Читать книгу The Rise of the Flying Machine - Hugo Byttebier - Страница 11

Sir George Cayley

Cayley worked and wrote down his observations at a time when those interested in aviation had shifted their ideals and become aware of the possibilities of an artefact they had long been aware of, a plaything they had looked at without seeing, as the French enthusiast de La Landelle aptly put it.

It suddenly dawned on a few privileged minds that the kite, the plaything referred to, was in fact a flying body governed by the same laws of aerodynamics that applied to birds, those same laws that had been formulated by Newton. The kite, it was now believed, would be able to lift man into the air in a more rational manner than could be achieved by trying to imitate birds, so the kite would become a mechanical bird.

It is generally believed that the kite was invented by the Chinese a few centuries before the Christian era, but another contender for the title of inventor is the Greek philosopher Archytas of Tarent, who lived in the 4th century BC.

When speculating about the kite’s origins, the theory that it could have been discovered accidentally by observing runaway sails or hats or something similar holds little ground because it overlooks the fact that a kite can only rise when it is firmly attached to the ground. It would be more logical to visualize some kind of sail tugging at the hand of someone who held it as tightly as he could.

Kites began to be regarded as subject to the laws of aerodynamics during the 18th century, and in 1756 the German mathematician Euler wrote: “The kite, this child’s toy, despised by the scholars, could nevertheless lead to the most profound reflections”.

Indeed, in order to conceive the kite as similar in characteristics to the bird, a great mental effort had to be made because it was necessary to understand that the forces acting upon the kite had to be inverted.

A kite flies by capturing the kinetic energy of the wind, which is air on the move, so that a kite in reality flies by the power of the sun and the traction on the line that holds it to the ground is a measure of that force.

At the end of the 18th century it began to be understood that the force measured by the traction on the line was to be replaced by a thrust created on board the kite, making it move and generate lift.

This was the great discovery, as Cayley explained in his celebrated “triple paper” published in William Nicholson’s Journal of Natural Philosophy, Chemistry and the Arts (known as Nicholson’s Journal) in 1809 and 1810: “It is perfectly indifferent whether the wind blows against the plane or the plane be driven with equal velocity against the air... If therefore a waft of surfaces advantageously moved, by any force within the machine, took place to the extent required, aerial navigation would be accomplished.”

For the first time, the pessimistic conclusions of Leonardo da Vinci, Borelli, Navier and many others were replaced by the belief that a man-made engine could work the miracle. Again, quoting Cayley: “I feel perfectly confident, that this noble art will soon be brought to man’s general convenience, and that we shall be able to transport ourselves and families, and their goods and chattels, more securely by air than by water... To produce this effect, it is only necessary to have a first mover which will generate more power in a given time, in proportion to its weight, than the animal system of muscles.”

Once the principles of dynamic flight had been formulated (“To make a surface support a given weight by the application of power to the resistance of air”), Cayley went on to invent the aeroplane practically single-handed and wrote down his findings in a magisterial essay first published in Nicholson’s Journal in November 1809 and February and March 1810.

Starting with the powerplant, he considered steam as motive fluid but explicitly rejected the unwieldy machines moved by atmospheric pressure which were built by Boulton and Watt and turned his mind to the newly devised engines of Richard Trevithick (who was a genius comparable to Cayley himself) and which worked with what Trevithick described as “pressure of steam”. In 1804, Trevithick had just built the first locomotives in Britain and in 1808 a steam-driven road wagon.

Pondering on the possibilities of making steam engines lighter and more powerful, Cayley proposed the water-tube boiler, which was indeed to become the most efficient and lightest generator, though it appeared many years later. But Cayley looked farther ahead and proposed that a lighter and better engine could be built by using internal combustion, by “firing inflammable air (gas) with a due portion of common air under a piston”, to quote his own words.

However, Cayley had not yet reached the limits of his vision. Once the machine flew, what would happen? It had to remain stable in the air and not behave like a dead leaf, it also had to be steerable and not zoom like an arrow. Incredibly, Cayley solved nearly all these problems too.

He had a good look at the then already known parachute, noted its lack of stability and concluded that lateral stability could only be achieved by an angular form of the wings. “With the apex downwards”, a dihedral angle, as it is called today. Cayley called this “the chief basis of stability in aerial navigation”.

He also considered the need for longitudinal stability and thought that a low centre of gravity and a kind of automatism in the travel of the centre of pressure according to the angle of attack of the wing would achieve the desired effect.

Steerage would be obtained by a horizontal rudder “in a similar position to the tail in the birds” and a “vertical sail ... capable of turning from side to side which, in addition with its other movements effects the complete steerage of the vessel”.

He also saw the need for streamlining the body in order to reduce parasitic drag, especially the rear part and also noted that “diagonal bracing” would make it possible to build structures “with a greater degree of strength and lightness than any made use of in the wings of the bird”. This was the principle of trussing which Chanute introduced with good effect in the construction of biplane wings during the late 1890s and which remained in use for nearly forty years.

Giving his imagination free rein, Cayley then prophesied: “By increasing the magnitude of the engine, 10, 50, or 500 men may equally well be conveyed; and convenience alone, regulated by the strength and size of the materials, will point out the limit for the size of vessels in aerial navigation.”

Cayley made several experiments himself, which have been described in other publications1 but his thoughts ran too far ahead of the possibilities of the moment to achieve any practical result. He even designed a kind of hot-air engine and experimented a couple of times with gunpowder but was moved to remark: “Who would take the risk of breaking their necks or being blown to atoms?”. Yet, gunpowder as engine fuel had been the first used and would continue to be proposed from time to time, which only shows that in the pursuit of their ideals, mankind will not avoid the most appalling risks.

Referring to the experiments made upon the resistance of air by Smeaton2 and corrected by him by careful and unrelenting observation of the crow and other birds, Cayley came to the conclusion that a wing loaded at 1 lb/sq ft would carry 1 lb of weight as soon as a horizontal speed of 35 ft/sec (equivalent to 21 knots or 24 mph or 38 kph) was reached. This was correct and is the take-off speed of most of the ultralight planes that have come into fashion. What nobody knew was how much power was needed to accelerate a winged machine of a certain weight until flying speed was reached.

Newton’s formula had led Navier to compute impossibly high figures but Cayley, again by observing birds, noted: “The perfect ease which some birds are suspended with in long horizontal flights without one waft of their wings, encourages the idea that a slight power only is necessary”. Sir George was possibly not the first and certainly not the last, to let the soaring birds beguile him with that “slight power only”.

Having calculated that a man running upstairs was able to generate about 2 hp for a short time, he took into account that no man could sustain this rate of power for a long period (“one minute” noted Cayley). Consequently, he calculated the output needed at take-off — the moment at which he believed, correctly, that the greatest effort would have to be made — as 5 hp with a specific weight that had to remain below 30 lbs per hp.

In his day, a steam engine of five hp was a machine of awesome proportions located in a building specially erected to house it. Even so, he was well below the real power requirements, as would be discovered a century later.

Cayley waited all his life for the aero engine to appear, and during long periods he left aeronautics alone and dedicated himself to some of his other manifold preoccupations. The last published reference to the missing powerplant was written in 1853, three years before he died at the age of 83: “It need scarcely be further remarked that, were we in possession of a sufficiently light prime mover to propel such vehicles ... mechanical aerial navigation would be at our command without further delay”. This proved correct, but the goal was still more than half a century away.

1. Sir George Cayley’s Aeronautics 1796-1855, by Charles H. Gibbs-Smith (Science Museum, London, 1962).

2. Smeaton disclosed his tables of pressures around 1750, after an extended visit to the Low Countries where he was able to observe the windmills there and their efficient wing-shapes, a result of centuries of practical experience.