I’m fairly new to surfacing in Rhino 8, working on a Blended Wing Body (BWB) light aircraft aerodynamics study project. I’m hitting a wall finding a clean workflow to blend my fuselage shoulder into the wing root.
Centerbody: Modeled in SubD for rapid form finding and fast editing revisions based on CFD data.
Wings: Lofted NURBS surfaces using an exact 2D airfoil profile.
Goal: Create a smooth, G2 (or at least clean G1) continuous wing-root transition/fillet that maintains the exact airfoil shape further outboard, exports cleanly for CFD (Flow5/OpenFOAM), and eventually yields a closed NURBS STEP file for structural CAD in Onshape.
What I’ve Tried So Far:
Converting SubD ToNurbs then BlendSrf: The dense multi-patch conversion creates a huge mismatch in control points, causing severe twisting/bulging along the blend patch.
Intersect & Trim / Pipe Fillet: Struggled with trajectory divergence, causing unwanted S bend bulges across the trimmed gap.
Pure SubD Workflow: Attempted matching edge counts and snapping SubD rings to airfoil curves, but struggled to keep the exact aerodynamic profile clean without distortion.
Pure NURBS Workflow: Failed due to not being able to recreate the complex centerbody shape (likely a problem caused by my lack of experience) and also a lack of editability later on.
I’ve attached a stripped down .3dm file with just the SubD body and the NURBS wing.
What is the industry-standard Rhino workflow for this? Is there a Grasshopper setup, a specific Rhino 8 SubD-to-NURBS technique, or a fairing/patch strategy that is suitable for this exact joint?
I assume you need to fill the gap between the SubD body and the NURBS wing.
The shape of the opening in the body is similar to but different than the airfoil profile.
SubD has a fundamental property . The curvature of the SubD body surface is ZERO normal to the open edges. This is a fundamental property of SubD surfaces.
Workflows are design specific.
I’d make the gap much larger, and consider creating the center body as a closed SubD object, converting it to NURBs and then triming the opening.
(The is a significant curvature discontinuity along the leading edge of the airfoil. Is that part of the design of the airfoil, or is it an artifact of how the airfoil profile was constructed? My experience is curvature needs to be continuous around the nose.)
As @davidcockey says, one solution is to convert the body to nurbs, trim it back to get a larger opening. You can then create blend curves across the gap and use edge surfaces to fill them.
I don’t know how precise you need to get. This isn’t quite perfect but isn’t bad for a few moments effort and probably needs control point tweaking to get much better:
I converted the subd to nurbs, then used isocurves to trim the opening back to where they offered a smooth curve (but not too close to the five surface meeting points):
I then created some longitudinal isocurves on the wings and used blend curves to bridge between them and the adjacent body surface edges, trying different isocurve positions until I got good looking blends:
@davidcockey Thank you for pointing out the zero curvature boundary property on open SubD edges and flagging the LE curvature graph. That explains why my initial BlendSrf attempts were twisting so violently. Closing the SubD body before converting to NURBS makes total sense.
@jeremy5 I really appreciate you loading up my file and demonstrating that manual patch! Seeing how you set up the 3D curves and split it into clean, 4 sided EdgeCrf patches makes the surfacing logic much clearer. Also, good catch on the 5 point star point dimple. I’ll try to trim the opening back slightly further past those meeting points to keep the blend area clean, and if that doesn’t work, I guess I will give making my own NURBS patch layout a shot!
