Directional stability
Directional stability = yaw-axis stability: tendency to weathervane back into the relative wind after a yaw disturbance (TP 1102: "the vertical stabilizer contributes to the directional stability of an aircraft").
Mechanism: side area aft of the CG.
- Vertical fin and fuselage side area aft of the CG act like arrow feathers.
- More side area aft of the CG than ahead; equal fore and aft gives little or no directional stability.
- Yaw gust presses on fin: checks yaw, turns nose back toward relative wind.
- Aircraft settles on a new heading; pilot must reestablish the old one.
Sweepback (small contribution, different mechanism than lateral stability; PHAK: "usually small"):
- Yaw swings one wing's leading edge more square to the airflow.
- That wing picks up more drag.
- Extra drag pulls it back, restoring heading.
Trap: assuming the fin restores the original heading. Fin only stops the yaw.
References
- TP 1102 Basic Principles of Flight, Directional Stability
- PHAK ch.5, Directional Stability (Yawing)