Dick Johnson methodology for measuring glider performance

I am curious about what pilots see on their post-flight analysis programs (like SeeYou) or in flight tools (like XCsoar) for their average L/D. Does it compare well with the advertised L/D of your ship? Or is it lower (or higher)?
For me, using XCsoar, the figure displayed is often lower than stated in my manual.

On Monday, July 11, 2016 at 9:46:10 AM UTC-7, Soartech wrote:
I am curious about what pilots see on their post-flight analysis programs (like SeeYou) or in flight tools (like XCsoar) for their average L/D. Does it compare well with the advertised L/D of your ship? Or is it lower (or higher)?
For me, using XCsoar, the figure displayed is often lower than stated in my manual.

I usually set "bugs" to 6% degradation to my official polar in SeeYou. Through trial and error, this seems to match what I actually see as performance on final glides. Glider is Ventus C with winglets

Matt

On Monday, July 11, 2016 at 11:46:10 AM UTC-5, Soartech wrote:
I am curious about what pilots see on their post-flight analysis programs (like SeeYou) or in flight tools (like XCsoar) for their average L/D. Does it compare well with the advertised L/D of your ship? Or is it lower (or higher)?
For me, using XCsoar, the figure displayed is often lower than stated in my manual.

Depends on how good a pilot you are. I'm not happy unless I see interthermal L/D numbers over 60 due to flying energy lines, often much higher than that, if possible negative (gaining alt.). Best polar L/D is meaningless, we never fly straight at that speed.

Unless you fly in completely stable air, the post flight analysis L/D has more to do with the soaring conditions and how well you utilize energy lines, than glider performance.

Ramy

On Monday, July 11, 2016 at 8:22:38 PM UTC-4, Ramy wrote:
Unless you fly in completely stable air, the post flight analysis L/D has more to do with the soaring conditions and how well you utilize energy lines, than glider performance.

Ramy

Ramy, Are "energy lines" actually lift streets? That name makes it sound like some sort of mystical experience.

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!).

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