inertia coupling

A form of instability that manifests itself during maneuvers, especially in the pitching and yawing motions. It is particularly common during rolling at very rapid rates and high g rolling pullouts either at very low indicated air speeds or at high altitudes. This tendency is common in aircraft that have a long fuselage and heavy wings with the weight concentration at the extremities. This leads to the large inertia in the pitch and yaw. During these maneuvers, the inertia forces are able to overcome the stabilizing aerodynamic forces, resulting in rapid oscillations in the pitch about the principal inertial axis, increasingly marked with attitude because of the divergence of this axis from the relative wind. This aerodynamic divergence may result in a catastrophic situation.

When aircraft is rolled in a pull-up maneuver, it acts as a gyroscope and yaws to the right while rolling to the right. Higher the rate of roll, higher is the rate of yaw. As it passes through 90 degree bank angle, the yawing action dominates the directional stability. As the aircraft becomes inverted, the longitudinal stability of the tailplane becomes dominant in pitching moment. Once aircraft is rolled through 270 degrees, the pitching moment is translated into yawing moment. At this moment, yaw divergence may take place if the aircraft does not have sufficient directional stability.