Best Overall Motorglider available today?

On Thu, 19 Nov 2020 08:23:43 -0800, Eric Greenwell wrote:

Dave Nadler wrote on 11/19/2020 6:59 AM:
On 11/19/2020 8:35 AM, Eric Greenwell wrote:
How odd the 63:1 only occurs at the high wing loading in 21M, and not
at the minimum wing loading, where the L/D is essentially the same for
the 18M and 21M wings.

Not odd at all. Reynolds number changes make a big difference
especially with tiny chord. IIRC the original Ventus 15m increased LD 2
points with ballast, and could not change wingloading as much as modern
gliders.


This does not seem to be about wing loading, as the chart shows the
18/21 versions at the same wing loading, so I repeat: It's still odd
that adding 10' to the span does not significantly change the
performance at any speed except in the lowest few knots of the polar.

If it's Reynolds number that's important: Shouldn't the 21 M wing have
more feet of wider chord than the 18M wing, and shouldn't we expect it
to do better, just based on that?

Aspect ratio is also beneficial and will increase the overall L/D ratio
at a given airspeed if the wing section and wing loading are unchanged.


--
--
Martin | martin at
Gregorie | gregorie dot org

Martin Gregorie wrote on 11/19/2020 9:01 AM:
On Thu, 19 Nov 2020 08:23:43 -0800, Eric Greenwell wrote:

Dave Nadler wrote on 11/19/2020 6:59 AM:
On 11/19/2020 8:35 AM, Eric Greenwell wrote:
How odd the 63:1 only occurs at the high wing loading in 21M, and not
at the minimum wing loading, where the L/D is essentially the same for
the 18M and 21M wings.

Not odd at all. Reynolds number changes make a big difference
especially with tiny chord. IIRC the original Ventus 15m increased LD 2
points with ballast, and could not change wingloading as much as modern
gliders.


This does not seem to be about wing loading, as the chart shows the
18/21 versions at the same wing loading, so I repeat: It's still odd
that adding 10' to the span does not significantly change the
performance at any speed except in the lowest few knots of the polar.

If it's Reynolds number that's important: Shouldn't the 21 M wing have
more feet of wider chord than the 18M wing, and shouldn't we expect it
to do better, just based on that?

Aspect ratio is also beneficial and will increase the overall L/D ratio
at a given airspeed if the wing section and wing loading are unchanged.

Exactly! And yet, despite an aspect ratio of 36 (21 M) vs 29 (18 M), the 21 M performance is
almost identical, according to the chart. How can that be?

--
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

Exactly! And yet, despite an aspect ratio of 36 (21 M) vs 29 (18 M), the 21 M performance is
almost identical, according to the chart. How can that be?

--
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[/quote]

Hi - The polar graph on the JS website starts at a higher airspeed than most others - the major performance difference between spans occurs around the thermalling speed region which is missing in the graph. I can attest from flying JS1's in both spans against others that there is very little noticable performance difference at similar wingloadings at cruise speeds.
That said I believe the ballasted 21m JS1 is an underrated weapon and the JS2 will be even better. The JS5 will be in a class of its own.