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#1
July 15th 16, 12:03 AM
posted to rec.aviation.soaring
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Dick Johnson methodology for measuring glider performance
My "non engineering" comment is..... A similar airfoil was used on the PIK-20, SGS-135 and others.
When clean, it worked well. When dirty (bugs, dust, rain) the bottom dropped out. Common cure was: -sand to 45* of chord with 600 grit -wipe D-tube with liquid dish soap and let dry Sanding allowed a turbulent boundary layer, soap sheeted water, thus killing drag! Long final glide in a "20" with others into Mifflin, a 1-35 (highly modified) showed the drag gain when we hit rain showers, he had a tough time, we made slight adjustments to final glide. So, that airfoil can be good clean,but really sucks when dirty. No, you DON'T want a high gloss finish!!! 
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