The Artificial Horizon — Page 184, Lesson 218

I want to walk you through the acceleration errors in the electric artificial horizon, and then the fast erection system. This is the electric version of the instrument, so the gyro is spun by an electric motor rather than by a vacuum pump, and that changes its behaviour in some very important ways. Let me start with why acceleration errors are minimal here. There are three distinct reasons, and I want you to hold all three in your head because they each attack a different source of error. First, the high rotor speed. An electric artificial horizon spins its gyro rotor very fast, and that gives it very high gyro rigidity. Rigidity is the gyro's resistance to being pushed out of its plane of rotation. With high rigidity you get very low precession rates — precession being the gyro's tendency to move its axis when a force is applied to it. So because the gyro is so rigid, there is less potential for it to move out of the earth's vertical. That's the fundamental defence. Second, the rotor housing is less bottom heavy in the electric horizon. Bottom-heaviness is what we call pendulosity — the mass distribution that makes the gyro want to hang with its heavy end down, which is what keeps it aligned with gravity. But that same pendulosity causes errors during acceleration. Because the electric horizon's housing is less bottom heavy, the roll error while accelerating is reduced. Less pendulosity, less acceleration-induced roll error. Third, and this is the clever part — pitch and roll cut-out switches. Let me explain the problem they solve. When an aircraft in a level attitude accelerates, the pitch levelling switch will falsely complete its circuit. The switch uses a mercury 'ball' in a tube, and during acceleration that mercury ball moves back in its tube due to inertia. That movement falsely completes the circuit, which would then cause the pitch torque motor to falsely precess the gyro out of the vertical. So to stop that, a pitch cut-out switch is included in the circuit, and it activates when an acceleration of 0.18g or greater is detected. So above 0.18g, the pitch levelling is cut out, and the false precession never happens. The same logic applies in a turn. In a turn, the roll mercury switch would falsely activate the roll torque motor — again because the mercury ball moves due to the turn. So a cut-out is incorporated in the circuit for the roll channel, and this one is activated at 10 degrees angle of bank. So once you bank past 10 degrees, the roll levelling is cut out. Now, the fast erection system. Many electric horizons include this, and its job is to give rapid initial erection — that's the gyro being brought up to the vertical when you first switch the instrument on — and also quick re-erection should the instrument topple. Toppling happens when you exceed the operating limits of the gyro, and the gyro physically hits its stops and loses its reference. Let me give you typical figures. The normal erection rate is 4 degrees per minute. That's how fast the gyro is slowly pulled back to the vertical by the erection torque motors. When you push the fast erection knob on the face of the instrument, that rate is increased to 120 degrees per minute. So from 4 degrees per minute to 120 degrees per minute — a massive increase. What the knob does mechanically is increase the voltage to the erection torque motors. More voltage, more torque, faster erection. And here's the elegant payoff. One of the advantages of having a fast erection system is that the pendulosity — the bottom-heaviness — of the gyro can be reduced. Because you no longer need the gyro to be heavily bottom-heavy to erect itself quickly, you can make it less bottom-heavy. And as we said at the start, less pendulosity means smaller turning and acceleration errors. So the fast erection system doesn't just recover the gyro quickly — it indirectly makes the whole instrument more accurate in normal flight. There's a note at the end of the excerpt that's cut off — it begins "When airbo…" — and that's clearly the start of a sentence about airborne behaviour, but the text stops there, so I'll leave that for when we meet it properly. Let me just tie it together. The electric horizon minimises acceleration errors three ways: high rotor speed giving high rigidity and low precession, reduced bottom-heaviness reducing roll error, and the pitch and roll cut-out switches that disable the levelling circuits during acceleration above 0.18g and in turns beyond 10 degrees of bank. And the fast erection system, driven by a knob that boosts voltage to the torque motors, raises erection from 4 to 120 degrees per minute, and lets you reduce pendulosity, which further shrinks turning and acceleration errors.