Reynald's number
lennyk,
I'll just add a couple comments. As MonkeyBoy said, the Reynold's number can be thought of as the ratio of inertial forces to viscous forces. Rather than thinking of the Reynold's number as a real precise measure, though, it is useful to think of it as a rough indicator of what kind of flow you have. I like to think of the Reynold's number, intuitively, as a measure of how 'energetic' the flow is. This is not at all precise, and it is not an analogy that works in all circumstances, but for aircraft, it has some value. One of the big reasons we consider Reynold's number is that the lower Reynold's number that applies to our models implies that the boundary layer ( the air that is right next to the surface ) is not as energetic, and is more likely to separate ( the flow phenomenon that causes stall, speaking roughly ) than the energetic flow that we have at higher Reynold's numbers. The news is not all bad though. The less energetic flow also is more likely to be laminar, rather than turbulent, which means that we have a little better chance than the full-scale guys to get the advantage of lower drag due to laminar flow. We don't take advantage of it very often, but it's always nice to know we could.
Maybe this helps, or maybe it sounds as incoherent to you as it does to me now that I read it.
banktoturn