Building a Flyable Airplane Entirely Out of Cardboard

Cardboard Beam Strength Testing

• 🔬 Cardboard beam testing was conducted to determine the optimal flute orientation for structural integrity.
• ⚠️ Vertical flutes resulted in the weakest beams, collapsing at approximately 4 cups of sand, due to crushing and tearing.
• 📈 Horizontal flutes demonstrated significantly more strength, holding up to 11.2 lbs of sand before failure.
• 📊 Mixed flutes (two horizontal, two vertical) performed comparably to horizontal flutes, failing at 9 cups of sand.
• 💡 Cap strips added to the top and bottom of beams did not significantly improve strength and were deemed ineffective for this application.

Wing Spar and Rib Construction

• 🛠️ Based on testing, the spar was constructed using horizontal flutes for maximum strength.
• 🚀 The spar, measuring 13.5 ft long, was reinforced with glued-on cap strips and hot glue for assembly.
• 🧩 Ribs were initially designed with a tab-and-notch system, but this proved weak due to cardboard's fluted nature.
• ✨ A revised rib design, incorporating doublers on the inside and a truss-like structure, significantly increased sturdiness.
• ⚙️ Laser cutting was instrumental in efficiently producing a dozen identical ribs in approximately 12 minutes.

Wing Assembly and Testing

• 🧱 The wing assembly involved gluing the ribs to the spar, with hot glue used for its filling capabilities and speed.
• ⚖️ The completed wing, before covering, weighed approximately 12.6 lbs.
• 📊 Initial load testing revealed weaknesses, with duct tape failing at around 100 lbs and the rear spar buckling at 180 lbs.
• 🛠️ Improvements were made, including reinforcing the rear spar with an I-beam design to address compression failure.
• 📈 Further testing with string and improved rigging showed the wing could withstand approximately 170 lbs before significant deflection and eventual failure.

Covering and Final Weight

• 💨 Cardboard skin was initially considered but found to be too heavy, adding an estimated 6-9 lbs per wing.
• 📄 The final covering strategy involved using brown craft paper sprayed with water to shrink and adhere, mimicking airplane covering techniques.
• ⚖️ The completed, paper-covered wing weighed 21.8 lbs, totaling 44 lbs for both wings.
• ✈️ This weight is considered heavy for an ultralight, with a target of 14-16 lbs per wing for traditional designs.
• ✅ Despite the weight, the wing is deemed structurally sound enough for a low-power, low-altitude flight.