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Take the billow axis from the section pair, not a global projection #264

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Merged
1-Bart-1 merged 1 commit into from
Aug 22, 2026
Merged

Take the billow axis from the section pair, not a global projection #264

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9 changes: 9 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
@@ -1,5 +1,14 @@
# Changelog

## Unreleased

### Fixed
- Billowing now takes its rotation axis from the section pair itself instead of
projecting the leading-edge span vector on `spanwise_direction`. Sections run
`+y` to `-y`, so the pair already fixes the sign. The projection was decided by
3 % of the vector on a C-shaped kite's tip panels — 11 mm of span between the
outermost sections — so a deforming tip could flip that one panel's billow.

## VortexStepMethod v4.1.1 2026-08-19

### Fixed
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10 changes: 3 additions & 7 deletions src/wing_geometry.jl
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Expand Up @@ -1535,18 +1535,14 @@ function refine_mesh_by_splitting_provided_sections!(
# Apply billowing by rotating chords around LE.
if billowing_percentage > 0 && idx > start_idx
s_l = sections[li]; s_r = sections[li + 1]
if dot(s_l.LE_point - s_r.LE_point, wing.spanwise_direction) >= 0
le_neg, le_pos = s_r.LE_point, s_l.LE_point
else
le_neg, le_pos = s_l.LE_point, s_r.LE_point
end
diff_vec = le_pos - le_neg
# Sections run +y to -y, so the pair fixes the axis sign on its own.
diff_vec = s_l.LE_point - s_r.LE_point
span_len = norm(diff_vec)
y_hat = diff_vec / span_len
apply_billowing_to_pair!(
wing.refined_sections,
start_idx, idx - 1,
y_hat, span_len, le_neg,
y_hat, span_len, s_r.LE_point,
s_l.TE_point, s_r.TE_point,
billowing_percentage)
end
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