other to propel, attached to a light chair. A man seated in the chair was expected to rotate these screws by means of gearing, and so raise himself through the air.

The first practical experiment known, however, is that of M. Launoy, a naturalist, and M. Bienvenu, a mechanician, who jointly exhibited before the French Academy of Sciences in 1784 the little apparatus shown in fig. 25. It consisted of two superposed screws, about one foot in diameter, each composed of four feathers inserted in sockets at the ends of a rotating axle. This axle was put into motion by the unwinding of a cord fastened to the two extremities of a bow and the report to the French Academy (May 1, 1784) says:

"The working of this machine is very simple. When the bow has been bent by winding the cord, and the axle placed in the desired direction of hight-say vertically, for instance- the machine is released. The unbending bow rotates rapidly, the upper wings one way and the lowerwings the other way, these wings being arranged so that the horizontal percussions of the air neutralize each other, and the vertical percussions combine to raise the machine. It therefore rises and falls back afterward from its own weight."

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FIG. 25. -- LAUNOY & BIENVENU-- 1784

Launoy & Bienvenu proposed also to build a large machine, and to go up in it themselves. It is not stated whether this was ever attempted but probably not, as a brief investigation must have satisfied them that they had no adequate primary motive power at hand to lift even its own weight in that way, and that with a secondary or stored power tile machine would fly but for a few seconds. Practically the same device was constructed by Sir George Cayley in 1795, and described by him in Nicholson's Journal for April, 1810; but whether he reinvented it or borrowed the idea from Launoy & Bienvenu is not stated. He mentions it merely as a toy, and his writings seem to indicate that he expected success to be achieved instead with an aeroplane to be driven by some sort of propelling apparatus, if only a sufficiently light first mover could be contrived.

Subsequently, Deghen, in 1816, Sarti, in 1823, and Dubochet, in 1334, all proposed and constructed models for flying machines on the vertical screw principle; but they did not discover the necessary light motor to transform their models into practical machines.

In 1842 Mr. Phillips, the inventor of the "Fire Annihilator," succeeded in raising into the air an apparatus weighing in the aggregate 2 lbs., by means of revolving fans inclined about 20° from the horizontal. The motive power was evolved by the combustion of charcoal, fire and gypsum making steam, as in the original fire annihilator and the engine consisted of rotating arms discharging steam direct into the atmosphere, and thus working by reaction, being the device known as the discovery of Hero, of Alexandria. Mr. Phillips exhibited a working model of his aerial machine at the Aeronautical Exhibition in London, in 1868; and in describing his experiment of 1842 he said:

"All being arranged, the steam was up in a few seconds, when the whole apparatus spun around like a top and mounted into the air faster than any bird; to what height it ascended I had no means of ascertaining. The distance traveled was across two fields, where, after a long search, I found the machine minus the wings, which had been torn off from contact with the ground." This is undoubtedly the first machine which has risen into the air by steam power; but the necessarily small capacity of the generator, and the wasteful though simple method of using the steam, limited its flight to a very few minutes, and removed it from the possibility of application on a practical scale.

In 1843 Mr. Bourne, the well-known English engineer, constructed some models of aerial screws, consisting of large fowl's feathers inserted in a cork, stuck on the top of a pine stick, to which a watch spring was attached, succeeded in making them rise by the force of the coiled spring to the height of some 20 ft.; but he recognized that the difficulty in the way of building a really navigable machine was to obtain "the right motive power."

This must also have been the conclusion of Mr. Cossus, who proposed, in 1845, the apparatus shown in fig. 26, which consists in three rotating aerial screws to be moved by steam power. The design is by no means devoid of merit, for by hinging the outer and smaller screws, and varying their angle with respect to the machine, the latter can be made to travel in any direction desired, while sustained by the rotation of the middle screw. It cannot be learned that Cossus tried any practical experiments, for a simple inquiry into the weights and relative energy of the steam engines of his day and an investigation as to the power required to sustain his apparatus must have speedily convinced him that it had better be abandoned.

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FIG. 26. -- COSSUS-- 1845.

Analogous proposals were made in 1851 by Mr. Aubaud, who combined several screws with an aeroplane; and by Le Bris, who designed a car surmounted by two screws turning in opposite directions, in order to overcome the tendency of the apparatus to rotate on its own axis, as the consequence of the horizontal component of the thrust of a single screw.

It was to overcome this same objection that, in 1859, Mr. Bright designed and patented the apparatus shown in fig. 27, the axles of the screws consisting of tubes, rotating in opposite directions, one inside of the other. Mr. Bright seems to have planned the machine to be suspended beneath a balloon, and to be worked by man power, in order to alter or to maintain the altitude at will, and thus save the expenditure of ballast in rising or of gas in descending. Its beneficial effects, however, seem to have proved so small- solely, it may be said, from the inadequacy of the motive power employed-that it has not come into practical use.

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FIG. 27. -- BRIGHT-- 1859.

These various efforts were somewhat desultory, and not followed up by anything like scientific experiments; but in 1863 there was in Paris a great "boom" in projects for navigating the air by means of aerial screws, and the French espoused its promotion with great enthusiasm. M. de Ponton d' Amécourt and M. de la Landelle had already studied the action of the screw upon the air, which, in July, 1863,, M. Nadar, a prominent photographer, invited to his reception rooms the élite of the press, of science, and of artists, and treated them to a first reading of his famous "Manifesto upon Aerial Automotion," which appeared the next day in the press, and was republished and commented upon throughout the whole of Europe.

In this manifesto, written with much eloquence, Nadar expressed the opinion that the principal obstruction in the way of navigating the air was the attention which had been given to balloons; that, in order to imitate nature, a flying machine must be made heavier than the air. Also that the surest means of success was the employment of the aerial screw -- "the sainted screw," as an illustrious mathematician called it, which was known to be capable of carrying up a mouse, and must, therefore, ˆ fortiori, be able to sustain an elephant.

The inanity of this argument was not apparent at the time; and Nadar proceeded to form a syndicate to promote "aviation" after the methods of opera bouffé. A journal was founded--the first Aéronaute--43 paying subscribers were obtained, and 100,000 copies of the first issue printed. This journal expired after its fifth issue. Then a monster balloon was built-the Géant-out of the exhibition of which it was expected to realize sufficient profits to build a screw machine which should put an end to ballooning forever. But the Géant met with all sorts of mishaps; it gave no profits, and entailed losses instead, which nearly ruined Nadar; and such experiments as were tried with aerial screws (outside of the little toys which were exhibited at the various meetings) demonstrated that the utmost weight which the exertion of one horse power could sustain, with a screw acting upon the air, was some 33 lbs., or, in other words, that if the apparatus were to weigh one ton, it would need 67 horse power continuously exerted to keep it afloat.

This is now clear enough to us. Assuming that in consequence of the rotation at high speed a smaller surface is required to sustain a given weight with a screw than with reciprocating wings or fixed aeroplanes, yet the motor for the screw would probably weigh about one-third of the whole weight of the apparatus (instead of one-quarter, as in the case of birds, and probably one-sixth in the case of aeroplanes), and so the utmost weight available for the motor of the screw and its supplies would be 1/3 of 33 lbs., or 11 lbs. to the horse power, while in 1863 there was no primary motor known then approximating such phenomenal lightness.

Now that Mr. Maxim has announced that he has built a steam engine, and its generator of 950 lbs. aggregate weight developing 120 actual horse power, or at the rate of 8 lbs. to the horse power, it is doubtless within his power to go up into the air with an aerial screw, and to perform therein various evolutions; but his trips would probably be short, and the consequences might be unpleasant were the machinery to break down while he is aloft.

He has, accordingly, with great good judgment, begun by applying his steam engine to an aeroplane, although this will involve greater difficulties in starting and in landing, as well as a less immediate demonstration.

Almost the only memento which now remains of the movement in favor of the aerial screw inaugurated by Nadar is the model of the flying machine designed in the Viscount de Ponton d' Amércourt, and which is shown in fig. 28. The following description is translated from that of M. Tissandier:

"M. de Ponton Amecourt constructed, in 1865, an aerial screw machine worked by steam, which was expected to rise with both its motor and its steam generator. This beautiful little model, which was exhibited at the London Aeronautical Exposition in 1868, is exquisitely finished. The boiler and frames are of aluminum, and the steam cylinders are of bronze. The reciprocating movement of the pistons is transmitted by gearing to a double pair of superposed screws of 41 sq. in. surface, one rotating in a different direction from the other. The apparatus, which is now in the collection of the French Society for Areial Navigation, weighs, without water or fuel, 6.1 lbs. The boiler is 3 1/4 in. high and 4 in. in diameter; the total height is 24 1/2 in. Unfortunately the boiler cannot be worked at sufficient pressure; when the machine is put into motion it possesses a certain ascensional force; it loses weight, but it does not rise."

 

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FIG. 28. -- D'AMÉCOURT -- 1863.

The illustrated papers also published about 1865 views of a great steam flying machine, attributed to M. de la Landelle. These showed a hull flanked with aeroplanes, and surmounted with two masts, each carrying four sets of screws, and also a partly folded umbrella, presumably to open into a parachute. It is to be found reproduced in most works upon aerial navigation, and in encyclopeadia articles, and is not given here, because it possesses no merit whatever, being probably a newspaper fancy, like the flying ship attributed to Mr. Edison, which went the rounds of the press some years ago, and which is also reproduced in M. Dieuaide's chart.

M. de la Landelle was a persevering man, as well as one of considerable scientific acquirements. He continued making experiments upon screws of various shapes long after M. Nadar and Ponton d' Amércourt had given them up in disgust; and he encouraged M. Pénaud, then a rising young man, to take up the study of Aviation. The latter produced in 1870 the little apparatus shown in fig. 29, which has remained the best of its kind.

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FIG. 29. -- PÉNAUD -- 1870.

Pénaud's flying screw, which is called by the French a "Helicoptère," consists of two superposed screws rotating in opposite directions, and actuated by the force of twisted rubber strings. The principle is the same as the apparatus of Launoy & Bienvenu and of Sir George Cayley, but the twisted rubber is far more effective than the bow, whether the latter be of whalebone or steel, and this change in the motor constituted the chief merit of Pénaud's modification. He first experimented with rubber in tension, but found that the increased weight of the frame (to resist the strains) more than compensated for the weaker effects of torsion, and that the latter application enabled him to construct models which were simple, cheap, efficacious, and not easily broken. These models, when built in varying proportions, would either rise like a dart to a height of some 50 ft., and then fall down, or sail obliquely in great circles, or, after rising some 20 or 25 ft., hover in the same spot for 15 or 20 seconds, and sometimes as many as 26 seconds, which was a much longer flight than had ever before been obtained with screws.

For lack of a sufficiently light primary motor, Pénaud's further experiments in this direction brought forth no practical results, and his apparatus has remained a toy, which has been varied in many ways.

The most popular of such toys have been the various single spinning screws, either of cardboard or metal, which are attached to a spindle around which a string is wound, and which are set in rapid rotation by briskly pulling and unwinding the cord These are of many shapes, with two, three, or more arms, and of various angles of pitch. Those with heavy rims are most effective, sometimes rising as much as 200 ft. into the air; but they have led to accidents and proved dangerous. In such devices the source of power is not taken up into the air, as in Pénaud's apparatus, but it is stored in the momentum of the screw and encircling ring (if any) by the original muscular effort. Mr. Wenham measured the force expended in unwinding the coiled string by attaching thereto a small spring steelyard, and noting the time of ascent of a flying screw of tin plate with three equidistant vanes. He computed that to maintain the flight of the instrument, weighing 396 grains, a constant force is required of near 60 foot-pounds per minute, or in the ratio of about 3 horse power for every 100 pounds.

Many modifications have also been made of the double screw arrangement, which takes up its secondary motor in the shape of twisted rubber. These have been produced by many people; but the cleverest are probably those of M. Dandrieux, who, in November, 1879, presented before the French Aeronautical Society⁵ no less than 10 different types, the best known of which is that of the butterfly, which is still to be found in the toy shops, and which comes to us both from Paris and from Japan. M. Dandrieux modified the shape and proportions of the screw, and effected a material improvement in its efficiency.

Flying screws driven by clock springs have been frequently made. Such an arrangement was constructed by Sir George Cayley, "the flying baronet," at the beginning of the century, and is described in his paper on "Aerial Navigation," in Vol. XV of Nicholson's Journal. Sometimes the attempt has been made to substitute man power. Of such was the experiment of Mr. Mayer, a stair-builder, who, about 1828, constructed an aerial screw proportioned to sustain 126 lbs., and rotated it with his own muscular power. In giving an account of the result, forty years later, he said, naively: ⁶

"The result was very flattering, though not perfectly successful. My pecuniary resources were exhausted, and other work in my own business being then offered to me, ascending by wings (screws) was abandoned until a more convenient season, and the more certain and substantial method of making stairs, and ascending them step by step, was substituted in its place."

⁵ L'Aéronaute, January 1880.
⁶ Third Annual Report Aeronautical Society of Great Britain.

 

SCREWS TO LIFT AND PROPEL

BY OCTAVE CHANUTE

 

Part II

April 1892.

 

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.

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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.

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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.

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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.

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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.

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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.

⁷ Aéronaute, March, 1870.

 

SCREWS TO LIFT AND PROPEL

BY OCTAVE CHANUTE

Part III

May 1892.

 

In 1886 and 1887 some experiments were tried at the Royal Dock Yards in Copenhagen, for the purpose of determining the relative efficiencies of screws operating in water, and those which should operate in the air. The experiments were in connection with marine, and not with aerial navigation; but it was found that not only would the aerial propeller develop as great a thrust as the water propeller, in proportion to the energy consumed, but that under certain conditions it would do slightly more, and greater thrusts per horse power were attained than in any previous experiments.

These very important results, for which most of our readers will be unprepared, warrant noticing the experiments at some length. They were described in a paper by H. C. Vogt, read before the British Association in 1888, and the first seems to have consisted in the careful measurement by Mr. Freninges, of Copenhagen, of the thrust and work done by a largish flying screw, two-bladed, 1 ft. in diameter and 1 ft. pitch, weighing 0.35 lbs. With 70 revolutions per second, it will rise 200 ft. into the air, and Mr. Freninges determined the efficacy or work done to be 63 per cent. of the kinetic energy imparted by the arm of the operator. At 52 revolutions per second, requiring the expenditure of 100 foot-pounds, the thrust of the screw against a stop was 6 lbs., and its efficiency therefore was (6 * 550)/100 = 33 lbs. per horse power, which agrees well with the measurements of Mr. Wenham and others.

The first dock-yard experiments were undertaken by Messrs. Dahlstrom & Lohman, and consisted in fitting out a launch 20 ft. long and 5 1/2 ft. beam with an aerial screw propeller of canvas 8 1/2 ft. in diameter, having 24 sq. ft. of area distributed over two ordinary canvas sails, the pitch of which could be varied. The engine was 1 1/2 horse power. Measured by a spring balance, the thrust of the air propeller was, in calm weather, 36.7 lbs. per indicated horse power, or the same as that of a water-screw turned by the same power. In windy weather this thrust was augmented through 75 per cent of the directions in which the wind could blow, thus illustrating the fact that if a current of air be blowing across the blades the efficiency of a propeller will be increased, because many more particles of air will be acted upon in the same space of time than in a calm. This fact promises important consequences for an aerial screw in propelling, should a true flying machine ever be compassed, for then the advancing screw would constantly have fresh particles of air to work upon, and there would be a reduction in the slip which necessarily must occur when its thrust is measured in a fixed position.

The next experiment was tried with the Government Dock Yard launch, which was 31 ft. long and 8 ft. beam. Its ordinary water screw was removed, and an air propeller of canvas was substituted, which was 20 ft. in diameter and had a total area of 250 sq. ft. This area was found much too large, but by reducing it to about 150 sq. ft. an average speed of nearly 7 knots was attained by the launch, whose speed with the ordinary water screw and the same power 11.3 indicated horse power) was a maximum of 7.3 knots per hour. There was, however, a slip of the driving rope which was estimated as wasting about 2 horse power. and the director estimated that the speed with the air propeller would have been 7.5 knots per hour if the gear had worked properly. As on previous trials, 75 per cent. of the winds increased the thrust of the propeller.

The apparatus for the next experiment, which was tried in 1887, was made by Messrs. Dahlstrom & Lohman, engineers, of Copenhagen. An air propeller with three vanes of thin sheet steel, and an area of about 5 sq ft., was fitted to a boat 16 ft. long and 4 1/2 ft. beam, and rotated by man power. It is stated to have produced a thrust of l0 lbs, with an effort of about 100 foot-pounds, or at the astonishing rate of 55 lbs. per horse power; but it must have been assisted by wind blowing athwart the blades, for Mr. H. C. Vogt, in a letter published in London Engineering, for December 4, 1891, says, in discussing Aerodynamics, that "with 1 indicated horse power it is not possible to obtain a thrust of over 40 lbs. to 45 lbs. with an air propeller- say 50 lbs. to 60 lbs. per brake horse power on the shaft -just the same in whatever manner area, pitch, and revolutions are varied."

On the basis of these Copenhagen experiments Mr. John P. Holland, in a very interesting letter, published in the New York Herald in November, 1890, claims that it is even now possible to navigate the air upon the screw principle, by simply combining things already tried and proved by various experimenters; and he gives the elements of a proposed steam apparatus, weighing some 7,000 lbs., and capable of carrying two men, with supplies of fuel, etc., sufficient to sail from 84 to 23.6 hours. Details of the design and method of operation are with held until a patent can be secured. As has already been said in referring to Mr. Maxim, it is probable that such a machine can be made to rise upon the air; but special appliances will be required to secure safety in case the machinery breaks down while under way, and in effecting a landing.

A somewhat similar proposal is made in a pamphlet published in 1891 by Mr. James Means, of Boston, but he gives only a scanty glimpse of the arrangement by which he thinks the problem could be solved. He proposes one screw on a vertical shaft, sustaining a car, with a pair of widely extended vertical planes, to prevent rotation of the apparatus, and concludes by saying: "If you want to bore through the air, the best way is to set up your borer and bore."

Our knowledge of the action of aerial screws is almost wholly experimental; and it would seem, in the present chaotic state of theory as applied to the screw, as it this remark of Mr. Means was almost as comprehensive and reliable as anything on the subject of aerial screws which has been published up to the present time. The writer feels quite certain that it contains in a condensed form as much reliable detailed solid information as several mathematical articles of considerable complexity which he has consulted, and it will be seen, by closely analyzing Mr. Means's suggestion, that after its entire adoption in the spirit in which it is made, there would be little left to be desired in the development of aerial screws.

Among the inventors who have most deeply and most intelligently studied the action of screws must be mentioned M. G. Trouvé of Paris, whose artificial flapping bird has already been noticed under the head of "Wings." He has proceeded almost wholly in the experimental way, and he has accomplished some very remarkable results. He began his experiments with marine screws applied to electric launches about 1881, and soon developed an electric motor weighing but 33 lbs. per horse power (primary battery not included), which rotated an improved marine screw some 2400 turns per minute.⁸

In 1886 he exhibited to the French Academy of Sciences a new method of constructing geometrically accurate screws by a process so simple that any workman can carry it out, and that the cost is very much reduced. He has also experimented, ever since 1867, with aerial screws, and has reached the conclusion that for the latter the best results are obtained when the pitch is equal to the diameter, or a little less⁹ contrary to marine practice, where pitch is generally 1.3 times the diameter.

In 1887, at the Scientific Congress at Toulouse, and in 1888, before the French Société de Physique, M. Trouvé exhibited the electric motor and aerial screw represented in fig. 35. The motor is the lightest ever built, weighing but 3.17 oz., and developing 868 foot-pounds per minute, or at the astonishing rate of 1 horse power for each 7.42 lbs. weight. It is wholly of aluminum, except the magnetic circuit, which is necessarily of very soft iron; and the armature is directly connected with a very light aerial screw, geometrically perfect, which was constructed by the process communicated to the French Academy of Sciences.

Click on Picture to enlarge

FIG 35.-TROUVÉ--1886.

 This apparatus, upon being placed in one pan of a pair of scales, and connected with a source of electricity of 40 Watts constant delivery, lightened itself of its entire weight by action upon the air. To make the experiment more striking, M. Trouvé then arranged it at the extremity of a balanced beam, as shown in the figure, connecting it with the electric supply through the standard, the knife edges and the beam. Then upon turning on the current, the screw began to revolve, and the balanced beam rose from the position A B into the position A' B', with the expenditure of 868 foot-pounds per minute, which M. Trouvé says is capable of raising it at the rate of 72 ft. per second.

Inasmuch as he estimates that this minute motor has only an efficiency of 20 per cent., and that a similar one of 50 to 100 horse power would possess an efficiency of 80 to 92 per cent., it would seem that M Trouvé now has it in his power to go up into the air with a pair of aerial screws, rotating in contrary directions in order to insure stability, moved by his wonderfully light motor, to float, to hover, and to move about at pleasure so long as he remains within the limits of length, of strength and of weight of a connecting wire to convey the electric force from a dynamo and steam engine, which remain on the ground, to the electric motor and aerial screw in the air.

This, as he points out, would be of practical use on the battle-field or in a besieged city, to observe the enemy; and it is not impossible that he will exhibit such an apparatus at some International Exposition; but he believes that he has now designed a still better solution of the problem; and we shall see, when we come to treat of aeroplanes, that he made the plans for an apparatus of that kind which seems to him to solve, both in arrangement and motive power, the all-important question of the navigation of the air.

For several years past series of experiments upon aerial screws, both for sustaining and for propelling, have been carried on by Commandant Renard, at the French Aeronautical War Establishment at Chalais. He published a preliminary paper in the Revue de L' Aéronautique in 1889 in which he gave a description of the machine used in testing, and of the results of the experiments with the screw of the war balloon La France, which is two-bladed, nearly 23 ft. in diameter, with an average pitch of 27.5 ft. and a surface of about 42 sq. ft.

He found that the efficacy of this screw, or its thrust in pounds divided by the foot- pounds exerted, varied from 48.4 lbs. per horse power at 17 turns per minute, down to 16.94 lbs. per horse power, with 48 turns per minute; and he calls attention to the fact that inasmuch as the thrust increases as the square of the velocity, while the power required grows as the cube, the proper method of comparing the efficiencies of various forms of screws is to compare the quotients obtained by dividing the cubes of the thrusts by the squares of the powers.

Commandant Renard seems to have so proceeded in comparing his experiments; and in a paper read by him before the French Society of Physics, in 1889 he stated that of seven forms of screws tried up to that time, one was much better than the others; and he added from theoretical considerations: "There must be a screw for which Thrust³/Power² = constant, is a maximum. This is confirmed by experiment; and it shows, moreover, that this maximum when plotted resembles a sharp peak, each side of which forms a veritable precipice. In other words, there is a screw very much better than others, and its form cannot be much departed from without producing very bad aerial screws."

None of the forms of screws experimented upon are published, save that of La France and that this is not the best may be inferred from the fact that Mr. Maxim who tested about fifty different forms of screws in his recent experiments, says: "The screw which gave the worst results was made exactly like those employed in the experiments of the French Government."

Mr. Maxim has published a popular account, all too brief, of his experiments, in the Century Magazine for October, 1891 but for obvious reasons does not go into scientific details. He has expressed the intention of eventually doing so, and this is sure to prove a very great addition to our present scanty knowledge, for his experiments on aerial screws have been more systematic and comprehensive than any heretofore tried.

From the foregoing it will be seen that comparatively few experiments have been made to compass artificial flight by means of sustaining aerial screws, and that much very much remains to be learned concerning the best form to be given to them, the proper area, velocity and pitch, as well as the power required, either for sustaining or for propelling a given weight in the air with a screw. Indeed, even for marine screws, our knowledge may be said to be wholly empirical-that is to say, based on experiment; and there is no mathematical theory of them which has found general acceptance, or which Connects their action with that of plane surfaces, so as to agree with the observed facts. Some calculations made by the present writer seem to indicate that it may be less difficult to do so, in the case of aerial screws; but it must be acknowledged that we really know but little about them, and that the most that we can say at present is that while a flying machine in which the sustaining power is to be obtained from rotating screws is likely to require less surface than an aeroplane to sustain the same weight, perhaps in the proportion of about one- third, yet et it is likely to require more power than the aeroplane to obtain the same speed of translation, and also to involve greater risks of accidents in ease of failure of any part of the machinery

It would seem to the writer as if the true function of aerial screws was to propel, leaving the sustaining power to be obtained in some other way, and we will therefore pass to the consideration of AEROPLANES.

⁸ Histoire d'un Inventeur-- Barral. Page 416.
⁹Ibid. Page 442.