Part 19 (2/2)
But before setting to work on the machine itself he made some useful experiments to determine the necessary size of his kites or aeroplanes, and the force requisite to move them.
He accordingly built a ”whirling-table,” consisting of a long arm mounted on a strong pivot at one end, and driven by a 10 horse-power engine. To the free end, which described a circle of 200 feet in circ.u.mference, he attached small aeroplanes, and by means of delicate balances discovered that at 40 miles an hour the aeroplane would lift 133 lbs. per horse-power, and at 60 miles per hour every square foot of surface sustained 8 lbs. weight. He, in common with other experimenters on the same lines, became aware of the fact that if it took a certain strain to suspend a stationary weight in the air, _to advance it rapidly as well as to suspend it took a smaller strain_.
Now, as on sea and land, increased speed means a very rapid increase in the force required, this is a point in favour of the flying-machine. Professor Langley found that a bra.s.s plate weighing a pound, when whirled at great speed, was supported in the air by a pulling pressure of less than one ounce. And, of course, as the speed increased the plate became more nearly horizontal, offering less resistance to the air.
It is on this behaviour of the aeroplane that the hopes of Maxim and others have been based. The swiftly moving aeroplane, coming constantly on to fresh air, the inertia of which had not been disturbed, would resemble the skater who can at high speed traverse ice that would not bear him at rest.
Maxim next turned his attention to the construction of the aeroplanes and engines. He made a special machine for testing fabrics, to decide which would be most suitable for stretching over strong frames to form the planes. The fabric must be light, very strong, and offer small frictional resistance to the air. The testing-machine was fitted with a nozzle, through which air was forced at a known pace on to the substance under trial, which met the air current at a certain angle and by means of indicators showed the strength of its ”lift” or tendency to rise, and that of its ”drift” or tendency to move horizontally in the direction of the air-current. A piece of tin, mounted at an angle of one in ten to the air-current, showed a ”lift”
of ten times its ”drift.” This proportion was made the standard.
Experiments conducted on velvet, plush, silk, cotton and woollen goods proved that the drift of c.r.a.pe was several times that of its lift, but that fine linen had a lift equal to nine times its drift; while a sample of Spencer's balloon fabric was as good as tin.
Accordingly he selected this balloon fabric to stretch over light but strong frames. The stretching of the material was no easy matter, as uneven tension distorted it; but eventually the aeroplanes were completed, tight as drumheads.
The large or central plane was 50 feet wide and 40 long; on either side were auxiliary planes, five pairs; giving a total area of 5400 square feet.
The steam-engine built to give the motive power was perhaps the most interesting feature of the whole construction. Maxim employed steam in preference to any other power as being one with which he was most familiar, and yielding most force in proportion to the weight of the apparatus. He designed and constructed a pair of high-pressure compound engines, the high-pressure cylinders 5 inches in diameter, the low-pressure 8 inches, and both 1 foot stroke. Steam was supplied to the high-pressure cylinders at 320 lbs. per square inch from a tubular boiler heated by a gasolene burner so powerful in its action as to raise the pressure from 100 to 200 lbs. in a minute. The total weight of the boiler, burner, and engines developing 350 horse-power was 2000 lbs., or about 6 lbs. per horse-power.
The two screw-propellers driven by the engine measured 17 feet 11 inches in diameter.
The completed flying-machine, weighing 7500 lbs., was mounted on a railway-truck of 9-foot gauge, in Baldwyn's Park, Kent, not far from the gun-factories for which Sir Hiram is famous. Outside and parallel to the 9-foot track was a second track, 35 feet across, with a reversed rail, so that as soon as the machine should rise from the inner track long spars furnished with f.l.a.n.g.ed wheels at their extremities should press against the under side of the outer track and prevent the machine from rising too far. Dynamometers, or instruments for measuring strains, were fitted to decide the driving and lifting power of the screws. Experiments proved that with the engines working at full power the screw-thrust against the air was 2200 lbs., and the lifting force of the aeroplanes 10,000 lbs., or 1500 in excess of the machine's weight.
Everything being ready the machine was fastened to a dynamometer and steam run up until it strained at its tether with maximum power; when the moorings were suddenly released and it bounded forward at a terrific pace, so suddenly that some of the crew were flung violently down on to the platform. When a speed of 42 miles was reached the inner wheels left their track, and the outer wheels came into play.
Unfortunately, the long 35-foot axletrees were too weak to bear the strain, and one of them broke. The upper track gave way, and for the first time in the history of the world a flying-machine actually left the ground fully equipped with engines, boiler, fuel, and a crew. The journey, however, was a short one, for part of the broken track fouled the screws, snapped a propeller blade and necessitated the shutting off of the steam, which done, the machine settled to earth, the wheels sinking into the sward and showing by the absence of any marks that it had come directly downwards and not run along the surface.
The inventor was prevented by other business, and by the want of a sufficiently large open s.p.a.ce, from continuing his experiments, which had demonstrated that a large machine heavier than air could be made to lift itself and move at high speed. Misfortune alone prevented its true capacities being shown.
Another experimenter on similar lines, but on a less heroic scale than Sir Hiram Maxim, is Professor S. P. Langley, the secretary of the Smithsonian Inst.i.tution, Was.h.i.+ngton. For sixteen years he has devoted himself to a persevering course of study of the flying-machine, and after oft-repeated failures has scored a decided success in his Aerodrome, which, though only a model, has made considerable flights.
His researches have proved beyond doubt that the amount of energy required for flight is but one-fiftieth of what was formerly regarded as a minimum. A French mathematician had proved by figures that a swallow must develop the power of a horse to maintain its rapid flight! Professor Langley's aerodrome has told a very different tale, affording another instance of the truth of the saying that an ounce of practice is worth a pound of theory.
A bird is nearly one thousand times heavier than the air it displaces.
As a motor it develops huge power for its weight, and consumes a very large amount of fuel in doing so. An observant naturalist has calculated that the homely robin devours per diem, in proportion to its size, what would be to a man a sausage two hundred feet long and three inches thick! Any one who has watched birds pulling worms out of the garden lawn and swallowing them wholesale can readily credit this.
Professor Langley therefore concentrated himself on the production of an extremely light and at the same time powerful machine. Like Maxim, he turned to steam for motive-power, and by rigid economy of weight constructed an engine with boilers weighing 5 lbs., cylinders of 26 ozs., and an energy of 1 to 1-1/2 horse-power! Surely a masterpiece of mechanical workmans.h.i.+p! This he enclosed in a boat-shaped cover which hung from two pairs of aeroplanes 12-1/2 feet from tip to tip. The whole apparatus weighed nearly 30 lbs., of which one quarter represented the machinery. Experiments with smaller aerodromes warned the Professor that rigidity and balance were the two most difficult things to attain; also that the starting of the machine on its aerial course was far from an easy matter.
A soaring bird does not rise straight from the ground, but opens its wings and runs along the ground until the pressure of the air raises it sufficiently to give a full stroke of its pinions. Also it rises _against_ the wind to get the full benefit of its lifting force.
Professor Langley hired a houseboat on the Potomac River, and on the top of it built an apparatus from which the aerodrome could be launched into s.p.a.ce at high velocity.
On May 6, 1896, after a long wait for propitious weather, the aerodrome was despatched on a trial trip. It rose in the face of the wind and travelled for over half a mile at the rate of twenty-five miles an hour. The water and fuel being then exhausted it settled lightly on the water and was again launched. Its flight on both occasions was steady, and limited only by the rapid consumption of its power-producing elements. The Professor believes that larger machines would remain in the air for a long period and travel at speeds. .h.i.therto unknown to us.
In both the machines that we have considered the propulsive power was a screw. No counterpart of it is seen in Nature. This is not a valid argument against its employment, since no animal is furnished with driving-wheels, nor does any fish carry a revolving propeller in its tail. But some inventors are strongly in favour of copying Nature as regards the employment of wings. Mr. Sydney H. Hollands, an enthusiastic aeromobilist, has devised an ingenious cylinder-motor so arranged as to flap a pair of long wings, giving them a much stronger impulse on the down than on the up stroke. The pectoral muscles of a bird are reproduced by two strong springs which are extended by the upward motion of the wings and store up energy for the down-stroke.
Close attention is also being paid to the actual shape of a bird's wing, which is not flat but hollow on its under side, and at the front has a slightly downward dip. ”Aerocurves” are therefore likely to supersede the ”aeroplane,” for Nature would not have built bird's wings as they are without an object. The theory of the aerocurve's action is this: that the front of the wing, on striking the air, gives it a downwards motion, and if the wing were quite flat its rear portion would strike air already in motion, and therefore less buoyant. The curvature of a floating bird's wings, which becomes more and more p.r.o.nounced towards the rear, counteracts this yielding of the air by pressing harder upon it as it pa.s.ses towards their hinder edge.
[Ill.u.s.tration: _M. Santos Dumont's Airs.h.i.+p returning to Longchamps after doubling the Eiffel Tower, October 19, 1901._]
The aerocurve has been used by a very interesting group of experimenters, those who, putting motors entirely aside, have floated on wings, and learnt some of the secrets of balancing in the air. For a man to propel himself by flapping wings moved by legs or arms is impossible. Sir Hiram Maxim, in addressing the Aeronautical Society, once said that for a man to successfully imitate a bird his lungs must weigh 40 lbs., to consume sufficient oxygen, his breast muscles 75 lbs., and his breast bone be extended in front 21 inches. And unless his total weight were increased his legs must dwindle to the size of broomsticks, his head to that of an apple! So that for the present we shall be content to remain as we are!
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