At 02:10 PM 6/9/2006, you wrote: >I read an explanation some years ago in the KR newsletters that the >WAF are clamped to the spars and generate their holding action by >friction. The 3/16 attach bolts are stressed primarily in tension >not in sheer. >Another newsletter article reported on KR-2 main spar testing. The >ultimate breaking force on the spar was 21g. (I don't know how hard >folks fly their KRs, but 6g would be much more than enough to break >parts of me.) One surprise was: The point of failure was about 12 >inches inboard of the WAF and not at the WAF. After the test the >WAFs were dismantled and examined for sheer damage. There was no >evidence of any sheer stress on the eight WAF nor the AN3 and AN6 >bolts. The report did not say there was not any sheer force, just no >evidence. I guess they did not find any dents in the bolts.
I wrote one article that you are referring too. That was in the mid 90s I think. My analysis never stated that the primary holding action of the wing attachment fittings was due to friction. That analysis shows that the wing attachment fitting, as designed, is not as strong as the spar when taken as a whole if clamping friction is not assumed. That analysis shows that the WAF-to-wood bolt interface will fail before the spar fails if there is no clamping friction. The bolt-to-wood reaction prior to localized failure is something like 70 percent of the ultimate strength of the spar. In my opinion, this a marginal design and can be improved with only minimal re-design. In the standard KR design, the 3/16" mounting bolts do have a tensile component since they are in single shear, but the primary component of the load is shear. A better design is to use one long bolt with a compression member between the WAF pairs instead of paired short bolts, placing the joint in a balance double shear. This is my considered opinion as a professional engineer. I do not remember any article that talked about actual load testing to failure of a KR spar. If you can tell me where that is I would like to read it. I do have the KR newsletters on CD but I do not remember it. Don Reid - donreid "at" peoplepc.com Bumpass, Va Visit my web sites at: AeroFoil, a 2-D Airfoil Design And Analysis Computer Program: http://aerofoilengineering.com KR2XL construction: http://aerofoilengineering.com/KR/KR2XL.htm Aviation Surplus: http://aerofoilengineering.com/PartsListing/Airparts.htm EAA Chapter 231: http://eaa231.org Ultralights: http://usua250.org VA EAA Regional Fly-in: http://vaeaa.org
