Static Stability Flight Test

Static Stability Flight Test (flight-test-operations/stability/static-stability-flight-test)

Use when the task is a static stability flight test: trim curve reduction from pitch control position versus speed, neutral point location, and static margin assessment.

Domain quick reference

• The trim curve is the elevator angle (pitch control position) needed to hold steady level trim at each test speed. Each speed V converts to a lift coefficient CL = 2 W / (rho V^2 S), so the test points become elevator angle versus CL.
• The trim curve slope b = d(delta_e)/dCL comes from a least squares fit delta_e = a + b CL. A negative slope is the signature of a statically stable aircraft: less up elevator is needed as speed increases.
• Stick fixed neutral point from the slope, the cg position h, and the elevator control power Cm_delta_e (per radian): h_n = h + b Cm_delta_e (pi/180), with static margin SM = h_n - h. A positive margin is stable, a negative margin is unstable (flag), and zero is neutral.
• Stick free neutral point shifts forward of the stick fixed value by (Cm_delta_e Ch_alpha) / (CL_alpha Ch_delta_e) from the free elevator hinge model (Ch_alpha and Ch_delta_e are the hinge moment derivatives, CL_alpha the lift curve slope, all per radian).
• Elevator angle per g: d(delta_e)/dn = (180/pi) CL SM / Cm_delta_e in degrees per g. A stable aircraft shows trailing-edge-up elevator movement with increasing load factor (negative value), reported as its magnitude.

Workflow

1. Collect the trimmed elevator angle at each steady level speed.
2. Convert the speeds to lift coefficients with lift_coefficients.
3. Fit the trim curve with trim_curve_fit (slope, intercept, R^2).
4. Locate the stick fixed neutral point and static margin with stick_fixed_neutral_point; flag an unstable slope.
5. With hinge moment data, find the stick free neutral point with stick_free_neutral_point.
6. Assess the elevator angle per g with elevator_angle_per_g.
7. Chain everything with static_stability_report for the final stability verdict.

Pitfalls

• Fitting elevator angle versus speed directly and reading the sign: convert to lift coefficient first; the slope convention b = d(delta_e)/dCL only holds after the conversion.
• Mixing units: Cm_delta_e and the hinge derivatives are per radian while the measured elevator angle is in degrees; the module converts with (pi/180).
• Forgetting the stick free shift: the free elevator neutral point is forward of the stick fixed value, so the stick free static margin is smaller.
• Treating the practice bands as regulation text: the numbers here are typical flight test practice; the certification criteria in the cited standards take precedence.

Behavior contract (gate 3)

The static stability logic is exercised by the gate 3 contract test: scripts/test_static_stability_flight_test.py against scripts/static_stability_flight_test_logic.py (stdlib unittest, offline). Run: python3 scripts/test_static_stability_flight_test.py

Compliance

• The trim curve reduction, the neutral point relations, and the elevator angle per g are common flight test methodology, paraphrased here. FAR-25 and CS-25 are cited as reference only for the stability demonstration context; no proprietary or copyrighted text is reproduced.
• compliance: STANDARDS-REF, gated: false.