◊[p207]CHAPTER VIIEQUILIBRIUM AND CONTROL
[p207]
In an aerodrome it is essential not only that its component parts shall be so disposed that the initial equilibrium is correct and highly stable, but also that some efficient means be provided for quickly and accurately restoring the equilibrium, if for any reason it is disturbed. If the aerodrome is of sufficient size and power to carry a human being it is, of course, possible merely to supply an efficient means of controlling the lateral and horizontal equilibrium of the machine and depend upon the intelligence and skill of the operator, as developed by practice and experience, to maintain the proper equilibrium of the machine while in the air. This method, however, is open to the objection that no matter how skilled the aviator may be there remains the probability of a serious if not fatal accident as the result of any momentary lapse or diversion of attention until the “sense of equilibrium” has been developed. One of the chief problems, therefore, which had impressed itself from the beginning of the work, was to devise some means by which the equilibrium of the aerodrome would be automatically maintained under the varying conditions of flight, so as to leave the aviator free, as far as possible, to control the direction of flight and to devote his attention to other important matters connected with the proper functioning of the various parts of the aerodrome. In the development of the models it had been absolutely necessary to develop some efficient automatic control, as they were far too small to carry an aviator, and the conditions of flight in the open air, even on the calmest day, were such that constant readjustments of the equilibrium were necessary. The success attained in the automatic control of the equilibrium of the models had been so great, and so much time would have been required for an aviator to acquire skill sufficient to control a machine without such automatic equilibrium, that it was considered both expedient and safe to embody in the large aerodrome the plans which had proved so successful in the models. It was necessary, however, to provide in addition in the large machine means whereby the aviator could quickly and accurately either modify the action of the automatic devices or, if desired, entirely supersede the automatic control by purely manual control. Three distinct problems were, therefore, encountered in connection with the equilibrium and control of the large aerodrome. In the first place, the machine as a whole had to be so designed, and its component parts so disposed as to secure a highly stable initial equilibrium; second, automatic means had to be provided for[p208]maintaining this equilibrium under the varying conditions of flight and for restoring it if for any reason it was disturbed, and, finally, provision had to be made for the quick and accurate control of the flight by the aviator. These problems, while intimately related, had to be met one by one and solved separately.
The general type of machine adopted was that which had been developed in the years of experiment with the steam-driven models. From the very first consideration of the large aerodrome, it seemed advisable to follow this type, which not only had shown itself to be distinguished by remarkable longitudinal and lateral stability in the tests, but was actually the only type in the world which had at that time shown any possibility of successful flight. There was, of course, a question whether single surface or superposed wings would be used, and in spite of the negative results obtained in the tests of the models with the superposed wings, it was felt that a considerable field for development was open in this direction. However, in spite of the advantages which theoretical considerations showed might be obtained through the introduction of this and various other modifications of the original type, the whole teaching of past experience in the construction of the model aerodromes had been that success was more certain to be achieved by following the course in which genuine practical results had been achieved. It was decided, therefore, that in the construction of the large aerodrome the design should follow as closely as constructional conditions would permit the lines of the successful model Aerodromes Nos. 5 and 6, which have already been fully described.
The longitudinal stability of an aerodrome is largely dependent upon the relation of three chief factors; the center of pressure, the center of gravity and the line of thrust. For an aerodrome of the “Langley” type, the relative positions of these which give the greatest degree of stability had been determined as far as possible through the years of experiment with the models. However, while it is the usual experience in designing machinery, or even scientific apparatus, that what appears theoretically to be the best plan has to be considerably modified for constructional reasons, yet in the design of an aerodrome this is particularly true, for not only must all the various parts function properly, both separately and as a whole, but this result must be secured for the very minimum of weight. Experience alone can enable one to appreciate thoroughly how seriously this consideration of weight complicates the problem.
In making the original designs for the large aerodrome it had been recognized that the relative positions of the line of thrust, center of pressure, and center of gravity were much better in model No. 6 than in model No. 5. From Data Sheet No. 1, for Aerodrome No. 5 when it made its flight on May 6, 1896, it will be noted that the line of thrust being assumed to be at the point 1500,43[p209]the center of gravity was at the point 1497, and that, assuming the rear wings to have two-thirds of the lifting effect of the front ones, the center of pressure was calculated to be at the point 1498, or one centimetre in front of the center of gravity, measured in the horizontal plane. In the vertical plane the center of pressure was calculated to be at the point 2536, and the center of gravity was found by test to be at the point 2501, when the line of thrust was assumed to be at the point 2500, the center of gravity being actually one centimetre above the line of thrust.
From the data sheet of Aerodrome No. 6, for its flight of November 28, 1896, it will be noted that the line of thrust being at the point 1500 the center of pressure was at the point 1487, and the center of gravity at the point 1484; that is, the center of pressure was three centimeters in front of the center of gravity, measured in the horizontal plane. In the vertical plane, taking the line of thrust at the point 2500, the center of pressure was at the point 2525, and the center of gravity at the point 2486, the center of gravity being 14 centimetresbelowthe line of thrust and 39 centimetres below the center of pressure, the distance from the center of pressure to the line of thrust being, therefore, 64 per cent of the distance between the center of pressure and the center of gravity.
As has been explained in Part I, while it is not desirable that the center of gravity be a great distance below the center of pressure, as such a relation tends to produce a special kind of rolling and pitching in varying currents of air, it is highly desirable that the center of gravity should lie some distance below the line of thrust in order that the three forces may be balanced. In a machine like model No. 5, where the center of gravity was actually, though very slightly above the line of thrust, there is a constant tendency to produce rotation of the aerodrome, if for any reason its equilibrium is disturbed, which is corrected in practice by the action of the Pénaud tail. In model No. 6, on the other hand, the disposition of the three factors was such that they tended to maintain, rather than to destroy, the initial equilibrium of the machine.
These desirable relative positions had been made possible in model No. 6 by the fact that the center of gravity and line of thrust could be located at practically any desired point, since with the use of steam the power plant consists of two separable parts, the boiler, with its fuel and water tanks, and the engine. These parts can, therefore, be placed in any part of the aerodrome that constructional or theoretical reasons demand. Furthermore, the engine constitutes such a relatively small portion of the weight of the entire machine that, if for any reason it is desirable to place the engine in the same plane as the line of thrust, its weight is not sufficient to alter materially the position of the center of gravity, since the boiler, water and fuel tanks can be placed as low as desirable and connected with the engine by suitable pipes.[p210]
With a gasoline engine, however, the conditions are very greatly altered. Here the engine constitutes practically the entire weight of the power plant, only such accessories as the ignition coil, batteries, and carburetor being available for lowering the center of gravity, unless the fuel, cooling water tanks and radiator be placed below the engine and the liquids forced up by means of a pump. In making the first designs for the large aerodrome, therefore, it was found that it would be practically impossible to make the relative positions of the center of gravity and line of thrust the same as had existed in model No. 6, however desirable it might be. The center of gravity could be brought appreciably lower than the line of thrust only by placing the gasoline engine in a plane considerably below that of the propellers, and this necessitated the addition of at least two more sets of gears with heavy bearings and braces. Besides this almost prohibitive factor of weight, it was also foreseen that great difficulty would be experienced in keeping even the two sets of bevel gears already necessary aligned and in proper condition for efficiently transmitting the power to the propellers unless the frame and other parts were made prohibitively heavy. It was, therefore, found necessary to bring the center of gravity practically in the same plane with the line of thrust, which made its general features as regards equilibrium more nearly resemble those of model No. 5 than of No. 6.
The weight of the aviator, it is true, constituted an appreciable part of the flying weight of the large machine, and it at first seemed possible to lower the center of gravity by placing him at a considerable distance below the line of thrust. But it was recognized from the beginning that the aviator would probably have to give a great deal of attention to any form of engine in order to insure its working properly, and his position must, therefore, be selected with a view to the proper supervision of the engine and without regard to its effect on the center of gravity.
Although the repeated successful flights of model No. 5 under varying conditions of wind and power inspired the belief that the minor adjustments, as well as the general plan of the large aerodrome, were such as to give highly stable equilibrium, nevertheless, more direct corroboration of this opinion was desired, and it was largely for this reason that the quarter-size model was constructed. In it every detail of the larger machine which in any way affected its equilibrium was exactly reproduced to scale, and the greatest care was taken that the same relative positions of the center of pressure, the center of gravity and the line of thrust which it was proposed to employ for the large aerodrome should be used on the model in its flight of August 8, 1903, which is later described. The entire success of this flight, so far as the balancing was concerned, in spite of the fact that the engine worked erratically and that the launching speed was much less than it should have been, removed every doubt[p211]that the equilibrium of the large aerodrome would be satisfactory under normal conditions.
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 Plate68,[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◊
PL. 68. AUTOMATIC EQUILIBRIUM DEVICES◊
FIG.1FIG.2PL. 69. MECHANISM OF CONTROL◊lgr
FIG.1FIG.2PL. 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 Plate69, 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.
Because of the difficulties involved in this or any other mechanical device for controlling the equilibrium, it was in every way advisable to retain in the large machine the Pénaud system, which, though itself imperfect in many ways, had been thoroughly tested in actual flight. In the models, it will be remembered, the combined Pénaud tail and rudder controlled the longitudinal equilibrium by movement in the vertical plane under the combined influence of its initial negative angle and the elasticity of its connection with the frame, the flight being kept as nearly as possible in a straight line by the vertical surfaces of the tail. Although it was necessary that the large aerodrome should be capable of being steered in a horizontal direction, it was felt to be unwise to give the combined Pénaud tail and rudder motion in the horizontal plane in order to attain this end, since the use of it for such a double function might very seriously interfere with its proper action in preserving the longitudinal stability. It was, therefore, at first thought best to dissociate the rudder and tail so that the rudder might be used for horizontal steering without in any way interfering with the proper functioning of the tail. But, as the main desideratum was to obtain a flight of the large machine as soon as possible, and perfection of steering control seemed secondary, it was decided, after further consideration, in order not to risk the unpredictable effects that might result from small changes, to duplicate on the large machine the combined Pénaud tail and rudder of the model, and to add another rudder for steering in the horizontal plane. Constructional requirements determined as the only available position for this rudder a rather disadvantageous one. As will be seen from Plate53, its efficiency was diminished by its being only about half as far from the center of gravity as the combined Pénaud tail and rudder, and by being located in the lee of a considerable portion of the frame, where it would be subject to the cross-currents of air created by the forward motion of the frame.[p215]
For the preservation of the equilibrium of the aerodrome, though the aviator might assist by such slight movements as he was able to make in the limited space of the aviator’s car, the main reliance was upon the Pénaud tail. But, in the absence of any data for determining the effect produced in passing from the model to the large machine, it could not be certain that calculations based upon the balancing of the model would accurately determine the proper balancing of the large machine. It was therefore decided to provide such attachment for the Pénaud tail that, while it would always have elastic connection with the main frame, yet its angle could be appreciably changed without affecting in any way the degree of elasticity of this connection. After many changes in plans for securing this result, it was finally decided to arrange it in the manner shown in the drawings. Referring to the general plans in Plates53and54, and to the details in Fig. 1 of Plate56, the main stem of the Pénaud tail is seen to be connected by a pin to the horn (17), which is brazed to the clamping thimble, by which it is mounted on the vertical tube (16), suitably connected and braced to the rear end of the midrod, the horn (17) being larger than the stem of the tail and set at an angle to the vertical tube (16), the pin connection permitting the tail to swing up and down. The bridle (40), connected to the center of the tail on its upper side, passes upward where it is connected to the spring (41), the other end of which is connected to a single wire rope (42), which passes over the pulley mounted on the top of the post (43), which is guyed to the upper guy-post by the wire (44). The wire rope (42), after passing over the pulley, is connected to the spring (45), around the two ends of which it forms a loop, and from there it passes down to the plane of the main frame and through suitable pulley blocks to the aviator’s control wheel (50), which is mounted on the starboard side of the main frame, convenient to the aviator’s right hand when he is facing forward. From this point the wire rope passes through the various pulley blocks towards the rear of the machine, and through the pulley block (46) mounted on the side and near the bottom of the rear lower guy-post. At a short distance beyond this pulley it is connected to a weaker spring (47), the other end of which is connected by a second bridle (48) to the under side of the Pénaud tail at its center. In order to prevent the springs (41), (45) and (47), which furnish the elasticity for the Pénaud-tail connection, from being strained beyond their elastic limit, either by a sudden gust of wind or by the aviator attempting to move so large an area of surface too suddenly, the wire rope (42) was made continuous around the springs, the portion between the points where it was joined to the two ends of the springs being made of such a length as to take the entire strain should the strain on the cord become greater than sufficient to stretch the springs 50 per cent of their original length.[p216]
In the construction of the equilibrium control wheel it was decided that some arrangement must be secured whereby the wheel would normally be inactive and maintain whatever position it had been set to, and at the same time could be moved by the aviator with one hand, the mere act of grasping it rendering it free to be moved, and whereby it must automatically lock itself in any position in which it might be when the aviator removed his hand from it. The multiplicity of things requiring the attention of the aviator made it desirable that his attention to any one of the important details, whether the engine, the equilibrium, or the steering, should never require more than one hand, thus leaving the other hand free either to hold on to the machine or to control some other detail at the same time. While an irreversible wheel, such as would be secured by the use of a worm and worm-wheel, at first seemed likely to answer the purpose, yet the movement of a worm-wheel by means of a worm is necessarily very slow if it is irreversible, and it here seemed desirable to so arrange the wheel that in case of emergency, of for rising or descending, the aviator could swing the Pénaud tail from its extreme upper position to its extreme lower one by a small motion of his hand, and thus small or large adjustments of the Pénaud tail could be intuitively felt to have been produced without the aviator having to remember how many turns he had made of the wheel.
The control of the steering rudder was effected by a steering wheel (51) similar in construction to the equilibrium control wheel (50), a continuous cord (52) passing from the steering wheel through suitable pulleys to either side of the steering rudder (r), springs being interposed in loops in the cord on either side of the steering rudder to give some elasticity to the control apparatus in order to prevent possible danger from the aviator attempting to move the rudder too suddenly. This steering rope passed directly through the steering rudder at the points where it was joined to it; so that, should one side of the cord in any way become entangled with the frame or with its pulleys, the strain produced by the aviator in attempting to move it in the opposite direction would be taken up by the cord and thereby avoid the possibility of destroying the rudder. For even should the cord become entangled on one side, the rudder could be given a slight amount of adjustment through the elasticity of the coiled springs.
The design of the combined Pénaud tail and rudder followed very closely that which had been used for the models, and its area of ninety-five square feet on the horizontal surface with a corresponding area of vertical surface bore the same relation to the area of the tail and rudder of the models that the area of the wings of the large machine bore to that of the wings of its prototype.
While the provisions for automatic equilibrium and manual control were not entirely ideal, even for the quiet atmospheric conditions under which it[p217]was proposed to make the first tests, nevertheless it was and still is believed that the provisions for such conditions were sufficient to enable a successful flight of a few miles to be obtained. It was thought to be very certain that, once a successful flight could be made, the funds for the further prosecution of the work would be readily forthcoming, and that when these funds were obtained the many problems of control, rising and alighting, could be undertaken.