Dick Johnson methodology for measuring glider performance

I guess people found out about the extreme loss of performance of a
wet or dirty LS-3 and spreat the word...


Cheers
Andreas

Indeed, many gliders from that era that used the same family of airfoils suffered thusly, perhaps none so reputed as the PIK 20. But I think the impact here in the U.S. was more from occasional rain than from the bugs that seem more a problem in Europe. Those of us who campaigned LS-3s here often left the wings sanded to a satin finish--without wax--so the rain would spread rather than bead up, which seemed to help significantly. 400 grit seemed about right although one top pilot left his at 220 grit--and joked about having to sand the occasional bugs off each day.

Just before I sold mine, I contoured the wings and polished them up, then flew a last contest that took me into rain one day. Nothing evil happened. I'd since heard from a PIK driver that using more positive flap than usual and flying a bit slower helped cure the "falling out of the sky" phenomenon for which PIKs were infamous. It seemed to work well for the LS-3, too.

To the question of CG position, I flew mine with the CG at the aft end of the range. That seemed to yield the best performance, the only negative being somewhat more pitch sensitivity in rough thermals.

Chip Bearden
ASW 24 "JB"

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!!!

Originally Posted by

I guess people found out about the extreme loss of performance of a
wet or dirty LS-3 and spreat the word...


Cheers
Andreas

Indeed, many gliders from that era that used the same family of airfoils suffered thusly, perhaps none so reputed as the PIK 20. But I think the impact here in the U.S. was more from occasional rain than from the bugs that seem more a problem in Europe. Those of us who campaigned LS-3s here often left the wings sanded to a satin finish--without wax--so the rain would spread rather than bead up, which seemed to help significantly. 400 grit seemed about right although one top pilot left his at 220 grit--and joked about having to sand the occasional bugs off each day.

Just before I sold mine, I contoured the wings and polished them up, then flew a last contest that took me into rain one day. Nothing evil happened. I'd since heard from a PIK driver that using more positive flap than usual and flying a bit slower helped cure the "falling out of the sky" phenomenon for which PIKs were infamous. It seemed to work well for the LS-3, too.

To the question of CG position, I flew mine with the CG at the aft end of the range. That seemed to yield the best performance, the only negative being somewhat more pitch sensitivity in rough thermals.

Chip Bearden
ASW 24 "JB"

Hi

FWIW I used to have a share in a Janus and I too found that having some extra positive of flap and flying a little slower helped to avoid the plummeting when wet. No hard figures to verify that but the airframe shaking went away as well so more comfortable for the poor old pilot.

:-) Colin