On Monday, January 28, 2019 at 3:17:19 AM UTC-5, nzhills wrote:
Hi, first time caller here, I note the first flight of the Pik-20 was in Oct-73, the Mosquito and Mini Nimbus in Sep 76. Grob came out with their Speed Astir in Apr 78. All competed in the 15 metre class. One major difference between them was the first 3 had the FX67k170/150 as a wing airfoil, whereas the last one, Speed Astir, had the Eppler 662. If you run an XFoil analysis of these two sections, (Profili provides a nice front end and visualisation tool), you'll see that the code predicts funny flows at the leading edge of the Wortmann section, (a reason for the bug problem maybe). Conversely the Eppler section is well behaved, i.e. the flows around the leading edge are well behaved. Both foils deliver very similar performance. So my question is, was the Eppler 662 directly a demand from someone asking Eppler to better the FX67k150 from Wortmann? Secondly, is one aspect of the development of airfoils simply getting laminar flow over greater and greater portions of the foil, (excepting that there are flow stability under varying conditions issues)? Regards Mark

$0.02: Both Eppler and Wortman have greatly contributed to the advancement of modern glider design with their airfoil design concepts.
Today, world class designers use 3D Computational Fluid Dynamics (CFD) using some form of Navier Stokes Equations, simulating the flow around the entire aircraft, even with the near wake, flap edge, and tip vortices.