Subsonic Airfoil Characteristics and Reynolds Number

There are two pages on airfoil characteristics. This one is about subsonic characteristics and the effect of Reynolds number. The other is "Mach Number Effect".

The Reynolds number is directly proportional to the product of airspeed and chord. Atmospheric variables also enter in and you can get an accurate calculation with the output of the program. For model aircraft the Reynolds numbers are much lower than those for full size aircraft. The Reynolds number is a measure of the effect of inertia forces compared to viscous forces. Small numbers mean viscous forces are more important. For models this means a profound change in the boundary layer.

These changes mean low maximum lift coefficient, low stalling angle of attack and high drag coefficients. It also often means that once an airfoil is in the stalled condition a significant reduction in angle of attack is required to "un-stall".

A Clark-Y section tested at low Re is an example of this. The test results for several different Re are given on the page CLARK_Y

The lower as F2A, then close attention needs to be paid to the thickness at the tip. Use this figure as a guide for how thin the blade should be at the tip.