Openvsp Geometry

Parametric Geometry (vehicle-design/conceptual/openvsp-geometry)

Use when the task is parametric aircraft geometry modeling in the spirit of OpenVSP: build the wing, fuselage, tail and nacelle geometry from user parameters, then derive the areas, volumes and centroids that feed sizing and mass estimation.

Domain quick reference

• Wing planform: trapezoid defined by span b, root chord c_r, tip chord c_t, leading edge sweep, dihedral and twist.
• Derived wing quantities: area S = (b/2)(c_r + c_t), aspect ratio AR = b^2/S, taper ratio lambda = c_t/c_r, mean geometric chord, mean aerodynamic chord (MAC) and its span station.
• Fuselage: body of revolution through stationwise radii; volume and wetted area come from the station integration.
• Wetted areas: lifting surfaces use two sides plus a thickness allowance; the fuselage uses the surface of revolution; nacelles use a cylinder with a hemispherical cap.
• Component volumes and centroids are the mass-property-relevant outputs; wetted-to-planform ratios support drag buildup.

Workflow

1. Collect the wing parameters (span, root and tip chord, sweep, dihedral, twist) and call wing_planform.
2. Define the fuselage as stationwise (x, r) pairs and call fuselage_from_stations (or use fuselage_cylinder for a plain cylinder).
3. Add tail surfaces as small trapezoid wings and nacelles as (length, diameter) pairs.
4. Assemble everything with build_geometry to get the geometry parameter table and the derived quantities.
5. Hand the derived areas, volumes and centroids to the sizing and mass properties leaves.

Geometry formulas

• Trapezoid area: S = (b/2)(c_r + c_t).
• Aspect ratio: AR = b^2/S.
• Taper ratio: lambda = c_t/c_r.
• Mean geometric chord: mgc = (c_r + c_t)/2.
• Mean aerodynamic chord (trapezoid): mac = (2/3) c_r (1 + lambda + lambda^2)/(1 + lambda).
• MAC span station: y_mac = (b/6)(1 + 2 lambda)/(1 + lambda).
• Sweep conversion: tan(Lambda_c4) = tan(Lambda_LE) - (c_r - c_t)/(2b).
• Wetted area of a lifting surface: S_wet = 2 S (1 + 0.2 t/c).
• Fuselage segment (truncated cone): volume V = pi dx (r1^2 + r1 r2 + r2^2)/3, wetted area S_wet = 2 pi r_avg dx sqrt(1 + (dr/dx)^2).
• Nacelle (cylinder with hemispherical cap): S_wet = pi D L, V = pi D^2 L/4 - pi D^3/24.
• Component centroid: lifting surfaces sit at the MAC station, mid-chord, offset in z by y_mac tan(dihedral); the fuselage centroid follows from the station volume integration.

Worked example

Transport-class geometry: wing b = 30 m, c_r = 4.5 m, c_t = 1.2 m, LE sweep 28 deg. Then S = 85.5 m^2, AR = 10.5, lambda = 0.267, mac = 3.31 m at y_mac = 6.37 m, quarter chord sweep 24.5 deg. Fuselage stations [(0, 0.9), (12, 1.9), (24, 1.9), (34, 0.7)] give volume ~ 300 m^3 and wetted area ~ 370 m^2; the wing wetted area is ~ 2 x 85.5 x 1.024 = 175 m^2 at t/c = 0.12. These derived quantities feed the weight and drag buildup.

Pitfalls

• Tip chord larger than root chord (negative taper is rejected).
• Zero or negative spans, chords, radii or lengths.
• Fuselage stations that do not advance in x.
• Nacelle length shorter than the cap radius.
• Confusing wetted area with planform area: lifting surfaces are wetted on both sides.

Behavior contract (gate 3)

The geometry logic is exercised by the gate 3 contract test: scripts/test_openvsp_geometry.py against scripts/openvsp_geometry_logic.py (stdlib unittest, offline). Run: python3 scripts/test_openvsp_geometry.py

Compliance

• Standards referenced, not reproduced: FAR-25 and CS-25 appear as reference-only context in standards-map.yaml; the geometry methods here are common conceptual design practice, summary-only.
• compliance: STANDARDS-REF, gated: false.