Right, let's pick this up with the transfer valve and the yaw damper actuator — Page 413, Lesson 509

Right, let's pick this up with the transfer valve and the yaw damper actuator. We've got the rate gyro sensing the yaw, the signal going through the computer, and now we're at the business end — actually moving the rudder. So, the transfer valve is the hydraulic muscle. It takes the electrical command from the yaw damper computer and converts it into hydraulic pressure. That pressure drives the yaw damper actuator, and the yaw damper actuator in turn drives the main rudder actuator. So you've got a chain: transfer valve, then yaw damper actuator, then the main actuator that physically moves the rudder. And here's the key limit — the maximum deflection of the rudder from this system is only 3° to 4° in either direction. That's a deliberate design limit. The yaw damper is there to smooth out oscillations, not to fly the aircraft, so it's given a very small authority over the rudder. Now, the feedback loop. We have a Linear Variable Differential Transformer — that's the LVDT. It's a position sensor. It measures the actual rudder position and sends that information back as a voltage. This position feedback is applied to SP2, which is a summing point in the circuit. Its job is to cancel out the processed rate gyro signal when the rudder position change is appropriate for the rate of yaw change. In other words, if the rudder has moved just the right amount to counter the yaw rate, the feedback cancels the command, and the rudder stops moving. But there's a second path. Position feedback is also applied through the energized relays of RL2, and that's fed back to SP2 as well. Now, the purpose of this second path is critical: it ensures the rudder will always return to the neutral position. Think about a crosswind. A strong crosswind can push against the rudder and hold it off-centre. In that case, the normal position feedback voltage might not be large enough to drive the rudder back to the centre. So what happens? The position feedback voltage now causes INT 2 — that's an integrator — to ramp up. That increases the voltage at SP2. This increased position feedback starts to drive the rudder back to neutral. And as the rudder returns, the position feedback voltage decreases, which allows the integrator to run back down. So the integrator is the persistent force that guarantees the rudder always centres itself, even against a crosswind. Now let's talk about testing the yaw damper. There's a test switch. When you actuate it — either to the right or to the left — it applies a voltage to the yaw damper rate gyro torquing coil. That torquing coil physically torques the rate gyro, which simulates aircraft movement without the aircraft actually moving. The rudder position indicator responds to this because there's an output from the position transducer. Here's the sequence: if you move the switch to the left, the indication will first move to the left, then back to the centre. On release of the switch, the indication will move to the right and back to the centre. And the reverse happens if you first move the switch to the right. So you get a symmetrical test — it checks the system can drive the rudder both ways and return it to centre. Finally, the indications. On a typical aircraft fitted with two yaw dampers, the main indications are a panel containing an on/off switch, a test switch or button, and a failure light for each of the two yaw dampers. That failure light is important — it indicates many faults. It could be loss of hydraulic power, loss of electrical power, a logic failure in the yaw damper computation system, or loss of input from the rate gyro. And the switch can then be used to isolate the faulty yaw damper. So to tie it together: the transfer valve drives the actuator, the LVDT feeds position back to cancel the command, the integrator guarantees return to neutral, the test switch torques the gyro to simulate motion, and the failure light tells you when to isolate the system. That's the complete yaw damper loop.