Directional Gyro Indicator (DGI) — Page 162, Lesson 196

I want to walk you through the latitude nut correction on the Directional Gyro Indicator — this is the mechanism that deals with one of the gyro's biggest enemies, the apparent wander caused by the Earth's rotation. Let's start with the hardware itself. The compensation device is called a latitude rider nut. It sits on a threaded stud that is fixed horizontally to the inner gimbal. The key word there is "adjustable" — you can screw this nut in or out along that threaded stud, and that movement is what creates the correction. Now, here's the clever part about its geometry. When the nut is in its central position, its effect is cancelled out by a counter-balance weight on the opposite side of the gimbal. So in the middle, the nut does nothing — it's balanced. The moment you screw it out a few turns, the nut applies a downward moment on the gimbal. And here's where the precession rule comes in. That downward force, with the usual precession rule applied, produces — viewed from above — an anticlockwise precession of the gyro, including its scale, in azimuth. And that anticlockwise precession causes the reading in the window to increase. Conversely, if you wind the nut in, you get clockwise precession, and that makes the readings decrease. So now you can see the whole point. The apparent drift due to the Earth's rotation can be cancelled for a given latitude by using the latitude nut to produce an equal and opposite real drift. The nut gives you a real, physical drift that opposes the apparent one. And because you can screw the nut in or out, you can compensate for either direction of apparent drift — increasing readings in the southern hemisphere, or decreasing readings in the northern hemisphere. Now, a critical limitation. This setting can only be changed under workshop conditions — not in the aircraft. That means the compensation will only be correct for the one latitude it was set for. But here's the saving grace: errors due to latitude changes are usually small compared with the random wander errors of the Directional Indicator itself. And if the aircraft is moved to a new operating area involving a latitude change on the order of 60°, a DGI with the appropriate latitude correction would probably be substituted — they'd swap the instrument rather than try to adjust it in the field. Let me make this concrete with the numbers. Figure 12.13 illustrates the effect of compensating a gyro for the apparent drift of minus 13°/h at 60°N. That minus 13 comes from the formula 15 sin 60° — 15 degrees per hour times the sine of the latitude. At 60° north, that's about 13 degrees per hour of apparent drift. The latitude nut introduces a real drift of plus 13°/h, so the resultant drift — assuming no random error — will be zero at 60°N. But — and this is the crucial insight — that compensation of plus 13°/h applied for 60°N is now present at all latitudes. The graph represents this by renumbering the drift scale. So the drift in the region of 60°N is negligible after compensation, but if the aircraft is moved to an area south of the equator, the drift values will actually be greater than if no compensation had been made at all. The compensation that was perfect at 60°N becomes a liability far away from it. Let me summarize the rules for you, because they're the takeaway. Flying north from the corrected latitude gives a decreasing reading — that's a minus drift rate. Flying south from the corrected latitude gives an increasing reading — a plus drift rate. Flying away from the corrected latitude results in the drift rate increasing. And flying towards the corrected latitude results in the drift rate decreasing. So the picture is: the correction is perfect only at the latitude it was set for, and the further you get from that latitude, the worse the residual drift becomes — and it's worse in the opposite hemisphere than it would have been with no correction at all. That's the trade-off you accept with a workshop-only adjustment.