Reading through this thread several times I seem to notice some things that might serve to settle the question. From Ben’s responses consider the following:
“But if you have two airplanes of similar light weights, low wing loadings, high power loadings etc. then you might also see the differences between a carefully selected airfoil and a WAG airfoil.”
“Dick you really need to instrument the airplanes to really know what the changes you have made resulted in doing. Visual sight and trying to remember just isn't going to hack it.”
“I try to push the concept that the vast majority of model designs that have been flying over the last 40 years are a vast source of knowledge. It is better than having a wind tunnel test data set. Pick something that is working very well and copy it whether a flying wing, canard or pattern ship. The evolution into today's RC airplanes has produced a fine bunch of flying machines. Copy and enjoy.”
“Dick the increase in goodness might just be a little but in a field such as aerodynamics where we have developed pretty good airfoils as of this date and where any performance increase, even a percent or two, is good, then someone with the proper tools can design an airfoil that is better. I don't know if we could say they are superior like it was twice as good or something but it can be just a little better and might make the difference in winning and coming in second.”
“We could say the modern airfoils are superior for our present pattern use. But a slight iteration on the present thin section might produce at best a slightly better airfoil, but doubtful that it would be superior.”
(underlining mine)
Several things stand out.
1. The vast majority of R/C models fly quite well regardless of whether the airfoil is scientifically designed or just a WAG.
2. An airplane designed for specific competitive criteria may gain a small advantage if the airfoil is based on actual test data rather than intuition.
3. The difference between a carefully designed airfoil and a WAG is generally small enough that it can’t be noticed by just observing flight.
When designing a model aircraft, even for competitive purposes, airfoil selection is probably the least critical factor. Such factors as wing and power loading, wing and tail planform and location, moment arms and force layout, cg, fuselage shape and cross section, etc., all have significant effect on the airplanes ability to perform the required maneuvers, and present the largest possibility of significant improvement.
Even in serious competition, the winner is not always the one with the best design, but skill and consistency play a very significant part. Some of the many hours spent doing scientific design might yield more results if spent in practice.
So if you are a serious competitor, pull out all stops and spend all the time and effort you want to design the perfect machine. But for most of us who just like to play with model airplanes, put together something that looks good, and doesn’t vary too much from other successful designs, and go have fun.