Fig. 2. Aerodynamic stability determined by the slope of measured aerodynamic torque about a rotational axis (here pitch) plotted as a function of the angular orientation about the same rotational axis. (A) Measured aerodynamic torques about the pitch axis for an aerodynamically stable frog. At the equilibrium angle of rotation, no torques are acting on the frog, so it remains at equilibrium. If a gust of wind pitches the frog's nose away from its initial position in either the upward or downward direction, this change in orientation relative to the airflow induces an aerodynamic torque that tends to rotate the frog back to its initial position. (B) Measured aerodynamic torques for a neutrally stable frog. If a gust of wind pitches the frog's nose away from its initial position in either the upward or downward direction, this change in orientation relative to the airflow induces no aerodynamic torque that tends to rotate the frog back to its initial orientation or to rotate the frog further away from its initial orientation, so the frog remains at the angular orientation to which it was pitched. (C) Measured aerodynamic torques for an aerodynamically unstable frog. If a gust of wind pitches the frog's nose away from its initial position in either the upward or downward direction, this change in orientation relative to the airflow induces an aerodynamic torque that tends to rotate the frog even further away from its initial position, causing the frog to diverge from its original flight path.