ashfordeOU Flutter Testing (flight-test-operations/flutter/flutter-testing)
Use when the task is flutter clearance for a flight test: the required flutter speed from the design dive speed factor, the extrapolated flutter speed from the damping trend, the frequency separation check between structural modes, and the FAR 25.629 flutter margin and damping margin verdicts.
Domain quick reference
- Required flutter speed: V_F_required = margin_factor * V_D with margin_factor default 1.2 and V_D the design dive speed in m/s. The flight test program must demonstrate freedom from flutter up to at least this speed.
- Flutter speed from the damping trend: fit damping versus test speed with a linear least squares line over n points (slope m, intercept b) and extrapolate to the speed where the fitted damping crosses zero, V_F = -b / m. Closed forms: m = (nsum(xy) - sum(x)sum(y)) / (nsum(x^2) - sum(x)^2) and b = (sum(y) - m*sum(x)) / n.
- Frequency separation: two structural modes must stay separated; the check is |f1 - f2| / ((f1 + f2) / 2) >= min_frac with min_frac default 0.10 (10 percent). Falling below the minimum indicates frequency coalescence, the flutter precursor.
- Flutter margin ratio: ratio = V_F_measured / V_D; per the FAR 25.629 context the clearance verdict passes when ratio >= 1.2 (the 1.2 factor is common certification practice; the exact basis comes from the flight test program).
- Damping margin at the maximum test speed: the measured damping at V_test must stay at or above the required minimum, default 0.03; negative damping means the mode already fluttered at the test speed.
Workflow
- Collect V_D (design dive speed) and compute the required flutter speed with required_flutter_speed(v_d).
- Gather the damping measurements (speeds, dampings) from the flight test and extrapolate the flutter speed with flutter_speed_from_damping(speeds, dampings); review the fit before trusting the crossing.
- Check each pair of structural modes with frequency_separation(f1, f2) and flag any pair below the 10 percent minimum.
- Compare the measured flutter speed with V_D using flutter_margin_ratio(v_f_measured, v_d); a ratio below 1.2 fails the clearance.
- Check the damping at the maximum test speed with damping_margin(damping_at_v_test); a value below the minimum fails the clearance.
- Gate the flutter test assessment on the margin ratio verdict and the damping margin verdict.
Pitfalls
- Feeding the damping points out of speed order: the speeds must be strictly increasing or the least squares fit is meaningless and raises ValueError.
- Extrapolating far past the tested range: the zero crossing is an extrapolation, so the farther V_F sits beyond the last test speed the less certain it is; the build-up program tests closer to the crossing before clearing.
- A flat or rising damping trend: no zero crossing exists and the function raises ValueError; investigate the measurement before claiming clearance.
- Confusing the two margins: the flutter margin ratio compares the flutter speed with V_D, while the damping margin compares damping at the maximum test speed with the minimum; each gates independently.
- Using a non-positive V_D: the required flutter speed and the margin ratio both raise ValueError instead of reporting a nonsense margin.
- Ignoring frequency coalescence: two modes converging toward the 10 percent separation minimum warn of a coupling that can drop the flutter speed below the extrapolated trend.
Behavior contract (gate 3)
The required flutter speed, damping trend extrapolation, frequency separation, flutter margin ratio, and damping margin logic is exercised by the gate 3 contract test: scripts/test_flutter_testing.py against scripts/flutter_testing_logic.py (stdlib unittest, offline). Run: python3 scripts/test_flutter_testing.py
Compliance
- Standards referenced, not reproduced: FAR-25 is US government work (public domain) and CS-25 is a free EASA download; the 1.2 flutter margin factor, the damping trend extrapolation, and the frequency separation practice sit in the FAR 25.629 context as common certification practice, summary-only per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.