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).

Written and reviewed by

Pablo Asensio · 2026-04-01

Aeronautical engineer. Passed the thirteen EASA ATPL(A) theoretical knowledge exams studying from these very notes. Writes and reviews each topic against the Part-FCL Learning Objectives.

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