The Artificial Horizon — Page 177, Lesson 208

Right, let's get into the artificial horizon. We've just looked at how the gyro works in principle, and now we're going to see how it's actually built and, crucially, what its limitations are. First, the limitations. The gyro itself wants to stay fixed in space, but the instrument case moves with the aircraft. The amount the case can move relative to the gyro is controlled by fixed stops. In older designs, the typical limits are ±60° in pitch and 110° each way in roll. So, 60 degrees nose up or nose down, and 110 degrees of bank to either side. Modern instruments are much more generous: there's complete freedom in roll, and up to 85° plus or minus in pitch. Now, here's the critical part. If those limits are exceeded, the gyro 'topples'. That gives you violent and erratic movements of the horizon bar. And unless a fast erection system is incorporated, you won't get accurate indications until the gyro has re-erected itself, and that takes a period of 10 to 15 minutes. So if you ever see that horizon bar thrashing about, you know the gyro has toppled, and you must not trust the instrument until it has settled back down. Now, the control systems. The rotor assembly is made very slightly bottom-heavy. Why? To keep down the time taken for initial erection when the gyro is first started up. But that alone isn't enough. A complex control system is required to maintain the rotor axis vertical in flight. There are two main types. A suction or air driven artificial horizon exhausts air through four slots which are normally half covered by four pendulous vanes. Electric artificial horizons use levelling or mercury switches and torque motors. So we have two families: air-driven and electric. Let's focus on the air-driven one, because that's where the detail is. In the air driven artificial horizon, an engine-driven suction pump—or a venturi tube in some light aircraft—is used to create a suction of about 4 inches of mercury in the instrument case. So we're pulling a vacuum inside the case. Replacement air, drawn in by this suction via a filter, is ducted through the outer and inner gimbals to enter the rotor case as a jet. That jet spins the rotor at up to 15,000 rpm. After driving the rotor, the air passes into the instrument case through slots at the base of the rotor housing. Now, the control system of the air driven artificial horizon consists of four slots and four pendulous, or hanging, vanes at the base of the rotor housing. The vanes hang down so that when the rotor axis is vertical, each slot is half covered by its vane. That means four equal jets of air emerge from the slots—fore and aft, and left and right. Because the four jets are of equal strength but in opposite directions, no force is exerted on the gyro, and therefore no precession occurs. The gyro rotor remains vertical. So the whole trick of the air-driven system is this: when the gyro is vertical, the vanes hang symmetrically, the jets balance, and nothing happens. The moment the gyro tilts relative to the case, the vanes shift, one jet gets stronger than its opposite, and that imbalance creates a force that precesses the gyro back to vertical. That's the erection system in action. That figure shows the equilibrium state—the four equal jets in balance. And you can see the nose-up and nose-down attitudes in the earlier figures to picture how the horizon bar moves relative to the aircraft symbol. So to tie it together: the rotor spins fast, the case can move within fixed limits, and if you exceed those limits the gyro topples and takes 10 to 15 minutes to re-erect. The air-driven system uses suction, a jet of air, and four pendulous vanes to keep the rotor axis vertical through balanced jets and precession. That's the core of the artificial horizon.