Dick Johnson methodology for measuring glider performance

On Tuesday, July 12, 2016 at 7:45:32 AM UTC+1, 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. They are just a few cm/s - so if you try to measure a 1:30 or 1:35 ship, that's not a big deal.

Now, if you are measuring a ship with 1:45, a 5 cm/s air mass movement will give you an error of 7%, or about 3 points.

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

Instead, we have the Idaflieg (which is kind of the federation of Akafliegs) doing their measurements in collaboration with DLR (the German Research Institute for Aerospace). These measurements are based on two key points:
One is a calibrated (the "sacred") glider, which has been an Open Cirrus, then a DG300-17 and now I think it is a Discus 2/18. This glider is measured ynd calibrated extensively (takes about 1-2 years).
Second key point is that the glider to be measured flies in parallal to the calibrated glider, and in the basic form of the measurement, pictures of both gliders are taken after each intervall of flying at a defined speed. By using the (precisely known) fuselage length of the calibrated glider, you can get the vertical distance between the gliders very precisely.

This method gets rid of any air mass movement and is very accurate. It comes at a steep cost though: a calibrated glider (which is used for nothing else), two tow planes to FL120 at 5 p.m., and one of the tow planes flying alongside the gliders during the descent.

Bert
Ventus cM "TW" (measured by Idaflieg to 1:47 ;-) )

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.

John Galloway

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.

It worked the other way, too. When DJ tested the ASW 20 and LS-3 (not the later LS-3a), they both had remarkable, essentially equal performance. This was confirmed by those who flew the two gliders in that first year or two (ours was delivered in 1978). For a brief period, they were equally favored by competition pilots in the U.S.

Then the rush to the ASW 20 began. At first this was a bit baffling. Except for the higher wing weight of the LS-3, what was there about the '20 that seemed to capture so many pilots' favor? Then it became evident that the two types were no longer equal. They still climbed together but the '20 had an advantage in glide.

I profiled the top surface of my LS-3 wings and discovered a "flat spot" where post curing had apparently shrunk the wing over the spar cap. Building up this flat spot fully restored the glide performance, equally dramatically compared with other types as well as other LS-3s. A few owners (e.g., Jim Cox, IIRC) went even further, building up the leading edge where the profile was apparently a bit too blunt compared with the published coordinates as well as fixing the flat spot, with similarly impressive results.

But by that point, not only had time passed the LS-3 by but DJ's tests of the LS-3a seemed to indicate that the later version had inferior glide performance right out of the box, allegedly because of a thicker profile caused by the molds not being completely stable (see heated exchange of letters in "Soaring" mag about that time). My own impression was that the early LS-3a gliders were quite good, but that impression didn't last.

In any case, the reputation of the LS-3 remains today as inferior to the original ASW 20a despite DJ's original published test.

Chip Bearden
ASW 24 "JB"

Before Richard Johnson's passing, I was communicating with him regarding a glider performance test (an ASW20) and he was kind enough to send me some Excel spreadsheets showing the parameters that he was focusing on. As expected, he used KTAS for the glide performance calculations. What caught me by surprise was his glide calculations used computed TRUE SINK as well. This might have made sense to me if he was measuring vertical speed with a rate instrument but unless I am mistaken, he measured sink rate using an altimeter and a stop watch.

I did not get the chance to ask him about this.

Nevertheless, he is missed.
Paul
ZZ

That is all well and good if you are in Europe, have connection to Akaflieg and want to spend a lot of money.
I did order the results for my Pilatus B4, and the cost was 10 euros. I do not call that "a lot of money" :-)

On Tue, 12 Jul 2016 10:04:30 -0700 (PDT),
wrote:


It worked the other way, too. When DJ tested the ASW 20 and LS-3 (not the later LS-3a), they both had remarkable, essentially equal performance. This was confirmed by those who flew the two gliders in that first year or two (ours was delivered in 1978). For a brief period, they were equally favored by competition pilots in the U.S.

Then the rush to the ASW 20 began. At first this was a bit baffling. Except for the higher wing weight of the LS-3, what was there about the '20 that seemed to capture so many pilots' favor?

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


Cheers
Andreas

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.

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!!!