AERIAL NAVIGATION
Effect of Wind.—Navigating in an airplane is complicated only on account of the fact that there is a wind blowing which may not be in the desired direction. While on the sea navigation is simple through the assistance of the magnetic compass (because side winds can not materially drift the ship sideways), in the air this is not the case; for if the pilot using the compass points the nose of the airplane directly north while a west wind is blowing, this wind will cause the machine to drift in an easterly direction so that in an hour of flight the airplane will be off its course by an amount equal to distance which the wind travels in 1 hr.; and the joint result of the motion of the airplane forward and the motion of the wind sideways will cause the machine to drift in a northeasterly direction at a speed quite different from its rated velocity, and in this case somewhat larger. Victor Carlstrom in his Chicago-New York flight found while he was over Cleveland that a side wind was deviating his course 17° away from what it should be, and if he had not had such landmarks as the shore of Lake Erie for guidance he might easily have lost considerable time.
The question of making allowance for this wind drift is very important where there are no landmarks, as in the case of night flying, flying over the sea, or flying over the clouds; and the only way the pilot can make allowances for these conditions is to figure them out before he starts from the airdrome, and plan to circumvent them. That is to say, the pilot in flight has no means, aside from visual observation of the ground, to determine whether or not the wind is blowing him off his course. He must determine the whole situation before he starts, and the process of doing so is as follows.
Graphical Method for Determining Direction to Steer.—The pilot will ascertain from the weather vane and anemometer of the airdrome (1) the velocity and (2) the direction of the wind, (3) the speed of the airplane he is to fly, (4) the compass bearing of the actual course which he desires to follow. With this data it is possible to construct a simple diagram and to determine the direction to be steered and the actual velocity which will result in the proposed journey. A draftsman’s scale, protractor and dividers, a pencil and a piece of paper are the necessary equipment.
When the wind blows at an angle with the desired course it is necessary to steer the airplane in such a direction that its own forward motion will neutralizer the side effect of the drift of the wind from moment to moment. The problem is to determine this direction for steering, as it is not known. We are not concerned with distances in this problem, for the direction is going to be the same whether our flight is of 100 or 200 miles. We are, however, vitally concerned with velocities; and we will assume that the velocity of the airplane is known to be 75 miles per hour, and from observation on a local anemometer the velocity of the wind is known to be 20 miles an hour. We also know, of course, the direction of the wind, which should be given in terms of an angle whose other leg points directly north. Now if the flight is to be made at a height of 2000 ft., as is usual in cross-country flight over average country, we will find that the speed of wind will increase as we rise up; moreover, that its direction will change. In the present case the wind will be 88 per cent. higher in 2000 ft. than it is on the ground; that is to say, the velocity at the altitude we are going to use is twenty times 1.88, or about 38 miles per hour. Moreover, as the height increases the direction of the wind changes, shifting around always in a clockwise direction as the height increases, in the present case shifting around 16° from its ground direction. (The change of velocity and direction for various heights is indicated on the subjoined table.) Thus a west wind becomes at a height of 2000 ft. a slightly northwest wind, or, to be exact, blows from a direction which is 74° west of north.
Our treatment of the problem then has for starting points: velocity of wind, 38 miles per hour; direction of the wind, 74° west of north; velocity of airplane 75 miles per hour; desired direction of flight (which has been determined by laying out on the map and reading the compass bearing with the protractor), say 60° east of north. In 1 hr. of flight the machine would travel in this unknown direction a distance of 75 miles were it not for the wind, but for every hour of such flying the wind is blowing it 38 miles sideways; and the desired direction must be such that its joint effect, together with the 38 mile sideways wind, will leave the machine exactly on its proper course at the end of the hour.
On the map or piece of paper denote the starting point by A (see Fig. [37]). From A draw a line parallel to the wind (that is to say, 74° west of north), and let this line represent, to any convenient scale, the speed of the wind, 38 miles per hour. The far end of the line may be called B, and may be given an arrow to represent the direction of wind. Now draw on the map a line from A to the desired destination (C), giving it, of course the proper compass bearing. Take the dividers, and with B as a center, describe an arc at such distance as to represent 75 miles per hour, the speed of the machine; this arc will intercept the line AC at D, and BD then gives the direction to steer, for it is that direction which will permit the airplane in 1 hour exactly to neutralize the sidewise drift of the wind. The distance AD on this diagram can be measured off and will give the actual velocity of movement along the line of flight in miles per hour. Notice that it is 97 miles per hour, quite different from the speed of the airplane.
Fig. 37.—Graphical method for determining direction to steer to counteract wind-drift.
Assuming that the pilot has determined the proper angle toward which the airplane nose must be pointed, has maintained this angle throughout his flight by means of the compass and has safely reached his objective; for the return trip this diagram must be completely reconstructed (unless the wind is exactly parallel to his course). The pilot should not make the mistake in returning to the starting point of steering the airplane nose in a direction exactly opposite to the outward trip; the reader may make this clear to himself by drawing the return diagram and comparing it with the outward-bound diagram.
To summarize flying when a cross wind is blowing, it will be said that the direction of actual travel will not be the direction indicated by the axis of the airplane; and that therefore while in a picture of the situation the airplane appears to skid sideways along the whole course it must be borne in mind that actually there is no skidding whatever but the air is meeting the airplane in normal manner. The situation is analogous to that of a fly going from one side to the other of the cabin of a moving ship, where the actual course through space of the fly is an apparent skid, due to the resultant of its own and the ship’s movement.
Variation of Velocity and Direction With Height
(25 miles per hour wind)
| Height in feet | At surface | 500 | 1000 | 2000 | 3000 | 4000 | 5000 |
| Velocity change in per cent | 100 | 135 | 172 | 188 | 196 | 200 | 200 |
| Clockwise deviation in degrees | 0 | 5 | 10 | 16 | 19 | 20 | 21 |
Effect of Wind on Radius of Action.—Not only is the direction of flight altered by the wind but also the radius of action from a standpoint of gasoline capacity is altered. In the above machine the gasoline capacity is sufficient for 3½ hr. of flight. How far can it go across country and return before the gasoline is used up? Always allow ½ hr. gasoline for climbing and for margin; this leaves 3 hr., which at 75 miles an hour is 225 miles, or 112 miles out and 112 miles back. Now suppose that a flight is to be made across country directly in the teeth of a 40-mile wind; the radius of flight will be altered as indicated by the following calculation: Speed outward is obviously 75 minus 40 or 35 miles per hour. Speed on the return trip is obviously 75 plus 40 or 115 miles per hour—3.29 times as fast—and occupying a time which may be designated by the letter x. The time on the outward trip may be designated by 3.29x, a total time of x + 3.29x which we know equals 180 min. before the gas runs out. Solve the equation x + 3.29x = 180 and we find that x is equal to 42 min., that is, the return trip requires 42 min., and the outward trip requires 138 min. The distance covered on the outward trip is then 138/60 of 35, which equals 80.5 miles. The radius is then reduced from 112 miles to 80.5 miles.
In cases where the wind is not parallel to the line of flight the actual velocity of course can not be obtained by adding up the airplane and wind velocities, but must be obtained by the graphical method mentioned above; thenceforward the calculation is the same.
Effect of Height.—Of course if one has to fly in the teeth of a wind and can choose one’s own altitude, it is desirable to fly low where the head wind has its smaller velocity, and when flying with the following wind to rise to considerable altitudes. The proper height at which to fly will be about 1500 to 3000 ft., for cross-country trips over ordinary country; but may be increased when the wind is unsteady or decreased when there are low-lying clouds. The steadiness as well as the speed of the wind increases with the height. The character of the country should be carefully investigated from the profile maps before starting; all hilly parts should be marked on the map as a warning against landing. Contour is not readily distinguished from a height of 2000 ft. and for this reason points may be indicated on the map where poor landing places make it desirable to fly high. The character of the country or the scarcity of landing places may make it advisable to fly at high altitudes for the following reasons: (1) in case of engine failure a good margin of height is necessary to provide length of glide to reach distant landing places; (2) there is then plenty of space for righting the airplane in case of bumps, side slips, etc.; (3) eddies or local currents due to inequalities of the ground do not exist to great heights; (4) landmarks can be better distinguished from high altitudes because the vision is better (however, one must never trust to landmarks only in navigating but should constantly use a compass if only as a check, and especially in passing through clouds). Having selected in advance the proper height to use during the trip climb to this height in circles; note the direction of wind drift meanwhile to check up your estimate. Pass directly over the point of departure and when over it point the nose of the airplane for a moment directly toward the desired objective (which can be done with the aid of the magnetic compass); select some distant object which is dead ahead, and therefore directly in the course; then head the nose of the machine up into the wind just enough so that the direction of movement will be straight toward this distant object. The direction of the nose of the machine thus set by a method distinct from the graphical method above mentioned should exactly correspond, however, with the calculated direction; and thus a means of checking is obtained.
Effect of Fog.—The effect of fog upon navigating an airplane is that it prevents the use of landmarks in aiding the pilot; also that it upsets the pilot’s sense of level. These two effects are, of course, independent of the fact that proper landing places are obscured, with resultant peril in case of engine failure. Therefore, a fog should be avoided whenever possible; when one comes up, the airplane should descend, and should never attempt to get above it, as in certain localities it may turn out to be a ground fog. If the fog is very bad, land at the earliest opportunity. It is on account of fog that the pilot avoids river valleys where frequently there is a haze from the ground up to a height of 700 ft., preventing the view of proper landing places in case of necessity.
Effect of Clouds on Navigation.—Flying in or above the clouds is a similar case, inasmuch as landmarks can not be seen. It is not wise to go above the clouds when on the sea coast, as offshore winds may, unknown to the pilot, carry him out to sea; and any flight over the sea which is to a distance greater than the safe return gliding distance is, of course, perilous.
Navigation by Means of the Drift Indicator.—The drift indicator is an instrument for determining directly the side drift of an airplane. It enables the pilot by looking through a telescope at the ground to determine exactly what his direction of motion is with relation to the ground. The telescope is mounted vertically and is rotatable about its own axis; it has a cross-hair which appears in the field of view during the pilot’s observation of the ground. As the airplane speeds overhead objects on the ground will appear through the telescope to slip backward in the given direction; and when accustomed to the use of this instrument the pilot can rotate the telescope until the cross-hair is exactly parallel to the apparent line of motion of objects on the ground. The telescope cross-hair is parallel to the axis of the airplane normally and the scale attached to the telescope will in this case read zero. When the pilot rotates the telescope so that the cross-hair becomes parallel to the relative backward motion of the ground the scale will read something different from zero and will give the angle between the actual line of motion and the axis of the airplane.
Such a drift indicator is, of course, useful only when the ground is visible. The pilot knowing the angle between the airplane axis and the line of motion and therefore knowing the deviation between the supposed course and the actual course is able to make corrections and steer the machine in its proper direction. This may be done by altering the “lubber-line” or his compass just enough to offset the side drift of the machine; after which the desired course may be followed by simply keeping to the proper compass bearing. An instrument has been devised wherein the rotation of the drift-indicator telescope simultaneously alters the lubber-line zero. The operator then has merely to take an occasional observation of the apparent drift line of the ground, which observation automatically shifts the lubber-line and navigation proceeds as if there were no side wind blowing whatever. Knowing the angle between the direction of movement and the airplane axis, the pilot may then compute the speed of motion in a manner analogous to the graphical method previously mentioned; or he can make use of a chart for the determination of this speed.
Navigation over Water.—In flying over water the presence of waves is a valuable guide to the aviator, for he knows that these waves extend in a direction normal to the wind. Moreover, he knows that the velocity of the waves bears some relation to the velocity of the wind. In order to estimate the velocity of the waves it is only necessary to know their wave length, that is, the distance between two consecutive wave crests. The rule is that for a wave length of 10 ft. the velocity is 10 miles per hour, and will vary as the square root of this wave length; that is, if the wave length is half, the velocity will be 10 divided by the square root of 2, or 7.1 miles per hour.
CHAPTER VI
THE RIGGING OF AIRPLANES
Object.—The object of this chapter is to teach the elementary principles of correct rigging. It is not expected that the student will become an expert mechanic, but with this treatment as a basis and through practice he will be able to judge whether or not a machine is correctly and safely rigged. In other words, he will not have to depend on someone else’s judgment as to whether panels, wires, controls, struts, etc., of a machine are in good order, but he will be able to observe understandingly that they are. If the engine goes wrong he can land, if the rigging goes wrong he is in great difficulty. Moreover, if the rigging is wrong, speed is lessened and the stability is uncertain.
The first thing to be learned in rigging is a knowledge of the peculiar terms which have come into use in aeronautics defining different parts of the machines. Our present list of terms is derived, partly from French, partly from English, and partly from American terms. Thus different names may refer to the same part.