Yaw Dampers — Page 405, Lesson 501

I want to walk you through yaw dampers, and to do that properly we have to start with the phenomenon they exist to fix: Dutch roll. Dutch roll is caused by the interplay between two different kinds of stability. First, lateral stability around the longitudinal axis — that's the axis running nose to tail, and stability there means the aircraft resists rolling. Second, longitudinal stability around the vertical axis — that's the axis running top to bottom through the aircraft, and stability there means the aircraft resists yawing, or the nose swinging left and right. Here's the key relationship: an aircraft with an excess of lateral stability will, by default, have poor directional stability. And that combination — strong roll resistance but weak yaw resistance — makes the aircraft susceptible to Dutch roll. Let me make sure the terms are clear. Stability is the aircraft's natural tendency to resist any disturbance and return to the same conditions that existed before the disturbance occurred. So if a gust pushes the nose off heading, a stable aircraft wants to come back to that original heading. Now, the mechanism. Consider an aircraft disturbed by a gust causing it to yaw. As the aircraft yaws, one wing travels slightly faster through the surrounding air and the other wing travels slightly slower. The fast wing produces slightly more lift than before, and the slower wing produces slightly less. That difference in lift obviously produces a roll. But it doesn't stop there. As lift increases, lift-induced drag increases too. So the faster, higher wing produces more drag, and the low wing produces less. That drag difference creates a yawing moment in opposition to the initial disturbance — it tries to swing the nose back the other way. And then the whole process reverses. That's the oscillation: yaw causes roll, roll causes yaw, back and forth. Swept wings make this tendency worse. Here's why: the forward-going wing undergoes a reduction in effective wing sweep, which further increases the lift it produces. The retreating wing experiences an increase in effective wing sweep, which again reduces its lift. So the roll effect is amplified on a swept-wing aircraft. Now, here's the important distinction. All airline aircraft are statically stable — they will naturally try to return to the undisturbed condition. But the amount they are dynamically stable depends on the amount of damping force available. Static stability is the tendency to return; dynamic stability is about whether that return is smooth or oscillatory. And that's exactly where the yaw damper comes in — it provides the damping that the aircraft's natural characteristics lack, to stop that Dutch roll oscillation from building. So to summarise the chain: a yaw disturbance creates a roll, the roll creates a yawing moment that opposes the original disturbance, the process reverses, and on a swept-wing aircraft this oscillation is amplified. The aircraft is statically stable, but without sufficient damping it's dynamically unstable — and that's the problem the yaw damper is designed to solve.