Читать книгу Airplane Photography - Herbert Eugene Ives - Страница 5

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An essential part of the equipment of either the aerial photographer or the designer of aerial photographic apparatus is a working knowledge of the principles and construction of the airplane, and considerable actual experience in the air. Conditions and requirements in the flying plane are far different from those of the shop bench or photographic studio. As a preliminary to undertaking any work on airplane instruments a good text-book on the principles of flight should be studied. Such general ideas as are necessary for understanding the purely photographic problems are, however, outlined in the next paragraphs.

Fig. 1.—The elements of the plane.

Construction of the Airplane.—The modern airplane (Fig. 1) consists of one or more planes, much longer across than in the direction of flight (aspect ratio). These are inclined slightly upward toward the direction of travel, and their rapid motion through the air, due to the pull of the propeller driven by the motor, causes them to rise from the earth, carrying the fuselage or body of the airplane. In the fuselage are carried the pilot, observer, and any other load. Wheels below the fuselage forming the under-carriage or landing gear serve to support the body when running along the ground in taking off or landing. The pilot, sitting in one of the cockpits, has in front of him the controls, by means of which the motion of the plane is guided (Figs. 2 and 3). These consist of the engine controls—the contacts for the ignition, the throttle, the oil and gasoline supply, air pressure, etc., and the steering controls—the rudder bar, the stick and the stabilizer control. The rudder bar, operated by the feet, controls both the rudder of the plane, which turns the plane to right or left in the air, and the tail skid, for steering on the ground. The stick is a vertical column in front of the pilot which, when pushed forward or back, depresses or raises the elevator and makes the machine dive or climb. Thrown to either side it operates the ailerons or wing tips, which cause the plane to roll about its fore and aft axis. The stabilizer control is usually a wheel at the side of the cockpit, whose turning varies the angle of incidence of the small stabilizing plane in front of the elevator, to correct the balance of the plane under different conditions of loading.

Fig. 2.—Forward cockpit of DeHaviland 4, showing instrument board.

Fig. 3.—Rear cockpit of DeHaviland 4, showing rear “stick” and rudder bar.

The fuselage consists usually of a light hollow framework of spruce or ash, divided into a series of bays or compartments by upright members, connecting the longerons, which are the four corner members, running fore and aft, of the plane. Diagonally across the sides and faces of these bays are stretched taut piano wires, and the whole structure is covered with canvas or linen fabric. Cross-wires and fabric are omitted from the top of one or more bays to permit their being used as cockpits for pilot and observer. In later designs of planes the wire and fabric construction has been superseded by ply-wood veneer, thereby strengthening the fuselage so that many of the diagonal bracing wires on the inside are dispensed with. This greatly increases the accessibility of the spaces in which cameras and other apparatus must be carried.

Fig. 4.—Biplane in flight.

The fuselage differs greatly in cross-section shape and in roominess according to the type of engine. In the majority of English and American planes, with their vertical cylinder or V type engines, the fuselage is narrow and rectangular in cross-section. In many French planes, radial or rotary engines are used and the fuselage is correspondingly almost circular, and so is much more spacious than the English and American planes of similar power. The shape and size of the plane body has an important bearing on the question of camera installation.

Fig. 5.—A single-seater.

Types of Planes.—The most common type of plane is the biplane (Fig. 4), with its two planes, connected by struts and wires, set not directly over each other, but staggered, usually with the upper plane leading. Monoplanes were in favor in the early days of aviation, and triplanes have been used to some extent. According to the position of the propeller planes are classified as tractors or pushers, tractors being at present the more common form. Planes are further classified as single-seaters (Fig. 5), two-seaters, and three-seaters. These motor and passenger methods of classification are now proving inadequate with the rapid development of planes carrying two, three, and even more motors, divided between pusher and tractor operation, and carrying increasingly large numbers of passengers. Aside from structure, planes may be further classified according to their uses, as scout, combat, reconnaissance, bombing, etc. Planes equipped with floats or pontoons for alighting on the water are called seaplanes (Fig. 182), and those in which the fuselage is boat-shaped, to permit of floating directly on the water, are flying boats (Fig. 183).

The Plane in the Air.—The first flight of the photographic observer or of the instrument expert who is to work upon airplane instruments is very profitably made as a “joy ride,” to familiarize him with conditions in the air. His experience will be somewhat as follows:

The plane is brought out of the hangar, carefully gone over by the mechanics, and the engine “warmed up.” The pilot minutely inspects all parts of the “ship,” then climbs up into the front cockpit. He wears helmet and goggles, and if the weather is cold or if he expects to fly high, a heavy wool-lined coat or suit, with thick gloves and moccasins, or an electrically heated suit. The passenger, likewise attired, climbs into the rear cockpit and straps himself into the seat. He finds himself sitting rather low down, with the sides of the cockpit nearly on a level with his eyes. To either side of his knees and feet are taut wires leading from the controls to the elevator, stabilizer, tail skid and rudder. If the machine is dual control, the stick is between his knees, the rudder bar before his feet. None of these must he let his body touch, so in the ordinary two-seater his quarters are badly cramped.

At the word “contact” the mechanics swing the propeller, and, sometimes only after several trials, the motor starts, with a roar and a rush of wind in the passenger's face. After a moment's slow running it is speeded up, the intermittent roar becomes a continuous note, the plane shakes and strains, while the mechanics hold down the tail to prevent a premature take-off. When the engine is properly warmed up it is throttled to a low speed, the chocks under the wheels are removed, the mechanics hold one end of the lower wing so that the plane swings around toward the field. It then “taxis” out to a favorable position facing into the wind with a clear stretch of field before it. After a careful look around to see that no other planes are landing, taking off, or in the air near by, the pilot opens out the engine, the roar increases its pitch, the plane moves slowly along the ground, gathers speed and rises smoothly into the air. Near the ground the air is apt to be “bumpy,” the plane may drop or rise abruptly, or tilt to either side. The pilot instantly corrects these deviations, and the plane continues to climb until steadier air is reached.

At first the passenger's chief impressions are apt to be the deafening noise of the motor, the heavy vibration, the terrific wind in his face. If he raises his hand above the edge of the cockpit he realizes the magnitude of wind resistance at the speed of the plane, and hence the importance of the stream-line section of all struts and projecting parts.

When he reaches the desired altitude the pilot levels off the plane and ceases to climb. Now his task is to maintain the plane on an even keel by means of the controls, correcting as soon as he notes it, any tendency to “pitch,” to “roll” or to “yaw” off the course. The resultant path is one which approximates to level straight flying to a degree conditioned by the steadiness of the air and the skill of the pilot. If he is not skilful or quick in his reactions he may keep the plane on its level course only by alternately climbing and gliding, by flying with first one wing down and then the other, by pointing to the right and then to the left. The skilled photographic pilot will hold a plane level in both directions to within a few degrees, but he will do this easily only if the plane is properly balanced. If the load on the plane is such as to move the center of gravity too far forward with respect to the center of lift the plane will be nose-heavy, if the load is too far back it will be tail-heavy. Either of these conditions can be corrected, at some cost in efficiency, by changing the inclination of the stabilizer. When the plane reaches high altitudes in rare air, where it can go no further, it is said to have reached its ceiling. It here travels level only by pointing its wings upward in the climbing position, so that the fuselage is no longer parallel to the direction of flight. An understanding of these peculiarities of the plane in flight is of prime importance in photographic map making, where the camera should be accurately vertical at all times.

The direction and velocity of the wind must be carefully considered by the pilot in making any predetermined course or objective. The progress of the plane due to the pull of the propeller is primarily with reference to the air. If this is in motion the plane's ground speed and direction will be altered accordingly. In flying with or against the wind the ground speed is the sum or difference, respectively, of the plane's air speed (determined by an air speed indicator) and the speed of the wind. If the predetermined course lies more or less across the wind the plane must be pointed into the wind, in which case its travel, with respect to the earth, is not in the line of its fore and aft axis. The effect of “crabbing,” as it is called, on photographic calculations is discussed later (Figs. 136 and 138).

When the plane has reached the end of its straight course and starts to turn, its level position is for the moment entirely given up in the operation of banking (Fig. 6). Just as the tracks on the curve of a railroad are raised on the outer side to oppose the tendency of the train to slip outward, so the plane must be tilted, by means of the ailerons, toward the inside of the turn. A point to be clearly kept in mind about a bank is that if correctly made a plumb line inside the fuselage will continue to hang vertical with respect to the floor of the plane, and not with respect to the earth, for the force acting on it is the combination of gravity and the acceleration outward due to the turn. Only some form of gyroscopically controlled pointer, keeping its direction in space, will indicate the inclination of the plane with respect to the true vertical. If the banking is insufficient the plane will side slip outward or skid; if too great, it will side slip inward.

Fig. 6.—Banking.

As part of the “joy ride” the pilot may do a few “stunts,” such as a “stall,” a “loop,” a “tail spin,” or an “Immelman.” From the photographic standpoint these are of interest in so far as they bear on the question of holding the camera in place in the plane. The thing to be noted here is that (particularly in the loop), if these maneuvers are properly performed, there is little tendency toward relative motion between plane and apparatus. In a perfect loop it would, for instance, be unnecessary, due to the centrifugal force outward, for the observer to strap himself in. It is, however, unwise to place implicit confidence in the perfection of the pilot's aerial gymnastics. No apparatus should be left entirely free, although, for the reason given, comparatively light fastenings are usually sufficient.

When nearing the landing field the pilot will throttle down the engine and commence to glide. If he is at a considerable altitude he may come down a large part of the distance in a rapid spiral. As the earth is approached the air pressure increases rapidly, and the passenger, if correctly instructed, will open his mouth and swallow frequently to equalize the air pressure on his ear drums. Just before the ground is reached the plane is leveled off, it loses speed, and, if the landing is perfect, touches and runs along the ground without bouncing or bumping. Frequently, however, the impact of the tail is sufficiently hard to cause it to bump badly, with a consequent considerable danger to apparatus of any weight or delicacy. This is especially apt to occur in hastily chosen and poorly leveled fields such as must often be utilized in war or in cross-country flying.

Appearance of the Earth from the Plane.—The view from the ordinary two-seater is greatly restricted by the engine in front and by the planes to either side and below (Figs. 7, 8, and 9). By craning his neck over the side, or by looking down through an opening in the floor, the passenger has an opportunity to learn the general appearance of the subject he is later to devote his attention to photographing. Perhaps the most striking impression he receives will be that of the flatness of the earth, both in the sense of absence of relief and in the sense of absence of extremes of light and shade. The absence of relief is due to the fact that at ordinary flying heights the elevations of natural objects are too small for the natural separation of the eyes to give any stereoscopic effect. The absence of extremes of light and shade is in part due to the fact that the natural surfaces of earth, grass and forest present no great range of brightness; in part to the small relative areas of the parts in shadow; in considerable part to the layer of atmospheric haze which lies as an illuminated veil between the observer and the earth at altitudes of 2000 meters and over (Figs. 10 and 11). Due to the combination of these factors the earth below presents the appearance of a delicate pastel.

As the gaze is directed away from the territory directly below, the thickness of atmosphere to be pierced rapidly increases, until toward the horizon (which lies level with the observer here as on the ground) all detail is apt to be obliterated to such an extent that only on very clear days can the horizon itself be definitely found or be distinguished from low lying haze or clouds (Fig. 4).

Fig. 7.—The view ahead.

Fig. 8.—The view astern.

Airplane Instruments.—Mounted on boards in front of the pilot and observer are various instruments to indicate the performance of engine and plane (Fig. 2). Those of interest to the photographic observer are the compass, the altimeter, the air speed indicator, the inclinometers.

Fig. 9.—The view between the wings.

Fig. 10.—Appearance of the earth from a low altitude—3000 feet or less.

The compass is usually a special airplane compass, with its “card” immersed in a damping liquid. Like most of the direction indicating instruments on a plane its indications are only of significance when the plane is pursuing a steady course. On turns or rapid changes of direction of any sort perturbations prevent accurate reading.

The altimeter is of the common aneroid barometer type. On American instruments it is usually graduated to read in 100-foot steps. While somewhat sluggish, it is quite satisfactory for all ordinary determinations of altitude in photographic work. Were primary map making to be undertaken, where the scale was determinable only from the altitude and focal length of the lens, the ordinary altimeter is hardly accurate enough.

Fig. 11.—Appearance of the earth from a high altitude—10,000 feet or more.

The air speed indicator consists of a combination of Venturi and Pitot tubes, producing a difference of pressure when in motion through the air which is measured on a scale calibrated in air speed. This instrument is important for determining, in combination with wind speed, the ground speed of the plane, on the basis of which is calculated the interval between exposures to secure overlapping photographs. Its accuracy is well above that necessary for the purpose.

Inclinometers for showing the lateral and fore and aft angle of the plane with the horizontal, are occasionally used, and have also been incorporated in cameras. The important point to remember about these instruments is that they are controlled not alone by gravity but as well by the acceleration of the plane in any direction. They consequently indicate correctly only when the plane is flying straight. On a bank the lateral indicator continues to indicate “vertical” if the bank is properly calculated for the turn.