# Directional Stability

> Stability around the vertical axis (Yaw). Weathercock effect.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-04 — Stability
- **Topic code:** 081-04-04
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/stability/static-directional-stability/

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## 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).

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