What is the main characteristic that differentiates a spiral dive from a spin?

The primary distinction between a spiral dive and a spin lies in the aircraft's aerodynamic state, specifically regarding whether the aircraft is stalled. In a spin, the aircraft has indeed entered a stalled condition, meaning that one wing is generating significantly less lift compared to the other. This creates a yawing motion, resulting in a nose-down attitude as the aircraft rotates around its vertical axis.

In contrast, a spiral dive does not involve a stall. During a spiral dive, the aircraft is in a coordinated flight condition, with both wings generating lift and maintaining airflow over their surfaces. The aircraft is descending at a high airspeed, and the flight path involves a tightening turn, which can increase load factors and lead to increased airspeed.

Factors such as flaps being extended or airspeed trends are not defining characteristics of one flight condition over the other. Both spirals and spins could potentially occur with flaps in various configurations, and while airspeed tends to increase in a spiral dive rather than decrease, this alone does not serve as the primary differentiator between the two maneuvers. The critical factor lies in the stall condition present in a spin, which is absent in a spiral dive.