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.
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.
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