Inertia coupling

In aeronautics, inertia coupling, also referred to as inertial coupling and inertial roll coupling, is a potentially catastrophic phenomenon of high-speed flight which caused the loss of aircraft and pilots before the design features to counter it (e.g. a big enough fin) were understood. It occurs when the inertia of a heavy fuselage exceeds the ability of the aerodynamic forces and moments generated by the wing and empennage to stabilize the aircraft. The problem became apparent as jet fighter aircraft and research aircraft were developed with narrow wingspans, that had relatively low roll inertia, caused by a long slender high-density fuselage, compared to the pitch and yaw inertias.

The term inertia/inertial coupling referring to divergence during a rolling maneuver has been described as misleading because the complete problem is one of aerodynamic as well as inertia coupling. Contributions to the maneuver are complex and include inertial coupling, aerodynamic coupling and the inertia ratios about the three axes, all of which occur simultaneously. However, inertial coupling has also been defined as essentially a gyroscopic effect, i.e. a tendency of a fuselage, when rolled rapidly, to swing away from the direction of flight and become broadside to the wind, and analyzed as such by Phillips. Inertial roll coupling has been defined as a resonant divergence in pitch or yaw when roll rate equals the lower of the pitch or yaw natural frequencies.