Yaw Dampers — Page 405, Lesson 504

I want to walk you through the yaw damper system now. This is one of those systems that sounds simple — a gyro senses yaw, a servo moves the rudder — but the cleverness is all in the signal processing. Let's start with the sensing side. The sensing element is a single-axis rate gyro, or in modern aeroplanes, an input from the IRS — that's the Inertial Reference System. Either way, the sensor is measuring the aeroplane's motion about the normal or vertical axis. That's the yawing axis, the one that points straight down through the floor. The whole objective is to sense the yawing motion as quickly as possible, and produce a correcting demand signal to the servo or actuator. That servo then feeds signals to the rudder control system, which applies rudder in opposition to the yaw. So the fundamental job: sense the yaw, push the rudder against it. Now, the problem is timing. If you wait until the disturbance has already built up, you're too late. That's where phase advance comes in. Phase advance is a means of applying the damping application as soon as possible. Why? Because damping must be applied when the rate of disturbance is at its greatest — not when the disturbance has moved to the point where the natural stability of the fin has already arrested it. Think about that. The fin has its own natural stability; it will eventually stop the yaw on its own. If you apply rudder at the same instant the fin starts to return the aircraft, the two forces combine and over-correct, and the aircraft overshoots. So the phase advance accelerates the signal so that the rudder input occurs at the point of maximum yaw, and it cancels the signal quickly so as not to exaggerate the yawing effect. In other words, the signal is pushed forward in time to catch the peak, then killed off fast so it doesn't add to the problem on the way back. Now here's the subtlety. The system as I've described it would also interpret a normally commanded turn as a yaw, and act in opposition. That would produce an unco-ordinated turn — in fact, the rudder would be exacerbating the co-ordination problem by introducing the full amount of its rudder authority in the opposite direction to that required. So the yaw damper needs to distinguish a commanded turn from a yaw due to a disturbance or apparent Dutch roll. Let's think about what happens in a turn. When an aircraft is turned, it rolls about the longitudinal axis in the direction of the turn, and it also yaws about the vertical axis. So a commanded turn produces yaw too. But the yaw damper is provided to dampen Dutch roll, not to prevent the aircraft following a turn command. Therefore, only the Dutch roll frequency must be acted upon by the yaw damper. That's the key concept — the Dutch roll filter. The Dutch roll frequency is based on the natural yawing frequency of the aircraft. It's relatively low, and it will differ slightly with aircraft type. The yaw damper must be designed to allow the Dutch roll frequency to control the rudder, but block all other frequencies. So the filter is a band-pass in effect — it passes the Dutch roll frequency and rejects everything else. Let me walk you through the behaviour. Initially, while the rate of turn is building up to the constant rate, the Dutch roll filter output also builds. Then it falls off to nothing when the rate of turn becomes constant. The reverse happens, with opposite polarity, as the filter capacitor discharges, when the aircraft levels out on completion of the turn. So whilst the turn is constant, the filter output is zero — and that results in no rudder demand. That's the beauty of it. A steady turn produces a constant yaw rate, and the filter blocks that. Only the oscillating Dutch roll frequency gets through. Now contrast that with an aircraft yawing at the Dutch roll frequency. Since the rate of turn is constantly changing, the output from the rate gyro is constantly changing. And the DC graph at the bottom of Figure 29.4 is the Dutch roll filter output — you can see it following that oscillation. That's the signal that drives the rudder to damp the Dutch roll. Let me show you the schematic. So to tie it all together: the rate gyro senses yaw rate, phase advance times the signal to hit the peak of the disturbance, and the Dutch roll filter ensures only the Dutch roll frequency actually drives the rudder — so a commanded turn doesn't get fought by the damper. That's the complete signal processing chain for a typical yaw damper.