AEROPLANE (from the Greek aer, air; pianos, wandering), a term now commonly used to define a °heavier than air° flying ma chine equipped with fixed aerofoils or main supporting surfaces and driven by suitable mo tive power.
Principle.— Everyone nowadays is familiar with the appearance of an aeroplane, but many there are, nevertheless, who do not know what, scientifically speaking, an aeroplane is. They see the machine on the ground; they observe someone giving frantic tugs at something that moves in Jerks; they hear a roar, which they know must come from an engine; they per ceive that, in starting, the machine runs for a while along the ground before rising gently into the air; but still they do not know why the aeroplane flies.
It has something to do with the wings, of course, but how? That is the question at which the average lay mind stops short, not for ability to understand the problem, but gen erally for lack of some appropriate explana tion that will bring what is fundamentally a very simple phenomenon out of its proper sphere of aeronautical science into the realm of everyday things that are comprehended by common sense.
There is an elusive aspect of the general view, and only one, that is apt to hide itself from the uninitiated unless brought prominent ly into the full light of the mind in the very first instance and that is the significance of a simple scientific expression much used in avia tion, namely, °relative motion.° If the man in the street saw an aeroplane apparently stand ing still in the air it would not occur to him to think that the machine must be flying through the wind at its full speed and that its relative motion in the air is quite unaffected by its motion relatively to the ground on which he is standing. Yet the same man knows very well that if he starts running on a calm day he will feel a slight breeze in his face, which is solely the result of his own relative motion through the air. He is also aware that if he puts his head out of an express railway car he will encounter half a gale of wind notwith standing that the leaves of the trees may show not so much as a tremor. If, instead of put ting his head out of the window, he were to take a sheet of stiff cardboard and put that outside he would have a still more practical demonstration of the force of the relative wind which supports the aeroplane in its flight. If the train is moving fast the card board will exhibit a violent tendency to flap upwards and downwards with the least variation from its truly edge-on horizontal position. It is at this point that the embryo scientist begins to think really hard. His
mind perceives an unsuspected fact that he senses to be of great importance. He has observed that by slightly raising the front edge of the piece of cardboard so that it is at a slight angle to its line of motion, instead of being truly edge-on, an extremely strong lift ing force acts on the cardboard, although its resistance to the air is but little more than it was when the cardboard was edge-on. So pronounced is the preponderating value of the lifting effort over the resistance at very small angles that anyone making this experiment would at once conclude that if he wished the wind to support a weight he would certainly arrange some sort of a surface beneath it, like a table, but tilted so as to have only a slight angle of inclination to the line of its flight through the air.
An aeroplane has its table-like supporting surfaces so arranged as to get the best lifting effect for the least effort, having regard, of course, for the conditions under which the machine is designed to fly. It is clear, merely from a glance at a number of aeroplanes, that they are not all exactly alike in this respect, but it will be noticed that they all have one point in common which is that the surface instead of being flat is cambered or slightly bellied like the sail of a yacht. This is an important analogy because a yacht is one of those commonplace objects that are so familiar that the man on the shore never stops to ask himself whether or not he understands how it sails. It will be the same with the aeroplane in a few years' time, which is why it is worth while troubling to appreciate an explanation now in order that one really may be informed as to the essential facts by the time aviation, in common with so many other interesting things, becomes veiled under the ever-spread ing pall of public indifference. The sail of a yacht is an aeroplane in principle but its use differs materially from the purpose of the wing of an aeroplane. When the wind blows obliquely on the sail of a yacht the pressure that it exerts is mostly directly toward cap sizing the boat, but owing to the set and cam ber of the sail the force is also directed slight ly forward toward the bows and the amount of this compbnent is sufficient to propel the boat. If a real wind were to blow obliquely from beneath on the wing surface of an aero plane the same propelling effect would be pro duced and the main force that tends to capsize the yacht would be turned to the useful pur pose of supporting the weight of the machine, which would continue to fly without using its engine so long as the conditions remained ap propriate.