History of Aviation - Chapter 5.2
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 17degree 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 sea, or flying over the clouds; and the only way pilot can make allowances for these conditions figure them out before he starts from the air and plan to circumvent them. That is to say, pilot in flight has no means, aside from observation of the ground, to determine whether not the wind is blowing him off his course. He must determine the whole situation before he starts, the process of doing so is as follows.
Graphical Method for Determining Direction Steer.-The pilot will ascertain from the weather vane and anemometer of the airdrome (1) velocity and (2) the direction of the wind, (3) 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 I steered and the actual velocity which will result in the proposed journey. A draftsman's scale, pro-tractor 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 neutralize 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 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 160 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 74degrees 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 degrees west of north; velocity of airplane 75 miles per hour; desired direction of. flight (which has been determined by laying out the map and reading the compass bearing with the protractor), say 60degrees east of north. In 1 hr flight the machine would travel in this unknown direction a distance of 75 miles were it not for wind, but for every. hour of such flying the wind 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 machine exactly on its proper course at the end the hour.
On the map or piece of paper denote the starting point by A (see Fig. 37). From A draw line parallel to the wind (that is to say,74degrees 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.
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 starting point of steering the airplane nose direction exactly opposite to the outward trip; reader may make this clear to himself by di the return diagram and comparing it with the ward-bound diagram.
To summarize flying when a cross wind is ing, it will be said that the direction of ac travel will not be the direction indicated by the of the airplane; and that therefore while in a ture of the situation the airplane appears to sideways along the whole course it must be in mind that actually there is no skidding whate'~ but the air is meeting the airplane in normal ner. The situation is analogous to that of a going from one side to the other of the cabin of moving ship, where the actual course through of the fly is an apparent skid, due to the resultant 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
tion 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 1/2 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 mm. before the gas runs out. Solve the equation x + 3.29x 180 and we find that x is equal to 42mm., that is, the return trip requires 42 mm., and -the outward trip requires 138 mm. The distance covered on the outward trip is then 13860 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 teeth of a wind and can choose one's own a] it is desirable to fly low where the head wind its smaller velocity, and when flying with the lowing wind to rise to considerable altitudes. proper height at which to fly will be about to 3000 ft., for cross-country trips over ordinary country; but may be increased when the wind unsteady or decreased when there are low. -clouds. The steadiness as well as the speed of wind increases with the height. The character the country should be carefully investigated f the profile maps before starting; all hilly should be marked on the map as a warning landing. Contour is not readily distinguished a height of 2000 ft. and for this reason points be indicated on the map where poor landing -make it desirable to fly high. The character of 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 inadvance the proper height to use during the trip climb to this height in circles; note the -' p1 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 of necessity.
Effect of Clouds on Navigation.-Flying in or above the clouds is a similar case, if landmarks can not be seen. It is not wise above the clouds when on the sea coast as c winds may, unknown to the pilot, carry him to sea; and any flight over the sea which is to 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 determine directly the side drift of an airplane. It enable the pilot by looking through a telescope at ground to determine exactly what his direction motion is with relation to the ground. The scope is mounted vertically and is rotatable about its own axis; it has a cross-hair which appears in 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 be 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 velocity of the waves it is only necessary to know wave length, that is, the distance between two consecutive wave crests. The rule is that for a length of 10 ft. the velocity is 10 miles per hour, will vary as the square root of this wave lei that is, if the wave length is half, the velocity be 10 divided by the square root of 2, or 7.1 per hour.