THE LEADING EDGE RADIUS

The reason for the low critical Re of thin profiles was, Schmitz argued, their combination of very small nose or leading edge radius and relatively small upper surface curvature. The stagnation point of the airflow near the leading edge of a wing at a positive angle of attack is, as Figure 2.2 shows, always slightly below the geometric leading edge. The boundary layer thus begins its journey over the upper surface by flowing around the leading edge itself. At high angles of attack, the flow in this neighbourhood is even slighdy upstream (Fig. 8.5). From near stagnation, the boundary layer thus moves towards a low pressure region on the upper surface, and accelerates. If the profile has a smoothly rounded leading edge of large radius, as thick aerofoils usually do, the boundary layer can follow this curve easily and remains laminar. If the leading edge radius is small, as on thin profiles, the boundary layer is compelled to flow round a very sharp curve or even a knife-like edge, changing direction very sharply while accelerating rapidly towards the low pressure point which, on profiles of this early kind, lies only a small distance behind the leading edge. The boundary layer inertia may be expected to overcome the viscous forces at the sudden change of direction, and separate from the wing surface. It re­attaches immediately the comer is passed, but a very small separation bubble, or what

Schmitz called a ‘rolled over vortex’ forms in the boundary layer. The small leading edge radius thus introduces some artificial turbulence into the airflow, and this encourages early transition. The transition and re-attachment is not instantaneous. A separation bubble forms, and the boundary layer re-attaches some distance aft of the leading edge.