Part 21

Mr. Henson designed his aërostat in 1843. “The chief feature of the invention was the very great expanse of its sustaining planes, which were larger in proportion to the weight it had to carry than those of many birds. The machine advanced _with its front edge a little raised_, the effect of which was to present its under surface to the air over which it passed, the resistance of which, acting upon it like a strong wind on the sails of a windmill, prevented the descent of the machine and its burden. The sustaining of the whole, therefore, depended upon _the speed at which it travelled through the air, and the angle at which its under surface impinged on the air in its front_.... The machine, fully prepared for flight, was started from the top of an inclined plane, in descending which it attained a velocity necessary to sustain it in its further progress. That velocity would be gradually destroyed by the resistance of the air to forward flight; it was, therefore, the office of the steam-engine and the vanes it actuated simply to repair the loss of velocity; it was made therefore only of the power and weight necessary for that small effect” (fig. 109). The editor of Newton’s Journal of Arts and Science speaks of it thus:--“The apparatus consists of a car containing the goods, passengers, engines, fuel, etc., to which a rectangular frame, made of wood or bamboo cane, and covered with canvas or oiled silk, is attached. This frame extends on either side of the car in a similar manner to the outstretched wings of a bird; but with this difference, that _the frame is immovable_. Behind the wings are two vertical fan wheels, furnished with oblique vanes, which are intended to propel the apparatus through the air. The rainbow-like circular wheels are the propellers, answering to the wheels of a steam-boat, and acting upon the air after the manner of a windmill. These wheels receive motion from bands and pulleys from a steam or other engine contained in the car. To an axis at the stern of the car a triangular frame is attached, resembling the tail of a bird, which is also covered with canvas or oiled silk. This may be expanded or contracted at pleasure, and is moved up and down for the purpose of causing the machine to ascend or descend. Beneath the tail is a rudder for directing the course of the machine to the right or to the left; and to facilitate the steering a sail is stretched between two masts which rise from the car. The amount of canvas or oiled silk necessary for buoying up the machine is stated to be equal to one square foot for each half pound of weight.”

Wenham[103] has advocated the employment of _superimposed planes_, with a view to augmenting the support furnished while it diminishes the horizontal space occupied by the planes. These planes Wenham designates _Aëroplanes_. They are inclined at a very slight angle to the horizon, and are wedged forward either by the weight to be elevated or by the employment of vertical screws. Wenham’s plan was adopted by Stringfellow in a model which he exhibited at the Aëronautical Society’s Exhibition, held at the Crystal Palace in the summer of 1868.

[103] “Aërial Locomotion,” by F. H. Wenham.--_World of Science_, June 1867.

The subjoined woodcut (fig. 110), taken from a photograph of Mr. Stringfellow’s model, gives a very good idea of the arrangement; _a b c_ representing the superimposed planes, _d_ the tail, and _e f_ the vertical screw propellers.

The superimposed planes (_a b c_) in this machine contained a sustaining area of twenty-eight square feet in addition to the tail (_d_).

Its engine represented a third of a horse power, and the weight of the whole (engine, boiler, water, fuel, superimposed planes, and propellers) was under 12 lbs. Its sustaining area, if that of the tail (_d_) be included, was something like thirty-six square feet, _i.e._ three square feet for every pound--the sustaining area of the gannet, it will be remembered (p. 134), being less than one square foot of wing for every two pounds of body.

The model was forced by its propellers along a wire at a great speed, but, so far as I could determine from observation, failed to lift itself notwithstanding its extreme lightness and the comparatively very great power employed.[104]

[104] Mr. Stringfellow stated that his machine occasionally left the wire, and was sustained by its superimposed planes alone.

The idea embodied by Henson, Wenham, and Stringfellow is plainly that of a boy’s kite sailing upon the wind. The kite, however, is a more perfect flying apparatus than that furnished by Henson, Wenham, and Stringfellow, inasmuch as the inclined plane formed by its body strikes the air at various angles--the angles varying according to the length of string, strength of breeze, length and weight of tail, etc. Henson’s, Wenham’s, and Stringfellow’s methods, although carefully tried, have hitherto failed. The objections are numerous. In the first place, the supporting planes (aëroplanes or otherwise) are not flexible and elastic as wings are, but _rigid_. This is a point to which I wish particularly to direct attention. Second, They strike the air _at a given angle_. Here, again, there is a departure from nature. Third, A machine so constructed must be precipitated from a height or driven along the surface of the land or water at a high speed to supply it with initial velocity. Fourth, It is unfitted for flying with the wind unless its speed greatly exceeds that of the wind. Fifth, It is unfitted for flying across the wind because of the surface exposed. Sixth, The sustaining surfaces are comparatively very large. They are, moreover, passive or dead surfaces, _i.e._ they have no power of moving or accommodating themselves to altered circumstances. Natural wings, on the contrary, present small flying surfaces, the great speed at which wings are propelled converting the space through which they are driven into what is practically a solid basis of support, as explained at pp. 118, 119, 151, and 152 (_vide_ figs. 64, 65, 66, 82, and 83, pp. 139 and 158). This arrangement enables natural wings to seize and utilize the air, and renders them superior to adventitious currents. Natural wings work up the air in which they move, but unless the flying animal desires it, they are scarcely, if at all, influenced by winds or currents which are not of their own forming. In this respect they entirely differ from the balloon and all forms of fixed aëroplanes. In nature, small wings driven at a high speed produce the same result as large wings driven at a slow speed (compare fig. 58, p. 125, with fig. 57, p. 124). In flight a certain space must be covered either by large wings spread out as a solid (fig. 57, p. 124), or by small wings vibrating rapidly (figs. 64, 65, and 66, p. 139).

_The Aërial Screw._--Our countryman, Sir George Cayley, gave the first practical illustration of the efficacy of the screw as applied to the air in 1796. In that year he constructed a small machine, consisting of two screws made of quill feathers (fig. 111). Sir George writes as under:--

“As it may be an amusement to some of your readers to see a machine rise in the air by mechanical means, I will conclude my present communication by describing an instrument of this kind, which any one can construct at the expense of ten minutes’ labour.

“_a_ and _b_ (fig. 111, p. 215) are two corks, into each of which are inserted four wing feathers from any bird, so as to be slightly inclined like the sails of a windmill, but in opposite directions in each set. A round shaft is fixed in the cork _a_, which ends in a sharp point. At the upper part of the cork _b_ is fixed a whalebone bow, having a small pivot hole in its centre to receive the point of the shaft. The bow is then to be strung equally on each side to the upper portion of the shaft, and the little machine is completed. Wind up the string by turning the flyers different ways, so that the spring of the bow may unwind them with their anterior edges ascending; then place the cork with the bow attached to it upon a table, and with a finger on the upper cork press strong enough to prevent the string from unwinding, and, taking it away suddenly, the instrument will rise to the ceiling.”

Cayley’s screws were peculiar, inasmuch as they were superimposed and rotated in opposite directions. He estimated that if the area of the screws was increased to 200 square feet, and moved by a man, they would elevate him. Cayley’s interesting experiment is described at length, and the apparatus figured in Nicholson’s Journal for 1809, p. 172. In 1842 Mr. Phillips also succeeded in elevating a model by means of revolving fans. Mr. Phillips’s model was made entirely of metal, and when complete and charged weighed 2 lbs. It consisted of a boiler or steam generator and four fans supported between eight arms. The fans were inclined to the horizon at an angle of 20°, and through the arms the steam rushed on the principle discovered by Hero of Alexandria. By the escape of steam from the arms, the fans were made to revolve with immense energy, so much so that the model rose to a great altitude, and flew across two fields before it alighted. The motive power employed in the present instance was obtained from the combustion of charcoal, nitre, and gypsum, as used in the original fire annihilator; the products of combustion mixing with water in the boiler, and forming gas charged steam, which was delivered at a high pressure from the extremities of the eight arms. This model is remarkable as being probably the first which actuated by steam has flown to a considerable distance.[105] The French have espoused the aërial screw with great enthusiasm, and within the last ten years (1863) MM. Nadar,[106] Pontin d’Amécourt, and de la Landelle have constructed clockwork models (_orthopteres_), which not only raise themselves into the air, but carry a certain amount of freight. These models are exceedingly fragile, and because of the prodigious force required to propel them usually break after a few trials. Fig. 112, p. 217, embodies M. de la Landelle’s ideas.

[105] Report on the First Exhibition of the Aëronautical Society of Great Britain, held at the Crystal Palace, London, in June 1868, p. 10.

[106] Mons. Nadar, in a paper written in 1863, enters very fully into the subject of artificial flight, as performed by the aid of the screw. Liberal extracts are given from Nadar’s paper in Astra Castra, by Captain Hatton Turner. London, 1865, p. 340. To Turner’s handsome volume the reader is referred for much curious and interesting information on the subject of Aërostation.

In the helicopteric models made by MM. Nadar, Pontin d’Amécourt, and de la Landelle, the screws (_m n o p q r s t_ of figure) are arranged in tiers, _i.e._ the one screw is placed above the other. In this respect they resemble the aëroplanes recommended by Mr. Wenham, and tested by Mr. Stringfellow (compare _m n o p q r s t_ of fig. 112, with _a b c_ of fig. 110, p. 213). The superimposed screws, as already explained, were first figured and described by Sir George Cayley (p. 215). The French screws, and that employed by Mr. Phillips, are _rigid or unyielding_, and strike the air _at a given angle_, and herein, I believe, consists their principal defect. This arrangement results in a ruinous expenditure of power, and is accompanied by a great amount of slip. The aërial screw, and the machine to be elevated by it, can be set in motion without any preliminary run, and in this respect it has the advantage over the machine supported by mere sustaining planes. It has, in fact, a certain amount of inherent motion, its screws revolving, and supplying it with active or moving surfaces. It is accordingly more independent than the machine designed by Henson, Wenham, and Stringfellow.

I may observe with regard to the system of rigid inclined planes wedged forward at a given angle in a straight line or in a circle, that it does not embody the principle carried out in nature.

The wing of a flying creature, as I have taken pains to show, is _not rigid_; neither does it always strike the air _at a given angle_. On the contrary, it is capable of moving in all its parts, and attacks the air at _an infinite variety of angles_ (pp. 151 to 154). Above all, the surface exposed by a natural wing, when compared with the great weight it is capable of elevating, is remarkably small (fig. 89, p. 171). This is accounted for by the length and the great range of motion of natural wings; the latter enabling the wings to convert large tracts of air into supporting areas (figs. 64, 65, and 66, p. 139). It is also accounted for by the multiplicity of the movements of natural wings, these enabling the pinions to create and rise upon currents of their own forming, and to avoid natural currents when not adapted for propelling or sustaining purposes (fig. 67, 68, 69, and 70, p. 141).

If any one watches an insect, a bat, or a bird when dressing its wings, he will observe that it can incline the under surface of the wing at a great variety of angles to the horizon. This it does by causing the posterior or thin margin of the wing to rotate around the anterior or thick margin as an axis. As a result of this movement, the two margins are forced into double and opposite curves, and the wing converted into _a plastic helix_ or _screw_. He will further observe that the bat and bird, and some insects, have, in addition, the power of folding and drawing the wing towards the body during the up stroke, and of pushing it away from the body and extending it during the down stroke, so as alternately to diminish and increase its area; arrangements necessary to decrease the amount of resistance experienced by the wing during its ascent, and increase it during its descent. It is scarcely requisite to add, that in the aëroplanes and aërial screws, as at present constructed, no provision whatever is made for suddenly increasing or diminishing the flying surface, of conferring elasticity upon it, or of giving to it that infinite variety of angles which would enable it to seize and disentangle itself from the air with the necessary rapidity. Many investigators are of opinion that flight is a mere question of levity and power, and that if a machine could only be made light enough and powerful enough, it must of necessity fly, whatever the nature of its flying surfaces. A grave fallacy lurks here. Birds are not more powerful than quadrupeds of equal size, and Stringfellow’s machine, which, as we have seen, only weighed 12 lbs., exerted _one-third of a horse power_. The probabilities therefore are, that flight is dependent to a great extent on the nature of the flying surfaces, and the mode of applying those surfaces to the air.

_Artificial Wings_ (Borelli’s Views).--With regard to the production of _flight by the flapping of wings_, much may and has been said. Of all the methods yet proposed, it is unquestionably by far the most ancient. Discrediting as apocryphal the famous story of Dædalus and his waxen wings, we certainly have a very graphic account of artificial wings in the De Motu Animalium of Borelli, published as far back as 1680, _i.e._ nearly two centuries ago.[107]

[107] Borelli, De Motu Animalium. Sm. 4to, 2 vols. Romæ, 1680.

Indeed it will not be too much to affirm, that to this distinguished physiologist and mathematician belongs almost all the knowledge we possessed of artificial wings up till 1865. He was well acquainted with the properties of the wedge, as applied to flight, and he was likewise cognisant of the flexible and elastic properties of the wing. To him is to be traced the purely mechanical theory of the wing’s action. He figured a bird with artificial wings, each wing consisting of _a rigid rod in front_ and _flexible feathers_ behind. I have thought fit to reproduce Borelli’s figure both because of its great antiquity, and because it is eminently illustrative of his text.[108]

[108] De Motu Animalium, Lugduni Batavorum apud Petrum Vander. Anno MDCLXXXV. Tab. XIII. figure 2. (New edition.)

The wings (_b c f_, _o e a_), are represented as striking vertically downwards (_g h_). They remarkably accord with those described by Straus-Durckheim, Girard, and quite recently by Professor Marey.[109]

[109] Revue des Cours Scientifiques de la France et de l’Etranger. Mars 1869.

Borelli is of opinion that flight results from the application of an inclined plane, which beats the air, and which has a wedge action. He, in fact, endeavours to prove that a bird wedges itself forward upon the air by the perpendicular vibration of its wings, the wings during their action forming a wedge, the base of which (_c b e_) is directed towards the head of the bird; the apex (_a f_) being directed towards the tail. This idea is worked out in propositions 195 and 196 of the first part of Borelli’s book. In proposition 195 he explains how, if a wedge be driven into a body, the wedge will tend to separate that body into two portions; but that if the two portions of the body be permitted to react upon the wedge, they will communicate _oblique impulses_ to the sides of the wedge, and expel it, base first, in a straight line.

Following up the analogy, Borelli endeavours to show in his 196th proposition, “that if the air acts obliquely upon the wings, or the wings obliquely upon the air (which is, of course, a wedge action), the result will be _a horizontal transference of the body of the bird_.” In the proposition referred to (196) Borelli states--“If the expanded wings of a bird suspended in the air shall strike the undisturbed air beneath it with a motion _perpendicular to the horizon_, the bird will fly _with a transverse motion_ in a plane parallel with the horizon.” In other words, if the wings _strike vertically downwards_, the bird will fly _horizontally forwards_. He bases his argument upon the belief that the anterior margins of the wings are _rigid and unyielding_, whereas the posterior and after parts of the wings are _more or less flexible_, and readily give way under pressure. “If,” he adds, “the wings of the bird be expanded, and the under surfaces of the wings be struck by the air _ascending perpendicularly to the horizon_, with such a force as shall prevent the bird gliding downwards (_i.e._ with a tendency to glide downwards) from falling, it will be urged _in a horizontal direction_. This follows because the two osseous rods (virgæ) forming the anterior margins of the wings resist the upward pressure of the air, and so retain their original form (literally extent or expansion), whereas the flexible after-parts of the wings (posterior margins) are pushed up and approximated to form a cone, the apex of which (_vide_ _a f_ of fig. 113) is directed towards the tail of the bird. In virtue of the air playing upon and compressing the sides of the wedge formed by the wings, the wedge is driven forwards in the direction of its base (_c b e_), which is equivalent to saying that the wings carry the body of the bird to which they are attached _in a horizontal direction_.”

Borelli restates the same argument in different words, as follows:--

“If,” he says, “the air under the wings be struck by the flexible portions of the wings (_flabella_, literally fly-flaps or small fans) with a motion perpendicular to the horizon, the sails (vela) and flexible portions of the wings (flabella) will yield in an upward direction, and form a wedge, the point of which is directed towards the tail. Whether, therefore, the air strikes the wings from below, or the wings strike the air from above, the result is the same--the posterior or flexible margins of the wings _yield in an upward direction_, and in so doing urge the bird in a _horizontal direction_.”

In his 197th proposition, Borelli follows up and amplifies the arguments contained in propositions 195 and 196. “Thus,” he observes, “it is evident that the object of flight is to impel birds upwards, and keep them suspended in the air, and also to enable them to wheel round in a plane parallel to the horizon. The first (or upward flight) could not be accomplished unless the bird were impelled upwards by frequent leaps or vibrations of the wings, and its descent prevented. And because the downward tendency of heavy bodies is perpendicular to the horizon, the vibration of the plain surfaces of the wings must be made by striking the air beneath them in a direction perpendicular to the horizon, and in this manner nature produces the suspension of birds in the air.”

“With regard to the second or transverse motion of birds (_i.e._ horizontal flight) some authors have strangely blundered; for they hold that it is like that of boats, which, being impelled by oars, moved horizontally in the direction of the stern, and pressing on the resisting water behind, leaps with a contrary motion, and so are carried forward. In the same manner, say they, the wings vibrate towards the tail with a horizontal motion, and likewise strike against the undisturbed air, by the resistance of which they are moved forward by a reflex motion. But this is contrary to the evidence of our sight as well as to reason; for we see that the larger kinds of birds, such as swans, geese, etc., never vibrate their wings when flying towards the tail with a horizontal motion like that of oars, but always bend them downwards, and so describe circles raised perpendicularly to the horizon.[110]

[110] It is clear from the above that Borelli did not know that the wings of birds strike _forwards_ as well as downwards during the down stroke, and _forwards_ as well as upwards during the up stroke. These points, as well as the twisting and untwisting figure-of-8 action of the wing, were first described by the author. Borelli seems to have been equally ignorant of the fact that the wings of insects vibrate in a more or less horizontal direction.

“Besides, in boats the horizontal motion of the oars is easily made, and a perpendicular stroke on the water would be perfectly useless, inasmuch as their descent would be impeded by the density of the water. But in birds, such a horizontal motion (which indeed would rather hinder flight) would be absurd, since it would cause the ponderous bird to fall headlong to the earth; whereas it can only be suspended in the air by constant vibration of the wings _perpendicular to the horizon_. Nature was thus forced to show her marvellous skill in producing a motion which, by one and the same action, should suspend the bird in the air, and carry it forward in a horizontal direction. This is effected by striking the air below perpendicularly to the horizon, but with oblique strokes--an action which is rendered possible only by the flexibility of the feathers, for the fans of the wings in the act of striking acquire the form of a wedge, by the forcing out of which the bird is necessarily moved forwards in a horizontal direction.”

The points which Borelli endeavours to establish are these:--

First, That the action of the wing is a wedge action.

Second, That the wing consists of two portions--_a rigid_ anterior portion, and a _non-rigid_ flexible portion. The rigid portion he represents in his artificial bird (fig. 113, p. 220) as consisting of _a rod_ (_e r_), the yielding portion of _feathers_ (_a o_).

Third, That if the air strikes the under surface of the wing perpendicularly in a direction from below upwards, the flexible portion of the wing will yield in an upward direction, and form a wedge with its neighbour.