Gyroscopes — Page 148, Lesson 174

Let's pick up with transport wander, because that's the last piece of the wander puzzle before we move on to how gyros are classified by function. So, transport wander. We've already looked at earth rate, which is the apparent wander caused by the earth spinning. Now I want you to imagine the earth is stopped in space, not rotating at all. We're going to ignore earth rate completely, just to isolate this one effect. Picture an aircraft flying from Los Angeles to London. At Los Angeles, the gyro is aligned with the local meridian — that's the line of true north as seen from Los Angeles. That's our solid white line in the figure. Now, as the aircraft flies to London, the gyro stays rigid in space. It keeps pointing at the same direction in inertial space — that's the dotted white line. But here's the key: when you arrive in London, the direction of true north from London is different. True north from London is the line connecting London to the North Pole, which is not the same line the gyro is holding. The difference between where the gyro is pointing and where true north actually is — that's the transport wander. And it's really just the difference in the alignment of the meridians at Los Angeles and London. The meridians converge at the poles, so as you move east or west, the local north direction changes. The gyro doesn't know that; it just holds its original direction in space. So transport wander is the apparent drift caused by you physically moving the gyro across the curved surface of the earth. Now, let's move on to the types of gyro, classified by function. This is a fundamental split. Some gyros measure angles — like 10 degrees of pitch, 5 degrees of bank, 30 degrees of heading change. These are called displacement gyros. Others measure angular rate — like a turn rate of 3 degrees per second. These are called rate gyros. The construction difference is critical. Displacement gyros have 2 gimbals and 2 degrees of freedom. Examples are the Directional Gyro Indicator, the DGI, and the Artificial Horizon. Rate gyros have one gimbal and one degree of freedom, and they're used in the Rate of Turn Indicator and in yaw dampers. Now, displacement gyros can be subdivided further into space gyros and tied gyros. A space gyro has gyroscopic inertia with reference to a point in space. It's free to wander, and if it does wander, nothing corrects it back to its original datum. So it needs to have a rate of real wander so low that it's negligible for practical purposes. That means space gyros have to be very accurate indeed — and they are correspondingly expensive. They're used in Inertial Navigation Systems. A tied gyro, on the other hand, if it wanders, is restored back to some orientation by an external force. It's maintained in some particular attitude or direction rather than in space. For instance, the directional gyro of a gyro-magnetic compass is slaved to remain oriented to magnetic north. So it's tied to a reference — an external force keeps pulling it back. Let me make sure you see the contrast clearly. Space gyro: free, wanders, nothing corrects it, must be extremely accurate, expensive, used in INS. Tied gyro: restored by external force, maintained in a particular attitude, like the slaved DGI to magnetic north. Take a look at the figures on this page — Figure 11.13 shows the horizontal transport wander with the red track from Los Angeles to London, and the solid and dotted white lines showing the gyro's alignment. That's the full picture of transport wander and the functional classification of gyros.