CHAPTER IV.PRINCIPALLY RELATING TO SCREWS.

Fig. 7.—The circulation of air produced by a difference in temperature.

The sketch (Fig. 7) shows a device consisting of a spirit lamp and a box of ice. The lamp heats the metallic plate, expands the air which rises and is cooled by convection on coming in contact with the top plate, and descends as shown. However, a fire is not necessary to accomplish this result; it is taking place all over the earth, all the time. A great number of plants depend upon a rising current ofair to transport their seeds to distant places. Seeds of the thistle and dandelion variety are sometimes able to travel hundreds of miles, to the great vexation of farmers; and there is a certain class of small spider known as “Balloon Spiders” which also depend upon a rising current of air to carry them from the place of their birth to some distant part where they, of course, hope to start a colony. When I was a boy of eight, I noticed small spiders webbing down from the sky. I was greatly puzzled; it appeared to me that they had attached their web to some stationary object high in the air and were spinning a web in order to lower themselves to the earth. What could that stationary object be? As the sky was clear, I was quite unable to understandthis phenomenon, but afterwards I learned from scientific books that there was a class of spiders that managed to rise high in the air by the aid of the wind. It appears that they climb a high tree until they have reached the uppermost extremity and then, from a leaf or twig that projects into the air, they wait for an ascending current of air. Although the spider is exceedingly small—the size of a pin’s head—it has about 200 spinnerets, its ordinary web being formed of no less than that number of extremely fine threads. These are spun out singly into the air until an almost invisible mass of fine webs interlacing each other in all directions and forming an approximately cylindrical network about half an inch in diameter and 18 inches long is produced. Whenever an upward draft of air approximately vertical occurs, it takes this weightless tangle of fine webs with it, and so soon as the spider finds there is sufficient pull to lift its weight, it lets go and ascends with the air. When the Nulli Secundus ascended at Farnborough and landed at the Crystal Palace, Mr. Cody, who was on board, reported what he supposed to be a very curious and unaccountable phenomenon. The balloon was covered with many thousands of minute spiders that it had picked up in the air on the voyage. Certainly this of itself is very strong evidence of the existence of these ascending currents of air.

Fig. 8.—arepresents a kite in a horizontal wind,e,e,e;b, the same kite in a rising column of air, the wind blowing in the direction shown atf,f. If the kite is a good one, it may pass over to the pointc.

When in Boston about fifteen years ago, I went to Blue Hill to witness the remarkable kite flying which was taking place at that time. The kites experimented with were of the Hargrave type, and of enormous dimensions. A steel wire and windlass worked by a steam engine was employed. I was told that on certain occasions the kites mounted extremely high, much higher than they were able to account for; but on this particular occasion, although they let out a great amount of wire, the kite did not mount very high. I have heard much discussion first and last regarding the flight of kites, and I think it is generally admitted that they do sometimes rise upwards and continue moving to the windward until they pass directly over the spot where they are attached to the earth. It was not, however, till about three years ago that I witnessed this phenomenon myself. Mr. Cody, who is the inventor of a very good kite, had been flying kites at the Crystal Palace for some months, and on one occasion I saw his kite rise, pass to the windward and directly over ourheads. I took hold of the cord with both hands, and was somewhat surprised to find what the lifting effect was. The kite was, however, of large dimensions, but by no means so large as Mr. Cody’s “man-lifting kites.” In the drawing (Fig. 8) I have shown, ata, the action of a kite in a horizontal wind, linese,e, showing the direction of the wind. A good kite will easily mount 45°, the angle shown, but on the occasion just mentioned, the sun had been shining brightly into the valley where the experimentstook place, and an upward current of air had been determined. The cooler air was, of course, rushing in from each side and mounting in about the centre of the valley, and Mr. Cody’s kite, instead of flying in a horizontal wind, soon reached a point where the wind was ascending at an angle, as shown atf,f. The kite would therefore mount until atb, where it presented the same angle to the wind as with the horizontal wind ata, and if it should be made to fly at a higher angle, it might pass over to the position shown atc. But it must not be imagined that this phenomenon can be witnessed every day in the year. It is only on rare occasions that one is fortunate enough to find a wind which is blowing at a sufficiently sharp upward trend to cause a kite to pass to the windward over the point of support. Neither must it be supposed that this favourable condition of things is of long duration. As the centre of the upward current is constantly moving, it is certain that very soon it will move away from the point from which the kite is being flown. What is true of kites is also true of flying machines. It is very difficult indeed to make a kite mount providing that it is in a descending current of air, and one is just as likely to find a descending current as any other. Flying machines will, therefore, have to be made with a considerable amount of reserve energy, so as to be able to put on a spurt when they encounter an adverse current. If a machine is made that is able to maintain itself in the air for any considerable length of time, it will not be a very difficult task to know when a current of air of this kind is encountered, because, if the engine is working up to speed, and everything is in perfect order, and still the machine is falling, it is very certain that an unfavourable current has been encountered, and efforts should be made to get out of it as soon as possible. Then, again, if the machine has an abnormal tendency to rise without any increase in the number of rotations made by the screws, the aeronaut may be certain that he has encountered an upward and favourable current of air which, unfortunately, will not last. It should, however, be borne in mind that, while the width of the upward current is not very great, nevertheless, it may extend in a practically straight line for many miles.

In 1887 I was approached by several wealthy gentlemen who asked me if I thought it was possible to make a flying machine. I said, “certainly; the domestic goose is able to fly and why should not man be able to do as well as a goose?” They then asked me what it would cost and how long it would take, and, without a moment’s hesitation, I said it would require my undivided attention for five years and might cost £100,000. A great deal of experimenting would be necessary; the first three years would be devoted to developing an internal combustion engine of the Brayton or Otto type, and the next two years to experimenting with aeroplanes and screws and building a machine. Even at that time I had a clear idea of the system that would be the best. However, nothing was then done, but in 1889 I employed for the purpose two very skilful American mechanics, and put them to work at Baldwyn’s Park, Kent. At that time the petroleum motor had not been reduced to its present degree of efficiency and lightness; it was not suitable for a flying machine, and I saw that it would require a lot of experimental work in order to develop it. After taking into consideration all the facts of the case, I decided to use a steam engine. Had I been able to obtain the light and efficient motors which have been recently developed, thanks to the builders of racing cars, I should not have had to experiment at all with engines and boilers, as I could have obtained the necessary motors at once. As it was, I was obliged to content myself with the steam engine.

Fig. 9.—Group of screws and other objects used in my experiments.

Fig. 10.—Some of the principal screws experimented with—h, a screw with very thick blades, andg, a screw made after a French model.

I found that there was a great deal of misunderstanding regarding the action of aeroplanes, and also of screws working in the air. I procured all the literature available on the subject, both English and French, and attempted to make a thorough study of the question; but I was not satisfied, on account of the wide difference in the views of the writers and the conflicting formulæ that were employed. I therefore decided to make experimentsmyself, and to ascertain what could be done without the use of anybody’s formula. Although this was nearly twenty years ago, I find that there is still a great deal of discussion regarding the action of aeroplanes and screws, in which the majority taking part in the discussion are in the wrong. However, several good works on the subject have recently been published.

Fig. 11.—The three best screws. The screw on the right has a uniform pitch throughout, the middle screw has increasing pitch, and the left screw compound increasing pitch.

Having designed and put my boiler and engine in hand, I commenced a series of experiments for the purpose of ascertaining the efficiency of screw propellers working in the air, and the form and size that would be best for my proposed machine. The illustrationFig. 9shows a photographic group of the screws and other objects with which I experimented.Fig. 10shows some of the leading types which, as will be seen, have blades of different shape, pitch, and size.Fig. 11shows three of the best screws employed. It will be observed that one has uniform pitch, another increasing pitch, and the third compound increasing pitch. In order to test the efficiency of my screws I made the apparatus shown inFig. 12. The power for running the screw was transmitted by means of a belt to the straight cylindrical pulleyc,c. Shaftb,bwas of steel, rather small in diameter, and ran smoothly, and practically without friction, through the two bearingsd,d. When the first screw,a,a, was run at a high velocity, the axial thrust pushed the shaftb,bback and elongated the spiral springe. The degree of screw thrust was indicated in pounds by the pointerg. The power was transmitted through a very accurate and sensitive dynamometer, so that the amount consumed could be easily observed by a pointer similar to the one employed for indicating the screw thrust. A tachometer was also employed to observe the number of turns that the screw was making in a minute. The whole apparatus was carefully and accurately made and worked exceedingly well. I was thus enabled, with my various forms of screws and other objects, to make very accurate measurements, some of which are exceedingly interesting.

Fig. 12.—Apparatus for testing the thrust of screws—a,a, the screw;b,b, shaft sliding freely in the bearingd,d;c, cylindrical pulley;e, spiral spring;f, steel rod;g, pointer for indicating the thrust in pounds.

Fig. 13.—Apparatus for testing the direction of air currents caused by a rapidly rotating screw. Silken threads were attached to the wirec,c, which indicated clearly the direction in which the air was moving.

In many of the treatises and books of that time it wasstated that a screw propeller, working in the air, was exceedingly wasteful of energy on account of producing a fan-blower action. Some inventors suggested that the screw should work in a stationary cylinder, or, better still, that the whole screw should be encased in a rotating cylinder, to prevent this outward motion of the air. In order to ascertain what the actual facts were, I attached a large number of red silk threads to a brass wire, which I placed completely around my screw (seeFig. 13). Upon starting up I found that, instead of the air being blown out at the periphery of the screw, it was in reality sucked in, as will be seen in the illustration. I was rather surprisedto see how sharp a line of demarkation there was between the air that was moving in the direction of the screw and the air that was moving in the opposite direction. The screw employed in these experiments was 18 inches in diameter and had a pitch of 24 inches. It was evident, however, if the pitch of the screw was coarse enough that there would be a fan-blower action. I therefore tried screws of various degrees of pitch, and found when the pitch was a little more than three times the diameter, giving to the outer end of the blade an angle of 45°, that a fan-blower action was produced—that is, part of the time when the screw was running, the air would alternate; sometimes it would pass inwards at the periphery and sometimes outwards. The change of direction, however, was always indicated by a difference in the pitch of the note given out, and also by the thrust. InFig. 14I have shown the extremities of the blades of some of the different forms of screws experimented with,in whichashows a plain screw, the front side being straight and of equal pitch from the periphery to the hub;bis a screw of practically the same pitch, but slightly curved so as to give what is known as an increasing pitch;cshows the extremity of a screw in which the curve is not the same throughout—that is, it is what is known as a compound increasing pitch;dis the shape of the screw that gave the angle of 45° above referred to.

Fig. 14.—This drawing shows the ends of screw blades in whichais a plain screw;b, screw with increasing pitch;c, screw with compound increasing pitch;d, end of screw blade 45°;e, screw with very thick blade;f, blade with no pitch at all;g, blade which gave a thrust in the direction of the convex side, no matter in which direction it was revolved;h, screw said to have been used in the French Government experiments.

The first screw experimented with wasa. This screw was run at a high velocity—about 2,500 revolutions per minute—until a screw thrust of 14 lbs. was obtained, and then the governor of the engine was set so that all screws of the same diameter could be run at the same speed. Wishing to ascertain the efficiency of the screw and how much was lost in skin friction, I multiplied the thrust in pounds by the pitch of the screw in feet and by the number of turns it was making in a minute. This, of course, gave the exact number of foot-pounds in energy that was being imparted to the air. I was somewhat surprised to find that it corresponded exactly with the readings of the dynamometer. I thought at first that I must have made some mistake. Again I went very carefully over all the figures, tested everything, and made another experiment and found, even if I changed the number of revolutions, that the readings of the dynamometer were always exactly the same as the energy imparted to the air. This seemed to indicate that the screw was working very well and that the skin friction must be very small indeed. In order to test this, I made what we will call, for the moment, a screw without any pitch at all—that is, the blades were of wood and of the exact thickness and width of the blades of the screwa, but without any pitch at all. The extremity of the blade is shown atf. I placed this screw on my machine in place ofa, and although my dynamometer was so sensitive that the pointer would move away from the zero pin by simply touching the tip of the finger to the shaft, it failed to indicate, and thus the screw appeared to consume no power at all. These experiments were repeated a considerable number of times. I then obtained a sheet of tin the same diameter as the screws, 18 inches, and upon running it at the same speed, I found that it did consume a measurable amount of power, certainly more than the two bladesf.This no doubt was due to the uneven surface of the tin. Had it been a well-made saw blade without teeth, perfectly smooth and true on both sides, it probably would not have required power enough to have shown on the dynamometer. However, it is quite possible that there is a little more skin friction with a polished metallic surface, than with a piece of smooth evenly lacquered wood. The screws which I employed were of American white pine such as used by patternmakers. This wood was free from blemishes of all kind, extremely light, uniform, and strong. When the screw had been formed, it was varnished on both sides with a solution of hot glue, which greatly increased the strength of the wood crosswise of the grain. When this glue was thoroughly dry, the wood was sand-papered until it was as smooth as glass; the whole thing was then carefully varnished with shellac, rubbed down again and revarnished with very thin shellac something like lacquer. In this way the surface of the screw was made very smooth. The screws, of course, were made with a great degree of accuracy and as free as possible from any unevenness. Having tested screwa, I next tested screwb. I found with the same number of revolutions per minute that this screw produced more thrust, but it required more power to run it, and when the energy imparted to the air was compared with the readings of the dynamometer, it was found that it did not do quite so well asa; still as the thrust was greater and the efficiency only slightly less, it appeared to be the better screw. Upon trying screwc, under the same conditions, the thrust was very much increased, but the power required was also increased to a still greater degree, showing that this form was not so favourable as eitheraorb. All the screws experimented with had very thin blades, and it occurred to me that the difference betweenaandbmight arise from the fact that, whenawas running at a very high velocity, the working side instead of being flat might have become convex to a slight extent, whereas withb, a slight bending back of the edges of the blade would still leave the working side concave. I therefore made the screw shown ate, which had the same pitch as the other three, but the working side was of the same shape asa. Of course the additional thickness of the blades made it impossible to give an easy curve to the back. Curiously enough I found thatedid nearly as well asa, and quite as well asb. The additional thickness did not interfere toany appreciable extent with its efficiency. I then made another propeller, shown atg, which was of the same thickness in the middle ase. Upon running this, I found that it required considerable power, and no matter which way it was run, the thrust was always in the direction of the convex side, which was quite the reverse from what one would have naturally supposed.

Fig. 15.—The manner of building up the large screws.

About the time that I was making these experiments, my duties called me to Paris, and while there I called on my old friend Gaston Tissandier. Through his influence I was permitted to see some models of the screws that were alleged to have been used by Captain Renard in his experiments for the French Government, and I was somewhat surprised to find the form of the blades, the same as shown ath,Fig. 14, and completely without any twist. On my return to England, I made a screw of this description. It is also shown in the photographic illustration,Fig. 9. Upon testing this screw, I found that its efficiency was only 40 per cent. of that ofa—that is, the energy or acceleration imparted to the air was only 40 per cent. of the readings of the dynamometer. It then occurred to me that this particular form of screw was probably the one that the French had for exhibition purposes, but not the one they intended to use. Having tried all the various forms of screws and other objects shown inFig. 9, I made some sheet metal screws; also a screw which consisted of a steel frame covered with woven fabric, and which was identical with screws that I had seen described in various worksrelating to aerial navigation. It was found quite impossible to keep the fabric taut and smooth, and the results were very bad indeed, it being only 40 per cent. as efficient as a well-made wooden screw.

Fig. 16.—A fabric-covered screw with a very low efficiency.

Having thus ascertained the best form of a screw, I built up my first large screws, which were 17 feet 10 inches in diameter, after the well-known manner of making wooden patterns for casting steamship propellers.Fig. 15shows the form of the end of the blade, the middle of the blade, and the hub. My first pair of large screws had a pitch of 24 feet, but these were too great a drag on the engine. I therefore made another pair with 16 feet pitch which greatly increased the piston speed, and permitted the engines to develop much more power; the screw thrust was also increased just in an inverse ratio to the pitch of the screws. Another pair of screwswas tried with 14 feet pitch and 12 feet in diameter, but these did not do so well. My large screws were made with a great degree of accuracy; they were perfectly smooth and even on both sides, the blades being thin and held in position by a strip of rigid wood on the back of the blade. In order to prevent the thrust from collapsing the blades, wires were extended backwards and attached to a prolongation of the shaft. Like the small screws, they were made of the very best kind of seasoned American white pine, and when finished were varnished on both sides with hot glue. When this was thoroughly dry, they were sand-papered again and made perfectly smooth and even. The blades were then covered with strong Irish linen fabric of the smoothest and best make. Glue was used for attaching the fabric, and when dry another coat of glue was applied, the surface rubbed down again and then painted with zinc white in the ordinary way and varnished. These screws worked exceedingly well. I had means of ascertaining, with a great degree of accuracy, the thrust of the screw, the number of turns per minute, the speed of the machine, and, in fact, all the events that were taking place on the machine. It was found that when the screw thrust in pounds was multiplied by the pitch in feet, and by the number of revolutions made in a minute of time, it exactly corresponded to the power that the engines were developing, and that the amount of loss in skin friction was so small as to be practically negligible.

Fig. 17.—The hub and one of the blades of the screw on the Farman machine. The bladec, is a sheet of metal riveted to the rodb, and forms a projection on the back of the blade which greatly reduces its efficiency. The peculiar form of hub employed makes it possible to change the diameter and pitch of this screw at will.

In connection with this subject I would say that many experimenters claim to have shown that the skin friction on screws is considerable, in fact, so great as to be a very important factor in the equation of flight. I am, however, of the opinion that these experimenters have not had well-made screws. If the surface of the screw is uneven, irregular, or rough, a considerable amount of energy is lost, as shown in the French screw and the fabric covered screw. It is simply a question of having a screw well-made. In those recently employed in France (seeFig. 17), the blades are of hammered sheet metal, the twist is not uniform or true, and what is worst of all, the armbprojects on the back of the blade and offers a good deal of resistance to the air. This form of screw, however, is very ingenious; as will be seen by the drawing, the pitch and diameter can be changed at will. It is, however, heavy,wasteful of power, and altogether too small for the work it has to do. The skin friction of screws in a steamship has led inventors to suppose that the same laws relate to screws running in air, but such is by no means the case. In designing a steamship, we have to make a compromise in regard to the size of the screw. If the screw is too small, an increase in diameter is, of course, an advantage, and it may also be an advantage, not only to increase thediameter, but also to reduce the pitch; however, a point is soon reached where the skin friction will more than neutralise the advantages of engaging a larger volume of water. This is because the water adheres to the surface; in fact, the skin friction of a ship and its screw consumes fully 80 per cent. of the total power of the engines, but with an air propeller its surface is not wetted and the air does not stick to its surface. If made of polished wood, the friction is so extremely small as to be almost unmeasurable. The diameter of a well-made screw running in air is therefore not limited in any degree by skin friction, as is the case with a screw running in water; in fact, it is rather a question of its weight, and its efficiency ought to increase in direct ratio with its diameter, because the area of the disc increases with the square of the diameter. The screw slip is therefore reduced by one-half by simply doubling the diameter of the screw. It will be understood that by doubling the diameter of the screw, four times as much airwill be engaged. If we push this back at half the speed, we shall have the same screw thrust, because the resistance of the air is in proportion to the square of the velocity that we impart to it, so that one just balances the other, and the diminution of wasteful slip is just in proportion to the increase in diameter. In all cases, the screw should be made as large as possible.

Fig. 18.—Section of screw blades having radial edges. With screws of this form, the blades, of course, become narrower as the hub is approached, and if it is a true screw and the edges radial, the sine of the angle will be the same at all points. It is 2 inches in this case.Fig. 18 enlarged(55 kB)

Fig. 18 enlarged(55 kB)

Fig. 19.—Shows the form of the blade of a screw propeller made of sheet metal. It is riveted at the edges and also to the arm of a screw with a stiffening piece at the extreme end. However, it is not necessary to rivet edges together. They may be welded with a name of acetylene oxygen gases.

Fig. 19a.—Shows the manner of welding and the finished edge.

Fig. 20.—A new form of hub, of great strength and lightness, for use on flying machines.

In the drawing (Fig. 18) I have shown screw blades of a proper shape to give the best results—that is, providing a metallic screw is employed. Instead of having the arm of the screw on the back of the blade to offer resistance to the air, the arm should be tubular, flattened, and covered on both sides with sheet metal. This particular formation not only prevents the air from striking the arm, but, at the same time, prevents the pressure of the air from deforming the blade, so, if a metallic screw is to be used, the form of blade which I have shown will be found much superior to that employed at the present time on continental flying machines. We should not lose sight of the fact that weight tells very seriously against the success of a flying machine, and that no expense should be spared to reduce the weight, providing that it is possible to do so without reducing the factor of safety. Suppose, for example, that we use an ordinary hub secured to a solid shaft by a common key. All the parts have to be made heavy in order to be sufficiently strong to withstand the strain. In the drawing (Fig. 20) I have shown a hub which I think is quite as light and strong as it is possible to make it. The action of the motor is often spasmodic and puts very great strain upon the parts, and there is a very strong tendency for the shaft to turn round in the hub. If a key is used, the hub has to be large and strong, and the key of considerable size, otherwise the parts would be deformed. In my own experiments, I have found considerable difficulty in securing a shaft to wooden screws. However, it will be seen in the drawings that a series of grooves is cut in the shaft and that the hub has internal projections, so that the one fits the other. This makes a very strong connection and is of extreme lightness. Both the hub and the shaft should be of tempered steel. The spokes should be hard drawn steel tubes with long fine threads, so as to withstand centrifugal force. To prevent them from rotating in the hub, the nutsd,dare provided, which compress the arms of the steel hub so as to grip the tube with anydegree of force required. It will be seen that with this system the pitch of the screw may be adjusted to some extent; however, it is better to have all parts of the screw, from hub to centre, of the same pitch. A slight deviation from this is admissible in the experimental stage, so long as the deviation from a true screw, caused by rotating the arm, is not greater than one half of the slip while in flight.

Many experimenters have imagined that a screw is just as efficient placed in front of a machine as at the rear, and it is quite probable that, in the early days of steamships, a similar state of things existed. For several years there were steamboats running on the Hudson River, New York, with screws at their bows instead of at their stern. Inventors of, and experimenters with, flying machines are not at all agreed by any means in regard to the best position for the screw. It would appear that many, having noticed that a horse-propelled carriage always has the horse attached to the front, and that the carriage is drawn instead of pushed, have come to the conclusion that, in a flying machine, the screw ought, in the very nature of things, to be attached to the front of the machine, so as to draw it through the air. Railway trains have their propelling power in front, and why should it not be the same with flying machines? But this is very bad reasoning. There is but one place for the screw, and that is in the immediate wake, and in the centre of the greatest atmospheric disturbance. While a machine is running, although there is a marked difference between water and air as far as skin friction is concerned, still the conditions are the same as far as thepositionof the screw is concerned. With a well-designed steamship, the efficiency of the screw is so great as to be almost unbelievable; in fact, if a steamship had never been made, and the design of one should be placed before the leading mathematicians of to-day, with the request that they should compute the efficiency of the screw, none of them would come anywhere near the mark. They would make it altogether too small. As before stated, when a steamship is being driven through the water, the water adheres to its sides and is moved forward by the ship—that is, it has acceleration imparted to it which exactly corresponds to the power consumed in driving the ship through the water. This, of course, retards it and we find in a well-designed ship, not run above its natural speed, that about 80 per cent. of thepower of the engine is consumed in skin friction, or in imparting a forward motion to the water. Suppose that we should take such a ship, remove the screw, and tow it through the water with a very long wire rope at a speed of, say, 20 miles an hour; we should find that the water at the stern of the ship was moving forward at a velocity of fully 6 miles an hour—that is, travelling in the same direction as the ship. By replacing the screw, and applying engine power sufficient to give the ship the same speed of 20 miles an hour, identical results would be produced. The skin friction still impels the water forward, so that the screw, instead of running in stationary water, is actually running in water moving in the same direction as the ship at a velocity of 6 miles an hour. If the slip of the screw should only be equal to this forward motion, the apparent slip would be nothing; in fact, the ship would be moving just as fast as it would move if the screw were running in a solid nut instead of in the yielding water. Curiously enough there have been cases of negative slip in which the actual slip of the screw in the water was less than the forward movement of the water, and in such cases a ship is said to have negative slip. A very noticeable case of this kind occurred in the Royal Navy in the sixties.[1]I was at the time engaged in a large shipbuilding establishment in New York, and remember distinctly the interest that the case created amongst the draughtsmen and engineers of that establishment. Of course, this apparently impossible phenomenon created a great deal of discussion on both sides of the Atlantic. It appears that this ship had been built under an Admiralty Specification which called for a screw of a certain diameter and pitch with a specified number of revolutions per minute, and for a certain number of knots per hour, also that the boiler pressure should not go above a certain number of pounds per square inch. When the ship was finished and went on her trial trip, it was found impossible to make the full number of turns called for in the specification with the boiler pressure allowable; nevertheless, the speed was greater than the specification called for, and as speed was the desideratum, and not the number of revolutions, the contractors thought their ship should be accepted. Then arose a discussion as to the diameter andpitch of the screw. It was claimed that a mistake must have occurred. A careful measurement was made in the dry dock, and all was found correct. Once more the ship was tried, and again her speed was in excess of the specification, notwithstanding that it was still impossible to get the specified number of revolutions per minute. Mathematicians then took the matter in hand, and it was found that the ship actually travelled faster than she would have done if the screw had been running in a solid nut. Instead of a positive slip, the screw had in reality a negative slip; but this was not believed at the time, and the discussion and controversy continued. The ship was tried again and again, and always with the same results. This apparently inexplicable phenomenon was accounted for in the following manner:—The hull of the ship was said to be rather imperfect and to cause a considerable drag in the water, so that, when the ship was moving at full speed, the water at the stern had imparted to it a forward velocity greater than the actual slip.

[1]The particulars relating to this event are taken from accounts published at the time in American papers.

What is true of ships is true of flying machines. Good results can never be obtained by placing the screw in front instead of in the rear of the machine. If the screw is in front, the backwash strikes the machine and certainly has a decidedly retarding action. The framework, motor, etc., offer a good deal of resistance to the passage of the air, and if the air has already had imparted to it a backward motion, the resistance is greatly increased. The framework will always require a considerable amount of energy to drive it through the air, and the whole of this energy is spent in imparting a forward motion to the air, so if we place the propelling screw at the rear of the machine in the centre of the greatest atmospheric resistance, it will recover a portion of the lost energy, as in the steamship referred to. It will, therefore, be seen that when the screw is at the rear, it is running in air which is already moving forward with a considerable velocity, which reduces the slip of the screw in a corresponding degree. I have made experiments with a view of proving this, which I shall mention further on, and which ought to leave no chance for future discussion.

Fig. 21.—Small apparatus for testing fabrics for aeroplanes, the material being subjected to an air blast in order to test its lifting effect as compared with its tendency to travel with the blast.

My first experiments had shown that wooden aeroplanes did much better than any of the fabric covered aeroplanes that I was able to make at that time, but as wood was quite out of the question on my large machine on accountof its weight, it was necessary for me to conduct experiments with a view of ascertaining the relative values of different fabrics. For this purposes, I made the little apparatus shown (Fig. 21). This was connected to a fan blower driven by a steam engine having a governor that worked directly on the point of cut-off. The speed was, therefore, quite uniform and the blast of air practically constant. I had a considerable number of little frames cut out of sheet steel, and to these I attached various kinds of fabric, such as ordinary satin, white silk, closely woven silk, linen, various kinds of woollen fabrics, including some very coarse tweeds, also glass-paper, tracing linen, and the best quality of Spencer’s balloon fabric. The blast of air was not large enough to cover the whole surface of the aeroplanes, so that the character of the back of the frames was of no account. The first object experimented with was a smooth piece of tin. When this was placed at an angle of 1 in 14, it was found that the drift or tendency to travel in the direction of the blast was just one-fourteenth part of the upward tendency, or lift. This was exactly as it should have been. Upon changing the angle to 1 in 10, a similar thing occurred; the lift was ten timesthe drift. I, therefore, considered the results obtained with the sheet of tin as unity, and gave to every other material experimented with, a coefficient of the unity thus established. Upon testing a frame covered with tightly-drawn white silk, a considerable amount of air passed through, and with an angle of 1 in 14, the lift was only about double the drift. A piece of very open fabric, a species of buckram, was next tried, and with this the lift and drift were about equal. With closely-woven, shiny satin the coefficient was about ·80; with a piece of ordinary sheeting the coefficient was ·90; with closely-woven, rough tweeds, ·70; and with glass-paper about ·75. With a piece of tracing linen very tightly drawn, results were obtained identical with those of a sheet of tin, and with Spencer’s balloon fabric the coefficient was about ·99. I, therefore, decided to cover my aeroplanes with this material. It will be observed that the apparatus is so arranged that both the lift and the drift can be easily measured.

Fig. 22.—Apparatus for testing the lifting effect of aeroplanes and condensers in an air blast.k,kshow two aeroplanes in position for being tested.Fig. 22 enlarged(110 kB)

Fig. 22.—Apparatus for testing the lifting effect of aeroplanes and condensers in an air blast.k,kshow two aeroplanes in position for being tested.

Fig. 22 enlarged(110 kB)

Fig. 23.—Apparatus for testing aeroplanes, condensers, etc., in an air blast. The opening is 3 feet square. Thin brass sustainers are shown in position for testing.

Fig. 24.—Cross-sections of bars of wood employed for ascertaining the coefficient of different forms.

Fig. 25.—Transverse sections of bars of wood experimented with for the purpose of ascertaining their coefficients as relates to a normal plane.

Fig. 26.—A flat aeroplane placed at different angles.

In order to ascertain the resistance encountered by various shaped bodies driven at various speeds through the air, the best form of aeroplanes, and the efficiency of atmospheric condensers, I made the apparatus shown inFigs. 22and23. The smaller and straight portion of thisapparatus was 12 feet long and exactly 3 feet square inside, and was connected as shown to a shorter box 4 feet square. Two strongly made wooden screwsb,bandd, were attached to the same shaft. These screws had two blades each, and while one pair of blades was in a vertical position, the other was in a horizontal position. I interposed between the screws, slats of thin wood arranged in the manner shown atd,d; this was to prevent rotation of the air. AteI placed vertical slats of thin wood, and horizontal slats of the same size atf. Atgtwo wide and thin boards, sharp at both edges and made in the form of the letter X, were placed in the box as shown in section XY. An engine of 100 H.P. with an automatic variable cut-off was employed which gave to the screws a uniform rate of rotation, and as the engine had no other work to do, the governor could be arranged to give varying speeds such as were required for the experiments. The objects to be tested were attached to the movable bars. In the drawing, the aeroplanek,kis shown in position for testing. This apparatus was provided with a rather complicated set of levers, which permitted not only the measurement of the lift of the objects experimented with, but also that of the drift. The principle employed in this apparatus was a modification of the ordinary weighing apparatus used by grocers, etc. The first object tested was a bar of wood exactly 2 inches square shown inFig. 24. This was placed in such a manner that the wind struck squarely against the side as shown in the drawing, and with a wind of 49 miles per hour, it was found that the drift or tendency to move with the air was 5·16 lbs.; at the same time, the wind on my instrument gave a pressure of 2 lbs. on a normal plane 6 inches square. The velocity of the wind was ascertained by an anemometer of the best London make.Upon turning the same bar of wood in the position shown atb, the drift mounted to 5·47 lbs. A round bar of wood, 2 inches in diameter, shown atc, gave a drift of 2·97 lbs. These experiments were repeated with a wind velocity of 40 miles per hour, when it was found that the drift ofawas 4·56 lbs., and that of the round bar, 2·80 lbs. It will be seen from these experiments that the power required for driving bars or rods through the air is considerably greater than one would have supposed. The next object experimented with wasa,Fig. 25. When this was subject to a wind of 40 miles an hour, the drift was 0·78 lb. Upon reversing this bar—that is, putting the thin edgeinstead of the thick edge next to the wind—the drift mounted to 1·22 lbs.;bshowed a drift of 0·28 lb. with the thick edge to the wind, and 0·42 lb. with the thin edge to the wind;cshowed a drift of 0·23 lb. with the thick edge to the wind, and 0·59 lb. with the thin edge to the wind; andd, which was the same thickness as the others and 12 inches wide, both edges being alike, showed a drift of only 0·19 lb. These experiments show in a most conclusive manner the shapes that are most advantageous to use in constructing the framework of flying machines. Aeroplanee,Fig. 26, when placed on the machine in a horizontal position showed neither lift nor drift, but upon placing it at an angle of 1 in 20, as shown atf, the lift was 3·98 lbs. and the drift 0·30 lb. with a wind velocity of 40 miles per hour. At this low angle the blade trembled slightly. Upon placing the same plane at an angle of 1 in 16 as shown atg, the lift was 4·59 lbs. and the drift 0·53 lb. It will be observed that the underneath side of this plane is perfectly flat. The next experiment was with planes slightly curved, as shown inFig. 27. The aeroplaneawas 16 inches wide, very thin, and only slightly curved. When set at a very low angle,it vibrated so as to make the readings very uncertain, but when set at an angle of 1 in 10 it lifted 9·94 lbs. with a drift of 1·12 lbs. By slightly changing the angle it was made to lift 10·34 lbs. with a drift of 1·23 lbs., the wind velocity being 41 miles per hour. Aeroplaneb, 12 inches wide,Fig. 27, when placed at an angle of 1 in 14 with an air blast of 41 miles per hour, gave a lift of 5·28 lbs. with a drift of 0·44 lb.; at an angle of 1 in 12 the lift was 5·82 lbs. and the drift 0·5 lb.; at an angle of 1 in 10 the lift was 6·75 lbs. and the drift 0·73 lb.; with an angle of 1 in 8 the lift was 7·75 lbs. and the drift 1 lb.; with an angle of 1 in 7 the lift was 8·5 lbs. and the drift 1·25 lbs.; at an angle of 1 in 6 the lift was 9·87 lbs. and the drift 1·71 lbs. Aeroplanec,Fig. 27, which had more curvature thanb, when run in a horizontal position, gave a considerable lift, and when raised to an angle of 1 in 12 it gave a lift of 6·12 lbs. with a drift of 0·54 lb. In another experiment at the same angle, it gave a lift of 6·41 lbs. with a drift of 0·56 lb.; at an angle of 1 in 16 it gave a lift of 5·47 lbs. with a drift of 0·37 lb.; at an angle of 1 in 10 it gave a lift of 6·97 lbs. and a drift of 0·70 lb.; at an angle of 1 in 8 it gave a lift of 8·22 lbs. with a driftof 1·08 lbs.; at an angle of 1 in 7 it gave a lift of 9·94 lbs. with a drift of 1·45 lbs.; at an angle of 1 in 6 it gave a lift of 10·34 lbs. and a drift of 1·75 lbs. This plane was then carefully set so that both the forward and aft edges were exactly the same height, and with a wind blast of 41 miles per hour it gave a lift of 2·09 lbs. with a drift of 0·21 lb. It was then pitched 1 in 18 in the wrong direction, and at this point, the lifting effect completely disappeared, while the drift was practically nothing.

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