Langley Memoir on Mechanical Flight, Parts I and II / Smithsonian Contributions to Knowledge, Volume 27 Number 3, Publication 1948, 1911

by S. P. Langley; Charles M. Manly

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 centimetres below the 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.