Remote Indicating Magnetic Compass — Page 211, Lesson 252

Let's start with the big picture. This is the Remote Indicating Magnetic Compass — and the key word is remote. In a simple aircraft compass, the sensing element sits right in front of you, in the cockpit. Here, the magnetic sensing is done somewhere else — out on the wing, typically — and the heading is shown to you on a separate indicator. So we have a detector unit, a heading indicator, and a link between them. Look at Figure 17.1 — the simple signal routing diagram. The chain runs like this: the Detector Unit, which is the flux valve, senses the earth's magnetic field. That signal goes to a Heading Indicator, which is driven by a Horizontal Gyro. The gyro turns a Direct Drive Shaft, and that shaft positions the compass card — the actual heading you read. And there's a Precession Motor sitting in the loop, plus an amplifier marked with a big A. Now, the heart of the whole system is the flux valve. Let me give you the exact professional name first: the Detector Unit, or flux valve. It senses the earth's magnetic field and reproduces it within the compass unit. So it doesn't just measure the field — it recreates that field information inside the instrument, where it can be compared with something else. That "something else" is the position of the gyro drive shaft. Here's the clever part. The gyro drive shaft also positions the compass card indicator — that's the heading you, the pilot, see. So we have two sources of heading information: the flux valve's reproduction of the earth's field, and the gyro's drive shaft position. The system compares them. If they're aligned, the compass card is reading the right heading, and nothing happens. But if the gyro drifts — and gyros do drift — the drive shaft will no longer be in alignment with the flux valve field. That misalignment generates an AC error signal. Let me slow down on that signal path, because it's the core of the whole operation. The AC error signal is passed to the precession amplifier — that's the big A in Figure 17.1. Inside that amplifier, the signal is amplified, phase detected, and rectified to DC. So we go from an AC error signal to an amplified DC signal. That DC signal then drives the precession motor. The precession motor turns the gyro — it physically precesses it, nudging it back into alignment. The gyro's output is fed via the direct drive shaft to the heading indicator, where it's compared again with the flux valve signal. Now, the operation I've just described assumes a steady heading. That's important. A steady heading gives a steady input from the flux valve. And it also assumes the compass has already been synchronized — meaning the gyro and the flux valve are already in agreement. Under those conditions, any difference between the flux valve field and the gyro alignment can only arise if the gyro drifts. So this whole correction loop exists to fix gyro drift. Let me trace the loop once more, end to end. Flux valve senses the earth's field, reproduces it in the compass unit. Compare with gyro drive shaft position. If aligned — no action, correct heading. If the gyro drifts — misalignment — AC error signal generated. That signal goes to the precession amplifier, where it's amplified, phase detected, and rectified to DC. The DC signal drives the precession motor, which turns the gyro. The gyro output feeds back through the direct drive shaft to the heading indicator for comparison with the flux valve signal. If they're aligned now, the compass is synchronized and no further action takes place. If not, the error correction continues until it is synchronized. So the system is a closed feedback loop. The flux valve is the reference — the truth, the earth's magnetic field. The gyro is the thing being corrected. And the precession motor is the actuator that does the correcting. The direct drive shaft is the mechanical link that carries the gyro's position to the heading indicator. Now, one thing I want to make sure you carry away: the signal changes form. It starts as an AC error signal, and it becomes a DC signal after the amplifier. That's not a trivial detail — it's how the system converts a sensed misalignment into a usable driving force for the motor. Let me also point you to the figures. Figure 17.2 shows the magnetic detector unit — the flux valve — positioned in a part of the aircraft least affected by on-board electrical interference. That's why it's remote, out on the wing. And Figure 17.3 shows the flux valve components — the circular plate screwed to the underside of the wing, with the black hemisphere protruding. So, to summarize the whole concept in one breath: the flux valve senses the earth's magnetic field, the gyro holds the heading, and whenever the gyro drifts away from the flux valve's truth, an error signal is generated, amplified, converted to DC, and used to precess the gyro back into alignment — and this continues until the compass is synchronized. That's the steady-heading correction for gyro drift.