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Aviation History
1925
1925 - 0377.PDF
JUNE 18, 1925 SMITH'S GYRO TURN INDICATOR New Instrument can be Driven Direct from Engine or by Small Windmill IN spite of the great improvements effected in recent yearsin the design and construction of compasses for aircraft it became evident long ago that the compass by itself no matterhow excellent its design, could not adequately fulfil the functions of a turn indicator under all conditions Manyare the inventors who have been at work on the design of an instrument intended as supplementary to the compassrather than as a substitute for it, and a number of turn indicators have been produced, some of which have beenvery excellent in theory, but of rather a complicated nature and making, perhaps, too great a demand on the pilotAmong those who have been working on the problem of a really suitable turn indicator are S. Smith & Sons (M.A.)Ltd., of Cricklewood, and that well-known firm has now produced an instrument which, it is felt, will meet all thedemands that could possibly be made on it. We recently paid a visit to the works of this firm at Crickle-wood and there saw one of the new Turn Indicators mounted on a turn-table in the laboratory. Although it is one thingfor an instrument to work in a laboratory, conditions are sometimes quite different for the same instrument to work inan aeroplane carrying out various violent manoeuvres, but there is no reason whatever to doubt that the Smith TurnIndicator will do all that is claimed for it, and as a matter of fact we understand that the instrument has been acceptedby the Air Ministry as suitable for fitting on commercial as well as service aeroplanes. The new Turn Indicator is of the gyroscopic type, andconsists fundamentally of two separate instruments, an alternating current generator and the gyroscopic indicator SMITH'S GYRO TURN INDICATOR : The three- phase alternating-current generator which provides the electricity for driving the Smith gyro turn indicator. itself, in which the gyroscope is formed by the electro-motor. It is well known, of course, that when a rotating mass has its axis deflected by some external force there is set up a tendency for the axis to move in a plane at right angles to that in which the deflection has occurred, this force being known as " precession," and it is upon this simple principle that the Smith Gyro Turn Indicator depends. When mounted in an aircraft, the axis of the gyroscope, or in other words the rotor of the electro-motor, is placed in the fore and aft direction, and the tilting frame or ring which carries the gyroscope is in turn fitted on a transverse axis, at right angles to the line of flight. The result of this arrangement is of course that the whole instrument can tilt round the transverse axis and is caused to do so by the slightest turn to right or left, and the rotor spindle is so arranged as to operate a pointer on the •circular dial mounted on the instrument board of the pilot's •cockpit. It may here be mentioned that means are provided for varying within very wide limits the sensitivity of the instrument, the control taking the form of a leaf spring •connected by a link to the tilting frame. Some idea of the sensitivity of the instrument is provided by the fact that -when set" to the most sensitive position the instrument will indicate a rate of turn as low as one complete turn in ten minutes. Chief among the novel features is, perhaps, the entire absence of sliding or moving electrical contacts, so that connections are always positive and there is nothing to clog or otherwise spoil the perfect nature of the contact. This has been made possible by so designing the three-phase •electro-motor that the rotor surrounds the stator, the three leads from the generator going to the stator, and having no connection whatever with the rotor. Incidentally there would seem to be another advantage in this arrangement in that from the very fact that it surrounds the stator thesize of the rotor is increased so that for a given rotor weight the moment of inertia will be greater than in the case of asmall-diameter rotor revolving inside the stator. In the Smith instrument the rotor normally runs at a speed ofabout 2,600 r.p.m. which speed, although sufficiently low to ensure that there will be no difficulty with bearings,produces a very considerable force. It is "due to this fact that the sensitivity of the instrument is such that it willindicate to the pilot even the slightest turn. The generator which supplies the current for the SmithGyro Turn indicator is of the three-phase alternating current type, and can either be mounted outside the fuselage anddriven by a windmill, or can be mounted inside and driven by the engine, via a small gear-box. One of our photographsshows the generator which, it will be seen, is of perfect streamline form so as to offer as little resistance as possible.It may be mentioned that the output of this generator is H • * I 11 SMITH GYRO TURN INDICATOR : In this view of the dial the knurled knob controlling the sensitivityis seen in the lower right-hand corner. not only ample for supplying the gyroscope, but, actually,there is an excess of output which can be used, if desired, for illuminating the cockpit. In another photograph isshown the dial of the turn indicator, and the control which enables the pilot to vary the sensitivity of the instrumentis in the form of a knurled knob in the lower right-hand corner. This knob alters the fulcrum of the leaf spring,and thus the force required to turn the pointer through a given angle. The advantage of this control over sensitivityis, of course, that the same instrument can be used in a variety of different types of machines, from the heavy andslow commercial freight carrier to the small single-seater fighter. The same sensitivity setting would not, of course,give the same reading on two such different types of machines, since the sensitivity is a function of the rate of turn, butsettings can be chosen which will give the same sensitivity in the slow and fast machine. There can be little dovibtthat pilots differ considerably in the amount of sensitivity they prefer and, consequently, the means of adjustmentprovided will enable the sensitivity to be regulated to suit the tastes of individual pilots, this being a very importantfeature in the design of the instrument. We should have liked to publish more detailed illustrations of Smith's gyro indi-cator but, at the moment, the manufacturers are not willing to illustrate the instrument in too great detail, and we musttherefore content ourselves with this very general description of the instrument. As regards workmanship and finish thegyro turn indicator is up to the usual high standard which one associates with all the products of this firm. In conclusionit might be mentioned that the weight of the gyro complete with generator and fitted with windmill drive attachmentis 9 lbs. 15| ozs., while the weight of gear-box and flex, when it is desired to use the engine drive, is 4 lbs. 11 ozs. 377
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