Directional stability

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Directional stability is the tendency of a vehicle or moving body to keep its orientation aligned with its direction of movement. When a car or an airplane gets turned a little relative to its direction of motion, it might correct itself, over-correct itself, or it might start to spin out of control. If it tends to correct itself, we say it's directionally stable, while if it tends to spin-out, we say it is directionally unstable. There are many factors that can effect dynamic stability including speed, weather and road conditions, vehicle shape and mass distribution, and tire properties.

Vehicle oscillations associated with dynamic stability are frequently called "weather vaning". When the vehicle's orientation is perturbed from the direction of motion, a restoring moment may be produced which is in a direction opposite to the orientation disturbance. This can lead to oscillations in orientation around the center of mass similar to a weather vane rotating about its (vertical) pivot.

With the exception of spacecraft, vehicles generally have a recognisable front and rear and are designed so that the front points more or less in the direction of motion. Without this stability, they may tumble end over end, spin or orient themselves at a high angle of attack, even broadside on to the direction of motion. At high angles of attack, drag forces may become excessive, the vehicle may be impossible to control, or may even experience structural failure. In general, land, sea, air and underwater vehicles are designed to have a natural tendency to point in the direction of motion.