On Thursday, 8 October 2020 at 19:39:27 UTC+1, Eric Greenwell wrote:
I was talking about comparison flights to determine performance relative to their
competitors, not absolute L/D measurements. If the JS1C is really a 63:1 glider,
they can relatively easily determine that it's significantly better than an ASG29,
Ventus 3, ASH 31. etc, by borrowing several of those gliders and doing the tests.
They do not need a calibrated glider for this type of test.

Do you find it easy to believe Jonkers calculations of max L/D are wrong and low
by 5%? I think they would have corrected their calculations by now if they
believed the Idaflieg's measurement of 63, so that the numbers for the JS2 would
be accurate, yet that is specified at only 63.
Tango Whisky wrote on 10/8/2020 8:45 AM:
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.

Bert
Ventus cM "TW"
Ex-Akaflieg Braunschweig

Le jeudi 8 octobre 2020 Ã* 16:17:30 UTC+2, Eric Greenwell a écrit :
It is very hard for me to believe Jonkers calculations are in error by 5% (which
is a lot!), so I suspect the error is the Idaflieg measurement. Jonkers can, and
likely has, easily do comparison glides itself, to confirm the performance of
their gliders. Since they stay with the 60:1 specification, why not accept their
numbers?
Which gliders have wildly optimistic best L/Ds?
--
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

I don't understand the focus on best LD but when you are dealing with very high performance gliders the difference in sink rate for a difference of 3 points is tiny. Even so why compare a 60:1 (or ?63:1) 21m JS1c with an 18m V3 or ASG29? Judging from Open Class contest results its only relevant competitor is the EB29