Let's talk about a few things.

1) Trying to determine if the ballooning is thrust or airspeed induced can be determined by getting really high, chopping the throttle, put the model in a straight down dive andobserve the model. (A fully symetrical wing model, well trimmed, should dive straight). Any deviation is attrributed to either incidence or eleavtor trim. With a flat bottom airfoil, expect that as the airspeed increases, the model WILL balloon.

Repeat the same but hit full throttle in the dive and see if the airplane noticably balloons as the power is added. Do the same with power on in a straight up climb but cut the power once the power and climb are established, and look to see how the model reacts.

It's one of the first true tests in trimming a pattern ship... The flat bottom airfoil will balloon with increased airspeed. The best you can do is set the angle of attack so the incidence angle between the wing and the stab give you something to live with.

2) . If your radio has the capability, try coupling the elevator function to the throttle function... adding downtrim as throttle increases. It works... until you fly inverted. (And in all reality, we'd love to have an airspeed detector onboard the airplane that would add down trim as an increase in airspeed was sensed).

3) "decalage" is the relative angle between the two wings in a biplane; not the angle difference between the wing and stab (incidence). http://en.mimi.hu/aviation/decalage.html

4) Somewhere in this thread I read in an attempt to reduce the ballooning effect, increase the prop pitch. Increasing prop pitch (if the engine can handle the additional load) will only exacerbate the ballooning as the increased pitch will fly the model faster, creating more lift and the ballooning effect. Reducing pitch will negate some ballooning.

5) Flat bottom airfoils are good for gobs of lift making drag that keeps the model from going really fast. A semi-symmetrical wing eliminates a lot of the nasty stuff learned beginning with a flat bottom airfoil. I'd rather teach someone to fly on a larger area, semi-symmetrical wing with moderate power than a flat bottom airfoil. (Sr. Falcon!)