Have scratch built an FT Flyer and Old Fogey, both are all up heavier than the original designs with paint, glue, reinforcements, landing gear, B pack motor, 3s850a and either 8 or 9x4.7 props, Zero thrust angle, have been swapping the same power pod between planes. They both required constant up elevator pressure unit I finally just mechanically trimmed them high so the plane would fly level hands off. The Flyer took a few dirt naps until figured this out, on the Old Fogey maiden I at least knew then, to pull back on the stick when it kept losing altitude. What flight characteristic is the cause: Are they too nose heavy? Too high of wing loading? Thrust angle? Errors in building? Photo below shows the elevator trimmed high to the point where the Old Fogey flys level, hands off the stick with light throttle.