# Can surface roughing effects on an airfoil Reynolds number be quantified?

I am building balsa free flight gliders with a chord of 0.10 meters and a velocity of around 3 meters per second. This works out to a Reynolds number of around 15,000.

The wings are around 1 meter in span, and are made with the wood grain running lengthwise. They are thin undercambered (no bottom covering) with maximum thickness of 7 mm at 30% of chord from leading edge.

Because of the roughness of the unfinished grain, is my Reynolds more like 50,000, or am I still down in the range where a flat plate might be better?

Flat plate friction diagram (picture source). Note the horizontal lines: They show that drag does not drop any more with increasing Reynolds number once the surface roughness exceeds a certain value. Roughness is given here relative to the chord length as $$\frac{\epsilon}{l}$$, with $$\epsilon$$ being the mean height of the roughness and $$l$$ the chord length.
• @Jimmy Don't confuse "increasing drag" and "increasing the drag coefficient". For a given structure, lower Re means lower velocity and the drag coefficient is multiplied by $v^2$ to get the drag force. But for laminar flow the drag force is proportional to $v$ not $v^2$, so the drag coefficient used in the conventional formula appears to be bigger at low Re even though the drag force is less. – alephzero Oct 19 at 0:01