I'm guessing the bellyboard is like the control line models I made out of
cardboard. Pull a flat sheet through the air at high speed, and a positive
angle of attack, and it will produce lift (fly). (In this case vortex lift.)
Equal and opposite reaction to the air mass accelerated downward is
accelerating the airframe upwards. That is what flaps do, bellyboards, and
parachutes too. Airfoils only redirect the force vectors backwards, creating a
positive L:D ratio forward. Start with a round parachute and you get an L:D of
1:1, a sport ram-air chute gives you 3:1, and a glider 40:1, they all descend
at 12-14ft/sec. Very cool! Peter
