Reynolds number is density times velocity times length divided by viscosity. Reynolds number is useful in comparing flow conditions. If the Reynolds numbers are the same then the flow will be the same around similar shapes. In a standard atmosphere at sea level the air density and viscosity are fixed and that leaves only the length and velocity to be compared. A particular airfoil will have exactly the same flow conditions as anothe example of itself of a different size has the product of chord times airspeed the same. For example, if a 10 inch chord NACA0009 at 50 MPH is compared to a 20 inch chord NACA0009 at 25 MPH, the flow conditions will be the same. that means that the lift coefficients and drag coefficients will be the same for the same angle of attack. It also means that the boundary layer thickness distribution along the chord will be the same and the location of the stagnation point will be the same. Also, the downwash and upwash patterns will be similar. The transition from laminar to turbulent flow will occur at the same percentage of chord and the seperation of flow will be at the same percent of chord.

However, if two examples of the same airfoil are operated at different Reynolds numbers the flow conditions may be somewhat different and the greater the difference between the reynolds numbers, the greater will be the differences in flow conditions. At model Reynolds numbers the rate of change of flow conditions with changes in Reynolds number is much greater than it is at full scale.