Читать книгу How It Flies; or, The Conquest of the Air - Richard Ferris - Страница 8

Degen’s apparatus to lift the man and his flying mechanism with the aid of a gas-balloon. See Chapter IV.

The airship is affected equally with the balloon by prevailing winds. A breeze blowing 10 miles an hour will carry a balloon at nearly that speed in the direction in which it is blowing. Suppose the aeronaut wishes to sail in the opposite direction? If the machinery will propel his airship only 10 miles an hour in a calm, it will virtually stand still in the 10-mile breeze. If the machinery will propel his airship 20 miles an hour in a calm, the ship will travel 10 miles an hour—as related to places on the earth’s surface—against the wind. But so far as the air is concerned, his speed through it is 20 miles an hour, and each increase of speed meets increased resistance from the air, and requires a greater expenditure of power to overcome. To reduce this resistance to the least possible amount, the globular form of the early balloon has been variously modified. Most modern airships have a “cigar-shaped” gas bag, so called because the ends look like the tip of a cigar. As far as is known, this is the balloon that offers less resistance to the air than any other.

Another mechanical means of getting up into the air was suggested by the flying of kites, a pastime dating back at least 2,000 years, perhaps longer. Ordinarily, a kite will not fly in a calm, but with even a little breeze it will mount into the air by the upward thrust of the rushing breeze against its inclined surface, being prevented from blowing away (drifting) by the pull of the kite-string. The same effect will be produced in a dead calm if the operator, holding the string, runs at a speed equal to that of the breeze—with this important difference: not only will the kite rise in the air, but it will travel in the direction in which the operator is running, a part of the energy of the runner’s pull upon the string producing a forward motion, provided he holds the string taut. If we suppose the pull on the string to be replaced by an engine and revolving propeller in the kite, exerting the same force, we have exactly the principle of the aeroplane.

As it is of the greatest importance to possess a clear understanding of the natural processes we propose to use, let us refer to any text-book on physics, and review briefly some of the natural laws relating to motion and force which apply to the problem of flight:

(a) Force is that power which changes or tends to change the position of a body, whether it is in motion or at rest.

(b) A given force will produce the same effect, whether the body on which it acts is acted upon by that force alone, or by other forces at the same time.

(c) A force may be represented graphically by a straight line—the point at which the force is applied being the beginning of the line; the direction of the force being expressed by the direction of the line; and the magnitude of the force being expressed by the length of the line.

(d) Two or more forces acting upon a body are called component forces, and the single force which would produce the same effect is called the resultant.

(e) When two component forces act in different directions the resultant may be found by applying the principle of the parallelogram of forces—the lines (c) representing the components being made adjacent sides of a parallelogram, and the diagonal drawn from the included angle representing the resultant in direction and magnitude.

(f) Conversely, a resultant motion may be resolved into its components by constructing a parallelogram upon it as the diagonal, either one of the components being known.

The Deutsch de la Muerthe dirigible balloon Ville-de-Paris; an example of the “cigar-shaped” gas envelope.

Taking up again the illustration of the kite flying in a calm, let us construct a few diagrams to show graphically the forces at work upon the kite. Let the heavy line AB represent the centre line of the kite from top to bottom, and C the point where the string is attached, at which point we may suppose all the forces concentrate their action upon the plane of the kite. Obviously, as the flyer of the kite is running in a horizontal direction, the line indicating the pull of the string is to be drawn horizontal. Let it be expressed by CD. The action of the air pressure being at right angles to the plane of the kite, we draw the line CE representing that force. But as this is a pressing force at the point C, we may express it as a pulling force on the other side of the kite by the line CF, equal to CE and in the opposite direction. Another force acting on the kite is its weight—the attraction of gravity acting directly downward, shown by CG. We have given, therefore, the three forces, CD, CF, and CG. We now wish to find the value of the pull on the kite-string, CD, in two other forces, one of which shall be a lifting force, acting directly upward, and the other a propelling force, acting in the direction in which we desire the kite to travel—supposing it to represent an aeroplane for the moment.

We first construct a parallelogram on CF and CG, and draw the diagonal CH, which represents the resultant of those two forces. We have then the two forces CD and CH acting on the point C. To avoid obscuring the diagram with too many lines, we draw a second figure, showing just these two forces acting on the point C. Upon these we construct a new parallelogram, and draw the diagonal CI, expressing their resultant. Again drawing a new diagram, showing this single force CI acting upon the point C, we resolve that force into two components—one, CJ, vertically upward, representing the lift; the other, CK, horizontal, representing the travelling power. If the lines expressing these forces in the diagrams had been accurately drawn to scale, the measurement of the two components last found would give definite results in pounds; but the weight of a kite is too small to be thus diagrammed, and only the principle was to be illustrated, to be used later in the discussion of the aeroplane.

Nor is the problem as simple as the illustration of the kite suggests, for the air is compressible, and is moreover set in motion in the form of a current by a body passing through it at anything like the ordinary speed of an aeroplane. This has caused the curving of the planes (from front to rear) of the flying machine, in contrast with the flat plane of the kite. The reasoning is along this line: Suppose the main plane of an aeroplane six feet in depth (from front to rear) to be passing rapidly through the air, inclined upward at a slight angle. By the time two feet of this depth has passed a certain point, the air at that point will have received a downward impulse or compression which will tend to make it flow in the direction of the angle of the plane. The second and third divisions in the depth, each of two feet, will therefore be moving with a partial vacuum beneath, the air having been drawn away by the first segment. At the same time, the pressure of the air from above remains the same, and the result is that only the front edge of the plane is supported, while two-thirds of its depth is pushed down. This condition not only reduces the supporting surface to that of a plane two feet in depth, but, what is much worse, releases a tipping force which tends to throw the plane over backward.

In order that the second section of the plane may bear upon the air beneath it with a pressure equal to that of the first, it must be inclined downward at double the angle (with the horizon) of the first section; this will in turn give to the air beneath it a new direction. The third section of the plane must then be set at a still deeper angle to give it support. Connecting these several directions with a smoothly flowing line without angles, we get the curved line of section to which the main planes of aeroplanes are bent.

With these principles in mind, it is in order to apply them to the understanding of how an aeroplane flies. Wilbur Wright, when asked what kept his machine up in the air—why it did not fall to the ground—replied: “It stays up because it doesn’t have time to fall.” Just what he meant by this may be illustrated by referring to the common sport of “skipping stones” upon the surface of still water. A flat stone is selected, and it is thrown at a high speed so that the flat surface touches the water. It continues “skipping,” again and again, until its speed is so reduced that the water where it touches last has time to get out of the way, and the weight of the stone carries it to the bottom. On the same principle, a person skating swiftly across very thin ice will pass safely over if he goes so fast that the ice hasn’t time to break and give way beneath his weight. This explains why an aeroplane must move swiftly to stay up in the air, which has much less density than either water or ice. The minimum speed at which an aeroplane can remain in the air depends largely upon its weight. The heavier it is, the faster it must go—just as a large man must move faster over thin ice than a small boy. At some aviation contests, prizes have been awarded for the slowest speed made by an aeroplane. So far, the slowest on record is that of 21.29 miles an hour, made by Captain Dickson at the Lanark meet, Scotland, in August, 1910. As the usual rate of speed is about 46 miles an hour, that is slow for an aeroplane; and as Dickson’s machine is much heavier than some others—the Curtiss machine, for instance—it is remarkably slow for that type of aeroplane.

Just what is to be gained by offering a prize for slowest speed is difficult to conjecture. It is like offering a prize to a crowd of boys for the one who can skate slowest over thin ice. The minimum speed is the most dangerous with the aeroplane as with the skater. Other things being equal, the highest speed is the safest for an aeroplane. Even when his engine stops in mid-air, the aviator is compelled to keep up speed sufficient to prevent a fall by gliding swiftly downward until the very moment of landing.

The air surface necessary to float a plane is spread out in one area in the monoplane, and divided into two areas, one above the other and 6 to 9 feet apart, in the biplane; if closer than this, the disturbance of the air by the passage of one plane affects the supporting power of the other. It has been suggested that better results in the line of carrying power would be secured by so placing the upper plane that its front edge is a little back of the rear edge of the lower plane, in order that it may enter air that is wholly free from any currents produced by the rushing of the lower plane.

As yet, there is a difference of opinion among the principal aeroplane builders as to where the propeller should be placed. All of the monoplanes have it in front of the main plane. Most of the biplanes have it behind the main plane; some have it between the two planes. If it is in front, it works in undisturbed air, but throws its wake upon the plane. If it is in the rear, the air is full of currents caused by the passage of the planes, but the planes have smooth air to glide into. As both types of machine are eminently successful, the question may not be so important as it seems to the disputants.

The exact form of curve for the planes has not been decided upon. Experience has proven that of two aeroplanes having the same surface and run at the same speed, one may be able to lift twice as much as the other because of the better curvature of its planes. The action of the air when surfaces are driven through it is not fully understood. Indeed, the form of plane shown in the accompanying figure is called the aeroplane paradox. If driven in either direction it leaves the air with a downward trend, and therefore exerts a proportional lifting power. If half of the plane is taken away, the other half is pressed downward. All of the lifting effect is in the curving of the top side. It seems desirable, therefore, that such essential factors should be thoroughly worked out, understood, and applied.

Section of the “paradox” aeroplane.