Well, my challenge didn't take too long for you guys to answer. There were glimmers in nearly every post. I'm no history expert, but as a lifelong modeler and an aero engineer, the pioneer era of aviation development has always fascinated me. Here is my construct for how we got to thin airfoils and later thick ones:

1. Early aviation pioneers of the 1800's and beginning 1900's (Pilcher, Lillenthal, Langley, Wrights, etc) looked to birds for guidance in their designs, especially since birds were the only successful game in town. And birds all seem to have quite thin wings.

2. Birds fly fine with thin airfoils because of the low Reynolds Numbers in which they operate.

3. The Wright brothers built a wind tunnel to measure lift and drag properties of candidate airfoils. Due to limitations in their tunnel, they made measurements at extremely low Reynolds Numbers, about the same regime in which birds fly. To make matters worse, the Wrights didn't yet know that testing at low Reynolds Numbers didn't necessarily translate to full sized prototypes operating at higher speeds. So based on their testing they failed to discover the benefits of thicker airfoil sections.

4. Following the lead of the early successful flyers, others used the same thin airfoils. They never realized the penalty in lift and drag that came with the thin airfoils.

5. By about 1914 a really substantial wind tunnel had been constructed at the University of Gottengen in Germany. It featured a much larger test section and much higher test speeds. In a short time scientists discovered the benefits of thicker airfoils. That's why many of the early thick airfoils are named, "Gottengen 123", etc. German designers such as Pfalz and Junkers quickly took advantage of this new technology. We see this in many of the WWI Fokkers and the Junkers monoplanes. There is an interesting paper given by Dr. Prandtl in the year 1920 that illustrates just how far the Germans had advanced during the WWI years. http://ntrs.nasa.gov/archive/nasa/ca...1993080860.pdf

6. Many designers stubbornly stuck with the thin airfoils, even into the 1930's. The British didn't really get into thick airfoils until the Hurricanes and Spitfires. Many American designers were equally unwilling to change.

7. So here we are today. Nearly everybody has embraced the thick airfoils. But only a few seem to realize that thick is of no particular benefit at very low speeds and small sizes. Certainly the birds haven't gotten the message ! But guys who fly hand launch gliders, Wakefield gliders and micro RC are doing just fine with thin airfoils because of the speed and size regime in which they fly.

8. Will a model of a WWI plane fly OK with its thin, original airfoil ? Yes it will, providing it's still down in the Reynolds range where fat airfoils don't help much. You may see some modest ill effects with the thin airfoils. More power required to fly, poor aileron response due to early airflow separation on the upper wing surface, etc. But if you are willing to put up with the small stuff, you can go ahead and use the thin airfoil.

One last comment and I'll shut up. BobH made the observation that most of the full size WWI airplanes were poor flyers. That's true. I help out occasionally with a large collection of WWI replicas. The two guys who fly them (I don't) tell me that they are mostly poor in the stability and control department. The DVII is perhaps the best of the bunch, but even it has slow ailerons. Most of the problems seem to be lateral/directional. Adverse yaw and slow roll response are pretty much universal among that genre. But that's mostly due to the tiny vertical stabilizers or the fact that some verticals are all-moving with no fixed fin.

Dick