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Aviation History
1939
1939 - 1917.PDF
/ FLIGHT, June 22, 1939. THE MODERN AIRCRAFT COMPASS Some Notes on Present Types and o?i Possible Developments in the Near Future By FRANCIS CHICHESTER. PILOTS and navigators are rebelling against the exist ing type of bowl compass with horizontal face, which is, in fact, almost the only kind of compass in use in Great Britain to-day. The chief complaint is the difficulty of reading it. With a horizontal face it must be placed in such a position that the pilot can lcok down on it, or, if it is inverted, look up at it. This limits its position. It must either be placed low at the side of the pilot's seat, in front of it, or above his head. The most popular of these positions has been that in front of the pilot's seat, where the compass is usually on a stand. Unfortunately, the control stick is then between it and the pilot. When the control was a plain column the pilot did not have so much difficulty in reading the com pass behind it, but to-day it usually has a spade or wheel grip and he must lean forward or to one side to read it. If the instrument is on the floor beside the seat, the pilot must screw round and look down every time. Also, in these positions, unless the grid wires are parallel or perpendicular to the vertical plane through the line of sight, the magnetic needle makes a different angle with them when the aeroplane is climbing or descending; and if the pilot keeps the needle apparently parallel with the grid wires, he may be off course. This parallax error is avoided in the inverted compass suspended from the roof. The pilot looks into a mirror beneath the compass, and the line of sight is vertical from the mirror through the glass bottom of the bowl. By this means the error of parallax, when the bowl is tilted in a climb or dive, is reduced to a minimum, which can, in general, be neglected. Taking deviation into considera tion as well, this position is far the best that has yet been found for a magnetic compass. Not only is the perform ance better there than in any other practicable situation, but also, the course can be read very accurately against a lubber line on an external verge ring as soon as the grid wires have been lined up with the needle. It has another advantage—it is in the most natural direction possible for a pilot to look, i.e., before his eyes and only slightly above. Retaining the View It is true that at first he may not like viewing the compass in a mirror, but this becomes quite natural as he gets accustomed to it. He may also object that it cuts out some of the view ; but if the mirror is close under the compass face he can see below it and to the side; if several inches below the compass, he can see well round it; the mirror can be swung up against the compass face when not required, so that the view is unobstructed for landing. In front of the pilot's seat, or beside it, the aperiodic compass often suffers badly in modern aircraft from change in deviation due to varying magnetic disturbance in the vicinity. When various electrically operated instruments are switched on, it may deviate several degrees, and, when the lights are put on, several degrees more. In a bomber every fresh load of bombs may cause a different deviation. In addition to the change caused by these, a further change of as much as 20 degrees can be caused by the generator when that is turned on. As far as the pitot heater, lighting, equipment, etc., are concerned, the com pass could be adjusted for deviation. For example, the aeroplane could be swung once with the pitot heater on, again with the pitot heater and airscrew anti-icer on, and a third time with the airscrew anti-icer and the night light ing on. But it would have to be done for so many com binations that it would be impracticable, and it is quite enough complication for the pilot if he has only two devia tion tables to refer to—one for when the aircraft has been swung with everything off, and the other for when every- The roof-mounted P.4 compass in the Short-Mayo Mercury. For taking-off or landing the mirror shuts up flat against the compass-face and does not obstruct the pilot's view. thing except the generator has been on. The generator disturbance sometimes cannot be allowed for because its output varies in flight as the batteries are charged, and in this case the generator must be turned off every time a fairly accurate compass reading is required. An inverted compass slung to the roof of the same aero plane and read at the same time as a compass beside the seat showed a total change in deviation of 4J degrees com pared with the i2| degrees of the compass beside the pilot's seat. But for electrical connections and switches near it in the cabin roof it might have been free from deviation. What the pilot would like, to my mind, is a vertical reading compass which can be set in the instrument panel amongst the other instrument dials. He wants it to show at a glance the course on which the aeroplane is flying, and he does not wish to have to adjust grid wires before he can do so. At the same time, he needs an instrument fitted with a course indicator to fulfil the function of grid wires and show at once if he is off course at all; it is too •much to expect a pilot to remember throughout a long flight the exact course which he is steering. There are two ways of satisfying this demand. One is to revert to an early type of magnetic compass used in aeroplanes; in its most familiar form it had a compass card rotating on a pivot and behind a glass face. The card had a vertical scale round the edge, which was visible at an aperture in the instrument panel. The course flown is indicated by a lubber line behind the glass face. The type was much used in the war, but was then in a crude state of development, and only indicated the cardinal points, North, North-east, East, etc. The navigation practised in the early stages of flying had no need of finer division oi the compass card, but this coarse division created an im- pression that the compass could not indicate more accu rately. This is not so in fact—it could be read accurately to 1 degree if the card were big enough to be so divided. One great advantage of this instrument over the usual compass is that it indicates at once, without interpretation or adjustment of the verge ring, the course on which tht aeroplane is flying. It has always been much in favour in the U.S.A. (five out of seven compasses in Pioneer's catalogue to-day are of this type), but that is no argu ment in its favour as far as the British pilot is concerned, because, rightly or wrongly, he considers the America;! mostly flies on radio ranges.
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