NACA Wing Project
The goal of this project was to design, build, and destructively test a lightweight aluminum wing that could carry a 97 lbf load at its tip without failing, and at the same time maximizing weight. The wing compared to other wing designs made by other groups and was scored on surviving the load, minimizing mass, and minimizing tip deflection. My group's final wing weighed 1 lb 10.8 oz, held the required 97 lbf with only minor skin rippling, and ultimately failed at roughly 260 lbf holding 2.7 times the required load, with a strength-to-weight ratio of about 148.
The Design
To design the wing I started from first principles, treating the wing as a cantilever beam and using hand calculations to find the minimum spar moment of inertia needed to carry the 97 lbf tip load with a 1.5 safety factor. I compared three spar cross-sections: a single I-beam, double C-beam, and double L-beam. I selected the tapered single I-beam for its bending stiffness per unit weight, then narrowed five candidate geometries down to the lightest one that still cleared the required stiffness. From there I moved into PTC Creo, modeling the full assembly and running static and buckling FEA under the 97 lbf tip load with the root bulkhead fixed. The simulations confirmed the hand calculations and, more importantly, revealed that skin buckling would govern failure, which redirected the rest of the design. I iterated the model against those results: tightening bulkhead spacing near the root where bending stress was highest, adding stringers along the lower compression surface to shorten the unsupported skin panels, and adding a small taper at the web-flange junction to relieve a stress concentration Creo flagged at the re-entrant corner. I also added a linear spanwise taper and lightening holes to strip material from the outboard sections where the bending moment approaches zero, cutting an additional 6.2% of the weight while holding the safety factor.
Building the wing
I machined the tapered I-beam spar from 7075-T6 aluminum on a Haas 3-axis CNC mill in two operations, profiling the top flange and lightening holes first, then flipping and shimming the workpiece on aluminum spacers to keep the central axis parallel to the bed before profiling the bottom flange and tip taper. The root and tip bulkheads were waterjet-cut from 6061 stock and the 10-32 mounting holes hand-tapped, while the five interior ribs were SLS 3D-printed in PA12 nylon. I cut and hand-formed the 0.016 in and 0.025 in 6061-T6 skins with aviation snips, then riveted the skins, stringers, and ribs to the spar using aluminum blind rivets at roughly 1 in spacing, matching rivet grip length to each local stack-up.
Testing the wing
The finished wing weighed 1 lb 10.8 oz and carried the required 97 lbf with only minor rippling in the skin. This matched the FEA prediction that skin buckling would be the first thing to show at the design load. It kept holding well past that, failing at 260 lbf, 2.7 times the requirement with a strength-to-weight ratio of about 148. The failure itself came down to a manufacturing mistake. During assembly a rivet had been driven through the web of the I-beam instead of the flange. The web carries the shear load along the entire span and has no redundant load path, so the stress concentration at that hole, combined with the already-thinned cross-section from the spanwise taper, caused the early failure.
