Editing Dutch roll (section)

==Stability==
In aircraft design, Dutch roll results from relatively weaker positive [[directional stability]] as opposed to positive [[Flight dynamics (fixed-wing aircraft)#Lateral modes|lateral stability]]. When an aircraft rolls around the longitudinal axis, a [[Slip (aerodynamic)|sideslip]] is introduced into the [[relative wind]] in the direction of the rolling motion (due to the lateral component of lift when the wings are not level). Strong lateral stability (due to the more-direct airflow past the down wing, which has been pivoted forward by the slip) begins to restore the aircraft to level flight. At the same time, somewhat weaker directional stability (due both to greater drag from the wing which is now generating greater lift, and by aerodynamic force on the vertical fin due to the yaw) attempts to correct the sideslip by aligning the aircraft with the perceived relative wind. Since directional stability is weaker than lateral stability for the particular aircraft, the restoring yaw motion lags significantly behind the restoring roll motion. The aircraft passes through level flight as the yawing motion is continuing in the direction of the original roll. At that point, the sideslip is introduced in the opposite direction and the process is reversed.

There is a trade-off between directional and lateral stability. Greater lateral stability leads to greater spiral stability and lower oscillatory stability. Greater directional stability leads to spiral instability but greater oscillatory stability.<ref>{{cite book |last1=Davies |first1=David P. |url=https://archive.org/details/handlingbigjetse0000davi/page/100/ |title=Handling the Big Jets: An Explanation of the Significant Differences in Flying Qualities Between Jet Transport Aeroplanes and Piston Engined Transport Aeroplanes, Together with Some Other Aspects of Jet Transport Handling |date=1971 |publisher=Air Registration Board |edition=3rd |page=100 |isbn=9780903083010 |access-date= |url-access=registration}}</ref>