Sir George Cayley’s forecasts—A steam-driven model which flew—The shape and curve of planes.

So passed the haphazard stage of flight; and now history moves to a second and more important period, that in which men of science were attracted to the problem. They worked upon theories, and made experiments with models; they studied the shape which Nature has given the birds; they sifted false notions and showed where error lay. But they did not fly. They were merely clearers of the ground, gathering information and classifying it, and paving the way for those daring workers who were to follow them—men who, by putting science to the test, were willing to risk their lives.

To England goes the distinction of the first practical attempts to solve the problems of flight; and it is the work of Sir George Cayley, an eminent scientist and engineer, that next merits attention. In a series of articles, published in Nicholson’s Journal during the years 1809-10, he forecasted many of the principles that go to the making of a modern-type aeroplane. He advised the construction of machines with fixed, outstretched wings like those of a bird; but he did more than this, for it is admitted generally he was first to point out that, to increase their lifting power as they were moved through the air, these wings should not be flat, but should be curved from front to back, or arched upward (Fig. 4). How important this suggestion was, subsequent experimenters were to show. Sir George Cayley realised also that a tail-plane, carried at the rear of a machine, would give it equilibrium, and might be moved up and down to control ascent or descent; and he used a rudder upon models, to steer them from side to side. He advised the use of steam engines as a motive power, and of revolving propellers to drive a craft through the air. But, like many another man, he was before his time. He built experimental craft—one, a model glider, which would sail down gracefully from the top of a hill; and another, a far larger machine, which would bear a man through the air, for a distance of several yards, if he ran forward with it against the wind. But the difficulty of obtaining a sufficiently light and practical motor, either of steam or other power, was an obstacle that proved insurmountable. One light engine, which Sir George Cayley planned, was to be driven by a series of gunpowder explosions in a cylinder; but the suggestion came to no practical issue.

This scientist did not write or work in vain. He compiled data which was invaluable, and interested and encouraged other men—even those, indeed, who in due course made the conquest. One of the first to work upon Sir George Cayley’s theories was an experimenter named Henson. He planned an ambitious machine weighing about a ton. It was to have planes of canvas stretched over a rigidly trussed frame of bamboo rods and hollow wooden spars; and these planes were to contain 4500 square feet of lifting surface, and be driven by screws operated by a steam engine of 30 h.p. (Fig. 5).

But this craft did not take practical shape, although in its appearance and many of its details it bore a resemblance to machines which ultimately were to fly. In the specification of the patent he took out for his invention, Henson indicated that it was for

“Improvements in locomotive apparatus and machinery for conveying letters, goods, and passengers from place to place through the air.”

Explaining his theories in this same specification he wrote:

“If any light and flat or nearly flat article be projected or thrown edgewise in a slightly inclined position, the same will rise into the air till the force exerted is expended, when the article so thrown or projected will descend; and it will readily be conceived that, if the article so projected or thrown possessed in itself a continuous power or force equal to that used in throwing or projecting it, the article would continue to ascend so long as the forward part of the surface was upwards in respect to the hinder part.”

Had Henson been able to carry out his ideas, it is almost certain that this experimental machine would have been wrecked in its tests, and probably several more after it, seeing that he would have had to learn to control them when in flight, and remembering also that, even with aircraft as they are built to-day, many details have to be studied and improved before a successful model is evolved.

All such work, of course, entails heavy expense. It was, indeed, the cost of experiments which prevented many an early inventor from building a full-sized machine. The designing and construction of a man-carrying craft, and the employment of skilled workmen and mechanics, to say nothing of repairs that may have to be made during a series of tests, represent an expenditure that may amount to thousands of pounds. As a rule, the inventor is not a man of wealth; and so far as flying was concerned, at any rate in the early days,—and to a more limited extent even at the present time,—people with money thought the difficulties so great that they would not advance funds for the carrying out of trials. So men with ideas had to do the best they could, and this resolved itself generally into writing and lecturing, and endeavouring to interest the public. But the public was not easily interested; ordinary folk did not believe that men would ever fly, while many people declared that it was going against Nature for us to try to imitate the birds, and that nothing but mischief would come of so doing.

Henson, failing to make definite progress with his scheme for a man-carrying craft—despite the fact that a company was floated to assist him—co-operated with another enthusiast named Stringfellow in a plan which was not so elaborate. They began to experiment with a series of models driven by tiny and most ingenious steam engines built by Stringfellow; and so cleverly did he construct them that the Aeronautical Society awarded him a prize of £100. The model which won him this recognition was a little plant which, while it weighed only 13 lbs., without water or fuel, would develop one horse-power.

What, by the way, is meant by a horse-power? The answer is as follows: in the early days of engineering, when it was found necessary to establish some well-recognised unit of power, a large number of experiments were carried out with horses, which were made to raise a weight from the ground by means of an arrangement of pulleys and ropes. The experiments showed this: that a horse can exert sufficient power to raise 33,000 lbs., or about 15 tons, to a height of 1 foot in the space of one minute. This, therefore, was called “one horse-power.”

In Stringfellow’s days, it must be remembered, there was no petrol engine; an engine so extremely light for the power that it will give, and with its liquid fuel and oil carried conveniently in tanks—an engine which, as Sir Hiram Maxim puts it, will give one horse-power of energy “for the weight of a barn-door fowl.”

The question of motive-power was, indeed, the great obstacle for the pioneers. When a man builds an aeroplane he must drive it through the air; and to drive it through the air he requires an engine. But he knows that his planes, owing to the small density or sustaining power of the air through which they pass, will raise only a limited load. And the machine itself, even if it is built of wood and canvas, represents an appreciable weight; to say nothing of that of the pilot. So, if his engine is heavy in proportion to the power it gives, and its fuel weighty, he may be prevented altogether from rising from the ground; or if he does rise, he may be able only to carry sufficient fuel for a flight of a short distance.

Henson and Stringfellow built in 1845 a model which weighed about 30 lbs. (Fig. 6); and although its stability was not perfect, it was an interesting machine—a forecast of the monoplane of the future. Here one saw the lifting planes take shape; the body between the wings; the tail-planes at the rear; and, above all, a suggestion of the means by which machines would be driven through the air: the fitting to the model, that is to say, of revolving propellers or screws. When an inventor has fitted an engine to an aircraft, means must be devised for using its power to drive the machine through the air; and to make the wings flap like those of a bird, has been found so complicated, owing to the mechanism necessary to imitate natural movements, that much of the power is wasted. Inventors such as Henson and Stringfellow, realising this difficulty, made wings that were outstretched and immovable, like those of a bird when it is soaring, and relied upon screw propellers—which they set spinning at great speed by means of their engines—to thrust their craft forward through the air.

In an early and simple form, the aerial propeller was as shown in Fig. 7. Here are two curved blades, so shaped that, when the propeller is made to revolve quickly, these blades will act powerfully upon the air. What the propeller does is to screw itself forward through the air, as one might revolve a corkscrew and drive it into a cork, or force a gimlet into a piece of wood. Each time you twist the gimlet for instance, as you drive it inwards, it forces itself a certain distance through the wood; and in a like manner the air-propeller, each time it revolves, tends to bore its way through the air (Fig. 8) and so push, or draw with it, the flying machine to which it is attached. But with air, seeing that its density is small, it is necessary to use a large screw, and to turn it fast, before power can be obtained.

In 1845, Stringfellow, who was now working alone—Henson having abandoned the tests and gone abroad—met with a definite success. He obtained actual flights with a steam-driven model in the form of a monoplane, weighing 8½ lbs. These tests attracted attention among scientists, but they led to nothing else—that is to say, no full-sized machine was the result. But Stringfellow’s model interested many people in the problems of flight. It showed, indeed, although in miniature, that a flying machine could be built and driven through the air; and so this patient experimenter did not labour in vain.

Following Stringfellow, upon the list of those who forged links in the aerial conquest, came Francis Herbert Wenham. His interest in flying, as with many other men, was aroused by watching the birds. Wenham, an engineer by profession, made a voyage up the Nile; and his study of the movement of birds, as they flew near his yacht, caused him to take up aviation in earnest, and carry out experiments for the Aeronautical Society. Wenham was interested largely in the lifting power of planes, and sought efficient shapes. He recommended the building of arched surfaces, so arranged that they had considerable span, but were narrow from front to back; and he suggested also that they should, when fitted to a machine, be placed one above another. Thus Wenham was the inventor of the biplane, as we know that craft to-day.

In explaining this point he wrote:

“Having remarked how thin a stratum of air is displaced between the wings of a bird in rapid flight, it follows that, in order to obtain the necessary length of plane for supporting heavy weights, the surfaces may be superposed, or placed in parallel rows with an interval between them” (Fig. 9).

To illustrate his theory, he built a model which had six long, narrow planes, arranged one above the other, rather like the slats of a Venetian blind. Wenham’s experiments were highly important, because they cleared a great deal of ground, and removed many misunderstandings. By showing that a long, narrow plane was more efficient—would, that is to say, carry a greater load through the air than one which was deep from front to back, owing to the fact that it is the front section of an inclined plane that provides the most “lift”; and by illustrating how, in a full-sized machine, such a row of planes could be arranged one above another, Wenham directed men’s thoughts towards a definite goal. By his work, and chiefly by his sifting of data, an outline was obtained of that aeroplane of the future which was actually to fly.

While Wenham was experimenting, an inventor named Penaud, in France, testing a series of models, made one which was driven by the twisting of elastic, and flew quite well. Penaud’s work in this respect is interesting, because small elastic-driven machines, such as he designed, were used afterwards in demonstration, and are flown to-day. For a miniature aeroplane, elastic is an ideal motive force—light and yet providing ample power, and with only one disadvantage: it unwinds itself rapidly, and then the model must descend.

In experiments of permanent value, after the discoveries of Wenham, important work was that of Horatio Phillips. Like Wenham, he devoted his attention mainly to a study of lifting planes, and tested many shapes and curves. Sir George Cayley, it may be remembered, had suggested a curved and not a flat plane; but Phillips went one better than this, for in 1881 he devised a plane with what has been termed a dipping front edge.

The shape and curve of a plane, is of vital importance. A machine may be built, and an engine and propellers fitted, but the question is: Will the planes support through the air the load they have been given to carry? Phillips made many experiments, and in the end he produced a wing-shape which he patented. He pointed out that an advantage might be gained in lifting effect if the main curve or camber was situated near the front edge of the plane, and not in the centre (Fig. 10). The theory Phillips worked upon was this—and it is interesting if it can be expressed clearly. Taking a plane curved as he recommended, with this “hump” towards the front, and forcing it through the air as would be the case were an aeroplane in flight, the rush of wind which meets the edge of the plane is split into two currents—one sweeping above and one below. The air current below the plane, following its curve, is thrust downward, and in being so thrust down imparts a lift to the plane; while the current thrown above the plane—rushing up and over the “hump” which, as has been shown, is situated close to the front edge—will sweep rearwards in such a way that there is a partial vacuum or air space between the fast-moving wind current and the curved-down section of the plane behind the “hump.” The value of such a vacuum is this: it has a raising effect upon the surface of the plane, which is thus not only pushed up from below, but drawn from above.

Fig. 11.—Suction above a Cambered Surface.