In aeronautics, inertia coupling, also referred to as inertial coupling and inertial roll coupling, is a potentially catastrophic phenomenon of high-speed flight in a long, thin aircraft, in which an intentional rotation of the aircraft about one axis prevents the aircraft's design from inhibiting other unintended rotations. The problem became apparent in the 1950s, when the first supersonic jet fighter aircraft and research aircraft were developed with narrow wingspans, and caused the loss of aircraft and pilots before the design features to counter it (e.g. a big enough fin) were understood. The term "inertia/inertial coupling" has been criticized as misleading, because the phenomenon is not solely an instability of inertial movement, like the Janibekov effect. Instead, the phenomenon arises because aerodynamic forces react too slowly to track an aircraft's orientation. At low speeds and thick air, aerodynamic forces match aircraft translational velocity to orientation, avoiding the dangerous dynamical regime. But at high speeds or thin air, the wing and empennage may not generate sufficient forces and moments to stabilize the aircraft.
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