Moulds for 70's gliders were not very stable, first serial numbers might have FX67-K170 and last ones "-K190" because of the deformation.


On Monday, 28 January 2019 15:47:27 UTC+2, Mike C wrote:
On Monday, January 28, 2019 at 1:17:19 AM UTC-7, 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

It may not be Eppler vs. Wortmann but Eppler vs. Drela.

Grob gliders used Eppler airfoils so I doubt (guessing) that it was a response for a better airfoil than the FX series airfoils you mentioned.

Another point to consider, if a person uses Dr. Eppler's 'Profil' program they would get different results than they would using Dr. Drela's 'XFoil' program. Martin Hepperle has a short comparison of the two programs that mentions a potential problem with leading edge analysis when using XFoil.

https://www.mh-aerotools.de/airfoils/index.htm

To add to the reality of 1960 era airfoils, airfoil accuracy in the 60' and 70's seems to have been a hit and miss proposition. When I tried to fit computer generated CNC cut templates on a PIK 20B, at no station was the airfoil close to being accurate. Using a station template on a profiled Mini Nimbus showed a very accurate FX 150K profile, but the template when put on a Mosquito, with the original un-profiled wing did not fit at all.

You may find it interesting to read the Johnson articles on modifying his PIK 20 to the proper leading edge profile, to get a better picture of the problem and resulting performance improvements.

Airfoil analysis may be one of the dark arts.

Mike