Now, once a shape is determined, how can we go about forming it into a structurally sound shape?
There are several methods of constructing the wings of our model planes. Perhaps the easiest to envision is a foam wing- this is a wing that is molded or cut from Styrofoam. External bracing is needed to provide sufficient strength and this can range from nothing more than fiber packing tape wrapped around the outside (lengthwise), through wooden or carbon fiber spars inserted into the surface of the wing, to fully sheeting the outer surface of the wing with something like balsa wood sheeting, very light plywood, or even fiberglass cloth or carbon fiber. All these methods will work but sheeting with balsa wood is probably the most common. It's quite easy and quick to make a foam wing- the usual method is called a "hot-wire"; this is a piece of thin wire that's heated and used to cut through the foam lengthwise along a set of patterns (although some CNC controlled hot-wire setups are in use). This foam 'blank' is then sheeted with glue and wood to finish the wing. Overall, this makes a nice wing that is <usually> very straight and strong. However, it's probably the heaviest method of wing construction due to the fact that the Styrofoam weighs at least 1/2 lb. per cubic foot, and more often, a full pound per cubic foot. Given a fairly thick wing this weight quickly adds up.
Another method is to form a high tech outer form for the wing. Sometimes this material (carbon fiber and / or Kevlar) is wrapped around a foam blank that has been hot-wired to shape. After these materials have hardened, the original blank of Styrofoam is 'melted' out of the wing using common petroleum based chemicals such as gasoline. This is called the 'lost foam' process and is fairly popular among those building very large, light and expensive planes. It would be a difficult process to master at home, and remains pretty costly due to the cost of the raw materials used. Further, these wings are almost always painted to finish them requiring further skills and equipment expense.
Now, the most common method of making wings on a small scale- built-up, wooden wings. Common materials are balsa wood, bass wood, spruce, and both lite and standard aircraft plywood. This is my preferred method because it's reasonably quick, reasonably easy, results in an adequately strong and extremely light wing, and generally allows any shape (within reason) to be built. Built-up wings are light because material is only added where necessary- this is the downfall of a foam wing because there is foam everywhere inside the wing but much of it is not necessary. We can add wood only where needed to resist a particular force; in the end, these places are relatively few and little wood is required to stand up to the rigors of flight. A downside of a built-up wing is that much of the outer sections are hollow and require covering of some sort but even this can be turned into an upside depending on the covering used.
The most conventional built-up wing is the "D-tube" wing, so called because the forward part resembles the letter "D" and is fully surrounded with thin material, like a tube. A very easy to make wing although warping the sheeting around the front can be a bit trying. There are other methods available though, and the ones I like best are those that use the same piece of wood for several purposes. For example, if the ribs are used at an angle instead of straight, they will begin to resist torsion loads much better. The control line people have been doing this for a lot of years. This is an example of such construction.
Note that there is no sheeting used on the forward part of the wing because it's not necessary. The angle of the ribs provides sufficient strength without covering it fully in balsa. Further, there is no solid trailing edge either- simply an upper and lower piece of 1/16" sheeting joined in the front by another strip of 1/16" wood.
This idea can be carried further by angling the ribs even more. This is where the beauty of an ellipse / tangent airfoil begins to really shine too because a true cosine airfoil would be very difficult to angle this much.
The ribs in this wing actually cross just behind the spars. This yields a very strong wing with very little weight; I believe this type of construction will compete with almost anything else available at any price. Even the spaces between the wood can be turned into a plus if covered with transparent or translucent film :-)
To carry this idea just a bit further, the last in a series is being built now (7/2001). It uses the properties of the earlier types but finishes the theme all the way to the leading edge. Oh, and by the way, the leading edge is not solid wood but rather some warped sheeting. This can be noted from the side view. Also note that all wings shown on this page are swept but not tapered. Put another way, they are basically straight wing halves that are joined in such a way as to make both the leading and trailing edges 'swept'. The basic reason for this is esthetics although flight characteristics do not seem to be hampered by such modifications.
Also notice in the above picture that shear webs are not used. Crossed-truss type construction is also used between the spars in place of the webs that most kit makers supply. The trusses are both lighter and stronger than full shear webs.
This wing has a span of 54", a chord of 16", and a maximum thickness of just under 3 3/4" (137 mm X 40.6 mm X 9.4 mm). All-up weight, as shown, is 8.7 ounces and the wing is immensely strong in both bending moment and torsion (resistance to wracking). All sheet materials are 1/16", and the spars are 1/4" square hard balsa. In my experience, nothing more is required or desired. More wood does not make a better wing, only a heavier one. This wing will not be flown gently, as a park flier, but rather as a fun-fly type with a couple of horsepower up front. As an aside, if 1 pound per cubic foot were used, the foam blank itself would weight about 16 ounces before any wood was added!
