Chapter II: Part 2

In order to utilize the air as a means of transit, the body in motion, whether it moves in virtue of the life it possesses, or because of a force super-added, must be heavier than air. It must tread with its wings and rise upon the air as a swimmer upon the water, or as a kite upon the wind. This is necessary for the simple reason that the body must be active, the air passive. The flying body must act against gravitation, and elevate and carry itself forward at the expense of the air and of the force which resides in it, whatever that may be. If it were otherwise--if it were rescued from the law of gravitation on the one hand, and bereft of independent movement on the other, it would float about uncontrolled and uncontrollable like an ordinary balloon.

In flight one of two things is necessary. Either the wings must attack the air with great violence, or the air in rapid motion must attack the wings: either suffices. If a bird attempts to fly in a calm, the wings must be made to smite the air after the manner of a boy's kite with great vigour and at a high speed. In this case the wings fly the bird. If, however, the bird is fairly launched in space and a stiff breeze is blowing, all that is required in many instances is to extend the wings at a slight upward angle to the horizon so that the under parts of the wings present kite-like surfaces. In these circumstances the rapidly moving air flies the bird. The flight of the albatross supplies the necessary illustration. If by any chance this magnificent bird alights upon the sea he must flap and beat the water and air with his wings with tremendous energy until he gets fairly launched. This done he extends his enormous pinions[11] and sails majestically along, seldom deigning to flap his wings, the breeze doing the work for him. A familiar illustration of the same principle may be witnessed any day when children are engaged in the pastime of kite-flying. If two boys attempt to fly a kite in a calm, the one must hold up the kite and let go when the other runs. In this case the under surface of the kite is made to strike the still air. If, however, a stiff autumn breeze be blowing, it suffices if the boy who formerly ran when the kite was let go stands still. In this case the air in rapid motion strikes the under surface of the kite and forces it up. The string and the hand are to the kite what the weight of the flying creature is to the inclined planes formed by its wings.

The area of the insect, bird and bat, when the wings are fully expanded, is greater than that of any other class of animal, their weight being proportionally less. As already stated, however, it ought never to be forgotten that even the lightest insect, bird or bat is vastly heavier than the air, and that no fixed relation exists between the weight of body and expanse of wing in any of the orders. We have thus light-bodied and large-winged insects and birds, as the butterfly and heron; and others with heavy bodies and small wings, as the beetle and partridge. Similar remarks are to be made of bats. Those apparent inconsistencies in the dimensions of the body and wings are readily explained by the greater muscular development of the heavy-bodied, small-winged insects, birds and bats, and the increased power and rapidity with which the wings in them are made to oscillate. This is of the utmost importance in the science of aviation, as showing that flight may be attained by a heavy powerful animal with comparatively small wings, as well as by a lighter one with greatly enlarged wings. While, therefore, there is apparently no correspondence between the area of the wing and the animal to be raised, there is, except in the case of sailing insects, birds and bats, an unvarying relation as to the weight and number of oscillations; so that the problem of flight would seem to resolve itself into one of weight, power, velocity and small surfaces, _versus_ buoyancy, debility, diminished speed and extensive surfaces--weight in either case being a _sine qua non_.

That no fixed relation exists between the area of the wings and the size and weight of the body to be elevated is evident on comparing the dimensions of the wings and bodies of the several orders of insects, bats and birds. If such comparison be made, it will be found that the pinions in some instances diminish while the bodies increase, and the converse. No practical good can therefore accrue to aviation from elaborate measurements of the wings and body of any flying thing; neither can any rule be laid down as to the extent of surface required for sustaining a given weight in the air. The statements here advanced are borne out by the fact that the wings of insects, bats and birds may be materially reduced without impairing their powers of flight. In such cases the speed with which the wings are driven is increased in the direct ratio of the mutilation. The inference to be deduced from the foregoing is plainly this, that even in large-bodied, small-winged insects and birds the wing-surface is greatly in excess, the surplus wing area supplying that degree of elevating and sustaining power which is necessary to prevent undue exertion on the part of the volant animal. In this we have a partial explanation of the buoyancy of insects, and the great lifting power possessed by birds and bats,--the bats carrying their young without inconvenience, the birds elevating surprising quantities of fish, game, carrion, &c. (fig. 28).

While as explained, no definite relation exists between the weight of a flying animal and the size of its flying surfaces, there being, as stated, heavy-bodied and small-winged insects, birds and bats, and the converse, and while, as has been shown, flight is possible within a wide range, the wings being, as a rule, in excess of what are required for the purposes of flight,--still it appears from the researches of L. de Lucy that there is a general law, to the effect that the larger the volant animal, the smaller, by comparison, are its flying surfaces. The existence of such a law is very encouraging so far as artificial flight is concerned, for it shows that the flying surfaces of a large, heavy, powerful flying machine will be comparatively small, and consequently comparatively compact and strong. This is a point of very considerable importance, as the object desiderated in a flying machine is elevating capacity.

De Lucy tabulated his results as under:--

+-------------------------------------------------+------------------------------------+ | INSECTS | BIRDS. | +---------------------------+---------------------+-------------------+----------------+ | | Flying Surface | | | | | referred to the | | Flying Surface | | | Kilogramme | | referred to the| | Names. | = 2 lb. 8 oz. 3 dwt.| Names. | Kilogramme. | | | 2 gr. avoird. | | | | | = 2 lb. 3 oz. 4.428 | | | | | dr. troy. | | | +---------------------------+---------------------+-------------------+----------------+ | | sq. | | sq. | | | yds. ft. in. | | yds. ft. in. | | Gnat | 11 8 92 | Swallow | 1 1 104-1/2 | | Dragon-fly (small) | 7 2 56 | Sparrow | 0 5 142-1/2 | | Coccinella (Lady-bird) | 5 13 87 | Turtle-dove | 0 4 100-1/2 | | Dragon-fly (common) | 5 2 89 | Pigeon | 0 2 113 | | Tipula, or Daddy-long-legs| 3 5 11 | Stork | 0 2 20 | | Bee | 1 2 74-1/2 | Vulture | 0 1 116 | | Meat-fly | 1 3 54-1/2 | Crane of Australia| 0 0 130 | | Drone (blue) | 1 2 20 | | | | Cockchafer | 1 2 50 | | | | / Stag-beetle \ | | | | | Lucanus < (female) / | 1 1 39-1/2 | | | | cervus | Stag-beetle \ | | | | | \ (male) / | 0 8 33 | | | | Rhinoceros-beetle | 0 6 122-1/2 | | | +---------------------------+---------------------+-------------------+----------------+

"It is easy, by the aid of this table, to follow the order, always
decreasing, of the surfaces, in proportion as the winged animal
increases in size and weight. Thus, in comparing the insects with one
another, we find that the gnat, which weighs 460 times less than the
stag-beetle, has 14 times more of surface. The lady-bird weighs 150
times less than the stag-beetle, and possesses 5 times more of
surface, &c. It is the same with the birds. The sparrow weighs about
10 times less than the pigeon, and has twice as much surface. The
pigeon weighs about 8 times less than the stork, and has twice as much
surface. The sparrow weighs 339 times less than the Australian crane,
and possesses 7 times more surface, &c. If now we compare the insects
and the birds, the gradation will become even much more striking. The
gnat, for example, weighs 97,000 times less than the pigeon, and has
40 times more surface; it weighs three millions of times less than the
crane of Australia, and possesses 140 times more of surface than this
latter, the weight of which is about 9 kilogrammes 500 grammes (25 lb.
5 oz. 9 dwt. troy, 20 lb. 15 oz. 2-1/4 dr. avoirdupois).

"The Australian crane, the heaviest bird weighed, is that which has
the smallest amount of surface, for, referred to the kilogramme, it
does not give us a surface of more than 899 square centimetres (139
sq. in.), that is to say, about an eleventh part of a square metre.
But every one knows that these grallatorial animals are excellent
birds of flight. Of all travelling birds they undertake the longest
and most remote journeys. They are, in addition, the eagle excepted,
the birds which elevate themselves the highest, and the flight of
which is the longest maintained."[12]

The way in which the natural wing rises and falls on the air, and reciprocates with the body of the flying creature, has a very obvious bearing upon artificial flight. In natural flight the body of the flying creature falls slightly forward in a curve when the wing ascends, and is slightly elevated in a curve when the wing descends. The wing and body are consequently always playing at cross purposes, the wing rising when the body is falling and vice versa. The alternate rise and fall of the body and wing of the bird are well seen when contemplating the flight of the gull from the stern of a steamboat, as the bird is following in the wake of the vessel. The complementary movements referred to are indicated at fig. 29, where the continuous waved line represents the trajectory made by the wing, and the dotted waved line that made by the body. As will be seen from this figure, _the wing advances both when it rises and when it falls_. It is a peculiarity of natural wings, and of artificial wings constructed on the principle of living wings, that when forcibly elevated or depressed, even in a strictly vertical direction, they inevitably dart forward. If, for instance, the wing is suddenly depressed in a vertical direction, as at a b of fig. 29, it at once darts downwards and forwards in a double curve (see continuous line of figure) to c, thus converting the vertical down stroke into a _down, oblique, forward stroke_. If, again, the wing be suddenly elevated in a strictly vertical direction, as at c d, the wing as certainly darts upwards and forwards in a double curve to e, thus converting the vertical up strokes into an _upward, oblique, forward stroke_. The same thing happens when the wing is depressed from e to f and elevated from g to h, the wing describing a _waved track_ as at e g, g i.

There are good reasons why the wings should always be in advance of the body. A bird when flying is a body in motion; but a body in motion tends to fall not vertically downwards, but _downwards and forwards_. The wings consequently must be made to strike _forwards_ and kept in advance of the body of the bird if they are to prevent the bird from falling _downwards and forwards_. If the wings were to strike backwards in aerial flight, the bird would turn a forward somersault.

That the wings invariably strike forwards during the down and up strokes in aerial flight is proved alike by observation and experiment. If any one watches a bird rising from the ground or the water, he cannot fail to perceive that the head and body are slightly tilted upwards, and that the wings are made to descend with great vigour in a downward and _forward_ direction. The dead natural wing and a properly constructed artificial wing act in precisely the same way. If the wing of a gannet, just shot, be removed and made to flap in what the operator believes to be a strictly vertical downward direction, the tip of the wing, in spite of him, will dart forwards between 2 and 3 ft.--the amount of forward movement being regulated by the rapidity of the down stroke. This is a very striking experiment. The same thing happens with a properly constructed artificial wing. The down stroke with the artificial as with the natural wing is invariably converted into an oblique, downward and forward stroke. No one ever saw a bird in the air flapping its wings towards its tail. The old idea was that the wings during the down stroke _pushed_ the body of the bird in an upward and forward direction; in reality the wings do not push but _pull_, and in order to pull they must always be in advance of the body to be flown. If the wings did not themselves fly _forward_, they could not possibly cause the body of the bird to fly forward. It is the wings which cause the bird to fly.

It only remains to be stated that the wing acts as a true kite, during both the down and the up strokes, its under concave or biting surface, in virtue of the forward travel communicated to it by the body of the flying creature, being closely applied to the air, during both its ascent and its descent. This explains how the wing furnishes a persistent buoyancy alike when it rises and when it falls (fig. 30).

The natural kite formed by the wing differs from the artificial kite only in this, that the former is capable of being moved in all its parts, and is more or less flexible and elastic, whereas the latter is comparatively rigid. The flexibility and elasticity of the kite formed by the natural wing are rendered necessary by the fact that the wing, as already stated, is practically hinged at its root and along its anterior margin, an arrangement which necessitates its several parts travelling at different degrees of speed, in proportion as they are removed from the axes of rotation. Thus the tip travels at a higher speed than the root, and the posterior margin than the anterior margin. This begets a _twisting diagonal movement_ of the wing on its long axis, which, but for the elasticity referred to, would break the wing into fragments. The elasticity contributes also to the continuous play of the wing, and ensures that no two parts of it shall reverse at exactly the same instant. If the wing was inelastic, every part of it would reverse at precisely the same moment, and its vibration would be characterized by pauses or dead points at the end of the down and up strokes which would be fatal to it as a flying organ. The elastic properties of the wing are absolutely essential, when the mechanism and movements of the pinion are taken into account. A rigid wing can never be an effective flying instrument.

The kite-like surfaces referred to in natural flight are those upon which the constructors of flying machines very properly ground their hopes of ultimate success. These surfaces may be conferred on artificial wings, aeroplanes, aerial screws or similar structures; and these structures, if we may judge from what we find in nature, _should be of moderate size and elastic_. The power of the flying organs will be increased if they are driven at a comparatively high speed, and particularly if they are made to reverse and reciprocate, as in this case they will practically create the currents upon which they are destined to rise and advance. The angles made by the kite-like surfaces with the horizon should vary according to circumstances. They should be small when the speed is high, and vice versa. This, as stated, is true of natural wings. It should also be true of artificial wings and their analogues.

_Artificial Flight_.--We are now in a position to enter upon a consideration of artificial wings and wing movements, and of artificial flight and flying machines.

We begin with artificial wings. The first properly authenticated account of an artificial wing was given by G.A. Borelli in 1670. This author, distinguished alike as a physiologist, mathematician and mechanician, describes and figures a bird with artificial wings, each of which consists of _a rigid rod in front and flexible feathers behind_. The wings are represented as striking _vertically downwards_, as the annexed duplicate of Borelli's figure shows (fig. 31).

Borelli was of opinion that flight resulted 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 (d). In the 196th proposition of his work (_De motu animalium_, Leiden, 1685) he states that--

"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." "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."

r e, Anterior margin of the right wing, consisting of a rigid rod.

o a, Posterior margin of the right wing, consisting of flexible
feathers.

b c, Anterior; and

f, Posterior margins of the left wing same as the right.

d, Tail of the bird.

r g, d h, Vertical direction of the down stroke of the wing.]

The same argument is restated in different words as under:--"If 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_."

There are three points in Borelli's argument to which it is necessary to draw attention: (1) the direction of the down stroke: it is stated to be _vertically downwards_; (2) the construction of the anterior margin of the wing: it is stated to consist of _a rigid rod_; (3) the function delegated to the posterior margin of the wing: it is said _to yield in an upward direction_ during the down stroke.

With regard to the first point. It is incorrect to say the wing strikes vertically downwards, for, as already explained, the body of a flying bird is a body in motion; but as a body in motion tends to fall downwards and forwards, the wing must strike downwards and forwards in order effectually to prevent its fall. Moreover, in point of fact, all natural wings, and all artificial wings constructed on the natural type, invariably strike downwards and forwards.

With regard to the second point, viz. the supposed rigidity of the anterior margin of the wing, it is only necessary to examine the anterior margins of natural wings to be convinced that they are in every case flexible and elastic. Similar remarks apply to properly constructed artificial wings. If the anterior margins of natural and artificial wings were rigid, it would be impossible to make them vibrate smoothly and continuously. This is a matter of experiment. If a rigid rod, or a wing with a rigid anterior margin, be made to vibrate, the vibration is characterized by an unequal jerky motion, at the end of the down and up strokes, which contrasts strangely with the smooth, steady fanning movement peculiar to natural wings.

As to the third point, viz. the upward bending of the posterior margin of the wing during the down stroke, it is necessary to remark that the statement is true if it means a slight upward bending, but that it is untrue if it means an extensive upward bending.

Borelli does not state the amount of upward bending, but one of his followers, E.J. Marey, maintains that during the down stroke the wing yields until its under surface makes a backward angle with the horizon of 45 deg. Marey further states that during the up stroke the wing yields to a corresponding extent in an opposite direction--the posterior margin of the wing, according to him, passing through an angle of 90 deg., plus or minus according to circumstances, every time the wing rises and falls.

That the posterior margin of the wing yields to a slight extent during both the down and up strokes will readily be admitted, alike because of the very delicate and highly elastic properties of the posterior margins of the wing, and because of the comparatively great force employed in its propulsion; but that it does not yield to the extent stated by Marey is a matter of absolute certainty. This admits of direct proof. If any one watches the horizontal or upward flight of a large bird he will observe that the posterior or flexible margin of the wing never rises during the down stroke to a perceptible extent, so that the under surface of the wing, as a whole, never looks backwards. On the contrary, he will perceive that the under surface of the wing (during the down stroke) invariably looks forwards and forms a true kite with the horizon, the angles made by the kite varying at every part of the down stroke, as shown more particularly at c d e f g, i j k l m of fig. 30.

The authors who have adopted Borelli's plan of artificial wing, and who have endorsed his mechanical views of the wing's action most fully, are J. Chabrier, H.E.G. Strauss-Durckheim and Marey. Borelli's artificial wing, it will be remembered, consists of a rigid rod in front and a flexible sail behind. It is also made to strike vertically downwards. According to Chabrier, the wing has only one period of activity. He believes that if the wing be suddenly lowered by the depressor muscles, it is elevated solely by the reaction of the air. There is one unanswerable objection to this theory: the birds and bats, and some if not all the insects, have distinct elevator muscles, and can elevate their wings at pleasure when not flying and when, consequently, the reaction of the air is not elicited. Strauss-Durckheim agrees with Borelli both as to the natural and the artificial wing. He is of opinion that the insect abstracts from the air by means of the inclined plane a component force (composant) which it employs to support and direct itself. In his theology of nature he describes a schematic wing as consisting of a rigid ribbing in front, and a flexible sail behind. A membrane so constructed will, according to him, be fit for flight. It will suffice if such a sail elevates and lowers itself successively. It will of its own accord dispose itself as an inclined plane, and receiving obliquely the reaction of the air, it transfers into tractile force a part of the vertical impulsion it has received. These two parts of the wing, moreover, are equally indispensable to each other.

Marey repeats Borelli and Durckheim with very trifling modifications, so late as 1869. He describes two artificial wings, the one composed of a rigid rod and sail--the rod representing the stiff anterior margin of the wing; the sail, which is made of paper bordered with cardboard, the flexible posterior margin. The other wing consists of a rigid nervure in front and behind of thin parchment which supports fine rods of steel. He states that if the wing only elevates and depresses itself, "the resistance of the air is sufficient to produce all the other movements. In effect (according to Marey) the wing of an insect has not the power of equal resistance in every part. On the anterior margin the extended nervures make it rigid, while behind it is fine and flexible. During the vigorous depression of the wing, the nervure has the power of remaining rigid, whereas the flexible portion, being pushed in an upward direction on account of the resistance it experiences from the air, assumes an oblique position which causes the upper surface of the wing to look forwards." The reverse of this, in Marey's opinion, takes place during the elevation of the wing--the resistance of the air from above causing the upper surface of the wing to look backwards.... "At first," he says, "the plane of the wing is parallel with the body of the animal. It lowers itself--the front part of the wing strongly resists, the sail which follows it being flexible yields. Carried by the ribbing (the anterior margin of the wing) which lowers itself, the sail or posterior margin of the wing being raised meanwhile by the air, which sets it straight again, the sail will take an intermediate position and incline itself about 45 deg. plus or minus according to circumstances.... The wing continues its movements of depression inclined to the horizon; but the impulse of the air, which continues its effect, and naturally acts upon the surface which it strikes, has the power of resolving itself into two forces, a vertical and a horizontal force; the first suffices to raise the animal, the second to move it along."[13] Marey, it will be observed, reproduces Borelli's artificial wing, and even his text, at a distance of nearly two centuries.

The artificial wing recommended by Pettigrew is a more exact imitation of nature than either of the foregoing. It is of a more or less triangular form, thick at the root and anterior margin, and thin at the tip and posterior margin. No part of it is rigid. It is, on the contrary, highly elastic and flexible throughout. It is furnished with springs at its root to contribute to its continued play, and is applied to the air by a direct piston action in such a way that it descends in a downward and forward direction during the down stroke, and ascends in an upward and forward direction during the up stroke. It elevates and propels both when it rises and falls. It, moreover, twists and untwists during its action and describes figure-of-8 and waved tracks in space, precisely as the natural wing does. The twisting is most marked at the tip and posterior margin, particularly that half of the posterior margin next the tip. The wing when in action may be divided into two portions by a line running diagonally between the tip of the wing anteriorly and the root of the wing posteriorly. The tip and posterior parts of the wing are more active than the root and anterior parts, from the fact that the tip and posterior parts (the wing is an eccentric) always travel through greater spaces, in a given time, than the root and anterior parts.

a b, Anterior margin of wing, to which the neurae or ribs are affixed.

c d, Posterior margin of wing crossing anterior one.

x, Ball-and-socket joint at root of wing, the wing being attached to
the side of the cylinder by the socket.

t, Cylinder.

r r, Piston, with cross heads (w, w) and piston head (s).

o o, Stuffing boxes.

e, f, Driving chains.

m, Superior elastic band, which assists in elevating the wing.

n, Inferior elastic band, which antagonizes m. The alternate
stretching of the superior and inferior elastic bands contributes to
the continuous play of the wing, by preventing dead points at the end
of the down and up strokes. The wing is free to move in a vertical and
horizontal direction and at any degree of obliquity.]

The wing is so constructed that the posterior margin yields freely in a downward direction during the up stroke, while it yields comparatively little in an upward direction during the down stroke; and this is a distinguishing feature, as the wing is thus made to fold and elude the air more or less completely during the up stroke, whereas it is made to expand and seize the air with avidity during the down stroke. The oblique line referred to as running diagonally across the wing virtually divides the wing into an active and a passive part, the former elevating and propelling, the latter sustaining.

It is not possible to determine with exactitude the precise function discharged by each part of the wing, but experiment tends to show that the tip of the wing elevates, the posterior margin propels, and the root sustains.

The wing--and this is important--is driven by a direct piston action with an irregular hammer-like movement, the pinion having communicated to it a smart click at the beginning of every down stroke--the up stroke being more uniform. The following is the arrangement (fig. 32). If the artificial wing here represented (fig. 32) be compared with the natural wing as depicted at fig. 33, it will be seen that there is nothing in the one which is not virtually reproduced in the other. In addition to the foregoing, Pettigrew recommended a double elastic wing to be applied to the air like a steam-hammer, by being fixed to the head of the piston. This wing, like the single wing described, twists and untwists as it rises and falls, and possesses all the characteristics of the natural wing (fig. 34).

a b, Anterior margin of left wing.

c d, Posterior margin of ditto.

d g, Primary or rowing feathers of left wing.

g a, Secondary feathers ditto.

x, Root of right wing with ball-and-socket joint.

l, Elbow joint.

m, Wrist joint,

n,o, Hand and finger joints.]

He also recommends an elastic aerial screw consisting of two blades, which taper and become thinner towards the tips and posterior margins. When the screw is made to rotate, the blades, because of their elasticity, assume a great variety of angles, the angles being least where the speed of the blades is greatest and vice versa. The pitch of the blades is thus regulated by the speed attained (fig. 35).

The peculiarity of Pettigrew's wings and screws consists in their elasticity, their twisting action, and their great comparative length and narrowness. They offer little resistance to the air when they are at rest, and when in motion the speed with which they are driven is such as to ensure that the comparatively large spaces through which they travel shall practically be converted into solid bases of support.

After Pettigrew enunciated his views (1867) as to the screw configuration and elastic properties of natural wings, and more especially after his introduction of spiral, elastic artificial wings, and elastic screws, a great revolution took place in the construction of flying models. Elastic aeroplanes were advocated by D.S. Brown,[14] elastic aerial screws by J. Armour,[15] and elastic aeroplanes, wings and screws by Alphonse Penaud.[16]

x, End of driving shaft.

v,w, Sockets in which the roots of the blades of the screw rotate, the
degree of rotation being limited by steel springs (z, s).

a b, e f, tapering elastic rods forming anterior or thick margins of
blades of screw.

d c, h g, Posterior or thin elastic margins of blades of screw. The
arrows m, n, o, p, q, r indicate the direction of travel.]

Penaud's experiments are alike interesting and instructive. He constructed models to fly by three different methods:--(a) by means of screws acting vertically upwards; (b) by aeroplanes propelled horizontally by screws; and (c) by wings which flapped in an upward and downward direction. An account of his helicoptere or screw model appeared in the _Aeronaut_ for January 1872, but before giving a description of it, it may be well to state very briefly what is known regarding the history of the screw as applied to the air.

The first suggestion on this subject was given by A.J.P. Paucton in 1768. This author, in his treatise on the _Theorie de la vis d'Archimede_, describes a machine provided with two screws which he calls a "pterophores." In 1796 Sir George Cayley gave a practical illustration of the efficacy of the screw as applied to the air by constructing a small machine, consisting of two screws made of quill feathers, a representation of which we annex (fig. 36). 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. 36, 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 sq. ft., and moved by a man, they would elevate him. His interesting experiment is described at length, and the apparatus figured in _Nicolson's Journal_, 1809, p. 172.

Other experimenters, such as J. Degen in 1816 and Ottoris Sarti in 1823, followed Cayley at moderate intervals, constructing flying models on the vertical screw principle. In 1842 W.H. Phillips succeeded, it is stated, in elevating a steam model by the aid of revolving fans, which according to his account flew across two fields after having attained a great altitude; and in 1859 H. Bright took out a patent for a machine to be sustained by vertical screws. In 1863 the subject of aviation by vertical screws received a fresh impulse from the experiments of Gustave de Ponton d'Amecourt, G. de la Landelle, and A. Nadar, who exhibited models driven by clock-work springs, which ascended with graduated weights a distance of from 10 to 12 ft. These models were so fragile that they usually broke in coming in contact with the ground in their descent. Their flight, moreover, was unsatisfactory, from the fact that it only lasted a few seconds.

Stimulated by the success of his spring models, Ponton d'Amecourt had a small steam model constructed. This model, which was shown at the exhibition of the Aeronautical Society of Great Britain at the Crystal Palace in 1868, consisted of two superposed screws propelled by an engine, the steam for which was generated (for lightness) in an aluminium boiler. This steam model proved a failure, inasmuch as it only lifted a third of its own weight. Fig. 37 embodies de la Landelle's ideas.

All the models referred to (Cayley's excepted[17]) were provided with rigid screws. In 1872 Penaud discarded the rigid screws in favour of elastic ones, as Pettigrew had done some years before.

Penaud also substituted india-rubber under torsion for the whalebone and clock springs of the smaller models, and the steam of the larger ones. His helicoptere or screw-model is remarkable for its lightness, simplicity and power. The accompanying sketch will serve to illustrate its construction (fig. 38). It consists of two superposed elastic screws (a a, b b), the upper of which (a a) is fixed in a vertical frame (c), which is pivoted in the central part (d) of the under screw. From the centre of the under screw an axle provided with a hook (e), which performs the part of a crank, projects in an upward direction. Between the hook or crank (e) and the centre of the upper screw (a a), the india-rubber in a state of torsion (f) extends. By fixing the lower screw and turning the upper one a sufficient number of times the requisite degree of torsion and power is obtained. The apparatus when liberated flies into the air sometimes to a height of 50 ft., and gyrates in large circles for a period varying from 15 to 30 seconds.

Penaud next directed his attention to the construction of a model, to be propelled by a screw and sustained by an elastic aeroplane extending horizontally. Sir George Cayley proposed such a machine in 1810, and W.S. Henson constructed and patented a similar machine in 1842. Several inventors succeeded in making models fly by the aid of aeroplanes and screws, as, e.g. J. Stringfellow in 1847,[18] and F. du Temple in 1857. These models flew in a haphazard sort of a way, it being found exceedingly difficult to confer on them the necessary degree of stability fore and aft and laterally. Penaud succeeded in overcoming the difficulty in question by the invention of what he designated an automatic rudder. This consisted of a small elastic aeroplane placed aft or behind the principal aeroplane which is also elastic. The two elastic aeroplanes extended horizontally and made a slight upward angle with the horizon, the angle made by the smaller aeroplane (the rudder) being slightly in excess of that made by the larger. The motive power was india-rubber in the condition of torsion; the propeller, a screw. The reader will understand the arrangement by a reference to the accompanying drawing (fig. 39).

Models on the aeroplane screw type may be propelled by two screws, one fore and one aft, rotating in opposite directions; and in the event of only one screw being employed it may be placed in front of or behind the aeroplane.

When such a model is wound up and let go it descends about 2 ft., after which, having acquired initial velocity, it rises and flies in a forward direction at a height of from 8 to 10 ft. from the ground for a distance of from 120 to 130 ft. It flies this distance in from 10 to 11 seconds, its mean speed being something like 12 ft. per second. From experiments made with this model, Penaud calculates that one horse-power would elevate and support 85 lb.

a a, Elastic aeroplane.

b b, Automatic rudder.

c c, Aerial screw centred at f.

d, Frame supporting aeroplane, rudder and screw.

e, India-rubber, in a state of torsion, attached to hook or crank at
f. By holding the aeroplane (a a) and turning the screw (c c) the
necessary power is obtained by torsion. (Penaud.)]

D.S. Brown also wrote (1874) in support of elastic aero-biplanes. His experiments proved that two elastic aeroplanes united by a central shaft or shafts, and separated by a wide interval, always produce increased stability. The production of flight by the vertical flapping of wings is in some respects the most difficult, but this also has been attempted and achieved. Penaud and A.H. de Villeneuve each constructed winged models. Marey was not so fortunate. He endeavoured to construct an artificial insect on the plan advocated by Borelli, Strauss-Durckheim and Chabrier, but signally failed, his insect never having been able to lift more than a third of its own weight.

a b c d, a' b' c' d', Elastic wings, which twist and untwist when made
to vibrate.

a b, a' b', Anterior margins of wings.

c d, c' d', Posterior margins of wings.

c, c', Inner portions of wings attached to central shaft of model by
elastic bands at e.

f, India-rubber in a state of torsion, which provides the motive
power, by causing the crank situated between the vertical wing
supports (g) to rotate; as the crank revolves the wings are made to
vibrate by means of two rods which extend between the crank and the
roots of the wings.

h, Tail of artificial bird.]

De Villeneuve and Penaud constructed their winged models on different types, the former selecting the bat, the latter the bird. De Villeneuve made the wings of his artificial bat conical in shape and comparatively rigid. He controlled the movements of the wings, and made them strike downwards and forwards in imitation of natural wings. His model possessed great power of rising. It elevated itself from the ground with ease, and flew in a horizontal direction for a distance of 24 ft., and at a velocity of 20 m. an hour. Penaud's model differed from de Villeneuve's in being provided with elastic wings, the posterior margins of which in addition to being elastic were free to move round the anterior margins as round axes (see fig. 24). India-rubber springs were made to extend between the inner posterior parts of the wings and the frame, corresponding to the backbone of the bird.

A vertical movement having been communicated by means of india-rubber in a state of torsion to the roots of the wings, the wings themselves, in virtue of their elasticity, and because of the resistance experienced from the air, twisted and untwisted and formed reciprocating screws, precisely analogous to those originally described and figured by Pettigrew in 1867. Penaud's arrangement is shown in fig. 40.

If the left wing of Penaud's model (a b, c d of fig. 40) be compared with the wing of the bat (fig. 18), or with Pettigrew's artificial wing (fig. 32), the identity of principle and application is at once apparent.

In Penaud's artificial bird the equilibrium is secured by the addition of a tail. The model cannot raise itself from the ground, but on being liberated from the hand it descends 2 ft. or so, when, having acquired initial velocity, it flies horizontally for a distance of 50 or more feet, and rises as it flies from 7 to 9 ft. The following are the measurements of the model in question:--length of wing from tip to tip, 32 in.; weight of wing, tail, frame, india-rubber, &c., 73 grammes (about 2-1/2 ounces). (J. B. P.)

_Flying Machines_.--Henson's flying machine, designed in 1843, was the earliest attempt at aviation on a great scale. Henson was one of the first to combine aerial screws with extensive supporting structures occupying a nearly horizontal position. The accompanying illustration explains the combination (fig. 41).

"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 the 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." The editor of Newton's _Journal of
Arts and Sciences_ speaks of it thus:--"The apparatus consists of a
car containing the goods, passengers, engines, fuel, &c., 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
steamboat, and acting upon the air after the manner of a windmill.
These wheels receive motions 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."

F.H. Wenham, thinking to improve upon Henson, invented in 1866 what he designated his aeroplanes.[19] These were thin, light, long, narrow structures, arranged above each other in tiers like so many shelves. They were tied together at a slight upward angle, and combined strength and lightness. The idea was to obtain great sustaining area in comparatively small space with comparative ease of control. It was hoped that when the aeroplanes were wedged forward in the air by vertical screws, or by the body to be flown, each aeroplane would rest or float upon a stratum of undisturbed air, and that practically the aeroplanes would give the same support as if spread out horizontally. The accompanying figures illustrate Wenham's views (figs. 42 and 43).

a, a, Thin planks, tapering at each end, and attached to a triangle.

b, Similar plank for supporting the aeronaut.

c, c, Thin bands of iron with truss planks a, a, and

d, d, Vertical rods. Between these are stretched five bands of holland
15 in. broad and 16 ft. long, the total length of the web being 80 ft.
This apparatus when caught by a gust of wind, actually lifted the
aeronaut.]

a, a, Main spar 16 ft. long;

b, b, Panels, with base board for aeronaut attached to main spar.

e, e, Thin tie-band of steel with struts starting from main spar. This
forms a strong light framework for the aeroplanes, consisting of six
webs of thin holland 15 in. broad. The aeroplanes are kept in parallel
plane by vertical divisions of holland 2 ft. wide.

c, c', Wing propellers driven by the feet.]

Stringfellow, who was originally associated with Henson, and built a successful flying model in 1847, made a second model in 1868, in which Wenham's aeroplanes were combined with aerial screws. This model was on view at the exhibition of the Aeronautical Society of Great Britain, held at the Crystal Palace, London, in 1868. It was remarkably compact, elegant and light, and obtained the L100 prize of the exhibition for its engine, which was the lightest and most powerful so far constructed. The illustration below (fig. 44), drawn from a photograph, gives a very good idea of the arrangement--a, b, c representing the superimposed aeroplanes, d the tail, e, f the screw propellers. The superimposed aeroplanes (a, b, c) in this machine contained a sustaining area of 28 sq. ft., 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 aeroplanes and propellers) was under 12 lb. Its sustaining area, if that of the tail (d) be included, was something like 36 sq. ft., i.e. 3 sq. ft. for every pound. The model was forced by its propellers along a wire at a great speed, but so far as an observer could determine, failed to lift itself, notwithstanding its extreme lightness and the comparatively very great power employed. Stringfellow, however, stated that it occasionally left the wire and was sustained by its aeroplanes alone.

The aerial steamer of Thomas Moy (fig. 45), designed in 1874, consisted of a light, powerful, skeleton frame resting on three wheels; a very effective light engine constructed on a new principle, which dispensed with the old-fashioned, cumbrous boiler; two long, narrow, horizontal aeroplanes; and two comparatively very large aerial screws. The idea was to get up the initial velocity by a preliminary run on the ground. This accomplished it was hoped that the weight of the machine would gradually be thrown upon the aeroplanes in the same way that the weight of certain birds--the eagle, e.g.--is thrown upon the wings after a few hops and leaps. Once in the air the aeroplanes, it was believed, would become effective in proportion to the speed attained. The machine, however, did not realize the high expectations formed of it, and like all its predecessors it was doomed to failure.

Two of the most famous of the next attempts to solve the problem of artificial flight, by means of aeroplanes, were those of Prof. S.P. Langley and Sir Hiram S. Maxim, who began their aerial experiments about the same time (1889-1890). By 1893-1894 both had embodied their views in models and large flying machines.

Langley, who occupied the position of secretary to the Smithsonian Institution, Washington, U.S.A., made many small flying models and one large one. These he designated "aerodromes." They were all constructed on a common principle, and were provided with extensive flying surfaces in the shape of rigid aeroplanes inclined at an upward angle to the horizon, and more or less fixed on the plan advocated by Henson. The cardinal idea was to force the aeroplanes (slightly elevated at their anterior margins) forwards, kite-fashion, by means of powerful vertical screw propellers driven at high speed--the greater the horizontal speed provided by the propellers, the greater, by implication, the lifting capacity of the aerodrome. The bodies, frames and aeroplanes of the aerodromes were strengthened by vertical and other supports, to which were attached aluminium wires to ensure absolute rigidity so far as that was possible. Langley aimed at great lightness of construction, and in this he succeeded to a remarkable extent. His aeroplanes were variously shaped, and were, as a rule, concavo-convex, the convex surface being directed upwards. He employed a competent staff of highly trained mechanics at the Smithsonian Institution, and great secrecy was observed as to his operations. He flew his smallest models in the great lecture room of the National Museum, and his larger ones on the Potomac river about 40 m. below Washington.

While Langley conducted his preliminary experiments in 1889, he did not construct and test his steam-driven flying models until 1893. These were made largely of steel and aluminium, and one of them in 1896 made the longest flight then recorded for a flying machine, namely, fully half a mile on the Potomac river. The largest aerodrome, intended to carry passengers and to be available for war purposes, was built to the order and at the expense of the American government, which granted a sum of fifty thousand dollars for its construction.