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

Let's pick up with the Directional Gyro Indicator — the DGI — and I want to walk you through the rest of its operation, because we've got the re-erection system, the caging device, the limitations, and the errors. First, let's finish the re-erection story. We said the rotor axis is kept in the yawing plane by a jet of air and a wedge plate. Now, here's the key point: that jet produces an effective force, which we call force 'P', and that force acts to re-erect the rotor axis back into the yawing plane. So if the gyro drifts slightly, the jet blows against the wedge and the force P pushes the axis back where it belongs. But here's the clever part — what happens if the gyro is displaced so far that the jet is nowhere near the wedge plate? Then the first system — the one that restores the gyro — takes over and brings it back to its correct position. So we have two levels of correction: the jet provides what we call coarse adjustment, and the wedge plate provides fine adjustment. Coarse first, then fine. Now let's look at the Caging Device. On the front of the instrument there's a caging knob. When you push it in, it moves a caging arm which locks the inner gimbal at right angles to the outer gimbal — that locks the rotor axis in the yawing plane. At the same time, a gear engages with the outer gimbal, so that by turning the knob you can rotate the gyro and synchronize the scale reading with — usually — the compass reading. Why is the caging device designed this way? There are four reasons. First, so the DGI can be synchronized with the compass and reset as required. Second, so the gyro will not topple during synchronization. Third, so that toppling and possible damage to the instrument can be prevented by caging before manoeuvres in which pitch and roll limits may be exceeded. And fourth, so the gyro can be instantly re-erected and re-synchronized if it has toppled. Now, the DGI Limitations. This is important. 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. That happens because the inner gimbal comes up against the stops, and the precession causes the outer gimbal and the scale to spin rapidly. But there are exceptions. If the rotor axis is athwartships — that is, pointing across the aircraft, wing to wing — then 360 degrees of aircraft rotation in the looping plane is possible without toppling the gyro. 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. The actual indications on the scale at which these two situations can arise depend on the vintage and manufacture of the instrument. Finally, the DGI Errors. It's virtually impossible for a DGI to remain synchronized with the compass, and there are several reasons. The most significant errors are: gimballing errors; random wander; apparent wander due to earth's rotation; errors resulting from varying rotor rpm; and apparent wander due to change of aircraft position — that's transport wander. We'll deal with each of these in the subsequent paragraphs. So to tie it together: the jet and wedge give you coarse and fine re-erection, the caging device lets you lock, synchronize, and protect the gyro, the limits tell you when it will topple, and the errors explain why it never stays perfectly aligned with the compass.