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Run npx skillmds@latest add ashfordeou/longitudinal-stability in your terminal (requires Node.js), paste this page's agent-chat prompt into Claude, Cursor, or any MCP-connected agent, or download the SKILL.md file and copy it into your agent's skills directory.

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Who published Longitudinal Stability?

ashfordeOU (@ashfordeou) published this skill. Their other Agent Skills are listed on their SkillMD profile.

Longitudinal Stability by ashfordeou

Name: Longitudinal Stability
Author: ashfordeOU

Longitudinal Stability

Use when you must assess static longitudinal stability of an aircraft: compute the neutral point from the wing aerodynamic center, the tail volume coefficient, the lift slope ratio, and the downwash gradient; derive the static margin at the current center of gravity; and determine whether the aircraft is longitudinally stable for pitch stability. Produces the neutral point location, the static margin, and the stability verdict that gate the longitudinal stability analysis. Trigger: static margin, neutral point, longitudinal stability, pitch stability, center of gravity.

ashfordeOU 18cf87e 3 files · 9.1 KB Updated Sep 17, 2026

File contents

Static Longitudinal Stability (flight-mechanics/stability-control/longitudinal-stability)

Use when the task is static longitudinal stability analysis: neutral point, static margin, and the pitch stability verdict at a given center of gravity.

Domain quick reference

The neutral point is the aerodynamic center of the whole aircraft (wing plus tail); all positions are fractions of the mean aerodynamic chord, dimensionless.
Neutral point: h_np = h_ac_w + V_h * (a_t / a_w) * (1 - depsilon/ dalpha), where h_ac_w is the wing aerodynamic center, V_h is the tail volume coefficient, a_t/a_w is the lift slope ratio, and depsilon/dalpha is the downwash gradient.
The tail volume coefficient is dimensionless; the lift slope ratio and downwash gradient are dimensionless too.
Static margin = neutral point - center of gravity; positive static margin means a stable configuration.
Longitudinally stable when the static margin meets the minimum margin (default 0.05).

Workflow

Collect the wing aerodynamic center, tail volume coefficient, lift slope ratio, and downwash gradient.
Compute the neutral point with neutral_point.
Take the center of gravity position and derive the static margin with static_margin.
Check the margin with longitudinally_stable.
Gate the pitch stability assessment on the verdict.

Pitfalls

Reversing the static margin sign: positive margin (neutral point aft of the center of gravity) is stable, not unstable.
Confusing the neutral point with the wing aerodynamic center: the neutral point includes the tail contribution, the aerodynamic center of the wing alone does not.
Zero tail volume coefficient: a configuration with no stabilizing tail surface cannot produce a meaningful neutral point offset.
Downwash gradient at or above 1.0: physically invalid for this model.

Behavior contract (gate 3)

The neutral point, static margin, and stability logic is exercised by the gate 3 contract test: scripts/test_longitudinal_stability.py against scripts/longitudinal_stability_logic.py (stdlib unittest, offline). Run: python3 scripts/test_longitudinal_stability.py

Compliance

Standards referenced, not reproduced: FAR-25 and CS-25 require positive static longitudinal stability for transport aeroplanes; the neutral point and static margin method is common flight mechanics methodology, summary-only per standards-map.yaml.
compliance: STANDARDS-REF, gated: false.

---name: longitudinal-stabilitydescription: Use when you must assess static longitudinal stability of an aircraft: compute the neutral point from the wing aerodynamic center, the tail volume coefficient, the lift slope ratio, and the downwash gradient; derive the static margin at the current center of gravity; and determine whether the aircraft is longitudinally stable for pitch stability. Produces the neutral point location, the static margin, and the stability verdict that gate the longitudinal stability analysis. Trigger: static margin, neutral point, longitudinal stability, pitch stability, center of gravity.license: Apache-2.0---# Static Longitudinal Stability (flight-mechanics/stability-control/longitudinal-stability)Use when the task is static longitudinal stability analysis: neutralpoint, static margin, and the pitch stability verdict at a givencenter of gravity.## Domain quick reference- The neutral point is the aerodynamic center of the whole aircraft (wing plus tail); all positions are fractions of the mean aerodynamic chord, dimensionless.- Neutral point: h_np = h_ac_w + V_h * (a_t / a_w) * (1 - depsilon/ dalpha), where h_ac_w is the wing aerodynamic center, V_h is the tail volume coefficient, a_t/a_w is the lift slope ratio, and depsilon/dalpha is the downwash gradient.- The tail volume coefficient is dimensionless; the lift slope ratio and downwash gradient are dimensionless too.- Static margin = neutral point - center of gravity; positive static margin means a stable configuration.- Longitudinally stable when the static margin meets the minimum margin (default 0.05).## Workflow1. Collect the wing aerodynamic center, tail volume coefficient, lift slope ratio, and downwash gradient.2. Compute the neutral point with neutral_point.3. Take the center of gravity position and derive the static margin with static_margin.4. Check the margin with longitudinally_stable.5. Gate the pitch stability assessment on the verdict.## Pitfalls- Reversing the static margin sign: positive margin (neutral point aft of the center of gravity) is stable, not unstable.- Confusing the neutral point with the wing aerodynamic center: the neutral point includes the tail contribution, the aerodynamic center of the wing alone does not.- Zero tail volume coefficient: a configuration with no stabilizing tail surface cannot produce a meaningful neutral point offset.- Downwash gradient at or above 1.0: physically invalid for this model.## Behavior contract (gate 3)The neutral point, static margin, and stability logic is exercised bythe gate 3 contract test: scripts/test_longitudinal_stability.pyagainst scripts/longitudinal_stability_logic.py (stdlib unittest,offline). Run:python3 scripts/test_longitudinal_stability.py## Compliance- Standards referenced, not reproduced: FAR-25 and CS-25 require positive static longitudinal stability for transport aeroplanes; the neutral point and static margin method is common flight mechanics methodology, summary-only per standards-map.yaml.- compliance: STANDARDS-REF, gated: false.

Frequently asked questions