The lifting principle must be analogous to that by which a kite is suspended. A kite is prevented from rising beyond a certain height by a string, and the pressure of the wind working against it at an angle tends to lift it, like a soft wedge continuously driven under it. In practice it makes no difference whether the kite be stationary in a wind or towed rapidly through a dead calm; the wedge-like action of the air remains the same.
Maxim decided upon constructing what was practically a huge compound kite driven by very powerful motors.
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 circumference, 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 brass 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 crape 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.