Pitch angle correct — Page 177, Lesson 215

Let's pick up with the pitch angle correction, because that's the last piece of the air-driven horizon's error story before we move to the electric instrument. We said the turning error tilts the gyro axis by about 2°. The fix is to build a compensation into the instrument so that in a correctly banked turn, the horizon bar is driven to show the true attitude. But here's the honest limitation: that compensation is set for one specific speed and one specific rate of turn. If you fly the turn at a different speed, or a different rate of turn, the compensation no longer matches the actual error. So you get small residual errors — small leftover errors. They're still there, but they're very much smaller than they would be if you had no compensation at all. So the design trades a perfect fix for a massive reduction in error. Now, rigidity. This is the property that makes a gyro resist being disturbed. In the suction-driven horizons, the rotor spins at up to 15 000 rpm. That high rotor speed gives high gyroscopic inertia — the gyro's resistance to having its axis moved. In the electric horizons, the rotor spins at 22 500 rpm, which is typical, and that gives even greater rigidity. Because the inertia is so high, the precession rates are low. Precession is the gyro's response to an applied torque — the axis moves slowly. So if a horizon topples — if the gyro axis gets knocked right over — it will take a significant period of time to re-erect itself, unless the instrument has a rapid erection device fitted. That's a device that speeds up the re-erection process. Now, serviceability checks. Before flight, you check that the horizon bar takes up a laterally level position — level side to side — with the correct pitch indication for the aircraft type. And you check that this indication is maintained when you taxi. If a caging device is fitted — a device that locks the gyro — you must uncage the instrument at least five minutes before take-off. That's to ensure the rotor axis has had time to reach alignment with the true vertical. In flight, the artificial horizon should give an immediate and correct indication of any change in pitch or roll attitude. Now let's move to the electric artificial horizon. The main advantage of the electric horizon over the air-driven one is its greater rigidity, because of its faster spin rate. That greater rigidity gives increased accuracy because the errors are reduced. The basic principle is the same as the air-driven horizon — it's still a vertical gyro. But instead of being tied to earth's gravity by pendulous vanes, it's tied by mercury levelling switches and torque motors. Let me explain that control system. The gravity-operated control system consists of mercury levelling switches, which are fixed to the base of the rotor, and electric torque motors. If a levelling switch is not level — if the gyro axis has wandered off the vertical — the mercury liquid ball moves from its central position. That movement closes an electrical circuit, which drives the torque motor. The torque motor provides the force, and that force is precessed to return the gyro axis back to the vertical. There are two levelling switches: one senses pitch, one senses roll. They activate the pitch and roll torque motors respectively, and those motors precess the gyro back to the vertical as soon as it starts to wander. Now, the 90° precession rule — this is the key to understanding which motor does what. Because of that rule, the torque motor on the side of the inner gimbal corrects wander in the rolling plane. It applies torque round the lateral axis to produce rotation about the longitudinal axis. Likewise, the pitch torque motor is on the outer — the longitudinal — gimbal, so that the precession is about the lateral axis to correct for pitch. And this control system, like the suction horizon, is designed to compensate for turning errors. It does this by maintaining the rotor axis slightly tilted away from the true vertical, and having the horizon bar compensated by a similar amount. So the same principle of compensation carries over from the air-driven instrument to the electric one.