Very interesting information Bert, thanks for sharing.
Mana

On Thursday, October 8, 2020 at 5:45:36 PM UTC+2, Tango Whisky wrote:
If you think that inflight measurements are easy to make, you are greatly mistaken.
Idaflieg has refined its procedure for over 40 years, and it is a huge effort:
You have to tow the glider to be measured, and a calibrated reference glider (formerly an Open Cirrus, then a DG300 modified to 17 m, now I think a Discus 2c 18m) in parallel up to 10'000+ ft very early in the morning on a day without convection and fly in free flight at the various speed points. One of the tow planes takes pictures along, with the refernce glider's fuselage length as the measure for the difference in altitude which will gradually develop, and the reference glider's polar as the base line. Typically, multiple flights on different days are conducted in order to get a decent set of data points. Nowadays, this is assisted by differential GPS data.

If you try to do measurements without a reference glider, you can't avoid air movements influencing the measurement (on a high pressure day without convection settled in, the airmass is sinking ever so slighly, and 3 cm/s will make for a huge difference at an L/D of 60.

So no, I wouldn't distrust the Idaflieg measurements, but I would distrust hearsay of the results of those measurements.

Idaflieg will never publish data for a specific reason: Such data could be used for commercial purposes by the manufacturer, or by their competitors (in the 80's, Nimbus 3 ans ASW22 have never been measured because the result would have risked to put one of the companies out of business). Using data for commercial purposes would lead to a situation were 5 years down the road, manufacturers would be very reluctant to put their gliders at disposition for measurements (and they are usually put at disposition by the manufacturers to give them a chance that this would be the serial number with the best shape ever). Idaflieg is about science, not publicity, and its driven by the students of the various Akafliegs.

And yes, you can contact them on their webpage and inquire about specific models, and they'll send you a hardcopy for 10 Euros per set. You are not allowed to spread it - they have no handle on that, but if it happend, nobody would get any copies anymore.