Write the panel aerodynamics once, as traceable functions

[1-Bart-1]

Why

SymbolicAWEModels re-expressed this package's per-panel force symbolically so it could carry it in an ODE right-hand side — the airfoil axes, the chord blend, the angle of attack, the lift and drag directions, the force and the pitching moment, all transcribed by hand into ModelingToolkit equations, plus twins of flow_curvature_cm and section_pitch_rate. Two spellings of one derivation, kept in step by hand. The recent billow-axis fix (#264) is exactly the kind of edit that changes one and not the other.

There were three copies inside this package too: update_panel_properties! built the axes, calc_forces! built the force directions, and calculate_results built them again.

What

src/panel_aerodynamics.jl holds that algebra once, as pure functions of the section geometry and the flow:

panel_axes(le_1, te_1, le_2, te_2, chord_weight, orient)
panel_inflow(axes, va_1, va_2, v_ind, dva_1, dva_2, deficiency)
panel_force_directions(axes, alpha_dir, spanwise)
panel_loads(axes, dirs, q_dyn, cl, cd, cm, scale)

plus panel_span_vector, panel_chord, panel_chord_weight, dynamic_pressure, panel_moment, panel_couple_force, effective_alpha, smooth_norm, and the relocated flow_curvature_cm / section_pitch_rate.

They are generic in the number type, free of in-place buffers, and branch-free apart from two ifelse guards, so one definition serves a Float64 solver, a ForwardDiff.Dual linearization, and a symbolic trace. update_panel_properties!, init_pos!, calc_forces! and calculate_results all call them.

The companion SymbolicAWEModels PR (OpenSourceAWE/SymbolicAWEModels.jl#284) deletes its transcription and calls these instead.

Numerical equivalence

Measured against main on a 12-panel POLAR_VECTORS wing over three inflow directions — 55 stored distributions plus the panel geometry, compared element by element:

fields compared	55
bitwise identical	47
differing	8
max relative difference	2.1e-16 (≈ 1 ulp)

Panel geometry (x_airf/y_airf/z_airf/chord/width/aero_center/control_point) is bitwise identical, which is the check that panel_axes and its offset chord blend reproduce update_panel_properties! exactly.

The eight that differ are lift_dist, drag_dist, panel_moment_dist, moment_dist, moment_coeff_dist, f_body_3D, m_body_3D and moment. The cause is reassociation, not a behaviour change: moment_dist was (f·ẑ·arm + m)·width and is now (f·width)·ẑ·arm + m·width, and the dynamic pressure is now factored out of the lift/drag products instead of being multiplied through. Out-of-polar-range inflow produces NaN in exactly the same places as before.

Allocations

	main	this branch
calc_forces!	0 B	0 B
solve_base! / solve!	1472 B	1472 B
calculate_results (via solve)	12928 B	12928 B
update_panel_properties!	64 B	0 B
reinit!(body_aero)	4208 B	4144 B

The two improvements come from the chord no longer subtracting MVectors.

Notes

• section_pitch_rate gains a three-argument form taking the trailing minus leading edge apparent wind directly, which is the shape a symbolic consumer has. The four-argument form is unchanged.
• effective_alpha and panel_inflow's deficiency argument carry an unsteady lag into the angle the polars are read at. Nothing in the solver has unsteady state to feed them; they are here so a symbolic consumer that does reads the same definition rather than growing its own.
• Norms inside the shared functions are floored (smooth_norm, 1e-12 m) instead of branch-guarded. flow_curvature_cm and section_pitch_rate keep an ifelse guard: a zero v_rel or zero chord must still report zero, and dropping that guard made a zero-chord section return -1e12 — caught by test_flow_curvature.
• update_panel_properties! previously indexed out of bounds for a single-panel wing; panel_chord_weight returns 0.5 there instead.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/panel_aerodynamics.jl	94.23%	3 Missing ⚠️
src/panel.jl	71.42%	2 Missing ⚠️
src/wing_geometry.jl	95.65%	1 Missing ⚠️

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