CHAPTER IIPRELIMINARY

◊[p005]CHAPTER IIPRELIMINARY

[p005]

Part I of the present work is intended to include an account of the experiments with actual flying models, made chiefly at or near Washington, from the earliest with rubber motors up to the construction of the steam aerodromes that performed the flights of May 6 and November 28, 1896.

An account of some observations conducted at Washington, with the whirling table, on the reaction of various surfaces upon the air, is relegated to a later part.

The experiments with working models, which led to the successful flights, were commenced in 1887, and it has seemed to me preferable to put them at first in chronological order, and to present to the reader what may seem instructive in their history, while not withholding from him the mistaken efforts which were necessarily made before the better path was found. In this same connection, I may say that I have no professional acquaintance with steam engineering, as will, indeed, be apparent from the present record, but it may be observed that none of the counsel which I obtained from those possessing more knowledge was useful in meeting the special problems which presented themselves to me, and which were solved, as far as they have been solved, by constant “trial and error.”

I shall, then, as far as practicable, follow the order of dates in presenting the work that has been done, but the reader will observe that after the preliminary investigations and since the close of 1893, at least four or five independent investigations, attended with constant experiment and radically distinct kinds of construction, have been going on simultaneously. We have, for instance, the work in the shop, which is of two essentially different kinds: first, that on the frames and engines, which finally led to the construction of an engine of unprecedented lightness; second, the experimental construction of the supporting and guiding surfaces, which has involved an entirely different set of considerations, concerned with equilibrium and support in flight. These constructions, however successful, are confined to the shop and are, as will be seen later, useless without a launching apparatus. The construction of a suitable launching apparatus itself involved difficulties which took years to overcome. And, finally, the whole had to be tested by actual flights in free air, which were conducted at a place some 30 miles distant from the shop where the original construction went on.[p006]

Simultaneously with these, original experiments with the whirling-table were being conducted along lines of research, which though necessary have only been indicated. We have, then, at least five subjects, so distinct that they can only be properly treated separately, and accordingly they will be found in Chapters VII, VIII, IX and X,◊and in Part Third [in preparation].

It is inevitable that in so complex a study some repetition should present itself, especially in the narrative form chosen as the best method of presenting the subject to the reader. Each of these chapters, then, will contain its own historical account of its own theme, so that each subject can be pursued continuously in the order of its actual development, while, since they were all interdependent and were actually going on simultaneously, the order of dates which is followed in each chapter will be a simple and sufficient method of reference from one to the other.

In order to understand how the need arises for such experiments in fixing conditions which it might appear were already determined in the work “Experiments in Aerodynamics,”8it is to be constantly borne in mind, as a consideration of the first importance, that the latter experiments, being conducted with the whirling-table, force the model to move in horizontal flight and at a constant angle. Now these are ideal conditions, as they avoid such practical difficulties as maintaining equilibrium and horizontality, and for this reason alone give results more favorable than are to be expected in free flight.

Besides this, the values given in “Aerodynamics” were obtained with rigid surfaces, and these surfaces themselves were small and therefore manageable, while larger surfaces, such as are used in actual flight, would need to be stiffened by guys and like means, which offer resistance to the air and still further reduce the results obtained. It is, therefore, fairly certain, that nothing like the lift of 200 pounds to the horse-power for a rate of 40 miles an hour,9obtained under these ideal conditions with the whirling-table, will be obtained in actual flight, at least with plane wings.

The data in “Aerodynamics” were, then, insufficient to determine the conditions of free flight, not alone because the apparatus compels the planes to move in horizontal flight, but because other ideally perfect conditions are obtained by surfaces rigidly attached to the whirling-table so as to present an angle to the wind of advance which is invariable during the course of the experiment, whereas the surfaces employed in actual flight may evidently change this angle and cause[p007]the aerodrome to move upward or downward, and thus depart from horizontal flight so widely as to bring prompt destruction.

To secure this balance, or equilibrium, we know in theory, that the center of gravity must be brought nearly under the center of pressure, by which latter expression we mean the resultant of all the forces which tend to sustain the aerodrome; but this center of pressure, as may in fact be inferred from “Aerodynamics,”10varies with the inclination of the surface. It varies also with the nature of the surface itself, and for one and the same surface is constantly shifted unless the whole be rigidly held, as it is on the whirling-table, and as it cannot be in free flight.

Here, then, are conditions of the utmost importance, our knowledge of which, as derived from ordinary aerodynamic experiments, is almost nothing. A consideration of this led me to remark in the conclusion of “Aerodynamics”:

“I have not asserted, without qualification, that mechanical flight is practically possible, since this involves questions as to the method of constructing the mechanism, of securing its safe ascent and descent, and also of securing the indispensable condition for the economic use of the power I have shown to be at our disposal—the condition, I mean, of our ability to guide it in the desiredhorizontaldirection during transport—questions which, in my opinion, are only to be answered by further experiment and which belong to the inchoate art or science ofaerodromicson which I do not enter.”

It is this inchoate art of aerodromics which is begun in the following experiments with actual flying machines.

In all discussions of flight, especially of soaring flight, the first source to which one naturally looks for information is birds. But here correct deductions from even the most accurate of observations are very difficult, because the observation cannot include all of the conditions under which the bird is doing its work. If we could but see the wind the problem would be greatly simplified, but as the matter stands, it may be said that much less assistance has been derived from studious observations on bird-flight than might have been anticipated, perhaps because it has been found thus far impossible to reproduce in the flying machine or aerostatic model the shape and condition of wing with its flexible and controllable connection with the body, and especially the instinctive control of the wing to meet the requirements of flight that are varying from second to second, and which no automatic adjustment can adequately meet.

At the time I commenced these experiments, almost the only flying-machine which had really flown was a toy-like model, suggested by A. Pénaud, a young Frenchman of singular mechanical genius, who contributed to the world many most original and valuable papers on Aeronautics, which may be found in the journal “L’Aeronaute.” His aeroplane is a toy in size, with a small propeller[p008]whose blades are usually made of two feathers, or of stiff paper, and whose motive power is a twisted strand of rubber. This power maintains it in the air for a few seconds and with an ordinary capacity for flight of 50 feet or so, but it embodies a device for automatically securing horizontal flight, which its inventor was the first to enunciate.11

Although Pénaud recognized that, theoretically, two screws are necessary in an aerial propeller, as the use of a single one tends to make the apparatus revolve on itself, he adopted the single screw on account of the greater simplicity of construction that it permitted. One of these little machines is shown as No. 11, Plates1and2.

ABis a stem about 2.5 mm. in diameter and 50 cm. long. It is bent down at each end, with an offset which supports the rubber and the shaft of the screw to which it is hooked. The screwHH1is 21 cm. in diameter, and has two blades made of stiff paper; two are preferable, among other reasons, because they can be made so that the machine will lie flat when it strikes in its descent. About the middle ofABthere is a “wing” surfaceDC, 45 cm. long and 11 cm. broad, the endsCandDbeing raised and a little curved. In front of the screw is the horizontal rudderGKhaving a shape like that of the first surface, with its ends also turned up, andinclined at a small negative angle with this wing surface. Along its center is a small fin-like vertical rudder that steers the device laterally, like the rudder of a ship.

The approximately, but not exactly, horizontal rudder serves to hold the device in horizontal flight, and its operation can best be understood from the side elevation. LetCDbe the wing plane set nearly in the line of the stem, which stem it is desired to maintain, in flying, at a small positive angle, α, with the horizon, α being so chosen that the tendency upward given by it will just counteract the action of gravity. The weight of the aeroplane, combined with the resistance due to the reaction of the air caused by its advance would, under these conditions, just keep it moving onward in a horizontal line, if there were no disturbance of the conditions. There is, however, in the wing no power of self-restoration to the horizontal if these conditions are disturbed. But such a power resides in the rudderGK, which is not set parallel to the wing, but at a negative angle (α1) with it equal to the positive angle of the wing with the horizon. It is obvious that, in horizontal flight, the rudder, being set at this angle, presents its edge to the wind of advance and consequently offers a minimum resistance as long as the flight is horizontal. If, however, for any reason the head drops down, the rear edge of the rudder is raised, and it is at once subjected to the action of the air upon its upper surface, which has a tendency to lower the rear of the machine and to restore horizontality. Should the head rise, the lower[p009]surface of the rudder is subjected to the impact of the air, the rear end is raised, and horizontality again attained. In addition to this, Pénaud appears to have contemplated giving the rudder-stem a certain elasticity, and in this shape it is perhaps as effective a control as art could devise with such simple means.

Of the flight of his little machine, thus directed, Pénaud says:

“If the screw be turned on itself 240 times and the whole left free in a horizontal position, it will first drop; then, upon attaining its speed, rise and perform a regular flight at 7 or 8 feet from the ground for a distance of about 40 metres, requiring about 11 seconds for its performance. Some have flown 60 metres and have remained in the air 13 seconds.12The rudder controls the inclination to ascend or descend, causing oscillations in the flight. Finally the apparatus descends gently in an oblique line, remaining itself horizontal.”

The motive power is a twisted hank of fine rubber strips, which weighs 5 grammes out of a total of 16 grammes for the whole machine, whose center of gravity should be in advance of the center of surfaceCD, as will be demonstrated in another place. This device attracted little notice, and I was unfamiliar with it when I began my own first constructions at Allegheny, in 1887.

My own earliest models employed a light wooden frame with two propellers, which were each driven by a strand of twisted rubber.13In later forms, the rubber was enclosed and the end strains taken up by the thinnest tin-plate tubes, or better still, paper tubes strengthened by shellac.

Little was known to me at that time as to the proper proportions between wing surface, weight and power; and while I at first sought to infer the relation between wing surface and weight from that of soaring birds, where it varies from12to 1 sq. ft. of wing surface to the pound, yet the ratio was successively increased in the earlier models, until it became 4 sq. ft. to 1 pound. It may be well to add, however, that the still later experiments with the steam-driven models, in which the supporting surface was approximately 2 sq. ft. to the pound, proved that the lack of ability of these early rubber-driven models to properly sustain themselves even with 4 sq. ft. of wing surface to the pound, was largely due to the fact that the wings themselves had not been stiff enough to prevent their being warped by the air pressure generated by their forward motion.

During the years I presently describe, these tentative constructions were[p010]renewed at intervals without any satisfactory result, though it became clear from repeated failures, that the motive power at command would not suffice, even for a few seconds’ flight for models of sufficient size to enable a real study to be made of the conditions necessary for successful flight.

In these earliest experiments everything had to be learned about the relative position of the center of gravity, and what I have called the center of pressure. In regard to the latter term, it might at first seem that since the upward pressure of the air is treated as concentrated at one point of the supporting surface, as the weight is at the center of gravity, this point should be always in the same position for the same supporting surface. This relation, however, is never constant. How paradoxical seems the statement that, ifabbe such a supporting surface in the form of a plane of uniform thickness and weight, suspended atc(acbeing somewhat greater thancb) and subjected to the pressure of a wind in the direction of the arrow, the pressure on the lesser armcbwill overpower that on the greater armac! We now know, however, that this must be so, and why, but as it was not known to the writer till determined by experiments published later in “Experiments in Aerodynamics,” all this was worked out by trial in the models.

FIG.1. Diagram of suspended plane showing position ofC. P.

It was also early seen that the surface of support could be advantageously divided into two, with one behind the other, or one over the other, and this was often, though not always, done in the models.

At the very beginning another difficulty was met which has proved a constant and ever-increasing one with larger models—the difficulty of launching them in the air. It is frequently proposed by those unfamiliar with this difficulty, to launch the aerodrome by placing it upon a platform car or upon the deck of a steamer, and running the car or boat at an increasing speed until the aerodrome, which is free to rise, is lifted by the wind of advance. But this is quite impracticable without means to prevent premature displacement, for the large surface and slight weight renders any model of considerable size unmanageable in the least wind, such as is always present in the open air. It is, therefore, necessary in any launching apparatus that the aerodrome be held rigidly until the very moment of release, and that instant and simultaneous release from the apparatus be made at all the sustaining points at the proper moment.[p011]

There is but a very partial analogy in this case to the launching of a ship, which is held to her ways by her great weight. Here, the “ship” is liable to rise from her ways or be turned over laterally at any instant, unless it is securely fastened to them in a manner to prevent its rising, but not to prevent its advancing.

The experiments with rubber-driven models commenced in April, 1887, at the Allegheny Observatory, were continued at intervals (partly there, but chiefly in Washington) for three or four years, during which time between thirty and forty independent models were constructed, which were so greatly altered in the course of experiment that more nearly one hundred models were in reality tried. The result of all this extended labor was wholly inconclusive, but as subsequent trials of other motors (such as compressed air, carbonic-acid gas, electric batteries, and the like) proved futile, and (before the steam engine) only the rubber gave results, however unsatisfactory, in actual flight, from which anything could be learned, I shall give some brief account of these experiments, which preceded and proved the necessity of using the steam engine, or other like energetic motor, even in experimental models.

An early attempt was made in April, 1887, with a model consisting of a frame formed of two wooden pieces, each about 1 metre long and 4 centimetres wide, made for lightness, of star-shaped section, braced with cross-pieces and carrying two long strips of rubber, each about 1 mm. thick, 30 mm. wide, 2 metres long, doubled, weighing 300 grammes. Each of these strips could be wound to about 300 turns, one end being made fast to the front of the frame, the other to the shaft of a four-bladed propeller 30 cm. in diameter. The wings were made of lightest pine frames, over which paper was stretched, and were double, one being superposed upon the other. Each was 15 cm. wide, and 120 cm. long. The distance between them was 12 cm. and the total surface a little more than 3600 sq. cm. (4 square feet). In flying, the rubber was so twisted that the propellers were run in opposite directions. The weight of the whole apparatus was not quite 1 kilogramme, or about 1 pound to 2 feet of sustaining surface, which proved to be entirely too great a weight for the power of support. When placed upon the whirling-table, it showed a tendency to soar at a speed of about ten miles an hour, but its own propellers were utterly insufficient to sustain it.

In this attempt, which was useful only in showing how much was to be learned of practical conditions, the primary difficulty lay in making the model light enough and sufficiently strong to support its power. This difficulty continued to be fundamental through every later form; but besides this, the adjustment of the center of gravity to the center of pressure of the wings, the disposition of the wings themselves, the size of the propellers, the inclination and number of their blades, and a great number of other details, presented themselves for examination.[p012]Even in the first model, the difficulty of launching the machine or giving it the necessary preliminary impulse was disclosed—a difficulty which may perhaps not appear serious to the reader, but which in fact required years of experiment to remove.

By June, 1887 two other models, embodying various changes that had suggested themselves, had been constructed. Each of these had a single propeller (one an1812-inchpropeller with eight adjustable blades, the other a 24-inch propeller with four adjustable blades) and was sustained by two pairs of curved wings 4 feet 7 inches long. It is, however, unnecessary to dwell further on these details, since these models also proved altogether too heavy in relation to their power, and neither of them ever made an actual flight.

At this period my time became so fully occupied with the experiments in aerodynamics (which are not here in question) that during the next two years little additional was done in making direct investigations in flight.

In June, 1889, however, new rubber-driven models were made in which the wooden frames were replaced by tubes of light metal, which, however, were still too heavy, and these subsequently by tubes of paper covered with shellac, which proved to be the lightest and best material in proportion to its strength that had been found. The twisted rubber was carried within these tubes, which were made just strong enough to withstand the end-strain it produced. The front end of the rubber being made fast to an extremity of the tube, the other end was attached directly to the shaft of the propeller, which in the early models was still supplied with four blades.

A detailed description of one of these early models, No. 26, shown in Plates1and4, follows:

In each of the two tubes of paper, stiffened with shellac, which form a part of the framing, is mounted a hank of twisted rubber, which connects with a propeller at the rear. There are two pairs of wings, superposed and inclined at an angle, the one above, the other below the frame. A light stem connected with the frame bears a triangular Pénaud tail and rudder.

Length of model105cm.Spread of wings83u+3003Width of upper wings14u+3003Width of lower wings19u+3003Diameter of propeller29u+3003Area of upper wings1134sq. cm.Area of lower wings1548u+3003Area of tail144u+3003Weight of wings51grammesWeight of tail7u+3003Weight of frame38u+3003Weight of wheels20u+3003Weight of rubber (.09 pound)40u+3003Total weight156u+3003No. of turns of rubber100Time of running down8secondsHorse-power from preceding data0.001HP

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The aerodromes made at this time were too heavy, as well as too large, to be easily launched by hand, and it was not until 1891 that the first one was constructed light enough to actually fly. This first flight was obtained from the north window of the dome of the Allegheny Observatory, on March 28, 1891, and imperfect as it was, served to show that the proper balancing of the aerodrome which would bring the center of gravity under the center of pressure, so as to give a horizontal flight, had yet to be obtained.

From this time on until 1893, experiments continued to be made with rubber-driven models, of which, as has been stated, nearly 40 were constructed, some with two propellers, some with one; some with one propeller in front and one behind; some with plane, some with curved, wings; some with single, some with superposed, wings; some with two pairs of wings, one preceding and one following; some with the Pénaud tail; and some with other forms. A few of these early forms are indicated on the accompanying Plates1to4, but it does not seem necessary to go into the details of their construction.

No. 11 with which an early flight was made, closely resembles the Pénaud model.

No. 13 has two propellers, one in front and one behind, with a single wing.

No. 14 has two propellers, nearly side by side, but one slightly in advance, with a single wing and a flat horizontal tail.

No. 15 has one leading propeller and two broad wings, placed one behind the other.

No. 30 has the propeller shafts at an angle, and one pair of wings.

No. 31 has the propeller shafts at an angle, and two pairs of wings superposed.

The wings in general were flat, but in some cases curved. The rubber was usually wound to about 100 turns, and trouble continually arose from its “kinking” and unequal unwinding, which often caused most erratic flights.

It is sufficient to say of these that, rude as they were, much was learned from them about the condition of the machines in free air, which could never be learned from the whirling-table or other constrained flight.

The advantages and also the dangers of curved wings as compared with plane ones, were shown, and the general disposition which would secure an even balance, was ascertained; but all this was done with extreme difficulty, since the brief flights were full of anomalies, arising from the imperfect conditions of observation. For instance, the motor power was apparently exhausted more rapidly when the propellers were allowed to turn with the model at rest, than when it was in motion, though in theory, in the latter case more power would seem to be expended and a greater speed of revolution obtained in a given time. The longest flights obtainable did not exceed 6 or 8 seconds in time, nor 80 to 100 feet in distance, and were not only so brief, but, owing to the spasmodic action of the rubber and other causes, so irregular, that it was extremely difficult to obtain even the imperfect results which were actually deduced from them.[p014]

The following rules and symbols were adopted for determining the relative position of points on the aerodrome, some of them during 1891, and some of them since. All are given here for convenience of reference, though their chief application is to the larger steam aerodromes described later. Those which immediately follow were meant to give some of the notation of descriptive geometry in untechnical language for the use of the workmen employed. LetX,YandZbe three lines at right angles to each other, and passing through the same point in space,O, lying at any convenient distance above the floor of the work-shop. The lineXlies North and South; the lineYlies East and West, and the lineZpoints to the zenith. Now place the aerodrome on the floor so that its principal axis lies horizontally in the planeXZ, with its head pointing North, and in such a position that a line passing through the center of the propellers shall coincide with the lineY.

FIG.2. Diagram showing mensural coordinates.

When measurements are made on or parallel to the lineX, the point of intersectionOwill be marked 1500 centimetres, and distances toward the South will be less than, and distances toward the North greater than 1500 centimetres.

When measurements are made on or parallel to the lineZ, the pointOwill be considered to be marked 2500 centimetres, and distances above will be greater than, and distances below will be less than 2500 centimetres.[p015]

Lastly, when measurements are made on or parallel to the lineY, the pointOwill be marked 3500 centimetres, and distances toward the East will be greater than, and distances toward the West will be less than 3500 centimetres. Measurements in these latter directions will be comparatively infrequent because the center of gravity and center of pressure both lie in the planeXZ.

In the figure the pointTin the tail, if 15 centimetres to the South ofO, would be graduated 1485 centimetres. A weight (W) 25 centimetres below the axis, would be graduated 2475 centimetres. A point 50 centimetres above the axis would be graduated 2550 centimetres, etc.

CGrepresents the Center of Gravity of the aerodrome, or (with subscript letters) of any specially designated part, or with reference to some indicated condition.

CG1CG2represent the Center of Gravity as referred to the first, or horizontal, and to the second, or vertical plane, respectively.

CPrepresents the Center of Pressure14of the whole aerodrome, or (with a subscript) of any specially designated part.

CFrepresents the Center of Figure of the aerodrome, or of any specially designated part.

Subscripts:

“A” represents the total area of the supporting surface; “a” represents the total area of the tail;HPrepresents the horse-power by Prony brake measurement. “Horse-power by formula” is given by Maxim’s formula:15

HP=rev. × diam. of propeller × pitch × thrust.33,000

(This formula was not in use at the time of the rubber-motor experiments, for which the thrust was not taken. It appears to assume the conditions where the screws from a fixed position move a mass of still air, are the same as those of free flight. Its results, however, are in better agreement with experiment than might be anticipated.)

“Flying-weight” means everything borne in actual flight, including fuel and water.[p016]

Remembering that the principal object of all these experiments is to be able to predict that setting of the wings and tail with reference to the center of gravity which will secure horizontal flight, we must understand that in the following tables (see No. 30) the figuresCPm= 1516.5 cm. mean a prediction that the center of pressure of the sustaining surfaces in motion (CPm) is to be found in a certain position 1516.5; that is, 16.5 cm. in advance of the line joining the propeller shafts. This prediction has been made by means of previous calculation joined with previous experimental adjustment. We know in a rough way where theCPwill fall on the wings when they are exposed independently if flat, and at a certain angle, and where it will fall on the tail. From these, we can find where the resultingCPof the whole sustaining surface will be.

It would seem that when we have obtained the center of gravity by a simple experiment, we have only to slide the wings or tail forward and back until the (calculated) center of pressure falls over this observed center of gravity. But in the very act of so adjusting the wings and tail, the center of gravity is itself altered, and the operation has to be several times repeated in order to get the two values (the center of pressure and center of gravity) as near each other as they are found in the above-mentioned table, our object being to predict the position which will make the actual flight itself horizontal. How far this result has been obtained, experiment in actual flight alone can show, and from a comparison of the prediction with the results of observation, we endeavor to improve the formula.

The difficulties of these long-continued early experiments were enhanced by the ever-present difficulty which continued through later ones, that it was almost impossible to build the model light enough to enable it to fly, and at the same time strong enough to withstand the strains which flight imposed upon it. The models were broken up by their falls after a few flights, and had to be continually renewed, while owing to the slightness of their construction, the conditions of observation could not be exactly repeated; and these flights themselves, as has already been stated, were so brief in time (usually less than six seconds), so limited in extent (usually less than twenty metres), and so wholly capricious and erratic, owing to the nature of the rubber motor and other causes, that very many experiments were insufficient to eliminate these causes of mal-observation.

It is not necessary to take the reader through many of them, but not to pass over altogether a labor which was so great in proportion to the results, but whose results, such as they were, were the foundation of all after knowledge, I will, as illustrations, take from an almost unlimited mass of such material the observations of November 20, 1891, which were conducted with Model No. 30 with a single pair of wings, shown in Plate1, and with another one, No. 31, also shown[p018]in Plate1, with superposed wings, which was used for the purpose of comparison. S. P. Langley was the observer, the place of observation the larger upper hall of the Smithsonian building, at Washington, the time being taken by a stop-watch, and the distance by a scale laid down upon the floor. The models were in every case held by an assistant and launched by hand, being thrown off with a slight initial velocity. In the case of No. 30, the preliminary calculation of the position of the center of pressure had been made by the process already described; the center of gravity, with reference to the horizontal plane, was determined by simply suspending the whole by a cord.

PL. 1. RUBBER-MOTOR MODEL AERODROMESNOS. 11, 13, 14, 15, 26, 30, 31◊

PL. 2. RUBBER-MOTOR MODEL AERODROMESNOS. 11, 13, 14◊

PL. 3. RUBBER-MOTOR MODEL AERODROMESNOS. 15, 24◊

PL. 4. RUBBER-MOTOR MODEL AERODROMENO. 26◊

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