Remote Indicating Magnetic Compass — Page 211, Lesson 253

Let’s pick up right where the flux valve and gyro leave off. I want to walk you through what happens when we take that gyro out of the loop, and why we absolutely don’t do that. In theory, we could cut out the gyro entirely. The flux valve’s magnetic field could be passed along for comparison with the compass card, and the resulting error signal could be sent to a motor that directly drives the compass card shaft. That would still give us an electromagnetic compass, with the detector unit remote from the major source of deviations, and its output could drive other systems. But here’s the catch: such a system would be overly responsive to any fluctuations in the field detected by the flux valve, and it would suffer significantly from turning and acceleration errors. That’s why we keep the gyro in. The gyro gives stability and rigidity, because the precession motor applies corrections to the drive shaft at a rate of only about 3° per minute. That slow correction rate is what smooths everything out. Now let’s look at operation in a turn. We’ll assume, initially, that the gyro does not drift during the turn. That’s not unreasonable, because even during a full 360° orbit, the turn will only take about 2 minutes. So the aircraft turns, but the gyro, having rigidity, does not. That gives relative rotation between the horizontal gyro and the instrument case, which operates the bevel gear and causes the direct drive shaft to rotate, changing the heading indication on the compass card. At the same time, the heading sensed by the flux valve, which is being passed to the drive shaft for comparison, is changing at the same rate. So no error signal is generated, and the compass should remain synchronized during the turn. If there is some gyro drift during the turn, on completion of the turn there will be a small error signal. That gets taken out as I described in the previous paragraph — the precession motor corrects it at that slow 3° per minute rate. Now, rapid synchronization. When the gyro is started up on initial switch-on, the alignment it adopts is random, and it’s unlikely to be in synchronization with the earth’s magnetic field. So an error signal is detected at the gyro drive shaft. The problem is that the precession motor’s normal correction rate is only 3° per minute. If the gyro happened to be 90° out, it would take 30 minutes to synchronize — obviously unacceptable. The solution is a rapid synchronization facility. It can be either a mechanical clutch operated by the pilot, as in the DGI, or, in later compasses, a high gain mode for the precession amplifier — similar in principle to rapid erection in the electric artificial horizon. This is operated by a 2-position switch, spring-loaded to the normal position, which has to be held against the spring for rapid alignment. Operating that switch increases the precession motor’s correction rate so that synchronization takes only a few seconds. So the key takeaway: the gyro is there to dampen out fluctuations and turning/acceleration errors, the flux valve senses the field, and the rapid sync switch lets us get aligned quickly on start-up instead of waiting half an hour.