Very interesting topic. Any reports/reviews yet?
Many variables seem to make it complicated. Just some amateur thoughts (I`m not an engineer or have a degree in this) …
Let’s pretend a situation which is most likely: Front wing has more surface area than rear wing and front wing can therefore generate more up/down lifting effect than the rear wing at a given speed. Rider creates between 60 and 120kg down force which has to be compensated by the wings to create overall positive lift.
In this scenario I think there are (at least?) four different situations/categories:
a) At “sub-riding” speed any AoA combination of front and rear wing creates too little lift to foil or at least one wing creates stall.
b) At the minimum speed that a foil starts to lift, both the front and rear wing would travel at uplifting AoA because the front wing alone would not have enough lift on its own. This is even the case at low/lower traveling speeds.
c) At “break even” speed one of the wings (mostly the rear one in commonly used configurations with the front wing being bigger and having more positive Aoa than the rear wing) travels at a zero-lifting AoA through the water because the other (front) wing just generates the right amount of lift on its own to keep the same flight level.
d) At even higher speeds the same wing (again mostly the rear one) travels at (slightly) down-lifting AoA through the water because it has to reduce the bigger lifting effect of the front wing and keep the combined/overall AoA of front and rear at the right angle to ride horizontally level.
Question in general: At what speed do you expect the “break even” for up to date foils with a rider of about 80kg?
Addition: Think about a configuration with the smaller rear wing having more positive AoA than the front wing.
Question: Is there practically a speed thinkable at which the front wing travels at zero-lifting AoA and the rear wing creates the needed uplift on its own?
In this whole theoretical scenario I did not consider any aspects like control, comfort or user-friendlyness. Neither was the position of cg important for me in this theoretical scenario.
Conclusions for me:
1) At lower speeds (almost?) all rear wings have to provide positive lift by their AoA, because the front wing alone does not provide enough lift to compensate the riders` weight for foiling.
2) The rear wing can very well have more positive AoA than the front one. But as long as the rear wing is (much) smaller than the front wing or has a more neutral profile than the front wing it does not necessarily generate more positive lift than the front wing.
Confusing? Wasn’t meant to be….