Rotate a twisted section's chord fully about the spanwise axis

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+## VortexStepMethod v4.1.1 2026-08-19
+
+### Fixed
+- Section twist now rotates the chord fully about the spanwise axis. The axial
+  Rodrigues term was missing, so a swept or dihedral section's along-span chord
+  component was scaled by `cos(theta)`: the chord shortened and tilted out of the
+  twist plane, growing as `theta^2`.
+
 ## VortexStepMethod v4.1.0 2026-08-17
 
 ### Added

Project.toml

@@ -1,7 +1,7 @@
 name = "VortexStepMethod"
 uuid = "ed3cd733-9f0f-46a9-93e0-89b8d4998dd9"
 authors = ["1-Bart-1 <bart@vandelint.net>", "Oriol Cayon and contributors"]
-version = "4.1.0"
+version = "4.1.1"
 
 [workspace]
 projects = ["examples", "docs", "test"]

src/wing_geometry.jl

@@ -634,6 +634,10 @@ end
 
 Rotate each refined section's TE point around its LE point by the given per-section
 twist angle, using `wing.non_deformed_sections` as the reference geometry.
+
+The rotation is a full Rodrigues rotation about the spanwise axis. A swept or
+dihedral section has a chord component along that axis, and dropping the axial
+term would scale it by `cos(theta)`, shortening the chord and tilting it.
 """
 function _apply_refined_section_thetas!(wing::Wing{P, T}, section_thetas) where {P, T}
     local_y = zeros(MVector{3, T})
@@ -677,8 +681,10 @@ function _apply_refined_section_thetas!(wing::Wing{P, T}, section_thetas) where
 
         chord .= section.TE_point .- le
         normal .= chord × local_y
+        axial = local_y ⋅ chord
         @. wing.refined_sections[i].TE_point = le +
-            cos(theta) * chord - sin(theta) * normal
+            cos(theta) * chord - sin(theta) * normal +
+            (1 - cos(theta)) * axial * local_y
     end
     return nothing
 end

test/yaml_geometry/test_yaml_wing_deformation.jl

@@ -271,4 +271,20 @@ using Test
         @test wing.refined_section_weight[1] ≈ 1.0
         @test wing.refined_section_weight[end] ≈ 0.0
     end
+
+    @testset "Twist rotates a swept chord rigidly" begin
+        # A swept or dihedral section's chord has a component along the twist
+        # axis, which only a full rotation carries through unchanged.
+        wing = Wing(8; n_unrefined_sections=3)
+        add_section!(wing, [0.0, 5.0, 1.0], [1.0, 4.6, 1.0], INVISCID)
+        add_section!(wing, [0.0, 0.0, 0.0], [1.0, 0.0, 0.0], INVISCID)
+        add_section!(wing, [0.0, -5.0, 1.0], [1.0, -4.6, 1.0], INVISCID)
+        refine!(wing)
+        chords = [norm(s.TE_point - s.LE_point) for s in wing.refined_sections]
+        for theta in (0.05, 0.2, 0.5)
+            VortexStepMethod.unrefined_deform!(wing, fill(theta, 3), nothing)
+            twisted = [norm(s.TE_point - s.LE_point) for s in wing.refined_sections]
+            @test maximum(abs.(twisted .- chords) ./ chords) < 1e-12
+        end
+    end
 end