woundedbear wrote: What is a Reynolds number ?
David Lewis wrote: As you shrink an airplane down, air molecules do NOT get smaller, therefore aerodynamic behavior between different scales is not similar. Generally drag goes up as Reynolds number (Re) decreases. Most often Re is used to compare the performance of a given airfoil at different scales or speeds. In that case it's called the "chord Reynolds number", and the characteristic length plugged into the formula is the chord of the airfoil or wing.
Full scale engineers rely on test flights or wind tunnel data collected at (or corrected for) the Re in the case at hand. With models, however, Re effects are more important, and data is rarely available and less useful. Even if you have two wings with identical Re, they may not fly the same because small differences, such as building tolerances and surface roughness, have a significant effect on aerodynamic behavior.