Dynamic Stability Flight Test

Dynamic Stability Flight Test (flight-test-operations/stability/dynamic-stability-flight-test)

Use when the task is a dynamic stability flight test: mode excitation, decaying oscillation reduction, and handling qualities verdicts.

Domain quick reference

• Each mode is excited with a control input: elevator doublet or pulse for the short period, elevator pulse for the phugoid, rudder pulse for the Dutch roll, aileron step for roll subsidence, rudder step for the spiral. The response time history is recorded until the oscillation damps (2 to 3 full cycles for the long-period phugoid).
• Log decrement from same-sign peak amplitudes n cycles apart: delta = (1/n) * ln(A0 / An). The mode must decay; equal or growing amplitudes raise a no-decay error.
• Damping ratio: zeta = delta / sqrt(delta^2 + 4 * pi^2). For small delta, zeta is about delta / (2 * pi).
• Frequencies: damped w_d = 2 * pi / T_d from the peak period, undamped w_n = w_d / sqrt(1 - zeta^2) for an under-damped mode.
• Decay times: time to half amplitude t_half = ln(2) / (zeta * w_n), cycles to half amplitude N = ln(2) / (2 * pi * zeta). A divergent mode (zeta < 0) has a time to double amplitude.
• Handling qualities verdicts use the typical flight test practice bands (certification values in FAR-25 / CS-25 take precedence): short period 0.3 to 2.0 acceptable, 0.08 to 0.3 marginal, below 0.08 inadequate; Dutch roll 0.08 to 2.0 acceptable, 0.02 to 0.08 marginal, below 0.02 inadequate; phugoid 0.04 to 2.0 acceptable; spiral must converge (zeta >= 0), else divergent.

Workflow

1. Choose the excitation for the mode with excitation_technique.
2. Record the response and extract the same-sign peak amplitudes and their timestamps (local_maxima helps find the peaks).
3. Reduce the sequence with mode_identification: log decrement, damping ratio, period, damped and undamped frequencies, and time to half amplitude.
4. Judge the result with handling_qualities_verdict against the band table.
5. For a divergent mode, report time_to_double_amplitude instead of a half-amplitude time.

Pitfalls

• Using peaks of alternating sign: the log decrement needs same-sign peaks n whole cycles apart.
• Reading the undamped frequency as the damped value: w_n is always larger than w_d for zeta > 0.
• Quoting a half-amplitude time for a divergent mode: apply the time to double amplitude.
• Treating the band values as regulation text: they are typical flight test practice; the certification criteria in the cited standards take precedence.

Behavior contract (gate 3)

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

Compliance

• The excitation techniques, log decrement reduction, and damping relations 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.