The second problem encountered in connection with the balancing and control of the large aerodrome was that of providing an efficient means for maintaining the equilibrium under varying atmospheric conditions. Although much had been done toward the solution of this problem in the development of the models, the whole question was reopened and thoroughly reconsidered in designing the large aerodrome. The Pénaud tail, when made elastic or when more or less rigid, but attached to the frame through an elastic connection, and normally set at a negative angle, furnishes a means of automatically controlling the equilibrium, which is sufficiently sensitive and accurate to enable a machine to fly for a considerable distance, at least in moderately calm weather, as is evidenced by the various flights of the model aerodromes, where there was no human intelligence to control them. But owing to the principle of action of the Pénaud tail, the flight of an aerodrome controlled by it must of necessity be more or less undulatory in its course. Furthermore, the tests with the models had indicated that, while the Pénaud tail served remarkably well as a means of controlling the equilibrium of the machine, provided the balancing had been rather accurately determined, and, further, provided nothing happened to affect seriously the equilibrium of the machine, it was limited in its effectiveness by its narrow range of action. It was thought that a control mechanism which should be more sensitive and at the same time should act more powerfully to prevent the upsetting of the equilibrium when the machine was subjected to rather strong disturbing forces was desirable for any machine which was to transport a human being and, therefore, involved the risk of a fatal accident.

In the earlier period of the work and before the correct application of the Pénaud tail to the model aerodromes had been found, Mr. Langley had planned a large number of different forms of automatic control for preserving the equilibrium of the machines. The more frequently recurring of these were devices for changing the angle of the wings or tail, and others for shifting the wings or tail bodily so as to shift the position of the center of pressure with respect to the center of gravity, the motive power for operating the devices being in some cases that derived from a gyroscope or a pendulum, and in others small electric motor apparatus controlled by a pendulum or a gyroscope. Most of these, however, never reached the stage of development where they were actually tried on the machines in flight, as the tests of some of them in the shop showed that they were unreliable, while others were abandoned either when partly built or when only the drawings for them had been made. Among the better-preserved models of devices for this purpose which were in existence when the writer became associated with the work are those shown in Plate [68], [p212] where the piece at the top is a pendulum (inverted or direct) which controls the movement of the horizontal tail by means of the cords and apparatus shown, actuating these through the small electro magnets and apparatus attached. Just below the rod, which represents a piece of the midrod, are three parts, the first of which is a group of six little batteries clustered in a circle, while next to it is a system of needles hung in gymbals, with electro-steering apparatus in cups which itself turns on a graduated base, these electric connections, together with the battery, controlling the vertical rudder. On the right of this is another piece of apparatus for actuating windlass cylinders which turn one way or the other as the contact is made by one side or the other of the pendulum or the needle. At the bottom, on the two rods, is a tail-piece which automatically throws the center of pressure forward or backward according as the aerodrome departs one way or the other from the horizontal.

In spite of the fact that all the early attempts of Mr. Langley to devise such a mechanical control had been very unsatisfactory, the idea that something of this kind was necessary had never really been abandoned by him. Here was to be seen one of his chief characteristics, which was never to abandon any idea that seemed valuable until it was brought to a successful issue or some very strong proof was developed that the idea was impracticable. While on a trip abroad during the summer of 1899, and especially while resting at Vallombrosa, Italy, Mr. Langley’s mind again turned to this problem, and he wrote a number of very interesting letters emphasizing the importance of devising such a mechanism which should be controlled by gravity. When he returned to the Institution in the fall he insisted upon the same idea.

PL. 68. AUTOMATIC EQUILIBRIUM DEVICES [◊]

FIG. 1 FIG. 2 PL. 69. MECHANISM OF CONTROL [◊] [lgr]

A mechanism which had been devised by the writer for another, but somewhat similar, purpose seemed to be well adapted to this end, and it was accordingly decided to construct a small model of such a size as would be suitable for use on one of the steam-driven models. The plan of control which it was proposed to follow was to have some mechanism which would control the angle of the tail through the action of gravity on a pendulum bob. Since it would require an exceedingly heavy pendulum should the deflections of it be directly utilized to produce corresponding movements of the tail, the most feasible plan seemed to be to have a light pendulum, which, while free to move under the action of gravity, would nevertheless by its movement cause some outside force to produce corresponding and simultaneous movements of the tail. The general scheme of arrangement is shown in Plate [69], Figs. 1 and 2. This device consists essentially of a cylinder (1) in which is mounted a piston with the piston rod (3) passing through the cylinder head and connected to the cord (5) which passes over the pulley (6), fastened to the tube (2), which is slidably mounted on the midrod (7), whence it is carried over the pulley (8) on the guy-post (9). From here it is connected to the spring (10) which is fastened by the [p213] bridle (11) to the upper side of the Pénaud tail (12). The other end of the piston rod (3) passes through the head in the other end of the cylinder, and has connected to it a cord (14) which passes over the pulley (15) fastened to the tube (2), whence it is continued over the pulley (16) and is joined to the spring (17), which is connected by the bridle (18) to the lower side of the tail. Mounted on top of the cylinder (1) is a valve chamber (20) having ports leading to the two ends of the cylinder. Mounted in the valve chamber is a rocking valve surrounded by a bushing having ports in it, and to which is fastened a rod (25) which passes through the said valve and the head of the valve chamber. Fastened to the rod (25) of the bushing is a lever (26), which by means of the link (27) is connected to the piston rod (3). Fastened to the rocking valve is a rod (28) which telescopes over the rod (25) and also passes through the same head of the valve chamber, and carries at its outer end a pendulum (29) on the lower end of which is the bob (30).

If steam or any other fluid under pressure is furnished to the valve chamber through the pipe (31), none will be admitted to the cylinder so long as the pendulum is vertical or at right angles to the axis of the cylinder; and the tail will be in its normal position, which we will suppose to be an upward inclination of five degrees. If, now, the front of the machine be depressed, thereby causing the pendulum to move to the right, such movement of the pendulum will cause the valve to open, admitting fluid to the left-hand end of the cylinder. This, acting on the piston, will force it towards the right, which, by means of the cord, will cause the angle of the tail to be increased, thereby causing the rear of the machine to be depressed and the front to be raised. But as soon as the piston begins to move under the action of the fluid pressure it simultaneously moves the bushing which surrounds the valve by means of the connecting links and levers, so that as soon as the piston has moved a distance proportional to the amount that the valve has been opened by the pendulum, it causes the bushing to shut off the port and thus prevents further fluid entering the cylinder. As soon as the aerodrome responds to the action of the tail the pendulum will, of course, begin to move back to its normal position of perpendicularity to the cylinder, and will then open the valve to the other port, thereby causing fluid to pass into the opposite end of the cylinder. This fluid acting on the piston will move it in the opposite direction and thereby cause the tail to be drawn back to its normal position at the same time that the pendulum gradually reaches its normal position, owing to the return of the aerodrome to its normal position. In the explanation given above it was assumed that the slidable tube (2) was in a fixed position. It was planned to have the equilibrium normally maintained automatically and at the same time permit the operator to modify the automatic control and even to assume full manual control. To secure this, the slidable tube (2) was connected at each end to an [p214] endless cord (20) which after passing over suitable pulleys was connected to the control wheel (51) at the aviator’s car.

A model of this device was constructed in the spring of 1900 and was tested with steam pressure in the shop. The test showed that the device acted immediately and with precision, the piston performing movements simultaneously and in exact accordance with the pendulum. The device, however, was never tried in a flight of any of the aerodromes owing to the lack of time necessary to properly install it on the machine. Furthermore, it was thought probable that the rapid acceleration of the aerodrome at the moment of launching would so disturb the pendulum as to cause it to be in a very different position from that of vertical, and also that the motion of the aerodrome through the air would itself be a somewhat disturbing factor.