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. Description Inertia coupling will tend to occur when an aircraft with the weight distribution described above is quickly rolled about an axis other than its roll axis. The tendency can be countered by a number of strategies which include increasing directional stability and reducing allowed roll rate and duration and limiting angle of attack for performing rolling maneuvers. The cause of the disturbing motion may be visualised by thinking of the aircraft mass being concentrated in two locations, "dumbbell-fashion" on its roll axis, one in front of the centre of gravity and the other behind. The aircraft will be flying along its aerodynamic or wind axis with the "dumbbell" at some angle of attack. Rolling about the aerodynamic axis will tend to cause the "off-axis" masses to move outwards. The trend in fighter aircraft design through the 1950s of short wing spans, fuselages of high density and flight at high altitude all tended to increase the inertia forces due to rolling in comparison with the aerodynamic restoring forces provided by