# Longitudinal Stability

> Stability around the lateral axis (Pitch). The role of the Center of Gravity (CG) and Tailplane.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-04 — Stability
- **Topic code:** 081-04-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/stability/static-and-dynamic-longitudinal-stability/

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## Key Forces
*   **Lift ($L$)**: Acts through the Aerodynamic Center (AC). Normally behind the CG.
*   **Weight ($W$)**: Acts through the Center of Gravity (CG).
*   **Tail Load**: Usually a downward force to balance the Nose Down moment caused by Lift being behind Weight.

## Static Stability Requirement
For positive static longitudinal stability, the Center of Gravity (CG) must be **forward** of the Aerodynamic Center (AC) of the whole aircraft (Neutral Point).
*   **If CG is too far Aft**: Stability is reduced. The aircraft becomes more maneuverable but harder to control (twitchy). If CG is behind the Neutral Point, the aircraft becomes unstable.
*   **If CG is too far Forward**: Stability is increased (very stable), but controllability is reduced (heavy elevator). Greater tail downforce is needed -> More Lift needed -> More Drag -> Higher Fuel Consumption.

## Factors Affecting Stability
1.  **CG Position**: Forward = Stable. Aft = Unstable.
2.  **Tailplane Area**: Larger tailplane = More stability.
3.  **Tail Moment Arm**: Longer distance between CG and Tail = More stability.

## Dynamic Longitudinal Stability
*   **Phugoid**: A long period oscillation (speed and altitude exchange, Angle of Attack constant). Lightly damped but easily controlled by pilot.
*   **Short Period Oscillation**: A rapid pitching oscillation (Angle of Attack changes). Heavily damped naturally. If undamped, results in PIO (Pilot Induced Oscillation).

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