Spiral divergence

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Spiral divergence is characterized by an airplance that is very stable directionally but not very stable laterally, e.g. a large finned airplance with no dihedral. In this case when the airplance is in a bank and sideslipping, the side force tends to turn the plance into the relative wind. Because of the greater directional stability, the yaw would be quickly eliminated by a stabilizing yaw moment set up by the keel surface. The outer wing travels faster, generates more lift, and the airplance will roll to still a higher bank angle. No lateral stability is present to negate this roll. The bank angle increases and causes the aircraft nose to drop into the direction of yawing. The airplane continues to turn into the sideslip in an ever-tightening spiral dive.

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