Before revision: Role of aerodynamic center in aircraft stability For longitudinal static stability: and For directional static stability: and Where: For a force acting away from the aerodynamic center, which is away from the reference point: Which for small angles and , , , simplifies to: General Case: From the definition of the AC it follows that .
After revision: Role of aerodynamic center in aircraft stability For longitudinal static stability: For directional static stability: Where: For a force acting away from the aerodynamic center which is away from the reference point: Which for small angles and , , , simplifies to: General Case: From the definition of the AC it follows that