ashfordeOU Spin Recovery (flight-mechanics/stability-control/spin-recovery)
Use when the task is spin entry, developed spin modes, autorotation, and the recovery control sequence for a stalled aircraft.
Domain quick reference
- A spin is a developed post-stall rotation about a near-vertical axis, sustained by autorotation: beyond the stall the lift curve slope turns negative, so the more stalled wing half loses lift and the asymmetric moments keep the rotation going.
- Post-stall lift model (linear drop): Cl = cl_max + m_post * (alpha - alpha_stall), m_post < 0. Worked example: cl_max = 1.4, alpha_stall = 16 deg, m_post = -0.02 per deg, alpha = 20 deg gives Cl = 1.32.
- Autorotative band: the alpha range on the negative-slope post-stall region, ending where the post-stall lift returns to zero, alpha_end = alpha_stall + cl_max / |m_post|. Worked example: 16 + 1.4 / 0.02 = 86 deg. The wing is autorotating only while alpha_stall < alpha < alpha_end.
- Spin descent rate: V_d = sqrt(2 * W / (rho * S * C_D_spin)) with C_D_spin the spin drag coefficient (flat spins near 1.0 to 1.6, steep spins lower). Worked example: W = 15000 N, S = 16 m^2, rho = 1.225, C_D_spin = 1.2 gives V_d = 35.7 m/s.
- Developed spin rotation rate: Omega = 2 * V_d * nu / b with nu the nondimensional rotation rate. Worked example: nu = 0.4, b = 10 m gives Omega = 2.86 rad/s.
- Spin mode by the flatness ratio nu = Omega * b / (2 * V_d): below 0.3 steep (descent-dominated), 0.3 to 0.5 developed, above 0.5 flat (rotation-dominated). Worked example: tip speed Omega * b / 2 = 14.3 m/s against V_d = 35.7 m/s gives nu = 0.4.
- Recovery sizing: altitude lost is V_d * t_rec (35.7 m/s for 3 s is 107 m), and the rotation decays exponentially, t_stop = tau * ln(Omega_0 / Omega_stop). Worked example: Omega_0 = 2.86 rad/s, tau = 1.5 s, Omega_stop = 0.2 rad/s gives 3.99 s.
- Standard recovery sequence: power to idle, ailerons neutral, rudder full opposite to the rotation, elevator forward to break the stall; hold the inputs until rotation stops, then recover from the dive.
Workflow
- Confirm the departure: check the stall penetration and the autorotative condition with stall_penetration_deg and autorotative_condition.
- Compute the post-stall lift and the band edge with post_stall_lift_coefficient and autorotation_band_end_deg.
- Estimate the spin descent rate and rotation rate with spin_descent_rate and spin_rotation_rate.
- Classify the mode with spin_flatness_ratio: below 0.3 steep, 0.3 to 0.5 developed, above 0.5 flat.
- Apply the recovery sequence: power idle, ailerons neutral, rudder opposite, elevator forward; hold until rotation stops.
- Size the recovery with recovery_altitude_loss and rotation_stop_time, and check the altitude lost against the minimum recovery altitude for the flight condition.
Pitfalls
- Routing takeoff questions here: ground roll, lift-off speed, and field length belong to the takeoff-performance sub-skill; the spin is a post-stall stability regime, not a takeoff phase.
- Routing landing questions here: approach speed, flare, ground roll, and stopping distance belong to the landing-performance sub-skill; the spin is not a landing regime.
- Routing drag breakdown questions here: the aerodynamics drag-polars leaves give the induced and parasite drag breakdown against lift, but they do not model the autorotative condition or the spin rotation rate.
- Treating any alpha above the stall angle as autorotative: the wing must sit inside the negative-slope band (alpha < alpha_end); past the band edge the linear post-stall model loses validity.
- Deflecting ailerons with the spin: standard recovery requires ailerons neutral; aileron with the rotation is a pro-spin input.
- Getting the rudder direction wrong: rudder goes opposite to the rotation, not with it; confuse spin direction with turn direction and the spin deepens.
- Using a free-air lift curve in the spin: the post-stall slope is negative and much flatter than the pre-stall value, so the lift estimate must use the post-stall model.
- Using one nondimensional rate for every spin: mixing the steep-spin nu near 0.2 with the flat-spin nu above 0.5 mis-sizes the rotation rate and the recovery altitude.
Behavior contract (gate 3)
The post-stall lift, autorotative band, spin descent and rotation rates, mode classification, and recovery sizing logic is exercised by the gate 3 contract test: scripts/test_spin_recovery.py against scripts/spin_recovery_logic.py (stdlib unittest, offline). Run: python3 scripts/test_spin_recovery.py
Compliance
- Standards referenced, not reproduced: FAR-25 and CS-25 set the spin-recovery demonstration requirement for transport aeroplanes; the simplified spin models are common flight-mechanics methodology, summary-only per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.