Washout filter

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Washout filter

A washout filter is a stable high-pass filter with zero static gain. It passes rapid changes in input but ignores slow, steady signals. Its common transfer function is G(s) = s / (s + d), where d is a time constant. This means a constant input yields zero output, and only transient, higher-frequency changes produce a response.

In aircraft yaw damping, washout filters are placed ahead of the yaw damper and rudder actuators. They filter out low-frequency (slow) signals in the feedback loop so steady pilot commands aren’t damped, while still letting rapid changes pass through to the damper. This helps the aircraft respond quickly to maneuvers without the yaw damper counteracting steady inputs. An example of this use is in yaw damper design for large jet aircraft.