foil design

[Kamikuza]

Point up or down or both ways, they all do the same thing. It just make more sense to point up on the airplane and point down on the hydrofoil.

That glider has a shroud around a landing wheel. Winglets to reduce tip vortices go up...

[Hawaiis]

Wikipedia:

Hoerner wing tips

Hoerner tip on a glider. A landing gear wheel is integrated
The earliest-known implementation of a Hoerner-style downward-angled "wingtip device" on a jet aircraft was the so-called "Lippisch-Ohren" (Lippisch-ears), allegedly attributed to the Messerschmitt Me 163's designer Alexander Lippisch, and first added to the M3 and M4 third and fourth prototypes of the Heinkel He 162A Spatz jet light fighter for evaluation - this was done in order to counteract the dutch roll characteristic the marked dihedral angle of the original He 162 design's wings possessed. As production of the Third Reich's chosen turbojet-powered emergency fighter was of prime importance at the start of 1945, disruption of the production line to make other types of changes to correct such a problem were not likely to have been available, and the added wingtip devices became a standard feature of the approximately 320 completed He 162A jet fighters built, with hundreds more He 162A airframes going unfinished by V-E Day.[6]

Following the end of World War II, Dr. Sighard F. Hoerner was a pioneer researcher in the field, having written a technical paper published in 1952[7] that called for drooped wingtips whose pointed rear tips focused the resulting wingtip vortex away from the upper wing surface. Drooped wingtips are often called "Hoerner tips" in his honor. Gliders and light aircraft have made use of Hoerner tips for many years.[8][9]

[Tiago1973]

again from memory - induced drag is proportional to aspect ratio in a reverse manner, ie, a low aspect ratio wing will have a higher induced drag & vice versa

winglets increase the AR of a certain wing (without having to extend it´s span), regarless if they go up or down

so i´m with Hawai here - up or down will work for induced drag reduction

but guess, as always, that the devil will be in the details

[Kamikuza]

Interesting! I knew the He162 had winglets to counter the nasty handling but always thought that as the vast majority of winglets go up that they were more effective... and the droopy ones were only to clean up the wing tip wheels.

I assumed the foils fins had the dipped tips for the same reason as the volksjaeger... stability.

[ozchrisb]

The main foils drop the tips down so that they don't pierce the surface. We have an extra dimension to deal with. Gilders and planes have other issues to deal with once their wings change from air to water.....