Inertial coupling is a dangerous effect which can undermine the performance of aircraft trying to manoeuvre at high speed. It is not an aerodynamic effect, but one caused by the simple laws of conservation of angular momentum – when an aircraft attempts to roll at high speed, centrifugal forces cause it to buck, pitch and yaw about all three axes. The effect was discovered when engineers and pilots tried to break the sound barrier in the Forties, and is made worse by long, thin aircraft designs with stubby wings and small tail planes. Enlarging these aerodynamic surfaces helped to negate inertial coupling’s effects.
Answered by Giles Sparrow