fix: include prebend and sweep in the rotor polar lever

The rotor polar moment took the section lever from the z span alone, so a
swept or prebent blade was undercounted: the mass integration follows the
full 3-D reference-axis arc, but the lever ignored the in-plane tangential
sweep and the prebend projection. Resolve each section onto the rotor axes
instead: in-plane radial r = hub_r + z*cos(cone) - x*sin(cone), tangential
t = y, axial a = z*sin(cone) + x*cos(cone). The polar lever is then the
full in-plane distance from the shaft axis r**2 + t**2, and the axial term
carries the prebend projection too. A straight blade (x = y = 0) is
unchanged.

Author: SMI-Lab-Inha

src/pybmodes/io/windio.py

@@ -843,9 +843,11 @@ def _blade_span_mass_inertia(
     (prebend / sweep) are optional and default to zero (a straight span).
 
     Returns the single-blade mass, the polar second moment about the rotor
-    axis ``∫ (dm/ds) · r(s)² ds`` (radial ``r = hub_r + z·cone_cos``), the
-    axial second moment ``∫ (dm/ds) · a(s)² ds`` (axial offset
-    ``a = z·cone_sin``), and the axial first moment ``∫ (dm/ds) · a(s) ds``.
+    axis ``∫ (dm/ds) · (r² + t²) ds`` (in-plane radial
+    ``r = hub_r + z·cone_cos − x·cone_sin`` and tangential ``t = y`` so
+    prebend / sweep are included), the axial second moment
+    ``∫ (dm/ds) · a(s)² ds`` (axial offset ``a = z·cone_sin + x·cone_cos``),
+    and the axial first moment ``∫ (dm/ds) · a(s) ds``.
     The caller builds the rotor's polar inertia from the second radial
     moment, the coned transverse (diametral) inertia from the radial and
     axial second moments, and places the rotor body at its coned centre of
@@ -922,14 +924,21 @@ def _blade_span_mass_inertia(
     seg = np.linalg.norm(np.diff(xyz, axis=0), axis=1)
     s = np.concatenate([[0.0], np.cumsum(seg)])
     mass = _trapezoid(mpl, s)
-    # The spanwise position resolves into a radial distance from the rotor
-    # axis (hub radius + span*cos(cone)) and, for a coned rotor, an axial
-    # offset along the shaft (span*sin(cone)). The radial part gives the
-    # polar second moment; the axial part adds to the transverse moment.
-    span = coords[2]
-    radial = hub_r + span * cone_cos
-    axial = span * cone_sin
-    polar_second_moment = _second_moment(mpl, radial, s)
+    # Resolve each section onto the rotor axes (r = in-plane radial from the
+    # shaft, t = in-plane tangential, a = along the shaft). The reference axis
+    # gives z spanwise, x prebend (flapwise, out of the rotor plane) and y
+    # sweep (edgewise, in the rotor plane). Coning rotates the span/prebend
+    # (radial/axial) plane by the cone angle about the edgewise axis; sweep is
+    # unaffected. The polar lever is the in-plane distance from the shaft axis
+    # ``r² + t²`` (so sweep and prebend are not dropped, issue #130); the
+    # axial offset feeds the coned transverse term.
+    x_off, y_off, span = coords[0], coords[1], coords[2]
+    radial = hub_r + span * cone_cos - x_off * cone_sin
+    tangential = y_off
+    axial = span * cone_sin + x_off * cone_cos
+    polar_second_moment = _second_moment(mpl, radial, s) + _second_moment(
+        mpl, tangential, s
+    )
     axial_second_moment = _second_moment(mpl, axial, s)
     axial_first_moment = _first_moment(mpl, axial, s)
     return mass, polar_second_moment, axial_second_moment, axial_first_moment

tests/test_windio_rna.py

@@ -200,6 +200,30 @@ def _mk(cone: float):
     assert coned.mass == pytest.approx(flat.mass)
 
 
+def test_rna_rotor_inertia_includes_sweep(tmp_path: pathlib.Path) -> None:
+    """Sweep (reference_axis.y) sets an in-plane tangential distance from the
+    shaft axis, so it adds ``y²`` to the rotor polar lever. A constant
+    tangential offset leaves the span arc length (and thus the blade mass)
+    unchanged but raises the rotor inertia; a lever built from the z span
+    alone would drop it (Codex review on #130)."""
+    pytest.importorskip("yaml")
+    from pybmodes.io.windio import read_windio_rna
+
+    def _mk(y_off: float):
+        o = _base_ontology()
+        ra = o["components"]["blade"]["elastic_properties_mb"]["six_x_six"][
+            "reference_axis"
+        ]
+        ra["y"] = {"grid": [0.0, 1.0], "values": [y_off, y_off]}
+        return read_windio_rna(_write(o, tmp_path, f"sw_{y_off}.yaml"))
+
+    straight = _mk(0.0)
+    swept = _mk(10.0)
+    assert swept.mass == pytest.approx(straight.mass)  # constant offset, same arc
+    assert swept.ixx > straight.ixx
+    assert swept.izz > straight.izz
+
+
 def test_rna_coned_cm_shift_with_uptilt(tmp_path: pathlib.Path) -> None:
     """A coned rotor's CM sits off the hub apex along the (tilted) shaft, so
     with uptilt the tower-top vertical CM shifts by m_blades·offset·sin(uptilt)