Chapter V: Construction of Frame of Large Aerodrome (2)

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.

WING CLAMPS]

[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.

FIG. 1

FIG. 2

FIG. 3

HOISTING AERODROME TO LAUNCHING-TRACK]

AERODROME ON LAUNCHING-CAR; FRONT WINGS IN PLACE, GUY-WIRES ADJUSTED]

DETAILS OF GUY-POSTS]

FIG. 1.

FIG. 2.

GUY-POST AND PIN ON LAUNCHING CAR]

[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.

[p188]