Over the course of time, I've had the pleasure of designing and test-flying a lot of wings. One cannot help but to learn a few lessons along the way. The first part of this is the consideration of the airfoil itself- the outside shape of the airfoil and its behavior when flying. The second part is about the mechanics of constructing wings in general.
Part Two- the mechanics of wing construction.
Part 1:
Without going into a lot of aerodynamics or scientific study, let's consider an airfoil in simple terms. A wing, any wing (even rotary wings like those found on helicopters, gyrocopters and airplane propellers), flies because of forces placed on it while air flows around it. While avoiding the age-old argument about the very specific reasons why aircraft fly, we can certainly say that the reason lift is generated is due to the movement of air as the wing (or airfoil) passes. Air will be accelerated downwards and this must impart an upwards force on the wing itself in homage to Newton's 'equal and opposite reaction' law. This force is easy enough to generate too- you can do it by placing your hand out of a moving car's window, set this hand at some angle relative to the wind, and the air moving past will definitely try to force that hand either up or down depending on the angle. In the end, anything that is longer than it is thick will fly given sufficient power; the oft quoted example is a piece of plywood which most certainly does not have an aerodynamic shape. The problem with something so badly shaped for our purposes is simple efficiency; to generate a given amount of lift, a sheet of wood will create tremendous drag and therefore take tremendous power to make it fly.
Enter the true airfoil. Its only claim to fame is that it will produce lift with little drag, or put another way, it's far more efficient than a flat piece of anything. A real mathematician's airfoil is a cosine function wrapped around a focal point.... as in the diagram below. All referenced airfoils are fully symmetrical because we're only interested in fully aerobatic planes- there are, of course, semi-symmetrical and asymmetrical airfoils also. These are even more efficient but only when the wing is 'upright' while fully symmetrical airfoils can be used equally well 'upright' or 'upside-down'. The first airfoil is a NACA 0010, meaning that it's 10 percent as thick as it is long.
We can easily vary the thickness to length ratio: the next airfoil is also a NACA 00XX series but is 20% thick and so is referred to as a NACA 0020. Ignore the fact that this shape is shorter than the one above, it's the length to thickness ratio that's important. Either airfoil could be scaled up or down to make a certain size wing but the relationship between length and thickness, in percentage, would remain the same.
The big difference between the behavior of these two wings can actually be summed up pretty simply- the thinner wing produces less drag but stalls sooner / easier. The thicker wing will resist stalling but will not allow as much top speed due to increased drag. It really is that simple too.... we just have to pick which characteristic we like the best (or hate the least <G>) and choose a design accordingly. As an aside, if a wing is on the thin side and also tapered, this accentuates the lousy stall behavior. Look around at any of the high-speed aerobats (Sukhoi, CAP, Extra, Giles, etc., etc.) and note how they suffer from accelerated stalls when a lot of elevator is used. Simply making the wing thicker will easily and finally solve this problem but it reduces top speed as well as inhibiting or prohibiting some wing stalled maneuvers such as snap rolls. Fortunately, the drag increases at a much slower rate than the benefits of a thicker wing do, at least in my opinion, so it's an easy choice. All of this assumes that some type of sport wing is being built; if the application is racing, then of course the most desirable wing is the thinnest and pointiest one possible. Also, the effects of wing thickness do NOT scale between aircraft sizes. What works great on a 40 lb., 120 inch wingspan aircraft will not work nearly as well if the wing thickness is scaled down to a ~.40 size plane. As the plane size is decreased, the wing thickness must be proportionally increased to maintain similar flight characteristics. A Boeing 747 might a pussycat when it weighs 3/4 of a million pounds but a 1/50th size model (true scale) would most certainly be a handful.
These mathematical airfoils are nice but can be difficult to work with for several reasons. Perhaps one of the most important reasons is that they do not stretch well, if one is trying to make a rib that will be precise even though used at some angle. Also, this shape can be difficult to build as it requires some type of alignment device during construction (most often seen as tabs on the rear of the ribs to hold it straight). If we modify the shape slightly, we can have one that is almost as efficient but far easier to use. What we can do is to replace the cosine function in the forward part of the wing with an ellipse and the rear section with two lines that are simple tangent to that ellipse. The next two shapes duplicate the first two with the changes mentioned. The first is again 10% while the second is much thicker.
Note that both of these shapes are very similar to the true airfoil in the forward section but have straight (tangent) lines in the back section. This allows the wing to be built flat on a building board. While there is slightly more drag associated with this type of shape, such a wing will always be easier to rotate so that maneuvers like snap rolls are easier and faster.
Finally, we can actually dispense with most of the curved shape altogether and move along toward something truly easy to build. The following shape is probably the easiest and fastest to form into a wing and believe it or not, they fly fine although they are certainly more 'draggy' than any of the above shapes.
I have actually produced wings using this shape with good results. They are very fast to scratch build because the ribs can be cut almost entirely with a razor and cutting guide. As I said, drag goes up but overall flight characteristics are actually quite good.
In the end though, the second series of shapes (ellipse and tangent lines) is the best compromise. I've seen lift to drag charts on these types of airfoils and they are remarkably close to the full cosine airfoils in efficiency. Certainly this difference would be lost in our toy planes even given reasonably accurate measuring equipment. Given that almost all of us simply watch the airplane fly to determine its efficiency, the difference simply disappears.
