Yaw Dampers — Page 405, Lesson 504

We're starting a new topic now: yaw dampers. Let's dig right in. The yaw damper is a system that senses the aeroplane's motion about its normal, or vertical, axis — that's the axis that runs vertically through the aircraft, the one it rotates around when it yaws. The sensing is done by a single-axis rate gyro, or in modern aeroplanes, by an input from the IRS — that's the Inertial Reference System. The whole objective is to sense the yawing motion as quickly as possible and produce a correcting demand signal to a servo or actuator, which then feeds signals to the rudder control system to apply rudder in opposition to the yaw. So the rudder pushes against the unwanted motion. Now, there's a critical piece of timing here called phase advance. 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 it: if you apply rudder at the same instant the fin starts to return the aircraft to its original heading, then the combined forces — the fin's natural restoring force plus your rudder input — will over-correct and cause the aircraft to overshoot. 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, it times the rudder application to hit exactly when the yaw rate is peaking, and then it stops the input before it adds to the problem. But here's the complication. The system as I've described it would also interpret a normally commanded turn as a yaw and act in opposition to it. 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 be able to distinguish commanded turn inputs from yaw due to a disturbance or apparent Dutch roll. Let me explain the difference. When an aircraft is turned, it rolls about the longitudinal axis — that's the axis running nose to tail — in the direction of the turn, and it also yaws about the vertical axis. So a turn involves both roll and yaw. 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. The Dutch roll frequency is based on the natural yawing frequency of the aircraft. That frequency is relatively low, and it will differ slightly with aircraft type. So yaw dampers must be designed to allow the Dutch roll frequency to control the rudder but block other frequencies. That's where the Dutch roll filter comes in. Let's look at what the filter does. 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, as the aircraft levels out on completion of the turn. So, whilst the turn is constant, the filter output is zero. That results in no rudder demand. The key idea is that a steady, commanded turn produces a constant rate of turn, and the filter cancels that out — it only passes the oscillating Dutch roll frequency. 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. The DC graph at the bottom of Figure 29.4 shows the Dutch roll filter output in that case — it's constantly varying, which is what drives the rudder to damp the oscillation. So to summarise the whole chain: the rate gyro or IRS senses yaw rate, phase advance times the rudder input to hit the peak of the disturbance, and the Dutch roll filter ensures the system only responds to the Dutch roll frequency and ignores commanded turns. That's the core of the yaw damper.