Gyroscopes — Page 139, Lesson 170

Let's pick up right where we left off — we've covered rigidity, the property that keeps a gyro axis fixed in space. Now I want to talk about what happens when that axis doesn't stay put. That's the concept of wander. Despite rigidity, the orientation of a gyro axis may alter over time. Any departure of a gyro axis from its original orientation is called wander. So wander is the umbrella term — any movement of the axis away from where it started. Now, wander can be either real or apparent, or a combination of both. I'll explain the difference in a moment, but first let's break wander down into its two subcategories, because that's where the real meat is. Wander is subdivided into drift and topple. If the gyro axis wanders in the horizontal plane, it's called drift — and that drift can be real or apparent. If the gyro axis wanders in the vertical plane, it's called topple — again, real or apparent. So think of it this way: drift is horizontal movement of the axis, topple is vertical movement. Two different planes, two different names. Now here's the key distinction between the two types of gyros. A horizontal axis gyro can both drift and topple — it can wander in either plane. But a vertical axis gyro can only topple. It can topple forwards, backwards, or sideways, but it cannot drift at all. That's a hard limitation — vertical axis gyros simply don't drift. Let me make sure you've got the picture. Imagine the gyro axis as a line in space. If that line swings around like the hand of a clock lying flat on a table, that's drift — horizontal wander. If the line tips up or down, like a see-saw, that's topple — vertical wander. Now, about real versus apparent — I want to be clear on this because it's a distinction you'll need. Real wander is an actual physical movement of the gyro axis caused by forces acting on it. Apparent wander is an illusion — the axis hasn't actually moved, but it appears to have moved because the reference frame itself is moving. The Earth's rotation causes apparent wander, for example. But I'll get into the specifics of that when we cover the causes in more detail. For now, the core takeaway is this: wander is any departure of the gyro axis from its original orientation. It's either real or apparent, or both. It's subdivided into drift — horizontal wander — and topple — vertical wander. Horizontal axis gyros can do both; vertical axis gyros can only topple, forwards, backwards, or sideways, never drift. That's the foundation. When you understand which plane the axis is wandering in, you know whether you're dealing with drift or topple — and that tells you a lot about what kind of gyro you're working with and what corrections it might need.