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[◊] Contents

SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOLUME 27 NUMBER 3

LANGLEY MEMOIR ON MECHANICAL FLIGHT

PART I. 1887 TO 1896

BY SAMUEL PIERPONT LANGLEY EDITED BY CHARLES M. MANLY

PART II. 1897 TO 1903

BY CHARLES M. MANLY Assistant in Charge of Experiments

(PUBLICATION 1948)

CITY OF WASHINGTON
PUBLISHED BY THE SMITHSONIAN INSTITUTION
1911

Commission to whom this Memoir
has been referred:

The Lord Baltimore Press BALTIMORE, MD., U. S. A.

[◊] ADVERTISEMENT

The present work, entitled “Langley Memoir on Mechanical Flight,” as planned by the late Secretary Samuel Pierpont Langley, follows his publications on “Experiments in Aerodynamics” and “The Internal Work of the Wind” printed in 1891 and 1893, respectively, as parts of Volume 27 of the Smithsonian Contributions to Knowledge.

This Memoir was in preparation at the time of Mr. Langley’s death in 1906, and Part I, recording experiments from 1887 to 1896, was written by him. Part II, on experiments from 1897 to 1903, has been written by Mr. Charles M. Manly, who became Mr. Langley’s Chief Assistant in June, 1898. The sources of information for this Part were the original carefully recorded accounts of the experiments described.

It is expected later to publish a third part of the present memoir, to consist largely of the extensive technical data of tests of the working of various types of curved surfaces, propellers, and other apparatus.

It is of interest here to note that experiments with the Langley type of aerodrome[1] did not actually cease in December, 1903, when he made his last trial with the man-carrying machine, but as recently as August 6, 1907, a French aviator made a flight of nearly 500 feet with an aerodrome of essentially the same design. (See Appendix.)

In accordance with the established custom of referring to experts in the subject treated, all manuscripts intended for publication in the Smithsonian Contributions to Knowledge, this work was examined and recommended by a Commission consisting of Mr. O. H. Tittman, Superintendent of the United States Coast and Geodetic Survey, who witnessed some of the field trials, George O. Squier, Ph. D. (Johns Hopkins), Major, Signal Corps, U. S. Army, and Albert Francis Zahm, Ph. D., of Washington City.

CHARLES D. WALCOTT,

Secretary of the Smithsonian Institution.

[1] The name “aerodrome” was given by Secretary Langley to the flying machine in 1893, from ἀεροδρομέω (to traverse the air) and ἀεροδρόμος air runner.—Internal Work of the Wind, p. 5.

[◊] PREFACE

The present volume on Mechanical Flight consists, as the title-page indicates, of two parts. The first, dealing with the long and notable series of early experiments with small models, was written almost entirely by Secretary Langley with the assistance of Mr. E. C. Huffaker and Mr. G. L. Fowler in 1897. Such chapters as were not complete have been finished by the writer and are easily noted as they are written in the third person. It has been subjected only to such revision as it would have received had Mr. Langley lived to supervise this publication, and has therefore the highest value as an historical record. The composition of the second part, dealing with the later experiments with the original and also new models and the construction of the larger aerodrome, has necessarily devolved upon me. This is in entire accordance with the plan formed by Mr. Langley when I began to work with him in 1898, but it is to me a matter of sincere regret that the manuscript in its final form has not had the advantage of his criticism and suggestions. If the reader should feel that any of the descriptions or statements in this part of the volume leave something to be desired in fullness of detail, it is hoped that some allowance may be made for the fact that it has been written in the scanty and scattered moments that could be snatched from work in other lines which made heavy demands upon the writer’s time and strength. It is believed, however, that sufficient data are given to enable any competent engineer to understand thoroughly even the most complicated phases of the work.

Persons who care only for the accomplished fact may be inclined to underrate the interest and value of this record. But even they may be reminded that but for such patient and unremitting devotion as is here enregistered, the now accomplished fact of mechanical flight would still remain the wild unrealized dream which it was for so many centuries.

To such men as Mr. Langley an unsuccessful experiment is not a failure but a means of instruction, a necessary and often an invaluable stepping-stone to the desired end. The trials of the large aerodrome in the autumn of 1903, to which the curiosity of the public and the sensationalism of the newspapers gave a character of finality never desired by Mr. Langley, were to him merely members of a long series of experiments, as much so as any trial of one of the small aerodromes or even of one of the earliest rubber-driven models. Had his health and strength been spared, he would have gone on with his experiments undiscouraged by these accidents in launching and undeterred by criticism and misunderstanding.

Moreover, it is to be borne in mind that Mr. Langley’s contribution to the solution of the problem is not to be measured solely by what he himself accomplished, important as that is. He began his investigations at a time when not only the general public but even the most progressive men of science thought of mechanical flight only as a subject for ridicule, and both by his epoch-making investigations in aerodynamics and by his own devotion to the subject of flight itself he helped to transform into a field of scientific inquiry what had before been almost entirely in the possession of visionaries.

The original plans for this publication provided for a third part covering the experimental data obtained in tests of curved surfaces and propellers. Owing to the pressure of other matters on the writer, the preparation of this third part is not yet complete and is reserved for later publication.

CHARLES M. MANLY.

NEW YORK CITY.

CONTENTS

LIST OF PLATES

[◊] REFERENCES

[◊] LANGLEY MEMOIR ON MECHANICAL FLIGHT PART I. 1887 TO 1896 BY S. P. LANGLEY EDITED BY CHARLES M. MANLY

CHAPTER I
INTRODUCTORY

I[2] announced in 1891,[3] as the result of experiments carried on by me through previous years, that it was possible to construct machines which would give such a velocity to inclined surfaces that bodies indefinitely heavier than the air could be sustained upon it, and moved through it with great velocity. In particular, it was stated that a plane surface in the form of a parallelogram of 76.2 cm. × 12.2 cm. (30 × 4.8 inches), weighing 500 grammes (1.1 lbs.), could be driven through the air with a velocity of 20 metres (65.6 feet) per second in absolutely horizontal flight, with an expenditure of 1/200 horse-power, or, in other terms, that 1 horse-power would propel and sustain in horizontal flight, at such a velocity (that is, about 40 miles an hour), a little over 200 pounds weight of such surface, where the specific gravity of the plane was a matter of secondary importance, the support being derived from the elasticity and inertia of the air upon which the body is made to run rapidly.

It was further specifically remarked that it was not asserted that planes of any kind were the best forms to be used in mechanical flight, nor was it asserted, without restrictions, that mechanical flight was absolutely possible, since this depended upon our ability to get horizontal flight during transport, and to leave the earth and to return to it in safety. Our ability actually to do this, it was added, would result from the practice of some unexplored art or science which might be termed Aerodromics, but on which I was not then prepared to enter.

I had at that time, however, made certain preliminary experiments with flying models, which have been continued up to the present year,[4] and at the same time I have continued experiments distinct from these, with the small whirling-table established at Washington. The results obtained from the latter being supplemental to those published in “Experiments in Aerodynamics,” and [p002] being more or less imperfect, were at first intended not for publication, but for my own information on matters where even an incomplete knowledge was better than the absence of any.

It is to be remembered that the mechanical difficulties of artificial flight have been so great that, so far as is known, never at any time in the history of the world previous to my experiment of May, 1896, had any such mechanism, however actuated, sustained itself in the air for more than a few seconds—never, for instance, a single half-minute—and those models which had sustained themselves for these few seconds, had been in almost every case actuated by rubber springs, and had been of such a size that they should hardly be described as more than toys. This refers to actual flights in free air, unguided by any track or arm, for, since the most economical flight must always be a horizontal one in a straight line,[5] the fact that a machine has lifted itself while pressed upward against an overhead track which compels the aerodrome to move horizontally and at the proper angle for equilibrium, is no proof at all of real “flight.”

I desire to ask the reader’s consideration of the fact that even ten years ago,[6] the whole subject of mechanical flight was so far from having attracted the general attention of physicists or engineers, that it was generally considered to be a field fitted rather for the pursuits of the charlatan than for those of the man of science. Consequently, he who was bold enough to enter it, found almost none of those experimental data which are ready to hand in every recognized and reputable field of scientific labor. Let me reiterate the statement, which even now seems strange, that such disrepute attached so lately to the attempt to make a “flying-machine,” that hardly any scientific men of position had made even preliminary investigations, and that almost every experiment to be made was made for the first time. To cover so vast a field as that which aerodromics is now seen to open, no lifetime would have sufficed. The preliminary experiments on the primary question of equilibrium and the intimately associated problems of the resistance of the sustaining surfaces, the power of the engines, the method of their application, the framing of the hull structure which held these, the construction of the propellers, the putting of the whole in initial motion, were all to be made, and could not be conducted with the exactness which would render them final models of accuracy.

I beg the reader, therefore, to recall as he reads, that everything here has been done with a view to putting a trial aerodrome successfully in flight within a few years, and thus giving an early demonstration of the only kind which is conclusive in the eyes of the scientific man, as well as of the general public—a demonstration that mechanical flight is possible-—by actually flying.

All that has been done, has been with an eye principally to this immediate [p003] result, and all the experiments given in this book are to be considered only as approximations to exact truth. All were made with a view, not to some remote future, but to an arrival within the compass of a few years at some result in actual flight that could not be gainsaid or mistaken.

Although many experimenters have addressed themselves to the problem within the last few years—and these have included men of education and skill—the general failure to arrive at any actual flight has seemed to throw a doubt over the conclusions which I had announced as theoretically possible.

When, therefore, I was able to state that on May 6, 1896, such a degree of success had been attained that an aerodrome, built chiefly of steel, and driven by a steam engine, had indeed flown for over half a mile—that this machine had alighted with safety, and had performed a second flight on the same day, it was felt that an advance had been made, so great as to constitute the long desired experimental demonstration of the possibility of mechanical flight. These results were communicated to the French Academy in the note given below.[7]

Independently of the preliminary experiments in aerodynamics already published, I had been engaged for seven years in the development of flying models. Although the work was discouraging and often resulted in failure, success was finally reached under the conditions just referred to, which obviously admitted of its being reached again, and on a larger scale, if desired. [p004]

In view of the great importance of these experiments, as demonstrating beyond question the practicability of the art of mechanical flight, and also in view of the yet inchoate state of this art, I have thought it worth while to publish an account of them somewhat in detail, even though they involve an account of failures; since it is from them, that those to whom it may fall to continue such constructions, will learn what to avoid, as well as the raison d’etre of the construction of the machines which have actually flown.

In an established art or science, this description of the essays and failures which preceded full knowledge would have chiefly an historical interest. Here almost nothing is yet established beyond the fact that mechanical flight has actually been attained. The history of failure is in this case, then, if I do not mistake, most necessary to an understanding of the road to future success, to which it led, and this has been my motive in presenting what I have next to say so largely in narrative form.

[◊]

[p005]

CHAPTER II
PRELIMINARY

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.

EXPERIMENTS WITH SMALL MODELS

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,”[8] it 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,[9] obtained 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,”[10] varies 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 desired horizontal direction during transport—questions which, in my opinion, are only to be answered by further experiment and which belong to the inchoate art or science of aerodromics on 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, Plates [1] and [2].

AB is 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 screw HH1 is 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 of AB there is a “wing” surface DC, 45 cm. long and 11 cm. broad, the ends C and D being raised and a little curved. In front of the screw is the horizontal rudder GK having a shape like that of the first surface, with its ends also turned up, and inclined 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. Let CD be 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 rudder GK, 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.[12] The 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 surface CD, 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.[13] In 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 from 12 to 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, if ab be such a supporting surface in the form of a plane of uniform thickness and weight, suspended at c (ac being somewhat greater than cb) and subjected to the pressure of a wind in the direction of the arrow, the pressure on the lesser arm cb will overpower that on the greater arm ac! 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 of C. 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 an 1812-inch propeller 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 Plates [1] and [4], 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 wings83
Width of upper wings14
Width of lower wings19
Diameter of propeller29
Area of upper wings1134sq. cm.
Area of lower wings1548
Area of tail144
Weight of wings51grammes
Weight of tail7
Weight of frame38
Weight of wheels20
Weight of rubber (.09 pound)40
Total weight156
No. of turns of rubber100
Time of running down8seconds
Horse-power from preceding data0.001HP

[p013]

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 Plates [1] to [4], 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]

ABBREVIATIONS AND SYMBOLS EMPLOYED

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. Let X, Y and Z be 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 line X lies North and South; the line Y lies East and West, and the line Z points to the zenith. Now place the aerodrome on the floor so that its principal axis lies horizontally in the plane XZ, with its head pointing North, and in such a position that a line passing through the center of the propellers shall coincide with the line Y.

FIG. 2. Diagram showing mensural coordinates.

When measurements are made on or parallel to the line X, the point of intersection O will 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 line Z, the point O will 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 line Y, the point O will 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 plane XZ.

EXAMPLE

In the figure the point T in the tail, if 15 centimetres to the South of O, 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.

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

CG1 CG2 represent the Center of Gravity as referred to the first, or horizontal, and to the second, or vertical plane, respectively.

CP represents the Center of Pressure[14] of the whole aerodrome, or (with a subscript) of any specially designated part.

CF represents 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; HP represents 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]

EXPERIMENTS WITH AERODROMES NOS. 30 AND 31.

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 figures CPm = 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 the CP will 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 resulting CP of 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 Plate [1], and with another one, No. 31, also shown [p018] in Plate [1], 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 AERODROMES NOS. 11, 13, 14, 15, 26, 30, 31 [◊]

PL. 2. RUBBER-MOTOR MODEL AERODROMES NOS. 11, 13, 14 [◊]

PL. 3. RUBBER-MOTOR MODEL AERODROMES NOS. 15, 24 [◊]

PL. 4. RUBBER-MOTOR MODEL AERODROME NO. 26 [◊]

OBSERVATION OF NOVEMBER 20, 1891.

OBSERVER, S.P.L.LOCALITY, UPPERHALL, SMITHSONIANBUILDING.
No. 30.
Single wings.		No. 31.
Superposed wings.
CPm1516.5cm.
CG115151517cm.
CFw1528cm.
Length (without fender)120cm. =3.94ft.120cm. =3.94ft.
Width over wing tips120cm. =3.94ft.120cm. =3.94ft.
Weight of rubber (72 grammes in each tube)144gr. =0.32lbs.144gr. =0.32lbs.
Total flying weight (including tail)432gr. =0.95lbs.506gr. =1.11lbs.
Turns of rubber3030
Diameter of propellers37cm. =1.21ft.37cm. =1.21ft.
Width of propellers7cm. =0.23ft.7cm. =0.23ft.
Pitch of propellers50cm. =1.64ft.50cm. =1.64ft.
Area of wings (each 992 sq.cm.)1984sq.cm.=2.13sq.ft.Each pair 1984 sq.cm.= 2.13 sq.ft. Total 3968 sq.cm.= 4.26 sq.ft.
Area of tail373sq.cm.=0.40sq.ft.373sq.cm.=0.40sq.ft.

Area of wings and tail in No. 30, 2357 sq. cm. = 2.53 sq. ft. 2.53 sq. ft. ÷ .95 = 2.7. Therefore, there are 2.7 or nearly 3 square feet of sustaining area to the pound.

Nov. 20, 1891.
Flight.Aerodrome.Results.
1No. 30With 30 turns of the rubber, flew low through 10 metres.
2No. 30Flew heavily through 12 metres.
3No. 31Flew high and turned to left; distance not noted.
4No. 31The right wing having been weighted (to depress it and correct the tendency to turn to the left), model flew high, but the rubber ran down when it had obtained a flight of 10 metres.
5No. 31The wings were moved backward until the CP stood at 1493. The model still turned to the left; flight lasted three and a-half seconds; distance not noted.
6No. 31Vertical tail was adjusted so as to further increase the tendency to go to the right. In spite of all this, the model turned sharply to the left, flying with a nearly horizontal motion; time of flight not noted; distance not noted.
7No. 30Straight horizontal flight; time three and three-fifth seconds, when rubber ran down; distance 13 metres.
8No. 30Straight flight as before; time two and four-fifth seconds; distance 13 metres.
9No. 30With a curved wing in the same position as the flat wing had previously occupied, model flew up and struck the ceiling (nearly 30 feet high), turning to right, with a flight whose curtate length was 10 metres.
10No. 30Wing having been carried back 5 centimetres, model still flew up, but not so high, and still turned to the right.
11No. 30Wings carried back 5 centimetres more; model still flew high; time two and two-fifths seconds; distance 13 metres.
12No. 30Wings carried back 4 centimetres more; model still flew high during a flight of 13 metres.

The observations now ceased, owing to the breaking up of the model.

The objects of these experiments, as of every other, were to find the practical conditions of equilibrium and of horizontal flight, and to compare the calculated with the observed positions of the center of pressure. They enable us to make a comparison of the performances given by earlier ones with a light rubber motor, with the relatively heavy motors used to-day, as well as a comparison of single flat, single curved, and superposed flat wings.

The average time of the running down of the rubber in flight was something like three seconds, while the average time of its running down when standing still was but one and a half seconds. It might have been expected from theory that it would take longer to run down when stationary, than in flight, and this was one of the many anomalies observed, whose explanation was found later in the inevitable defects of such apparatus.

The immediate inferences from the day’s work were:

1. That the calculated position of the CP at rest, as related to the CG, is trustworthy only in the case of the plane wing.

2. The formula altogether failed with the curved wing, for which the CP had to be carried indefinitely further backward.

On comparing the previous flights of November 14, with these, it seems that with the old rubber motor of 35 grammes and 50 turns, the single wing, either plane or curved, is altogether inferior to the double wing; while with the increased motor power of this day, the single wing, whether plane or curved, seems to be as good as the double wing. It also seems that the curved wing was rather more efficient than the plane one.

The weight of the rubber in each tube was 72 grammes, or 0.16 pounds; mean speed of flight in horizontal distance 412 metres (about 15 feet) per second.[16]

From experiments already referred to, there were found available 300 foot-pounds of energy in a pound of rubber as employed, and in 0.16 of a pound, 48 foot-pounds of energy were used; 4833,000 or 0.00145 = the horse-power exerted in [p019] one minute, but as the power was in fact expended in 1/20 of that time we have 20 × 0.00145 = 0.029; that is, during the brief flight, about 0.03 of a horse-power was exerted, and this sustained a total weight of only about a pound.

In comparing this flight with the ideal conditions of horizontal flight in “Aerodynamics,” it will be remembered that this model’s flight was so irregular and so far from horizontal, that in one case it flew up and struck the lofty ceiling. The angle with the horizon is, of course, so variable as to be practically unknown, and therefore no direct comparison can be instituted with the data given on page 107 of “Experiments in Aerodynamics,” but we find from these that at the lowest speed there given of about 35 feet per second, 0.03 of a horse-power exerted for three seconds would carry nearly one pound through a distance of somewhat over 100 feet in horizontal flight.

The number of turns of the propellers multiplied by the pitch corresponds to a flight of about 16 metres, while the mean actual flight was about 12. It is probable, however, that there was really more slip than this part of the observation would indicate. It was also observed that there seemed to be very little additional compensatory gain in the steering of No. 30 for the weight of the long rudder-tail it carried. It may be remarked that in subsequent observations the superiority of the curved wing in lifting power was confirmed, though it was found more liable to accident than the flatter one, tending to turn the model over unless it was very carefully adjusted.

It may also be observed that these and subsequent observations show, as might have been anticipated, that as the motor power increased, the necessary wing surface diminished, but that it was in general an easier and more efficient employment of power to carry a surface of four feet sustaining area to the pound than one of three, while one of two feet to the pound was nearly the limit that could be used with the rubber motor.[17]

It may be remarked that the flights this day, reckoned in horizontal distance, were exceptionally short, but that the best flights at other times obtained with these models (30 and 31) did not exceed 25 metres. Such observations were continued in hundreds of trials, without any much more conclusive results. [p020]

The final results, then, of the observations with rubber-driven models (which were commenced as early as 1887, continued actively through the greater portion of the year 1891 and resumed, as will be seen later, even as late as 1895), were not such as to give information proportioned to their trouble and cost, and it was decided to commence experiments with a steam-driven aerodrome on a large scale.

[◊]

[p021]

CHAPTER III
AVAILABLE MOTORS

In the introductory chapter to “Experiments in Aerodynamics,” it was asserted that

“These researches have led to the result that mechanical sustentation of heavy bodies in the air, combined with very great speeds, is not only possible, but within the reach of mechanical means we actually possess.”

It was, however, necessary to make a proper selection in order to secure that source of power which is best adapted to the requirements of mechanical flight. Pénaud had used india rubber as the cheapest and at the same time the most available motor for the toys with which he was experimenting, but when models were constructed that were heavier than anything made prior to 1887, it appeared, after the exhaustive trials with rubber referred to in the preceding chapter, that something which could give longer and steadier flights must be used as a motor, even for the preliminary trials, and the construction of the large steam-driven model known as No. 0, and elsewhere described, was begun. Even before the completion of this, the probability of its failure grew so strong that experiments were commenced with other motors, which it was hoped might be consistent with a lighter construction.

These experiments which commenced in the spring of 1892 and continued for nearly a twelvemonth, were made upon the use of compressed air, carbonic-acid gas, electricity in primary and storage batteries, and numerous other contrivances, with the result that the steam engine was finally returned to, as being the only one that gave any promise of immediate success in supporting a machine which would teach the conditions of flight by actual trial, though it may be added that the gas engine which was not tried at this time on account of engineering difficulties, was regarded from the first as being the best in theory and likely to be ultimately resorted to. All others were fundamentally too heavy, and weight was always the greatest enemy.

It is the purpose of this chapter to pass in brief review the work that was done and the amount of energy that was obtained with these several types of motors, as well as the obstacles which they presented to practical application upon working aerodromes.

INDIA RUBBER

India rubber is the source of power to which the designer of a working model naturally turns, where it is desirable that it shall be, above all, light and free from the necessity of using complicated mechanism. Rubber motors were, [p022] therefore, used on all of the earlier models, and served as the basis of calculations made to determine the amount of power that would be required to propel aerodromes with other sources of energy.

Some of the disadvantages inherent in the use of rubber are at once apparent, such as the limited time during which its action is available, the small total amount of power, and the variability in the amount of power put forth in a unit of time between the moment of release and the exhaustion of the power. In addition, serious, though less obvious difficulties, present themselves in practice.

There are two ways in which rubber can be used; one by twisting a hank of strands, and, while one end is held fast, allowing the other to revolve; the other, by a direct longitudinal stretching of the rubber, one end being held fast and the other attached to the moving parts of the mechanism. The former method was adopted by Pénaud, and was also used in all of my early constructions, but while it is most convenient and simple in its (theoretical) application, it has, in addition to the above drawbacks, that of knotting or kinking, when wound too many turns, in such a way as to cause friction on any containing tube not made impracticably large, and also that of unwinding so irregularly as to make the result of one experiment useless for comparison with another.

In 1895, some experiments were made in which the latter method was used, but this was found to involve an almost impracticable weight, because of the frame (which must be strong enough to withstand the end pull of the rubber) and the mechanism needed to convert the pull into a movement of rotation.

As the power put forth in a unit of time varies, so there is a corresponding variation according to the original tension to which the rubber is subjected. Thus in some experiments made in 1889 with a six-bladed propeller 18.8 inches in diameter, driven by a rubber spring 1.3 inches wide, 0.12 inch thick and 3 feet long, doubled, and weighing 0.38 pound, the following results were obtained:

Number of twists of rubber5075100
Time required to run down7sec.10sec.12sec.
Foot-pounds developed37.563.0124.6
Foot-pounds developed per min.321.4378.0623.0
Horse-power developed0.00970.01150.0189

Thus we see that, with twice the number of turns, more than three times the amount of work was done and almost twice the amount of power developed, giving as a maximum for this particular instance 328 foot-pounds per pound of rubber.

The usual method of employing the twisted rubber was to use a number of fine strands formed into a hank looped at each end. One of these hanks, consisting of 162 single or 81 double strands of rubber, and weighing 73 grammes, when given 51 turns developed 55 foot-pounds of work, which was put out in 4 seconds. This corresponds to 0.01 horse-power per minute for one pound of rubber. [p023]

The results of a large number of tests show that one pound of twisted rubber can put forth from 450 to 500 or more foot-pounds of work, but at the cost of an overstrain, and that a safe working factor can hardly be taken at higher than 300 foot-pounds, if we are to avoid the “fatigue” of the rubber, which otherwise becomes as marked as that of a human muscle.

While twisting is an exceedingly convenient form of application of the resilience of rubber to the turning of propelling wheels, the direct stretch is, as has been remarked, much more efficient in foot-pounds of energy developed by the same weight of rubber. It was found that rubber could not, without undue “fatigue,” be stretched to more than four and a half times its original length, though experiments were made to determine the amount of work that a rubber band, weighing one pound, was capable of doing, the stretching being carried to seven times its original length. The results varied with the rubber used and the conditions of temperature under which the experiments were tried, ranging from 1543 foot-pounds to 2600 foot-pounds. The tests led to the conclusion that, for average working, one pound of rubber so stretched, is capable of doing 2000 foot-pounds of work, but, owing to the weight of the supporting frame and of the mechanism, this result can be obtained only under conditions impracticable for a flying machine. In the more practicable twisted form it furnishes, as has been said, less than a fifth of that amount.

The conclusions reached from these experiments are:

1. The length of the unstretched rubber remaining the same, the sustaining power will be directly proportional to the weight of rubber;

2. With a given weight of rubber, the end strain is inversely proportional to the length of the unstretched rubber;

3. With a given weight of rubber, the work done is constant, whatever the form; hence if we let w = the work in foot-pounds, g = the weight of the rubber in pounds, and k = a constant taken at 2000 as given above, we have

w = kg = 2000 g foot-pounds.

This is for an extension of seven units of length, so that for a unit of extension we would have approximately

w = 300 g foot-pounds

which for four units of extension corresponds very closely to the 1300 foot-pounds which Pénaud claims to have obtained.

4. The end strain varies with the cross-section for a given unit of extension.

These results can lead to but one conclusion; that for the development of the same amount of power when that amount shall be 1 horse-power or more, rubber weighs enormously more than a steam engine, besides being less reliable [p024] for a sustained effort, and, therefore, cannot be used for propelling aerodromes intended for a flight that is to be prolonged beyond a few seconds.[18]

It may be desirable to present a tabular view of the theoretical energy of available motors, which it will be noticed is a wholly different thing from the results obtained in practice. Thus, we represent the weight of rubber only, without regard to the weight of the frame required to hold it. In the steam engine, we consider the theoretical efficiency per pound of fuel, without regarding the enormous waste of weight in water in such small engines as these, or the weight of the engine itself. We treat the hot-water engine in like manner, and in regard to carbonic acid and compressed air, we take no note of the weight of the containing vessel, or of the cylinders and moving parts. In the same way we have the theoretical potency of electricity in primary and storage batteries, without counting the weight of the necessary electromotors; and of the inertia-engine without discussing that of the mechanism needed to transmit its power.

Foot-pounds of energy in one pound of

Gasoline15,625,280
Alcohol9,721,806
Gunpowder960,000
Hot water, under pressure of 100 atmospheres383,712
Air, under pressure of 100 atmospheres, isothermal expansion120,584
Liquid carbonic acid, at temperature of 30° and pressure of 100 atmospheres78,800
Electric battery; short-lived, thin walled; chromic acid and platinum75,000
Steel ring, 8 inches in diameter, at speed of 3000 turns per minute19,000
Storage battery17,560
Rubber, pulled2,000
Rubber, twisted300

It may be interesting to consider next, in even a roughly approximate way, what may be expected from these various sources of energy in practice.

STEAM ENGINE

The steam engine on a small scale, and under the actual restrictions of the model, must necessarily be extremely wasteful of power. If we suppose it to realize 2 per cent of the theoretical energy contained in the fuel, we shall be assuming more than was actually obtained. The energy of the fuel cannot be obtained at all, of course, without boiler and engine, whose weight, for the purpose of the following calculation, must be added to that of the fuel; and if we suppose the weight of the boilers, engines and water, for a single minute’s flight, to be collectively ten pounds, we shall take an optimistic view of what may be expected under ordinary conditions. We have in this view 1/500 of the [p025] theoretical capacity possibly realizable under such conditions, but if we take 1/1000 we shall probably be nearer the mark. Even in this case we have, when using gasoline as fuel, 15,625 foot-pounds per minute, or nearly 0.50 horse-power, as against .0091 horse-power in the case of the rubber, so that even with this waste and with the weight of the engines necessary for a single minute’s service, the unit weight of fuel employed in the steam engine gives 55 times the result we get with rubber.

PL. 5. RUBBER-PULL MODEL AERODROME [◊]

PL. 6. RUBBER-PULL MODEL AERODROME [◊]

PL. 7. RUBBER-PULL MODEL AERODROME [◊]

PL. 8. RUBBER-PULL MODEL AERODROME [◊]

PL. 9. RUBBER-PULL MODEL AERODROME [◊]

With alcohol we have about 23 the result that is furnished by gasoline, since nearly the same boiler and engine will be used in either case. Certain difficulties which at first appeared to be attendant on the use of gasoline on a small scale induced me to make the initial experiments with alcohol. This was continued because of its convenience during a considerable time, but it was finally displaced in favor of gasoline, not so much on account of the superior theoretical efficiency of the latter, as for certain practical advantages, such as its maintaining its flame while exposed to wind, and like considerations.

GUNPOWDER

Although there are other explosives possessing a much greater energy in proportion to their weight than gunpowder, this is the only one which could be considered in relation to the present work, and the conclusion was finally reached that it involved so great a weight in the containing apparatus and so much experiment, that, although the simplicity of its action is in its favor where crude means are necessary, experiments with it had better be deferred until other things had been tried.

HOT-WATER ENGINE

A great deal of attention was given to the hot-water engine, but it was never put to practical use in the construction of an aerodrome, partly on account of the necessary weight of a sufficiently strong containing vessel.

COMPRESSED AIR

Compressed air, like the other possible sources of power, was investigated, but calculations from well-authenticated data showed that this system of propelling engines would probably be inadequate to sustain even the models in long flights. As the chief difficulty lies in the weight, not of the air, but of the containing vessel, numerous experiments were made in the construction of one at once strong and light. The best result obtained was with a steel tube 40 mm. in diameter, 428 mm. in length, closed at the ends by heads united by wires, which safely contained 538 cubic cm. of air at an initial pressure of 100 atmospheres for a weight of 521 grammes. [p026]

If we suppose this to be used, by means of a proper reducing valve, at a mean pressure of 100 pounds, for such an engine as that of Aerodrome No. 5, which takes 60 cubic cm. of air at each stroke, we find that (if we take no account of the loss by expansion) we have 18,329 foot-pounds of energy available, which on the engine described will give 302 revolutions of the propellers.

There are such limits of weight, and the engines must be driven at such high speeds, that the increased economy that might be obtained by re-heating the air would be out of the question. The principal object in using it would have been the avoidance of fire upon the aerodrome, and the expansion of the unheated air would probably have caused trouble with freezing, while the use of hot (i. e. superheated) water was impracticable. So when, after a careful computation, it was found that, having regard to the weight of the containing vessel, only enough compressed air could be stored at 72 atmospheres and used at 4, to run a pair of engines with cylinders 0.9 inch in diameter by 1.6 inches stroke, at a speed of 1200 revolutions per minute for 20 seconds, all further consideration of its adaptation to the immediate purpose was definitely abandoned. This course, however, was not taken until after a model aerodrome for using compressed air had been designed and partially built. Then, after due consideration, it was decided to make the test with carbonic-acid gas instead.

GAS

The gas engine possesses great theoretical advantages. At the time of these experiments, the gas engine most available for the special purposes of the models was one driven by air drawn through gasoline. As the builders could not agree to reduce the weight of a one horse-power engine more than one-half of the then usual model, and as the weight of the standard engine was 470 pounds, it was obvious that to reduce this weight to the limit of less than 3 pounds was impracticable under the existing conditions, and all consideration of the use of gas was abandoned provisionally, although a gasoline engine of elementary simplicity was designed but never built. I purposed, however, to return to this attractive form of power if I were ever able to realize its theoretical advantages on the larger scale which would be desirable.

ELECTRICITY

As it was not intended to build the model aerodromes for a long flight, it was thought that the electric motor driven by a primary or storage battery might possibly be utilized. It therefore occurred to me that a battery might be constructed to give great power in proportion to its weight on condition of being short-lived, and that in this form a battery might perhaps advantageously take the place of the dangerous compressed-air tubes that were at the time (1893) [p027] under consideration for driving the models. I assumed that the longest flight of the model would be less than five minutes. Any weight of battery, then, that the model carried in consumable parts lasting beyond this five minutes would be lost, and hence it was proposed to build a battery, the whole active life of which would be comprised in this time, to actuate a motor or motors driving one or two propellers.

According to Daniell, when energy is stored in secondary batteries, over 300,000 megergs per kilogramme of weight can be recovered and utilized if freshly charged.

300,000 megergs = 0.696 horse-power for 1 min.

300,000 megergs = 0.139 horse-power for 5 min.

In a zinc and copper primary battery with sulphuric acid and water, one kilogramme of zinc, oxidized, furnishes at least 1200 calories as against 8000 for one kilogramme of carbon, but it is stated that the zinc energy comes in so much more utilizable a form that the zinc, weight for weight, gives practically, that is in work, 40 per cent that of carbon. The kilogramme of carbon gives about 8000 heat units, each equal to 107 kilogrammetres, or about 6,176,000 foot-pounds. Of this, in light engines, from 5 to 10 per cent, or at least 308,800 foot-pounds, is utilized, and 25 of this, or about 124,000 foot-pounds, would seem to be what the kilogramme of zinc would give in actual work. But to form the battery, we must have a larger weight of fluid than of zinc, and something must be allowed for copper. If we suppose these to bring the weight up to 1 kilogramme, we might still hope to have 50,000 foot-pounds or 1.5 horse-power for one minute, or 0.3 horse-power for 5 minutes.

Storage batteries were offered with a capacity of .25 horse-power for 5 minutes per kilogramme, but according to Daniell one cannot expect to get more than 0.139 horse-power from a freshly charged battery of that weight for the same time.

The plan of constructing a battery of a long roll of extremely thin zinc or magnesium, winding it up with a narrower roll of copper or platinized silver, insulating the two metals and then pouring over enough acid to consume the major portion of the zinc in 5 minutes, was carefully considered, but the difficulties were so discouraging, that the work was not undertaken.

The lightest motors of 1 horse-power capacity of which any trace could be found weighed 25 pounds, and a prominent electrician stated that he would not attempt to construct one of that weight.

In trials with a 12 horse-power motor driving an 80 cm. propeller of 1.00 pitch-ratio, I apparently obtained a development of 0.56 indicated horse-power at 1265 revolutions; but at lower speeds when tried with the Prony brake, the brake horse-power fell to 0.10 at 546 revolutions, and even at 1650 revolutions [p028] it was but 0.262 indicated, with a brake horse-power of 0.144, or 55 per cent of that indicated.

With these results both of theoretical calculation and practical experiment, all thought of propelling the proposed aerodrome by electricity was necessarily abandoned.

CARBONIC-ACID GAS

At the first inception of the idea, it seemed that carbonic-acid gas would be the motive power best adapted for short flights. It can be obtained in the liquid form, is compact, gives off the gas at a uniform pressure dependent upon the temperature, and can be used in the ordinary steam engine without any essential modifications. The only provision that it seemed, in advance, necessary to make, was that of some sort of a heater between the reservoir of liquid and the engine, in order to prevent freezing, unless the liquid itself could be heated previous to launching.

The engines in which it was first intended to use carbonic acid were the little oscillating cylinder engines belonging to Aerodrome No. 1. The capacity of each cylinder was 21.2 cu. cm., so that 84.8 cu. cm. of gas would be required to turn the propellers one revolution when admitted for the full stroke, and 101,760 cubic cm. for 1200 revolutions. The density of the liquid at a temperature of 24° C. was taken as .72, and as 1 volume of liquid gives 180 volumes of gas at a pressure of 212 atmospheres, we have 101,760180 = 565 cu. cm. of liquid, or 407 grammes required for 1200 revolutions of the engines.

Thus, a theoretical calculation seemed to indicate that a kilogramme of liquid carbonic acid would be an ample supply for a run of two minutes. The experiments were, at first, somewhat encouraging. The speed and apparent power of the engines were sufficient for the purpose, but the length of time during which power could be obtained was limited.

In 1892, 415 grammes of carbonic acid drove the engines of Aerodrome No. 3 700 revolutions in 60 seconds, 900 in 75, and 1000 in 85 seconds, at the end of which time the gas was entirely expended. The diameter of these cylinders was 2.4 cm., the stroke of the pistons 7 cm., and the work done, that of driving a pair of 50 cm. propellers, when taken in comparison with the propeller tests detailed elsewhere, amounted to an effective horse-power of about 0.10 for the output of the engine.

The difficulties, however, that were experienced were those partially foreseen. The expansion of the gas made such serious inroads upon the latent heat of the liquid, that lumps of solid acid were formed in the reservoir, and could be heard rattling against the sides when the latter was shaken, while the expansion of the exhaust caused such a lowering of temperature at that point, that the [p029] pipes were soon covered with a thick layer of ice, and the free exit of the escaping gas was prevented.

Such difficulties are to be expected with this material, but here they were enhanced by the small scale of the construction and the constant demand for lightness. And it was found to be very hard to fill the small reservoirs intended to carry the supply for the engines. When they were screwed to the large case in which the liquid was received and the whole inverted, the small reservoir would be filled from one-third to one-half full, and nothing that could be done would force any more liquid to enter.

In view of these difficulties, and the objections to using a heater of any sort for the gas, as well as the absolute lack of success attendant upon the experiments of others who were attempting to use liquid CO2 as a motive power on a large scale elsewhere, experiments were at first temporarily and afterwards permanently abandoned.

The above experiments extended over nearly a year in time, chiefly during 1892, and involved the construction and use of the small aerodromes Nos. 1, 2, and 3, presently described.

[◊]

[p030]

CHAPTER IV
EARLY STEAM MOTORS AND OTHER MODELS

In dealing with the development of the aerodrome, subsequent to the early rubber-driven models, the very considerable work done and the failures incurred with other types of motors than steam, have been briefly dealt with in the preceding chapter, but are scarcely mentioned here, as no attempts at long flights were ever successful with any other motor than steam, and no information was gained from any of the experiments made with compressed air, gas, carbonic acid, or electricity, that was of much value in the development of the successful steam machines.

In November, 1891, after the long and unsatisfactory experiments with rubber-driven models already referred to, and before most of the experiments with other available motors than steam had been made, I commenced the construction of the engines and the design of the hull of a steam-driven aerodrome, which was intended to supplement the experiments given in “Aerodynamics” by others made under the conditions of actual flight.

In designing this first aerodrome, here called No. 0, there was no precedent or example, and except for the purely theoretical conditions ascertained by the experiments described in “Aerodynamics,” everything was unknown. Next to nothing was known as to the size or form, as to the requisite strength, or as to the way of attaching the sustaining surfaces; almost nothing was known as to the weight permissible, and nothing as to the proper scale on which to build the aerodrome, even if the design had been obtained, while everything which related to the actual construction of boiler and engines working under such unprecedented conditions was yet to be determined by experiment.

The scale of the actual construction was adopted under the belief that it must be large enough to carry certain automatic steering apparatus which I had designed, and which possessed considerable weight. I decided that a flying machine if not large enough to carry a manager, should in the absence of a human directing intelligence, have some sort of automatic substitute for it, and be large enough to have the means of maintaining a long and steady flight, during which the problems (which the rubber-driven models so imperfectly answered) could be effectually solved.

When, in 1891, it was decided to attempt to build this steam aerodrome, the only engine that had been made up to that time with any claim to the lightness and power I was seeking, was the Stringfellow engine, exhibited at the Crystal Palace in London, in 1868, which it was then announced developed 1 horse-power [p031] for a total weight (boiler and engines) of 13 pounds. The original engine came into the possession of the Institution in 1889 as an historical curiosity, but on examination, it was at once evident that it never had developed, and never could develop the power that had been attributed to it, and probably not one-tenth so much.

With the results obtained on the whirling-table at Allegheny as a basis, a theoretical computation of the weight which 1 horse-power would cause to soar showed that, with a plane whose efficiency should be equal to that of a 30 × 4.8 inch plane set at an angle of 5° and moving at a speed of 34 miles an hour, 1 horse-power would support 120 pounds.[19] With a smaller angle even better results could be obtained, but as the difficulties of guidance increase as the angle diminishes, I did not venture to aim at less than this. In this computation, no allowance was made for the fact that these results were obtained by a mechanism which forcibly maintained the supporting surface in the ideal condition of the best attainable angle of attack as if in perfect equilibrium, and above all in the equally ideal condition of perfectly horizontal flight.

Besides this, I had to consider in actual flight the air resistance due to the guy wires and hull, but after making an allowance of as much as three-quarters for these differences between the conditions of experiment and those of free flight, I hoped that 1 horse-power would serve to carry 30 pounds through the air if a supporting surface as large as 3 feet to the pound could be provided, and this was the basis of the construction which I will now describe.

The general form of this Aerodrome No. 0, without wings or propellers, is shown in the accompanying photograph in Plate [10]. Its dimensions and its weights, as first designed, and as finally found necessary, are as follows:

COMPARISON OF ESTIMATED AND ACTUAL WEIGHTS OF PARTS OF AERODROME “0”—IN POUNDS AND OUNCES.
EstimatedActual
lbs.oz.lbs.oz.
Engines4041
Boilers and Burners8111314
Pumps and Attachments00110
Steering Apparatus0600
Frame of Hull and Braces, including bowsprit and tail tube77811
Oil tank covering and pipes00013
Shafts, ball bearings (2:1) and wooden propellers (1:7)11438
Wings (5:4) and guys (0:9)40513
Tail1522
Jacket at prow0040
Total without oil or water2711448

(The weights attained in the actual making were, as is seen, nearly double those first estimated, and this constant increase of weight under the exigencies of construction was a feature which could never be wholly eliminated.) [p032]

After studying various forms for the hull or body of the prospective aerodrome, I was led to adopt the lines which Nature has used in the mackerel as most advantageous so far as the resistance of the air was concerned, but it proved to be difficult in construction to make the lines of the bow materially different from those of the stern, and in this first model the figure was symmetrical throughout.

As I wish that my experience may be of benefit to the reader, even in its failures, I will add that I made the not unnatural mistake of building on the plan on which the hull of an ordinary ship is constructed; that is, making the hull support the projecting bowsprit and other parts. In the aerodrome, what corresponds to the bowsprit must project far in advance of the hull to sustain the front wings, and a like piece must project behind it to sustain the rear wings and the tail, or the supporting surfaces of whatever kind. The mistake of the construction lay in disjoining these two and connecting them indirectly by the insufficiently strong hull which supported them. This hull was formed of longitudinal U-shaped ribs of thin steel, which rested on rings made of an alloy of aluminum, which possessed the lightness of the latter metal with very considerable toughness, but which was finally unsatisfactory. I may say parenthetically that in none of the subsequent constructions has the lightness of aluminum been found to compensate for its very many disadvantages. The two rods, which were each 1 metre in length, were with difficulty kept rigorously in line, owing to the yielding of the constructionally weak hull. It would have been better, in fact, to have carried the rod straight through at any inconvenience to the disposition of the boilers and the engine.

PL. 10. STEEL FRAMES OF AERODROMES NOS. 0, 1, 2, 3, 1891 AND 1892 [◊]

I may add that the sustaining surfaces, which were to be nearly flat wings, composed of silk stretched from a steel tube with wooden attachments, were to [p033] have been carried on the front rod, but, as subsequent experience has shown, these wings would have been inadequate to the work, both from their insufficient size and their lack of rigidity.

The propellers, which were to be 80 cm. in diameter, 1.25 pitch-ratio, and which were expected to make from five to six hundred revolutions a minute, were carried on the end of long tubular shafts, not parallel, but making with each other an angle of 25 degrees, and united by gears near the bow of the vessel in the manner shown in Plate [10].

The first engines were of the oscillating type, with the piston-rod connected directly to the crank; were very light, and were unprovided with many of the usual fittings belonging to a steam engine, such as rod or piston packing; and their construction was crude in comparison with their successors. They were tested with the Prony brake and found to be deficient in power, for with a steam pressure of 80 pounds to the square inch, they ran at the rate of 1170 revolutions per minute, and developed only .363 horse-power. It soon became evident that they were too light for the work that it was intended that they should do, and steps were taken, even before the completion of these tests, for the construction of a pair of more powerful cylinders, which should also be provided with a special boiler for the generation of the steam. Acting upon the supposition, in a saving of steam, it was decided to work with compounded cylinders. As two propellers were to be used, they were each fitted with a distinct pair of cylinders working directly upon the shaft, but so connected by gearing that they were compelled to turn at the same rate of speed.

The cylinders were of the inverted oscillating type, like the first pair of engines, but, unlike them, they were single-acting. The dimensions were: diameter of high-pressure cylinder 1.25 inches; low pressure, 1.94 inches, with a common stroke of 2 inches, and with cranks set opposite to each other so that one cylinder was always at work. The cylinders were held at their upper ends by a strap passing around a hollow conical trunk, which served the double purpose of a support for the cylinders and an intermediate receiver between them. This receiver had a mean inside diameter of 1.25 inches, with a length of 4.75 inches, so that it had about twice the cubical capacity of the high-pressure cylinder, while the displacement of the low-pressure cylinder was about 2.5 times that of the high; ratios that would have given satisfactory results, perhaps, had the steam pressure and other conditions been favorable to the use of the compound principle in this place. There were no valves for the admission of the steam, for, inasmuch as the engines were single-acting, it was possible to make ports in the cylinder-head act as the admission and exhaust ports as the cylinder oscillated, and thus avoid the complication and weight of eccentric and valves. [p034]

These cylinders were set in a light frame at an angle of 25° with each other, or 12.5° with the median line of the aerodrome, and drove the long propeller shafts as shown in Plate [10], No. 0. At the extreme forward end of the crank-shafts there was a pair of intermeshing bevel gears which served to maintain the rate of revolution of the two propellers the same.

FIG. 3. Boilers in use in 1891–1892.

The boiler built for this work was a beehive-shaped arrangement of coils of pipe. It consisted at first, as shown in Fig. 3, of three double coils of 38-inch copper pipe coiled up in the shape of a truncated cone, carrying in the central portion a pear-shaped receiver into the upper portion of which the water circulating through the coils discharged. Each of these receivers was connected at the top with the bottom of a long cylindrical drum, with hemispherical ends, which formed a steam space from which supply for the engines was drawn. The lower ends of the coils were connected with an injection pipe supplying the water. Each “beehive” had 23 turns of tubing, and had a base of 7.5 inches and a top diameter of 6 inches, the steam drum being 2.5 inches in diameter. I may here say that in the selection of the general type of boiler for the work to be done, there was never any hesitation regarding the use of the water-tube variety. Their superiority for the quick generation of large volumes of steam had been so pronounced that nothing else seemed capable of competing with [p035] them in this respect, regardless of the absolute economy of fuel that might or might not be exhibited. Hence, to the end of my experiments nothing else was used.

Even before the “beehive” boiler was completed, I was anxious to ascertain what could be done with a coil of pipe with a stream of water circulating through it, as well as with various forms of burners, for I realized that the success of the apparatus depended not only upon getting an exceedingly effective heating surface, but also an equally effective flame to do the heating.

For fuel I naturally turned to the liquids as being more compact and readily regulated. Whether to use some of the more volatile hydrocarbons or alcohol, was still an unsolved problem, but my opinion at the time was that, on the limited scale of the model, better results could probably be obtained with alcohol.

In the experiments made with a coil preliminary to the trial of the “beehive” boiler, I tried a simple horizontal coil of 38-inch copper pipe into which two forked burners working on the Bunsen principle and using city illuminating gas, were thrust. The jets were about 12 inch apart. The arrangement primed so badly that the engines could not get rid of the entrained water, and would only make a few turns.

I then tried the same coil with two 1.25-inch drums in the inside and with five longitudinal water tubes at the bottom, beneath which were the same two forked burners used in the previous experiment. The coils were covered with a sheet of asbestos, and two round burners were added. This boiler would hold a steam pressure of about 15 pounds and run the engine slowly; but if the pressure were allowed to rise to 60 pounds, the engine would drive a 2-foot propeller of 18-inch pitch at the rate of about 650 turns per minute for from 80 to 90 seconds, while the steam ran down to 10 pounds, showing that this boiler, at least, was too small. This was further shown in a trial of the plain coil made in October, 1891; 6 pounds of water were evaporated in 32 minutes under a pressure of 60 pounds. This was at the rate of 11.25 pounds per hour, or, taking the U. S. Centennial standard of 30 pounds of evaporation per horse-power, gave an available output of less than 13 horse-power.

With these results before me, I decided to make a trial of the “beehive” principle upon a smaller scale than in the boiler designed for Aerodrome No. 0. I used a small boiler of which the inner coil consisted of 8 turns of 38-inch copper tube about 28 gauge thick, and the outer coil of 11 turns of 14-inch copper pipe. This gave 12 feet of 38-inch, and 16 feet of 14-inch tubing. The drum was of No. 27 gauge, hard planished copper. With this boiler consuming 6 oz. of fuel, 80.3 oz. of water were evaporated in 28 minutes, or at the rate of about 10.75 pounds per hour. As these coils contained but 2.22 square feet of heating surface, and as the three to be built would contain 3.7 square feet each, it was estimated the [p036] 10 square feet afforded by them could safely be depended upon to provide steam for a 1 horse-power engine. As far as fuel consumption was concerned, the rate of evaporation was about 15.6 pounds of water per pound of gasoline, all of which was satisfactory.

The burner originally designed for use in connection with the “beehive” boilers, consisted of a small tank in which a quantity of gasoline was placed, the space above being filled with compressed air. Rising from the bottom of this tank was a small pipe coiling back and down and ending in an upturned jet from which the gas generated in the coil would issue. The burner thus served to generate its own gas and act as a heater for the boilers at the same time.

In the construction of Aerodrome No. 0, four of the “beehive” coils were placed in a line fore and aft. The fuel tank was located immediately back of the rear coil and consisted of a copper cylinder 11 cm. in diameter and 9 cm. long. The engines were placed immediately in front of the coils, all the apparatus being enclosed in a light framing, as shown in the photograph (Plate [10]).

Extending front and back from the hull were the tubes for supporting the wings and tail, each one metre in length. The cross-framing for carrying the propeller shafts was built of tubing 1.5 cm. diameter, and the shafts themselves were of the same size. The ribs of the hull were rings made of angle-irons measuring 1.50 × 1.75 cm., which were held in place longitudinally by five 0.7 cm. channel bars.

As it had been learned in the preliminary experiments with the model “beehive” boiler that the heated water would not of itself cause a sufficiently rapid circulation to be maintained through the tubes to prevent them from becoming red-hot, two circulating pumps were added for forcing the water through the coils of the two forward and two rear boilers respectively, the water being taken from the lower side of the drum and delivered into the bottom of the coils, which were united at that point for the purpose. A worm was placed upon each of the propeller shafts, just back of the engines, meshing in with a gear on a crank-shaft from which the pumps were driven. This shaft rotated at the rate of 1 to 24, so that for 1200 revolutions of the engine, it would make but 50, driving a single-acting plunger 1.2 cm. in diameter and 2 cm. stroke.

Apparently all was going well until I began to try the apparatus. First, there was a difficulty with the burner, which could not be made to give forth the relative amount of heat that had been obtained from the smaller model, and steam could not be maintained. With one “beehive” connected with the compound engine, and a 70 cm. propeller on the shaft, there were about 250 turns per minute for a space of about 50 seconds, in which time the steam would fall from 90 pounds to 25 pounds, and the engine would stop. Then, as we had no air-chamber on the pumps at the time, they would not drive the water through the coils. Subsequent experiments, however, showed that the boilers could be [p037] depended upon to supply the steam that the compound engines would require; but after the whole was completed, the weight, if nothing else, was prohibitory.

I had gone on from one thing to another, adding a little here and a little there, strengthening this part and that, until when the hull was finally completed with the engines and boilers in place, ready for the application of the wings, the weight of the whole was found (allowing 7 pounds for the weight of the wings and tail) to be almost exactly 45 pounds, and nearly 52 pounds with fuel and water. To this excessive weight would have to be added that of the propellers, and as the wings would necessarily have to be made very large in order to carry the machine, and as the difficulties of launching had still to be met, nothing was attempted in the way of field trials, and with great disappointment the decision was made in May, 1892 (wisely, as it subsequently appeared) to proceed no further with this special apparatus.

However, inasmuch as this aerodrome with its engines and boilers had been completed at considerable expense, it was decided to use the apparatus as far as it might be practicable, in order to learn what must be done to secure a greater amount of success in the future. The fundamental trouble was to get heat. In the first place there was trouble with the burners, for it seemed to be impossible to get one that would vaporize the gasoline in sufficient quantity to do the work, and various forms were successively tried.

All of the early part of 1892 was passed in trying to get the boilers to work at a steam pressure of 100 pounds per square inch. On account of the defects in the tubes and elsewhere this required much patient labor. The writer, even thus early, devised a plan of using a sort of aeolipile, which should actuate its own blast, but this had to be abandoned on account of the fact that the pear-shaped receivers would not stand the heat. This necessitated a number of experiments in the distillation of gas, in the course of which there was trouble with the pumps, and a continual series of breakages and leakages, so that the middle of April came before I had secured any further satisfaction than to demonstrate that possibly the boilers might have a capacity sufficient for the work laid out for them to do; but subsequent experiments showed that even in this I was mistaken, for it was only after additional jets had been put in between the coils that I succeeded in getting an effective horse-power of 0.43 out of the combination.

Finally, on the 14th of April, after having reduced the capacity of the pumps to the dimensions given above (for the stroke was originally 1.25 inch) I obtained the development of 1 full horse-power by the engine for 41 seconds, with a steam pressure of 100 pounds per square inch, and a rate of revolution of 720 per minute. But at the end of this brief period, the shafts sprung and the worm was thrown out of gear. [p038]

I pass over numerous other experiments, for their only result was to make it clear that the aerodrome, as it had been constructed, could not be made to work efficiently, even if its great weight had not served as a bar to its flight. It was, therefore, decided to proceed with the construction of another.

After the failure of the first steam-driven model No. 0, which has just been described, subsequent light models were constructed. These, three in number, made with a view to the employment of carbonic acid or compressed air, but also to the possible use of steam, are shown in Plate [10], Nos. 1, 2, 3; on the same scale as the larger model which had preceded them. In describing these, it will be well to mention constructive features which were experimented on in them, as well as to describe the engines used.

In No. 1, which was intended to be on about 25 the linear scale of No. 0, the constructive fault of the latter, that of making the support depend on a too flexible hull, was avoided, and the straight steel tube (“midrod” it will hereafter be called) was carried through from end to end, though at the cost of inconvenience in the placing of the machinery, in what may be called the hull, which now became simply a protective case built around this midrod. The mistaken device of the long shafts meeting at an angle, was, however, retained, and the engines first tried were a pair of very light ones of crude construction.

These were later replaced by a pair of oscillating engines, each 3 cm. diameter by 3 cm. stroke, with a combined capacity of 42 cubic cm. and without cut-off. The midrod was made of light steel tubing 2 cm. outside diameter. The framing for the hull was formed by a single ring of U section, 8 cm. across and 18 cm. in depth, stayed by five ribs of wood measuring 0.7 × 0.3 cm. The inclined propeller shafts, which were connected by a pair of bevel gears as in No. 0, were made of tubing 0.5 cm. outside diameter, and were intended to turn propellers of from 40 to 45 cm. in diameter. The weight, without engine or reservoir for gas, was 1161 grammes. With a weight equivalent to that of the intended reservoir and engines plus that of the proposed supporting surfaces, the whole weight, independent of fuel or water, was 2.2 kilogrammes.

The engines, which were not strong enough to sustain a pressure of over 2 atmospheres, at an actual pressure of 20 pounds drove the 45 cm. propellers through the long V shafts and lifted only about 17 of the flying weight of the machine. The power developed at the Prony brake was collectively only about .04 horse-power, giving 1200 turns a minute to two 40 cm. propellers. This was the best result obtained.

This aerodrome was completed in June, 1892, but changes in the engines and other attempted improvements kept it under experiment until November of that year, when it appeared to be inexpedient to do anything more with it.

Aerodrome No. 2 (see Plate [10]), was a still smaller and still lighter construction, in which, however, the midrod was bent (not clearly shown in the [p039] photograph), so as to afford more room in the hull. This introduced a constructional weakness which was not compensated by the added convenience, but the principal improvement was the abandonment of the inclined propeller shafts, which was done at the suggestion of Mr. J. E. Watkins, so that the propellers were carried on parallel shafts as in marine practice. These parallel shafts were driven by two very small engines with cylinders 2.3 cm. in diameter by 4 cm. stroke, with a collective capacity of 33 cu. cm. and without cut-off, which were mounted on a cross-frame attached to the midrod at right angles near the rear end of the hull.

These engines, driven either by steam or by carbonic-acid gas developed 0.035 horse-power at the Prony brake, giving 750 revolutions of the 45 cm. propellers, and lifting about 15 of the total weight which it was necessary to provide for in actual flight. A higher rate of revolution and a better lift were occasionally obtained, but there was little more hope with this than with the preceding models of obtaining power enough to support the actual weight in flight, although such sacrifices had been made for lightness that every portion of the little model had been reduced to what seemed the limit of possible frailty consistent with anything like safety. Thus the midrod was lighter than that of No. 1, being only 1 cm. in outside diameter. The frame was made of thin wooden strips 5 mm. × 3.5 mm., united by light steel rings. The cross framing carrying the engines was also of wood, and was formed of four strips, each 7 mm. × 3 mm. The shafts were but 4 mm. in diameter.

As these engines did not give results that were satisfactory, when using carbonic-acid gas, experiments were commenced to secure a boiler that would furnish the requisite steam. As the “beehive” boiler had proved to be too heavy, and as the steam obtained from it had been inadequate to the requirements, something else had to be devised. A few of the boilers used in 1892 are shown in Fig. 3. The one marked A is one of the “beehives,” while an element of another form tried is that marked B. It consisted of 38-inch copper tubes joined to a drum of 10-oz. copper. This was made in May, 1892, and was tested to a pressure of 50 atmospheres, when it burst without any tearing of the metal.

In July another boiler like that shown at C in Fig. 3 was made. This was formed of tubes 3 cm. in diameter, and weighed 348 grammes. It carried about 300 grammes of water and stood a steam pressure of 125 pounds per square inch, but failed to maintain sufficient steam pressure.

Accordingly, in the same month, a third boiler like that shown at D was built. It consisted of a tube 12 inches long to which were attached fifteen 14-inch tubes each 7 inches long, in the manner shown. The heating surface of this boiler, including the tubes and the lower half of the drum, amounted to 750 square cm., and it was thought that this would be sufficient to supply steam for a flight of a [p040] minute and a half. But when a test was made, it also was found to be deficient in steaming power even after changes were made in it which occupied much time.

By the first of October, 1892, there had been built one large aerodrome that could not possibly fly, a smaller one, No. 1, on 25 the linear scale of No. 0, with a pair of engines but no means of driving them, and the still smaller No. 2 with a boiler that was yet untried.

Aerodrome No. 3 (Plate [10]) was an attempt to obtain better conditions than had existed in the preceding model without any radical change except that of moving the cross frame, which carried the engines and propellers, nearer the front of the machine. Instead of the oscillatory engines used up to this time, two stationary cylinder engines, each 2.4 cm. in diameter and 4 cm. stroke, having a combined capacity of 36 cu. cm. without cut-off were employed for driving the propellers. The engines, though occasionally run in trials with steam from a stationary boiler, were intended to be actuated either by compressed air or carbonic-acid gas contained in a reservoir which was not actually constructed, but whose weight was provisionally estimated at 1 kilogramme. The weight of the aerodrome without this reservoir was but 1050 grammes, including the estimated weight of the sustaining surfaces, which consisted principally of two wings, each about 1 metre in length by 30 cm. in breadth and which were in fact so slight in their construction, that it is now certain that they could not have retained their shape in actual flight.

The only trials made with this aerodrome, then, were in the shop, of which it is sufficient to cite those of November 22, 1892, when under a pressure of 30 pounds, the maximum which the engines would bear, two 50 cm. propellers were driven at 900 revolutions per minute, with an estimated horse-power of 0.07, about 35 per cent of the weight of the whole machine being lifted. This was a much more encouraging result than any which had preceded, and indicated that it was possible to make an actual flight with the aerodrome if the boilers could be ignored, the best result having been obtained only with carbonic acid supplied without limit from a neighboring ample reservoir.

This aerodrome was also tested while mounted upon a whirling-arm and allowed to operate during its advance through the air. The conclusion reached with it at the close of 1892, after a large part of the year passed in experiments with carbonic-acid gas and compressed air, was that it was necessary to revert to steam, and that whatever difficulties lay in the way, some means must be found of getting sufficient power without the weight which had proved prohibitory in No. 0.

With this chapter, then, and with the end of the year 1892, I close this very brief account of between one and two years of fruitless experiment in the construction of models supplied with various motors, subsequent to and on a larger scale indeed than the toy-like ones of india rubber, but not even so efficient as those had been, since they had never procured a single actual flight.

[◊]

[p041]

CHAPTER V
ON SUSTAINING SURFACES

The following general considerations may conveniently precede the particular description of the balancing of the aerodrome.

In “Experiments in Aerodynamics,” I have given the result of trials, showing that the pressure (or total resistance) of a wind on a surface 1 foot square, moving normally at the velocity of 1 foot per second, is 0.00166 pounds, and that this pressure increases directly as the surface of the plane, and (within our experimental condition) as the square of the velocity,[20] results in general accordance with those of earlier observers.

I have further shown by independent investigations that while the shape of the plane is of secondary importance if its movement be normal, the shape and “aspect” greatly affect the resultant pressure when the plane is inclined at a small angle, and propelled by such a force that its flight is horizontal, that is, under the actual conditions of soaring flight.

I have given on page 60 of “Aerodynamics,” the primary equations,

Pα = P90F(α) = kAV2F(α),

W = Pα cos α = kAV2F(α) cos α,

R = Pα sin α = kAV2F(α) sin α,

where W is the weight of the plane under examination (sometimes called the “lift”); R the horizontal component of pressure (sometimes called the “drift”); k is the constant already given; A the area in square feet; V the velocity in feet per second; F a function of α (to be determined by experiment); α the angle which, under these conditions, gives horizontal flight.

I have also given on page 66 of the same work the following table showing the actual values obtained by experiment on a plane, 30 × 4.8 inches (= 1 sq. ft.), weighing 500 grammes (1.1 pounds):