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Aviation History
1912
1912 - 0891.PDF
OCTOBER 5, 1912. ^JJGHT] THE EVOLUTION OF THE AERO COMPASS. By M. B. FIELD and F. A. KING WITH the rapid development of cross-country flying, the necessity pattern of about this diameter, for an adequate aero compass becomes more and more obvious, and the aviator naturally turns to the nautical navigator for the results of experience gained during many years of hard practical work. It is true that a marine-type compass as it stands would be most unsuitable on the aeroplane, but nevertheless it embodies the embryo from which the final aero compass must develop. In nautical work compasses fall under one of two heads—(A) the dry card system and (B) the liquid system. Practically all dry cards are now constructed on Lord Kelvin's (then Sir William Thomson) original model. The essential charac teristics of such a card are :— (1) Lightness, to eliminate pivot friction errors, and to minimise the deterioration of pivot and jewel consequent on jarring and vibration. (2) A sufficiently large magnetic moment of the needles, so that the directive couple exerted on the card, when deflected through The compass complete, and the card and magnetic system. very small angles from its true position, is great in comparison with the pivot frictional torque. (3) A long period of vibration to secure steadiness. K Now, increase in the strength of the needles decreases the period of oscillation, so that condition (2) militates against con dition (3). The period of oscillation is increased by increasing the moment of inertia of the moving card, but this again militates against condition (1). Thus one is faced by a number of conflicting conditions. Sir Wm. Thomson solved the difficulty by producing an exceedingly light card, but with the mass so distributed that its moment of inertia was great in relation to the magnetic moment, while the directive A card of this description would obviously be too bulky for aeroplane work. When, however, the diameter of the card is much reduced, the steadiness is so much impaired that the other system mentioned above, i.e., the liquid system, must be resorted to. In this type of compass the card is immersed in a chamber filled with colourless liquid, and by a special arrangement of air-tilled dome, round which the card is assembled, the pivot is relieved of the principal weight of the card by flotation effects. Capt. the Hon. Wentworth Chetwynd, R.N., has devoted an immense amount of attention to the elaboration of compasses of this description, and has shown that they are more suitable to withstand the shock of gunfire and the drastic treatment to which they must be subjected in warships, than are the dry card compasses, with the result that they are now universally adopted by the British Admiralty and the majority of foreign admiralties. In this connection, and more especially, perhaps, in relation to aero-compasses, acknowledgment is due to (apt. Creagh- Osborne, R.N., who has carried out a large amount of pioneer experimental work. The well-known firm of Kelvin and James White, Ltd., of Glasgow and London, are makers of the liquid compasses embodying Capt. Chetwynd's improvements, and this firm has now turned its attention to the manu facture of these compasses for aeroplane work. The cards are supported on pivots tipped with " Osmiridium," a hard native alloy, which is ground and polished to a fine point, thereby reducing the friction between the pointer and the sapphire jewel mounted on the card to a minimum. The card is housed in a light spinning, which, as previously stated, is filled with a colourless liquid. To allow for the varying volume of the fluid due to changes of temperature, the bottom ol the bowl is made of an elastic metal diaphragm, which can yield with the expansion or contraction of the liquid. One of the fundamental improvements introduced by Capt. Chetwynd is the use of a card of small diameter in relation to that of the containing bowl, the object being that the edge of the card is a considerable distance away from any portion of the surrounding containing case. It is well known that when a bowl filled with a viscous fluid is suddenly rotated, the liquid in the immediate vicinity of the walls of the bowl rotates with it, while that in the centre lags behind, and will only be set in motion if the rotation be continued for a considerable time in the same direction. If now the compass card approach the edge of the bowl with any sudden turning motion, the ring of liquid, dragged round by the walls of the bowl, will drag the card with it, and thus will set up a swinging motion of the card and consequent unsteadiness with each turning movement of the aeroplane. The object then of the small card is to remove the edges of the same from all possibility of being acted upon by that portion of the liquid which is carried round by the l»w The outer case, with shock-damping device, the inner bowl, and the card. force again was great in relation to the exceedingly reduced pivot friction, even for very small angles of deviation. It is obvious, however, that with these conflicting conditions, an unrestricted choice of the diameter of the card is not possible, and experience has shown that the requisite combination of attributes is obtained in the most advantageous manner in a card of about 10 inches diameter, and hence we find that the majority of compasses used in the Merchant Marine have dry cards of the Lord Kelvin 891 when sudden turning motions are effected. This principle, clearly appreciated by Capt. Chetwynd and provided for in his liquid compasses, is one of the causes for their remarkable steadiness. By this means it is possible to obtain a remarkably steady small card, and one which, nevertheless, shows remarkably little pivot friction, in fact, all the attributes originally aimed at in the design of the original 10-inch dry nautical card are attainable. It might also be pointed out here that the well-known difficulty of
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