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Aviation History
1915
1915 - 0591.PDF
AUGUST 13, 1915. Turning now to the method of marking the compass card, this is somewhat different from that usually adopted for nautical compasses, due, mainly, to the difficulties of keeping an aeroplane dead on its course for any length of time in anything short of a flat calm. Two of the accompanying sketches show typical marine and aero compass cards. Like the former the card of the aeroplane compass is usually divided into 3600, of which only every fifth is marked, and every tenth numbered, it being almost impossible to steer with greater accuracy. The sequence, however, of the numbering is different in the two cases. From the illustration of the ship's compass card, it will be seen that the numbering runs from o° at N. to 90° at E., and then downwards again to 0° at S. The aero compass, on the other hand, is marked progressively round so that E. becomes 900, S. 180°, and W. 2700 — N. being 3600. In addition to the numbers the cardinal points and quadrantal points are given. The advantage of this method is that it is extremely easy to remember, there being no question as to whether a certain course is N.N.W. -^W. or N. by W. -J W. In some cases when it is desired to place the compass so high up on the instrument board that it would be difficult for the pilot to look directly on the compass card, a small mirror is placed above the compass and on the side farthest away from the pilot, who then watches the reflection in the mirror instead of the compass card itself. Frequently a small prism is employed in place of the mirror so as to show an enlarged image of the numbers. In compasses designed to be used in this manner the figures on the card are reversed so that when looking at the reflection in the mirror they are seen in the usual way. For use at night the compasses of Service pilots are often illuminated by a small electric light placed inside the glass covering of the compass, where it lights up the card without dazzling the pilot. The necessary current is furnished by a small battery, which generally has a capacity of three or four hours. After this brief description of the compass itself, the procedure of placing it in the aeroplane and adjusting it will be dealt with. Placing the Compass. Firstly it must be remembered that the north point of the compass does not point to the geographical North Pole, but serves to indicate the direction of the magnetic north or magnetic meridian. The difference between the " true" north and the magnetic north is known as the variation, i.e., the angle between the " true " and magnetic meridians at various points of the earth, and is •called easterly or westerly variation according to whether the magnetic meridian is to the right or left of the " true " meridian. It must also be remembered that the north point of a •compass very seldom points to the magnetic north when placed in an aeroplane, in consequence of the usually large amount of magnetic material used in the construc tion of the machine, this said material attracting the compass needle one way or the other, and thus making it read incorrectly; this error is known as deviation. And therefore it is important that when laying out a •course deviation and variation must be allowed for, unless deviation is eliminated by the means shown later under "Adjusting the Compass." The position of the compass in the machine with reference to its surroundings is very important, and where possible the following points should be observed :— I. The compass must be in a position where the pilot lias an unobstructed view of the instrument. [/tiGHT] II. If possible the compass should be placed on centre line, fore and aft, of the machine in order that the magnetic material surrounding the compass may l>e more or less symmetrical, and thus more easily compensated for when adjusting the instrument. III. The pilot should be seated directly behind the compass so as to prevent errors in reading due to parallax. (The apparent angular shifting of an object caused by the change in position of the observer.) IV. If possible the moving control levers, &c, in close proximity to the compass should be made of non-magnetic material; and it should be remembered that the ends of iron or steel rods are especially liable to have a disturbing effect on the instrument. V. The compass should be placed as far as possible from the engine and magneto. Adjusting the Compass. Having placed the compass in the best available posi tion, having in mind the points mentioned in the previous paragraph, if it is not possible to arrange the compass on the fore and aft line of the machine, but to one side of it, great care must be taken that the lubber line is exactly parallel to the fore and aft line of the machine. The object of compass adjustment is :— 1. To balance up the directive forces in whichever direction the machine is heading. 2. To reduce the deviation to the least possible values. To adjust the compass proceed as follows :— With the aid of another compass, lay out on the ground by means of pegs and cord the cardinal points N., S., E. and W., and the quadrantal points N.E., S.E., N.W., S.W., according to the direction indicated by the compass (not the compass in the machine). w- Sketch showing method of laying out the compass points on the ground by means of cords and pegs. For testing the aero compass the machine is placed with its centre line parallel to the various cords. This second compass may now be dispensed with, and the aeroplane compass corrected from the cardinal points now represented by cords on the ground. A most important point to remember, however, when 591 D 3
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