Aileron Reversal

Aileron Reversal (flight-mechanics/stability-control/aileron-reversal)

Use when the task is the aeroelastic control check of a wing: the reversal dynamic pressure and reversal speed from the torsional stiffness, the aileron effectiveness fraction at the flight dynamic pressure, and the control reversal verdict against the dive speed limit.

Domain quick reference

• Reversal dynamic pressure from the torsional stiffness: q_rev = k_t / (C_l_alpha * eta * S * c * e), with k_t the wing torsional stiffness about the elastic axis in N m / rad, C_l_alpha the lift curve slope in per radian, eta the dimensionless aileron effectiveness factor (0 < eta <= 1), S the wing area in m^2, c the mean chord in m, and e the elastic axis to aerodynamic center offset in m; q_rev comes out in Pa.
• Reversal true airspeed from the dynamic pressure: V_rev = sqrt(2 q_rev / rho), with rho the air density in kg/m^3 and the speed in m/s. Lower density at altitude raises the reversal speed.
• Aileron effectiveness fraction at a flight dynamic pressure: eff = 1 - q / q_rev. Unity at zero speed, zero at the reversal point, and negative beyond it: negative effectiveness is control reversal.
• Reversed verdict: the ailerons reverse when the flight dynamic pressure q exceeds q_rev, which the design dive speed must never reach. FAR-25.629 requires the airplane to be free from control reversal within the design envelope (CS-25.629 mirrors it).
• The method is the classical simplified aeroelastic estimate of the NACA TR-799 lineage; it assumes the aileron lift acts at the aerodynamic center, offset e behind the elastic axis.

Workflow

1. Collect the torsional stiffness k_t, the lift curve slope, the aileron effectiveness factor eta, the wing area S, the mean chord c, and the offset e.
2. Compute the reversal dynamic pressure with reversal_dynamic_pressure.
3. Convert it to the reversal speed with reversal_speed at the flight density, or use reversal_speed_from_stiffness for the direct answer.
4. Compute the effectiveness fraction at the flight dynamic pressure q with aileron_effectiveness.
5. Check the verdict with is_reversed against the dive speed dynamic pressure q = 0.5 rho V_dive^2.
6. If reversed, raise k_t (stiffen the wing) or reduce the offset e and re-evaluate until the dive limit clears the reversal speed.

Pitfalls

• Using the stiffness per unit span or the beam bending stiffness where the formula takes the total torsional stiffness k_t in N m / rad about the elastic axis.
• Setting eta above 1 or at 0: eta is a dimensionless effectiveness factor in (0, 1], and reversal_dynamic_pressure raises ValueError outside that range.
• Confusing the elastic axis with the aerodynamic center: e is the distance from the elastic axis to the aerodynamic center, positive when the aerodynamic center lies aft of the elastic axis.
• Quoting the reversal speed as an indicated or calibrated airspeed: V_rev is a true airspeed at the flight density rho, so it changes with altitude.
• Declaring reversal from a single negative effectiveness reading: the verdict must compare the dive speed dynamic pressure of the design envelope against q_rev.
• Mixing units: chord, area, and offset in m and m^2, stiffness in N m / rad, density in kg/m^3, angles in radians.

Behavior contract (gate 3)

The aileron reversal logic is exercised by the gate 3 contract test: scripts/test_aileron_reversal.py against scripts/aileron_reversal_logic.py (stdlib unittest, offline). Run: python3 scripts/test_aileron_reversal.py

Compliance

• Standards referenced, not reproduced: FAR-25 is US government work (public domain) and CS-25 is a free EASA download; the reversal estimate is common aeroelastic methodology in the NACA TR-799 lineage, and FAR-25.629 / CS-25.629 set the control reversal requirement, all summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.