Assumptions with spar sizing:

• MTOW of 55 lbs
• 3.8 g’s
• safety factor of 1.5
• Wing span of 120 in
• Main wing generates entirety of total lift
• Elliptical lift distribution
• Main spar handles all of the bending and shear loads on the wing
• Made of 6061 aluminum
  • Tensile/compressive/bending yield strength of 40000 psi
  • Shear strength of 21000 psi

Checking a Vertical Spar Height of 0.75 in (the smallest square dimensions)

Inputting these parameters into a MATLAB code, guessing a vertical spar height of 0.75 in, yields the following results:

Assuming a thickness of 0.125 in (using https://www.metalsupermarkets.com/product/aluminum-channel-6061/ as a reference), a flange cross-section of 0.0863 in^2 correlates to a flange length of 0.68 in. Because these aluminum channels typically come in a square form, this means that the 0.75”x0.75”x0.125” aluminum trim channel is a valid spar for our assumptions because the necessary flange length of 0.68” is marginally less than the unmodified 0.75” from this commercially available option.

Mass Estimation for this Spar

Using the basic mass estimation process outlined in Spar Aluminum Trim Channels, this spar would have a linear mass density of 7.5E-4 slug/in, a linear weight density of 0.0241 lb/in, and a total weight of 2.895 lb if it covers entire 120 in span.