A failed gantry lets me continue my development of my new foilset this weekend. Did a little research and thinking about winglets. Only an overview here, and only the stuff I consider of relevant and of significant effect.
Winglet
What is the effect of adding winglets? The major effect is to reduce downwash, make it harder for the high pressure side to leak around to the low pressure side.
This increases apparent aspect ratio of wing reducing induced drag. But it comes with the penalty of adding parasitic drag without adding any extra lift.
As a consiquence of the above you should always add wing span in preference to winglets.
The induced drag effect is much more significant at high lift coefficients (slow speeds, when your foils are struggling to provide enough lift) so you could see the effect of adding winglets (or extra span) as helping you take off sooner, but it will reduce your top speed.
Wingtip Fence
When are winglets used? Always when wing span is limited
ie by structural considerations
ie by outside influences, such as 15m class in gliders or aerobridges for airliners.
So should winglets be added to your moth?Only if you need to take off at a slower speed and
1. You are using the same production hydrofoil as everyone else.
2. Your proto foil design is aready solid carbon and at its structural limit.
3. If your moth doesn't look enough like the bat boat yet (but I suggest adding winglets in a more visible spot).
In all other cases, add span, not wingtips, and add more reinforcing if necessary.
Spiroid Wingtip
