@ Derk - flat profiled front wings work great, if you curve them, even better! I believe that at least a part of the strong longitudinal stability of the tandem relies on the fact that the front wing has a high AoA and curved underside, to provide a lot of RAM air lift. The rear wing has a rather shallow angle of attack, a flat bottom and a véry thick profile. In this configuration, the rear wing is able to make quite some lift too, but it doesn't rely so much on the Ground effect.
If the craft increases it's flight altitude a bit, the strong RAM-air effect decreases quickly and the front wing will lose quite a lot of lift, allthough the rear wing won't notice this altitude change as much as the front wing and it will still provide most of it's lift. --> the craft tends to pitch down. If you add this to the fact that placing two wings in strong ground effect gives a very nice pitch damping, which gives the tandem this 'robust' flight handling, you'll get a very stable craft, as long as your close to the ground.
Anyway, the model looks cool, if it tends to flip, just shove the CG forward and forward till it stops, don't start cutting in the wings directly.
@ Gabriel, I found the Airfish to be quite stable, or at least very well controllable on pitch, everytime I accidentely flew it out of GE. It did however start to roll over everytime, because of the negative dihedral.
and how the **** they want to achieve 1,2 meters, I don't know. clearly, it is possible, as long as you stabilse it as a tandem winged airplane, not as a jörg boat. But then, if you're still able to fly it without elevator, I don't know