Remote Indicating Magnetic Compass — Page 220, Lesson 255

I want to walk you through the remote indicating magnetic compass, starting with the heart of the system: the detector unit, which is also called the flux valve. First, let's talk about where this unit lives and why. The detector unit is positioned in a part of the aircraft least affected by on-board electrical fields. In practice, that's usually the wing tip or the tail fin, where any aircraft-generated magnetic disturbances are at a minimum. The reason is simple: its whole job is to sense the direction of the earth's magnetic field, so we have to get it as far away as possible from the aircraft's own magnetic noise. Inside this unit, we have a pendulous magnetic detecting element. "Pendulous" means it hangs, like a pendulum. It's mounted on a Hooke's Joint, which is a type of universal joint. This joint lets the detector swing within limits of 25 degrees about the pitch and roll axes, but it allows no rotation in azimuth. Let me unpack that. Pitch is the nose up and down motion, roll is the wing up and down motion. So the element can tilt up to 25 degrees in either of those directions to stay aligned with the earth's field. But azimuth is the compass direction — the heading — and it cannot rotate at all in that plane. That's critical, because any rotation in azimuth would corrupt the heading reading. The whole unit is contained in a sealed case that is partially filled with oil. That oil is there to dampen any oscillations created during flight. So if the aircraft bumps or vibrates, the oil absorbs that motion and stops the element from swinging wildly. Now, let's look at the physical installation. The circular plate is screwed to the underside of the wing. The black hemisphere protrudes out into the airflow, and that hemisphere is simply a protective cover for the flux valve inside. The cable carrying the signals passes along inside the structure of the wing. So the signals generated by the flux valve travel through that cable back to the instruments in the cockpit. The primary component here is the flux valve itself. It's a 3-spoked device, fixed in azimuth but with some freedom in the vertical to allow alignment with the plane of the earth's magnetic field. So again, it can tilt vertically to match the field, but it cannot rotate around the vertical axis. Now, let's look at the legs of the flux valve. Each of the 3 legs ends in a curved shape called a 'rams' horn'. Those curved horns are simply to improve magnetic flux gathering efficiency. They make the leg better at collecting the magnetic field, but they do not affect the principle — the flux valve would detect even without them. So they're an efficiency improvement, not essential to the operation. To understand how the flux valve works, we start with just a single leg, ignoring the rams' horns. A simplified diagram of a flux valve leg shows a coil wound around a centre post. Alternating current is fed to that coil. That alternating current produces fields of opposite sign in the top and bottom legs of the flux valve. So at any instant, if the top leg has a north polarity, the bottom leg has a south polarity, and then it flips as the AC alternates. That's the foundation. The alternating current creates opposing fields in the two halves of the leg, and the earth's magnetic field interacts with that. We'll build on this to see how the three legs together produce a heading signal, but first, make sure you're comfortable with this single-leg concept: the AC-driven coil, the opposite-sign fields, and the role of the rams' horns as an efficiency aid.