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Aviation History
1915
1915 - 0590.PDF
I/O GHT AUGUST 13, J915. THE AEROPLANE COMPASS. By "GUIDANCE." WITH the increasing frequency and distance of aerial raids mostly over unknown country, the means available for use by our pilots in setting and maintaining a true course assumes a greater importance than ever before, and a few notes regarding that modern pathfinder, the aviator's compass, should therefore be of interest to the readers of Flight. Let it be understood at the outset that the subject is of such a nature that in an article like the present it can only be dealt with in an elemen tary way, and that its object is to help those who are not familiar with the general principles of the compass to an understanding of the fundamental idea of its con- struction and use. Briefly, the magnetic compass in its simplest form consists of a magnetised bar of steel suspended from its centre, the earth's magnetism causing the bar, or needle as it is generally called, to point to the magnetic poles of the earth. To be of any use on board a ship or an aeroplane, however, a considerably more complicated arrangement is necessary, and the various types of com passes in use, however much they may differ in detail design, may be classified under two headings : the dry card and the liquid type. The former sometimes con- to be made liquid tight and provision made for allowing of expansion and contraction of the liquid. Disregarding differences in detail design according to individual makers, the liquid type of aeroplane compass consists essentially of a small metal float to which the magnetic needle or needles are fixed and on which also the com pass card is mounted. Surrounding this float is a larger reservoir containing the liquid. In this connection it should be pointed out that the reason for leaving a considerable space between the edge of the compass card and the sides of the con tainer depends on the well-known phenomenon that when a receptacle containing a viscous fluid is suddenly rotated the portion of the liquid nearest the walls will rotate with it whilst that in the centre will remain undisturbed for an appreciable period unless the receptacle be kept rotating in the same direction for some considerable time. The float, which sometimes takes the form of an air-filled dome in the centre of the compass card, is made of such, a size that it will almost but not quite support the magnetic needles and the card, the remainder of the weight being taken by a central pivot tipped with some hard alloy ground and polished to a very fine point in *^*^^^^^l^l^^^^\^^^ oWer type of ship's card. The sketch in the centre compass, in whld.^are inserted the bar magnets used for correcting the sists simply of a magnetic needle pivoted on a steel centre, and contained in a casing, on the bottom or sides of which are marked the cardinal points N E S W and the quadrantal points, N.E., S.E., S.W.,'and NW " attach^ L^ °tf ,hC ^ rd COn'Pass has the «* attached to and turning with the needle. The fixed point on the outer casing or a line on'the glass roverL ermea the lubber point and the lubber line rS rg!ve'£r;eii;ourse'and the *- - -s especia ly suitable, as it can be made extremely accurate and dehcate, ,t would be practically useless on board a„ aeroplane, since ,t takes a considerable time to come to rest and is seriously affected by vibration or any foTm of movement One purpose for which the dry car7c0 ™ ss may be advantageously employed is for Ci g ouTfhe cardinal and quadrantal points on the around from S^81 * ***** ^^eae4anUencd0mpaS Compasses of the liquid tvDe are a onnH ,wi complicated, chiefly owingtoX EtXfhetll^ order to reduce to a minimum the friction between the point and the jewel in the centre of the card. Like those employed on board ship, the aeroplane compasses are invariably gymballed so as to keep the cam always in a horizontal position, no matter what the attitude of the machine. The gymballs take the form of two rings each p,voted on two points 1800 apart, but the points on one ring being 90° from the points of the other. As any vibration is likely to have a highly disturbing enect on the compass, some form of anti-vibrational arrangement is always provided. This may consist of a ShL fi °T srrounding ^e compass bowl, the space I6™ tbe bowl a"d the outer casing being 'filled with i SgLMlerial as> for instance; horse hair. nr nt"1 °d Wh',Ch is Used t0 supplement the one band?^ "? °ned' ?t0 slinS the C™P™ from elastic an3 the iP^ P0SC "*?" pads betwee" the c°mPass mounted &t P°vT ,of the aer°Plane ™ which h is en^e L i "5 be f?Und occasionally that at certain the8 vuW ht nC/dle Wil1 SWlng in apathy with utUitv of th°n' Wlth' °f C°UrSe' d«rimental result to the triln'rSbber^6 ~*fa ** iS » ** 590
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