# Autodesk CFD neutral baseline

This is an **unpowered, neutral-control, gear-down geometry baseline**, not a completed CFD study or an aerodynamically optimized aircraft.

- `airframe-neutral.step`: one connected closed aircraft solid. Control gaps, vents and hatch seams are sealed; internal electronics and propeller blades are omitted. Wing support saddles are extended to touch the wing.
- `external-air-volume.step`: surrounding air with the aircraft already subtracted. Import this alone when solving only external airflow.
- `autodesk-cfd-neutral.step`: both named aircraft and air components. Check region assignments after import; do not create another overlapping external volume.
- `candidate.f3d`: separate editable Fusion source.
- `geometry-report.json`: native Fusion measurements.
- `coarse-fluid-preflight.msh`: independent Gmsh import/meshing check; **not an Autodesk CFD mesh**.

## Import and test

Use a supported Windows installation of Autodesk CFD. Import STEP, confirm aircraft span **1,400 mm** and nose-tail extent **888 mm**, and verify the air region surrounds rather than overlaps the solid. Assign air to the fluid region, aircraft surfaces as stationary no-slip walls, a chosen inlet velocity, and an outlet pressure. Set far-field side/top/bottom conditions appropriate to an external-flow case. Record temperature, density, viscosity and reference area/chord; do not infer performance from geometry alone.

Create local refinement at leading/trailing edges, wing-body junctions, struts and wake; add suitable near-wall layers. Run at least three progressively refined meshes and check force/moment convergence, conservation and sensitivity to domain size and turbulence treatment. The independent coarse mesh has no wall layers and its worst element quality is low, so it is unsuitable for reporting lift/drag.

The supplied domain extends from X=-4,250 to 9,350 mm and Y/Z=±3,500 mm. The independent import found one fluid volume and no free boundary curves; a coarse 90,133-tetrahedron mesh had positive minimum SICN quality 0.02369. No Autodesk CFD solve, CL/CD result, powered rotor analysis, stall assessment or flight validation has been performed. Deflected surfaces and powered VTOL require separate cases; these sealed neutral surfaces cannot represent them.

Official references: [supported CAD/STEP](https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/Simulation-CFD-Supported-CAD.html), [external volume](https://help.autodesk.com/cloudhelp/2026/ENU/SimCFD-UsersGuide/files/GUID-337A13C8-EC51-4AB3-BB83-A37A007CD65C.htm), [system requirements](https://help.autodesk.com/cloudhelp/2026/ENU/SimCFD-Install-Notes/files/GUID-A15C448E-107C-447F-9F6D-0A585D5058C4.htm).