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Best Overall Motorglider available today?
At 21:12 08 October 2020, Eric Greenwell wrote:
John Galloway wrote on 10/8/2020 1:09 PM: On Thursday, 8 October 2020 at 19:39:27 UTC+1, Eric Greenwell wrote: I was talking about comparison flights to determine performance relati= ve 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 th= an 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 wro= ng 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 gre= atly mistaken. Idaflieg has refined its procedure for over 40 years, and it is a hug= e 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 th= ink a Discus 2c 18m) in parallel up to 10'000+ ft very early in the morni= ng on a day without convection and fly in free flight at the various spee= d points.. One of the tow planes takes pictures along, with the refernce = glider's fuselage length as the measure for the difference in altitude wh= ich will gradually develop, and the reference glider's polar as the base = line. Typically, multiple flights on different days are conducted in orde= r to get a decent set of data points. Nowadays, this is assisted by diffe= rential GPS data. If you try to do measurements without a reference glider, you can't a= void air movements influencing the measurement (on a high pressure day wi= thout 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 dis= trust hearsay of the results of those measurements. Idaflieg will never publish data for a specific reason: Such data cou= ld be used for commercial purposes by the manufacturer, or by their compe= titors (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)= =2E Using data for commercial purposes would lead to a situation were 5 y= ears down the road, manufacturers would be very reluctant to put their gl= iders at disposition for measurements (and they are usually put at dispos= ition by the manufacturers to give them a chance that this would be the s= erial number with the best shape ever). Idaflieg is about science, not pu= blicity, and its driven by the students of the various Akafliegs. And yes, you can contact them on their webpage and inquire about spec= ific models, and they'll send you a hardcopy for 10 Euros per set. You ar= e not allowed to spread it - they have no handle on that, but if it happe= nd, nobody would get any copies anymore. Bert Ventus cM "TW" Ex-Akaflieg Braunschweig Le jeudi 8 octobre 2020 =E0 16:17:30 UTC+2, Eric Greenwell a =E9crit = : 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. Jonk= ers can, and likely has, easily do comparison glides itself, to confirm the perfo= rmance of their gliders. Since they stay with the 60:1 specification, why not = accept their numbers? Which gliders have wildly optimistic best L/Ds? --=20 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...cations/downlo ad-the= -guide-1 =20 I don't understand the focus on best LD but when you are dealing with v= ery 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 wi= th an 18m V3 or ASG29? Judging from Open Class contest results its only= relevant competitor is the EB29 =20 My mistake: I looked at the wrong chart on their website. The JS1C/21M (t= he 60:1=20 glider) should be compared to other 21M gliders, of course. I agree the f= ull polar=20 is important when estimating contest performance, but the max L/D is ofte= n a good=20 indication of the rest of the polar for modern sailplanes. --=20 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...cations/downlo ad-th= e-guide-1 As I understand it, the Js1 only really beats other gliders when the wing loading is high.At one point I was considering buying one and talking with the agent about the benefits of the 21 M tips. Their opinion was 21 M was often slower than a well blasted 18. I looked at the polars and was frankly baffled. At that time they were only fitting panels and enclosures for jet engines that still had not been certified. I opted to save a lot of money and buy a second hand Ventus2ct which I love but has hardly flown this year because of Covid.(could of rented a light twin for less by the hour!!!) |
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