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 



^LusseR line 



Norlh f>S- ^-4 



16 



F,^2 



Fiq. 2 A 



f 



%5 



Fi^3A 



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 



