Model airplanes can be designed in a number of ways, but the easiest way
is to take a known model with good flight characteristics, simply take the strategic
measures from it and change the outline of it to something more appealing.

  Take for example one op my favorites, the Astro Hog, keep wingsize,dihedral, fuselage length, noselength, stab- and finsize.Now you can sketch up a new design held back only by these parameters !

For instance changing the outline of the fin and stab, but maintaining size and control surface size, should keep the characteristics about the same.

Move the cockpit backwards, make a tandem two seater, or make a modern cockpit, like an Extra 300, and maybe alter the outline of the wingtips.

This is all very simple and much more rewarding than to build from a kit. This may be considered more like Customizing than designing, but never mind, the main goal is to have fun!