Paul T wrote on 10/7/2020 10:26 PM:
At 04:12 08 October 2020, 2G wrote:
On Tuesday, October 6, 2020 at 1:58:06 PM UTC-7, Eric Greenwell
wrote:
The JS website says it's 60 for the JS1C/21M; 63 for the JS2/21M.

jfitch wrote on 10/6/2020 1:37 PM:
Link? Google produces zero results.
On Tuesday, October 6, 2020 at 1
1:00:06 AM UTC-7, Paul T wrote:
At 15:39 06 October 2020, jfitch wrote:
They are claiming 63:1, that is 7 points higher than AS claim of
56:1. I
th=
ink it is best explained by a mistake in their math. I'd be
interested in
s=
eeing the test data proving it.=20

On Tuesday, October 6, 2020 at 12:53:25 AM UTC-7, Carlo
Orsini
wrote:
Thank you for your first hand infos, those are good news for
me (I
don't
=
understand why they advertise these dimensions in a different
way).
JS2
see=
ms to be a nice project overall. Hard to me to understand
where they
strech=
ed out those +4 points of efficency in 21m, according to their
calculated
p=
olars, compared to ASH31 (yes I know that '31 profiles are a
bit
superseede=
d and the aspect ratio is a factor too but 4 points are a huge
amount!!).
Those two dimensions for the JS2 and the ASH31 are
clearly not
be
compa=
rable - one internal cockpit rim and one external I guess. The
525mm
should=
er width for the JS2 is exactly the same as the quoted figure
for the
JS1
(=
and JS3) and the JS1 cockpit roominess is fully equal to the
ASH26/31
from
=
which it was derived. I have 4 years in a JS1 followed by 2 in
the
ASH
26e
=
and they are so similar that it would be hard to know which
cockpit I
was
i=
n with my eyes closed. Looking at the JS2 cockpit photographs
it
obvious
th=
at its structural cockpit rim design is the same as the 31, 26
and
JS1.
JS
=
do not make small cockpits.
I believe the Idafleig measured a JS1C at 63:1......so the JS2
witH
its
few improvements on the JS1C should achieve that....


Someone might ask them how they came up with these L/D figures
because I
understand that they aren't from actual flight tests.

Tom


THE JS1C TO MY KNOWLEDGE HAS BEEN MEASURED AT 63:1 BY THE
IDAFLEIG IN ITS FLIGHT TESTS- PROBABLY THE MOST RELIABLE WAY
OF MEASURING A GLIDERS POLAR. (I THINK THE JONKERS BOYS
WHERE SUPRISED BY THAT FIGURE AS IT IS RARE FOR A GLIDER TO
EXCEED ITS THEOROTICAL FIGURE -BUT IT DOES HAPPEN.) ALL
MANUFACTURERS FIGURES ARE AT BEST 'THEORETICAL' UNTIL
MEASURED.................. SOME ARE WILDLY OPTIMISTIC - BUT BEST L/D
DOESNT MEAN MUCH THESE DAYS REALLY................. ITS NOT AS
THOUFGH THIS AIRCRAFT AS NOT PROVED ITSELF IN THE CONTEST
ARENA IS IT.......

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