Chapter VII: History of Construction of Frame and Engines of Aerodromes (2)

A summary of these experiments, together with some account of the difficulties encountered and the results finally obtained with the apparatus in use at the end of the year, is given in the following report, which was prepared by Dr. Barus in December, 1894.

“If water be sprayed upon a surface kept in a permanent state of ignition, any quantity of steam might be generated per time unit. Similarly advantageous conditions would be given if threads of water could be passed through a flame. In practice this method would encounter two serious difficulties, the importance of which is accentuated when the boiler apparatus is to be kept within the degree of lightness essential in aerodromics. These difficulties are (1) the danger of chilling the flame below the point of ignition or of combustion of the gases, and (2) the practical impossibility of maintaining threads of water in the flame. For it is clear that the threads must be joined in multiple arc, so as to allow a large bulk of water to circulate through the boiler, whereas even when there are but two independent passages for the water through the furnace, it is hard to keep both supplied with liquid without unduly straining the pump. If the water be even slightly deficient, circumstances will arise in which one of [p071] the passages is better than the other. This conduit will then generate more steam and drive the water under force through the other passage, increasing the temperature discrepancy between them. Eventually the hot passage reaches ignition and either bursts or melts. This is what sooner or later takes place in boilers adapted for flying machines and consisting of tubes joined in multiple arc, when a single moderately strong circulating pump supplies the system.

“To avoid these annoyances, ‹i. e.›, to increase the length of life of the boiler, the boiler tubes are joined in series to the effect that a single current of water may flow successively through all of them. It is needful therefore to select wide tubes, such as will admit of an easy circulation in consideration of the length of tubing employed without straining the pump and at the same time to allow sufficient room for the efflux of steam. Other considerations enter here, the bearing of which will be seen presently: if the tube be too wide the difficulty of coiling it on a mandrel of small diameter is increased, while at the same time the tube loses strength (‹cæt. par.›) in virtue of the increased width.

Diagram 1. Diagram 2.]

“It is from considerations such as these that, in the course of many experiments, copper tubing about 8 mm. in diameter has been adopted. Copper is selected because of its freedom from internal corrosion, easy coiling, and because of its availability in the market. The thinnest tube to be had (walls only 0.1 mm. thick) will withstand more pressure than can be entrusted to the larger steam receivers in circuit with the boiler. The boiler weight is thus a negligible factor, and it is quite feasible to reduce the thickness of boiler tubing, by the superficial application of moderately strong nitric acid, to 200–400 grammes per horse-power of steam supplied. External corrosion due to flames occurs only in case of deficient water, and if the boiler be made of tubing with the walls 0.2 mm. thick, it is in view of the possibility of such accidents. Boilers may then be tested to 25 atm. without endangering the metal.

“Boilers are wound or coiled with regard to the two points above suggested, viz.: to avoid chilling the flame the successive turns are spaced on all sides, and to bring the water as nearly into the flame as possible, the diameter of the coils is chosen as small as expedient. Further reasons for this will presently be adduced. The type of boiler eventually adopted is shown in the accompanying diagrams, 1 and 2, Fig. 11.

“Diagram 1, is a perspective diagram showing the plan of winding and Diagram 2, an end view. The circulation is indicated. There are two inner coils [p072] each containing about 17 turns, wound on a mandrel 5 cm. in diameter. The turns are spaced so as to allow about 1 cm. clear between successive turns. The outer coil envelopes both, and in this there are about 3 cm. between successive turns, and 8 turns in all. Length, say, 30 cm., breadth 16 cm., thickness 10 cm., give the external dimensions of the boiler. The shell space between outer and inner layers of tubing must nowhere be less than 1 cm. When so wound, the inner coils (here as in other boiler forms) raise about 80 per cent or more of the steam; the outer or enveloping coil, while not quite useless, make the most effective frame work for the boiler jacket which has been devised. The coils are brazed together by blind tubes, as shown in Diagram 2, to keep the whole in shape. Weight with couplings and cover when complete 535 grammes.

“The cover is preferably of mica, through which the flame within the boiler may be seen, and in which lightness, nonconduction, and resistance to the disintegrating effects of high temperature are met with in a pronounced degree. This jacket is held down by copper bands and the end band is continuous with the long smoke-stack, as will be presently shown.

“The wide form of boiler with two coils within the envelope is not absolutely essential. The same amount of steam can be generated from one coil in an envelope in other respects equal to Diagram 1 if a sufficiently hot flame be passed axially through the coils. Such a flame, however, is unstable, and for this reason two milder flames with a good air access are to be preferred on practical grounds even if the weight is thereby increased.

“To further understand the boiler construction it is advisable to consider the action of the flame. Inasmuch as wide tubes must be used, the problem of evaporating water as fast as possible is equivalent to getting heat into the current (water and steam circulating through the coils) as fast as possible from without. If, therefore, ‹t› is the mean temperature of the fluids within the coils, and ‹T› the effective temperature surrounding the tube, then the rate at which heat will flow into the tubes is proportional to ‹T›−‹t›. Now ‹t› the temperature of the steam is nearly constant (100°–150°) whereas ‹T› the effective flame temperature may vary from 800° to, say, 1600°. It is for this reason that the heat sponged up by the boiler depends almost directly on the flame temperature.

“What conditions, therefore, will make the flame effectively hot?

“(1) The coils must obviously be brought as nearly into the flame as feasible: for this purpose the cylindrical helix is better than any other form. But

“(2) The turns and coils must not be so crowded together as to chill the flame into imperfect combustion in various parts of its extent. Hence the loose form of winding. Again

“(3) There must be oxygen enough to allow complete combustion, and

“(4) The flame itself must be hot and the radiation checked by good jacketing.

“To take up the last points: the effective heat of the flame depends not only on the combustion heat of the fuel used; it depends also, among other things, on the speed with which this combustion takes place. A flame burning from a low pressure of alcohol gas will be at low temperature as compared with a flame burning from high pressures of the gas. If the flame be burnt from a Bunsen burner in the usual way it is an interesting question to know how flame temperature will vary with gas pressure. At present we know it merely in steam pressures incidently produced in a given engine (No. 4) as for instance:

Flame pressure, 10 lbs., 20 lbs., 30 lbs. ┐ Steam pressure, 40 lbs., 80 lbs., 120 lbs. ┘ in the running engine.

“Unfortunately there is a limit set to this process of increasing the steam supply, quite aside from conditions inherent in the method. This is due to the fact that a certain speed of efflux cannot be exceeded without putting the flame out. Suppose, for instance, in Fig. 12, that a gas generated from a liquid is ignited at the end of the Bunsen burner ‹F›; then if the velocity of efflux of mixed gas and air in the direction ‹AB› from the mouth of ‹F› exceeds the velocity of combustion in the direction ‹BA›, the flame will obviously be carried away from the mouth of the tube and dissipated. This state of things is actually realized at pressures exceeding about 15 lbs., depending on the degree of mixture of the combustible gases used, and therefore on apparently haphazard conditions connected with the jet, the air holes, the air supply, etc.

“If, however, the velocity of the jet at the point of efflux be checked by an obstruction like a cylinder ‹C›, Fig. 13, placed co-axially with the burner tube ‹F›, the speed of combustion will no longer be exceeded (supposing ‹C› properly chosen) and flames will then burn from high-pressure gas. In this way flames were maintained generated from alcohol gas at even 40 lbs. and above.

“The gas escaping from the Bunsen burner is never sufficiently aërated to burn completely. Otherwise there would (in general) be explosions in the tube ‹F›. A part of this air is supplied at the mouth of the boiler ‹B›, Fig. 14, and the amount available here will depend on the velocity of the jet ‹F›. Hence it does not follow that a high-pressure burner like that in Fig. 11 will supply a proportionate amount of heat, since its jet suction is not intense and the combustion within the boiler is incomplete. This difficulty may be remedied by placing [p074] air holes in the jacket of the boiler, provided the boiler be wrapped loosely enough not to chill the flame below ignition. It is with reference to this effect that the boilers, Fig. 11, were wound. A number of rifts ‹aaa›, Fig. 15, are then left in the jacket through which air may enter in virtue of the burner flame acting as a jet at the mouth of the boiler.

“When so constructed the flame at first enters the inner coil only; but after a little while it suddenly spreads out throughout the whole interior space and envelops the coils. This sudden expansion is due, probably, to the assumption of the spheroidal state by the water within the coils, the current now flaring on an enveloping cushion of steam. The pump must work well, for deficient water means a hot tube and deficient steam, or eventually a rupture of the tube.

“Thus far the dependence for draft has been on the burner jet and the suction of the smoke-stack in virtue of the inertia of the moving gases. But even with this ventilated boiler, this method is limited to certain dimensions of the boiler. Thus a boiler 80 cm. long yielded about the same quantity of steam as a boiler half as long and otherwise similar. Only the initial parts of the boiler are, therefore, relatively efficient, and the reason of this seems to be that, apart from shape, etc., the flame as a heat-producing agent is practically defunct, when a certain amount of heat has been taken out of it: in other words, even with fair ventilation the flame is eventually chilled off by the voluminous products of combustion continually accumulating in the boiler. The same choking action accompanies the presence of unburnt gases. If, for instance, the flame be burnt in the air, it is slender and much smaller in volume than in the boiler. The flame is also of small volume and burns completely in a wide boiler, but the steam is always deficient, because of the distance between flame and coils (see above). With the above apparatus about 1/2 lb. of dry steam per minute per square foot of heating surface was attained.

“This introduces the final condition for rapid steam generation. There must be artificial suction at the smoke-stack. By passing the exhaust steam in the form of a central jet through the smoke-stack the yield of steam was increased 20 to 30 per cent. In fact as the supply of gas from the burner is given, the artificial suction in question means more air in the boiler for the same amount of gas and it means also a more rapid removal of the exhaust gases. The experiments with steam suction are yet to be completed, and with them the boiler question is to be finally laid at rest. The chief points at issue are these:

“1. Seeing that the jet suction increases with the length of the smoke-stack, up to a certain length at least, how long and how wide must the efficient smoke-stack be made? Thus a smoke-stack 10 cm. long is all but useless. Good results are obtained when the stack measures 30 cm. in length beyond the end of the steam jet. [p075]

“2. What is the relative efficiency of the initial and final halves of the length of the boiler? This will show in how far it is useful to increase the length of the boiler for a given burner and steam jet. It will also show what advantage is to be gained from triplicate boilers with three burners, as compared with duplicate boilers with two burners, or single boilers with one burner, ‹when the same weight› of tubing is used throughout.

“3. What is the effect of pressure on the aeolipile tank, or in how far does the steam generated depend on what may be called the pressure of the flame? This is also an important point which remains for quantitative solution. It can be approached in two ways: either by finding the steam evaporated in terms of the tank pressure, or by finding the temperature of the flame pyrometrically.

“4. What speed of water circulation best conduces to steam generation? A good pump is now installed by which the circulation can be varied. If water can be put into the boiler just fast enough to come out dry steam at the other end, the efficiency ought to be a maximum, but it does not follow that it will be so, for one can imagine a wet circulation sponging up more heat than one which is just dry at the end.”

1895

During January and February, 1895, the experiments with boilers and burners were continued and even better and more uniform results than those given above were obtained. The boilers of Aerodrome No. 5 were finally brought to such a state of efficiency, that under favorable conditions a lift of nearly sixty per cent of the flying weight was secured. This was much more than was required for flight, but it was decided to postpone the trials until No. 4 could also be made ready for a test and the frame of No. 5 could itself be strengthened in many weak places.

Upon examining No. 4, which had been put aside since the trials in December, it was found to have rusted so badly throughout and to be so unfit in every way for trial, that a complete reconstruction of the whole would be necessary. So many advantages had been gained in No. 5 by the double midrod type of construction that it was decided to rebuild No. 4 on a modification of the same plan, as shown in Plate 11, retaining, however, the same engines which had been used before.

In this a very guarded return was made to the type which had proved so unsatisfactory in No. 0, that is, making the hull support rods at the front and rear for attaching the wings and tail. In this case, however, the hull was constructed very rigidly, and the tubes at the front and rear were firmly attached and braced so that they could withstand a considerable strain without undue distortion. The work on this frame was completed in March, but the other parts were not in entirely efficient condition even in May, when the aerodromes were taken to Quantico for trial. Moreover, it was found that the weight of this aerodrome had increased far beyond the original estimates. [p076]

In view of the disasters from trials in the field, due to inability to obtain automatic equilibrium in flight and to the flexure of the large wings rather than to defects of the engines, the conditions at this time, after three years of failure, seemed so nearly hopeless, that without abandoning the work on these steam aerodromes, I again had recourse to the early plan of constructing smaller models driven by India rubber, in which the small wings employed could be made of the requisite stiffness. Instead of employing twisted rubber, however, the defects of which had been amply proved in previous trials, these new constructions were meant to employ rubber directly stretched and pulling. In this condition the rubber exercises nearly six times the power in proportion to weight that it does when twisted, but on the other hand it requires a very strong frame and subordinate parts.

I spent an inordinate amount of time and labor during this year in attempting to employ this latter form of construction and finally got a few useful results from it, but none in proportion to the labor expended.

During March, Aerodrome No. 5, the frame of which had proved on test to be radically weak, was completely refinished except for the wings. The propellers had hitherto been made of wood, but in May, I commenced a new construction of steel, wood and cloth, on a plan giving a figure which, though not rigorously helicoidal, was practically near enough to the theoretical form and was also both lighter and more elastic than the wooden construction.

On May 8 and June 7 Aerodrome No. 5 was again tried at Quantico, and although the tests were unsuccessful, in that the aerodrome failed to fly, partly because of the fact that so much time was spent in raising steam that practically the entire supply of fuel and water was exhausted before the aerodrome was actually launched, yet it had come so much nearer flying than any machine had previously done, that it was felt that if either the power could be increased or the weight decreased even a slight amount, the aerodrome would probably fly. In view of the great care that had been exercised in keeping down the weight, it seemed almost hopeless to attempt to reduce it, and it also seemed equally hopeless to attempt to get more power without increasing the weight. However, something had to be done to increase the ratio of power to weight, and as it was seen that this would involve extensive changes in No. 5, it was decided to entirely rebuild No. 4 with this idea in view, though it was evident that it involved a plan of construction even lighter than the dangerously light plan on which No. 4 had already been constructed.

During Mr. Langley’s absence in Europe in the summer, Aerodrome No. 4 was entirely reconstructed and made to embody many new characteristics, the changes introduced being so radical that this model was henceforth designated as “New No. 4.” The new characteristics of this model were its unprecedentedly [p077] light frame and the elevation of the transverse frame 12 centimeters above the midrod, whereby the position of the line of thrust was raised so that it was 20 centimetres from the center of pressure, which from theory seemed to be very nearly its correct position. The total flying weight was but 6400 grammes (14 pounds), with a total supporting surface of fifty-four square feet, equivalent to very nearly four square feet per pound. It was hoped that with this extremely light construction the “dead lift” would amount to a large percentage of the flying weight, and as much as sixty per cent was actually lifted on the pendulum. As, however, the aerodrome approached completion it became more and more evident that the construction was hopelessly fragile, the frame being scarcely able to support itself in the shop. By November this conclusion became certain, and this aerodrome (New No. 4) was never put to an actual test in the field. The very expensive set of wings covered with gold beater’s skin, which were also constructed at this time for this model, proved so weak under test that they were entirely abandoned.

When Mr. Langley returned to Washington in the fall, many important points, which had been under special consideration during the past year, particularly those relating to the disposition of sustaining surfaces, and the provision of automatic equilibrium, were still not definitely determined. It was not yet decided whether two sets of wings of equal area should be used for the aerodrome, or what the efficiency per unit of area of the following surfaces was in comparison with the leading surfaces. To aid in determining these and other important points concerning the relative position of the center of gravity and the center of pressure in the horizontal planes, he had several small gliding models made, which could be used with either one or two pairs of wings, and afforded an opportunity for testing and comparing several types of curved surfaces.

These models were built so that the center of gravity could be adjusted to any desired point, and had in addition, as a means of assisting in preserving equilibrium, a small tail-rudder, shaped somewhat like a child’s dart, which was intended to support no part of the weight.

The tests with these models were very satisfactory and aided greatly in the final development of what is known as the “Langley type.” Indeed, in the single month of November all the points, which had hitherto been more or less indefinite, were finally decided upon, and the tests of the following spring proved these decisions correct.

Two sets of wings of equal area were hereafter provided for every aerodrome, which not only greatly increased the stability, but also overcame the difficulty hitherto experienced in bringing the ‹CP› over the ‹CG›. The tail-rudder, formed of planes intersecting at right angles, was adopted as the means of control. In use on the aerodromes it was set at a negative angle, and given a certain [p078] degree of elasticity, which was at first provided in the frame of the rudder, but was later given by a flat wooden spring, by which it was attached to the aerodrome. The tail in this form now became the sole means of controlling the equilibrium, and the results obtained with it were so very satisfactory that no further attention was given either to the gyroscopic control built during the previous summer, or to any of the electrical forms of control constructed prior to that time, all of which involved more or less delicate apparatus.

The definite form into which these ideas crystallized is perhaps best exemplified in the letter of instructions issued by Mr. Langley on November 30, 1895 to the men employed on the work. The text of this letter is given in the Appendix, and the forms referred to in it for recording the weights and adjustments of the aerodromes are those used in the data sheets after this time.

In October work was resumed on Aerodrome No. 5, on which nothing had been done since its test on June 7. The reconstruction of “Old No. 4” into “New No. 4” which had occupied the entire summer, and the final result of which was the production of a machine so radically weak as to be useless, had been so discouraging that it seemed vain to attempt in any way to decrease the weight of No. 5. The addition of the rear wings in place of the tail had, however, so greatly increased the supporting surface that it seemed possible that No. 5 might now be able to fly with no greater engine power than it had on June 7. Some weak places in its frame were, therefore, strengthened and the midrod at the front was raised five centimetres in order to raise the center of pressure farther above the center of gravity and give the front wings a greater range of adjustment. Some slight changes were also made in the gearing which drove the pump, so as to make it work faster, and new burners, boilers and a gasoline tank were constructed during November. Later the midrod, which had formerly consisted of two separate pieces attached at the front and rear respectively of the main frame, was made continuous, and in order to avoid passing it through the smoke-stack, the stack was made to fork at this point. These changes are clearly shown in Plates 14 and 15, which are photographs taken on December 3. This plan was, however, soon changed so that the midrod passed through the smoke-stack and was rigidly attached to the frame at several points, and a new pump and new boilers were substituted for those which had been worn out. Aside from these changes, which although small, added very materially to the general strength of the frame, no important changes were made in No. 5 prior to its remarkable flight of May 6, 1896.

AERODROME NO. 5, DECEMBER 3, 1895. PLAN VIEW. RUDDER REMOVED]

AERODROME NO. 5, DECEMBER 3, 1895. SIDE VIEW]

While these changes were being made in No. 5, similar ones were also being carried out in New No. 4, and the addition of the rear wings to No. 4, together with other slight changes, made it such a distinctively different machine from what it had been, that it was now designated as No. 6. After making extensive [p079] repairs to the extremely light frame of No. 6 (formerly New No. 4) it was thought to be in suitable condition for flight and was accordingly boxed preparatory to sending it to Quantico.

The year, therefore, closed with No. 6 apparently in condition for test, but it was decided not to take it to Quantico until No. 5, which was still undergoing repairs, could also be got ready.

1896

A few days after the beginning of the new year, while the repairs on No. 5 were being completed, it was decided that the frame of No. 6 which had been boxed ready to be carried into the field for trial, was so weak that before putting it to an actual test in flight it would be best to make some tests on the strength of its frame. While testing the frame for torsional strength, it broke under the moderate test of a weight of 500 grammes placed at the tips of the wings, the angle of deflection just prior to its breaking being 35°, while the frame of Old No. 4 in March, 1895, had shown a deflection of only 10.5° under a similar test. This breaking of the frame showed very plainly that the worst fears in regard to it had been realized and that by some means or other the frame must be strengthened. This was finally accomplished by making the midrod continuous through the smoke-stack as had already been done in No. 5, and at the same time an additional improvement was made in the means of attaching the Pénaud tail, whereby it was lowered in order to give it a greater clearance in passing under the launching car in actual test. Later the boilers proved defective and new ones were substituted, but except for some minute details no further changes were made in Aerodrome No. 6 prior to its test in May.

On May 6, No. 6 was unsuccessfully tried at Quantico just prior to the very successful test of No. 5. In this test no serious damage was done to the frame, but before going to Europe in the summer, Mr. Langley ordered that both aerodromes be completely overhauled and put in condition for further experiments in the fall. In this remodelling practically no changes were introduced in the frame of either No. 5 or No. 6, but the engines of No. 6 were refitted and a new boiler was substituted, which, with slight improvements in the burner, resulted in a somewhat increased power in the engines.

A complete description, giving all essential details of both Aerodromes Nos. 5 and 6, will be found in Chapter X.

[p080]

CHAPTER VIII HISTORY OF CONSTRUCTION OF SUSTAINING AND GUIDING SURFACES OF AERODROMES 4, 5 AND 6