Learning to fly in the U.S. Army

by E. N. Fales

Of course, you will engage a guard to watch the machine all night; see that a rope is strung around the airplane to keep off the crowd which may collect.

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.