I’m going to take a contrary view and say that in my opinion, if you are designing a model of conventional proportions, building a foam model is a waste of time, and here are the reasons.

If you stay reasonably close to the areas, moments, and airfoils used in other successful aircraft, your design can’t help but fly successfully. The real challenge to designing from scratch isn’t as much the aerodynamics, as the structure. Ramrod25’s stick is a good example. As long as the weight and balance is within the normal range for that type of aircraft, it will fly just like every other “stick” style model. What sets it apart as an original is the beautifully engineered structure. He has gone to great lengths to save weight with contoured cutouts in the ribs, judicious use of stub ribs, built up tail surfaces, flaps and ailerons, etc. Of course some of this is necessary due to the size which also makes it a little easier to do. Sometimes your goal may be to simplify the construction without adding too much additional weight. In which case you would avoid extensive cutouts, and use different density balsa and lite ply to control the weight. If you were designing a combat model, all that beautiful structure would seem kinda foolish. One of the benefits of building a lot of kits before launching out with original design, is to see different approaches to structure.

Even with the best effort, the proof is still in the testing. The little aircraft in my avitar is an original design of mine. It flew right off the board with no adjustments, and performs even better than I had hoped. However on the first hard landing the wing attach point broke. After it happened a second time, I designed a new support that didn’t add much weight but considerable strength. After the modification it has been a pretty rugged flying machine.

There is really no magic formula. An airplane design is a series of compromises depending on the mission and based on experience, appearance, past designs and a little luck. When your creation that you conceived, designed, and built takes to the air you will be a proud flier indeed.