# Theory

## What exactly is a Reynolds number?

What does X-Plane do with the high and low Reynolds number that you can enter in PlaneMaker?

Reynolds number is air density x true airspeed x wing chord divided by the air's viscosity. (Viscosity is a measure of the "stickiness" of a fluid. Alcohol has a lower viscosity than honey.)

HUH????

More to the point, the Reynolds number is a measure of how "fine grained" the turbulence of the airflow is. At very low speeds, fluid flow is laminar, that is, it flows in smooth layers. At higher speeds (i.e., higher Reynolds numbers) the flow becomes increasingly turbulent at increasingly smaller dimensions.

Aifoils behave differently at different Reynolds numbers; in general you get a higher stall angle and correspondingly higher lift from an airfoil at higher Reynolds numbers. You can see this from most airfoil performance charts, like those in Theory of Wing Sections.

Each .AFL file has a Reynolds number. When you create a .AFL, you should enter the Reynolds number that's given for the airfoil chart you're using. X-Plane computes the Reynolds number for each airfoil section as you fly. If you've specified two different airfoils for the high and low Reynolds number entries (PlaneMaker Special -> Foils), X-Plane will compute airfoil performance from the two airfoils by interpolating according to the current Reynolds number. For example, if your current Reynolds number is halfway between the Reynolds numbers of the two airfoils, X-Plane will take the average of their performance.

## How do I assign Reynolds numbers when I create an airfoil?

Here's the ideal way to do it: