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!!!)