Ollie

I think it is instructive to look at reynolds number from the point of view of its impact on historical events. The Wright brothers built a wind tunnel to test various airfoils to see which was best. The wind tunnel airfoil models were small to fit the small wind tunnel. The speed of the air in the wind tunnel wasn't very high either. As a result of these test conditions the Wright brothers selected a thin highly cambered airfoil for their early gliders and their Wright Flyer. Their selection did not take into account all the effects of differences in scale and velocity (reynolds number related effects) between the wind tunnel and their full scale application. This resulted in wings that were overkill from a lift capability point of view and which had severe high speed limitations. It also resulted in the need for a wire braced biplane to meet the structural requirements. It wasn't until late in WWI that German aeronautical research revealed the superiority, at full scale (high reynolds numbers), of monoplane wings with thick airfoils that allowed strong enough spars to do away with wire bracing. Airfoils similar to the Wright brothers airfoil are still used in wire braced indoor freeflight wings where their high camber and thinness give superior performance at low reynolds numbers.