CAD and Sketchup
Computer Aided Design is almost like cheating. Almost. With the aid of a computer it is possible to draw, design, plan, and if necessary make change on a level that is difficult to conceptualize. In a very short amount of time I can draw a model for a single engine plane with the use of free software. The advantages in using a PC are immediately noticeable when it comes time to start cutting the foam. My program of choice is a surface modeling program called Sketchup and it's brought to you by the good people at Google.
The best things in life are free
Don't assume that since Sketchup is free it's a useless program. On the contrary, it's interface is very easy to use, making it easy to learn, yet it's very powerful and can be upgraded for free with various plugins. Chances are if there is something you want it to do, and it won't, somebody has made a plugin that will do it. There are two plugins that will allow a scratch builder to make full use of this program. The first is a script called jf_unfoldtool and will allow you to unfold the surface models you have drawn. This script is what makes this program instantly better then others I have used. The next script is for drawing complex curves know as Bézier curves and is called bezierspline. These plugins can be found at www.crai.archi.fr/RubyLibraryDepot
I don't want to make a complete tutorial on how to use Sketchup. I will show a few tricks on how to model quickly and efficiently. If you are struggling with sketchup I higly recommend you browse on over to youtube and check out some of the videos done by Kram242. He has been generous enough to show exactly how to get started with profile planes and most of what he does can be directly used in 3D drawings since much is done the same way.
To start out grab yourself a 3-View drawing of the plane you want to make off the net. I like this site I don't speak Russian, but they've got some nice drawings over there. Next download and install the Gimp. Easy to use, free, and powerful. Break down the views into separate images, and if you're lucky enough to get a drawing that has section views, split them up as well since it will enhance your scale. Save the files as tiff or jpeg images and import them into Sketchup.
A complicated sectionalized view of the FW-190
The next step is to draw the ribs, the more ribs the more complex your final model will be. Keep it as simple as possible. The above example is very detailed and I would not recommend it for the first time builder. There are about twice as many profiles as there needs to be. The final result needs to be simple. It took considerable time to break this drawing down to a usable level, the process however, is the same. Once the section is laid in, the next step is to trace the rib. This is done with circles and arcs, simple circles and arcs. Do not use the bezier curve plugin. When you select the arc curve, or the circle, you will see in the lower corner the number of sides it will draw. For circles I type 8, for arcs I type 3. The alternative is to draw lines, again, as few as possible. Trace out each profile in this manner. If you do not have profiles to work with, buy a model for reference, and approximate a section on the side view where the fuse changes shape.
The next step is to run spars from end point to end point. Obviously the fewer end points, the faster this will go. After the spars are run start making triangles. Each time you make a triangle you will create a surface. In the end your model will simply be a mesh of triangles. In the above example I had to further simplify the model by rotating it about and deciding what sections I could do without. From there I deleted, redrew, and remade my meshes. In addition to that, I used the move command, to move endpoints together to further simplify the model.
FW-190 (in Blue Print Style) with Sketchup
FW-190 (Normal) with Sketchup
Originally this 190 had ten sections, I brought it down to seven, still maintaining the proper shapes. This one went from a very difficult model to something that will unfold simply, and go together easy as well. Here is a picture of another model I have made using this process:
F6F Hellcat showing it's triangle skin
The stabilizer is modeled by tracing out one side, then mirroring to the other (I just select it, right clicked, and chose flip along green). The rudder is made the same way, only without the mirror. This is where the bezier curves can come in handy. The next step is to draw out the main wing and decide on the airfoil. This can be another complete ordeal, depending on how you want to make your wing. I draw two airfoils, one for the root, another for the tip, using the bezier tool and then make metal templates to hot wire cut my wings. More detail is provided in the airfoil design section.
The final result of all this work is the unfold tool. This allows you to unfold (and then print out) the exact shape that you can cut out of the foam product of your choice. Another neat thing to do would be to cut out the shapes from monocoat or fiberglass to form exact shapes to cover your model (although this will add weight).
F6F Hellcat and FW-190 unfolded, ready to print
To get a better understanding on how this all goes together, check out the build log on the P-51D Mustang, especially this part about rough shapes, with sharp corners. As foam is bent it naturally will take a rounded shape, all that you as a designer need to do is manage the profile, or rib, it forms to. The best way to do that is to offset the thickness of your foam onto the section from which the model was created.