Refer to figure.
SPIRAL DIVERGENCE/INSTABILITY

Spiral divergence will exist when static directional stability is very large when compared to the lateral stability. The character of spiral divergence is not violent.

When a small sideslip is introduced, the strong directional stability tends to restore the nose into the wind while the relatively weak “dihedral effect” lags in restoring the aeroplane laterally. The rate of divergence in the spiral motion is usually so gradual that the pilot can control the tendency without difficulty.

Spiral instability is normally considered less objectionable than oscillatory instability, so most aircraft are designed to be spirally unstable.

With this characteristic, when the aircraft is yawed, either by the prolonged application of rudder or asymmetric power, a rolling moment in the direction of yaw occurs and the aircraft quickly enters a spiral dive.

This can occur when flying at low airspeeds under asymmetric power conditions, when excessive yaw due to too much power is coupled with insufficient rudder control to balance it, and may quickly place an aircraft in a dangerous attitude.