Dick Johnson methodology for measuring glider performance

Energy lines are not necessarily "lift streets' but rather the selection of a path through the air that results in the least energy lost. Many times a reduction in sinking airmass is the gain.

Hi

Thank you to all that replied. Glider RN and a reply I have received privately, provided all the information I require. To conduct such a test is clearly an exacting endeavor, requiring a thorough preparation.

On the positive side, I have thoroughly enjoyed a flight in utterly still air ( I have not yet experienced wave flight). Also thanks Martin for suggesting Akafliegs, I am sure their method would also be informative.

Cheers

Paul

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 ;-) )

The problem with the Idaflieg/DLR measurements is that they are not simply made available to the larger public.

There seems to be a kind of secrecy agreement between the Akafliegs/Idaflieg and the (mainly German) sailplane manufacturers to keep the results under wraps for a number of years (at least two, if my information is correct), and even then, you won't find them simply in the soaring press, you'll have to ask them to the Idaflieg...

The German (and other) manufacturers no longer publish the polars of new sailplanes in the flight manual either (fear of being proven too optimistic?).. So if you want to calibrate a flight computer, you have to guess what the true polar might be.

That's why, however imperfect, the Johnson measurements have been widely disseminated in the past. They were readily available: as soon as they were published in Soaring, they were translated and published in the international soaring press... much to the chagrin of most manufacturers, because they usually were worse than the claimed values (yes, manufacturers used to put an "official" polar in the flight manual in the old days!).

On Tuesday, July 12, 2016 at 7:58:20 PM UTC+12, wrote:
The problem with the Idaflieg/DLR measurements is that they are not simply made available to the larger public.

There seems to be a kind of secrecy agreement between the Akafliegs/Idaflieg and the (mainly German) sailplane manufacturers to keep the results under wraps for a number of years (at least two, if my information is correct), and even then, you won't find them simply in the soaring press, you'll have to ask them to the Idaflieg...

The German (and other) manufacturers no longer publish the polars of new sailplanes in the flight manual either (fear of being proven too optimistic?). So if you want to calibrate a flight computer, you have to guess what the true polar might be.

That's why, however imperfect, the Johnson measurements have been widely disseminated in the past. They were readily available: as soon as they were published in Soaring, they were translated and published in the international soaring press... much to the chagrin of most manufacturers, because they usually were worse than the claimed values (yes, manufacturers used to put an "official" polar in the flight manual in the old days!).

The good news is that it is not actually necessary to know the precise performance of your particular glider or even glider type in order to fly it close enough to optimally. Any roughly similar polar (and they are all roughly similar in a given class) will do the job. The information you want is "how fast should I fly" and that is neither very critical (+/- 5 knots is fine) nor very different from one glider to another.

What does vary is the total height loss over an extended run e.g. a final glide. I don't think there's any option but to see if you consistently come out above or below your expected height loss and use that to make a correction in the programmed polar.

The air mass in an anticyclone generally sinks over its whole area. That's what generates the wind which turns clockwise around an anticyclone (northern hemisphere).

I agree that just for programming the polar into the flight computer, the actual precision does not matter much.

As to the Idaflieg measurements - there is no secrecy involved. Manufacturers put their gliders at disposition for those measurements, and no data will be published in the first two years. This stems from the times of Nimbus 3 vs ASW22 - with people looking only at best L/D, one of the two gliders being published with 2 points more would have made the other manufacturer go bankrupt.

The data is not published anywhere. If you're interested in a measurement of your type of glider, you send Idaflieg a request by email. If it exists, they will send you a paper copy, and a bill of about 10-15 Euros.

Thanks Bert

That is all well and good if you are in Europe, have connection to Akaflieg and want to spend a lot of money. I guess it may make sense to review a glider, but it simply is not practical elsewhere. So I guess if you wish to obtain a reasonable measurements you may as well use the best method available that fits the conditions where one flies. I have not yet verified your assertion regarding the error with your stated parameters regarding the vertical movement of the airmass one is flying through. Nor am I a good enough meteorologist to know how likely is it that the whole airmass would be uniformly moving up a down over a distance required to loose 1000ft over flat lands without wind that may cause sheerwave.


Finally, I would think that enough people with requisite knowledge looked at Dick J work, yet there is not a great deal of criticisms floating about. But as you say, Concordia may not be a suitable candidate.

Cheers

Paul

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 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"