Yaw Dampers — Page 413, Lesson 508

I want to walk you through yaw dampers now — this is a system that stops the aircraft from oscillating around the vertical axis, the yaw axis. Let's start with the split rudder concept, because that's where the excerpt opens. On aircraft with a split rudder surface — that means the rudder is actually two separate halves — if one yaw damper system fails, the aircraft only has half the yaw damper protection it originally had. That's a design consideration, and it's allowed for in the design and operation of the aircraft. But here's the catch: even that small remaining input can cause overstress in certain flying conditions. So some aircraft have an input to the yaw damper computer from the CADC — that's the Central Air Data Computer — and that input schedules the gain of the yaw damper inputs for the ambient flying conditions. In plain terms, the CADC tells the yaw damper computer how much authority it's allowed to use, based on the current flight conditions. At high speed, the yaw damper authority may be even further reduced, specifically to avoid that overstress condition. Now let's look at the operating modes. There are two: Synchronization and Engaged Mode. Synchronization exists to prevent yaw axis engagement transients. A transient is a sudden, unwanted spike in the system when you switch it on. The way it's prevented is by cancelling servomotor outputs using an inverting integrator. Let me unpack that. There's a relay called RL1 — you can see it in Figure 29.6, the series yaw damper schematic diagram. RL1 is energized prior to yaw damper engagement. While it's energized, any amplifier output is fed back through the integrator. The inversion through the integrator cancels any transients that are present. So the system is effectively zeroing itself out before you actually engage it. Then, on engagement of the yaw damper, RL1 is de-energized — it drops out, and the system goes live. Now the Engaged Mode. Providing the interlock logic is good — that's a safety check that the conditions are right — the engage solenoid engages, allowing the yaw damper elements of the Power Control Unit, the PCU, to pressurize. That's the hydraulic muscle. From there, the signal path is quite specific. The rate gyro signal is phase advanced and applied to the demodulator. The demodulator converts the signal to a DC signal, and the output polarity represents the AC signal input phase. So the demodulator is turning an AC signal into DC, and the polarity of that DC tells you the phase of the original AC signal. Then the yaw damper frequency is passed by the bandpass filter, which blocks all other frequencies. This is the clever part. Because only the yaw damper frequency gets through, the yaw damper does not oppose normal turn manoeuvres, and it does not respond to aircraft vibration and bending. Those are different frequencies, so they're filtered out. The modulator then restores the AC signal, maintaining the appropriate phase. So you've gone AC to DC and back to AC, keeping the phase information intact. The servo amplifies the signal and applies it to the transfer valve, which in turn drives the yaw damper actuator, which drives the main actuator. So it's a chain: servo, transfer valve, yaw damper actuator, main actuator. And here's the key limit — the maximum deflection of the rudder is 3 degrees. That's the authority limit of the yaw damper. It can only move the rudder three degrees, which is why the system can be scheduled down at high speed — it's a small authority, but even that small input can overstress the structure in certain conditions. So to tie it together: the yaw damper senses yaw rate with a rate gyro, processes that signal through demodulator, filter, and modulator, and drives the rudder through a limited 3-degree authority, with the gain scheduled by the CADC to protect the airframe.