DIRECTIONAL STABILITY

DIRECTIONAL STABILITY

Directional stability is stability around the vertical or normal axis.
The most important feature that affects directional stability is the vertical tail surface, that is, the fin and rudder. Keel effect and sweepback also contribute to directional stability to some degree.

The Fin

An airplane has the tendency always to fly head-on into the relative airflow. This tendency which might be described as weather vaning is directly attributable to the vertical tail fin and to some extent also the vertical side areas of the fuselage. If the airplane yaws away from its course, the airflow strikes the vertical tail surface from the side and forces it back to its original line of flight. In order for the tail surfaces to function properly in this weather vaning capacity, the side area of the airplane aft of the center of gravity must be greater than the side area of the airplane forward of the C.G. If it were otherwise, the airplane would tend to rotate about its vertical axis.

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