Dutch Roll Filter

Short summary of Dutch Roll, Yaw Damper, and the Dutch Roll Filter, all in one

✈️ DUTCH ROLL — Simple Explanation

Dutch roll is a side-to-side swaying motion that combines both:

• Yawing (nose moving left and right), and
• Rolling (wingtip to wingtip rocking).

It happens because of the interaction between lateral and directional stability. Aircraft with strong roll stability but weak yaw stability (like swept-wing jets) are most prone to it.

So the aircraft oscillates like it’s “wobbling” through the air — rolling and yawing in opposite directions.

⚙️ YAW DAMPER

To prevent Dutch roll, jets are fitted with a yaw damper system.

• It automatically detects yaw rate using a rate gyro (or IRS).
• It then sends small, rapid rudder inputs to counteract the motion and stop the oscillation.
• The yaw damper operates continuously during flight — even when the autopilot is off.

✅ Other functions:

• Keeps turns coordinated
• Helps runway alignment on approach
• Reduces rudder input needed during engine-out flight

🧠 DUTCH ROLL FILTER

The yaw damper must not react to normal pilot-commanded turns — only to unwanted yaw oscillations.

To achieve this, it uses a Dutch roll filter, which is:

A narrow band-pass filter that only passes signals at the natural Dutch roll frequency of the aircraft.

• When the aircraft is in a steady turn, the yaw rate is constant → filter output is zero → no rudder movement.
• When Dutch roll starts, the yaw rate oscillates at the Dutch roll frequency → filter detects it → rudder moves to stop it.

✅ In short:

✈️ Summary Line:

Dutch roll is an unwanted yaw–roll oscillation. The yaw damper automatically cancels it using the rudder, and the Dutch roll filter ensures it only reacts to Dutch roll — not pilot-made turns.