CHAPTER V
CONSTRUCTION OF FRAME OF LARGE AERODROME
The general plan for the large aerodrome was never a matter of uncertainty. At the time when the first general designs were made there had been in the history of mankind only one type of machine, that of the steam-driven Langley models, which had proved capable of flight for any considerable distance. Furthermore, the selection of this type had been the result not of sudden fancy or of purely theoretical consideration, but of years of the most careful experimentation, in the course of which nearly every conceivable style of machine had been tested with some form of power. It would have been worse than folly, therefore, if the one clear path had been left to seek some unknown way.
It was fully realized from the first, however, that the increase in size alone would make necessary in the design for the large aerodrome a great many modifications from the designs of the steam-driven models. It was not possible here, as in nearly every other kind of structure, simply to magnify uniformly the parts and proportions of the small machine in order to obtain a successful large one. This is particularly true in the case of the aerodrome, because the rapid increase of weight in the larger structure is out of all proportion to the increase in strength, while it is very desirable that the more expensive machine which is designed to carry a human being shall be relatively even stronger than the easily replaced model. This problem of increasing size without sacrificing strength and stability, it was known from the beginning, would be encountered in a particularly difficult form in designing the frame of the large machine, and was to be solved not by the discovery of some new and wonderfully strong material, but by improvements both in the general plan and the details of the machine. Here, as is often the case, it was not the large changes in the design but the improvements in small and sometimes seemingly unimportant details which demanded the most careful consideration and, as a whole, contributed most to the final result. For this reason, as well as because the large changes, when pointed out, are usually easily understood, the present chapter is for the most part a description of the improvement of details.
From the experience gained in the construction of the frames of the several steam-driven models, it was decided that the frame for the large aerodrome must consist essentially of two principal parts. First, a rigid backbone was required, extending from the point of attachment of the front wings to the point of attachment of the rear wings; and this backbone, for convenience designated [p165] the “main frame,” must support the second principal part, the “transverse frame,” which formed a cross with the main frame, and at the ends of which the propellers were mounted. While it was necessary that this transverse frame should have considerable rigidity and strength in a vertical direction, yet its main strength and stiffness was required in the horizontal plane for withstanding the thrust of the propellers. It had been possible to construct the frames of the later steam-driven models stiff enough, and at the same time light enough, by the use of properly proportioned steel tubing, but calculation very soon showed that in order to secure sufficient rigidity for the frame of the large aerodrome and at the same time keep the weight within the permissible limit, it would be necessary to depend very largely on guy-wires and to use tubing only for forming the struts against which the guy-wires should act. But this obviously introduced a new series of problems. The extensive system of guy-wires necessary would add materially to the head resistance of the aerodrome, and this might conceivably be so great as to require more propulsive power than would be required for a frame heavier but unincumbered by the head resistance of the wires. It became necessary to consider these problems, but no data were accessible from which the head resistance could be computed with any confidence. The coefficient of resistance for a cylindrical body moving through the air in a direction perpendicular to its length may in general be taken as one-half that of a flat body of the same cross-section; but it was thought very certain that, owing to the fact that tightly stretched wires are in constant vibration when the aerodrome is in the air, the resistance of the wires must be considerably greater than would be calculated from treating them as cylinders having a coefficient of 0.5. Unfortunately, no data on the resistance of vibrating wires were at hand. Before proceeding with the designs for the guying of the frame, therefore, the following brief series of tests was made in November, 1898, on the whirling table, in order to learn approximately the resistance that the proposed system of guy-wires for the large aerodrome would offer:
MEASUREMENTS OF THE RESISTANCE OF GUY-WIRES, USING FRAME ATTACHED TO “BALANCE.”
RESISTANCE OF FRAME WITHOUT WIRES.
Frame consists of: 4 tubes, 1 cm. diameter, 14.5 cm long; 2 tubes, 1 cm. diameter, 41 cm. long; 2 tubes, 1 cm. diameter, 101 cm. long.
Revolutions of Velocity of Resistance. Calculated resistance turn-table per frame. Feet Grammes. of frame. minute. per minute. ‹r.› Grammes. 6.75 608 11.5 14.2 9.75 877 34.0 29.6 12.0 1080 51.8 44.8 16.35 1475 97.0 83.8 19.75 1775 134.0 121.3 22.7 2045 168.0 161.2 25.5 2290 205.0 202.0
RESISTANCE OF FRAME WITH 1ST SET OF WIRES.
First set of wires: 16 wires, 0.6 mm. diameter, 102 cm. long; 6 wires, 0.6 mm. diameter, 42 cm. long.
Revolutions Velocity of Resistance Resistance Calculated of turn-table wires. Feet of frame of wires. resistance per minute. per minute. and wires. R_1−r=r_1. of wires. Grammes. Grammes. R_1. 9.75 877 47.5 13.5 8.88 12.0 1080 73.5 21.7 13.47 13.75 1237 93.5 25.5 17.65 17.25 1550 144.0 37.0 27.7 20.25 1822 187.0 45.5 38.4 22.50 2025 216.0 47.5 47.4 22.875 2060 225.0 52.0 49.0 24.56 2215 250.0 56.5 56.7
RESISTANCE OF FRAME WITH 2D SET OF WIRES.
Second set of wires: 15 wires, 1.2 mm. diameter, 102 cm. long; 2 wires, 1.2 mm. diameter, 42 cm. long.
Revolutions Velocity of Resistance Resistance Calculated of turn-table wires. Feet of frame of wires. resistance per minute. per minute. and wires. R_2−r=r_2. of wires. Grammes. Grammes. R_2. 9.25 833 54.0 26.5 15.35 9.35 841 55.0 27.0 15.4 11.3 1018 82.0 36.75 22.65 11.5 1035 82.0 35.25 23.4 13.0 1170 104.5 43.0 29.9 13.15 1185 105.0 42.5 30.60 16.7 1505 160.0 59.0 49.5 16.75 1510 160.0 58.0 49.9 19.5 1755 196.0 64.0 67.4 19.7 1770 203.0 69.5 68.5 21.60 1945 236.0 77.0 82.6 21.65 1950 237.0 77.75 83.2 21.75 1957 235.0 75.5 83.7
RESISTANCE OF FRAME WITH 3D SET OF WIRES.
Third set of wires: 15 wires, 2 mm. diameter, 102 cm. long; 2 wires, 2 mm. diameter, 42 cm. long.
Revolutions Velocity of Resistance Resistance Calculated of turn-table wires. Feet of frame of wires. resistance per minute. per minute. and wires. R_3−r=r_3. of wires. Grammes. Grammes. R_3.
9.25 833 65 37.5 25.2 11.55 1040 91 43.5 39.35 11.55 1040 101 53.5 39.35 15.25 1375 160 75.0 68.7 18.1 1630 203 86.5 96.6 19.25 1735 221 92.0 109.5 19.25 1735 216 87.0 109.5 20.63 1860 237 90.5 125.8
The last column of these tables is calculated for a coefficient of form equal to 0.5, which has been found to be approximately correct for a rigid cylindrical body.
These tables are not sufficiently extensive to determine accurately the exact resistance that wires of various sizes will offer at given velocities, or to serve as the basis for the deduction of formulæ, and were not made for that purpose. However, from the above data, and the curves plotted in Plate 44, it will be seen that some unexpected results were obtained.
[p167]
These results are fairly well summarized in the following general statements: First, that the coefficient of resistance increases to some degree as the size of the wire is decreased; second, that in the case of wires of the size which it was expected to use, and at approximately the soaring speed of the aerodrome, the resistance is certainly not greater than 75 per cent, and more probably less than 50 per cent of the resistance encountered by a flat surface of the same projected area; third, that the coefficient of resistance did not seem to be increased by the vibration of the wires. On the contrary, it was noted during the experiments that when they reached a speed which just caused them to “sing,” there was a marked diminution in the resistance. This statement is made, however, with some reserve, for it is probable that the singing of the wires was due to vibration in the horizontal plane, and it is not definitely known what the effect would be of vibration in the vertical plane.
To make the very extensive experiments necessary to determine these propositions conclusively would have required much more time than could at this period be spared from the actual constructional work on the aerodrome. Nevertheless, the data did seem to indicate that it was at least not unwise to employ the extensive system of guying which had been planned in order to give the necessary strength to the frame of the large aerodrome. This plan of construction was, therefore, definitely adopted, and as a result of later experience the system of guying was still further extended.
As the transverse frame had to be made comparatively rigid in order to prevent undue binding of the bearings of the transmission and propeller shafts, it was necessary to make it intrinsically stronger and, therefore, heavier in proportion to its size than the main frame. The main frame, although requiring great strength to enable it to withstand the strains, both torsional and direct, which were imposed upon it by the weights which it supported, did not need excessive rigidity, and could, indeed, be distorted an appreciable amount without danger of any serious effect on the action of the wings or rudder; but even a small amount of distortion in the transverse frame might easily cause such friction at the bearings of the shafts as to absorb fifty per cent or more of the engine power.
In the photographs, Plates 45 to 48, which show the actual condition of the frame on January 31 and February 1, 1900, the letters ‹A›, ‹B›, ‹C›, ‹D›, ‹E›, ‹F›, ‹G›, ‹H› and ‹I› designate parts of the main frame, ‹A› and ‹H› being the rear and front midrods, respectively, to which the wings were to be attached. ‹B› and ‹I› are curved extensions of the starboard main tube, the port main tube being exactly similar, and ‹C›, ‹D›, ‹E›, ‹F› and ‹G› are cross-tubes which connect the midrods to the port and starboard tubes. ‹R› is the front main tube of the transverse frame, the rear main tube being exactly similar, and both being connected to the main tubes of the main frame where they cross them. The ends of the main tubes of the [p168] transverse frame are joined together by the “bed plates” ‹L›, which are of I-beam section, and have mounted on their outer faces the bearings which support the propeller shafts. At ‹V› are bevel gears mounted on the propeller shafts, which are driven by co-acting bevel gears, ‹M›, mounted on the outer ends of the transmission shafts, ‹O›, the latter being at this point firmly supported in bearings mounted on the inner faces of the bed plates and steadied by the intermediate bearings, ‹N›. The two transmission shafts are seen to be not in line, the rotary cylinder engine that was then under construction requiring this arrangement. The bed plates, ‹L›, are further stiffened by the brace tubes, ‹K›, and the transverse frame is braced against the thrust of the propellers by the tubes ‹J›. The four tubes, ‹P›, unite at their upper ends to form what was designated as the upper “pyramid,” and the wires, ‹S› and ‹T›, radiate from its apex to the rear and front, respectively, of the main frame. The lower “pyramid,” on the under side of the frame, also has similar wires running fore and aft. The main portions of both frames are further strengthened by their sub-frames, which merge together, and the main tubes of the main frame are individually stiffened in the vertical plane by a minor system of guying. The scales shown in the photographs are calibrated in metres.
It is to be particularly noted that the midrod, which had heretofore formed the backbone of the main frame, was now made to act merely as a means of attaching the wings to the frame, the main strength of the frame being furnished by the two parallel fifty millimetre tubes which extended the entire length of the frame and which, reinforced by the guy-wires, formed a truss not only more rigid transversely, but also many times stronger in its ability to resist torsional strains than could be secured by a single tube of equal weight. In this plan of constructing the main frame, the pyramids constituted a very important element, for with the guy-wires arranged as they were it was impossible for any portion of the frame to experience a stress which was not transmitted in some way to the pyramids. In the frame, as here shown, these pyramids were formed of tubes 15 mm. in diameter, 0.5 mm. thick, stiffened against buckling under the end pressure by means of the cross-braces, which united them near their midpoints. While the sole function of the upper pyramid was to serve in the system of guying the frame, the lower pyramid not only served a similar purpose, but also provided a means for holding the aerodrome to the launching car in the process of launching it, the clutch-hooks gripping around the short horizontal tube at the apex of the pyramid and thus drawing the “bearing points” of the machine firmly against the uprights on the car. In fact, the particular arrangement of these pyramids was largely determined by this necessity for providing means for holding the aerodrome to the launching car, and the form which seemed best suited to the purpose was duplicated on the upper side of the frame.
[p169]
The “bearing points” were not attached to the frame at the time these photographs were taken, but are seen leaning against the scales in the foreground of Plate 46. Their position on the frame will be more clearly seen in later photographs, where it will be noted that they were made use of in the more elaborate system of guying which was adopted.
While, in general, the frame at this time seemed to be reasonably stiff and strong, yet it was subjected to a very thorough test by supporting it at different points and suspending from it weights to represent the various parts, such as engine, aviator, wings, rudder and so forth, the deflections which were produced by these weights being carefully noted. It was further tested by subjecting it to vibratory strains, such as it would be likely to meet in actual use. After this the whole frame was tested against torsional strains, such as would be caused by the wind twisting one set of wings more than the other. As a result of these tests it was decided that the frame should be strengthened as far as it was possible to do so without greatly increasing the weight, which even now was found to be rapidly increasing beyond what had been calculated as permissible. The main guy-wires were replaced by heavier and stronger ones, and while these were found to add somewhat to the stiffness of the frame, yet something more seemed necessary to insure safety.
The delay in securing the engine, which had been contracted for with a guarantee that it would be delivered in February, 1899, had become so serious and had delayed the completion of the frame to such an extent that the question of building an exact duplicate of the large machine, but of one-quarter its linear dimensions was being carefully considered at this time, and it was decided to make no further changes in the guying of the large frame until after the small one was built. On account of its smaller size changes could be more readily and cheaply made on it, and the advantages of different methods of guying could be just as well studied. Later, when this was completed, it was found that, with the same system of guying that had been used in the larger frame, the model was so very stiff that it did not require any further strengthening, the smaller scale, of course, accounting for the difference. What was thought to be the best system to follow in strengthening the frame of the large machine was, however, first tried on the smaller one, and it was found that for a very slight increase in weight a very great increase in strength could be obtained. This change in the system of guying consisted essentially of building a “trestle” of tubing at a point on the upper side, midway between the pyramid and the rear end of the frame. One of the former sets of guy-wires which passed to the rear of the frame was then replaced by a set which started at the foot of the rear tubes of the upper pyramid, passed over and was fastened to the trestle, and from there passed to the rear end of the frame at the points where the longer guy-wires from the pyramid had formerly been attached. The [p170] guy-wires on the lower side of the frame, at the rear, were correspondingly changed so that the upper and lower systems should be similar, the wires which started from the main tubes at the foot of the pyramid passing to the bearing points, and from there to the rear end of the frame.
In order to keep the main frame of the large aerodrome as short as possible, it had originally been planned to make the distance between the center of pressure of the front wings and the center of pressure of the rear equal to five metres. When these same proportions were followed in the quarter-size model, it was found that it brought the rear wings so close to the propellers that their lifting effect was certain to be interfered with by the blast of air created by the slip of the propellers. It was therefore decided that all things considered it would be best to increase this distance between the wings, even though this involved an increase in weight, partly on account of the increased amount of tubing, and still more on account of the guy-wires which it would be necessary to add in order to make up for the weakness due to increased length. The large aerodrome frame was accordingly lengthened 2.5 feet (76.2 cm.), and the guy-wire system was changed to that clearly shown by the photographs of July 10, 1902, Plates 49, 50 and 51, the black cross-lines on the background being 50 centimetres apart. From an inspection of these photographs it will be seen that two sets of guy-wires were carried from the upper and lower pyramids, respectively, towards the rear of the frame, the first set being carried to the main tubes at the foot of the “trestle” and the bearing points, and the second set to these same main tubes at the second cross-tube. The sets of wires which started from the feet of the pyramids were carried over the “trestle” on the upper side and the bearing points on the lower side, and both joined to the main tubes at the rear cross-tube. Additional cross-guy-wires for stiffening the frame sideways were added in each of the squares formed by the junction of the cross-tubes with the main tubes. A secondary system of truss guy-wires running over short guy-posts attached to the tubes of the main frame also contributed to the strength and rigidity of the whole.
Although the pyramids had shown no signs of weakness, nevertheless, because of increased strains due to the lengthening of the main frame, it was thought advisable to make them stronger. Instead of the 15-mm. tubing, which had formerly been used, 25-mm. tubing of the same thickness was therefore substituted, and additional cross-braces were added, as will be seen from the photographs, and from the scale drawings in Plates 52, 53 and 54, which show the aerodrome as it was when completed. The numerals attached to these drawings refer to the detail drawings shown in later plates.
In order to secure the proper adjustment of the guy-wires, not only of the frame but of many other parts, notably the wings, propellers and rudder, it was necessary to use a large number of turn-buckles. As almost every wire [p171] required at least one, and in some cases two turn-buckles, the weight represented by this single item rapidly became so formidable as to require serious attention. In the construction of the models, it had been necessary to employ some special turn-buckles in connecting the guy-wires of the wings to their guy-posts in order to secure the minute adjustment of the wires necessary to prevent the wings from being warped and distorted by unequal and improper adjustment. These turn-buckles had been made in the Institution shops, as the very lightest ones which could be secured in the market were from ten to twenty times as heavy as it was necessary for them to be to provide ample strength. In the construction of the large aerodrome, however, the large number required, and the desire to complete the machine at the earliest moment, made it advisable to procure the turn-buckles, if possible, from outside sources, and a very careful search was accordingly made among the various dealers. After much delay some bronze turn-buckles were secured which were very much stronger for their weight than any others on the market, but upon testing them it was found that while they weighed 45 grammes, their average breaking strength was only 593 pounds. Previous experience had shown that turn-buckles which would not break under a less load than 750 pounds could certainly be made to weigh not more than 18 grammes. As even at this time it was realized that at least 100 turn-buckles would be necessary for the entire machine, the excess weight which the heavy turn-buckles would add was felt to be absolutely prohibitory, and the construction of steel turn-buckles was immediately begun in the Institution shops. These turn-buckles were at first made in several sizes, and while some few were at first made “double ended,” most of them were threaded at only one end, the other end being provided with a swivel-hook, or eye. They were at first made of mild steel, the swivel-hooks, in fact, being made of wire nails in order to utilize the head of the nail as a shoulder without the expense of machining rod steel of a size large enough to form the shoulder. It was found, however, that the weak point of this type of turn-buckle was the swivel end, and most of those which were then on hand were made double ended by removing the hook, tapping a left-hand thread into this end of the shank, and fitting a threaded eye-socket in it. The guy-wires themselves were attached to the eyes of the turn-buckles and to the fittings on the frame by twisting loops at the ends of the wires, and although the very greatest difference in the strength of a completed guy-wire may result from the way in which the loops are twisted, yet, after much training, the workmen were taught to twist these very uniformly, following the plan which can be best understood by an inspection of the drawings in Plate 55 which show the loops more clearly than they can be described. After the loops had been properly twisted, soft solder was run all through the twist in order to unite firmly the twists of the wire. Although special grades of wire were found which showed very high tensile strength when the wire was [p172] tested without having loops formed in its ends, yet it appeared that the twisting of these high-grade wires so seriously affected them that in the case of guy-wires with loops at the ends, better final results could be obtained by using softer grades of steel. The wire which was actually found best, after much experiment, was a good grade of Bessemer steel of a medium hardness, which had been “coppered” to prevent rusting. However, even with the softer grades of steel wire, it was found that there were sometimes hard spots in the wire which revealed themselves only upon test, and that when a hard spot occurred in the twisted portion where the loop was formed, the final strength of the completed guy-wire was sometimes only twenty-five per cent of what it should be. The precaution was then taken to subject each of the completed guys to a test strain at least twenty-five per cent greater than it was calculated the wire would have to stand in actual use, so that no accident from defective wires would be likely to occur.
Later on, however, much trouble was caused by the loops in the ends of some of the guy-wires slipping, owing to the giving way of the solder which had been run through the joint, the amount of slipping, while small, being sufficient to alter completely the relative stresses on the various wires, thus causing distortion of the framework itself. In order to avoid this difficulty a new method was devised of attaching the guy-wires to the turn-buckles and to the fittings by which they were carried to the frame. This method consisted in threading the ends of the guy-wires so that they could be inserted directly in the threaded ends of the turn-buckles. The wires when connected in this way to the turn-buckles showed absolutely no slip, and the entire system gained greatly in strength thereby. The only disadvantage which was found in this new method of attaching the guy-wires to their fittings, was that if the wire was bent very close to the fitting, it would break in the screw thread very easily. But since most of the guy-wires when once attached to the machine are always tight, and in fact, under more or less strain, there was in most cases no likelihood of the wires being endangered by being bent close to the fittings. Since the screw threads, which it was necessary to adopt in this new plan of connecting the guy-wires, had to be very much finer than the threads which had been used in the turn-buckles previously constructed, it was necessary to make new turn-buckles, the others being too thin to permit of their being bored out, bushed and re-threaded. The new turn-buckles were made of a much higher grade of steel, and probably represent very nearly the maximum of strength for the minimum of weight possible without the use of some of the very much higher-grade steels which have recently come on the market, but which are exceedingly expensive to work. By means of this improved plan of attaching the wires,[42] it [p173] was found possible to gain practically fifty per cent in the strength of the entire system of guy-wires used on the frame.
Many small changes were from time to time made in the various small fittings by which the guy-wires were attached to the frame, nearly all of these fittings having been originally made of a very mild grade of steel owing to the fact that it was so very much easier to work. At the time these fittings were made it was constantly expected that a trial of the aerodrome would be possible very soon, and it seemed necessary to expedite the work as much as possible and avoid the delay involved in using grades of steel that would have been materially harder to work. As is always the case in work of this kind, retrospect shows many instances where what was supposed to be a short cut to results actually proved to be the longest path, but the work as a whole was remarkably free from imperfect parts which necessitated reconstruction.
In the construction of the frames of the models it had been customary to fit the tubing accurately at the joints and to join it permanently together by brazing, as this was not only the lightest form of joint that could be made, but also the most expeditious method consistent with securing a strength of the joint comparable with that of the tubing itself. The construction of the frame by this method of brazing the joints together permanently, offered, however, several serious drawbacks: among them, that when a tube got injured it was a considerable task to replace it, while the brazing of the new tube in place required extreme care to prevent the frame from being warped when completed, as the tube became longer while very hot and contracted after the joint had set. Furthermore, the great heat required destroyed to a considerable degree the desirable qualities due to the tube being “cold drawn,” a reduction of strength of something like 25 per cent being almost inevitable, even when the brazing was most carefully done. It was, therefore, decided that in the construction of the large machine all of the main joints should be made by a system of “thimbles,” and it was planned at first to make these thimbles by brazing short pieces of steel tubing into the proper shapes and angles so that they would accurately fit the tubes which were to be joined. The construction of the thimbles in this manner, however, seemed to involve an excessive amount of work; and, as it was found that very thin castings of aluminum-bronze could be obtained, which would show a tensile strength very nearly as great as steel, it was decided to make up patterns for the thimbles and cast them of aluminum-bronze.
The aluminum-bronze castings were obtained and properly machined to fit the tubes, but when it was attempted to “tin” the interior walls of the thimbles it was found that the solder could not be made to stick to the bronze. As a considerable amount of work had been expended on the machine work of these thimbles much time and effort was spent in attempting to devise “fluxes” [p174] and solders which could be made to work with the aluminum-bronze, but the final result was that the aluminum-bronze thimbles had to be abandoned. They were replaced by similar castings of gun-metal of a slightly heavier section, which at the time were thought to be very suitable for the purpose.
But, in finally assembling the frame after the changes described above had been made, steel thimbles, built up of short pieces of tubing, as had originally been planned, were substituted for the gun-metal thimbles. This change was made not only because of the great increase in strength, but more particularly because many of the gun-metal fittings had been imperfectly constructed, so that it was extremely difficult to align the frame. The steel thimbles, which were made in the Institution shops proved thoroughly satisfactory and gave no trouble of any kind. Many of these thimbles and the method of attaching the guy-wire fittings to them are shown in Plates 56 and 57, as well as in Plate 55.
TRANSVERSE FRAME
It will be recalled from the description of the models Nos. 5 and 6, in Part I, that the position of the line of thrust, with respect to the positions of the center of pressure and center of gravity in the vertical plane was, theoretically, very much better in No. 6 than in No. 5. In designing the large aerodrome, it was desired to reproduce as nearly as possible the relative position of the line of thrust with reference to the center of pressure and center of gravity which existed in No. 6, but for constructional reasons it was found impossible to do so. In fact it appeared that without seriously complicating the construction of the frame it was impossible to raise the line of thrust with respect to the center of gravity materially higher than it was in No. 5. In No. 6 the line of thrust was 12 centimetres above the midrod, this being effected by placing the engines some distance from the boiler, and at the extreme ends of the transverse frame where they were connected directly to the propellers. In the case of the steam engine the weight of the engine proper is a relatively small portion of the entire weight of the power plant, and it is, therefore, possible to put the engine almost anywhere without materially affecting the center of gravity. But where a gas engine is used the engine itself constitutes the greater part of the weight of the power plant, and any raising of the engine, therefore, materially raises the center of gravity of the whole machine. The line of thrust in the large aerodrome was, therefore, practically in the plane of the main frame, and consequently very little higher than the center of gravity.
The use of one engine to drive two propellers mounted at opposite ends of the transverse frame, and in a direction perpendicular to the crank shaft of the engine, necessitates the use of a pair of bevel gears between each of the propeller shafts and the shafts by which the power is conveyed to them from [p175] the engine shaft. Since the efficient transmission of power through bevel gears requires that they be very accurately placed with reference to each other, and maintained very accurately in this position while they are at work, it was necessary to make the transverse frame very rigid, especially at its extreme ends. This was accomplished by the use of what were called “propeller-shaft bed plates.” They are designated by the numeral 27 in Plate 54, and are shown in detail in Plate 58 as of a very deep I-beam section, having very narrow flanges top and bottom, the web of the I-beam furnishing the strength in a vertical direction, while sufficient stiffness laterally was obtained from the flanges, assisted by the brace tubes, which acted as struts between the bed plates and the main tubes of the transverse frame. These struts, while very light, added enormously not only to the lateral stiffness of the propeller bed plates, but furnished for a minimum weight a maximum prevention against twisting of the plates. The propeller-shaft bed plates were originally planned to be made of sheet metal with the flanges brazed to the web. But at the time that they were constructed the pressure of the work was so great in the Institution shops that it was found necessary to have some of the work done outside, and the parties who undertook the construction of these bed plates were unwilling to attempt to braze them up, and accordingly worked them from steel forgings made for the purpose. The expense of this plan of construction proved large and unnecessary, as both previous and later experience proved that it was not only practicable to braze up bed plates more complicated in their design than these, but that equal strength for equal weight could thus be obtained for less than one-quarter the cost of constructing them from solid forgings. Furthermore, where such parts are made from the solid, changes which later tests prove advisable can frequently not be carried out without very serious cost and delay, while with the bed plates formed by brazing less hesitancy is felt in removing parts which are brazed thereto and substituting new parts, or even discarding the bed plates altogether and substituting new ones. Particular emphasis is laid on this point for the reason that much expense and delay would have been avoided had these very expensive propeller-shaft bed plates been discarded as early as 1901 and replaced by others which would have permitted a considerable strengthening of the ball-bearings, which, while strong enough to stand even more power than they were originally designed for, were far too weak to be safe when working under the greatly increased stress due to the very much higher engine power which was later used. Instead of discarding these bed plates then for new ones, they were strengthened by brazing to them crescent-shaped pieces, as shown in the drawings and photographs. This strengthening was made necessary by the larger hole cut in the bed plates for the larger bevel gears. The bed plates for the engine, which are later described, besides other bed plates which were made for other purposes, were all [p176] formed by the use of sheet metal and tubing properly brazed together, and none of them ever gave any trouble.
In the early photographs of the aerodrome frame, especially that of January 31, 1900, Plate 45, it will be noted that the two transmission shafts, which extend from the propeller-shaft bed plates towards the center, are not in line, the port transmission shaft being at the center of the transverse frame, while the starboard shaft is three inches to one side. This arrangement was necessary in order to connect the shafts to the rotary cylinder engine which was being constructed under contract, and which was almost momentarily expected for more than a year after its original promise of delivery on February 28, 1899. Later, when this engine was finally found to be a failure, and the writer constructed the engine in the Institution shops, the starboard transmission shaft was moved over to the center line and the crank shaft of the engine, which was carried through on the center line of the transverse frame, was then connected directly to the inner ends of the transmission shafts.
These shafts, as well as the propeller shafts, were originally constructed of steel tubing 1.5 inches in diameter and 1/16 of an inch thick, but on account of the increased power of the large engine it was found necessary to increase the thickness of the shafts to 1/8 of an inch. Difficulty was also found with the tubing of which the shafts were made. This, though not exactly straight when received from the factory, could be pretty accurately straightened in the lathe by exercising proper care, but the moment any real strain was put upon it in the transmission of power, it again went out of shape and caused serious damage to the bearings by whirling, buckling, and so forth. As the skin of the tubing is really the strongest part, owing to the cold-drawing process to which it has been subjected, great care was taken to secure shafts which were sufficiently straight for use without machining, but it was finally found impossible to rely on the unmachined shafts, and all the later shafts for the aerodrome were made by getting tubing a sixty-fourth of an inch thicker than was calculated to be necessary and turning off this extra metal in a lathe.
Suitable flanges and collars were brazed to the propeller shafts; but, for convenience in assembling, the flanges by which the main transmission shafts were connected to the crank shaft of the engine were at first fastened to the shafts by screw-threads, the threads being in the proper direction to cause the flanges to jam against the shoulders of the shafts when the engine turned in its normal direction. This method of fastening, however, caused serious trouble, owing to the flanges jamming so tight that it became impossible to unscrew them after they had once been used in driving the propellers. The usual provisions of keys and key-ways adopted in general engineering practice, where solid shafts are employed, were, of course, out of the question, since the shaft would have to be greatly increased in thickness throughout its entire length [p177] merely to provide the extra metal at the small place in which the key-ways were formed. Taper pins either sheared off or very soon stretched the holes so badly as to leave the parts loose, and were otherwise very unsatisfactory. The method finally adopted, which proved very successful, was that of forming integral with the couplings shallow internal tongues and grooves which fitted corresponding tongues and grooves either in the exterior surface of the shafts or in collars brazed to them at the proper point. The form of flange coupling, in which bolts draw the two flanges tightly together, was also a source of considerable trouble and delay, which was finally overcome by forming shallow tongues and grooves in the faces of the flanges, the tongues taking up the torsion and relieving the bolts which held the flanges together of all strain except one of slight tension. The same difficulties experienced in mounting the couplings on the shafts were met with in connection with the gears, both on the propeller and transmission shafts, and were finally obviated in a manner similar to that described above.
The bevel gears originally constructed for transmitting the power from the transmission shafts to the propeller shafts, were made of case-hardened steel and were eight-pitch, twenty-five teeth, with three-quarter inch width of face. The gears were very accurately planed to give as perfect a form of tooth as possible, in order to avoid loss of power in transmission, and although the manufacturer who cut the teeth on them asserted at the time they were made that they would not be capable of transmitting more than five horse-power, yet they actually did transmit considerably more than twelve horse-power on each set; but they were not strong enough to transmit the full power of the large engine which was finally used. The gears that were finally used were similarly constructed of mild steel which was case hardened 1/64 of an inch deep after they were finished, there being thirty-one teeth in the gear on the transmission shaft and forty teeth in the one on the propeller shaft, the teeth being eight-pitch, three-quarters of an inch face. These light gears proved amply strong, and several times stood the strain which they accidentally received when one of the propellers broke while the engine was under full power, and thus threw the entire fifty horse-power over on the other propeller, which was consequently driven at a greatly increased speed.
Plain bronze bearings had been used throughout on the model aerodromes, but in the construction of the large aerodrome ball-bearings were used on all of the propeller and transmission shafts, not only on account of the decreased loss through friction, but also because ball-bearings can be built much lighter than solid bronze ones, and, furthermore, do not present such great difficulties in lubrication. However, owing to the limited size which it was possible to secure for these bearings, because of their having been originally designed for only twenty-four horse-power, and without any margin for a later increase of the [p178] space in which they had to be applied, they were never really large enough for the work they had to do when transmitting the full power of the large engine. They gave continual trouble, and were the source of delay which, while it cannot be accurately measured, since there were often other causes, yet might be conservatively estimated at not less than three or four months. Such a delay, when reckoned in retrospect, can easily be seen to have caused an expense which would have sufficed for almost any change in the bearings, bed plates, etc., had the change been made immediately after the bearings were found to give trouble. With the better steel which it is now possible to obtain for the races of the bearings, and with the high-grade balls now obtainable, the bearings could be readily replaced without changing any other parts and still be amply strong for the work.
PROPELLERS
Both the tests on the whirling-table and the actual results with the models had shown that propellers which were true helices formed out of wood were rather more efficient than those constructed by the use of a hub in which were inserted wooden arms, forming a framing over which cloth was tightly drawn. But the very great difference in the cost of construction and the facility with which the latter type could be repaired in case of damage—the wooden ones were practically of no use if once they were much injured-—made it seem advisable to construct all the propellers for the large aerodrome in the manner just explained. Several pair of small propellers had been built on this plan, some as early as 1895, and one very important advantage had been found to be possessed by this type besides cheapness and facility of repair. Wooden propellers of even so small a diameter as one metre had been found to suffer a quite appreciable bending of the blades, due to the thrust produced by them, even though the blades had been made of considerable thickness. In planning a propeller 2.5 metres in diameter for the large aerodrome it was seen that in order to make the blade sufficiently strong to withstand its own thrust it would be necessary to make it inordinately thick, which, of course, would mean a considerable increase in weight. In fact, it was seen that the weight of the larger propellers would increase practically as the cube of the diameter; which, for the 2.5-metre propeller, would involve a weight of something over fifteen times the weight of those one metre in diameter. The other type, which for convenience we will call “canvas covered,” permitted the bending moment produced on the blade by the thrust to be taken up by guy-wires running from the corners of the blades to a central post projecting from the hub of the propeller, and it was found that in this way a considerable saving in weight could be effected. [p179]
In November, 1897, in order to obtain by actual test some data on propellers, such as it was planned to use on the large aerodrome in case it was later built, it was decided to construct one propeller 2.5 metres in diameter and 1.25-pitch ratio with two blades, each covering the sector of 36 degrees on the projected circle. About this same time an engine builder, who some years before had made some experimental model engines in the Institution shops, proposed to construct a gasoline engine for the proposed large aerodrome. As past experience, not only with such engines but with all other forms of explosive motors, had not been very reassuring it was thought best to make brake tests of one of the heavier engines which he was at this time building, and at the same time make tests with one of these large propellers. A first series of tests was made at several different speeds, and then a second series was made with the engine driving the propeller at the same speeds. The engine varied so much, however, in the power developed at any speed that the data obtained were of little value. As it was also desired to learn just how much thrust could be obtained from these propellers, when driven by a given horse-power, a special hand car was fitted up to carry the engine, which was connected to a shaft on which the propeller was mounted. The propeller was raised above the floor of the car and projected over the rear end of it so as to be as little disturbed as possible by the deflection of the air currents caused by the car. This car, with the engine and propeller, was tested on a track near Mount Holly, N. J., in November, 1897, but the results were very unsatisfactory. In the first place, the car with the engine mounted on it was so very heavy and offered such a strong tractive resistance that very little speed of propulsion could be obtained. In the second place, the engine, which was said to have furnished over six horse-power on Prony-brake tests, evidently did not furnish anything like this amount of power at this time. And in the third place, the propeller was evidently far too large to permit the engine to run at the speed at which it would develop a reasonable amount of power unless some reduction gearing were interposed between it and the propeller. As the tests, for various reasons, had to be made at a great distance from Washington, and the supervision of them had to be entrusted by Mr. Langley to others, who either did not understand or appreciate the value of obtaining accurate data, it was found impracticable to continue them.
The large propeller used in these tests was built without special regard to weight, since it was expected that it would be subjected to rather rough usage under the very sudden strains produced by the irregular working of the gas engine. Its hub was made of brass tubing, the horns being brazed to rings which were slid over a central tube, the rings being finally soldered to the tube after the arms had been adjusted to the positions which would give the blade the correct shape and dimensions. The wooden arms were 1.5 inches in [p180] diameter at the hub end, tapering to 1.25 inches at the end of the blade. The blade was exceedingly stiff as regards pressure produced by thrust, but it was found to be considerably strengthened and made very much safer when guy-wires were added, in the manner explained above. This general type of construction was adhered to in all the future propellers for the aerodrome, though slight modifications, both as to the size of the arms and the number and position of the cross-pieces which formed the framing of the blade, were adopted from time to time. A pair of heavy propellers, 2.5 metre, 1.25-pitch ratio, 36-degree blade, the hubs of which were formed of brass castings, was, however, constructed for experimental purposes, where weight was not an important factor.
When these propellers were designed, the calculations as to their size and the horse-power which would be required to drive them at a certain speed were based on the very incomplete data obtained from the various propeller tests conducted during the preceding years. When later calculations were made for them, on the data obtained in the more accurate tests made in the summer of 1898, it was found that the power of the engines with which it was proposed to equip the aerodrome would not be sufficient to drive the propellers at anything like the speed which the former calculations had shown would be possible; and that, therefore, either the ratio of the gearing between the propellers and the engine would have to be changed so as to permit the engine to run at a very much higher speed than the propellers, or that propellers, having either less pitch or a smaller diameter, and possibly both, would have to be substituted for these larger ones.
Since it was easier to change the propellers then to change the gearing, a new set of propellers was designed which were of 2 metres diameter, with a pitch ratio of unity, and with a width of blade of only 30 degrees. It was calculated that 20 horse-power would drive these two propellers at a speed of 640 R. P. M., when the aerodrome was flying at a speed of 35 feet per second and the propellers were slipping about 50 per cent, this being found to be about the speed at which the engines might be expected to develop their maximum power. As the larger propellers having the brass hubs were thought to be excessively heavy, the hubs weighing 10.25 pounds each, and as any change either in size, pitch, or width of blade necessitated a new set of patterns in case the hubs were cast, it was decided to construct the new hubs of steel tubing. The weight was further reduced by decreasing the size of the wooden arms to 1-1/4 inch in diameter at the hub, tapering to 1 inch at the end of the blade.
After the engine builder in New York had been unable to fulfil his contract on the engine, and it had been condemned, propeller tests were made with the experimental engine built in the Institution shops. These tests showed: First, that the results which might be expected from larger propellers could be very safely predicted by extrapolation from the results of the propeller tests of 1898; [p181] and, second, that in order to get a thrust which would equal fifty per cent of the flying weight of the aerodrome it would be necessary to use propellers larger than two metres in diameter unless a very large surplus of power were provided. It was accordingly decided to make a set of propellers intermediate between the two-metre, unit-pitch ratio, thirty-degree blade ones, and the original ones which were two and one-half metres, one and one-quarter-pitch ratio, thirty-six-degree blade. A set was, therefore, designed two and one-half metres in diameter, unit-pitch ratio, and thirty-degree width of blade, the hubs being made of steel tubing brazed up in the same manner as the two-metre ones, and the wooden arms of the blades being one and three-eighths inches in diameter at the hub end, and tapering to one inch at the end of the blade.
Later, when the larger engine was actually tested in the frame, the inability of the original transmission and propeller shafts to stand the extra strain caused by the engine starting up very suddenly at times, together with the unsatisfactoriness of the screw-thread method of fastening the gears and couplings to the shafts made it necessary to provide new shafts, gears, couplings, etc. It was then decided to change the ratio of gearing between the engine and the propellers, which had been one to one, so that the engine might run faster and, therefore, permit the use of larger propellers. For constructional reasons the ratio chosen was thirty-one to forty, thus making the engine run approximately one-third faster than the propellers.
In the various tests made of the engine working in the frame there were two or three instances in which the propellers were damaged either by the sudden starting of the engine or by their not being able to stand the strain to which they were subjected by the power absorbed, but in every case such breakages were found to be due to imperfections of the brazing in the joints. While, therefore, it would have been desirable to make the propellers somewhat heavier, yet since the total weight of the aerodrome had been growing so very rapidly, it was felt that this need not be done, as a pair of propellers which had stood quite severe service in shop tests might reasonably be expected to stand the strain of actually propelling the aerodrome through the air.
Nevertheless, when in the summer of 1903 the actual trials of the large aerodrome were started, it was found that the very important difference between a propeller working in a closed room and one working in the open air had not been given due consideration. Several sets of propellers, 2.5 metres in diameter, unit-pitch ratio, 30-degree blade had been constructed and were on hand, in order that no delays might be caused through a lack of such extra parts. On September 9, 1903, when the aerodrome frame without the wings was mounted on the launching car on top of the boat for some trial runs with the engine to make sure that everything was again in readiness, before the engine had made 500 revolutions, the port propeller broke; and a few minutes [p182] later, when a new propeller had been substituted for this and the engine was again started up, the starboard propeller also broke. When, upon further trials and replacements of propellers, all had been so thoroughly demolished that there was not a complete set remaining, it was seen very clearly that the strains produced on a propeller working in the open air are very much greater than those produced in shop tests, where the air is necessarily quiet. These open-air tests of the propellers had demonstrated that their weakest point was where the steel tubes which received the wooden arms of the blade terminated, and that another, though not so serious, point of weakness was where the steel arms were brazed to the central hub, the thin metal tending to tear loose even before the brazed joint would give way. It was, therefore, decided to construct immediately a new set of propellers in which the steel arms should be made of much heavier tubing, that is, a sixteenth of an inch thick at the end where it was brazed to the central hub, and tapering in thickness to one-thirty-second of an inch at the other end. These arms were further made twelve inches long in place of being only three inches long as before. This added length carried the steel out beyond the point where the first section brace joined the three arms together, and where they were further strengthened by having the cloth covering tightly stretched around them. In order to utilize such of the hubs of the former propellers as had not been seriously damaged when the propellers broke, it was also decided to try the effect of merely adding an extra length of tube to the short arms by means of a thimble slipped over and brazed to the two parts, which would make these arms twelve inches long. The construction of these propellers was pushed as rapidly as possible; and after their completion no further trouble was at any later time caused by insufficient strength of the propellers. Even in the test of October 7, 1903, when the aerodrome came down in the water at a speed of something like fifty miles an hour, and at an angle of approximately forty-five degrees, no break occurred in either propeller until, when the aerodrome was plunging through the water, a blade of one propeller was broken by the terrific blow which it received when it struck the water under the impulse of the engine driving it at full speed. The severity of this blow is attested by the fact that the shaft, which was of steel tubing one-eighth inch thick, was twisted about ninety degrees.
This experience with propellers very strongly emphasizes the fact that on any flying machine the strains which are apt to be met with in the open air must be allowed for in the proportioning of the parts of the machine. But since an indiscriminate increase of strength in all the various parts of the machine would entail a prohibitory weight, very careful judgment, based on experience, will have to be exercised in deciding just where added strength must be employed, and also where the “live strains” are not apt to exceed very appreciably the calculation for statical conditions.
[p183]
Owing to Mr. Langley’s belief that the tests of the man-carrying aerodrome must not only be made over the water, but that it was necessary that the machine be launched from a car running on a track at a considerable elevation in order to permit the machine to drop a short distance after being launched in case it was not quite up to soaring speed when launched, it was necessary that the aerodrome be so constructed that it could be readily transported to the launching track from the interior of the house-boat where it was stored. This plan of storing the main body of the machine in the interior of the boat and hoisting it to the launching track just before attempting a flight (some of the difficulties of which may be more clearly appreciated by an inspection of Plate 60), made it necessary that the wings, tail and guy-posts be so constructed as to be readily attachable to and detachable from the main frame, and since the weather conditions are seldom suitable for a test for more than a couple of hours at a time, it was necessary that the mechanism employed for attaching these parts be so arranged that the proper settings of the different parts could be quickly obtained, and without requiring the exercise of judgment which past experience had shown did not often manifest itself during the hurry of the preparations for a test. While the wings, therefore, were made removable, yet all of the sockets, guy-wires, etc., which were loosened in removing them, were made with positive stops on them so that each fitting that was to be tightened up in assembling could be adjusted to its definitely determined position.
As all of the models had been constructed with these same parts removable in order to permit them to be readily shipped back and forth in the many trips which had been made with them from Washington to Chopawamsic Island, the same details of arrangement were used for attaching these parts on the large aerodrome, though the actual fittings by which the parts were attached in the latter case became more elaborate.
In the drawings, Plates 52, 53 and 54, the method of attaching the wings to the frame is clearly shown. Each of the two main ribs of each wing was secured to the midrod of the frame by a wing clamp, shown in detail in Figs. 1, 2, 5, 6 and 7 of Plate 59. Figs. 1 and 2 show the clamp for the middle main rib of each pair of wings, and Figs. 5 and 6 show the clamp for the main front rib, the latter being so constructed that the wings could be rocked on the midrib clamp as a pivot and secured at any angle of lift desired from 6-1/2 degrees to 15 degrees. The horns on each clamp merely acted as receiving sockets for the ends of the ribs, and were not in any way intended to do anything more than merely hold the ends of the ribs in their correct positions. The wings were fastened to the frame by the guy-wires which ran from two points on each main rib to an upper and a lower guy-post mounted on the midrod. The system of guy-wires for the wings is clearly shown in Plates 52, 53 and 54, and [p184] in Plate 61, which shows the aerodrome mounted on its launching car at the rear end of the track, and with the front pair of wings in place and all the guy-wires adjusted. The details of the guy-posts are shown in Plate 62, where it will be noted that the lower guy-post was of wood, with metal fittings, and was 2 metres long from the center of the midrod to the bottom, while the upper guy-post was a steel tube 109 centimetres long from the center of the midrod to its top. The guy-wires from the middle rib of each of the pair of wings were fastened to the fittings at the bottom of the lower guy-post, while the wires from the front main rib were fastened to the fittings which were brazed and riveted to the slidable collar, which was mounted on the steel tube forming the cap on this guy-post. This collar was made slidable to permit the angle of lift of the wings to be readily changed without affecting the length of the guy-wires. This collar, when once set for any particular angle of the wing, was prevented from sliding by a taper pin (not shown) which passed through it and the guy-post. In order to secure the wings more rigidly to the main frame and thereby throw on it all torsional strains from the wings, which it was specially designed to take, each of the middle main ribs was secured to one of the main tubes of the main frame by an auxiliary clamp at the point where this rib crossed the main tube. These auxiliary clamps are clearly shown in Figs. 3 and 4 of Plate 59.
Projecting from the lower end of each of the lower guy-posts was a five-sixteenth-inch steel rod about one inch long, as clearly seen in Plate 62. Brazed to the side of this rod, in such a position that it would project towards the rear of the aerodrome when the guy-post was in position, was a small arm or bracket. When the guy-post was in place with the aerodrome on the launching car, this pin was in a slot formed in a metal cap on the top of the small folding upright at the front or rear of the car, as seen in Fig. 1, Plate 63, while Fig. 2 of Plate 63 shows the pin just being inserted into this slot as the guy-wires of the guy-post are being fastened. This small arm or bracket on this rod projected under the cap to prevent the rod of the guy-post from being lifted out of the slot in the folding upright, when the wind acting under the wings tended to lift the aerodrome from the car. Particular attention is here called to this apparently insignificant detail, for it was this arm or bracket on this small rod of the front guy-post which, hanging in the cap on top of the folding upright, caused the accident in the launching of the aerodrome on October 7, 1903. Certain it is that but for the accident due to this apparently insignificant detail, success would have crowned the efforts of Mr. Langley, who above all men deserved success in this field of work, which his labors had so greatly enriched.
[p185]
AVIATOR’S CAR
In determining on a suitable car for the aviator various designs were made, differing all the way from that in which the aviator occupied a sitting position facing directly ahead and with practically no freedom of movement, but was even strapped to the machine to avoid the possibility of being thrown out, to the one finally adopted, in which he was provided with the greatest freedom of movement, could either stand or sit, as the occasion seemed to demand, and could face in any direction for giving proper attention to any of the multitudinous things which might at any time require his attention, and could, if agile, even climb from the extreme front of the machine to the rear. The wisdom of giving the aviator complete freedom without hampering him in any way by provisions for preventing his being thrown out of the machine was amply justified, as will later be seen in the description of the tests of the machine, where freedom of movement and agility prevented a fatal accident.
The aviator’s car was therefore designed to occupy the entire available space between the engine and the front bearing points, and between the two main tubes of the main frame, thus allowing him a space of something like three feet by five feet. The car itself was shaped like a flat-bottomed boat, the bottom being approximately level with the bottom of the lower pyramid. It had a guard rail of steel tubing eighteen inches above the floor, with a cloth covering drawn over the frame to decrease the head resistance of the appurtenances of the engine which were placed at the rear end of the car. The car was supported by vertical wires passing from its bottom up to the main frame, and was prevented from longitudinal or side motion by being fastened at the front to the cross-rod connecting the front bearing points, and at the rear to the lower pyramid. A light wooden seat extended fore and aft of the car at a height of about two feet from the floor, this seat resting on blocks of sponge rubber to absorb some of the tremor which existed in the whole aerodrome when the engine and propellers were working at high speed. The aviator was thus free to stand, to sit sidewise or to straddle the seat, and while the network of wires surrounding him prevented any great possibility of his being thrown out, yet there was a comparatively large opening between the guy-wires passing overhead which permitted him to climb out of the machine.
In order to enable the aviator to know exactly how the engine was operating, a tachometer, giving instantaneous readings of the number of revolutions, was connected by a suitable gear to one of the transmission shafts and placed where it could readily be seen.
During 1898 and 1899 considerable time and attention had been given to designing an instrument to be carried by the aerodrome which would automatically record the number of revolutions of the engine, the velocity and direction [p186] of the wind relative to the machine, the height of the aerodrome as shown by a specially sensitive aneroid barometer, and the angle of the machine with the horizontal plane of the earth. The construction of this instrument was undertaken by a noted firm of instrument makers, but after many months of delay, during which it was several times delivered as being complete, only to be returned for further work, it was finally condemned as unsatisfactory, and it was decided not to encumber the machine with such a delicate apparatus, which, even if perfectly made, could not be depended on to work properly when mounted on the aerodrome frame, in which there was a constant, though minute, tremor due to the high speed and power of the engine.
The completed frame, which is perhaps best shown in Plates 49, 50 and 51, and Plate 60, Figs. 1, 2 and 3, in spite of its size gave an appearance of grace and strength which is inadequately represented in the photographs. In making the designs for the large aerodrome no data were available for use in calculating the strains that would come on the different parts of the frame while in the air, and the size and thickness of the tubes and the strength of the guy-wires were consequently determined almost entirely by “rule of thumb,” backed by experience with the models. Although the dimensions, shape, and arrangement of most of the auxiliary parts of the machine were considerably changed during the course of construction in accordance with the indications of the exhaustive series of shop tests, the fundamental features of the construction were practically unaltered, but the changes in the guy-wire system and in the fittings by which they were attached, made the frame as a whole several times as strong as it was originally, and it was felt that the direction of further improvements in it would be shown only by actual test of it in flight where any weaknesses would be certain to manifest themselves.
It may be well to remark here that even with the data which were later obtained, judgment based on experience proved after all to be the safest guide for proportioning the strength of the various parts. It can be assumed that a live stress will produce a strain ten times as great as that due to a static stress on the part when the machine is stationary. For greater safety, it would be still better to assume a strain twenty times as great. If one is building bridges, houses, and similar structures, where weight is not a prime consideration, it would be criminal negligence to fail to provide a sufficient “factor of safety,” or what in many instances may be more properly termed a “factor of ignorance,” while at the present time the insistence on large factors of safety in machines intended to fly would so enormously increase the weight that, before one-half the necessary parts were provided, the weight would be many times what could possibly be supported in the air. Later, no doubt, as experience is gained in properly handling the machine in the air, increased strength entailing [p187] increased weight may be added in proportion to the skill acquired; and there is no doubt that man will acquire this skill with marvelous rapidity, approaching, if not equaling, that exhibited by him in the use of the bicycle, which, when first ridden, requires not only all of the rider’s skill but that of a couple of assistants, but when once mastered requires hardly more thought for its proper manipulation than even the act of walking involves, the balancing and guiding being done intuitively merely by the motion of the body and with practically no exertion.
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