Types of Structures and Material Choices
Before you ever draw lines on paper, use your CAD program, or cut any parts, spend the time to consider what you are building and how you intend to fly it. You should adopt a completely different design, building, and flying philosophy for each type of model you build. Obviously a high-speed aerobatic jet aircraft demands a different approach than a low-speed, non aerobatic trainer. This is the extreme example, but you get the idea. Ask yourself a few simple questions before you start, and save yourself a lot of headaches (and possibly heartaches!) later. Planning may be the most important stage of the design.
Here are some things to consider:
What is the purpose of the model? (Competition, Weekend fun flying, Test of a new design, etc.) Does this model need to be repeatable? (Will you draw detailed plans, make molds, etc.?) What are the structural requirements of the model? (extra strong for speed and/or aerobatics, simpler for a trainer) Based on the above, and other needs, what size will the model be? (Consider wing loading, power required, servo sizes, transportation, etc.) How long do you have to complete the model? (" Forever" if just for fun, maybe a deadline if for competition) If this is going to be a long-term project, are you willing to stick with it until it's finished, or will you "run out of gas" part way through? Can you do it yourself, or does it have qualities that will require some research or help from others?
You could extend this list, but it's a starting point. If you can answer all these questions, and still want to proceed, then let's go!
If you want the model to be repeatable, either to insure against mishaps or perhaps to allow kits to be produced, you must be more thorough during the pre-building phase. That is, you must draw detailed and accurate plans, and you must think through the assembly process, so you remember how to do it again, and so that you can explain it to someone who has never seen it before. We all dislike bad construction manuals, don't we?
There are basically two approaches to repeatability, which is to either plan for cutting a lot of parts out of wood, or in the case of fiberglass and foam, the creation of templates and molds. I've mostly settled into the fiberglass fuselage/sheeted foam wings style of building, but if you have die-cutting or laser-cutting capabilities, those are certainly good options. Look at the extremely high quality of Bob Violett's models for example, many of which have balsa built-up structures.
The choice often depends on specific design considerations. For example, if you are going to put a lot of items in a wing, such as aileron and flap controls, retracts, gear doors, and possibly other things, then I recommend a built-up structure, because you simply have more open space inside the wing to put these things without cutting your material. If, on the other hand, you're making a relatively simple wing, with only ailerons, then maybe a sheeted foam core is easier. Either of these choices can be built light and strong, if you're careful.
If you are going to build a small, simple, "boxy" structure, such as a non-scale trainer, there isn't a lot of justification in making it out of fiberglass, constructing molds, etc.. If you're building a scale project, perhaps a jet-type aircraft, and want to save yourself a lot of time later on the details, fiberglass is a good choice. If you're going to put scale panel lines, precision-fitting hatches, simulated rivets, etc. into a model, why not just do it once, when you make the plugs and molds? Personally, I don't think I could do all that work over and over, if I happened to crash such a model, because my enthusiasm would probably fade. With molds, you can reproduce a part immediately, with all the same detail you put into the first one. Properly constructed molds have many advantages, including consistent alignment of parts, repeatability of the details mentioned above, the ability to produce at least a few "kits" for friends, and a huge time savings. Once you've experienced this for yourself, it's hard to go back to balsa. Check the "Fiberglassing Techniques" page for information on the differences in Epoxy versus Polyester resin, types of cloth to use, etc..
Properly constructed molds can be used in a variety of ways. You could do a traditional cloth and resin layup in female molds, and produce fiberglass "shells". You also have the opportunity during the layup to add Kevlar or carbon fiber reinforcements and/or install formers during the process. Another option is to actually build a fuselage "from the outside in". There is a successful competition plane being built now, called the "Angel's Shadow" that uses a very clever process that results in a light and strong aircraft. In this case, the designer uses two traditional female mold halves for the fuselage, but in a unique way. First, a layer of clear finishing resin or "gel coat" is put in the mold. Then this layer is painted in the mold! Following that, the designer adds lightweight fiberglass cloth, and finally balsa sheeting, and a few key internal components. Finally, the whole thing is allowed to cure, under pressure. When it comes out of the molds, you have a fully finished fuselage that is gorgeous! I saw one in person at a recent pattern competition, and it is really first class.. (Noel Barrett Models)