ORIGINAL: adam_one
Banktoturn
Yes, some definitions should be clarified so we are able to understand each other…
Let's try:
The airfoil thickness as well as the camber, are normally expressed as % of the chord, right?
So, let's assume that an airfoil without camber (e.g. a flat plate) that has 3% thickness is considered thin.
If the wing has a chord of nine inches, the absolute thickness should in this case be about 1/4 inch.
According to the chart above, a thin flat plate is best at Re's <5000, so the actual wing chord at such a low Re is probably half the value mentioned above, so the absolute thickness would get down to 1/8 inch.
A cambered airfoil doesn't necessary have to be thicker than a flat plate, but if you have a camber of say 4%, the airfoil thickness may be more or less higher depending on what sort of airfoil, wing structure and covering material used.
E.g. indoors often use thin covering film over ribs between the spars, which make the absolute camber value almost equal to the absolute thickness.
As for the flat plate being the best airfoil…, I realise that you still are sceptical, but think that it applies only at very low Re's <5,000 where the airfoil's thickness starts getting closer to the size of the air molecules….
The picture below is just to ilustrate some terms despite you may already know them well..
adam_one,
I don't think we have yet succeeded in identifying common terminology. You make the comment that for a wing made of a thin film, absolute camber is almost the same as absolute thickness. This is not true, because thickness and camber are completely independent. Thickness is the distance between the upper and lower surface at a particular chordwise location, and the maximum thickness is the maximum value that occurs anywhere on the chord. Camber is the maximum distance between the chord line and the mean line. Consider a constant-chord wing made of an infinitesimally thin sheet of material, curved to, say, 10% camber. This wing has 0% maximum thickness, and 10% camber. Incidentally, it is also a plate, since the thickness is constant, zero, all along the chord.
Having said that, I agree that a cambered airfoil need not be thicker than a flat plate, but I don't know what point you are trying to make.
I am indeed still skeptical that a flat or curved plate is the 'best' airfoil at any Reynold's number. If you want to convince me, try to construct an intuitive argument that the chordwise variation in thickness somehow degrades the performance of an airfoil at low Reynold's numbers. I have tried to do this for myself, and failed. It seems very unlikely to me that the optimum airfoil section would suddenly be a constant thickness section, at some low Reynold's number. As I have mentioned before, I think that the confusion is caused by the fact that 'optimum' airfoils for lower Reynold's numbers do tend to be thinner, and thinner airfoils do begin to bear a visual resemblance to plates (curved or flat). In other words, the maximum thickness of the 'best' airfoil approaches zero as the Reynold's number approaches zero, but that does not mean that the thickness is constant over the chord at any given Reynold's number. In any case, this has nothing whatsoever to do with the ratio of the wing thickness to the size of the air molecules. Reynold's number effects are completely unrelated to the size of the molecules.
banktoturn