REYNOLD'S NUMBER COMPARISON - Derby Tech, June, 1985

Dennis Brown of Wichita, Kansas, sent me the following:


I think the comparison of the 65-418 and the 0012 is a little misleading.

At low Reynolds numbers the amount of laminar flow tends to be greater than at higher Reynolds numbers. However the tendency for a sudden shift from laminar flow to separated flow is greatly increased. Separation causes a great deal more drag than transition from laminar to turbulent flow. For consistency in wind tunnel data (taken at low Reynold's numbers) , it is common practice to put "roughness strips" near the leading edge of the model so that the uncertainties of laminar separation are avoided. The thickness of these turbulent, low Reynolds number, sections has a large influence on the drag. I've used a tunnel to compare airfoil shapes ranging from symmetrical Eppler sections with very far forward maximum thicknesses to the NACA 67 series airfoils. The difference in drag due to shape was minimal and within the data scatter. The difference in drag due to thickness was quite noticeable. This is the main reason your data shows the 0012 with less drag than the 65-418.

Another item to consider: The leading edge of a section that has a forward maximum thickness location is usually quite round. This means that leading edge separation at higher angles of attack is less likely than with sections with sharper leading edges. In a body of revolution like a soap box derby racer, the problem is different, but the effect is similar.


The following statements are excerpts from the aerodynamics book "Theory of Wing Sections" by Abbott. The numbers in parenthesis represent the page numbers where these statements can be found.

Skin friction is higher for turbulent boundary layer flow than for laminar flow. (83)

When inappreciable separation occurs, the wing profile drag is largely caused by skin friction, and the drag coefficient depends mostly on the relative amounts of laminar and turbulent flow. (84)

Turbulent boundary layers are more resistant to separation than are laminar layers. (84)

Laminar boundary layers can exist for only a relatively short distance in a region in which the pressure increases in the direction of flow.


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