a088008 is a lot closer to the true story than the rest of you guys.

It's where the model transitions from a high energy wind zone to a lower energy zone and gains speed by virtue of the inertia of the model reacting with the wind speed change. The model sets up a climbing and diving circle or oblique looping path that presents the wings to the wind shear area in a way such that it accelerates the model. The best spot to do this is at a sharp lip at the top of a slope where there is a severe wind direction and speed change right at the lip. The model dives into the back rotor and then shoots forward through the shear area and into the high energy area and changes it's flight attitude to gain the most speed energy out of the transition.

Done right the models easily exceed 100 mph and many flights have hit well over that up to, I seem to recall, 180 to 200 mph. This is obviously serious stuff and Gentle Lady owners need not apply....

Any of the slope racing airfoils would work well. Low camber airfoils in the 1% range combined with 7 to 8% thicknesses will really work at these speeds. I don't have my Profili at hand just now but I seem to recall that the HN or HS series of airfoils look like they would work very well at this task. There's some 1 to 1.5% camber RG and Selig airfoils that would probably work quite well too.

If no one chimes in with some more specific thoughts or choices I'll move this thread to Sailplanes.