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Aviation History
1923
1923 - 0783.PDF
DECEMBER 27, 1923 necessarily deviate to the right or left, as well as following thecontour of the ground over hills and through valleys, it becomes important to be able to ascertain when such dipsand rises are taking place. Otherwise, the pilot would receive a wrong impression of his position. Fig. 4 indicates how anaircraft approaching a hill over which runs the guide cable will be made aware of the fact. The dotted line indicates thepath followed by the aircraft, and the crosses represent the coils of the detector circuit. While the cable is runninghorizontally the maximum is heard in the horizontal coil, while no signal is picked up by the vertical coil. On approach-ing the slope of the hill the vertical coil begins to pick up the signals, which here are produced by a changed magnetic field.As the brow of the hill is approached there is a short period during which neither coil picks up any signal, as the lines offlux diverge at this point. Finally, as the machine arrives off the horizontal top of the hill, or plateau, the signals againbecome normal—i.e., maximum in the horizontal coil and nil in the vertical. This is the diagrammatic explanation of whathappens. In practice, of course, the matter is more com- plicated i.e., when the coils are at right angles to the flux), and aminimum when the coils are at right angles to the cable. With this fundamental fact kept in mind, the action of theguide cable becomes fairly evident, at any rate in its main principles. Let us suppose that the aircraft is flying verticallyabove the cable. The greatest induced current will then be found in the vertical longitudinal and in the horizontal framesor coils, while there will be no current in the vertical transverse coil. The manner of ascertaining the signal strength in thevarious coils is by a series of switches joining up the coils in different ways. A slide of this wiring diagram was shown, but,unfortunately, this slide has lines so faint as to be entirely unsuitable for reproduction. It is, however, easily seen thatby various combinations it is possible to ascertain the maximum signal strength, and from knowing in which coilsthis is obtained to discover the position of the aircraft in relation to the cable. Briefly, then, any deviation of the aeroplane from the guidecable—i.e., turning to the right or left—is discovered by alter- nately joining in series, in opposed directions, the verticallongitudinal and vertical transverse coils. According to i' . THE LOTH GUIDE CABLE: Diagrams representing basic principle of the invention. 1. The cable supportedon poles, an alternating current being generated in it by the alternator shown on the left. 2. Submarine cable operating on similar principle, but the return being through the water instead of through the earth. 3. Linesrepresenting the shape of the magnetic field surrounding the cable. The crosses represent the coils of the detector circuit mounted on the aircraft. 4. Shows the manner in which a pilot is made aware of his approach to a hill. We will now turn to the receiving circuit mounted on theaircraft. As already indicated, this consists of three separate coils, and, of course, of the necessary detecting and amplifyingapparatus. The coils are of the frame or panel type, not unlike the frame aerial familiar to most wireless amateurs.The function of these frame coils is, in fact, the same as that of the frame aerial used in direction finding, but, instead ofmovable frames, which would be too cumbersome to be used on an aircraft, the frames are permanently fixed, and thechanges in direction are determined by coupling in series any two of the three coils. These are : one vertical longitudinalcoil placed irt the fore-and-aft direction of the aircraft ; one vertical transverse coil ; and one horizontal coil. Perhapsa better mental picture may be formed if we state that the longitudinal vertical coil might be, and we believe actually is,placed inside the vertical fin on the tail of the aircraft. Similarly, the horizontal coil may be placed inside the fixedtail plane, while the transverse vertical coil is incorporated in the tail plane bracing. The placing of the three coils in ornear the tail is chosen in order to get the coils as far removed as possible from local disturbances caused by the magnetos,generators, etc., carried on the aircraft. Reference to this will be made later. In trying to follow the action of the guide cable anddetecting coils, it should be remembered that, as the flux produced by the guide cable is at right angles to the cable,the current induced in the receiving coils on the aircraft will be a maximum when the coils are parallel to the cable whether the machine is turning to right or left, the signalstrength will depend upon whether the coils are so joined as to oppose or assist each other. Thus any deviation of themachine is indicated at once. Not only the fact that the machine has turned, but also the direction in which it hasturned. If the aircraft has been drifted away, sideways, from theguide line, but is still flying parallel to it, an indication of the fact is provided in the first place by the reduction in signalstrength. It is, however, also possible to detect this drift by means of the coils. In this case use is made of the longi-tudinal vertical frame and the horizontal frame, which are. joined in series, first in one direction and then in the oppositedirection, by means of the switches provided. Just as in the case of angular deviation from the cable the right or left wereindicated by the manner in which the coils had to be joined, so in the case of drift without angular deviation can the direc-tion in which the machine has drifted—i.e., right or left—be ascertained. There now remains a third contingency, the approach to orreceding from the cable in a vertical direction. This may occur when the cable is horizontal by the aeroplane gainingor losing height, or it may occnr while the machine is following a straight horizontal flight path, but the cable itself is runningover a hill or through a valley. Diagram 4 shows how the magnetic field is changed in the latter case. For detectingsuch a change in the direction of the cable use is made of the transverse vertical frame and the horizontal frame. As 783
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