Directional Gyro Indicator (DGI) — Page 169, Lesson 199

Let’s pick this up right where the DGI story gets interesting. We’ve already seen that a directional gyro drifts because the earth rotates under it. Now I want to walk you through what happens when that drift is compensated, and why the compensation is never perfect. First, the effect of changing aircraft latitude on a compensated DGI. We’ve stated that the apparent drift rate due to the earth’s rotation varies with the sine of the latitude. So at the equator, sine of zero is zero — no apparent drift. As you move toward the pole, sine increases, so the drift rate increases. Now picture an aircraft tracking due north, starting from the equator. The initial apparent drift rate of an uncorrected gyro is zero. As the flight progresses, the DGI reading decreases. By the time the aircraft reaches 30°N, the reading is decreasing at a rate of 7½ degrees per hour. At 60°N, it’s decreasing at 13 degrees per hour. And if you keep going, the value of the apparent drift rate increases from zero up to 15 sin latitude degrees per hour at the pole. Notice that rate of increase is not linear — it’s a sine function, so it accelerates as latitude increases. And here’s the key point: the same applies if a compensated gyro is transported north or south of its latitude of correction. The compensation is set for one latitude, so as you leave that latitude, the compensation no longer matches the actual drift. Next, errors due to unstable rotor rpm. The rate of precession of a gyro depends on rotor rpm, and in a suction-driven DGI, we don’t have precise control over that rpm. So the latitude nut compensation is only approximate. Here’s the mechanism. At high altitude with inadequate suction, the rotor rpm will be lower than the design value. Lower rpm means reduced gyroscopic rigidity. Now, the latitude nut produces a precession rate that’s too high relative to the actual drift, so it over-corrects the apparent drift. Conversely, if rpm exceeds the design figure — which is less likely — rigidity increases, and the latitude nut produces a lower rate of precession, so it under-corrects the apparent drift. Now, transport wander. At any latitude other than the equator, meridians — which define local north — are not parallel. If the gyro is aligned to one meridian, then flown east to west, the new meridian will be inclined to the old one by transport wander. So as you fly east or west, the local definition of north changes, and the gyro, holding its old reference, shows that difference as wander. Finally, let’s do a drift rate calculation. Example 1: an aircraft is stationary at 60°N. Calculate the hourly wander rate for an uncompensated gyro. The solution is apparent wander equals minus 15 times sine of 60 degrees, decreasing, degrees per hour, which equals minus 12.99 degrees per hour. The minus sign indicates the reading is decreasing — that’s the direction of the wander. And then we have a practice question about the spin axis of a directional gyro — how it’s maintained in a particular plane, by air jets in an air-driven gyro and by a different means in an electrically driven gyro. That’s one of the book’s practice questions, so let’s try it one at a time.