Directional Stability

081-04-04Pablo Asensio Martínez2026-04-011 min

Mechanism

Directional stability is provided primarily by the Vertical Fin (Vertical Stabilizer).

  • If the aircraft yaws (sideslips), the fin meets the airflow at an angle of attack.
  • This generates a sideways aerodynamic force (lift) on the fin.
  • Since the fin is behind the CG, this force creates a moment that rotates the nose back into the wind (Weathercock Effect).

Factors Affecting Directional Stability

  1. Fin Area: Larger fin = Greater stability.
  2. Fin Moment Arm: Distance from CG to Fin. Longer = Greater stability.
  3. Sweepback: Increases directional stability. A swept wing advancing into the wind produces more drag than the retreating wing, creating a restoring moment.
  4. Keel Area: Area of fuselage/fin behind the CG aids stability. Area ahead of the CG (long nose) destabilizes.

Dorsal / Ventral Fins

Additional surfaces added to increase keel area aft of the CG to prevent directional instability (e.g., at high angles of attack or due to fuselage blanking).