fix: reject one- and two-bladed rotors in the auto-RNA inertia path

The rotor tensor uses the axisymmetric transverse split I_diam = I_polar/2
+ N*integral(dm*a**2), which is exact only for three or more evenly spaced
blades, whose mass ring has isotropic in-plane inertia. A one- or two-
bladed rotor lies on a single diameter, so its transverse inertia is
azimuth-dependent and no single static tower-top lump represents it (one
in-plane axis carries almost all of I_polar, the orthogonal one almost
none). Rather than emit a corrupted fore-aft / side-side inertia, reject
those blade counts with a message pointing at an explicit tip_mass. Zero
blades (bare tower) and the usual three-bladed rotors are unaffected.

Author: SMI-Lab-Inha

src/pybmodes/io/windio.py

@@ -1093,6 +1093,21 @@ def _nac_geom(key: str) -> float:
             f"assembly.number_of_blades must be a non-negative integer; got "
             f"{n_blades!r}."
         )
+    if n_blades in (1, 2):
+        # The rotor tensor uses the axisymmetric transverse split
+        # ``I_diam = I_polar/2 + …``, which is exact only for three or more
+        # evenly spaced blades (a symmetric mass ring has isotropic in-plane
+        # inertia). A one- or two-bladed rotor lies on a single diameter, so
+        # its transverse inertia is azimuth-dependent and no single static
+        # tower-top lump represents it. Reject rather than emit a corrupted
+        # fore-aft / side-side inertia (issue #130 review).
+        raise ValueError(
+            f"assembly.number_of_blades = {n_blades}: the auto-RNA rotor "
+            f"inertia assembly assumes an azimuthally symmetric rotor (three "
+            f"or more evenly spaced blades). A one- or two-bladed rotor has an "
+            f"azimuth-dependent transverse inertia that a single rigid tower-"
+            f"top lump cannot represent; pass an explicit tip_mass instead."
+        )
     orientation = str(assembly.get("rotor_orientation", "upwind")).strip().lower()
     if orientation not in {"upwind", "downwind"}:
         raise ValueError(

tests/test_windio_rna.py

@@ -200,6 +200,22 @@ def _mk(cone: float):
     assert coned.mass == pytest.approx(flat.mass)
 
 
+@pytest.mark.parametrize("n_bl", [1, 2])
+def test_rna_rejects_one_or_two_bladed_rotor(
+    tmp_path: pathlib.Path, n_bl: int
+) -> None:
+    """A one- or two-bladed rotor is not azimuthally symmetric, so the
+    ``I_polar/2`` transverse split is invalid and the auto-RNA rejects it
+    with a message pointing at an explicit tip_mass (Codex review on #130)."""
+    pytest.importorskip("yaml")
+    from pybmodes.io.windio import read_windio_rna
+
+    o = _base_ontology()
+    o["assembly"]["number_of_blades"] = n_bl
+    with pytest.raises(ValueError, match="azimuthally symmetric"):
+        read_windio_rna(_write(o, tmp_path, f"nb_{n_bl}.yaml"))
+
+
 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