THE BOUNDARY LAYER Re NUMBER

Reynolds number applied to a wing chord is not the same as the Re inside the boundary layer itself. As the airflow meets the wing near the leading edge, there is a point, called the stagnation point (Fig. 2.2) where the flow divides, some to pass above and some below the wing. The Re in the boundary layer at this point is zero, since the distance covered over the surface is nil. The boundary layer flow moves from the stagnation point along the skin of the wing and the Reynolds number at each point is based on the distance of that point measured round the aerofoil profile, from the stagnation point Hence the Re in the boundary layer increases as the distance from the stagnation point increases.

By the time the boundary layer reaches the trailing edge its Re will be higher, because of the greater distance covered, than the average worked out crudely using the wing chord, which is the straight line distance from leading edge to trailing edge. Since most aerofoils have different contours on upper and lower surfaces, and the wing is normally operating at some angle of attack, the boundary layer Re at opposite stations on top and bottom will differ a little. In what follows it is important to distinguish the so-called ‘critical Re’ of an aerofoil profile from the ‘critical Re’ in the boundary layer itself.