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!!!

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

wrote on 7/12/2016 8:26 AM:
I don't disagree with this post but I would say that, overall, DJ's
testing was good at pointing out the gliders that performed
particularly well compared with its competitors - and this was often
confirmed by the choices of top competition pilots. Example: ASW20
versus Mini Nimbus and LS3a.

One of the flaws in DJ's method (and the Akafliegs) is it doesn't test
the performance dynamically; ie, while dolphin soaring or flying in
turbulent air and thermals.

I once noticed the superior dolphin ability of my ASW 20 vs a Venus
while we were under clouds, quickly gaining on him. As soon as we
started a 10 mile glide in the blue, he slowly rose above me, over 800'
or so by the time we got to the next cloud.

The early ASW24, and others with that generation of airfoils (like my
ASH 26E), showed the problems induced by a "flat spot" in the L/D curve
when the air isn't smooth.

For competition purposes, I suggest competitions are the best test for a
glider. For recreational purposes, ultimate performance is unimportant
compared the other features of the glider.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

On Tuesday, July 12, 2016 at 2:45:32 AM UTC-4, Tango Whisky wrote:
The method by Dick Johnson works for gliders with limited performance.
Even in "still air conditions", which are typically found in meterological
high pressure regions, there are wide-area vertical movements of air.
There is no way that the method by DJ can work them out...

In addition to airmass movement, there are a couple other serious problems
with the methods used:

1) The CG of the plane was not controlled.
- what is the effect of forward vs. aft CG on max LD?
- what popular plane was measured with a very low LD due to above?
Discuss amongst yourselves...

2) The number of (and selection of) datapoints is not adequate
Discuss amongst yourselves...



...That's why in Europe, DJ measurements are widely disregarded.

Yup.

On Thursday, July 14, 2016 at 9:00:22 AM UTC-5, Dave Nadler wrote:
On Tuesday, July 12, 2016 at 2:45:32 AM UTC-4, Tango Whisky wrote:
The method by Dick Johnson works for gliders with limited performance.
Even in "still air conditions", which are typically found in meterological
high pressure regions, there are wide-area vertical movements of air.
There is no way that the method by DJ can work them out...

In addition to airmass movement, there are a couple other serious problems
with the methods used:

1) The CG of the plane was not controlled.
- what is the effect of forward vs. aft CG on max LD?
- what popular plane was measured with a very low LD due to above?
Discuss amongst yourselves...

2) The number of (and selection of) datapoints is not adequate
Discuss amongst yourselves...



...That's why in Europe, DJ measurements are widely disregarded.

Yup.

For reference see Paul Bikle's "Polars of Eight" in Soaring magazine.

Dave Nadler said
1) The CG of the plane was not controlled.
- what is the effect of forward vs. aft CG on max LD?
- what popular plane was measured with a very low LD due to above?
Discuss amongst yourselves...

So what is the typical percentage change in L/D from full forward CG to maximum rearward CG?
Is it a huge change (10%) or minor change? I have no idea.