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Aviation History
1915
1915 - 0629.PDF
THE SPERRY DRIFT INDICATOR. As was pointed out in an article on the aeroplane compass that appeared in our issue of August 13th, one of the greatest difficulties of aerial navigation by com pass is that of ascertaining and making allowance for the amount of drift caused by a wind blowing at an angle to the course of the aeroplane. For short flights over known territory a pilot may, by picking out familiar landmarks, check his course and make corrections for drift, until in the course of his flight he passes over the particular places lying along the course line marked off on his chart. When it comes to long flights over unknown country, however, the matter is O ^ far more complicated, and it R\ f~~ s-—^0 \ will be easily seen that a slight v. 1—1 •— error in estimation of drift may quite conceivably cause the pilot to completely miss his objective. In the accompanying diagram OF is the direction in which the aeroplane is steering, and would be the direction of travel Fig. 1. if there were no side wind. The line, OS, represents the side wind in direction, and its length bears the same ratio to the length of OF as the velocity of the side wind bears to the speed of the aeroplane. OR is the resultant of the two forces, and indicates the true direction in which the aeroplane is travelling. To ascertain the exact direction of travel is the object of the Sperry drift indicator, for the illustration of which we are indebted to our New York contemporary, Aerial ground can be seen. When looking through it five fine parallel hair lines are seen across the field of vision. The telescope is so mounted that it can be rotated about its longitudinal axis (which is, of course, always vertical) by means of the handle, 1), until the hairlines are parallel to the path of terrestrial objects across the glasses of the telescope. A pointer is provided by means of which it is possible to read on a graduated scale attached to the frame the angle between the true course taken by the aeroplane and the course indicated by the compass. The telescope is connected by Bowden cables to the adjust able lubber line of the compass, and in a two-seater machine the telescope may be placed in the observer's cockpit, while the compass is situated in front of the pilot. The observer will thus be constantly checking the drift by means of the telescope, and in adjusting it will at the same time adjust the lubber line by means of the Bowden cables. When thus dividing the task of naviga tion all the pilot has to do is to steer according to the lubber line, and the aeroplane will always be steering the desired course. The compass used in this set is the Sperry adjustable lubber line compass, in which the bowl is supported from the outer casing by springs so as to protect it against vibration and shock on landing. The compass card and lubber line are painted with a luminous radium compound which permits of reading the card in the dark at a dis tance of three feet. In addition a small electric lamp is provided, current for which is furnished by a small dry cell battery. The weight of the drift indicator and Fig. 2.—Sperry drift syn chronised set, consisting of the telescopic drift indicator, C, and a Sperry adjustable lubber line compass, A, synchronously connected by double Bowden wires, B, so that when the drift of the aeroplane is determined by the telescope the compass Is automatically set. Age. This new instrument for aerial navigation is the product of the Sperry Gyroscope Co. of Brooklyn, N.Y., and consists essentially of a short prismatic monocular telescope through which the path of objects on the compass complete is only 7 lbs., so that it can be carried on any machine, and would appear to be an invaluable addition to the outfit of an aeroplane used for cross country work. 629 D 3
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