On Tue, 12 Dec 2017 01:52:24 +0000 (UTC), Kiwi User
wrote:

Hi Martin,

I'm rather worried about spanwise flow originating at the wing root
(similar to the SB-13) - but let's wait and see. The guys (and gals)
know their stuff.

The idea was that the LE sweep promoted spanwise flow toward the tip,
which was encouraged to oppose the tip vortex rotation as it slid over
the convex tip profile. The pointed at the end of the TE anchors the tip
vortex while the roll-down of top surface flowing spanwise out along the
panel and down over the tip shape will tend to move the tip vortex
outward.

I used this tip design for many years on competition free flight F1A
gliders. It worked for me. It was notable that, while models with
conventionally rounded tips needed a lot of tip washout to prevent tip
stalling, my design worked best with unwarped [flat] tip panels.
Directional stability was good too. Minimal fin area is beneficial to F1A
performance and thermal centering, the optimum being just big enough to
kill dutch rolling tendencies. On my design the fin had to be reduced to
a surprisingly small size before the first signs of dutch roll appeared.
Benefits of solid balsa fins: you keep chopping bits off until the dutch
roll appears and then stick the last bit back on.