The aft CG is also the "up" elevator . The plane is designed to fly in circles; if it flies straight a head the nose goes up and it stalls, but in a turn the pitch up is enough to keep it from falling out of the turn. Making it a regular rc plane requires you to move the CG forward so it can stop the pitch up stall routine. The decalage is a speed control for the glide; if set right it finds a happy medium to fly at; if you move the CG forward without zeroing the decalage you tuned the stab into up elevator so it wants to loop.