Yaw Dampers — Page 405, Lesson 506

Let’s pick this up right where the yaw damper story gets interesting. I want to walk you through what happens after the rate gyro senses the aircraft’s motion, because that’s where the Dutch roll filter and the rudder authority limits come in. First, the key idea: the rate gyro produces an output for every turn the aircraft makes. That’s a problem, because we don’t want the yaw damper fighting the pilot on a normal, deliberate turn. We only want it to act on the oscillatory motion called Dutch roll. So we put a filter in the signal path. That filter is called the Dutch roll filter, and it’s a narrow band pass filter. Let me unpack that. A band pass filter lets through only signals within a specific frequency band and rejects everything else. “Narrow band” means that band is very tight. So this filter is designed to pass only signals that change at the frequency of the Dutch roll. In other words, the rate gyro sends signals for all turns, but only the components that match the Dutch roll frequency will appear at the input to the servo amplifier. That servo amplifier then drives the rudder servomotor. So the rudder only gets commanded to move when the aircraft is actually oscillating in Dutch roll, not during normal manoeuvres. Now, there’s a subtlety about the signal polarity. The DC polarities — that is, the direction of the electrical signal — are greatest when the rate of turn is greatest, and they reverse when the direction of turn reverses. So as the aircraft yaws one way, the signal builds up to a maximum, then as the yaw reverses, the polarity flips. That’s exactly what the filter needs to track the oscillation. Let me also mention the figure reference here. Figure 29.5 is a super simplified yaw damper, and its whole point is to illustrate mainly the Dutch roll filter. And Figure 29.4 shows the filter output during Dutch roll. I’ll put the schematic up for you: Now, the second big topic: Rudder Control Computing Authority. This is about how much rudder movement the yaw damper is allowed to command. Here’s the reasoning. Removing the oscillations does not require a large rudder movement — Dutch roll is a small, rapid oscillation, so a little rudder deflection is enough. Also, there’s a safety hazard: if the yaw damper runs away, that is, it malfunctions and drives the rudder hard in one direction, a large authority would be dangerous. So to reduce that hazard, yaw damper authority is normally only about 3 to 6 degrees left and right of centre. Now here’s an important detail about multiple systems. If two yaw damper systems are operating on a single span rudder, their authority is accumulative. That means each system’s authority adds up. So singly, each system could move the rudder by 3 degrees, but together they can move it by 6 degrees. That’s a critical point for understanding system design — you can’t just assume two systems give you redundancy with the same total authority; they combine. So to summarise what we’ve covered: the rate gyro senses all turns, the narrow band Dutch roll filter passes only the Dutch roll frequency to the servo amplifier, the amplifier drives the rudder servomotor, and the rudder authority is deliberately limited to about 3–6 degrees each side of centre, with two systems’ authorities adding together. That’s the complete picture of how the yaw damper is controlled and limited.