Spiral Instability
An airplane may display spiral instability in case it has strong directional stability relative to its lateral stability. When a wind gust makes an airplane with spiral instability to side-slip, powerful directional stability tends to yaw the airplane back into alignment with the relative wind. In this case the outside wing will generate more lift as it travels faster. This tends to roll the airplane in the direction of the yaw. The rolling force overcomes the weak dihedral effect, allowing further roll. As the vertical compo-nent of lift decreases, the relative wind has an upward component with the airplane swinging in alignment with the air stream, the nose begins to drop. This results in a gradually increasing spiral dive (figure 1.1).

  Figure 1.1 Spiral Instability

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