Which aircraft characteristics contribute to spiral instability?

In the context of aircraft stability, spiral instability is influenced significantly by the aircraft's directional stability and its dihedral effect. When considering these characteristics, strong static directional stability combined with weak dihedral effect creates a scenario that promotes spiral instability.

Static directional stability refers to the aircraft's ability to return to straight and level flight after a disturbance, such as a yawing motion. An aircraft with strong static directional stability will tend to correct itself quickly when disturbed in a sideways direction. However, if the dihedral effect is weak, the aircraft lacks sufficient lateral stability to counteract bank angles that may result from a side slip. This combination can lead to the aircraft entering a spiral descent or turn, as the strong directional stability encourages yaw correction which increases the bank angle further, and the weak dihedral effect provides insufficient resistance to this bank.

In contrast, the other options do not appropriately align with the dynamics of spiral instability. For example, having strong dihedral effect tends to improve lateral stability, which helps to counteract unwanted turns and maintain a straight flight path, thus reducing the propensity for spiral instability. The presence of equal stability in all directions would not promote spiral instability since it implies that the aircraft can respond uniformly to disturbances without favoring a particular direction.