The Artificial Horizon — Page 184, Lesson 218

Let’s pick up with the electric artificial horizon and look at why it behaves better than the pneumatic one under acceleration. I want you to hold three ideas in your head: gyro rigidity, precession, and the levelling switches. Those are the whole story here. First, why are acceleration errors minimal in the electric horizon? The first reason is rotor speed. The electric horizon spins its gyro rotor much faster than the pneumatic instrument can manage with airflow. That high rotor speed gives very high gyro rigidity. Rigidity is the gyro’s resistance to being pushed out of its plane of rotation. With high rigidity, precession rates are very low. Precession is the gyro’s tendency to move its axis when a force is applied to it. So with low precession, there is less potential for the gyro to move out of the earth’s vertical. In other words, the gyro stays put, aligned with the true vertical, even when the aircraft accelerates. The second reason is about the rotor housing. In the electric horizon, the rotor housing is less bottom heavy than in the pneumatic one. Bottom heaviness is what makes the gyro self-erect, but it also causes errors during acceleration. Because the housing is less bottom heavy, the roll error is reduced while accelerating. So the instrument is less prone to showing a false bank during acceleration. Now the third reason is the clever part — the pitch and roll cut-out switches. Let me explain the problem first. When an aircraft in a level attitude accelerates, the mercury ‘ball’ in the pitch levelling switch moves back in its tube due to inertia. Inertia is the tendency of the mercury to stay where it was while the aircraft moves forward. That movement falsely completes the pitch levelling circuit. If that circuit completed, the pitch torque motor would falsely precess the gyro out of the vertical. That would be a false pitch indication. So the designers added a pitch cut-out switch in the circuit. That switch activates when an acceleration of 0.18g or greater is detected. So above 0.18g, the pitch levelling circuit is cut out, and the false precession is prevented. The same logic applies in a turn. In a turn, the roll mercury switch would falsely activate the roll torque motor. So a cut-out is incorporated in the circuit, and this one is activated at 10 degrees angle of bank. So when the aircraft banks more than 10 degrees, the roll levelling circuit is cut out, preventing false precession from the roll torque motor. Now let’s move to the fast erection system. Many electric horizons include a fast erect system. Its purpose is to give rapid initial erection when the instrument is first powered up, and quick re-erection should the instrument topple. Topple means the gyro has exceeded its operating limits and tumbled out of its normal range. Quoting typical figures: the normal erection rate is 4 degrees per minute. By pushing the fast erection knob on the face of the instrument, that rate is increased to 120 degrees per minute. That is a huge jump. The action increases the voltage to the erection torque motors. More voltage means more torque, so the gyro erects much faster. There is a real advantage to having a fast erection system. Because the system can erect the gyro quickly, the pendulosity of the gyro can be reduced. Pendulosity is the bottom-heaviness of the gyro. If you can reduce the bottom-heaviness, you decrease the turning and acceleration errors. So the fast erection system is not just about speed of erection — it also allows the manufacturer to make the gyro less bottom heavy, which directly reduces the errors we talked about earlier. Let me tie this together. The electric horizon minimises acceleration errors through three mechanisms: high rotor speed giving high rigidity and low precession, a less bottom heavy rotor housing reducing roll error, and the pitch and roll cut-out switches that disable the levelling circuits during acceleration above 0.18g and during bank beyond 10 degrees. And the fast erection system gives you rapid erection at 120 degrees per minute versus the normal 4 degrees per minute, by boosting voltage to the erection torque motors, and it lets you reduce pendulosity, which further cuts turning and acceleration errors. One note — the excerpt cuts off mid-sentence with 'Note: When airbo…' — that appears to be the start of a note about airborne behaviour, but the text stops there. So we’ll leave that for when the material continues.