Directional Gyro Indicator (DGI) — Page 162, Lesson 189

I want to walk you through the Directional Gyro Indicator — the DGI — and I want to start by finishing the thought that's already in motion, because it's the heart of how this instrument keeps itself upright. We were looking at the erecting system. There's a jet of air and a wedge plate, and the key idea is that these two parts work together to pull the rotor axis back into the yawing plane. The yawing plane is the horizontal plane of the aircraft — the plane you turn in when you yaw. If the rotor axis tilts out of that plane, an effective force, which we call force 'P', acts to re-erect the rotor axis back into the yawing plane. That force 'P' is the restoring force — it's what pushes the axis back where it belongs. Now here's the clever bit about the two stages. If the gyro was displaced so far that the jet was nowhere near the wedge plate — meaning the displacement is so large the fine mechanism can't even see it — then the first system, the jet, would restore the gyro to its correct position. So the jet provides what we call coarse adjustment, and the wedge plate provides fine adjustment. Coarse first, fine second. That's the division of labour inside the erecting system. Now let's move to the front of the instrument, because that's where the pilot actually interacts with it. There's a caging knob. When you push it in, it moves a caging arm, and that arm locks the inner gimbal at right angles to the outer gimbal. Let me make sure you've got the gimbal picture. A gyro is mounted in two rings — an inner gimbal and an outer gimbal. When the caging arm locks the inner gimbal at right angles to the outer gimbal, it locks the rotor axis in the yawing plane. So the rotor is now held firmly horizontal. At the same time, a gear engages with the outer gimbal. So now, by turning the knob, you can rotate the gyro, and the scale reading gets synchronized with — usually — the compass reading. That's the whole point of the caging knob: it's how you align the DGI to the compass. Now, why is the caging device designed this way? There are four reasons, and I want you to hold onto all four. First, the DGI can be synchronized with the compass and reset as required. Second, the gyro will not topple during synchronization — that's why you cage it before you rotate it, so it can't fall over while you're turning it. Third, toppling and possible damage to the instrument can be prevented by caging before manoeuvres in which pitch and roll limits may be exceeded. So if you know you're about to do something aggressive, you cage first. And fourth, the gyro can be instantly re-erected and re-synchronized if it has toppled. So caging is both a prevention tool and a recovery tool. Now let's talk about the DGI limitations, because this is where the numbers matter. If the aircraft exceeds the pitch or roll limits of 85 degrees — and note, it's 55 degrees in an air-driven gyro DI — the gyro will topple. Why? Because the inner gimbal comes up against the stops. When it hits the stop, the precession causes the outer gimbal and the scale to spin rapidly. That's the classic topple — the scale whips around. But there are exceptions, and these are important. If the rotor axis is athwartships — that means pointing across the aircraft, wingtip to wingtip — then 360 degrees of aircraft rotation in the looping plane is possible without toppling the gyro. Looping plane is the vertical plane you loop in, nose over tail. And if the rotor axis is fore and aft — pointing along the aircraft, nose to tail — then 360 degrees of roll is possible without toppling. So in those two special orientations, you can rotate a full circle in that particular plane and the gyro survives. Now, the actual indications on the scale at which these two situations can arise — meaning exactly which scale reading corresponds to the athwartships or fore-and-aft rotor position — depend on the vintage and manufacture of the instrument. Different instruments are built differently, so you can't quote one universal number for that. Finally, let's look at the DGI errors, because it's virtually impossible for a DGI to remain synchronized with the compass. There are several reasons, and the most significant ones are listed here. First, gimballing errors. Second, random wander. Third, apparent wander due to earth's rotation. Fourth, errors resulting from varying rotor rpm. And fifth, apparent wander due to change of aircraft position — which is also called transport wander. I want to pause on the word 'apparent' in two of those, because it's a subtle point. Apparent wander due to earth's rotation and apparent wander due to change of aircraft position — these aren't the gyro actually drifting on its own. They're apparent, meaning they appear to the observer because of the reference frame. The earth rotates under the gyro, or the aircraft moves over the earth, and that makes the gyro seem to wander relative to the compass even though the rotor itself is holding its orientation in space. That's a distinction worth keeping straight as we go deeper into each error in the coming paragraphs. So to tie it together: the erecting system uses the jet for coarse and the wedge plate for fine adjustment, the caging knob locks the gimbals and lets you sync to the compass, the topple limit is 85 degrees — 55 for air-driven — with two special orientations that allow full rotation, and there are five families of error that keep the DGI from staying matched to the compass. That's the complete picture of the DGI as it stands.