Popular Science Monthly
��265
��Aeroplane Drift and What It Means
WHEN making a flight between two distant points separated by water, or over strange ground on which there are no famihar landmarks, an aviator uses a compass Hke any sailor. He may find his bearings at any time during the trip by plotting a line on his chart in the direction in which he has been travelling. Then by estimating his rate of speed and the length of time he has been flying he obtains a point on this line which repre- sents his position at the moment. Such was the plan which Lieutenant Porte originally intended using in navigating the America on his proposed trans- atlantic flight.
Serious errors are possible in steering by compass, because no correction is made for drift with the wind. Of course, there would be no drift in a per- fectly calm atmosphere ; but the air is unfortunately a very unstable medium, filled with currents of varying velocity and direction, which insidiously divert air craft from their supposed line of flight.
This is illustrated in Fig. 1. where the aeroplane is shown heading due north and the aviator naturally supposes that he is flying in that direction. A strong east wind is blowing and carrying him northwest. He cannot feel this wind be- cause he is moving with it and the longer he flies the farther he drif'is from his objective. This matter had never re- ceived very serious consideration until the transatlantic flight of the America was planned, and then it loomed up as a serious problem. A gyroscopic stabil- izer had been installed and automatic control ensured, thereby relieving the aviators of much responsibility, save that of "setting the course." Yet with the America well on her way there would have been no certainty as to where she would have landed, although the pilot might have kept her absolute]}' upon the compass course.
The air compass, like the mariner's compass, is provided with a mark known as the "lubber-line," a line usually en- graved on the comp.ss '-ase and repre- senting the bow of the ship. Cenerally there is a corresponding line 180° dis- tant representing the ship's stern. While
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��Showing different wind conditions met by
aviators during a flight, and on the right
the corresponding readings on the drift
indicator in each case
��the "compass needle," is frequently re- ferred to, nautical compasses are pro- vided with a card to which several "needles" are affixed on the under side. This card, bearing the cardinal points, is held toward the north through the influ- ence of the earth's magnetism. It will be seen, therefore, that when a northerly course is to be sailed the ship must be so maneuvred as to bring the "N" on the card directly opposite the "lubber- line," as shown in Fig. lA.
Now let us again consider drift. Suppose we set our course as described and the conditions are those disclosed in Fig. 1. If we are flying at a reasonable height we see below us so much of the earth's surface that we appear to be standing perfectly still in space ; we know we are progressing because we would fall if we were not. We can rely only upon the compass for our sense of
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