Description
Inertial coupling occurs when an aircraft such as that described above is quickly put into a roll, resulting in violent pitching and yawing, and loss of control as the aircraft rotates on all three axes.
The phenomenon itself is not aerodynamic, and is caused by general conservation of angular momentum acting on mass whose radial distribution is not symmetric about the axis of rotation. It can be visualized by imagining a uniform long rod, at each end of which is a perpendicular extension, each pointing opposite the other. (If the rod is horizontal, one points up and the other points down.) At the end of each extension is a weight. The extensions and weights are identical, so the center of mass and the axis of rotation along the length of the rod are unaffected by the weights. If the rod is then spun about its axis, the centrifugal forces on the two weights will cause the entire assembly to tilt relative to its initial axis of rotation.
Although a typical jet aircraft has most of its mass distributed close to its centerline, and the aerodynamics in planes designed to be stable (such that small fluctuations in control tend to return it to attitude equilibrium) provide some stabilization, it is important to remember that aircraft realistically always fly with a small non-zero random rate of yawing and pitching. Inertia coupling on an aircraft usually manifests itself as a downward pitching; rolling causes the tail mass to be flung upward and thus the nose to tip down. The pitching can in turn cause gyroscopic yawing.
Read more about this topic: Inertia Coupling
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