Dutch Roll vs Spiral Instability

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Dutch Rollž

Positive Dynamic Stability

›Lateral positive

–Strong Dihedral Effect

›Directional negative

–Weaker Weathervaing Tendency

Spiral instability

žDynamic Stability

›Lateral negative

–Weak Dihedral Effect   Caused by Weak Dihedral

›Directional positive

–Strong Weathervaning Tendency

Spiral instability exists when the static directional stability of the airplane is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium. When the lateral equilibrium of the airplane is disturbed by a gust of air and a sideslip is introduced, the strong directional stability tends to yaw the nose into the resultant relative wind while the comparatively weak dihedral lags in restoring the lateral balance. Due to this yaw, the wing on the outside of the turning moment travels forward faster than the inside wing and as a consequence, its lift becomes greater. This produces an overbanking tendency which, if not corrected by the pilot, will result in the bank angle becoming steeper and steeper. At the same time, the strong directional stability which yaws the airplane into the relative wind is actually forcing the nose to a lower pitch attitude. We then have the start of a slow downward spiral which, if not counteracted by the pilot, will gradually increase into a steep spiral dive. Usually the rate of divergence in the spiral motion is so gradual that the pilot can control the tendency without any difficulty.