Folding. The sheet is a set of rigid flat faces. A fold line drawn in top view defines a vertical cutting plane; every face it crosses is split, the paper on the chosen side is grouped into connected flaps (by connectivity on the unfolded sheet), and the flaps you pick rotate about the hinge. Flaps still attached to the other side by an earlier crease are blocked because they would tear. Rotate the model to fold along other axes.
Imperfection. Every fold is replayed with seeded noise: the line shifts and skews slightly, partial folds miss their angle, and flat folds spring back a degree or two. The same design folded with a different seed flies differently.
Baking. Stacked layers are merged into lifting surfaces (surfaces meeting at a shallow crease, like a wing and its elevon, share one continuous lattice) and rasterised into roughly 500 vortex-ring panels. For each wind direction (59 angles of attack × 27 sideslip angles) a vortex-lattice system is solved for attached flow, blended above a Reynolds-dependent stall angle with a flat-plate separated-flow model, plus laminar skin friction. Friction and stall depend on Reynolds number, so tables are baked at 8 airspeeds (1–20 m/s). Forces are stored per surface zone, so each zone sees its own local airflow in flight. That gives roll, pitch and yaw damping.
Flight. A 6-DOF rigid body (mass and inertia from the paper layers plus clip) is integrated at 2 kHz with the baked tables. Every paper vertex is a contact point against flat concrete, with restitution and Coulomb friction (μ≈0.6).
Keys: T toggle top/3D · Enter fold · F other side · Esc cancel · Ctrl+Z undo · Space throw