Reynolds number is useful in comparing flows around similar shaped objects. If two objects have different sizes but similar shapes and orientations to the flow, the pattern of air flow will be the same in the two cases if the reynolds numbers are the same. This is a handy concept when using lift and drag coefficient measurements from a wind tunnel model or computer simulation of an airfoil. If the air density is the same and the product of chord times velocity are the same, the reynolds numbers will be the same and the data can be applied with the assurance that the results will be valid. For example, if the wind tunnel used a 12 inch chord, at an angle of attack of zero degrees and a velocity of 100 feet per second, then the data could be applied to a model whose chord was 6 inches flying at a speed of 200 feet per second at an angle of attack of zero degrees.

When comparing two wings of the same area and airfoil but different aspect ratios, the frontal area of the two cases will be exactly the same. In this example, as the aspect ratio ratio increases, the thickness decreases such that the frontal area remains the same. If the two wings are operating at the same speed then the reynolds numbers will be different because of the difference in chords.

As the reynolds number decreases the profile drag increases slowly because the boundry layer thickens some with decreasing reynolds number. In the case of the S6063 airfoil at zero degrees angle of attack, the coefficient of profile drag increases from 0.006 to about 0.0068 when the reynolds number decreases from 300,000 to 200,000. At an angle of attack of two degrees the profile drag coefficient increase is from 0.0065 to 0.008 when the renolds numbers decrease from 300,000 to 200,000.

The detailed behavior of the boundry layer is quite complex with changes in reynolds number or angle of attack because of changes in the location on the airfoil surface where turbulence sets in and where laminar seperation bubbles occur when the angle of attack or reynolds number changes. The airfoil polars reveal the effects of this complex behavior.