Using the Building Block Approach above, I will give you more insight into my test program. Typical product testing in sporting goods industries occurs at the top, using full scale foils. But you can learn a lot, and save a lot of money, buy testing smaller specimens like those at the middle or base of the pyramid.
Fortunately, I didn’t have to do any coupon testing, because most material vendors will supply data sheets containing necessary material properties like modulus and strength. When developing a new composite material, the amount of testing required to generate necessary design values can cost years and millions of dollars, especially if it’s intended for a commercial airplane like the 787. It took Boeing well over 10 years to generate the necessary data to accurately and safely size the Dreamliner.
Moving on, element testing was critical to determining the layup of Project Cedrus. When using high temperature cure composites, warping can occur as a result of mold design or material, oven cure cycle, and/or layup. Since I rely on the flanges as bonding surfaces and indexing features for the edges, I need perfectly parallel surfaces out of the mold. To determine which layup was the optimal balance of structural performance and minimized warp, small elements were fabricated and evaluated.
Detail testing was also extensive, particularly with respect to adhesives and topcoats. I have spent a large portion of my career solving adhesives problems, getting dissimilar materials to stick together through large thermal gradients, thermal cycles, other environmental conditions, or simple structural loading is half art half science. Perhaps the most important aspect of adhesive bonding is surface prep, making sure there is no sebum (oil from your hands) or debris on the joint surfaces. Wiping with solvents like IPA or Acetone, and including chemicals like primers are equally as important as adhesive selection. With little experience in PVC polymers, I wanted to test the adhesion properties of polyurethane topcoat. To do this, I applied a thick coat to a number of small elements with different durometer PVC, and hung them from a pier for a month during the peak of Seattle summer. They got a cyclic exposure to salt water thanks to the tides, and 12+ hours of UV radiation, which can be quite damaging to some polymers.
Full scale testing was obviously extensive, and carried out over the last year. It helped me validate product reliability and durability, with lots of loadings in the car, flights, and sessions. Fuselage lengths were evaluated (another subjective variable) and critical flaws like ventilation were discovered. Structurally, testing wen’t pretty well. Admittedly, over the Holidays my stomach dropped when I got a text from Stringy that simply said “total loss of mast” while riding in La Ventana. It was followed by the below picture, as soon as the wifi could handle it.
Bonding dissimilar materials is always a challenge. You have issues like mismatched CTE (coefficient of thermal expansion), galvanic corrosion, and some adhesives simply stick well to some substrates but not the other. Aluminum and carbon joints however, have become quite common. Aluminum is a great material for aero structures like ribs and engine mounts, and commonly bonded to carbon wing skins. The most important aspect of joint design is arguably surface prep, ensuring that the substrate is free of oil from your hands, dirt, cutting fluid for the CNC machine, etc. Unfortunately in one of my prototype masts, we had clear evidence of an adhesive failure to the aluminum. Aside from introducing tighter process control at my shop, including new primers and surface pre-treatments, the next iteration of Project Cedrus will see the adhesive surface area double, and the length of the bracket increase to reduce the short couple at the joint. I am confident that this is a solvable problem, and am grateful I was made aware of the issue during prototype testing as opposed to after launching my production units.
Many thanks to Stringy’s girlfriend, who despite his sorrow and assumption that the mast was gone forever, found it snorkeling the next day in the deep water of Ventana Bay! The recovery allowed me to further understand the failure mode, and most importantly, I should be able to repair it and get the foil back in service for you soon!