until the wright brothers definitely solved the problem of flight and virtually gave the aeroplane its present place in aeronautics, there were three definite schools of experiment. the first of these was that which sought to imitate nature by means of the ornithopter or flapping-wing machines directly imitative of bird flight; the second school was that which believed in the helicopter or lifting screw; the third and eventually successful school is that which followed up the principle enunciated by cayley, that of opposing a plane surface to the resistance of the air by supplying suitable motive power to drive it at the requisite angle for support.
engineering problems generally go to prove that too close an imitation of nature in her forms of reciprocating motion is not advantageous; it is impossible to copy the minutiae of a bird’s wing effectively, and the bird in flight depends on the tiniest details of its feathers just as much as on the general principle on which the whole wing is constructed. bird flight, however, has attracted many experimenters, including even lilienthal; among others may be mentioned f. w. brearey, who invented what he called the ‘pectoral cord,’ which stored energy on each upstroke of the artificial wing; e. p. frost; major r. moore, and especially hureau de villeneuve, a most enthusiastic student of this form84 of flight, who began his experiments about 1865, and altogether designed and made nearly 300 artificial birds. one of his later constructions was a machine in bird form with a wing span of about 50 ft.; the motive power for this was supplied by steam from a boiler which, being stationary on the ground, was connected by a length of hose to the machine. de villeneuve, turning on steam for his first trial, obtained sufficient power to make the wings beat very forcibly; with the inventor on the machine the latter rose several feet into the air, whereupon de villeneuve grew nervous and turned off the steam supply. the machine fell to the earth, breaking one of its wings, and it does not appear that de villeneuve troubled to reconstruct it. this experiment remains as the greatest success yet achieved by any machine constructed on the ornithopter principle.
it may be that, as forecasted by the prophet wells, the flapping-wing machine will yet come to its own and compete with the aeroplane in efficiency. against this, however, are the practical advantages of the rotary mechanism of the aeroplane propeller as compared with the movement of a bird’s wing, which, according to marey, moves in a figure of eight. the force derived from a propeller is of necessity continual, while it is equally obvious that that derived from a flapping movement is intermittent, and, in the recovery of a wing after completion of one stroke for the next, there is necessarily a certain cessation, if not loss, of power.
the matter of experiment along any lines in connection with aviation is primarily one of hard cash. throughout the whole history of flight up to the outbreak of the european war development has been handicapped on the score of finance, and, since the85 arrival of the aeroplane, both ornithopter and helicopter schools have been handicapped by this consideration. thus serious study of the efficiency of wings in imitation of those of the living bird has not been carried to a point that might win success for this method of propulsion. even wilbur wright studied this subject and propounded certain theories, while a later and possibly more scientific student, f. w. lanchester, has also contributed empirical conclusions. another and earlier student was lawrence hargrave, who made a wing-propelled model which achieved successful flight, and in 1885 was exhibited before the royal society of new south wales. hargrave called the principle on which his propeller worked that of a ‘trochoided plane’; it was, in effect, similar to the feathering of an oar.
hargrave, to diverge for a brief while from the machine to the man, was one who, although he achieved nothing worthy of special remark, contributed a great deal of painstaking work to the science of flight. he made a series of experiments with man-lifting kites in addition to making a study of flapping-wing flight. it cannot be said that he set forth any new principle; his work was mainly imitative, but at the same time by developing ideas originated in great measure by others he helped toward the solution of the problem.
attempts at flight on the helicopter principle consist in the work of de la landelle and others already mentioned. the possibility of flight by this method is modified by a very definite disadvantage of which lovers of the helicopter seem to take little account. it is always claimed for a machine of this type that it possesses great advantages both in rising and in landing, since, if it were effective, it would obviously be able to86 rise from and alight on any ground capable of containing its own bulk; a further advantage claimed is that the helicopter would be able to remain stationary in the air, maintaining itself in any position by the vertical lift of its propeller.
these potential assets do not take into consideration the fact that efficiency is required not only in rising, landing, and remaining stationary in the air, but also in actual flight. it must be evident that if a certain amount of the motive force is used in maintaining the machine off the ground, that amount of force is missing from the total of horizontal driving power. again, it is often assumed by advocates of this form of flight that the rapidity of climb of the helicopter would be far greater than that of the driven plane; this view overlooks the fact that the maintenance of aerodynamic support would claim the greater part of the engine-power; the rate of ascent would be governed by the amount of power that could be developed surplus to that required for maintenance.
this is best explained by actual figures: assuming that a propeller 15 ft. in diameter is used, almost 50 horse-power would be required to get an upward lift of 1,000 pounds; this amount of horse-power would be continually absorbed in maintaining the machine in the air at any given level; for actual lift from one level to another at a speed of eleven feet per second a further 20 horse-power would be required, which means that 70 horse-power must be constantly provided for; this absorption of power in the mere maintenance of aerodynamic support is a permanent drawback.
the attraction of the helicopter lies, probably, in the ease with which flight is demonstrated by means87 of models constructed on this principle, but one truism with regard to the principles of flight is that the problems change remarkably, and often unexpectedly, with the size of the machine constructed for experiment. berriman, in a brief but very interesting manual entitled principles of flight, assumed that ‘there is a significant dimension of which the effective area is an expression of the second power, while the weight became an expression of the third power. then once again we have the two-thirds power law militating against the successful construction of large helicopters, on the ground that the essential weight increases disproportionately fast to the effective area. from a consideration of the structural features of propellers it is evident that this particular relationship does not apply in practice, but it seems reasonable that some such governing factor should exist as an explanation of the apparent failure of all full-sized machines that have been constructed. among models there is nothing more strikingly successful than the toy helicopter, in which the essential weight is so small compared with the effective area.’
de la landelle’s work, already mentioned, was carried on a few years later by another frenchman, castel, who constructed a machine with eight propellers arranged in two fours and driven by a compressed air motor or engine. the model with which castel experimented had a total weight of only 49 lbs.; it rose in the air and smashed itself by driving against a wall, and the inventor does not seem to have proceeded further. contemporary with castel was professor forlanini, whose design was for a machine very similar to de la landelle’s, with two superposed screws. this88 machine ranks as the second on the helicopter principle to achieve flight; it remained in the air for no less than the third of a minute in one of its trials.
later experimenters in this direction were kress, a german; professor wellner, an austrian; and w. r. kimball, an american. kress, like most germans, set to the development of an idea which others had originated; he followed de la landelle and forlanini by fitting two superposed propellers revolving in opposite directions, and with this machine he achieved good results as regards horse-power to weight; kimball, it appears, did not get beyond the rubber-driven model stage, and any success he may have achieved was modified by the theory enunciated by berriman and quoted above.
comparing these two schools of thought, the helicopter and bird-flight schools, it appears that the latter has the greater chance of eventual success—that is, if either should ever come into competition with the aeroplane as effective means of flight. so far, the aeroplane holds the field, but the whole science of flight is so new and so full of unexpected developments that this is no reason for assuming that other means may not give equal effect, when money and brains are diverted from the driven plane to a closer imitation of natural flight.
reverting from non-success to success, from consideration of the two methods mentioned above to the direction in which practical flight has been achieved, it is to be noted that between the time of le bris, stringfellow, and their contemporaries, and the nineties of last century, there was much plodding work carried out with little visible result, more especially so far as89 english students were concerned. among the incidents of those years is one of the most pathetic tragedies in the whole history of aviation, that of alphonse penaud, who, in his thirty years of life, condensed the experience of his predecessors and combined it with his own genius to state in a published patent what the aeroplane of to-day should be. consider the following abstract of penaud’s design as published in his patent of 1876, and comparison of this with the aeroplane that now exists will show very few divergences except for those forced on the inventor by the fact that the internal combustion engine had not then developed. the double-surfaced planes were to be built with wooden ribs and arranged with a slight dihedral angle; there was to be a large aspect ratio and the wings were cambered as in stringfellow’s later models. provision was made for warping the wings while in flight, and the trailing edges were so designed as to be capable of upward twist while the machine was in the air. the planes were to be placed above the car, and provision was even made for a glass wind-screen to give protection to the pilot during flight. steering was to be accomplished by means of lateral and vertical planes forming a tail; these controlled by a single lever corresponding to the ‘joy stick’ of the present day plane.
penaud conceived this machine as driven by two propellers; alternatively these could be driven by petrol or steam-fed motor, and the centre of gravity of the machine while in flight was in the front fifth of the wings. penaud estimated from 20 to 30 horse-power sufficient to drive this machine, weighing with pilot and passenger 2,600 lbs., through the air at a speed of 60 miles an hour, with the wings set at an angle of90 incidence of two degrees. so complete was the design that it even included instruments, consisting of an aneroid, pressure indicator, an anemometer, a compass, and a level. there, with few alterations, is the aeroplane as we know it—and penaud was twenty-seven when his patent was published.
for three years longer he worked, experimenting with models, contributing essays and other valuable data to french papers on the subject of aeronautics. his gains were ill health, poverty, and neglect, and at the age of thirty a pistol shot put an end to what had promised to be one of the most brilliant careers in all the history of flight.
two years before the publication of penaud’s patent thomas moy experimented at the crystal palace with a twin-propelled aeroplane, steam driven, which seems to have failed mainly because the internal combustion engine had not yet come to give sufficient power for weight. moy anchored his machine to a pole running on a prepared circular track; his engine weighed 80 lbs. and, developing only three horse-power, gave him a speed of 12 miles an hour. he himself estimated that the machine would not rise until he could get a speed of 35 miles an hour, and his estimate was correct. two six-bladed propellers were placed side by side between the two main planes of the machine, which was supported on a triangular wheeled undercarriage and steered by fairly conventional tail planes. moy realised that he could not get sufficient power to achieve flight, but he went on experimenting in various directions, and left much data concerning his experiments which has not yet been deemed worthy of publication, but which still contains a mass of information91 that is of practical utility, embodying as it does a vast amount of painstaking work.
penaud and moy were followed by goupil, a frenchman, who, in place of attempting to fit a motor to an aeroplane, experimented by making the wind his motor. he anchored his machine to the ground, allowing it two feet of lift, and merely waited for a wind to come along and lift it. the machine was stream lined, and the wings, curving as in the early german patterns of war aeroplanes, gave a total lifting surface of about 290 sq. ft. anchored to the ground and facing a wind of 19 feet per second, goupil’s machine lifted its own weight and that of two men as well to the limit of its anchorage. although this took place as late as 1883 the inventor went no further in practical work. he published a book, however, entitled la locomotion aérienne, which is still of great importance, more especially on the subject of inherent stability.
in 1884 came the first patents of horatio phillips, whose work lay mainly in the direction of investigation into the curvature of plane surfaces, with a view to obtaining the greatest amount of support. phillips was one of the first to treat the problem of curvature of planes as a matter for scientific experiment, and, great as has been the development of the driven plane in the 36 years that have passed since he began, there is still room for investigation into the subject which he studied so persistently and with such valuable result.
at this point it may be noted that, with the solitary exception of le bris, practically every student of flight had so far set about constructing the means of launching humanity into the air without any attempt at ascertaining92 the nature and peculiarities of the sustaining medium. the attitude of experimenters in general might be compared to that of a man who from boyhood had grown up away from open water, and, at the first sight of an expanse of water, set to work to construct a boat with a vague idea that, since wood would float, only sufficient power was required to make him an efficient navigator. accident, perhaps, in the shape of lack of means of procuring driving power, drove le bris to the form of experiment which he actually carried out; it remained for the later years of the nineteenth century to produce men who were content to ascertain the nature of the support the air would afford before attempting to drive themselves through it.
of the age in which these men lived and worked, giving their all in many cases to the science they loved, even to life itself, it may be said with truth that ‘there were giants on the earth in those days,’ as far as aeronautics is in question. it was an age of giants who lived and dared and died, venturing into uncharted space, knowing nothing of its dangers, giving, as a man gives to his mistress, without stint and for the joy of the giving. the science of to-day, compared with the glimmerings that were in that age of the giants, is a fixed and certain thing; the problems of to-day are minor problems, for the great major problem vanished in solution when the wright brothers made their first ascent. in that age of the giants was evolved the flying man, the new type in human species which found full expression and came to full development in the days of the war, achieving feats of daring and endurance which leave the commonplace landsman staggered at thought of that of which his fellows prove themselves capable.93 he is a new type, this flying man, a being of self-forgetfulness; of such was lilienthal, of such was pilcher; of such in later days were farman, bleriot, hamel, rolls, and their fellows; great names that will live for as long as man flies, adventurers equally with those of the spacious days of elizabeth. to each of these came the call, and he worked and dared and passed, having, perhaps, advanced one little step in the long march that has led toward the perfecting of flight.
it is not yet twenty years since man first flew, but into that twenty years have been compressed a century or so of progress, while, in the two decades that preceded it, was compressed still more. we have only to recall and recount the work of four men: lilienthal, langley, pilcher, and clement ader to see the immense stride that was made between the time when penaud pulled a trigger for the last time and the wright brothers first left the earth. into those two decades was compressed the investigation that meant knowledge of the qualities of the air, together with the development of the one prime mover that rendered flight a possibility—the internal combustion engine. the coming and progress of this latter is a thing apart, to be detailed separately; for the present we are concerned with the evolution of the driven plane, and with it the evolution of that daring being, the flying man. the two are inseparable, for the men gave themselves to their art; the story of lilienthal’s life and death is the story of his work; the story of pilcher’s work is that of his life and death.
considering the flying man as he appeared in the war period, there entered into his composition a new94 element—patriotism—which brought about a modification of the type, or, perhaps, made it appear that certain men belonged to the type who in reality were commonplace mortals, animated, under normal conditions, by normal motives, but driven by the stress of the time to take rank with the last expression of human energy, the flying type. however that may be, what may be termed the mathematising of aeronautics has rendered the type itself evanescent; your pilot of to-day knows his craft, once he is trained, much in the manner that a driver of a motor-lorry knows his vehicle; design has been systematised, capabilities have been tabulated; camber, dihedral angle, aspect ratio, engine power, and plane surface, are business items of drawing office and machine shop; there is room for enterprise, for genius, and for skill; once and again there is room for daring, as in the first atlantic flight. yet that again was a thing of mathematical calculation and petrol storage, allied to a certain stark courage which may be found even in landsmen. for the ventures into the unknown, the limit of daring, the work for work’s sake, with the almost certainty that the final reward was death, we must look back to the age of the giants, the age when flying was not a business, but romance.
lilienthal with his glider folded after a glide.
lilienthal’s biplane glider alighting.
pilcher’s ‘bat.’
the ‘bat’, side view.