Yaw Dampers — Page 413, Lesson 508

I want to walk you through yaw dampers, and I'll start with a design point that's specific to aircraft with a split rudder surface. On those aircraft, the rudder is effectively divided into two halves, and each half is driven by its own yaw damper system. So if one of those systems fails, you're left with only half the yaw damper protection you originally had. That's a deliberate design allowance — the aircraft is certified and operated knowing that a single failure leaves you with half authority. Now, here's the subtle part. Even that reduced half-authority input can cause overstress in certain flying conditions. So some aircraft feed an input into 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 according to the ambient flying conditions. In plain terms, the computer adjusts how much authority the yaw damper gets based on the air data it senses. At high speed, the yaw damper authority may be reduced even further, specifically to avoid an overstress condition. So the gain scheduling is a protective measure. Let me now walk you through the operating modes. There are two: Synchronization and Engaged Mode. First, Synchronization. Its purpose is to prevent yaw axis engagement transients. A transient here is a sudden, unwanted movement or 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 labelled 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, and the inversion through the integrator cancels any transients that are present. So the integrator effectively absorbs and cancels the unwanted signal. Then, on engagement of the yaw damper, RL1 is de-energized, and the system runs normally. Now the Engaged Mode. This begins with the interlock logic. Providing the interlock logic is good — that's a safety check that conditions are correct — the engage solenoid engages, and that allows the yaw damper elements of the Power Control Unit, the PCU, to pressurize. So the PCU is the unit that provides hydraulic power to move the rudder. From there, the signal path is what I want you to follow carefully. 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 of that DC signal represents the AC signal input phase. So the demodulator is turning an AC signal into DC, and the polarity tells you the phase. Next, the yaw damper frequency is passed by the bandpass filter, which blocks all other frequencies. This is a crucial design feature. 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. Then the modulator restores the AC signal, maintaining the appropriate phase. So we went AC to DC and back to AC, preserving phase information. The servo then amplifies the signal and applies it to the transfer valve. The transfer valve 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 limitation: the maximum deflection of the rudder is 3 degrees. That's the authority limit of the yaw damper — it can only move the rudder up to 3 degrees, which is enough to damp yaw oscillations but not enough to interfere with the pilot's control. So to tie it together: the yaw damper senses yaw rate with a gyro, filters out everything except the yaw damper frequency, converts and restores the signal while preserving phase, amplifies it, and drives the rudder through a limited 3-degree authority, all while the CADC can reduce that authority at high speed to prevent overstress.