What determines aircraft spin characteristics and recovery?

Spin is autorotation about a near-vertical axis with both wings stalled, one more deeply than the other. Entry requires stall plus yaw (intentional or from asymmetric conditions). Spin characteristics depend on: Mass distribution (inertia ratio Ix/Iz - nose-heavy promotes flatter spins, harder recovery), Aerodynamic damping (tail effectiveness in disrupting autorotation), and Control power available. Recovery involves: Opposite rudder to stop rotation, Forward stick to break stall, and Neutral ailerons (or as specified). Some aircraft are unrecoverable from certain spin modes. Certification (FAR 23.221, 25.143) requires demonstrated recovery or spin resistance. Design for spin resistance includes strakes, proper tail sizing, and stall behavior management.