PL. 43. FRONT END OF TRACK JUST PREPARATORY TO LAUNCHING AERODROME [◊]
On the large launching car the arrangement of the struts against which the bearing points of the frame were tightly drawn by the clutch was similar in all respects to that used on the model car, there being only slight differences in details. The details of the uprights on which the bearing points of the aerodrome frame rested are clearly shown in Figs. 6, 7, 8, and 9 of Plate [40]. From the photographs (Plate [41], Figs. 1, 2, and 3) which show the large frame mounted on the launching car, the general arrangement of the struts and the clutch-hook can be readily seen; and from Plate [42], Figs. 1, 2, and 3, which show in detail most of the important features of the clutch-post and its clutch, a very good idea of the size of the different parts may be had by observing that the distance from the fulcrum of each half of the hook to the pin by which it was connected through the universal joint to the vertical rods is five inches. As previously stated, this clutch-hook gripped the lower pyramid and pulled the bearing points of the frame firmly against the forward and rear struts of the launching car, and in launching the aerodrome the triggers arranged on the bottom of the car, which at the proper time pull on the vertical rods and thereby force the two halves of the clutch-hook apart, are so arranged that they strike a cross-beam at the front end of the track one inch before the triggers, which keep the struts from being pulled down by their springs, which tend to fold them up and force them down against the car. The triggers, which prevent the struts from being folded down, strike a cross-beam in the track one foot before the buffer pistons on the end of the car begin to enter the buffer cylinders at the end of the track, and, consequently, one foot before the folding prop, which supports the front end of the track, is knocked out by the car striking a special trigger which allows this folding prop to swing forward when the front end of the track folds down to insure that the aerodrome will not become entangled with the car, even though the aerodrome be not quite up to soaring speed at the moment of launching. The manner in which this front end of the track folds down can be very readily seen by comparing Plate [43] with Plate [95] of Chapter XII [◊], the former showing the front end of the track in horizontal position, with [p161] the aerodrome at the extreme rear end just preparatory to launching, and the latter showing the front end of the track folded down with the hinged prop standing outward in its downward path and the aerodrome just launched. These photographs will be more particularly referred to later, but attention is here called to them so that the description immediately following may be more easily understood.
Although this method of launching the aerodrome seemed to Mr. Langley, both theoretically and from the experience with the models, to be a satisfactory and feasible plan, there were two very important respects in which it seemed from the very first open to objection. In the first place, it was necessary that the aerodrome should be launched as nearly at its soaring speed as possible, because either an excess or deficiency of speed interfered to some extent with the equilibrium of the machine. So many factors were involved in the determination of what this final velocity should be that it seemed almost impossible to be sure of the results until at least one test of the aerodrome had been made. In the second place it was not known whether the rapid acceleration of the car would seriously interfere with the equilibrium of the aviator.
In reference to the first question it was, of course, known that a freely falling body acquires a speed of 32 feet per second at the end of the first second after having fallen a distance of 16 feet. It was proposed to launch the aerodrome at approximately 35 feet per second; and, since the distance over which the car would pass in acquiring this speed was approximately 60 feet, the rate of acceleration would, of course, be less than that for a freely falling body. The conditions in the two cases, however, are quite different. In the case of the freely falling body there is the constant force of gravity which causes the acceleration. In the case of the aerodrome the car is initially standing still but ready to be acted upon by the combined force of the thrust of the propellers and the tension of the springs. The propeller thrust is approximately 450 pounds at the moment of releasing the car, while the spring tension adds approximately 400 pounds more pull, giving a total pull of 850 pounds acting on the car at the start. The weight of the aerodrome including the aeronaut being approximately 850 pounds, and the weight of the car being approximately 450 pounds, the total weight to be accelerated is 1300 pounds. The resistance of the car and the aerodrome is zero at the moment the car is released, and increases approximately as the square of the velocity until it reaches approximately 300 pounds at the soaring speed of the aerodrome; while on the other hand the spring tension decreases uniformly from 400 pounds at the start to approximately 76 pounds at the end of the track, and the thrust due to the propellers decreases from 450 pounds at the start to approximately 250 pounds at the moment of launching. Consequently, it is in a general way clear that the rate of acceleration of the aerodrome and car decrease, probably in a geometric ratio, the rate of acceleration [p162] at the moment of launching the aerodrome being much less than that of a freely falling body. Since so many factors enter into the problem no confidence was felt in calculations as to what the rate of acceleration would be. It was, therefore, decided to determine it experimentally at the same time that tests were made on the car to determine what spring tension would be necessary to enable the aerodrome and car to acquire soaring speed by the time they reached the end of the track.
It was obviously impossible to make this initial test with the aerodrome mounted on the launching car, as the aerodrome would certainly wreck both itself and the car were it allowed to remain fastened when the car was stopped at the end of the track. It was, therefore, decided to make the tests by mounting on the car boards which would have a head resistance equal to that of the aerodrome. In order to minimize as much as possible the blow due to the car striking the buffers at the end of the track, the car had been made as light as possible. On this account it was felt to be unwise to risk adding to it a weight of 850 pounds to represent the aerodrome, and supplying an additional spring tension to represent the thrust of the propellers, as the total effect of the added weight and the added pull would certainly completely demolish the car. By calculation it was found that the omission of the 850 pounds weight of the aerodrome and the spring tension to represent the thrust of the propellers would practically counterbalance each other; and that if sufficient spring tension were provided to cause the car, with the light boards representing the head resistance of the aerodrome, to reach the soaring speed by the time it arrived at the end of the track, it would be safe to assume that this spring tension would be sufficient for use in launching the aerodrome.
The method of measuring the final speed of the launching car for the models consisted in fastening a strip of smoked paper to the launching car in such a position that it was drawn past a stylus fastened to the end of a vibrating tuning fork placed at the end of the track. This had proved perfectly successful, but it gave a record merely of the final speed attained by the car at the moment of launching the aerodrome. In the case of the large aerodrome it was desirable to have a record of the speed of the car during the first few feet, and also at several other points in its travel down the launching track, and the more numerous these points the better. Short strips of copper were accordingly placed every twelve inches along the length of the track, and these were connected by a wire to one terminal of a small electric battery. Mounted on the car, in such a way that it would be drawn across these contact strips, was a copper brush arranged to make continuous contact with another wire stretched along the track, this second wire being connected to the other terminal of the electric battery and having in its circuit the magnet which actuated a pen on a chronograph. Since the rate of revolution of the chronograph barrel was known, the [p163] distance between the marks which the magnet would cause the pen to make when its circuit was closed by the brush on the car passing across the contact strips on the track would give correct measures of the time consumed by the car in passing over each twelve inches of its travel. Upon test, however, it was found impossible to get the chronograph magnets to work rapidly enough to respond to the very rapid opening and closing of the circuit after the car had passed over the first one-quarter of its length of travel. As a large part of the slowness of action seemed to be due to the weight of the fountain pens, they were replaced by small glass tubes drawn out to a fine point and containing a small amount of ink. These seemed, however, to be still too heavy to respond to the rapid closing of the circuit unless the contacts were made unduly long. The contacts were finally made three inches long and placed only every three feet along the track, but just as these contacts were completed and placed in position the clock-work of the chronograph itself became deranged. Before it could be repaired, the tests were discontinued, as everything was in readiness for the boat to proceed down the river where the actual tests in free flight were to be made. Tests of the final speed of the car were, however, made by the tuning-fork method, and the springs were adjusted until their tension was sufficient to cause the car to attain a speed of thirty-five feet a second at a point three inches in front of the point at which the aerodrome would be released from the car.
[p164]