Samuel wrote-
>has anyone used thinner than 15%. Would thinner mean more speed? I don't know about the speed, but I can tell you that there was a considerable amount of credible and documented (and flight-proven) study and research done to arrive at the AS50xx-series of airfoils that are specifically tailored to the KR2/2S. Although you may get higher speed by tweaking the airfoil to get lower induced drag, you'll be giving up something else in the exchange, either in stall speed/sharpness, higher approach and landing speed as a result, or something else. It sounds like you're already familiar with the development of the AS50xx airfoils, but you may want to revisit the design goals and achievements here- http://www.krnet.org/as504x/ . As to using a thinner wing, I believe one of the objectives of the AS airfoil development was to utilize the stock KR spars so that there would not need to be a whole new structural analysis done and so that the airfoil could be retrofitted to existing KRs without modifying the spars. Bear in mind that the RAF48, the original KR/KR2S airfoil, is a 15% (actually closer to 14.9%, but you get the idea), and that the strength of a spar in bending generally goes as the square of its height (tallness). As an example using numbers that are easy to deal with, let's say that a box beam spar is 5" tall and 1" wide and it's constructed in such a way as to result in its having a section modulus of 4 in^3. If we reduce its height by 1", all else being equal, the section modulus is reduced to 2.56 in^3 or a reduction of 36%... more than a third. Just to continue down this theoretical line, let's say the 15% airfoil spar was designed for Normal category, +3.8G, with a 50% margin of safety (5.7G ultimate). If you reduce that by 36% you now have a spar that is at ultimate loading at just about 3.7G and your margin of safety is gone. You'll want to redesign the shallower spar to try to restore the margin of safety. Disclaimer: the numbers above are simplified, they are not specific to the KR box spar, but I hope I've made the point. Oscar Zuniga Medford, OR ________________________________ _______________________________________________ Search the KRnet Archives at https://www.mail-archive.com/[email protected]/. Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change options. To UNsubscribe from KRnet, send a message to [email protected]
