OK, time to fess up..

A couple weeks ago I was getting ready to begin making my rib stiffeners and as I was preparing to, there was an article in Kitplanes magazine about scalloping non-critical parts for weight savings
https://www.kitplanes.com/building-the-750sd-xtreme-part-5/
After reading it, I built a test piece and figured that I could save a bit over a pound of weight by doing that for all of the .025 rib stiffeners without affecting the strength. With the goal being to build as light as practical it seemed like a great idea. Over a three day weekend I built all of the .025 stiffeners with scallops.

My initial thinking was that it should not affect the strength of the parts since the scalloped leg was riveted to the rib. Well, not so fast... The more I thought about the mechanics of it, the more I began to doubt the advice in the article and my initial thinking. Since the stiffener angles are an L shape and we are riveting along one leg of the L offset from the bend it is going to impart a rotational moment along the rivet line as the rib flexes with the wing. This in turn is going to force the length of the stiffener to change to match which will impart a stress concentration in the valley of the scallop.... And thus, increase the likelihood of a crack forming....

So based on that, I started digging into it further and asking myself the question, why, if the only obstacle to scalloping is the amount of labor, and Airbus and Boeing have extreme CNC capabilities, do they not do it....

From my research the reasons are threefold....

First, as I had considered, a scalloped edge cannot carry significant lengthwise tension or compression without significant increases in peak-to-valley stress concentrations - compression buckles concentrating at wave valleys.

Second, Transverse bearing-tear-out occurs on a 40 degree angles from hole center. Scalloped edges reduce tear-out strength by reducing the hole-edge-to-free-edge distance. "Analysis and Design of Flight Vehicle Structures" by E. F. Bruhn​

Third, scalloped edges are significantly longer than a straight edge and cracks tend to form at edges; so the longer the edge, the greater the potential for cracks​.

If the rib stiffeners were parts that were easily inspected and easy to replace, I would likely use them. Also, if I was building a STOL competition aircraft the reward may outweigh the risks. For my purpose, I do not think the risk is worth the reward and I am nearly finished building a full set of .025 stiffeners per the plans.

The moral of the story here, is to make sure to do the research before putting the effort into any plans deviations and if there is any question, stick to the damn plans!

If anyone is building a no-limits STOL competition BH, I have a set of rib stiffeners for you!