Remote Indicating Magnetic Compass — Page 211, Lesson 253

Let me walk you through the remote indicating magnetic compass. This is the system that finally gets the compass detector away from the cockpit, where all the electrical interference lives, and puts it somewhere clean — but it introduces a whole new set of problems we have to solve. First, the theoretical shortcut. In principle, you could cut the gyro out entirely. The flux valve would sense the magnetic field, you'd compare that signal with the compass card, and any difference — any error signal — would drive a motor that directly turns the compass card shaft. That would still be an electromagnetic compass, with the detector unit remote from the major source of deviations, and its output could drive other systems. Sounds elegant. But here's the catch: such a system would be overly responsive to any fluctuation in the field detected by the flux valve, and it would suffer significantly from turning and acceleration errors. The field is never perfectly steady, and the aircraft is never perfectly still, so the card would jitter and lag and swing. That's why we keep the gyro. 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 the whole secret — it smooths out the noise. Now let's think about what happens in a turn. Assume, to start, that the gyro doesn't drift during the turn. That's not unreasonable, because even a full 360° orbit only takes about 2 minutes. The aircraft turns, but the gyro, having rigidity, does not. So you get relative rotation between the horizontal gyro and the instrument case. That relative motion operates the bevel gear, which rotates the direct drive shaft, changing the heading indication on the compass card. But at the same time, the heading sensed by the flux valve — which is being passed to the drive shaft for comparison — is changing at exactly the same rate. So no error signal is generated, and the compass stays synchronized throughout the turn. The gyro and the flux valve agree, because they're both seeing the same heading change. Now, if there is some gyro drift during the turn, then on completion of the turn there will be a small error signal. And that small error gets taken out by the precession motor at its normal 3° per minute rate, just as I described. So a little drift is fine — it self-corrects. But now we hit the real problem: rapid synchronization. When you first switch the gyro on, the alignment it adopts is random. It is very unlikely to be in synchronization with the earth's magnetic field. So the moment you power up, there's a large error signal detected at the gyro drive shaft. And here's the trouble — the precession motor's normal correction rate is only 3° per minute. If the gyro happened to be 90° out of sync, it would take 30 minutes to synchronize. That is obviously unacceptable for a pre-flight start-up. So the solution is a rapid synchronization facility. There are two versions. In the older compasses, like the DGI, it's a mechanical clutch operated by the pilot. In later compasses, it's a high gain mode for the precession amplifier — similar in principle to rapid erection in the electric artificial horizon. Either way, it's operated by a 2-position switch, spring-loaded to the normal position. You have to hold it against the spring for rapid alignment. Operating that switch increases the precession motor's correction rate so that synchronization takes only a few seconds instead of half an hour. So the whole design philosophy here is: use the gyro for stability, correct it slowly at 3° per minute to filter out noise, and give the pilot a manual override to slam it into sync quickly at start-up. That's the remote indicating magnetic compass in a nutshell.