When to begin scratch building?
Youv'e already built a few planes. when you boil it all down, on a high plane the proportions are basically the same. S o use a rib pattern from one you have because you know it works. so use the outline you like [Cessna] . Keep the distances the same, Like from front of wing to front of engine, back of wing to front of stab,etc. keep same cg. same rudder, stab areas. just change the shape. this a place to start. Read about different airfoils and what they do and try them out. A couple of good books are, Basics of R/C Model Aircraft Design published by Model Airplane News. Design and Build Your Own R/C aircraft by Ken L Smith.