The Artificial Horizon — Page 177, Lesson 213

Let's pick up with the artificial horizon, and I want to walk you through the errors that show up when the aircraft accelerates or turns. These are the classic limitations of the air-driven instrument, and you need to know exactly what the gyro does and why. First, the roll error due to acceleration. Think about what's inside the instrument: the rotor housing has a weighted base, and that weight gives the gyro its pendulous, or gravity-seeking, behaviour. Now, when the aircraft accelerates, that weighted base has inertia — it wants to stay where it is, to lag behind. But here's the key: that lagging force doesn't just push the base back. Because the gyro is spinning, the force gets precessed, which means the force is applied 90 degrees around from where you'd expect. The result is that the base of the rotor housing moves to starboard, to the right, and the gyro axis precesses out of the vertical. That rotates the whole rotor and gimbal assembly about the longitudinal axis, and the instrument gives you a right wing down indication. So during acceleration, you get a false right-wing-down. Now, deceleration is the opposite. It causes a nose-down, left wing low error — exactly the reverse of the acceleration indication. And I need to be very clear about the assumption here: these errors assume the rotor is rotating anticlockwise when viewed from the top. That's the case for British air-driven artificial horizons. But most electric horizons, and some American air-driven ones, have clockwise rotor spin, and that gives you the opposite errors. So the direction of the error depends entirely on the direction of rotor spin. Let me show you what these attitudes look like. That's a nose-up attitude, and is a nose-down attitude. Keep those in mind as we talk about turning errors. Now, turning errors in the air-driven artificial horizon. Whenever an aircraft turns, there must be an acceleration towards the centre of the turn — that's the centripetal force. Because the pendulous vanes are now affected by a horizontal acceleration as well as the acceleration due to gravity, you get errors in both pitch and roll indications. During the turn, the centrifugal force acts on two things: the fore and aft pendulous vanes, which gives you what's called the erection error, and the weighted base of the rotor housing, which gives you the pendulosity error. These errors are complex, and they change as the turn progresses. They cancel out after a full 360-degree turn. The magnitude varies with speed, with rate of turn, and with the type of horizon. For a chosen speed and rate of turn, you can compensate for these errors by tilting the top of the rotor axis slightly forward — that corrects the erection error — and slightly to the left — that corrects the pendulosity error. But in an uncorrected instrument, here's what you'll see. Assume the classic instrument: air-driven, gyro rotating anticlockwise when viewed from above. Turning through 90 degrees, the instrument under-reads bank angle, and there's a pitch error indicating a climb. Turning through 180 degrees, the bank angle is correct, but the pitch error still indicates a climb. Turning through 270 degrees, the instrument over-reads bank angle, and again the pitch error indicates a climb. And turning through 360 degrees, the bank angle is correct again. So the pattern to remember: the pitch error always indicates a climb throughout the turn, while the bank angle error changes — under-reading at 90 degrees, correct at 180, over-reading at 270, and correct again at 360. That's the classic behaviour of the uncorrected air-driven artificial horizon in a turn.