DesignMan

There is a nice, empirical method you can use. Build a miniature solid balsa glider of the model. It turns out that stability stays pretty much the same despite gross changes in Reynolds number. The cg found by experiment with the miniature will be extremely close to the correct position for the final design.

Play with decalage (wing-tail angle differece) and cg location until you get a model that just pulls out of a dive slowly. If it porpoises, you have the cg too far forward and too much decalage. If it tucks under, go for more forward cg and more decalage.

Remember, angle of attack is measured from most forward point of the leading edge to the most aft point on the trailing edge. Second, the airfoil symmetry or lack thereof determines at what angle the airfoil generates no lift. For most non-symmetrical airfoils, you actually generate significant lift at 0 degree angle of attack. It is even worse with a "flat bottom" airfoil. Most people think the bottom is the determiner of the angle of attack, but it is actually several degrees below the datum line. And then you add the 0 angle lift, and you can easily end up with way too much decalage.

Check out the Lanier ST40 trainer. I put a semi symmetrical airfoil on it to control the "zoom" when you change power levels. It is an extremely easy plane to fly as a result. Very little downthrust was required as a result. Flat bottom airfoil trainers will climb drastically when you goose the power and drop like a stone when you cut it. The ST-40 is far better behaved.[8D]