During the years 1892 and 1893, it will be recalled, four aerodromes, known as Nos. 0, 1, 2, and 3, had been built, which were of two general types of construction. First, that represented by No. 0, in which a radically weak hull was made to support rods at the front and rear, to which the wings and tail were attached. This aerodrome was abandoned on account of the inability to provide it with sufficient power, as well as because of its constructional weakness. Second, that type represented by Nos. 1, 2, and 3, in which a midrod was carried through from front to rear, around which the hull supporting the machinery was built. These models were much lighter than No. 0, but were all abandoned because it was found impossible to propel even the lightest of them. While all these machines were in the strictest sense failures, inasmuch as none of them was ever equipped with supporting surfaces, yet the experience gained in the construction of them was of the very greatest value in determining the points at which strength was needed, and in indicating the mode of construction by which strength and rigidity could be obtained.26
Another aerodrome, known as No. 4 (shown in Plate 11), was designed in the latter part of 1892, and by the end of March, 1893, its construction was well under way. It was of the second type, in that the midrod was continuous, but it differed from the preceding forms in having the machinery (boilers, burners, and tanks) attached directly to the midrod, the hull now taking the form of a mere protective sheathing. As in Nos. 2 and 3, two engines were used, which were mounted on a cross-frame of light tubing attached to the midrod at right angles. It had, as at first constructed, no provision for the generation of steam, but only for carrying a reservoir of carbonic acid to supply gas for the engines.
The whole, including wings, tail, and engines, but without the carbonic acid reservoir, weighed 1898 grammes (4.18 lbs.). A cylindrical reservoir, weighing 521 grammes (1.14 lbs.) and capable of holding 1506 cu. cm. (92 cu. in.) was constructed for this purpose, and tested for 30 minutes with a pressure of 100 [p054] atmospheres. If the weight of the cylinder, with its contents and adjuncts, be taken as 800 grammes (1.76 lbs.), the total weight of the aerodrome was 2698 grammes (5.95 lbs.). The wings were plane surfaces of silk, stretched over a very light frame, with no intermediate ribs to prevent the wing from being completely distorted by the upward pressure of the air. Even if they had been sufficiently strong and stiff, the total surface of both wings and tail was but 2601 sq. cm. (2.8 sq. ft.) or approximately 0.5 sq. ft. of supporting surface to the pound, much less than was found adequate, even under the most favorable circumstances. The weight was much more than had been contemplated when the wings were designed, yet, if all the other features of the aerodrome had been satisfactory, and sufficient power had been secured, the work of providing suitable supporting surfaces would have been attempted. But as it was found that the engines when supplied with carbonic-acid gas were unable to develop anything like the power necessary to propel the aerodrome, and that the construction could be greatly improved in many other ways, this aerodrome was entirely rebuilt. The work of the engines with carbonic acid had been so completely unsatisfactory that the idea was entirely abandoned, and no further attempts to develop an efficient motor other than steam were made.
It now became realized more completely than ever before that the primary requisite was to secure sufficient power, and that this could be obtained only by the use of steam. This involved a number of problems, all of which would have to be solved before any hope of a successful machine could be entertained. In the first place, engines of sufficient power and strength, but of the lightest possible construction, must be built. Second, a boiler must be constructed of the least possible weight, which would develop quickly and maintain steadily steam at a high enough pressure to drive the engines. This demanded some form of heating apparatus, which could work under the adverse condition of enclosure in a narrow hull, and steadily supply enough heat to develop the relatively large quantity of steam required by the engines.
The first of these problems, that of procuring suitable engines, was at least temporarily solved by the construction of two engines with brass cylinders, which had a diameter of 2.4 cm. (0.95 in.), and a piston stroke of 5 cm. (1.97 in.). The valve was a simple slide-valve of the piston type, arranged to cut off steam at one-half stroke. No packing was used for the piston or the valve, which were turned to an accurate fit to the cylinder and the steam-chest respectively. In the engines built up to this time, the parts had frequently been soldered together, and a great deal of trouble and delay had arisen from this cause. In these new engines, however, as strong and careful a construction was made as was possible within the very narrow limits of weight, with the result that the engines, though by no means as efficient as those constructed later, were used in all the experiments of 1893 and also during the first part of 1894. [p055]
As soon as these engines were completed, in February, 1893, a test was made of one of the cylinders, steam being supplied from the boiler of the shop-engine. The experiments were made with the Prony brake, and showed that at a speed of 1000 revolutions per minute, the power developed from a single cylinder was 0.208 H. P., with a mean effective pressure in the cylinder of only about 21 pounds per square inch of piston area, allowing a loss of 25 per cent for the internal resistance of the engine. This pressure was so much less than should have been obtained with the steam pressure used, that it now seems evident that the steam passages and ports were too small to admit and exhaust the steam with sufficient rapidity to do the work with the same efficiency that is obtained in common practice. This, however, was not immediately recognized. The piston speed at 1000 R. P. M. was 328 feet per minute, at which speed the steam at a pressure of 80 pounds should have been able to follow up the piston and maintain almost, if not quite, full boiler pressure to the point of cut-off, but it did not do so.
The problem of generating steam was much more difficult and required a long and tedious series of experiments, which consumed the greater part of the year before any considerable degree of success had been attained. In the course of these experiments many unexpected difficulties were encountered, which necessitated the construction of special forms of apparatus, which will be described at the proper point. Numerous features of construction, which seemed to be of value when first conceived, but which proved useless when rigorously tested, will be noted here, whenever a knowledge of their valuelessness may seem to be of advantage to the reader.
The boiler was necessarily developed simultaneously with the development of the heating apparatus, and in the following pages, as far as possible, they will be treated together; but often for the sake of clearness and to avoid repetition, separate treatment will be necessary.
At the beginning of these experiments, there was much doubt as to whether alcohol or gasoline would be found most suitable for the immediate purpose. An alcohol burner had been used in connection with the earliest aerodrome, No. 0, but from the results obtained with it at that time, there seemed to be little reason to hope for success with it. It is to be premised that the problem, which at first seemed insoluble, was no less than to produce steam for something like 1 H. P. by a fire-grate, which should occupy only a few cubic inches (about the size of a clenched hand) and weigh but a few ounces. It had to be attacked, however, and as alcohol offered the great advantage of high calorific properties with freedom from all danger of explosion, it was at first used.
Early in 1893, it occurred to me to modify the burner so as to make it essentially an aeolipile, and in April of that year the first experimental aeolipile model shown at A (Plate 12) was made. It was very small and intended for the [p056] demonstration of a principle rather than for actual service, but the construction of this small aeolipile was an epoch in the history of the aerodrome. It furnished immensely more heat than anything that had preceded it, and weighed so little and worked so well that in May the aeolipile marked B was made. In this design two pipes were led from the upper portion of the cylinder, one to a large Bunsen burner which heated the boiler, the other to a small burner placed under the tank to vaporize the alcohol. This was followed by the one shown at C, wherein the heating burner was smaller and the gas pipe, leading to the main burner, larger.
Figures D, E, F, and G (Plate 12) were really continuations and improvements of the same idea. In C there was simply a tube or flue through the tank; in F, however, this tube discharged into a smoke-stack fastened to the end of the cylinder, while in G the flue turned upward within the tank itself and discharged into the short stack on top. The object of these changes was to increase the draft and heating power of the small flame, so that the gas would be more rapidly generated and a greater quantity be thus made available for use under the boiler in a unit of time. They were, however, though improvements in a construction which was itself a great advance, still inadequate to give out a sufficient amount of heat to meet the excessive demands of the required quantity of steam. The boilers in connection with which these aeolipiles were used must now be considered.
The first boiler E (Plate 13) made during this year was a double-coil boiler of the Serpollet type, formed of 19 feet of copper tubing having an internal diameter of about 18 inch. Attached to the boiler was a small vertical drum, from the top of which steam was led to the engine, a pipe from the bottom leading to the pump. This boiler was tested in April with an alcohol heater, the pump in this trial being worked by hand. This apparatus developed a steam pressure varying from 25 to 75 pounds, which caused the engines to drive a 60 cm. propeller of 1.25 pitch-ratio 565 revolutions per minute. The greatest difficulty was experienced in securing a sufficient and uniform circulation in the boiler coils. The action in the present case was extremely irregular, as the pressure sometimes rose to 150 pounds, driving the engines at a dangerous speed and bending the eccentric rod, while at other times it would fall so low that the engines stopped completely.
As the pump used in this trial had proved so unsatisfactory and unreliable, it was replaced by a reservoir of water having an air-chamber charged to 10 atmospheres, the flow from which could apparently be regulated with the greatest nicety by a needle valve at the point of egress; but for some reason its performance was unsatisfactory and remained so after weeks of experiment.
There was used in connection with this device the double-coil boiler shown at F (Plate 13) which was made of tubes flattened so as to be nearly capillary. The idea of this was to obtain a larger heating surface and a smaller volume of [p057] water, so that by proper regulation at the needle valve, just that quantity would be delivered which could be converted into steam in its passage through the coils, and be ready for use in the engines as it left the boiler at the farther extremity. The results obtained from this were an improvement over those from the original coil, and a third set of coils (G, in Plate 13) was made. This boiler consisted of three flattened tubes superposed one over another.
These two boilers were tried by placing them in a charcoal fire and turning on an alcohol blast, while water from a reservoir under constant air pressure was forced through them past a pin valve. The result was that the two-stranded coil supplied steam at from 10 to 40 pounds pressure to run the engines at about 400 revolutions per minute. The pressure rose steadily for about 40 seconds and then suddenly fell away, though the coils were red-hot, and neither the water nor the alcohol was exhausted—apparently because of the irregularity of the supply of water, due to the time taken by it after passing the valve to fill the considerable space intervening between that point and the boiler.
An attempt was made to overcome this difficulty by putting a stop-cock directly in front of the boiler so that the water, while still under the control of the needle valve, could be turned in at once; the alcohol blast was also arranged to be turned on or off at pleasure, and provision was made, by taking out the end of the flue inclosing the boiler, to provide for an increased air supply. With this arrangement a flame eight or nine inches long was obtained, but a test showed that not more than 25 grammes of water per minute passed through the tubes, which was not enough.
Further tests with these boilers were so far satisfactory as to show that with the flattened-tube Serpollet boiler, comprising from 60 to 80 feet of tubing, from 80 to 100 pounds pressure of steam could be maintained, but not steadily. As there were difficulties in flattening the tubes to make a boiler of this sort, a compromise was effected in the construction of the one shown at H (Plate 13), which was made of light copper tubes 5 mm. in diameter, laid up in three lengths of 6 metres each. The ends of these coils were so attached to each other that the water entering at one end of the smallest coil would pass through it and then enter the middle coil, whence it passed through the third or outer coil. Two sets of these coils were made and placed in the thin sheathings shown in the photograph. Repeated experiments with these boilers demonstrated that the pressure did not rise high enough in proportion to the heat applied, and that even the pressures obtained were irregular and untrustworthy. The principal difficulty still lay in maintaining an active and uniform circulation through the coils, and for this purpose the water reservoir under constant air pressure had proved itself inadequate. This pointed to a return to the use of the force pump, the construction of which had hitherto presented so many special difficulties that it had been temporarily abandoned. [p058]
A further difficulty experienced in the use of these boilers had been that of obtaining dry steam for the engines, as during the early experiments the steam had been delivered directly into the engines from the boiler coils. But in August the writer devised a chamber, known as the “separator,” where it had an opportunity to separate from the water and issue as dry steam, or at least approximately dry steam. This was an arrangement familiar in principle to steam engineers under another form, but it was one of the many things which, in the ignorance of steam engineering the writer has already freely admitted, he had to reinvent for himself.
At about the same time, a new pump was designed to drive the water from the bottom of the separator, which served the double purpose of steam drum and reservoir, into the coils. This pump had a diameter of 4.8 cm., and was run at 180 strokes per minute.
The result of the first experiments with these improvements demonstrated that, within certain limits, the amount of water evaporated is proportional to the circulation, and in this boiler the circulation was still the thing that was at fault. Finally, the results of the experiments with the two-stranded, triple-coil boiler may be summed up in the statement that it was possible to maintain a pressure of 80 pounds, and that with it the engines could be made to develop from 0.3 to 0.4 H. P. at best. It weighed 650 grammes (1.43 pounds) without the asbestos jacket.
About this time the writer had the good fortune to secure the temporary services of Dr. Carl Barus, an accomplished physicist, with whose aid a great variety of boilers were experimented on.
The next form of boiler tested was that shown at N (Plate 13), made on a system of coils in parallel, of which there were twenty complete turns. In the first test it generated but 20 pounds of steam, because the flame refused to work in the colder coils. The work of this boiler was very unsatisfactory, and it was only with the greatest difficulty that more than ten pounds pressure could be maintained. There was trouble, too, with the circulation, in that when the flame was in full play the pump seemed to meet an almost solid resistance, so that it could not be made to do its work.
A new boiler was accordingly made, consisting of three coils of four strands each. With this the pump worked easily, but whereas it was expected to get 120 pounds pressure, the best that could be obtained was 70 pounds. The outer coil was then stripped off, and a trial made in which everything ran smoothly and the pressure mounted momentarily to 90 pounds. After some adjustment, a mean pressure of 80 pounds was obtained, giving 730 revolutions of the engine per minute, with an indicated horse-power of 0.32.
It was shown in this work that, within certain limits, steam is generated most rapidly when it is used most rapidly, so that two engines could be used [p059] almost as well as one, the reason apparently being that the rapid circulation increased the steam generating power of the boiler, and that the engines worked best at about 80 pounds. It was also found that a larger tubing was better than the small, weight for weight, this fact being due to the greater ease with which circulation could be maintained, since fewer coils were necessary in order to obtain the same external heating surface. The pressure in the coils and the separator was also much more nearly equalized. The result was that the boiler temporarily approved was one made of tubing 6.35 mm. (0.25 inch) in diameter, bent into a two-coil, two-stranded boiler, having sixteen complete turns for each strand in each coil. The total weight was 560 grammes (1.23 pounds) with a total heating surface of 1300 sq. cm. (1.4 sq. ft.).
The separator used in the experiments made during August and September was of a form in which the water was forced below a series of partitions that prevented it from following the steam over into the cylinders of the engines. It weighed 410 grammes (0.9 pound) and was most conveniently worked with 700 grammes (1.54 pounds) of water. The boiler and separator together weighed 970 grammes (2.1 pounds).
A new separator was, however, designed, which was horizontal instead of vertical, as it was intended that it should be placed just below the midrod. Another form, devised for constructional reasons, consisted of a cylinder in which a pump was imbedded. Heretofore the pump used had been single-acting, but it was now proposed to make a double-acting pump. Upon testing this apparatus, it was found that when using an aeolipile, it took 150 grammes of alcohol to evaporate 600 grammes of water. It was evident that the latter was used very wastefully, so that the thermal efficiency of the engine was not over one per cent; but it was also evident that, under the necessity of sacrificing everything to lightness, this waste was largely inevitable.
About the middle of October, another boiler (O, Plate 13) was made, which consisted of two coils wound in right and left hand screw-threads, one fitting loosely over the other, so as to make a cylindrical lattice-work 32 cm. (12.6 in.) long. Each coil contained two strands of copper tube 0.3 mm. thick, and weighing 54 grammes to the metre (0.036 pound to the foot). The inner coil had a diameter of 5.63 cm. (2.22 in.), with nine turns of tube to the strand, the two strands making a length of 319 cm. (10.5 feet) for the coil. The outer coil had a mean diameter of 6.88 cm. (2.71 in.) and a length of 388 cm. (12.7 feet) for the two strands. The total length of the two coils was, therefore, 707 cm. (23.2 feet), with a heating surface of about 1415 sq. cm. (1.52 sq. ft.) and a total weight of 382 grammes (0.84 pound).
The results obtained with this boiler were so far satisfactory as to show that, under the most favorable conditions, when air was supplied in unlimited quantities and there were no disturbing currents to put out or interfere with the work [p060] of the burners, steam could be supplied at a sufficient pressure to run the engines. It was realized, however, that the conditions in flight would be very different, and that in order to protect the apparatus from the wind, some sort of protecting covering would have to be devised, which would of itself introduce new difficulties in providing the burners with a proper and uniform draft.
The hull, as at first constructed, consisted of a cylindrical sheathing open in front, through the rear end of which the boiler and aeolipile projected inward, so that the air taken in at the front would be drawn through the boiler and hearth to the exclusion of lateral currents. In the first tests, however, after the hull had been applied, it was impossible to secure a proper rate of combustion, nearly the whole hull being filled with a bluish flame, while only a very small portion of the gases of combustion passed into the coils of the boiler. The remedy for this lay in obtaining an increased draft, and a small stack was, therefore, arranged to carry off the products of combustion. This proved inadequate, and it was only after several weeks of experiment with various types of smoke-stack, and constant alteration of the aeolipile, that it was possible to make the apparatus work [p061] efficiently when it was inside the hull. Finally such a degree of success was attained that the burners could be kept lighted even when the aerodrome was placed in a considerable artificial breeze, created by a blower in the shop.
In connection with these tests of the engines and boilers, some method was desired, in addition to the Prony brake tests, by which the thrust of the propellers when driven by the engines at various speeds could be measured accurately and in terms which would be readily available in judging whether the aerodromes were ready to be given an actual trial in free flight. Such a method was found in the use of an apparatus known as the “pendulum,” which was introduced near the end of 1892, but was not generally used until the end of 1893. After this time, however, this test was made a condition prerequisite to taking any of the aerodromes into the field, and proved of the greatest assistance in estimating the probable outcome of the trials.
The apparatus used, which is diagrammatically shown in Fig. 10, was extremely simple both in theory and operation. It consisted primarily of a horizontal arm AC carrying the knife-edge B by which it is pivoted on each side on supporting beams not shown. Depending from AC is the light vertical arm DE, rigidly joined to it and carrying the lower horizontal arm FG, all of which are braced together so as to maintain the arm DE constantly perpendicular to AC. To this arm FG the model was rigidly attached with its center of gravity in line with the vertical arm DE and its weight increased by the addition of properly disposed flat weights, in order to make the angle of lift for a given thrust of the propellers smaller and less likely to interfere with the working of the boiler and separator.
Before the actual test of the “lift” could be made, it was necessary to know the exact distance of the vertical center of gravity of the model and the extra weights from the knife-edge B. This was determined by the following method: A known weight was suspended from the arm AB at some arbitrarily selected distance from the point B. This weight caused the perpendicular arms AB and DE to rotate through an angle, θ, which was measured on the scale KL. Knowing, then, the weight on the arm AB, its point of application, the weight of the aerodrome suspended on the arm DE, and the angle of rotation, it is easy, by a simple application of trigonometric functions, to determine the distance of the center of gravity of the model from the point B.
In a test of Aerodrome No. 6 made on September 23, 1898, the weight suspended from AB was 10,000 grammes, its point of application 50 cm., the model was weighted to 20,450 grammes, and the angle of rotation, θ, was 7° 2′. Letting y equal the distance of the CG from B, we may equate the balanced forces thus:
Having determined this distance, the weight on AB was removed and the aerodrome was allowed to regain its former position. The distance of the center of thrust from B was then measured. The engine was next started and the number of revolutions of the propellers counted by a tachometer. The thrust of the propellers, acting perpendicularly to the arm BD, produced rotation around the point B, the angle of which was measured as above.
In the power test of No. 6, the following data were obtained:
As the propeller thrust and the weight of the model are forces acting in opposite directions at known distances from a center of rotation, letting L equal the “dead lift,” we may express the equation thus:
The flying weight of Aerodrome No. 6 was 12,064 grammes, and the per cent of this weight lifted was, therefore,
This was much more than was necessary for flight, but in order to insure successful flights and avoid delay, the rule was made in 1895 that no aerodrome was to be launched until it had previously demonstrated its ability to generate enough power to maintain for at least two minutes a lift of 50 per cent of the total flying weight. At the same time other important data were obtained, such as the steam-pressure, the time required to raise sufficient steam, the total time of the run, and the general working of the boilers and engines.
As will easily be seen, these tests afforded a most satisfactory basis of judging what the aerodromes might be expected to do in actual flight if the balancing were correct.
At this time, October, 1893, the aerodrome (Old No. 4) was practically complete, and the most anxious thought was given to lightening it in every way consistent with the ever-present demand for more power, which necessitated an increase in the weight of both burners and boilers to supply the requisite steam.
On November 14, when the aerodrome was prepared to be shipped to Quantico for trial, its condition was about as follows. The steam-generating apparatus—the parts of which were of substantially the forms last described, although some slight improvements had been introduced—had been developed to [p063] such a point that a pressure of from 70 to 80 pounds of steam could be maintained for 70 seconds, when it was tested in the shop. What it would do under the unfavorable conditions imposed by flight was to be learned only by trial.
At this pressure, the engines, the efficiency of which had been increased by an improvement in packing, would develop approximately 0.4 indicated H. P., while at 105 pounds pressure they at times developed as much as 0.8 H. P. When the aerodrome was tested on the pendulum, these engines, when making less than 700 revolutions per minute, lifted over 40 per cent of the total flying weight.
The propellers used at this time were accurate helices, having a diameter of 60 cm., a width of blade of approximately 36 degrees, and a pitch-ratio of 1.25. They were formed of wood, and were bushed with brass where they were attached to the shafts.